The Internet Engineering Task Force IETF

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.