Note: Descriptions are shown in the official language in which they were submitted.
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NETWORK SYNCHRONIZATION ARCHITECTURE FOR A
BROADBAND LOOP CARRIER (BLC) SYSTEM
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] NOT APPLICABLE
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] NOT APPLICABLE
REFERENCE TO A "SEQUENCE LISTING," A TABLE, OR A COMPUTER
PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK.
[0003] NOT APPLICABLE
1 S BACKGROUND OF THE INVENTION
[0004] The present invention relates generally to a system combining a
synchronization
function of a network element with a network interface function.
[0005] A Digital Loop Carrier (DLC) System is a component of a communications
network
that bundles a number of individual phone line signals into a single
multiplexed digital signal for
local traffic between a telephone company central office and a business
complex, subscriber, or
other outlying service area. Digital loop Garners can carry traffic for
regular phone calls (Plain
Old Telephone Service or POTS) and Integrated Services Digital Network (ISDN)
service. More
recently, approaches have been developed for using DLCs to handle the higher
bandwidth of
Digital Subscriber Loop (DSL) service. Such approaches are often referred to
as Broadband
Loop Carrier (BLC) systems. Figure 1 generally shows such a communications
network.
[0006] In order for the BLC system to function, it must remain synchronous
with respect to
network data signal timing. Further, each network element in the network
requires a stable clock
signal synchronous to the network data signals in order to maintain proper
signal timing. For
this reason, network elements such as BLC systems often extract
synchronization signals either
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from a signal source or from a dedicated timing source such as a Building
Integrated Timing
Source (BITS) generator.
[0007] The signal sources, or data signal inputs, to a network element can be
in the form of
optical signals, such as OCN (Optical Carrier level N), or electrical signals,
such as Tl. Both
types of signal carry data in the form of data bits and synchronization
information in the form of
signal transitions. Network elements typically extract both data and clock
information from their
inputs. The synchronization information typically requires local processing in
the form of
filtering in order to make it suitable for synchronization with the local
network element. Since
the signal has passed over an imperfect transmission network, periodic or
random variations in
the period of the digital signal may occur. A phase-locked loop (PLL) and
stable local oscillator
perform this function. The filter removes short and long-term variations in
frequency of the
recovered synchronization signals. The local stabilized oscillator also allows
the network
element to remain synchronized to the network if a connection to the network
is temporarily lost,
within tolerances specified in BellCore standard GR-1244-CORE.
[0008] In network elements serving a large number of end users, reliability of
the
synchronization system is typically designed to be more robust than that found
in smaller
systems. In order to achieve the required level of reliability, these systems
typically employ a
form of redundant synchronization hardware along with redundant
synchronization inputs from
the network. Along with a control element, these redundancy schemes are able
to automatically
recover either from synchronization input failures or hardware failures by
switching to the
backup. The requirements for various levels of redundancy are specified in
BellCore documents
GR-1244-CORE, GR-303-CORE, and GR-253-CORE.
(0009] In a typical network element, dedicated hardware modules in the form of
circuit boards
perform the synchronization. These modules receive synchronization signals
from the
communications network and provide the network element with the necessary
synchronization
signals to provide whatever service it is intended to provide. Since
synchronization uses
dedicated equipment, the extra equipment occupies space in the system cabinet
or shelf, which
could otherwise be used to increase functionality. It also adds cost,
mechanical complexity, and
increased points of potential failure, thereby reducing product reliability.
(0010] Therefore, it is an object of the present invention to obviate or
mitigate at least some of
the above-mentioned disadvantages.
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BRIEF SUMMARY OF THE INVENTION
[0011] In accordance with an aspect of the present invention, there is
provided a combined
wide area network (WAN) port/synchronization unit for receiving inputs
including data and
S timing information and for synchronizing the data for transmission. The unit
includes the
following components. A network interface receives the input and recovers data
and primary
timing information from the input. A data-path function processes the data. A
reference
selection unit receives timing information from the network interface as well
as timing
information from a secondary combined WAN port/synchronization unit. A
synchronization
control unit selects the most reliable timing information from the plurality
of timing information
inputs to the reference selection unit. A multiplexor multiplexes the timing
information with the
processed data across a link.
(0012] In accordance with another aspect of the present invention, there is
provided a
redundant network synchronization system. The system includes the following
components. A
primary combined WAN port/synchronization unit receives and synchronizes data
from a
network. A secondary combined WAN portlsynchronization unit receives and
synchronizes data
from the network. The secondary unit is coupled with the primary unit for
communicating
timing information, received from the network, therebetween. A primary system
switch element
receives data and timing information from one of the primary and the secondary
combined WAN
port/synchronization units for switching data to a predefined destination.
