Note: Descriptions are shown in the official language in which they were submitted.
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Instrument for Test and Measurement
of ATM Network Virtual Connections
This is a divisional of applicatiou Serial No. 2,237,986 filed November 15,
1996.
Field of the Invention
This invention relates to a test instrument and
methods of use for testing the operation of asynchronous
transfer mode (ATM) telecommunication networks. More
particularly, the invention relates to test instruments for
testing transmission of cells belonging to individual
virtual connections through the network.
Background of the Invention
With the increase in use of computing facilities
throughout modern society; and in particular with increased
communication over optical fiber-linked networks having far
higher transmission speeds than previous conductive wire
connections, there is substantial interest in new methods
of communication. More particularly, previous
communication tended to be segregated between voice and
data communication, with different networks being provided
for each. Typically voice communication took place over
the telephone system, while high speed data communication
took place over dedicated lines; data communication is also
possible in the telephone system, but only at substantially
slower rates than provided by dedicated lines. More
recently, digital video and image communications have
become of increased interest, particularly for so-called
multimedia applications.
The result is that substantial improvements in
flexibility of communication techniques are needed, in
particular, to permit convenient future upgrading of
communication facilities over time as additional data
sources become available. Still more specifically, it is
imperative that standards be developed and implemented
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allowing voice, video, in6ges, and data to be transmitted
more or less interchangeably over varying transmission
media, such that equipment installed at a particular time
will not soon be obsolete, but can continue to be used for
communication as overall communication speeds are increased
in the future.
These needs are largely expected to be met through
broad implementation of so-called asynchronous transfer
mode (ATM) communication networks. As distinguished from
synchronous transfer mode (STM) communications, ATM allows
the traffic rate from a particular source to be increased
or decreased upon demand when communication is desirable.
By comparison, in STM, a particular user is assigned
particular synchronous time slots on a particular
communications medium, limiting the flexibility of the
system. The significance of this distinction between
asynchronous and synchronous transmission to the invention
is discussed further below.
ATM networks are in the process of being installed in
conformity with internationally-agreed upon standards for
transmission of "cells" of data, including in "data" as
used herein digitized 'voice communication, digitized
images, and digitized video, as well as data per so. In
ATM, all types of messages to be transmitted are divided
into fixed length "cells", each cell including a header
including cell payload type and routing information, and a
fixed length "payload". The payload of each cell typically
contains a relatively small portion of an overall message
to be transmitted from a source to a destination. The ATM
cells are transmitted by way of a source node into a
network comprising a large number of switching nodes
connected by communication links. Accordingly, an overall
message to be transmitted from an originating source to an
ultimate destination is divided into a number of cells,
transmitted in sequence over a "virtual connection"
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established when the communication is established. Each
cell transmitted as part of an ATM message transits the
same virtual connection, that is, is routed through the
same sequence of switching nodes and connecting links.
It is important in effective implementation of ATM
networks that the specific type of communication lin}cs
included in each virtual connection not be a constraint on
the format of the ATM cell. That is, the cells of a
particular ATM message may be transmitted by wire, by fiber
optic cable, by satellite, or by combinations of these.
The cell format itself remains unchanged. In this way,
flexibility of the network configuration and implementation
of future faster communications media can be provided
without, for example, rendering obsolete the equipment used
to generate cells from messages to be transmitted.
By comparison, according to another modern day
communication technology, data is commonly transmitted in
the so-called "frame relay" mode, wherein each "packet"
transmitted includes the entire message. Hence each packet
is of different length. In frame relay transmissions, the
packet of data corresponding to the message is preceded by
a single header, such that the entire message is
transmitted in one long burst over a predetermined route
through a series of nodes from a source node to a
destination node. This system remains workable, but is
relatively inflexible as traffic needs change from time to
time. Further, frame relay transmission is best suited for
communication of data per se, which tends to be "bursty".
Voice and video communication have different intrinsic
requirements.
More specifically, communication of data per se is
typically time-insensitive, in that some time delay, and
considerable variation in the time delay experienced by
successive messages or segments of messages, between the
time of transmission and the time of reception does not
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interfere with utility oflcommunication. Voice and video
transmissions are by comparison very sensitive to
transmission time, in that all portions of the message must
be received at a rate closely proportional to the rate at
which they are transmitted, if important information is not
to be lost. Variation in the delay between segments of a
voice or video transmission is particularly disturbing to
the receiving party.
In ATM, as noted, messages are divided into
relatively small cells which are individually transmitted.
For example, a voice communication may be transmitted in
cells each effectively encoding single words, or even
single syllables. The ATM format allows the individual
cells of the message to be transmitted relatively
instantaneously, such that they can be reassembled and
delivered to a listener or viewer at the destination
without perceptible delay. More specifically, because each
message is transmitted over its own virtual connection, set
up only after determination that the nodes involved have
sufficient bandwidth to accommodate the anticipated cell
rate, overloads can be avoided and the cell delay
minimized. i
It therefore will be appreciated that, in essence, an
ATM communication involves setting up a virtual connection
identifying a sequence of nodes extending between a source
and a destination, and dividing the message into cells of
equal length. The cells of the message are subsequently
transmitted over the virtual connection, which may include
each or all of wire conductors, optical links, or satellite
relay links. At the ultimate destination, the headers of
the cells are removed, the payloads are reassembled,
returned to analog format when appropriate (e.g., in voice
communication), and delivered to the user.
The basic format of the fixed-length ATM cell (a
"cell" corresponding generally to a "packet", as that term
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is usually used), includes a header consisting of five eight-bit bytes (or
"octets"), these including cell payload type and routing information,
followed by 48 bytes of payload. Various standards organizations have
agreed on the format of the header and the overall cell structure. See,
for example, "ATM Pocket Guide", Publication No, 908-0119-01,
revision B, July 1994 of Tekelec Corporation of Calabasas, California.
As shown therein, and as reproduced by Fig. 4 hereof, the ATM header
of each cell includes at least 24 total bits of routing information,
comprising 8 bits of "virtual path identifier" (VPI) information and 16
bits of "virtual channel identifier" (VCI) information.
In transmission of ATM cells, the VPI and VCI routing
information in each cell header is updated at each node, responsive to
predetermined information stored by each node at call origination. The
VPI and VCI information stored in each cell at any given time is used
by each node to route the cell to the next node in the series of nodes
making up the virtual connection, as established at call origination.
More specifically, unlike a frame relay transmission, wherein the
header information is unchanged as the packet trans its the entire route
from its originating source to its ultimate destination, in ATM, the VPI
and VCI routing information in the header of each cell is updated as
each intermediate node is transited.
As noted, each cell of any given ATM message transits the same
virtual connection, that is, the same sequence of nodes. As part of the
call origination process, information is stored at each node in the virtual
connection, providing VPI and VCI information used to identify each
incoming cell and update its VPI and VCI, so that the cell is properly
switched to its next destination node in the virtual connection. Thus, as
each cell is received at a node, its individual VPI and VCI information
is examined by the node, and stored VPI and VCI information
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needed to convey that cell to the next node is used to
update the header accordingly. It will therefore be
appreciated that each node in an ATM network includes means
for examining the header of each cell received and updating
the VPI and VCI information accordingly.
The call origination process in ATM is well defined
and need not be detailed here except to mention that when
a call is originated, a series of "signaling" messages are
passed back and forth between the originating source node,
the intermediate nodes, and the ultimate destination node.
The call origination process involves the sending of a
message of predetermined format, indicating the relevant
cell parameters, e.g., the total number of cells to be
transmitted, and their anticipated rate of supply. Each
node which receives this call set-up message considers the
requirements of the call, e.g., the anticipated cell
density, and the like, to determine whether it has
bandwidth - that is, communications capability - sufficient
to handle the anticipated number of cells. During this
process each of the intermediate nodes ultimately forming
part of the virtual connection to be thus established must
in effect agree to the traffic requirements anticipated,
and must store sufficient information to allow updating of
the VPI and VCI as the cells of that message transit that
particular node.
Quality of Service Parameters
ATM users typically will agree with a service provider
to provide a certain quality of service, involving, for
example, pre-agreed limitations on the cell error ratio,
that is, the number of cells including errors that can be
tolerated for a given number of cells transmitted, the cell
loss ratio, that is, the number of cells that the network
may fail to transmit for a given number of cells
transmitted, as may occur due to oversubscription, and
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other parameters of operation. These parameters are agreed
upon depending on the anticipated traffic. For example,
voice and video communications typically can be effectuated
allowing rather higher bit error rates than data
communications; voice and video, however, are more
sensitive to variation in cell delay than are data
communications. Accordingly, these and other parameters
must be measured in use, with respect to specific virtual
connections, in order to ensure that the service contracted
for is met by both user and service provider.
Furthermore, it is desirable to measure specific
statistics of the network's operation, such as the
frequency of occurrence of various types of cells, in order
to optimize network utilization. For example, the cell
headers include indications of cell loss priority which can
be raised by the network when a user exceeds the parameters
of the corresponding traffic contract; the frequency of
occurrence of high cell loss priority indication can
accordingly be monitored to ensure that the network is not
being overutilized.
