DATA TRANSMISSION

Transmission Channels

Whereas the need for nationwide Picturephone trans-mission is far from pressing as yet, nationwide data

JUNE 1979

Transmission Channels transmISSIon over the digital channel is an urgent

present-day requirement. It is far more efficient to send data over the PCM telephone channels than to convert it to analog form and send it over analog channels. AT & T has a service called DDS, the Data-phone Digital Service, in which data can be trans-mitted from subscriber to subscriber in direct digital form-i.e., no modems.

To provide an end-to-end data service, several com-ponents are needed as well as the T 1 and T2 trunks we have described. First, a data-carrying local loop must be established from the subscriber to his local central office. Second, if that central office does not yet have PCM trunks reaching it, the digital signal must be transmitted over other trunks. Third, to give a nation-wide network, the segments of data-carrying PCM system must be interconnected.

An economical means to transmit data over analog trunks has come into use on the Bell System, called Data Under Voice, D UV. As we have commented, analog voice channels are frequency-division multi-plexed together to form groups, which are commonly transmitted over a microwave link. A mastergroup, for example, consists of 600 voice channels, a jumbo-group of 3600. When these jumbo-groups are sent over a microwave link, there is a gap underneath them in the transmitted radio band. The gap is 564 KHz wide, and nothing is transmitted in it except for a radio pilot, which is used to indicate radio continuity (Figure 7).

The gap exists because the signal at the bottom edge of the band is too variable for good quality speech transmission, Data, however, can be transmitted in the gap with high accuracy.

When the T 1 carrier bit stream is encoded as in Figure 4, its spectrum is too wide to fit in the gap, as shown in Figure 7. It is therefore recoded using a 7-level code.

A data pilot is added, to monitor the signal, and the signal fits comfortably underneath the lowest group of telephone circuits. The radio continuity pilot has to be moved out of the way to a higher frequency.

Data Under Voice, DUV, made it possible to inter-connect the digital carriers without building new physical links and to rapidly build a nationwide data network. DUV will fill the gap until nationwide T4 links come into existence. Much of its potential value lies in the fact that microwave links go to most cities, including small ones.

DUV is designed to give a low error rate. A design objective is that on a 4000 mile connection better than 99.75% of all customer channel seconds shall be free of error. A 4000 mile connection will normally go over many different multihop radio systems; 16 radio sys-tems would typically connect in tandem.

DUV and the TI and T2 carriers made possible the AT&T Dataphone Digital Service (DDS) offering.

Underneath the mastergroup transmitted by microwave are 564 KHz unused except for a pilot signal used to indicate radio continuity:

The baseband spectrum of data transmitted by the T1 carrier (1.544 mb/s) is as shown here:

The data is compressed by converting it from the bipolar representation of Fig. 27.5 to a 7-level code. When this is done it fits underneath the mastergroup. The position of the radio continuity pilot has to be changed:

Frequency,

---+-Frequency,

kilohertz-Frequency, kilohertz ~

Figure 7. On the Bell System, segments of Tl carrier can be linked together nationwide b.v using DUV, Data Under Voice, on micro-wave channels

Customers leased channels at speeds up to 56,000 b / s, which are digital end-to-end and hence require no modems. DDS could presumably handle data rates much higher than 56,000 b

I

s were it not for the limited capacity of the local telephone loops.

By putting digital repeaters on the local loop, some-what like the TI carrier, digital services could be exoanded UD to 1.544 mb / s. It is also possible to make digital services switchable.

-Western Union, like AT&T, requires nationwide digital facilities super-imposing on an existing nation-wide analog network. Substantially more than half of Western Union's traffic is digital in nature. Western Union is therefore modifying existing analog micro-wave routes to make them hybrid micromicro-wave carrying 6.3 mb / s in the lower part of their spectrum. On other microwave routes, digital radio is being added, using the same towers and antennas and operating initially at 20 mb / s. The latter approach is called digital

CS10-411-108 Basic Concepts

The Nature and Organization of Digital Transmission Channels

build. Western Union's WESTAR satellite trans-ponders also transmit either analog groups or high-speed digital bit rates. Figure 8 shows the Western Union digital hierarchy.

SYNCHRONIZATION

Synchronization is vital for digital transmission. It is essential for the receiving machine to know which bit is which. This is not too much of a problem on point-to-point lines. The Bell T 1 carrier solved it by adding one extra bit per frame, thereby obtaining an 8000-bit-per-second signal that carries a distinctive pattern. If synchronization slips, then this pattern is searched for, and synchronization can usually be restored in a few milliseconds.

This is fine so long as the channel bank remains intact.

If, however, the bank were split up into its constituent channels and these channels were transmitted separ-ately by pulse code modulation, then it would be advantageous to have a synchronization bit sequence for each channel rather than for the group of channels.

