Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
2000-11-07
2004-03-23
Tran, Congvan (Department: 2683)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S428000, C370S350000, C370S355000
Reexamination Certificate
active
06711411
ABSTRACT:
BACKGROUND
The invention relates to synchronization in telecommunication systems.
Digital communication networks often require a common timing reference to operate accurately. That is, the clocks in one node of the network should operate at the same speed as the clocks in other nodes of the network. Failure to provide synchronized clocks will lead to Jitter and Wander, which in turn can lead to such problems as transmission errors and buffer under/overflow. A network cannot maintain low error rates under such conditions, and ultimately may require some degree of unavailability to rectify the situation.
To provide for a common timing reference, digital communication networks include synchronization networks, whose job it is to ensure that a common timing reference is used throughout the network. One such synchronization network is described in European Telecommunication Standards Institute (ETSI) document European Guide (EG) 201 793 v1.1.1 (2000-10), entitled “Transmission and Multiplexing (TM); Synchronization Network Engineering”, which is hereby incorporated herein by reference in its entirety. This document describes the various elements that make up a synchronization network, and the principles of operation by which such a network distributes accurate timing information from so-called Primary Reference Clocks (PRCs) to the clocks located in other pieces of equipment throughout the network. PRCs are the highest quality clocks in a network, and are usually based on a free-running Caesium Beam oscillator giving a very accurate clock performance.
FIG. 1
is a block diagram of an exemplary digital communication network
100
that includes a synchronization network. For purposes of illustration, the exemplary network
100
is a telecommunications network, and therefore includes, at network nodes, equipment that is well-known in the art. In the figure, transport links are indicated by solid lines, and synchronization reference links are shown by dashed lines that include an arrow at one end to indicate the source and recipient of the reference clock signal. Where a node has the possibility of receiving a reference clock from more than one source, primary reference links (i.e., those synchronization links that are preferred to be used for supplying a reference clock from one node to another) are denoted by the number “1” next to the dashed line indicating the link. Secondary reference links (i.e., those synchronization links that are used when the primary synchronization link is unavailable) are denoted by the number “2” next to the dashed line indicating the link.
The exemplary network
100
utilizes the Synchronous Digital Hierarchy (SDH), which is a standard technology for synchronous data transmission on optical media. It is the international equivalent of the Synchronous Optical Network (SONET). To facilitate the following discussion, the various nodes of the network are given reference characters A, B, C, D, E, F, G, H, I, L, M, and N.
In a fully synchronized network, all sources should be ultimately traceable to a PRC. In the exemplary network, this is the PRC A. The PRC A supplies its high quality clocking signal (“clock”) to Stand Alone Synchronization Equipment (SASE) B. A SASE is a piece of synchronization equipment that contains a Synchronization Supply Unit (SSU), which is a high quality slave clock. The SASE B distributes its clock signal to a Digital Exchange C (which, in alternative embodiments, could be a Telephone Exchange) and also to an SDH multiplexer (MUX) D.
The SDH MUX D distributes its clock signal to an SDH Digital Cross Connect unit (SDH DXC) E, which in turn distributes its clock signal to an SDH Add Drop Multiplexer (ADM) F. The clock supplied by the SDH ADM F is then supplied to each of two more SDH ADMs G and I. The reference link between the SDH ADM F and the SDH ADM G is a primary link.
Rather than using the supplied clock signal itself, the SDH ADM I operates in a “bypass” mode (commonly named “NON-SETS locked”, where “SETS” stands for “Synchronous Equipment Timing Source”), whereby the synchronization clock is merely forwarded directly to the SASE L. This is common when, for example, the ADM and SASE are implemented in the same building. Essentially, the SASE L is the real recipient of the synchronization clock supplied by SDH ADM F, and this clock is treated as a secondary link. In the exemplary embodiment, the SASE L's primary link is supplied (through the SDH ADM I operating in “bypass” mode) by an SDH ADM H.
Despite its bypass function, the SDH ADM I does require a synchronization clock, and in the exemplary embodiment this is supplied by the SASE L.
The SDH ADM I supplies its synchronization clock to the SDH ADM H, and this is treated as a secondary link. The SDH ADM H's primary link is supplied by the SDH ADM G. To permit reconfigurability, the SDH ADM H is also coupled to supply a synchronization clock to the SDH ADM G, and this is treated as a secondary link by the SDH ADM G.
In accordance with the exemplary embodiment, the SDH ADM H also supplies a synchronization clock to a digital switch M, which also receives a synchronization clock from the digital switch N. The remainder of the exemplary network is not shown, since this is not important to understanding the invention.
It is very important that the synchronization network be planned in such a way so as to avoid the occurrence of timing loops, both during normal operation as well as when a malfunction prevents one or more nodes from supplying their reference clocks to their planned recipient nodes. A timing loop is created when a clock is directly or indirectly synchronized to itself. In a timing loop situation, all the clocks belonging to the loop can show a large frequency offset compared to nominal frequency and are likely to be isolated from the rest of the synchronization network. To avoid timing loops, elements in a ring should be provided with means that enable the possible generation of timing loops to be discovered. Such elements are usually connected such that they each have at least two synchronization sources, so that when one source is discovered to cause a timing loop, there is at least the possibility of avoiding it by selecting one of the alternative sources. For example, suppose that the reference link between nodes F and G is cut. In this situation, the SDH ADM G will respond by looking to node H to supply the necessary reference clock. However, under normal circumstances, node H expects to receive its reference clock from node G. It is apparent that a timing loop will occur here unless node H also responds to the break between nodes F and G by looking to another source for its reference clock. It is important that the clock supplied by this alternative source also not ultimately be derived from the clock at node G or from the clock at node H in order to avoid a timing loop.
In SDH networks, the use of Synchronous Status Messages (SSMs) provides some help with avoiding timing loops. An SSM is a signal that is passed over a synchronization interface to indicate the Quality-Level of the clock that the interface is ultimately traceable to; that is, the grade-of-clock to which it is synchronized directly or indirectly via a chain of network element clocks (the “synchronization trail”), however long this chain of clocks is. In a fully synchronized network, all sources should ultimately be traceable to a PRC, and there is a predefined code to indicate this. Another code, “Do Not Use for Synchronization”, is used to prevent timing loops and is transmitted in the opposite direction on interfaces used to synchronize an equipment's clock.
Although the SSM algorithm is a good concept in some applications like SDH or SONET rings, it is unable to guarantee that all timing loops will be prevented, because it only provides information about the quality of the traceable synchronization reference source, but not information about the actual physical source. See, for example, chapter 4.13 of ETS 300 417-6-1, “Generic requirements of transport functionality of equipment
Telefonaktiebolaget LM Ericsson (publ)
Tran Congvan
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