U.S. Patent 5,343,463 to van Tetering et al discusses
methods for measuring the performance characteristics of a
path of an ATM telecommunications switching network by
generically transmitting test packets (cells) while other
live traffic is operating. U.S. Patent 5,271,000 to
Engbersen et al discloses a method and apparatus for
testing and evaluating distributed networks by transmitting
generic test packets through the network to a test packet
analyzer where the transmitters are located at
geographically distributed locations with respect to the
analyzer. U.S. patent 5,457,700 to Merchant et al shows a
test cell generator for inserting test cells in place of
idle cells in an ATM network. As idle cells do not include
VPI and VCI information assigning them to a virtual
connection, Merchant, as in the cases of Van Tetering and
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Engbersen, is not capaile of measurement of network
performance with respect to a particular virtual
connection. None of these references show systems capable
of tracking transmission of a particular cell through the
network.
Objects of the invention
It is therefore an object of the invention to provide
an instrument and method for identifying the cells
belonging to an individual virtual connection, for
replacing these specific cells at a first test access point
with predetermined test cells, and for detecting these test
cells at a second test access point.
For example, in order to measure cell transfer delay,
two-point cell delay variation, round trip travel time, the
quality of service parameters enumerated above, and other
relevant parameters of network operation with respect to
one or more selected virtual connections, "live" cells
identified in a cell sequence as belonging to a selected
virtual connection may be replaced by predetermined test
cells injected into the cell stream at a first access point
by a first test instrument and detected by a second
instrument at another access point to monitor their
transmission. Due to the asynchronous nature of ATM
communication, the rate of transmission of cells with
respect to any virtual connection may change
instantaneously, and may vary widely subject to the maximum
rate possible on the specific transmission medium. It is
accordingly an object of the invention to provide a first
test instrument capable of'inserting test cells in place of
the identified cells of a selected virtual connection, in
order to avoid disturbance of the cell transmission rate of
the virtual connection. This process performed with
respect to a particular virtual connection according to the
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invention is referred to as "rate-matched cell
identification and replacement".
More specifically, normally continuous streams of
cells are transmitted in both directions between pairs of
nodes in the network. When there is no user traffic to be
transmitted, "unassigned" or "idle" cells are transmitted.
In order to avoid loss of user cells, and in order to avoid
corrupting user data, it is often preferable to transmit
test cells in the "cell slots" in which unassigned or idle
cells would otherwise be transmitted. Therefore, it is
also an object of the invention to provide an instrument
and method for identifying "unassigned" or "idle" cells in
addition to the cells belonging to an individual virtual
connection. Preferably,.the unassigned or idle cells are
replaced with test cells configured as "operations,
administration, and maintenance" (OAM) cells, which are
treated differently by nodes in the network than ordinary
"user" cells. The OAM-configured test cells perform
functions associated with OAM cells, e.g., measuring the
quality of service parameters associated with an in-service
virtual connection. This process performed according to
the invention is referred to as "rate-matched insertion" of
test cells into a virtual connection.
According to a further object of the invention, one or
more bits of a user cell may be modified, constituting that
cell a test cell. For example, a bit of the header may be
altered to test the header error correction circuitry
provided at the individual nodes.
Summary of the Invention
The above objects of the invention and others which
will appear as the discussion below proceeds are provided
by the present invention, according to which a test
instrument implementing the present invention is provided
for connection to a node of a network. Such a node may be
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an originating source o4 an ultimate destination node,
typically termed a gateway node, or may be an intermediate
node.
In contrast to the van Tetering et al, Engbersen et
al, and Merchant et al disclosures mentioned above, this
invention provides a method for replacement of "live"
traffic cells with test cells, modification of live traffic
cells, or insertion of OAM cells into a live traffic cell
stream by a first test instrument connected at any node
forming part of an active virtual connection. Thus, this
invention allows for matching the flow rate of cells
transmitted by a live traffic source located within the
network upstream of the test instrument containing the
invention. The test cells are detected (for example) by a
second test instrument at a second node. The actual
transmission performance experienced by the test cells is
monitored to evaluate the quality of service.
More specifically, the test instrument of the
invention may be operated (1) to replace identified cells
belonging to a specific virtual connection with test cells
having predetermined information stored therein, such as
sequence numbers, time stamps indicating the time of
transmission, or cyclic redundancy checks used for the
detection of bit errors within each cell; (2) to replace
unassigned or idle cells with OAM-configured test cells
including VPI and VCI routing information conforming to a
specified virtual connection, for performing OAM functions
with respect to that virtual connection; or (3) to modify
the information within cells identified as belonging to a
specific virtual connection, e.g., for testing the response
of the network to the modifications. These modifications
may include injecting single or multiple bit errors into
the cell header, changing the payload type indicator and/or
cell loss priority fields, and changing any of the
specified fields within the cell payload. Modifications
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may be applied to the cell stream at a periodic rate, at a
rate that is a function of the virtual connectionrs cell
rate, to a burst of consecutive cells, or to one cell only.
The test instrument of the invention comprises a cell
filter for examining headers and partial payloads of all
cells transiting the network, to identify, for example,
assigned cells belonging to specific virtual connections,
or for identifying operations, administration, and
maintenance (OAM) cells also transmitted by the network
from time to time. The cell filter may be implemented by
combinatorial logic, such as a number of exclusive-OR
gates, or by a content-addressable memory (CAM).
In response to identification by the cell filter of a
cell belonging to a particular virtual connection, a test
cell from a test cell queue or a modified version of the
received cell may be transmitted. An OAM-configured test
cell may be transmitted in lieu of an unassigned or idle
cell identified by the cell filter.
Brief Description of the Drawings
The invention will be better understood if reference
is made to the accompanying drawings, in which:
Fig. Z shows a schematic overview of a network for ATM
traffic communication;
Fig. 2 shows a schematic diagram of the mechanism
whereby a node updates the VPI and VCI information of each
ATM cell it processes;
Fig. 3, including Figs. 3(a) - 3(c), shows three
possible methods of connecting a test instrument to the
test access port of a network node;
Fig. 4 shows schematically the layout of a typical ATM
cell according to the standardized ATM format used in the
industry;
Fig. 5 shows schematically the components of a test
instrument according to the invention;
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Fig. 6 is a block dia6m of the principal components
of the test processor of a test instrument according to the
invention;
Fig. 7 shows a diagram indicating the way in which ATM
communication conforms to the industry-standard open
systems interconnection (OSI) model;
Fig. 8 shows a schematic diagram of the principal
components and steps involved .in rate-matched cell
identification, cell modification, cell replacement, or OAM
cell insertion processing according to the invention;
Fig. 9 shows a block diagram of a test processor as
provided in an implementation of the instrument of the
invention as used for collecting statistics characterizing
network operations; and
Fig. 10 shows a detail of a microsequencer comprised
by the test processor of Fig. 9.
Description of the Preferred Embodiment
As discussed above, the present invention relates to
a test instrument and associated methods for measuring
operational parameters of interest in an ATM communication
network. I
The instrument of the invention can be used to
identify cells belonging to a specified virtual connection
and either modify the identified cells, replace the cells
with test cells, or augment the cells with additional OAM-
conf igured test cells. The test cells are then transmitted
over the virtual connection previously established. The
test cells are typically detected at a second node, so as
to monitor the pertinent parameters of their transmission.
The following provides certain additional information
helpful in understanding the precise nature of the
technical problems to be addressed by the invention, prior
to discussion of the manner in which the invention resolves
these problems.
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As indicated above, asynchronous transfer mode (ATM)
transmission of data (including in "data" voice, images,
and video, digitized as necessary, as well as data per se)
involves the segmenting of all messages to be transmitted
into equal-length "cells" that are time-multiplexed over a
communication link (i.e., cells from plural sources are
transmitted in "cell slots" as available) at a source.
Each cell of a given message is transmitted over the same
sequence of nodes and links, termed a "virtual connection",
to the destination of the particular message. (It will be
appreciated that a "virtual connection" as used herein is
referred to as a "logical channel" in certain references.)
The virtual connection is established at a call setup time,
begun when the originating source node transmits a first
message including information as to the bandwidth required
for transmission of the message. Candidate intermediate
nodes then determine whether they can satisfy the bandwidth
requirements of the traffic, and may negotiate these
parameters if necessary. Ultimately, a virtual connection
is established by storing next-node destination information
in content-addressable memories (CAMs) or other circuitry
comprised by each of the nodes; that is, VPI and VCI
information pertaining to each segment of the virtual
connection of which a given node is a component is stored
at call origination, such that the routing information of
each cell is updated as it passes through each of the nodes
included in the virtual connection.
As the individual cells of a particular message
transit the network, they may be multiplexed several times,
e.g., from a relatively low speed local area network into
a much higher speed wide area network. The links
connecting the nodes may include conductive wires, optical
fibers, and/or satellite transmission links. The streams
of cells received at the ultimate destination nodes are
demultiplexed and presented to the users connected to the
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ultimate destination node.+ Because each of the cells of a
particular message transits the same sequence of nodes
making up the virtual connection, each of the cells should
arrive in its proper order. However, commonly the ultimate
destination node will assemble the various cells of the
message in a buffer, and strip off the header information
and other non-message components of the cells, such that
the entire message can be accessed in a single operation.
