Note: Descriptions are shown in the official language in which they were submitted.
, CA 02270100 1999-04-23
SPECIFICATION
TITLE
METHOD FOR CONNECTING COMMUNICATION SYSTEMS
VIA A PACKET-ORIENTED DATA TRANSMISSION LINK
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a method for connecting communication
systems via a packet-oriented data transmission link, wherein the
communication
systems provide multiplex connections respectively formed of a periodic
sequence of
channel-individual information segments such that data packets serving for
data
transmission are sub-divided into sub-structure elements which are, in turn,
allocated
to channels of the multiplex connections.
Descriation of the Prior Art
Sonderausgabe telcom report and Siemens-Magazin COM: ISDN im Buro -
HICOM", Siemens AG, Berlin and Munich, 1985, particularly pages 50 through 57,
discloses a communication system which includes a primary multiplex terminal,
often
referred to as an S2m interface, for a connection to either a communication
network or
a further communication system. In general, an SZM interfaces includes, first,
30
payload data channels that are designed as ISDN-oriented B-channels
(Integrated
Services Digital Network) with a transmission rate of 64 kBitls and, second, a
signaling
channel that is designed as an ISDN-oriented D-channel with a transmission
rate of 64
kBit/s.
The data transmission rate of 2 Mbitls is thus available for a data
transmission
between two communication systems. The data transmission rate of 2 Mbitls is
thereby
permanently off; i.e., independent of the transmission capacity actually
utilized.
Unused, free transmission capacities cannot be utilized in some other way.
As a result of the increasing need for the transmission of video information
in
modern communication technology such as, for example, still and motion
pictures in
picture telephony applications, the significance of transmission and switching
technologies for high and variable data transmission rates (above 100 Mbit/s)
is
increasing. A known data transmission method for high data rates is what is
referred
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CA 02270100 1999-04-23
to as the asynchronous transfer mode (ATM). A data transmission on the basis
of the
asynchronous mode currently enables a variable transmission bit rate of up to
622
Mbitls.
The present invention is directed to a method with which such a packet-
oriented
data transmission can be realized between a plurality of communication
systems.
SUMMARY OF THE INVENTION
For a better understanding of the functioning of a packet-oriented data
transmission between communication systems, known principles will be discussed
in
greater detail.
In the data transmission method known as the asynchronous transfer mode
(ATM), data packets of a fixed length, what are referred to as ATM cells, are
used for
the data transport. An ATM cell is composed of a cell header, what is referred
to as the
"header", that is five bytes long and contains switching data relevant for the
transport
of an ATM cell and of a payload data field, or "payload", that is 48 bytes
long.
A data transmission that is internal within the communication system, for
example between a switching network module and a network terminal unit,
usually
occurs via what is referred to as a "PCM highway" according to the TDM method
(Time
Division Multiplex). Given a data transmission via an ATM network connected to
the
network terminal unit, a conversion of the continuous data stream internal
within the
communication system into data packets according to the ATM format is
necessary -
also often referred to as "ATM layer" in the literature.
A conversion of the continuous data stream based on the TDM method onto the
ATM format occurs according to the ATM adaptation layer AAL type 1 (ATM
Adaption
Layer). An adaptation of the "ATM layer" to the switching layer (layer 3)
occurs
according to the OSI reference model (Open Systems Interconnection) with the
ATM
adaption layer AAL.
All 32 channels succeeding one another in time in a TDM frame communicated
via a PCM highway are converted by the ATM adaption layer AAL (type 1 whereas
each having respectively one byte of payload data information) onto the ATM
cell
format in the way described below.
