The MPLS and Frame Relay Alliance The Optical Internetworking Forum OIF

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 22 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP F Time slot 1 Time slot 2 Time slot 3 Time slot 24 . . . Figure 2.2 The DS1 format. 64-Kbps voice. For five successive frames, an 8-bit PCM sample is used. In the sixth frame, a 7-bit sample is used, and the eighth extra bit is robbed for signaling. The total transmission rate of the DS1 format is 24 × 8 + 1 = 193 bits per 125 µsec corresponding to 1.544 Mbps, with each voice channel carrying a 64-Kbps voice. In the international hierarchy, the Level 1 format for voice consists of thirty-two 8-bit time slots, resulting to a total transmission rate of 2.048 Mbps. Of these time slots, thirty are used for voice and the remaining two are used for synchronization and control.

2.1.1 Fractional T1E1

Fractional T1 or fractional E1 allows a user to purchase only a fraction of the T1 or E1 capacity. Fractional T1 services are offered on an N × 64 Kbps or an N × 56 Kbps basis, where N = 2, 4, 6, 8, or 12. For example, if N = 2, then only two time slots are used per frame, which corresponds to a channel with a total bandwidth of 128 2 × 64 Kbps. With fractional T1E1, users can reduce costs because they pay only for the number of time slots that matches their bandwidth requirements.

2.1.2 Unchannelized Framed Signal

In an unchannelized framed signal, the time slot boundaries are ignored by the sending and receiving equipment. For instance, in the unchannelized T1 framed signal, all 192 bits are used to transport data followed by the 193rd framing bit. This approach permits more flexibility in transmitting at different rates. This scheme is implemented using proprietary solutions. It is also possible to use the entire frame including the framing bit in an unchannelized manner.

2.2 SONETSDH

The synchronous optical network SONET was first proposed by Bellcore now Telecor- dia in 1985, and was further developed and standardized by ANSI’s T1X1 committee. SONET was designed to multiplex DSn signals and to transmit them optically between equipment made by different manufacturers. SONET was not designed, however, to address the needs of the European community, which used the ITU-T PDH signals. In view of this, ITU-T adopted the synchronous digital hierarchy SDH as the international standard, which enables the efficient multiplexing of 34.368-Mbps PDH signals ITU-T’s Level 3. SONET is compliant with SDH. SONET and SDH were also defined to carry ATM cells and PPP and HDLC frames. The information transmitted by SONETSDH is organized into frames. These frames are transmitted continuously one after the other. Each frame consists of a collection of overhead fields and a payload. SONETSDH equipment constructs these frames in the electrical domain and then transmits them out optically. At the receiving end, the