Rate Matching in a Virtual Connection
The transmission rates of the ATM cells in aggregate
vary greatly with the medium involved in each link. At a
local area network level, the transmission rate may be
typically up to 10 Mbits/sec, that is, 10,000,000 bits per
second, the standard "Ethernet" rate typically used for
twisted-pair copper wire communication. Over high speed
optical data fibers conforming to the known SONET optical
network protocol, the bit rate may be up to 622 Mbits/sec.
Higher rates are also planned.
Pairs of nodes in the network connected by links
normally transmit a continuous stream of assigned and
unassigned ATM cells at a' fixed rate in both directions
over the links, in order to maintain system synchronism.
When there is no user traffic to be transmitted,
"unassigned" or "idle" cells are transmitted. Such
unassigned or idle cells may be replaced with OAM test
cells according to the invention. More specifically, and
according to an important aspect of the invention, cells
belonging to a specific virtual connection in the
continuous stream of cells passing between pairs of nodes
in the network are modified, augmented with OAM cells, or
replaced by test cells.
As noted above, a distinction exists between
asynchronous communication, i.e., ATM, wherein various
users may seek to transmit messages at any time, so that
cells are assigned time slots as needed, and synchronous
communication, i.e., STM, where all users retain
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permanently assigned time slots in a sequence thereof. In
STM, it is a simple matter to perform rate-matched
processing; the correct time slot is identified by counting
time slots from a synchronizing indication that is
regularly transmitted, and a test packet inserted in the
stream at the corresponding point in time. Transmission of
the test packet through the network can be traced
similarly. In ATM, the cells of a particular virtual
connection cannot be expected to be located in any
particular time slot and must be identified by individual
examination thereof.
Furthermore, as noted, intrinsic to ATM communication
is the fact that cells need not be transmitted between any
identified source and destination at any given time.
Therefore, various important network transmission
parameters, such as the average cell transfer delay, the
cell delay variation, and the size and rate of bursts of
cells, each to be measured with respect to a particular
virtual connection, can only be meaningfully monitored if
the rate of transmission of cells is not varied as a
consequence of the measurement process. Accordingly, in
order that a test instrument containing the invention can
be employed without excessive interference with network
operations, it is necessary that the cell modification or
cell replacement process be performed in a"rate-matched"
fashion, i.e., test cells configured as user cells must be
replacements for or modifications of user cells, in order
to match their rate of transmission. This process,
referred to herein as "rate-matched" cell identification and
cell modification or cell replacement, depending on the
operation to be performed, allows known test cells to be
inserted into the stream for detection elsewhere in the
network for monitoring network parameters. For certain
tests, the stream of user cells need not be altered; in
these cases, unassigned or idle cells can be replaced with
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OAM-configured test cells tithout affecting transmission of
user cells. Further, in order that the sequence of user
cells is not disrupted, all cells passing through the node
to which the test instrument is connected must be delayed
identically, if at all.
In order to satisfy the objects of the invention
mentioned above, it is therefore necessary to provide a
test instrument having the capability of recognizing cells
with respect to their respective virtual connection and
replacing or modifying the identified cells at speeds of up
to the 622 Mbits/sec and higher rates available over
optically-linked nodes. The high speed recognition
capability provided by the test instrument of the invention
is employed for rate-matched modification, replacement, or
insertion modes of operation, e.g., in order to recognize
cells belonging to a virtual connection of interest and
replacement thereof in the message stream by test cells.
Topology of an ATM network
Fig. 1 shows in schematic form the overall arrangement
of an ATM network, exemplified by the connection of a local
area network 10, that is, a number of individual computers
12 interconnected by well-known local area network
hardware, to a wide area ATM network indicated generally at
22. For example, a local area network 10 may exist on a
college campus or the like, as indicated. The local area
network (LAN) comprises a number of individual computers 12
each connected by LAN interface units 14 to ATM LAN
switches 16. The ATM LAN switches 16 provide communication
between computers 12 of the LAN 10, and also identify cells
intended for destinations outside the LAN 10 and convert
these to ATM cells, a process which will be detailed
further below. LAN 10 is connected by an ATM user network
interface 18 to wide area network 22 by way of an ATM
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switch 20. Network ATM switch 20 is thus a"gateway" node
to the wide area ATM network 22. Network 22 comprises a
large number of intermediate nodes 23 connected by a large
number of links 24. ATM traffic may also be originated by
digital telephone equipment, video conference equipment, or
other known devices.
As indicated above, various nodes 23 transmit at
differing transmission rates and are connected by
correspondingly-varied media, i.e., links 24. Low speed
nodes may be connected by wire conductors; more commonly,
and especially in new installations, optical fibers are
being used to connect high speed nodes so as to enable very
high speed data transmissions from point to point.
Satellite links may also connect various nodes.
As noted above, as a rule, an overall message to be
transmitted over the ATM network, which may be a few
seconds of digitized voice or video, or data per se, is
divided into a large number of cells of identical format.
The individual cells are generated by an ATM switch serving
as a gateway node 20. Each cell is provided with initial
VPI and VCI information at the gateway node, which is used
to direct it to the first intermediate node in its virtual
connection. As the cells transit the wide area network 22,
their VPI and VCI information is updated at each node 23
until the cells reach a similar network ATM switch serving
as a destination node 28 connected to the ultimate
destination of a particular message. The ultimate
destination may be a computer 25, similarly part of a local
area network by virtue of being connected to a LAN router
26, in turn connected to destination node 28. As
indicated, the structure and operation of the ATM network
is well known and is defined by a variety of different
standards describing the interfaces between the various
classes of nodes, links, LANs, and other components
involved.
{
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Fig. 2 shows schematically the updating of each cell
at each node in a normal ATM connection. Cells arriving on
an incoming line 30 reach the node 32. More speci'Lically,
streams of incoming cells are received over a plurality of
links 24, and are multiplexed by a switch 29 to provide a
single stream of cells via line 30, to each node 32. Each
incoming stream of cells will typically include cells from
a number of virtual connections. As discussed above, and
in further detail below, each ATM cell includes a 5-byte
header, including VPI and VCI routing information
identifying the next node in the virtual connection
established for the cells of each message.
The VPI and VCI of the incoming cells, shown
schematically at 34, are supplied to the comparand register
35 of (typically) a content-addressable memory (CAM) 36.
When correspondence between the VPI and VCI of the
incoming cell and the contents of the CAM 36 is detected,
the CAM outputs updated VPI and VCI routing information
from data stored at 38 at call set-up with respect to each
virtual connection. The new VPI and VCI then become part
of the header of the outgoing cell, and are used to
similarly identify the icell at the next node. A
multiplexing switch 39 fornling the connection of each node
to a plurality of outgoing links 24 is controlled such that
each cell is transmitted over the correct link to reach the
next node in the virtual connection.
Thus, as discussed above, at each node in a virtual
connection, the VPI and VCI of the incoming message are
updated to indicate the next node in the network to which
that particular cell is to be transmitted. The
correspondence between the "incoming" and "outgoing" VPI
and VCI information is established at call origination and
stored in each node forming part of a virtual connection
established for each message by exchange of a sequence of
call set-up messages between an ultimate source node and an
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ultimate destination node. While the capability of
updating the VPI and VCI routing information of each cell
at each node can be provided using other circuit
components, currently-preferred node designs typically use
CAMs in each node to provide the VPI and VCI updating at
very high speed. Nodes having this capability are within
the skill of the art as of the time of filing of this
application. See U.S. patents 5,414,701 to Shtayer and
5,422,838 to Lin.
In the embodiment shown, the nodes themselves each
comprise routing intelligence, that is, for responding to
call set-up messages to establish virtual connections.
However, the invention would also be useful in evaluating
a network designed such that one or more central routers
determined the sequence of nodes and links to be traversed
by the cells of : each message, i. e. , to define the virtual
connections.
Methods of Test Access
Figs. 3(a) - 3(c) show three different methods whereby
a test instrument according to the invention can be
connected to a conventional preexisting node 42 of an ATM
network, using, in this example, electrical wire
connections. As shown, node 42 is connected to two
incoming lines 44 and 46 and two corresponding outgoing
lines 48 and 50, respectively. As shown in Fig. 3(a), the
test instrument 54 may be connected to two test port
terminals 52 in a "monitor" mode of operation of the test
instrument of the invention. In this mode, network service
is undisturbed; the test instrument simply monitors traffic
through the node and may perform various tests, maintain
network operation statistics, and the like, without
affecting flow of traffic.
In an "emulate and terminate" mode shown in Fig. 3(b),
the test instrument effectively terminates incoming line 46
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and originates traffic over~transmit line 48. In this mode
of operation, the test instrument effectively takes the
node out of service.
Finally, in a third "through" mode, shown in Fig.