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CA 02270100 1999-04-23
In addition to the "header", the first byte within the "payload" area of an
ATM cell
is employed as a pointer. This pointer serves the purpose of restoring the
synchronization between transmitter and receiver in case one or more ATM cells
has
been lost due, for example, to a transmission error. The communication of the
payload
data information begins with the second byte of the payload area. The payload
data
bytes allocated to the individual channels of the TDM frame are thereby
communicated
successively. A transmission of the first byte (the byte allocated to the
channel number
0) of the TDM successor frame occurs immediately after a communication of the
last
byte (the byte allocated to the channel number 31 ) of a TDM frame. The
pointer
thereby points to the first byte of a TDM frame beginning within the payload
area of an
ATM cell. Fig. 1 shows an illustration of this structure.
An allocation of the bytes stored in the payload area of an ATM cell to a
channel
of a TDM frame thus occurs via the position of the byte in the payload area of
the ATM
cell. A transmission of the data occurs independently of the occupancy of a
channel,
as a result whereof the existing network resources are definitely
unnecessarily
occupied.
Added thereto is the fact that the filling time for an ATM cell amounts to 6
msec
(125Ns per byte). As a result, echo problems can occur in the network due to
the
transmission delay - also often referred to as "delay" in the literature. This
effect is
intensified by the employment of compression algorithms as employed, for
example,
within the framework of mobile radio telephony. Given a compression of 1:10,
the filling
time for an ATM cell thus amounts to 60 msec, the echo problems being yet
further
intensified as a result thereof.
Given the employment of compression algorithms that leave the information to
be communicated unmodified due to the removal of redundant information
sequences,
a variable data stream arises from the continuous data stream dependent on the
existing redundancy. Given a data transmission with a constant transmission
rate such
as, for example, given a packet-oriented data transmission according to ATM
adaption
layer AAL Type 1, this leads to problems. For this reason, compression
algorithms are
preferably employed that generate a continuous stream; for example, on the
basis of
statistical predictions. Currently, however, non-compression algorithms can
lead to a
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CA 02270100 1999-04-23
falsification of the information to be communicated in certain situations, so
that these
compression algorithms can, in fact, be employed for a transmission of speech
but are
unsuitable for a transmission of facsimile or data.
A critical advantage of the present invention, therefore, is that only sub-
structure
elements are transmitted via the ATM network whose allocated channels are
currently
occupied. This leads to an improved utilization of the existing network
resources.
A further advantage of the present invention is that, due to the transmission
of
an individually adjustable plurality of payload data bytes allocated to a
terminal
equipment connection in a sub-structure element of a data packet, a data
transmission
with a variable transmission rate can be realized. This enables the employment
of
compression algorithms that, dependent on the redundancy present in the data
to be
transmitted, generate a variable data stream without falsification of the
information from
a continuous data stream.
Due to the definition of the first payload data segment of an ATM cell as a
pointer
that references the starting address of a first sub-structure element located
in the
payload area of the ATM cell, a synchronization of the communication system
given a
loss of one or more ATM cells can be realized in a simple way.
Additional features and advantages of the present invention are described in,
and will be apparent from, the Detailed Description of the Preferred
Embodiments and
the Drawings.
DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a schematic illustration of the data format according to the TDM
method and of the corresponding ATM data format according to the ATM adaption
layer
AAL Type 1;
Fig. 2 shows a schematic illustration of the corresponding ATM data format and
of the data format of a sub-structure element according to the ATM adaption
layer AAL
Type 2; and
Fig. 3 shows a schematic illustration of the corresponding ATM data format,
according to the ATM adaption layer AAL Type 5.
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CA 02270100 1999-04-23
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 shows a schematic illustration of two TDM frames R1, R2 and of two
corresponding ATM cells ATM-Z1, ATM-Z2 according to the ATM adaption layer AAL
Type 1. A TDM frame R1, R2 includes 32 channels via which a data transmission
is
possible in the framework of 30 terminal equipment connections, and wherein an
allocation of 30 channels for a transmission of payload information and of two
channels
far a transmission of signaling information exists. Given a conversion of a
continuous
data stream based on the TDM method onto the ATM format according to the ATM
adaption layer AAL Type 1, all 32 channels succeeding one another in time in a
TDM
frame, with respect to the one byte payload data information, are converted
onto the
ATM cell format in the following way.
