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In this chapter, we focus on the SONET/SDH transport technology. The G.709 standard is described in Section 9.3, since the reader is required to have knowledge of the WDM optical technology. We first start with a description of T1 and E1, and then we present in detail the SONET/SDH hierarchy, the SONET STS-1 frame structure, overheads, payload, and the SONET STS-3 frame structure. Subsequently, we describe the SONET/SDH devices and SONET/SDH rings.
One of the main features of SONET/SDH rings is that they are self-healing. That is, a SONET/SDH ring can recover automatically when a fiber link fails. This failure can occur when a fiber is accidentally cut, when the optical components used to transmit on a fiber fail, or the SONET/SDH switch fails. We will describe various architectures for self-healing rings, such as two-fiber and four-fiber protection schemes.
We conclude this chapter with a description of the generic framing procedure (GFP) and data over SONET/SDH (DoS). GFP is a lightweight adaptation scheme that permits the transmission of different types of traffic over SONET/SDH and, in the future, over
Connection-oriented Networks Harry Perros © 2005 John Wiley & Sons, Ltd ISBN: 0-470-02163-2
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
G.709. DoS is a network architecture that uses GFP (together with two other mechanisms) to provide an efficient transport of integrated data services over SONET/SDH.
Time-division multiplexing permits a data link to be used by many sender/receiver pairs (see Figure 2.1). A multiplexer combines the digital signals from N incoming links into a single composite digital signal, which is transmitted to the demultiplexer over a link. The demultiplexer then breaks out the composite signal into the N individual digital signals and distributes them to their corresponding output links. In the multiplexer, there is a small buffer for each input link that holds incoming data. The N buffers are scanned sequentially and each buffer is emptied out at the rate at which the data arrives.
The transmission of the multiplexed signal between the multiplexer and the demultiplexer is organized into frames. Each frame contains a fixed number of time slots, and each time slot is preassigned to a specific input link. The duration of a time slot is either a bit or a byte. If the buffer of an input link has no data, then its associated time slot is transmitted empty. The data rate of the link between the multiplexer and the demultiplexer that carries the multiplexed data streams is at least equal to the sum of the data rates of the incoming links. A time slot dedicated to an input link repeats continuously frame after frame, thus forming a channel or a trunk.
TDM is used in the telephone system. The voice analog signals are digitized at the end office using the pulse code modulation (PCM) technique. That is, the voice signal is sampled 8000 times per second (i.e. every 125 ^sec), and the amplitude of the signal is approximated by an 8-bit number, thus producing a 64-Kbps stream. At the destination end office, the original voice signal is reconstructed from this stream. Because of this sampling mechanism, most time intervals within the telephone system are multiples of 125 ^sec.
The North American standard that specifies how to multiplex several voice calls onto a single link is known as the digital signal level standard, or the DS standard. This is a generic digital standard, independent of the medium over which it is transmitted. The DS standard specifies a hierarchy of different data rates (see Table 2.1). The nomenclature of this hierarchy is DS followed by the level of multiplexing. For instance, DS0 refers to a single voice channel corresponding to 64 Kbps, while DS1 multiplexes 24 voice channels and has a data rate of 1.544 Mbps. The higher levels in the hierarchy are integer multiples of the DS1 data rate. The letter C stands for concatenation. For instance, the concatenated signal DS1C consists of two DS1 signals pasted together for transmission purposes.
Figure 2.1 Synchronous time-division multiplexing (TDM).
Table 2.1 The North American hierarchy.
Digital signal Voice Data rate
number channels (Mbps)
DS0 1 0.064
DS1 24 1.544
DS1C 48 3.152
DS2 96 6.312
DS3 672 44.736
DS3C 1344 91.053
DS4 4032 274.176
Table 2.2 The international (ITU-T) hierarchy.
Level Voice Data rate
number channels (Mbps)
0 1 0.064
1 30 2.048
2 120 8.448
3 480 34.368
4 1920 139.264
5 7680 565.148
The DS standard is a North American standard. The international hierarchy standardized by ITU-T is different, and consists of various levels of multiplexing (see Table 2.2). For instance, Level 1 multiplexes 30 voice channels and has a data rate of 2.048 Mbps; Level 2 multiplexes 120 voice channels and has a data rate of 8.448 Mbps; and so on.