Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1999-11-02
2003-05-06
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C370S503000, C375S354000
Reexamination Certificate
active
06560245
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of synchronous hierarchy communications networks, for example synchronous digital hierarchy (SDH) and SONET networks and to the synchronisation of network elements within such networks.
A synchronous hierarchy communications network comprises a number of interconnected nodes or network elements (NE), e.g. SDH equipment (SE), arranged to exchange data, synchronisation and control signalling according to a synchronous hierarchy, as set out for example in the synchronous digital hierarchy (SDH) or SONET standards. Typically a synchronisation signal will be passed from one NE to the next, along with the data signal, so creating a synchronisation path through communicating NEs via data ports. At least some NE's are also provided with NE clock interfaces for receipt and transmission of synchronisation signals. The synchronisation is carried by virtue of the synchronisation signal's aggregate bit rate rather than the data it contains. The signal also carries the synchronisation status message (SSM).
For brevity in the following reference will be made to SDH, it being understood that the invention is also applicable to SONET. A significant feature of SDH systems is the ability of networks to automatically recover from synchronisation failures. To support this feature each NE requires a pre-configured synchronisation source priority table and each synchronisation signal must carry an indication of the quality of the timing source from which it is derived. In SDH networks, this indication of quality is carried in the SSM. For any one STM-N output, the NE will autonomously select from the SSM values of the available sources the one with the highest quality. The selection of which source to use to synchronise data signals sent out from different (STM-N) ports of an NE is also controlled by the pre-configured priority table of that equipment. For any one STM-N output, one or a number of synchronisation sources can each be assigned a priority and the NE can use the priority table to identify the source with the highest priority. In practice priority is only used to select a synchronisation source if there are more than one available with the same, highest quality, or if a valid SSM is not available.
The NE clock interfaces include one or more synchronisation signal inputs (SSI, often called ‘T3’ inputs) and synchronisation signal outputs SSO, often called ‘T4’ outputs). The T
3
inputs are used to provide one or more external synchronisation reference signals for injecting into a network of NEs. One specific use of the NE clock interfaces is to provide access to a Stand-Alone Synchronisation Equipment (SASE) which is employed at certain points in a SDH network to both monitor and improve the quality of the synchronisation signal at intermediate points in the synchronisation path.
Data (STM-N) signals comprise data, synchronisation information and synchronisation signal quality information (ie SSMs). In conventional NEs the clock rate of the synchronisation signal selected from the STM-N inputs is divided down within the NE to 2.048 MHz (i.e. 2.048 Mbit/s if carrying data) for output at the T
4
port. Thus the T
4
synchronisation signal does not have the same frequency or bit rate as the STM-N signal but does contain synchronisation information derived from it. It follows that the T
3
input synchronisation signal is expected to also have a frequency (or data rate) of 2.048 MHz (or Mbit/s) and this signal frequency is multiplied within the NE to yield a synchronisation signal at the STM-N rate and synchronised to the T
3
input synchronisation signal for output with STM-N data signals.
The SSM has been defined for carrying signal quality information since before 1990 for STM-N signals but only recently has the SSM been defined for NE clock interfaces which operate at 2.048 Mbit/s. Consequently very few existing NEs support the SSM at NE clock interfaces. This lack of support means that such NE clock interfaces cannot be used to pass SSM signals.
A problem exists in current NEs where the SSM is not supported at these NE clock interfaces. It is difficult to build a synchronisation network which is able to automatically recover from many types of fault unless all parts of the synchronisation path can support the SSM feature. There is therefore a need for NEs to be able to propagate a received SSM even though the NEs may not be capable of sending or receiving SSMs on NE clock interfaces (e.g. via a SASE).
A similar situation exists in SONET networks. SONET uses 51.84 MHz Mbit/s STS-N “synchronous transport signal” in place of STM-N and 1.544 MHz (Mbit/s) clock interfaces in place of 2.048 MHz (Mbit/s).
SUMMARY OF THE INVENTION
The present invention provides a telecommunications system comprising a plurality of interconnected network elements (NE), in which each NE comprises one or more ports; each NE for inputting from the or each other NE via the port or ports a signal comprising data and a synchronisation signal for synchronising the data and a quality level indication (QLI) determined by the quality of the source of the synchronisation signal; each NE for outputting to the or each other NE via the port or ports a signal comprising data and a synchronisation signal for synchronising the data and a quality level indication (QLI); in which a particular one of the plurality of NEs comprises a synchronisation signal output (SSO) for outputting a particular synchronisation signal received as part of the signal at the port or at a particular one of the ports; in which the particular NE comprises a synchronisation signal input (SSI) for inputting the particular synchronisation signal output from the SSO with no QLI, in which the particular NE comprises means for sending the particular synchronisation signal input at the SSI, as part of the signal output from the or each port for synchronising the data therein; and in which the NE also comprises QLI means for associating the particular one of the input QLIs determined by the quality of the source of the particular synchronisation signal with the particular synchronisation signal input at the SSI for sending as part of the signal output from the or each port.
REFERENCES:
patent: 5886996 (1999-03-01), Wolf
patent: 5956347 (1999-09-01), Slater
patent: 6163551 (2000-12-01), Wolf
patent: 6411633 (2002-06-01), Wolf
patent: 6418151 (2002-07-01), Walter
patent: 6317475 (2002-11-01), Kasurinen
Kirschstein et al.
Marconi Communications Limited
Olms Douglas
Pizarro Ricardo M.
LandOfFree
Telecommunications system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Telecommunications system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Telecommunications system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3070883