The ATM Forum STANDARDS COMMITTEES

20 SONETSDH 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 SONETSDH.

2.1 T1E1

Time-division multiplexing permits a data link to be used by many senderreceiver 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 demulti- plexer 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 µsec, 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 µsec. 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. M U X D E M U X N input links N output links link 1 2 N 1 2 N • • • • • • Figure 2.1 Synchronous time-division multiplexing TDM. T1E1 21 Table 2.1 The North American hierarchy. Digital signal number Voice channels Data rate 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 number Voice channels Data rate Mbps 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 standard- ized 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. The DS and ITU-T hierarchies are both known as the plesiochronous digital hierarchy PDH. Plesiochronous means nearly synchronous; it is derived from the Greek words plesion, which means nearly the same, and chronos, which means time. The digital signal is carried over a carrier system, or simply a carrier. A carrier consists of a transmission component, an interface component, and a termination component. The T carrier system is used in North America to carry the DS signal, and the E carrier system is used to carry the international digital hierarchy. T1 carries the DS1 signal, T2 the DS2 signal, T3 the DS3 signal, and so on. Similarly, E1 carries the Level 1 signal; E2 carries the Level 2 signal; and so on. Typically, the T and DS nomenclatures are used interchangeably. For instance, one does not distinguish between a T1 line and the DS1 signal. The same applies for the international hierarchy. In addition to the T and E carrier systems, there is the J system used in Japan; this system is very similar to the T carrier system. The DS1 format has twenty-four 8-bit time slots and one 1-bit time slot slot F for frame synchronization see Figure 2.2. On the F channel, the frame synchronization pattern 1010101. . . is transmitted. Each of the twenty-four time slots carries a single