In addition to the cell header H of the ATM cell ATM- Z1, ATM-Z2, the first
byte
within the payload area, referred to below as payload data area, is defined as
pointer
Z. This pointer Z points to the first byte of a TDM frame R1, R2 beginning
within the
payload data area of an ATM cell ATM-Z1, ATM-Z2. With this pointer Z, a
restoration
of the synchronization between transmitter and receiver is possible in case
one or more
ATM cells ATM-Z1, ATM-Z2 have been lost due, for example, to a transmission
error.
The transmission of the payload data information contained in a TDM frame R1,
R2
occurs beginning with the second byte of the payload data area of an ATM cell
ATM-Z1,
ATM-Z2. The payload data bytes allocated to the individual channels of the TDM
frame
R1, R2 are thereby communicated in sequence. Immediately after a transmission
of
the last byte (the byte allocated to channel 31 ) of a TDM frame R1, a
transmission of
the first byte (the byte allocated to the channel 0) of the TDM successor
frame R2
occurs. An allocation of the payload data bytes of an ATM cell ATM-Z1, ATM-Z2
to a
channel of a TDM frame R1, R2 thus occurs via the position of the byte in the
payload
data area of the ATM cell ATM-Z1, ATM-Z2.
Fig. 2 shows a schematic illustration of two ATM cells ATM-Z1, ATM-Z2 and of
a sub-structure element SE according to the ATM adaption layer AAL Type 2.
Within
the framework of the ATM adaption layer AAL Type 2, there is the possibility
of
subdividing the payload data area of an ATM cell ATM-Z1, ATM-Z2 into sub-
structure
elements SE. A sub-structure element SE according to the ATM adaption layer
AAL
CA 02270100 1999-04-23
type 2 is composed of a cell header SH that is 3 bytes long and of a payload
data area
I of variable length (0 through 64 bytes). The cell header of a sub-structure
element SE
according to the ATM adaption layer AAL Type 2 is subdivided into a channel
identification CID (Channel Identifier) that is 8 bits long, into a length
indicator LI
(Length Indicator) that is 6 bits long, a sender-receiver identification UUI
(User-to-User
Indication) that is 5 bits long and a 5 bit long cell header checksum HEC
(Header Error
Control).
Corresponding to an ATM cell ATM-Z1, ATM-Z2 according to the ATM adaption
layer AAL Type 1, the first byte in the payload data area of an ATM cell ATM-
Z1, ATM-
Z2 is defined as pointer Z. This pointer Z indicates the starting address of
the first sub-
structure element SE whose cell header SH lies in the payload data area of the
ATM
cell ATM-Z1, ATM-Z2. A restoration of the synchronization between transmitter
and
receiver is possible with this pointer Z in case one or more ATM cells ATM-Z1,
ATM-Z2
have been lost due, for example, to a transmission error.
Due to the sub-division of an ATM cell ATM-Z1, ATM-Z2 into sub-structure
elements SE, a plurality of channels that are all addressed with the same ATM
address,
composed of a VPI _ _ -(Virtual Path Identifier) and of a VCI (Virtual Channel
Identifier), can
be defined within an ATM connection on the basis of the channel identification
CID.
Within the framework of an interworking of communication systems, thus, there
is the
possibility of defining a sub-structure element SE for a transmission of
signaling
information; i.e., for a transmission of D-channel data. A further sub-
structure element
SE can be defined for every required B-channel for a transmission of payload
data
information, i.e. for a transmission of B-channel data, so that the
transmission capacity
can be exactly adapted to the current requirements. In Fig. 2, for example,
only sub-
structure elements SEO, SE4, and SE9 are defined for the channels 0, 4 and 9;
i.e., that
data are currently communicated between the communication systems only via
three
existing terminal equipment connections. In contrast to an ATM cell ATM-Z1,
ATM-Z2
according to the ATM adaption layer AAL Type 1, an allocation of a useful data
byte to
a channel of a TDM frame R1, R2 given an ATM cell ATM-Z1, ATM-Z2 according to
the
ATM adaption layer AAL Type 2 does not occur via the position of the payload
data
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CA 02270100 1999-04-23
byte in the payload data area of the ATM cell ATM-Z1, ATM-Z2 but via the
channel
identifier CID.
