Fig. 10.20. Frame format of STS-3.

In SONET, multiplexing is used when multiple lower order path-layer signals are adapted into a higher-order path signal, or when the higher order path signals are adapted into the line overhead. Lower rate STS's can be multiplexed to make them compatible with higher rate systems. The STS-3 can be generated by multiplexing three STS-1 signals, four STS-3's can be multiplexed into one STS-12 and so on. For the case of 3 STS-1 multiplexed into one STS-3, the STS-3 frame is made of 3 x 90 or 270 columns and 9 row with the total of 2430 bytes. Thus the data rate of STS-3 is 2430 bytes x 8 bits x 8000 frames/sec = 155.52 Mbps. Fig. 10.20 shows the frame format of STS-3.

Hence the general format for STS-n is made up of lower rate STS's. For example, general format of STS-9 is made up of lower rate of STS = 1 with 90 x 9 = 810 columns and 9 rows.

The multiplexing of three STS-1's to generate STS-3 is shown in Fig. 10.21.

Fig. 10.21. SONET Multiplexing.

In the Fig. 10.21 three STS-1 signals are multiplexed to obtain one STS-3 signals. Similarly 3 STS-3 signals can be multiplexed to obtain STS-9. Any type of service, ranging from voice to high speed data and video, can be accepted by various types of service adapters. A service adapter maps the signal into the payload envelope of the STS-1 or VT. New services and signals can be transported by adding new service adapters at the edge of the SONET network.

All inputs are eventually converted to a base format of a synchronous STS-1 signal (51.84 Mbps or higher). Lower speed inputs such as DS-1s are first bit or byte multiplexed into VTs. Several synchronous STS-1s are then multiplexed together in either a single or two stage process to form an electrical STS-N signal (N > 1). STS multiplexing is performed at the byte interleave synchronous multiplexer. Basically the bytes are interleaved together in a format such that the low-speed signals are visible. No additional signal processing occurs except a direct conversion from electrical to optical to form an OC-N signal.

Add/drop Multiplexer (ADM). ADMs are used for extracting or inserting lower rate signals from or into higher rate multiplexed signals without completely demultiplexing the SONET signals. SONET does not restrict manufacturers to providing a single type of product, not require them to provide all types. For example, one vendor might offer an ADM with access at DS-1 only, whereas another might offer simultaneous access at DS-1 and DS-3 rates. Fig. 10.22 shows the ADM.

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