The result would be one bit per character rather than one bit per frame, as in Figure 3. One bit per character is already used for network control signaling, and the result is 8000 bits per second in this case. This rate seems far too much for any purposes that can be fore-seen at the moment. Network signaling consists mainly of sending routing addresses (the number you dial) and disconnect signals (when you replace your receiver). Therefore it has been suggested that this bit position should be shared between the network con-trol signaling function and the synchronization function.

Synchronization becomes a much more difficult prob-lem when a large switched network is considered.

Signals are transmitted long distances over different

300 in-evitably differ slightly even if the transmitting loca-tions are synchronized. This problem must be solved before a nationwide PCM network can be established.

There are two types of solutions. The first is a fully synchronous approach in which an attempt is made to synchronize the clocks of the different switching offices and to compensate for any drift in synchroniza-tion of the informasynchroniza-tion transmitted. The clocks of the different offices in the network must all operate at exactly the same speed. The second is a quasi-synchronous approach in which close but not perfect clock correlation is accepted, and the mUltiplexing operations must be designed to cope with the im-perfections. For the time being the latter approach is the more practicable one.

Two bit streams, then, that are to be multiplexed to-gether on a T 1 or higher level channel have very slightly different bit rates. To achieve the multiplex-ing, more pulses are available on the outgoing line than on the incoming lines. The excess time slots are filled with dummy pulses. The presence of the dummy pulses is signaled, and they are removed at the receiv-ing end. This technique is referred to as pulse stuffreceiv-ing.

The more accurate the clocks, the less pulse stuffing is required. As the design of the digital hierarchy evolves, the accuracy of the clocks employed may increase, and the degree of pulse stuffing needed may decrease~

AT&T'S T4M SYSTEM

In most cities the ducts beneath the streets for tele-communications channels are almost full, and more capacity is needed. Digging up the streets to lay down larger ducts is extremely expensive. A more attractive option is to replace some of today's cables with digital cables that carry a higher traffic volume.

1.544 6.312 20

Time MB/s Time MB/s Time MB/s

division division division

multiplex _~ multiplex multiplex Digital overbuild microwave

~ ~ybrid T1 carrier T2 carrier microwave

JUNE 1979

The Nature and Organization of Digital Transmission Channels

The Bell System T4M coaxial cable system comes in two versions, one designed to fit into existing 3Y2-inch ducts and the other to fit into the newer 4-inch ducts.

The former has 18 coaxial tubes per cable, and the latter has 22 tubes per cable. The cables are the same as those used on today's analog long-haul systems, but the electronics are entirely different. Each tube can transmit 274 mbl s, carrying 4032 voice channels.

Two tubes in a. cable are spare and can be auto-matically switched into operation if a failure occurs on another tube. Of the remaining tubes, half transmit in one direction and half in the other. Thus, the smaller T4M cable transmits a total of 2,192 billion b

I

s in each direction, carrying up to 32,256 two-way telephone conversations; the larger transmits a total of 2,740 billion bls carrying up to 40,320 two-way telephone conversations.

The central conductor of each coaxial tube carries the De current, which is used to power the re-generators until the next maintenance office is reached. The span between such offices can be up to

180 kilometers.

In addition to occupying a dedicated cable, the T4tv1 bit streams are sometimes sent over certain coaxial tubes in cables that also carry other types of signals.

Some composite cables, for example, have 8 coaxial tubes and 750 pairs of wires.

EVOLUTION

An enormous amount of capital is tied up in national analog telephone networks. So much money is in-volved that they cannot be converted to all~igital

networks in a few years. The proportion of digital links will grow slowly. They will be installed first where they can be most profitable or where the pres-sure for extra circuits is greatest, as in crowded urban and suburban areas. The T4M system was first in-stalled in the New York to Newark area and was pressed into service early after a catastrophic fire in 1975. It had to be tailored to the available cable ducts and traffic requirements.

Figure 9 shows how peM lines might be used in to-day's typical urban environment. It presents a highly simplified picture of part of the telephone network in London. The top half of the picture shows inter-connections between local, tandem, and subtandem exchanges. The bottom half shows how the network could be simplified with the use of peM links. The simplification shown by the illustration would have appeared far more striking if it had been drawn with the 200 or so local exchanges that exist rather than with the small number in the diagram.

The saving would be much greater, agaIn, if peM concentrators were also used.

Local exchange

o

Subtandem exchange

o

Toll/trunk outlet Junction to subtandem Junction to tandem Junction to toll/trunk outlet

A highly Simplified diagram of the present London step -by -step tandem network catering for subscriber trunk dialing (direct distance dialing In American parlance) Only a limited number of exchanges and routings are shown, in order to lessen the compleXity of the figure All direct junctions between local exchanges have been omitted

With integrated PCM between exchanges, the routing might be Simplified as follows

If the drawing had not been so highly Simplified, the saving would have appeared much greater

Figure 9. Redrawn with permission from Techniques of Pulse Code Modulation in Modification Networks, by C. C. Hartley, P. Morret, F Ralph, and D. J. Tarran, Cambridge Universi(v Press, Cambridge, 1967

Im Dokument COMMUNICATIONS SOLUTIONS (Seite 101-104)