A payload data field I of variable length (0 through 26) can be defined by the
six-
bit long length identifier LI in the cell header of a sub-structure element.
The possibility
connected therewith of realizing a data transmission with variable
transmission capacity
allows the employment of compression algorithms that, due to a removal of
redundant
information, generate a variable data stream from a constant data stream.
Given a conversion of a continuous data stream based on the TDM method onto
the ATM data format according to ATM adaption layer AAL Type 2, the payload
data
field I with a constant length of 8 bytes is employed for the sub-structure
elements SE.
A sub-structure element SE according to ATM adaption layer AAL Type 2 is thus
11
bytes long, a cell header with a length of 3 bytes and 8 bytes of payload
data. In this
case, the cell overhang, which is often referred to as "overhead" in the
references, of
a sub-structure element SE according to ATM adaption layer AAL Type 2 is more
than
37% of the payload. In order to reduce this cell overhead, a data transmission
can be
realized with the assistance of the ATM adaption layer AAL Type 5.
Fig. 3 shows a schematic illustration of an AAL Type 5 frame R5 and of two ATM
cells ATM-Z1, ATM-Z2 according to the ATM adaption layer AAL Type 5. An AAL
type
frame R5 has a length of 0 through 2'6 bytes. The information contained in an
AAL
Type 5 frame R5 is divided into one or more ATM cells ATM-Z1, ATM-Z2 according
to
the length of the AAL type 5 frame R5. For a restoration of the
synchronization
between transmitter and receiver if one or more ATM cells ATM-Z1, ATM-Z2 have
been
lost due, for example, to a transmission error, the data contained in an AAL
Type 5
frame R5 is divided such into one or more ATM cells ATM-Z1, ATM-Z2 that the
first byte
of the AAL Type 5 R5 is allocated to the first byte of the payload data area
of an ATM
cell ATM-Z1. As a result of a marking F in the cell header of the last ATM
cell ATM-Z2
allocated to an AAL Type 5 frame R5) it can be recognized that the next-
following ATM
cell that contains the same ATM address, composed of VPI and VCI, in the cell
header
H is the first ATM cell allocated to the AAL Type 5 successive frame.
For a subdivision of an AAL Type 5 frame R5 into sub-structure elements SE,
sub-structure element-individual pointers L (with a length of one byte) are
defined which
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CA 02270100 1999-04-23
indicate the length of the individual sub-structure elements SE. Sub-structure
elements
SE up to a length of 2a = 256 bytes therefore can be realized. Within the
framework of
an interworking of communication systems, there is the possibility, via a
separate
signaling, of defining an allocation between the channels of a TDM frame and
individual
sub-structure elements SE; for example, via the length of the sub-structure
element SE
or via the sequence of the individual sub-structure elements SE. In Fig. 3,
for example,
only sub-structure elements SEO, SE4 and SE9 for the channel 0, 4 and 9 are
defined;
i.e., that data is being currently communicated between the communication
systems
only via three existing terminal equipment connections.
Given a conversion of the data format according to the TDM method onto the
ATM data format according to ATM adaption layer AAL Type 5, a payload data
field
having constant length of 8 bytes is employed for the sub-structure elements
SE. A
sub-structure element SE according to ATM adaption layer AAL Type 5 is thus 9
bytes
long, a pointer Z having a length of 1 byte and 8 bytes of payload data. In
this case,
the cell overhead of a sub-structure element SE according to ATM adaption
layer AAL
Type 5 only amounts to 12.5% of the payload.
Although the present invention has been described with reference to specific
embodiments, those of skill in the art will recognize that changes may be made
thereto
without departing from the spirit and scope of the invention as set forth in
the hereafter
appended claims.
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