3(c), the test instrument is interposed in the traffic path
between the incoming lines 44 and 46 and the corresponding
outgoing lines 48 and 50. R a t e- m a t c h e d c e 1 1
identification and modification, and replacement and
insertion of test cells according to the present invention,
are performed with the instrument connected in the
"through" test access mode of Fig. 3(c). For purposes of
the present invention, the test instrument can be
interposed in the cell flow path before or after the
updating of VPI and VCI. information at each node, although
it will be apparent that the VPI and VCI information
supplied to the instrument will vary accordingly.
In the "through" mode, all traffic passing through
node 42 is delayed by a fixed period of time, typically an
integral number of cell time slots, varying with the rate
of transmission of the cells at that particular point in
the network, to enable test instrument 54 to carry out the
appropriate processing steps. It will be apparent that it
is desirable that all traffic be delayed identically so
that the order of cell transmission is not disturbed.
Further, it is important that the delay be as short as
possible.
Note that networks linked by high speed fiber optic
lines may be accessed in a conceptually similar way,
although there are no currently standardized methods for
providing test access to nodes in fiber optic networks.
In each of Figs. 3(a) - 3(c), and below, the ports at
which the test instrument 54 receives traffic from the node
are labeled RX1 and RX2, while the ports through which the
test instrument transmits data back to the node for
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transmission over the network are referred to as TX1 and
TX2.
Fornzat of an ATM Cell
Fig. 4 shows the format of the typical ATM user cell.
As discussed above, each ATM cell includes 53 8-bit bytes,
that is, includes a 5-byte header and 48 bytes of payload.
The payload may contain user data in the case of normal
user cells or control information, in the case of
operations, administration, and maintenance (oAM) or
resource management (RM) cells. The individual cells in a
stream of cells may be separated by additional cell
delineation bytes of predetermined format, depending on the
medium of transmission being employed.
Within a LAN, for example, and at the ATM User Network
Interface (UNI) 18 (Fig. 1), the first byte of the header
includes four bits of generic flow control (GFC)
information, followed by four bits of virtual path
identifier (VPI) information. Within the wide area network,
the GFC bits are typically replaced with four additional
VPI bits. VPI information also makes up the first four
bits of the second byte, which is followed by four bits of
virtual channel identifier (VCI) information. All eight
bits of the third byte of the header include VCI, as do the
first four bits of the fourth byte. The 24 (or 28) total
bits of VPI and VCI together comprise routing information
for the cell. Where the cell is an unassigned or idle
cell, the VPI and VCI bits are all set to zero; the test
instrument of the invention is capable of recognizing this
and inserting an oAM-configured test cell in place of an
unassigned or idle cell. The fifth, sixth, and seventh
bits of the fourth byte of the cell header are payload type
identifier (PTI) bits, which typically indicate whether the
payload of the cell includes user data, whether the
segments of the network through which the cell has traveled
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have experienced congestiAn and the like, or whether the
cell is a"OAM" cell, used for control of network
operation, administration, and maintenance. The last bit of
the fourth byte is a cell loss priority (CLP) bit, a"1"
indicating that the cell is subject to discard in the event
of network congestion or the like. Where the VPI and VCI
fields are all zeroes, the CLP bit differentiates between
idle and unassigned cells. Finally, the fifth byte of the
header includes header error control (HEC) data used to
reconstruct the header if a single bit error is detected
within the cell header by any of the nodes along a virtual
connection.
Again, as discussed above, the VPI and VCI information
included in the header of each ATM cell is updated at each
node as the cell transits its virtual connection from its
originating source node to its ultimate destination node.
More specifically, each node stores VPI and VCI information
corresponding to each virtual connection then being
supported. When the sequence of nodes making up the
virtual connection is determined at call origination, the
VPI and VCI information of the incoming cells is stored by
each node in association with the corresponding VPI and VCI
information to be written into the header of each cell, so
as to update the network routing information of each cell
as each node is transited.
The test instrument of the invention is'therefore
capable of examining incoming cell VPI and VCI information
to identify the cells belonging to particular virtual
connections, e.g., for replacing such cells, modifying one
or more bits thereof, or augmenting the stream of cells
with OAM-configured cells, thus providing a stream of test
or OAM cells for being similarly identified by a second
instrument elsewhere in the virtual connection. In this
case, the test instrument of the invention is connected in
the through mode of Fig. 3(c), wherein all traffic through
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a particular node passes through the test instrument. For
example, some or all of the cells from a particular virtual
connection may be detected and replaced with test cells, to
monitor the quality of service and other characteristics
between particular nodes in the network. Alternatively,
unassigned cells may be identified and replaced by oAM-
configured cells to avoid impacting an in-service virtual
connection, e.g., to measure cell error ratio or the like.
The cell payload type information included in the PTI field
can be examined by the instrument of the invention at each
test access point to differentiate between user and OAM
cells. This facility is employed by the invention, e.g.,
to measure network performance with respect to a particular
virtual, connection, e.,g., cell loss ratio, cell
misinsertion rate and theilike.
Contrasting Switched and Permanent Virtual Connections
The VPI and VCI information is normally arbitrary, in
that it cannot be analyzed to identify the ultimate source
of the cells, to identify the position of a cell in a
sequence of cells, or the like. The VPI and VCI information
also does not include a virtual connection identifier per
se. However, it will be appreciated by those of skill in
the art that certain VPI and VCI values are "reserved",
e.g., for call origination messages. Candidate nodes
identify the reserved VPI and VCI of call origination
messages in order to respond to the call set-up query,
i.e., to determine whether they can serve as part of a
proposed virtual connection. Certain "permanent" virtual
connections (as opposed to "switched" virtual connections
established at call origination, as described) may also be
established by permanent VPI and VCI assignments. Further,
individual service providers may employ parts of the VPI
field to indicate levels of service and the like.
~
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Test Instrument FunctionallDescription
Fig. 5 shows a block diagram of the principal
components of a test instrument 59 for carrying out the
functions provided according to the invention. The
processing steps required to carry out the various tests
made possible according to the invention are performed in
main part by a test processor 60 monitoring cells received
from an associated node and transmitting a stream of user
and test cells to the node. The test instrument is
connected to the network via a network test access point
61, such as the test port 52 of Fig. 3(a). When desired,
the test access point may include dual transmit and receive
ports to allow bidirectional cell monitoring, as shown by
Fig. 3(c). The same functionality can be provided in many
cases by a single receiver port and single transmitter port
test instrument as well.
The specific operations to be carried out by the test
processor 60 are controlled by a user providing commands by
way of a user interface 62 which is in turn connected to
test processor 60 by a host processor 64. The host
processor 64 provides the user with, for example, the
ability to specify a virtual connection of interest, and to
specify the type of test' to be performed. The host
processor also provides substantial processing capability
with respect to test results and the like received by way
of test processor 60. Test instruments including a user
interface, a test processor, and a host processor are
generally known; the invention in this case generally
resides in the specific functions provided by and structure
of the test processor 60.
As discussed, the test processor 60 identifies
specific cells in a stream of cells by comparison of the
VPI and VCI fields of each cell to information stored in
the test processor. The stored information is provided by
the user, who (for example) would key the information into
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the host 64 by way of a keyboard comprised by the user
interface 62. The user in turn may obtain the VPI and VCI
information from a system administrator, who assigns this
information to each new virtual connection. The instrument
may also store VPIs and VCIs received for a period of time,
building a list of active connections. The user may then
select one for analysis. A virtual connection may also be
established specifically in order to test specific aspects
of network operation, and the VPI and VCI then communicated
to the user at the test instrument.
Test Processor Block Diagram
Fig. 6 shows a functional block diagram of the
principal components of the test processor 60. The test
processor examines each I incoming cell and carries out
appropriate action in real time, that is, without
significant delay of cells, long term storage, or the like.
In a presently preferred embodiment, the test processor 60
comprises a "real-time embedded system" comprisiiig
dedicated hardware, one presently-preferred implementation
being shown in further detail in Fig. S.
Incoming cells are received by a receiver 66. The VPI
and VCI fields of each cell are compared by a cell filter
68 to VPI and VCI information pertaining to a virtual
connection of interest, as supplied by the user via the
host 64, connected in turn to a microprocessor 84. For
example, traffic between a particular source and a
particular destination may have been identified as
including an unacceptable cell error ratio or the like.
Accordingly, the cell filter 68 would be supplied with the
VPI and VCI information of the corresponding virtual
connection, for comparison to that of incoming cells, to
locate those belonging to the particular virtual connection
of interest.
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Where correspondencef is detected, such that a cell
belonging to a particular virtual connection is identified,
an Insert Test Cell signal indicated at 70 is provided by
the cell filter 68 to a cell stream multiplexer unit 72.
The cell stream multiplexer unit 72 selects a test cell
from one of several buffers or queues. For example, an
OAM-configured cell provided by an OAM cell generator 74 to
cell stream multiplexer 72 may be selected for insertion.
That is, the inserted cell may include PTI information
identifying the cell as an OAM cell (see Fig. 4), or may be
configured as a normal user cell, depending on the specific
test to be carried out. If a test cell configured as a
normal user cell is to be transmitted, multiplexer 72 may
select the test cell from test cell generator 76. One or
more bits of the identified cell may also be modified by a
cell modifier 73 connected to multiplexer 72, to configure
that cell as a test cell.
