2.4.7. Trunks and multiplexing

Telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trounce. This multiplexing schemes can be divided into two basic categories:

1. Frequency Division Multiplexing (FDM) - the frequency spectrum is divided among the logical channels, single frequency bands are allocated to different users. As an example from another area of life, we can take radio broadcasting where different frequencies are allocated to different radio stations.
2. Time Division Multiplexing (TDM) - the users take turns (in a round robin), each one periodically getting the entire bandwidth for a little burst of time. Compare the burst of music alternated by the burst of advertising in radio broadcasting as an illustration.

When 3000 Hz wide voice-grade telephone channels are multiplexed using FDM, 4000 Hz is allocated to each channel to keep them well separated. First, the voice channels are raised in frequency, each by a different amount, and then they are combined (Fig. 2-24).

Fig. 2-24. Frequency division multiplexing. (a) The original bandwidths.
(b) The bandwidths raised in frequency. (c) The multiplexed channel.

The FDM schemes used around the world are to some degree standardized. A widespread standard is 12 4000 Hz voice channels multiplexed into 60 to 108 kHz band. This unit is called a group. Five groups can be multiplexed to form a supergroup, five supergroups form a mastergroup.

For fiber optic channels, a variation of frequency division multiplexing called Wavelength Division Multiplexing (WDM) is used (Fig. 2-25).

Fig. 2-25. Wavelength division multiplexing.

FDM requires analog circuitry and cannot be performed by a computer. In contrast, TDM can be handled entirely by digital electronics, so it has become far more widespread in recent years. But it can only be used for digital data. Since the local loops produce analog signals, they must be converted in the end offices to be combined onto outgoing trunks.

The analog signals are digitized in the end office by a device called a codec (coder-decoder), producing a 7 or 8 bit number (see Fig. 2-17). The codec makes 8000 samples per second (125 (sec/sample) because the Nyquist theorem says that this is sufficient to capture all the information from the 4 kHz telephone channel bandwidth. This technique is called Pulse Code Modulation - PCM. PCM forms the heart of the modern telephone system.

There are a variety of incompatible schemes in use for PCM in different countries around the world. One method that is in widespread use in North America and Japan is the T1 carrier (Fig. 2-26). It consists of 24 voice channels multiplexed together. 24 analog signals are sampled on a round-robin basis during each 125 (sec interval each channel getting 8 bits (possibly 7 bits of data and one for control) into the output stream. The resulting frame contains 24 x 8 = 192 bits plus one extra bit for framing, yielding 193 bits every 125 (sec. This gives a gross data rate 1.544 Mbps. The 193rd bit used for synchronization takes on the pattern 010101010101 .. . When the T1 system is being used entirely for data, only 23 of the channels are used for data. The 24th one is used for a special synchronization pattern, to allow faster recovery in the event of error.

Fig. 2-26. The T1 carrier (1.544 Mbps).

There is also a CCITT recommendation for a PCM carrier at 2.048 Mbps called E1. This carrier has 32 8 bits data samples packed into the basic 125 (sec frame. This is in widespread use outside North America and Japan.

Once the voice signal has been digitalized, different compaction methods have been developed to reduce the member of bits needed per channel. All are based upon the principle that the signal changes relatively slowly compared to the sampling frequency, so that much of the information in the 7 or 8 bit digital level is redundant.

TDM allows multiple T1 carriers to be multiplexed into higher-order carriers (Fig. 2-28).

Fig. 2-28. Multiplexing T1 streams onto higher carriers.

After AT&T was broken up in 1984, the need for standardization for long-distance carriers became obvious. In 1985, Bellcore began working on a standard called SONET (Synchronous Optical NETwork). Later, CCITT joined the effort which resulted in SONET standard and a set of parallel CCITT recommendations (G.707, G.708, and G.709) in 1989. The CCITT recommendations are called SDH (Synchronous Digital Hierarchy) that differs from SONET only in minor ways. In fact all the long-distance telephone traffic in the US, and much of it elsewhere now using trunks running SONET in the physical layer. As SONET chips become cheaper, SONET interface boards for computers may become more widespread enabling to plug computers directly into the heart of the telephone network over special leased lines.