[0013] It is an advantage of the present invention that an increased number of
subscribers can
be served from the same volume of equipment. Space in a system, which would
otherwise be
occupied by a timing module, can now be used to house additional subscriber
line interfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] An embodiment of the invention will now be described by way of example
only, with
reference to the following drawings in which:
[0015] Figure 1 is block diagram of a communication network;
[0016] Figure 2 is a schematic diagram of a combined WAN-synchronization port;
[0017] Figure 3 is a schematic diagram of a redundancy scheme using the WAN
port
illustrated in Figure 2; and
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[0018] Figure 4 is block diagram of a method according to an embodiment of the
present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] For convenience, like numerals in the description refer to like
structures in the
drawings. Refernng to Figure 2, a combined wide area network (WAN) port and
synchronization unit is illustrated generally by numeral 200. The port/unit
200 includes a
network interface 202, a timing termination unit 204, a reference selection
unit 206, a
synchronization control unit 210, a synchronization unit 212, a data-path
function 208, and a
multiplexor 214.
[0020] The network interface 202 has an input for coupling to an incoming
network signal and
outputs coupled to the reference selection unit 206 and the data-path function
208. The output
from the data-path function 208 is coupled to the multiplexor 214. The timing
termination unit
204 has an input for coupling to a building integrated timing source (BITS),
and an output
coupled to the reference selection unit 206. The reference selection unit 206
additionally has
inputs for coupling to timing signals from other WAN ports, and an input
coupled to the
synchronization control unit 210. The output from the reference selection unit
206 is coupled to
an input of the synchronization unit 212, which has another input coupled to
the synchronization
control unit 210. The output of the synchronization unit 212 is coupled to the
multiplexor 214.
The multiplexor 214 has an output for coupling to a local system (not shown).
[0021] The network interface 202 terminates a signal from a network in order
to provide
telephony and data service to end-users, or subscribers (not shown).
Typically, the signal is an
electrical or optical signal. Electrical Garner standards include the T-Garner
system, such as T1
and T3. Optical signals include optical signals such as OC x, where the
transmission speed is x
times the base speed OC-1 of 51.84 Mbps. Furthermore, the standards include
electrical signals
that are converted to optical signals, such as Synchronous Transport Signal
(STS) and
Synchronous Transfer Mode (STM). These signals are generally provided for
protocols such as
Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH).
Other
possible protocols include Ethernet over fiber, Resilient Packet Ring (RPR),
and the like, as will
be appreciated by a person skilled in the art.
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[0022] The network interface 202 recovers network synchronization signals 205
from the input
signal and transmits them to the reference selection unit 206. The data from
the input signals is
transmitted through standard data-path functions 208, where they are processed
for the local
system. The data-path functions 208 may vary from application to application,
but such
functions are known in the art and need not be described in detail. Further,
the timing
termination unit 204 extracts timing information 207 from the BITS and
transmits it to the
reference selection unit 206. Both the timing information 207 and the network
synchronization
signals 205 are also transmitted to the other WAN ports to be coupled with
their reference
selection units.
[0023] Thus, the reference selection unit 206 also receives timing signals
from other sources,
such as backup or secondary WAN ports. The reference selection unit 206
selects a
synchronization source from the plurality of received timing signals. The
selection is made by
the synchronization control unit 210 in accordance with either operator
intervention or
automatically using preset rules.
[0024] For example, signals transmitted in accordance with the GR-1244-CORE
standard
include a message within the signal for indicating the suitability of the
synchronization signal. A
DS 1 timing signal is considered to have failed or to be unavailable under the
following
conditions: a loss of signal energy, which is indicated by a loss of signal
(LOS) indicator at the
local receiver; an alarm condition, which is indicated by a defined bit
pattern referred to as the
DS1 Alarm Indication Signal (AIS); and frame mismatch, indicated by out of
frame (OOF) or
loss of frame (LOF) indicators. These messages are stripped from the incoming
signal at the
network interface 202 and sent to the reference selection unit 206 along with
the timing
information.
[0025] In yet another example, signals transmitted in accordance with the GR-
436-CORE
standard include a message within the signal for indicating the suitability of
the synchronization
source. These messages, referred to as Synchronization Status Messages (SSMs),
indicate the
suitability of using an input signal as a synchronization source. SSMs can be
carried in the
messaging channels of SONET and DS 1 links, and the like. These messages can
convey whether
or not the link should be used for synchronization as determined by the
sourcing network
element. Further, these messages convey the quality level of a clock source as
indicated by the
sourcing network element.
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[0026] The above examples illustrate two methods for determining the validity
of the
synchronization signals. Other methods are known in the art and will be
apparent to a person
skilled in the art.
[0027] The synchronization control unit 210 is normally set up to use one of
two timing
signals, PRIMARY and BACKUP. The PRIMARY and BACKUP timing references are
derived
from like interfaces. For example, if the PRIMARY timing reference is
extracted from the input
signal, the BACKUP timing reference is extracted from the input signal.
Similarly, if the
PRIMARY timing reference is retrieved from the BITS, the BACKUP timing
reference is
retrieved from the BITS. There is normally no provision to switch types of
timing sources
although this could be done. Generally, the timing architecture is deemed to
be reliably
engineered and thus multiple backups are not typically provided. In the
present embodiment, a
single alternate combined WAN port/synchronization unit is provided as a
backup.