Cell stream multiplexer 72 is supplied with the stream
of cells passing through the cell modifier via a first line
77, with user-configured test cells to be inserted in place
of identified cells in the stream of test cells from the
test cell generator 76 via a second line 79, and with OAM-
configured test cells from generator 74 via a third line
75. In the absence of the Insert Test Cell signal 70, that
is, unless a test cell is to be interposed into the stream
of cells, the stream of cells passes unaltered from the
receiver 66 through cell modifier 73 to cell stream
multiplexer 72. The stream of cells flows from the
multiplexer 72 to a transmitter 78, where the stream of
cells is reconfigured for serial transmission as necessary
and returned to the network via port TX1.
The particular test operation to be performed is
controlled responsive to user commands provided to host
processor 64 (Fig. 5) connected in turn to microprocessor
84, communicating control signals to the principal
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components of the test processor via bus 86. For example,
where a time stamp is to be provided as part of measurement
of the round trip travel time of a test cell, this
information is inserted into the payload of the test cell
by a time stamp generator 82 connected downstream of the
multiplexer 72, as shown. The test instrument of the
invention may be employed for measuring various statistics
of interest in monitoring network operation.
The cell filter 68 can be implemented in at least two
ways. The basic function of the cell filter is to compare
stored VPI and VCI information, for example, representative
of cells belonging to a particular virtual connection, to
the VPI and VCI of each incoming cell. Where cells
belonging to a particularvirtual connection need simply be
identified, e.g., for replacement or modification, or for
measurement of the size and rate of bursts of cells from a
particular virtual connection, the cell filter may be
implemented using combinatorial logic and associated
registers, as shown in Fig. 8 and discussed in detail
below. Where one or more characteristics of the cells are
to be monitored, or where cells from a number of virtual
connections are to be identified, a content-addressable
memory (CAM) as shown in Fig. 9 is the preferred
implementation of the cell filter 68.
The OSI Seven Layer Model
The receiver 66 and transmitter 78 in Fig. 6 both
perform functions usually termed part of the "physical
layer" of a communication device. The "physical layer"
nomenclature refers to the first layer of an industry
standard model of communications referred to as the "Open
Systems Interconnection" (OSI) model. According to the OSI
model, communications functions are divided between seven
"layers", each comprising hardware and software and having
standard interfaces, defined such that the components
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(hardware and software) implementing each layer can be
replaced and upgraded as necessary without affecting the
others. The principal functions of the physical layer are
deframing, physical overhead removal, and cell delineation.
The physical layer thus identifies the cells within the
continuous bit stream received from the node and presents
these cells to the ATM layer. Layer 7, the highest layer
in the OSI model, is referred to as the application layer,
and may for example constitute software whereby a human
user indicates that a message is to be sent. Layers 6, 5,
4, and 3 are increasingly-detailed layers the specifics of
which are not of direct relevance to the present invention.
The ATM protocol processing occupies the link layer, layer
2.
Fig. 7 shows an example of the application of ATM
within the OSI model. As noted, the lowest layer, layer 1,
is the physical layer. in the example of Fig. 7, this
physical layer is exemplified as an optical fiber
transmitter indicated at 94. At the second or ATM layer
90, ATM cells are created and formatted according to the
particular protocol employed by the medium. in this
example, the cells are formatted into "frames" conforming
to a SONET protocol used commonly at the physical layer for
optical fiber transmission. More specifically, as
indicated further in Fig. 7, the higher layers 95 generate
a large data frame 96; that is, the message to be
transmitted is provided by the higher layers 95. At the
ATM adaptation layer 97, the frame 96 is fragmented into
48-byte segments, corresponding to payloads of cells, as
indicated at 98; headers are added at 99, providing the VPI
and VCI address information, and completing the cells. The
Fig. 7 example presumes the cells are to be transmitted
over optical fiber according to the SONET protocol.
Accordingly, the cells are placed in a SONET frame at 92
(whereby a number of cells may be transmitted between SONET
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nodes, without individual routing, for example) and
transmitted by transmitter 94 at the physical layer 93, as
indicated.
Correspondingly, when a cell is received by the test
instrument, the physical layer device of the node organizes
the serial stream of bits transmitted by the physical
medium as an 8-bit wide by 53 byte long cell as shown in
Fig. 4.
Rate-Matched Cell Modifi.cation, Replacement, and Insertion
As noted, user cell modification, replacement of user
cells identified as belonging to particular virtual
connections, and replacement of unassigned or idle cells
with test cells, which may be configured as OAM cells, all
to be performed in an appropriately rate-matched manner,
are particular objects of the invention. Fig. 8 shows a
detailed functional block diagram of one implementation of
the test processor employed to perform the relevant
functions, as well as the flow of cells and control
signals. The Fig. 8 implementation corresponds generally
to the block diagram of Fig. 6; it will be recognized that
numerous alternatives are possible.
As shown in Fig. 8, the incoming data is received by
a physical layer receiver 66. The data is supplied to a
cell identification and filtering unit 68. Where
correspondence between a cell and stored VPI and VCI
information is detected, a"VC-Detected" control signal on
line 100 is provided to a cell stream multiplexer input
selector 136. More specifically, selected VPI and VCI
values are provided at 102, e.g., by the user via
microprocessor 84, and stored in register 106. (Where an
unassigned or idle cell is to be replaced, these values are
all zeroes, as noted.) The cell identification and
filtering unit 68 compares the stored cell identification
information to the corresponding fields of each cell
}
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received. When correspondAce is detected, the VC-Detected
signal is provided on line 100 and triggers, for example,
modification of the cell or its replacement by a test cell.
The cell identification and filtering unit 68 can similarly
identify unassigned or idle cells, and trigger insertion of
an OAM-configured test cell in their place.
In the embodiment shown in Fig. 8, the cell filter 68
is implemented by combinatorial logic, that is, discrete
circuit elements, in this case 32 exclusive-OR gates 104.
As is well known by those of skill in the art, the OR of
the outputs of a set of exclusive-OR gates provide a
positive active-low output only when all of the inputs to
the exclusive-OR gate match a specified pattern. In this
application, the selected VCI/VPI values are stored in a
first input register 106 and form one set of inputs
supplied to the exclusive-OR gates on a 32-bit input bus
108. The corresponding VPI and VCI values from the
incoming cells are stored in a second input register 110
and are supplied by a second 32-bit input bus 112 to the
exclusive-OR gates 104. When all of the bits stored in the
two registers 106 and 110 match, the exclusive-OR gates 104
provide 32 identical output signals to an OR gate 114,
which in turn provides a one-bit output signal to a latch
116. The "low" output of latch 116 comprises the VC-
Detected signal 100, and is also supplied as a Field
Modification control signal 118 to a cell field modifier
circuit 120, and to a user cell counter 111.
The incoming cells are also supplied by way of line
122 to a pipelining first-in first-out (FIFO) buffer 124,
where they are stored for a'fixed number of cell slots. In
the event the cells are not to be replaced or modified,
they are supplied from buffer 124 via "through-mode cells"
line 126 to cell stream multiplexer 128. Multiplexer 128
is a switching device for supplying the cell to be
transmitted in each cell slot to transmitter 78.
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Whether a particular identified cell (or all of a
group of similar cells) is to be modified or replaced is
controlled by a Fields to Modify signal provided by
microprocessor 84 on line 130 responsive to user commands.
Where one or more received cells are to be modified
responsive to the Field Modification control signal 118,
and responsive to the Fields To Modify signal on line 130,
the cell then stored in the pipelining FIFO buffer 124 is
directed to the cell field modifier circuit 120 by way of
a Cells to Modify line 132. One or more bits of the
incoming cell are accordingly set, reset, or inverted by
the cell field modifier circuit 120 and provided as a
modified cell on line 134 to multiplexer 128.
By comparison, where the identified cells are to be
replaced with test cells, multiplexer 128 may select an
appropriate test cell from several sources, responsive to
an Input Selection signal provided by a cell stream
multiplexer input selector 136, again responsive to a test
selection made by the user. For example, where an
unassigned cell is detected by the cell filter 68, as
indicated at 138, an OAM-configured test cell can be placed
in the cell stream in place of the unassigned cell. OAM-
configured test cells can also be inserted into the cell
stream at regular intervals measured by counter 111, which
maintains a count of the user cells detected between OAM
cells corresponding to a particular virtual connection. A
test and/or OAM cell generator 140 provides a sequence of
test cells, which may be configured as user or OAM cells,
to a test and/or OAM cell FIFO queue unit 142. The test
and/or OAM cells are provided to the multiplexer 128 via
line 144. Alternatively, a dummy test cell can be provided
from a buffer 146 to the multiplexer 128, in the event no
test cell is currently available from the queue unit 142.
