Fig. 10.22. Add/Drop Multiplexer.

At an ADM site, three possible functions are performed. They are listed below :

1. At ADM, only those signals that need to be accessed are dropped or inserted without demultiplexing the entire signals. ADM uses header information such as addresses and pointers to identify individual streams.

2. ADMs can also be configured as a survivable ring. SONET enables drop and repeat or drop and continue with drop and repeat, a signal terminates at one node is duplicated (repeated) and is then sent to the next and subsequent nodes. The ring survivability has telephony and cable TV applications. It also provides alternate routing, if the connection cannot be made through one of the nodes. For example, when transporting video, it can be delivered (dropped) at a node and repeated for delivery to the next node-channels not treminating at a node can be passed through without physical intervention to other nodes.

3. The ADM provides interfaces between the different network signals and SONET signals. At this site, it can drop lower rate signals to be transported on different facilities, or it can add lower rate signals into the higher rate STS-N signal. The rest of the traffic simply continues straight through.

Digital cross connects (DCS). DCS is ideally used at a SONET hub. It accepts various optical carrier rates, accesses the STS-1 signals and switches. One major difference between a cross connect and an add/drop multiplexer is that a cross connect may be used to interconnect a much larger number of STS-1s. There are two types of DCS. They are wideband digital cross connects (W-DCS) and Broad band digital cross connects (B-DCS). Both W-DCS and B-DCS are shown is Fig. 10.23.

The W-DCS terminates SONET and DS-3 signals and has the basic functionality of VT and DS-1 level cross connection. It is the SONET equivalent to the DS-3/DS 1 DCS and accepts optical OC-N signals as well as STS-1s, DS-1s and DS-3s. In W-DCS, the switching is done at

VT level. As W-DCS can automatically cross connect VTs and DS-1s, the W-DCS can be used as a network management system. Thus W-DCS is ideal for grooming at a hub location.

DS 1 switch matrix

|VT 1.5 JDS 1

|DS 1

Jds 1

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