The DSL Forum STANDARDS COMMITTEES

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 SONETSDH 23 SONETSDH equipment receives the optical signal and converts it to the electrical domain in order to process the frames. The electrical side of the SONET signal is known as the synchronous transport signal STS , and the electrical side of the SDH is known as the synchronous transport module STM . The optical side of a SONETSDH signal is known as the optical carrier OC. SONET’s basic rate is 51.84 Mbps, whereas the SDH’s basic rate is 155.52 Mbps. SONET’s basic rate enables the efficient transport of a DS3 signal; SDH’s basic rate enables the efficient multiplexing of ITU-T’s Level 3 signals. A hierarchy of different levels can be constructed see Table 2.3. The first column gives the optical carrier level OC-N; the next two columns give the equivalent STS and STM levels, respectively. N can take a value between 1 and 255. The data rate, overhead rate, and payload rate associated with each level is shown in columns 4, 5, and 6, respectively. In SONET, a data rate is typically referred to by its optical level; in SDH, it is typically referred to by its electrical level. For instance, a 155.520 Mbps data rate is referred to in SONET as OC-3 , whereas it is referred to in SDH as STM-1. Not all levels are economically viable; existing products such as OC-3STM-1, OC- 12STM-4, and OC-48STM-16 are indicated by bold formatting. As can be seen, each level can be obtained from the previous one by multiplying it by four. SONETSDH is channelized. For example, in SONET, STS-3 is constructed by mul- tiplexing three STS-1 basic rate streams; STS-12 is constructed by multiplexing twelve STS-1 streams; and so on. Likewise, in SDH, STM-4 is constructed by multiplexing four STM-1 basic rate streams; STM-16 is constructed by multiplexing 16 STM-1 streams; and so on. As indicated in the last row of Table 2.3, the STM level for OC-N is obtained by dividing N by three. The data rate of STS-1 is 51.840 Mbps, of which 1.728 Mbps is used for overheads and the remaining 50.112 Mbps for payload information. The data rate for STS-3 is obtained by multiplying the corresponding data rate of STS-1 by three. Likewise, STS-3’s overhead and payload data rates can be obtained by multiplying those from STS-1 by three. As Table 2.3 SONETSDH hierarchy. Optical level SONET level electrical SDH level electrical Data rate Mbps Overhead rate Mbps Payload rate Mbps OC-1 STS-1 – 51.840 1.728 50.112 OC-3 STS-3 STM-1 155.520 5.184 150.336 OC-9 STS-9 STM-3 466.560 15.552 451.008 OC-12 STS-12 STM-4 622.080 20.736 601.344 OC-18 STS-18 STM-6 933.120 31.104 902.016 OC-24 STS-24 STM-8 1244.160 41.472 1202.688 OC-36 STS-36 STM-12 1866.240 62.208 1804.932 OC-48 STS-48 STM-16 2488.320 82.944 2405.376 OC-96 STS-96 STM-32 4976.640 165.888 4810.752 OC-192 STS-192 STM-64 9953.280 331.776 9621.504 OC-768 STS-768 STM-256 39813.120 1327.104 38486.016 OC-N STS-N STM-N3 N51.840 N1.728 N50.112 24 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP we go up the hierarchy, the percent of the overhead remains constant: it corresponds to 3.33 of the data rate or to 3.45 of the payload rate. As mentioned above, a channelized STS-N consists of N STS-1s. Each STS-1, as demonstrated below, is constructed using a specific combination of DSn and E1 signals. In addition to this channelized structure, it is possible to simply fill the STS-3 payload with ATM cells or IP packets packed in PPP or HDLC frames. The resulting structure is called concatenated, and is indicated by adding a c after the optical level name e.g. OC-3c and OC-12c and after the electrical level name STS-3c and STS-12c. Similar concatenation takes place in SDH. Concatenated SONETSDH links are commonly used to interconnect ATM switches. In fact, the ATM architecture was originally defined to run at OC-3c. They are also used in packet over SONET PoS. In PoS, IP routers are interconnected with SONETSDH links and the IP packets are transported directly over SONETSDH after they have been encapsulated in PPP or HDLC frames. 2.3 THE SONET STS-1 FRAME STRUCTURE The SONET STS-1 frame consists of 810 bytes and is transmitted 8000 times per sec- ond i.e., every 125 µsec. This gives a total data rate of 8000 × 810 × 8 bitssec i.e., 51.84 Mbps. A single byte commonly referred to as a time slot in the frame is trans- mitted 8000 times per second, thus giving a data rate of 64 Kbps, as in the case of a time slot in the DS1 signal. This permits SONET to transmit uncompressed PCM voice. The SONET STS-1 frame is graphically displayed in a matrix form, consisting of nine rows and 90 columns see Figure 2.3. Each cell in the matrix corresponds to a byte. Starting from byte 1, the frame is transmitted out row by row. The frame consists of the overhead section and the payload section. The overhead section, called the transport over- head TOH , occupies the first three columns. The payload section occupies the remaining 87 columns i.e. columns 4 to 90, and it carries the synchronous payload envelope SPE. 90 1 1 1 90 2 91 92 93 94 95 96 2 3 4 5 6 2 3 4 5 6 … … … 180 3 181 182 183 184 185 186 … 270 4 271 272 273 274 275 276 … 360 5 361 362 363 364 365 366 … 450 6 451 452 453 454 455 456 … 560 7 561 562 563 564 565 566 … 630 8 631 632 633 634 635 636 … 720 9 721 722 723 724 725 726 … 810 Figure 2.3 The SONET STS-1 frame structure.