Where, for example, a particular virtual connection
has been identified as experiencing problems, the VPI and
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VCI fields of each cill are examined by the cell
identification filter 68 to identify cells from that
particular virtual connection. The "VC-Detected" signal
indicating detection of a match is provided to cell stream
multiplexer input selector 136. The cell stream
multiplexer input selector 136 is also supplied with a Cell
Available signal on line 137 from test/OAM cell FIFO queue
142. Thus, when both the VC-Detected and Cell Available
signals, on lines 100 and 137 respectively, are active, the
multiplexer 128 injects a test cell from the test cell
queue 142 in place of the corresponding user cell in the
stream of through mode cells on line 126, thereby replacing
the user cell.
Similarly, if user input has been provided indicating
that oAM-configured test cells are to be transmitted when
possible, when both the Unassigned Cell Detected and Cell
Available signals, on lines 138 and 137 respectively, are
active, the multiplexer 128 injects an OAM-configured cell
from the OAM cell queue 142 in place of the corresponding
unassigned cell in the stream of through mode cells,
thereby inserting an OAM-configured test cell into the cell
stream. OAM cells can ialso be transmitted at regular
intervals determined by counter ill, as discussed further
below.
As noted, dummy test cells stored in buffer 146 are
selected by multiplexer 128 to replace dropped user cells
when the test and/or OAM cell generator 140 is not capable
of cell generation at the rate of the virtual connection
being dropped.
Finally, in the absence of either of the VC-Detected
or Unassigned Cell Detected signals, multiplexer 128 passes
the through mode received cells from the cell pipelining
FIFO buffer 124 unaltered to the physical layer transmitter
78.
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In some cases, alteration of one or more bits of a
particular cell or cells within a virtual connection
constitutes that cell a test cell for the purposes of the
present invention. For example, a header bit may be
altered to test the next node's header error detection and
correction facility. To this end, a cell field
modification unit 120 is provided. Cell field modification
unit 120 is connected between cell pipelining FIFO buffer
124 and the cell stream multiplexer 128 as shown. Each
cell received by the test processor is stored in cell
pipelining FIFO buffer 124 for a fixed number of cell
slots, while the cell is simultaneously examined by cell
filter 68, until it is time for the cell to be transmitted.
An altered or replacement test cell is transmitted, as
above, or the cell stored in cell pipelining FIFO buffer
124 is transmitted unaltered. As noted, one or more bits
may be altered by the cell field modification unit 120,
where this is sufficient to constitute the modified cell a
suitable test cell.
Finally, as noted, a continuous stream of ATM cells is
transmitted between each linked pair of nodes at any given
time, each cell occupying a cell slot, that is, a unit of
time. In order that the system can remain synchronized, it
is important that a continuous stream of cells is
transmitted. When there is no user traffic to be
transmitted, "unassigned" or "idle" cells may be
transmitted; such unassigned and idle cells include an
indication in their headers that they do not contain user
data, i.e., their VPI and VCI are all zeroes. Such cells
are the first choice for replacement by OAM-configured test
cells according to the invention. Use of OAM-configured
test cells is discussed further below.
Summarizing the operation of the instrument according
to the invention, all cells passing through a particular
node are examined by the cell filter 68. The cells of one
~
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or more virtual connectioJ are replaced by test cells, may
have one or more bits modified, may have a time stamp
inserted, may be augmented with or reconfigured as OAM
cells, may have individual identifiers inserted in their
payloads, or the like. In order that network synchronism
is not disturbed, all cells passing through the node are
delayed by an equal finite deterministic period, typically
four cell slots, to provide time, for example, for
replacement of identified cells with test cells according
to the invention. For this reason, cells to pass through
the node unaffected, as well as cells to be modified, are
stored in the cell pipelining FIFO buffer 124 so that they
can be transmitted at the appropriate time.
Depending on the specific test cell to be inserted
into the cell 'stream, as noted above, either a single bit
of a received cell may be altered, multiple bits may be
altered, or the entire cell may be dropped and replaced
with a predetermined test cell. Commonly, but not in every
case, such predetermined test cells are configured as OAM
cells, as OAM cells can be inserted into an in-service
virtual connection without affecting transmission of user
cells. Test cells may alsolbe configured as user cells, or
as resource management (RM) 'cells, depending on the test to
be performed. RM cells may be transmitted for recognition
at a virtual source/virtual destination (VS/VD)-capable
node within the network. A VS/VD-capable node has the
ability to loop-back RM cells (i.e., transmit it back to
the node from which it was received). Various tests can
therefore be carried out using a single instrument
according to the invention by judicious selection of RM-
configured test cells.
Accordingly, when a particular cell is identified in
the stream of cells by the cell identification filter 68,
the cell, having been simultaneously stored in the cell
pipelining FIFO buffer, may be altered in a cell field
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modification unit 120 in response to a field modification
control signal 118, or the cell may be discarded entirely
and replaced responsive to VC-Detected signal 100. In the
latter case, a test and/or OAM cell may be selected from a
queue thereof stored at 142, having been generated by a
test and/or OAM cell generator unit 140. In response to
the control signals received, the cell stream multiplexer
128 selects for each cell slot the original cell stored in
the cell pipelining buffer 124, an altered cell received
from cell field modification unit 120, a test cell or an
OAM cell received from the queue 142, or a dummy test cell
from buffer 146. The selected cell is directed to the
transmitter 78 for re-serialization and transmission to the
node for being released into the network in sequence.
Important features ofithis aspect of the invention are
detection of specific cells (either identification of cells
belonging to a virtual connection of interest, or
unassigned or idle cells) by the cell filter 68 and control
responsive to user instructions of the options of passing
the cell through the instrument unaltered, modifying the
cell, or dropping the cell and inserting a test cell.
These functions are selected responsive to the field
modification control signal 118 and VC-detected control
signal 100 provided to the multiplexer 128, both responsive
to user input.
It will be appreciated that the function of the cell
filter 68 in the embodiment shown in Fig. 8 is simply to
identify those received cells having VPI and VCI
information conforming to stored corresponding data, or
similarly to identify unassigned or idle cells. In this
circumstance, the exclusive-OR embodiment of the cell
filter 68 is adequate. Where, however, a number of virtual
connections may require simultaneous monitoring, or where
several characteristics of the cells are to be monitored,
for example, to measure certain statistics indicative of
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the overall operation of ~the methods, the cell filter is
preferably implemented by a content-addressable memory
(CAM). As is understood by those of skill in the art, a
CAM has the capability of simultaneously comparing input
data supplied to a comparand register to a number of stored
values and outputting correspondence signals and/or data
responsive to the comparison. Use of a CAM therefore
provides significant additional flexibility to the
instrument of the invention and is preferred in numerous
embodiments thereof, depending on the exact functions to be
performed by the instrument. Figs. 9 and 10, discussed
below, illustrate a CAM - containing embodiment of the
instrument, as used for monitoring statistics of operation
of the network.
Use of the Invention in Monitoring Network Performance
As mentioned above, a number of parameters describing
network operation are of interest, in particular in
monitoring whether the service being provided meets quality
of service standards. Parameters of network operation
relevant to such determinations include measurement of cell
transfer delay, cell delay variation, cell error ratio,
cell loss ratio, and cell misinsertion rate, among others.
The rate-matched cell identification, replacement,
modification, or insertion process according to the
invention just discussed is employed in measuring many of
these parameters. For example, a particular virtual
connection may include a very large number of cells being
transmitted between a particular originating source and an
ultimate destination. It is not possible to measure the
cell transfer delay, for example, without identifying a
particular one of the cells and measuring its particular
delay. For this reason, the test instrument containing the
invention is provided with the ability to replace the cells
from that virtual connection wit}- test cells including
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identifiers, time-of-departure time stamps, and sequence
numbers. A second test instrument can identify individual
test cells, for example, in order to append a time-of-
arrival time stamp, and measure the cell transfer delay by
computing the difference between the two time stamps. A
number of similar tests can be performed with respect to
successive cells from a particular virtual connection and
the results compared to measure the cell delay variation,
a very important parameter in voice and video transmission,
as noted.
The test cell is identified at a second node of
interest by a second generally similar test instrument
similarly monitoring the headers of each cell for detecting
cells belonging to the virtual connection and then
examining the header, for example, and optionally part of
the payload, for identification of a test or OAM cell. For
example, a stream of user cells flowing through the test
instrument of the invention may be modified, using the cell
field modifier circuit, by setting all of the cell loss
priority (CLP) bits to 1.
Other modifications are also possible. For example,
as noted, individual cell identifiers may be added to the
payloads of test cells and detected by a similar
instrument, effectively tracking individual cells through
the network. In order to avoid the necessity of
differentiating such test cells configured as user cells
from actual user cells, testing may be performed with
respect to particular virtual connections only when they
are out of service, i.e., fully configured but not in
immediate use for transmission of user cells.
Overview of OAM Performance Management Processing
As discussed above, certain cells provided in the
network are termed OAM (operations, administration, and
maintenance) cells. Several types of OAM cells are defined
~
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- performance management (~PM), fault management (FM), and
others. Bits in the PTI f ield of the header are set to
identify the OAM cells for identification by the nodes.
These OAM cells accordingly do not contain user data, but
include payload information provided for other purposes.
As noted, OAM cell types include perforntance
management (PM), fault management (FM), and other types.