[0028] Yet further, if none of the timing signals provided to the reference
selection unit is
deemed to be valid or lost due to cable cuts, a fall back mode is provided. In
the fall back mode,
a local network element returns to the previous valid clock setting, which is
maintained by a
local PLL. This mode is referred to as a HOLDOVER mode.
[0029] The timing signal selected by the reference selection unit 206 is
output to the
synchronization unit 212. The synchronization unit 212 provides signal
conditioning to the
recovered network synchronization signals in order to satisfy the service
quality requirements
imposed by relevant standards such as GR-1244-CORE, GR-303-CORE, and GR-253-
CORE.
The operation performed by the synchronization unit 212 is known in the art
and thus need not
be described in detail.
[0030] The selection of timing signals from a plurality of available signals
provides a
mechanism for synchronization path redundancy. This allows a network element
to continue
operating, even in the presence of line and equipment failures. (This concept
will be explained
in greater detail with reference to Figure 3.)
[0031] The multiplexor 214 multiplexes the data output from the data-path
function 208 with
the timing signal received from the synchronization unit 212 and distributes
the combined data
and synchronization signal to the local network element system over a system
backplane.
[0032] Referring to Figure 3, a simplified schematic of a network element
system is illustrated
generally by numeral 300. Figure 3 illustrates various synchronization paths
as well as the
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redundant nature of those paths for the purpose of illustrating the function
of the combined WAN
port synchronization unit 200 (see Figure 2). The network element system 300
comprises a
primary 302a and a secondary 302b combined WAN port/synchronization unit
(corresponding to
the port/unit 200 of Figure 2), dual redundant system switch elements 304, and
multiple, non-
redundant, subscriber interfaces 306.
(0033] Each of the WAN port/synchronization units 302 is coupled with each of
the system
switch elements 304. Similarly, each of the system switch elements 304 is
coupled to each of the
subscriber interface elements 306. Each WAN port/synchronization unit 302
recovers network
reference timing signals from its respective optical/electrical data
interfaces (see Figure 2).
These extracted timing signals are made available to the other WAN
port/synchronization units
for providing redundant timing paths (as previously described above with
reference to Figure 2).
[0034] The system switch elements 304 in the present embodiment provide two
functions.
Firstly, they provide a redundant switch element for data and voice signals.
Therefore, if one of
the switches 304a is malfunctioning, the other switch 304b is used to route
the data. Secondly,
they provide redundant timing paths for network reference timing signals.
Thus, if the network
reference timing signal is disturbed en route to a system switch element 304a,
the network
reference timing signal can be routed through an alternate system switch
element 304b. The
system switch elements 304, either through operator intervention or autonomous
control (based
on predefined parameters), can choose either of the combined WAN
port/synchronization unit
outputs to use as its input. Typically, the system switch elements 304 select
the output of the
primary combined WAN portlsynchronization unit 302a as their input. If,
however, the system
switch elements 304 do not receive such a signal, they switch their input to
the output of the
secondary combined WAN port/synchronization unit 302b.
[0035] In the present embodiment, the subscriber interfaces 306 provide both
Plain Old
Telephone Service (POTS), or analog voice, and asynchronous digital subscriber
loop (ADSL)
functions. Other types of subscriber interfaces will be apparent to a person
skilled in the art.
Each subscriber interface 306 receives network reference timing signals from
each of the system
switch elements 304. Similarly to the system switch elements 304, the
subscriber interfaces 306
select the output of the primary system switch element 304a as their input.
If, however, the
subscriber interfaces 306 do not receive such a signal, they switch their
input to the output of the
secondary system switch element 304b.
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[0036] Figure 4 illustrates a method 400 according to an embodiment of the
present invention.
Such method may be implemented in hardware, software, firmware, or a
combination thereof, or
in similar manners. In step 402, the input is received and the data and
primary timing
information is recovered from the input. In step 404, the data is processed.
[0037] In step 406, a plurality of timing information is received. The
plurality of timing
information includes the primary timing information from the network interface
and secondary
timing information from a secondary combined WAN port/synchronization unit.
[0038] ~ In step 408, the most reliable timing information is selected from
the plurality of timing
information. In step 410 the most reliable timing information is multiplexed
with the data
processed in step 404 across a link. The details of these steps are more fully
discussed above
with reference to Figure 2.
[0039] It should be noted that the network element could be provisioned with
any of the
redundant elements or without any, depending on customer requirements. It
should also be noted
that the architecture towards the subscriber, from the subscriber interface
unit, is well understood
by a person skilled in the art and, thus, is not described here.
[0040] Although the invention has been described with reference to certain
specific
embodiments, various modifications thereof will be apparent to those skilled
in the art without
departing from the spirit and scope of the invention as outlined in the claims
appended hereto.
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