The Institute of Electrical and Electronics Engineering IEEE

2 SONETSDH and the Generic Frame Procedure GFP So far, we have witnessed the development of three generations of digital transport tech- nologies for telephony: PDH, SONETSDH, and G.709. The first generation of digital transport technology was the plesiochronous digital hierarchy PDH, of which the North American standard T1 and the ITU-T equivalent standard E1 are probably the most well-known transport schemes. T1E1 was first deployed in the early 1960s to transport voice traffic. The synchronous optical net work SONET was proposed by Bellcore now Telecordia in 1985, and it can be seen as the second generation of digital transport networks. SONET was designed to multiplex PDH signals and 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 plesiochronous digital hierarchy. In view of this, ITU-T adopted the synchronous digital hierarchy SDH as the international standard. SONET is compliant with SDH. SONET and SDH were also defined to carry ATM cells and PPP and HDLC frames. The third generation digital transport network is the ITU-T standard G.709, other- wise known as the digital wrapper. This is a new standard that takes advantage of the wavelength division multiplexing WDM technology. It can carry IP packets, ATM cells, Ethernet frames, and SONETSDH synchronous traffic. In this chapter, we focus on the SONETSDH 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 SONETSDH hierarchy, the SONET STS-1 frame structure, overheads, payload, and the SONET STS-3 frame structure. Subsequently, we describe the SONETSDH devices and SONETSDH rings. One of the main features of SONETSDH rings is that they are self-healing. That is, a SONETSDH 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 SONETSDH 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 SONETSDH DoS. GFP is a lightweight adaptation scheme that permits the transmission of different types of traffic over SONETSDH and, in the future, over Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2 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.