The different types of OAM cells are detected and treated
differently by the nodes. For example, OAM-FM cells are
removed by the nodes from the path traversed by user cells;
accordingly, for example, measurements of cell delay
determined using test cells configured as OAM-FM cells
would not accurately track the cell delays experienced by
user cells. OAM-PM cells, by comparison, transit the same
path as user cells, and measurement made using test cells
configured as OAM-PM cells are representative of
transmission of user cells.
OAM-configured test cells may be transmitted, e.g.,
between intermediate nodes, to measure cell loss ratios at
regular intervals between the selected nodes. OAM-PM cells
would normally be preferred for this purpose. For example,
an instrument according to the invention connected to a
given node may transmit an'OAM cell after every, say, 256
user cells belonging to a specified virtual connection, and
a second instrument connected to a receiving node may count
the cells received with respect to that virtual connection
between detection of OAM cells. If the result is not 256,
it is apparent that a cell has been lost or misinserted.
More specifically, user cell counter 111 may be
employed to count the user cells received with respect to
a particular virtual connection and cause multiplexer 128
to select an OAM-configured test cell for transmission at
regular intervals, e.g., every 256 cells, as noted. In
many circumstances, the cells immediately following the
256th user cell will also be user cells, and an unassigned
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or idle cell, as normally preferred for insertion of OAM-
configured test cells, will not be available. In order to
avoid interference with user traffic, the 256th and all
subsequent user cells may be delayed by one cell slot, to
permit insertion of the OAM cell. The "lost" cell slot can
be recovered upon detection of the next idle cell.
However, in busy virtual connections, this may take some
time. An alternative is to transmit an OAM cell in place
of the next detected idle cell, together with an indication
of the actual number of user cells counted by counter 111
in the interval since transmission of the prior OAM-
configured test cell. The receiving instrument can then
compare the count value provided with its own similar count
of the cells received from that virtual connection between
successive OAM cells.
I7se in Measuring Round Trip Time
Those of skill in the art will be aware that there are
available "virtual source/virtual destination" (VS/VD)-
capable nodes, wherein intermediate nodes can be operated
in some respects as if they were originating source or
ultimate destinations, effectively segmenting the network.
The round trip travel time can be measured between such
VS/VD-capable nodes, essentially using the process just
described. That is, two test instruments according to the
invention are connected to two VS/VD-capable nodes of
interest, and cells including time stamps are originated at
a first VS/VD-capable node, looped back at a receiving
VS/VD node, and detected at the first to measure the round
trip travel time of the message between the two nodes. The
capability of identifying specific cells provided according
to the present invention is essential to such test
capability. Round Trip Time (RTT) can also be measured
between an ATM source and a virtual destination, such as a
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VS/VD-capable node or betieen an ATM virtual source, again
a VS/VD-capable node, and its destination.
Use of Instrument for Statistics Measurement
a. Microsequencing Technology
Figs. 9 and 10 provide details of an implementation of
the instrument of the invention as employed for measuring
certain statistics important in monitoring the overall
performance of the ATM network. Fig. 9 shows this test
processor, which includes a microsequencer 200.
As shown in Fig. 10, a microsequencer 200 is typically
composed of a control memory 202, which stores a
microprogram composed of routines, and an address
generation circuit 204. Each stored routine is composed of
a set of micr.oinstructions dedicated to perform a task.
Each microinstruction has two components, a part which
executes a micro-operation, and another part fed back to
the address generator 204, to select the next
microinstruction. The address generation circuit 204 then
provides the next address to the control memory 202, which
provides the next microinstruction, and so on. The initial
"vector" inputs to the acidress generator 204 come from
external circuits, providing a full or a partial starting
address to the microsequencer. The feedback signal also
affects the choice of the next address. For example, the
feedback signal can cause the address generator 204 to
increment the prior address to access the next
microinstruction, or to branch to a new address in control
memory 200.
As employed in a preferred implementation of the
present invention, a microsequencer 200 receives vector
inputs from a CAM. (See Fig. 9.) The vectors are provided
to an address generator 204 responsive to examination of
each incoming cell, to select the correct starting address
in a control memory 202. Control memory 202 accordingly
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provides microinstructions controlling incrementing of
appropriate memory locations corresponding to the cell
types, virtual connection types, and aggregate network cell
types corresponding to the cell identification. Table I
provides an exemplary list of all types of cells as to
which statistics are maintained according to one
implementation of the invention.
More specifically, the control memory 202 provides
Count Offset, Incrementer Control, and Feedback signals.
The Count Offset signal forms the less significant part of
the address to a memory device storing running totals of
the counts, that is, the Count Offset signal specifies the
address of the stored count value(s) to be incremented.
The Increment Control signal initiates the process of
incrementing the count being addressed by the Count Offset
signal. The Feedback signal is supplied to address
generator 204 to control the addressing of the next
microinstruction in control memory 202.
b. Statistics Processor
Fig. 9 provides a block diagram of the principal
components of the test processor 60 as used in an
instrument implementing statistics processing functions
according to the present invention. An ATM physical layer
device 210 is used to extract cells from the physical layer
of the network. The physical layer device 210 provides
certain indicator control signals to the a controller 212.
These signals indicate the occurrence of certain types of
error, including cell payload transmission (CRC-10) errors
detected using cyclic redundancy check (CRC) bits,
correctable or uncorrectable errors in the header, both
detected using header error control (HEC) bits (Fig. 4), or
loss of cell delineation (i.e., inability to locate the
beginning and endings of successive cells). A Loss Of Cell
Delineation signal halts all processing of the statistics
~
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processor, until cell t delineation is reestablished.
Uncorrectable and Correctable HEC error indications are
passed on to the microsequencer 200 to be counted;
uncorrectable HEC error indicators further invalidate the
filtering process carried out by the CAM on all cells, such
that no other counters are incremented with respect to that
particular cell. CRC-10 cell payload errors, that is,
signals indicating detection of errors in the cell payload,
are also counted by the microsequencer 200, and disable the
counts associated with cell payloads, e.g., the counts
maintained of various subtypes of OAM cells, RM cells, aiid
AAL3/4 cells, the latter referring to classes of service
provided.
Controller 212 can be implemented as a state machine,
i.e., a series of logical elements arranged to step along
possible predetermined paths and having specified outputs
at each step in response to sets of particular input
values. The output signals provided by controller 212 may
include an Enable Comparison signal controlling comparison
of valid received cell data to stored categories of data by
the CAM 214, and an Enable Operation signal provided to
microsequencer 200, as shown. Controller 212 may also pass
HEC Error signals, CRC 'Error signals and the like to
microsequencer 200 for counting.
Each received cell is stored in a first in-first out
(FIFO) buffer memory 216 in preparation for cell filtering
by a content-addressable memory (CAM) 214. The CAM 214
effectively filters the cells in order to identify those of
particular interest. As discussed above, cell
identification and filtering are performed by supplying a
portion of each cell (in a preferred embodiment, including
the VPI and VCI, the payload type identifier and cell loss
priority bits, and part of the cell payload) to the
comparand, i.e., address, register of the CAM 214. Cell
identifying information, again at least the VPI and VCI of
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cells belonging to virtual connections of interest, is
stored in CAM 214, typically responsive to user input from
user interface 62 (Fig. 6), provided to the CAY, via
embedded processor controller 224, as indicated at 225.
CAM 214 determines whether the corresponding bits of
each received cell match any of the information stored in
the CAM. If a match is detected, indicating, for example,
detection of an OAM cell received from a particular virtual
connection of interest, a VC Region Index signal and a
Counts Vector signal are provided by CAM 214, that is,
those signals form the associated data field of the matched
entry. The Counts Vector signal is stored by a rate-
decoupling FIFO 218, and becomes an input signai to an
address generator 204 comprised by microsequencer 200 (see
Fig. 10); that is, the Counts Vector signal becomes a
Vector input signal selecting a microsequencer subroutine
needed to increment the appropriate counter values. The VC
Region Index signal is effectively a compressed version of
the VCI/VPI address, and is used to select a memory region
in a dual port random access memory (DPRAM)220. DPRAM 220
contains the count values used to record network
operational statistics; those associated with a particular
virtual connection are all accessed by the VC Region
signal, with individual counts selected by the Count Offset
signal. Stated differently, the VC Region index signal is
used to select the region in memory 220 maintaining
counters for the aggregate network traffic. It will be
appreciated that the number of virtual connections which
can be supported is limited by the size of the VC Region
Index, e.g. an 8-bit VC Region Index signal supports
simultaneous maintenance of count values with respect to
each of up to 256 virtual connections.
Microsequencer 200 operates to maintain various
statistics stored by the dual-port random access memory
(DPRAM) 220 by generating the appropriate count offset
~
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addresses corresponding to the statistical counts to be
incremented, causing the previously stored count values to
be read from the addressed memory locations in DPRAM 220,
and supplied via a 16-bit data bus 221 to an incrementer
222 f or incrementing. The incremented values are then
stored back in their original locations in DPRAM 220.