The SONET design had four major goals:

1. to make it possible for different carriers to interwork,
2. to unify U.S., European, and Japanese digital systems all working on the base of 64 kbps PCM channel but combining them in different ways,
3. to provide a way to multiplex multiple digital channel together. At the time SONET was devised, T4 was the highest channel as for speed. It was necessary to extend the scale higher.
4. to provide support for operation, administration and maintenance (OAM).

SONET is a synchronous system. It is controlled by a master clock. Bits on a SONET line are sent out at precise intervals, controlled by master clock.

Fig. 2-29. A SONET path.

A SONET system consists of switches, multiplexers, and repeaters, all connected by fiber (Fig. 2-29). SONET terminology:

• section = a fiber going directly from any device to any other device, with nothing in between,
• line = a run between two multiplexers, possibly with one or more repeaters in the middle,
• path = a connection between the source and destination, possibly with one or more multiplexers and repeaters.

The basic SONET frame is a block of 810 bytes put out every 125 (sec. The 8000 frame/sec exactly matches the sampling rate of PCM channels used in all digital telephony systems.

The 810 byte SONET frames are best described as a rectangle of bytes, 90 columns wide by 9 rows high. The gross data rate is 8 x 810 x 8000 = 51.84 Mbps. This is a basic SONET channel called STS-1 (Synchronous Transport Signal -1). All SONET trunks are a multiple of STS - 1.

Fig. 2-30. Two back-to-back SONET frames.

The first three columns of each frame are reserved for system management information (Fig. 2-30). The first three rows contain the section overhead, the next six contain the line overhead.

The remaining 87 columns hold 50.112 Mbps of user data. However, the user data, called the Synchronous Payload Envelope - SPE - do not always begin in row 1, column 4. They can begin anywhere within the frame. A pointer to the first byte is contained in the first row of the line overhead. The first column of the SPE is the path overhead (i.e., header for the end-to-end path sublayer protocol).

The multiplexing of data streams, called tributaries, is illustrated in Fig. 2-31. The final output stream is STS-12 having 12 times the capacity of the STS - 1 stream. At this point the signal is scrambled, to prevent long runs of 0s or 1s from interfering with the clocking, and converted from an electrical to an optical signal. Multiplexing is done byte for byte.

Fig. 2-31. Multiplexing in SONET.

The SONET multiplexing hierarchy is shown in Fig. 2-32. The optical carrier corresponding to STS-n is called OC-n. The SDH names are different, and they start at OC-3 because CCITT-based systems do not have a rate near 51.84 Mbps. The gross data rate includes all the overhead, the SPE data rate excludes the line and section overhead. The user data rate excludes all overhead.

Fig. 2-32. SONET and SDH multiplex rates.

When a carrier, such as OC-3, is not multiplexed, but carries the data only from a single source, the letter c (concatenated) is appended to the designation. So OC-3c indicates a data stream from a single source at 155.52 Mbps. The amount of user data in an OC-3c stream is slightly higher than in OC-3 stream (149.760 Mbps versus 148.608 Mbps) because the path overhead column is included inside the SPE only once, instead of three times in case of three independent OC-1 streams.

By now it should be clear why ATM runs at 155 Mbps: the intention is to carry ATM cells over SONET OC-3c trunks. It should also be clear that the 155 Mbps is the gross rate, including the SONET overhead.

The SONET physical layer is divided up into four sublayers (Fig. 2-33):

• The photonic sublayer is concerned with specifying the physical properties of the light and fiber to be used.
• The section sublayer handles a single point-to-point fiber run, generating a standard frame at one end and processing it at the other.
• The line sublayer is concerned with multiplexing multiple tributaries onto a single line and demultiplexing them at the other end.
• The path sublayer and protocol deal with end-to-end issues.

Fig. 2-33. The SONET architecture.