An embedded processor controller 224 extracts the
stored results from DPRAM 220 periodically, responsive to
a toggling signal provided by a results update interval
timer 226. For example, in monitoring a 155.52 Mbps
network, the results update interval (i.e., the interval at
which the stored values are read and reset to zero) must be
at least equal to 8 cycles per second, to assure that none
of the 16-bit counter values stored by DPRAM 220 will
overflow. Adjustments to counter value sizes in DPRAM 220
and results update intervals are required to support lower
or higher speed networks.
As indicated, the DPRAM 220 is bank-switched, that is,
two copies of all of the counted values are maintained by
the DPRAM corresponding to the virtual connections being
monitored. Each copy occupies a different bank. This
allows microsequencer 100 to continue to increment counter
values in one bank while embedded processor controller 224
extracts the results from the other bank. As noted, a
clock signal from results update interval timer 226 causes
the banks to toggle.
The monitored counts, that is, the statistics that are
maintained, are typically used to track network
utilization, such as the number of high priority cells
received with respect to each virtual connection being
monitored. Table I following identifies one set of
statistics that may be maintained, for monitoring operation
of the network according to the invention.
As indicated by the three columns of Table I, cell
type statistics are maintained for each virtual connection
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(VC) (column 1) for reserved VCs, for monitoring use of
reserved VCs (e.g., for call set-up messages, OAM cells,
and the like (column 2), and in the aggregate (column 3).
The individual cell types listed and selected by the
counter offset address in each row of the table are
identified by the ATM publications mentioned above and will
be familiar to those of skill in the art.
The counts contained in Table I are accumulated in the
test processor 60 of Fig. 5 and passed as test results to
the host processor 64 at regular time intervals. The host
processor 64 will add test results from the latest time
interval to previously accumulated results. In addition,
the host processor 64 performs various arithmetic
operations such as adding, subtracting, multiplying, or
dividing categories of results, in order to derive other
result categories. The host processor 64 then filters and
formats the test results and passes test and measurement
information to the user interface 62.
The following is an example of the results processing
which may be provided by the test instrument of the
invention. As shown in the first two rows of Table I,
corresponding to counter offset addresses 0 and 1, CLP=O
and CLP=1 counts (that is, counts of cells having their CLP
bit set to 0 or 1 respectively) are accumulated with
respect to a particular VC in column 1; this category is
therefore called Cell Type Counter Maintained for each VC
Region Address. The information is stored in DPRAM 220 of
Fig. 9 during the time interval controlled by the results
update interval timer 226. At the end of the results
update interval as controlled by timer 226, the CLP=O and
CLP=1 counts (and others) are read by embedded processor
controller 224 and added to the corresponding previous
CLP=O and CLP=1 counts. The new CLP=O and CLP=1 counts are
added together to derive a total VC cell count. The CLP=O
count is divided by the total VC cell count and multiplied
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by 200 to derive the ptrcentage of cells having high
priority. The percentage of high priority cells is
presented to the user interface for display.
TABLE I
1 2 3
Counter Cell Type Reserved Aggregate
Offset Counters Virtual Network Cell
Address Maintained For Connection Cell Type Counters
Each VC Region Type Counters
Address
0 CLP=O CLP=0 CLP=O
1 CLP=l CLP=l CLP=1
2 Uncongested Uncongested Uncongested
3 Congested Congested Congested
4 AAL5 EOM AAL5 EOM AAL5 EOM
OAM F5 Segment OAM F5 Seginent OAM F5 Segment
6 OAM F5 End-to- OAM F5 End-to- OAM F5 End-to-
End End End
7 In-Band RM In-Band RM In-Band RM
8 out-of-Band RM Out-of-Band RM Out-of-Band RM
9 Reserved Reserved Reserved
OAM PM OAM PM OAM PM
11 OAM FM OAM FM OAM FM
12 OAM A/D OAM A/D OAIvI A/D
13 OAM SM (Sys OAM SM (Sys OAM SM (Sys
Mgmnt) Mgmnt) Mgmnt)
14 AAL3/4 BOM AAL3/4 BOM Unused
AAL3/4 COM AAL3/4 COM Unused
16 AAL3/4 SSM AAL3/4 SSM Unused
17 AAL3/4 EOM AAL3/4 EOM Unused
18 Congested RM Congested RM Congested RM
19 No Increase RM No Increase RM No Increase RM
ACR Increase RM ACR Increase RM ACR Increase RM
21 Forward RM Forward RI4 Forward RM
22 Backward RM Backward RM Backward RM
23 Unused Unused Unused
24 Unused Unused Unused
Unused Unused Invalid Cell
(different
types)
26 Unused Unused GFC /= 0
27 RM CRC-10 Error RM CRC-10 Error RM CRC-10 Error
28 OAM CRC-10 OAM CRC-10 OAM CRC-10
Error Error Error
29 AAL3/4 CRC-10 AAL3/4 CRC-10 Unused
Error Error
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30 AAL3/4 Length AAL3/4 Length Uncorrectable
Error Error HEC
31 Correctable HEC Correctable HEC Correctable HEC
It will thus be appreciated that statistics are
maintained according to the invention using a CAM 214 to
identify cells by comparing at least part of the header
(and in some cases part of the payload) of each received
cell to stored information identifying, for example,
particular virtual connections and cell characteristics of
interest. When a match is detected by CAM 214, a Counts
Vector signal provides an address to a microprogram stored
by microsequencer 200. The microsequencer 200 reads each
Counts Vector from the FIFO 218 in order to select and
activate a corresponding microprogram. Each microprogram
in turn provides a list of addresses of the appropriate
counts in DPRAM 220 to be incremented. Thus the counts
stored by the DPRA.M 220 corresponding to the cells of
interest are incremented by the microsequencer 200. The
current value for each count to be incremented is retrieved
from DPRAM 220, incremented by an incrementer 222, and
written back to its assigned location in DPRAM 220. Stated
more generally, the CAM 214 examines the pertinent fields
of each incoming cell and provides the microsequencer 210
with information identifying the counts to be incremented.
The microsequencer then causes the correct counts located
in DPRAM 220 to be read therefrom, incremented, and stored
back into DPRAM 220.
At intervals controlled by timer 226, the values for
each count are read and cleared from DPRAM 220 by
controller 224. In the preferred embodiment, DPRAM 220 is
implemented as a dual-port random access memory, divided
into two banks which are organized identically; the most
significant address line is used to toggle access between
the banks. Toggling is initiated by a Toggle Banks signal
{
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provided by the results uphate interval timer, implementing
a "bank switching" function. The bank switching function
allows one bank of the memory to be read by the embedded
processor controller while the microsequencer is
simultaneously incrementing counts stored in the other
bank. The toggling interval, as controlled by the results
update interval timer, must occur frequently enough to
prevent the counters contained inside the DPRA.M froni
overflowing, resulting in a loss of data, e.g., due to a
continuous stream of cells of a single type. The embedded
processor controller 224 may be used to generate additional
"derived statistics" by mathematically combining the
results read from the dual port memory.
Finally, it should be understood that the basic
architecture of the statistics processor shown in Fig. 9 is
also useful in monitoring statistics of non-ATM
communications networks which also operate at high speeds.
For example, statistics may be maintained with respect to
low-speed frame relay networks using conventional
microprocessor technology to recognize the individual
packets by examination of their Data Link Connection
Identification (DLCI) bits, that is, by examination of
header information, analogous to the VPI and VCI
information of ATM cells, comprised by each franle relay
packet. However, microprocessor speeds are inadequate to
monitor statistics in high-speed "DS3" frame relay
networks, employing data rates of 45 mbits/sec. With
appropriate modifications which will be apparent to those
of skill in the art, the statistics processor architecture
of Fig. 2, employing a CAM to examine header information at
high rates, a microsequencer to increment counts
corresponding to data of interest, and a dual port RAM to
store the counts while permitting data to be read out, is
capable of successfully monitoring statistics concerning
operation of high-speed frame relay communications links.
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Conclusion
It will thus be appreciated that the instrument of the
invention, having the capability of identifying individual
cells in a stream of ATM cells by examining the cell header
information of each, is therefore enabled to perform
operations not possible in the prior art. More
specifically, the ability to identify cells from a
particular virtual connection and modify, replace, or
augment them with test or OAM cells recognizable elsewhere
in the network by a second test instrument, or looped-back
by an OAM-capable node, allows the transmission of
individual cells to be monitored. This ability in turn
allows direct measurement of cell transfer delay and other
important parameters, even though a particular virtual
connection may include thousands of cells, none of which
indicate under normal circumstances their position in the
sequence of cells transmitted over the virtual connection.
In absence of the capability of the instrument of the
invention to add individual identifiers to the cells of
particular virtual connections, it would not be possible to
measure actual transmission parameters with respect to
individual ATM cells. By comparison, in STM, where cells
from a particular connection are constrained to occupy
specific time slots in an overall sequence, there is no
need to identify cells belonging to an individual
connection by examining each cell passing through a node.
While a preferred embodiment of the invention has been
described in detail, and numerous examples of its operation
have been given, it will be appreciated by those of skill
in the art that these are merely exemplary and that other
implementations of these and various further aspects of the
invention are also within its scope and extent. The
invention is therefore not to be limited by the above
exemplary disclosure, but only by the following claims.
49
