Biblio5

170 SIGNALING

Figure 7.6 Quasi-associated channel signaling, typical of E-1 channel 16. As shown, the signaling travels on a separate channel but associated with its group of traffic channels for which it serves. If it were conventional analog signaling, it would be just one solid line, where the signaling is embedded with its associated traffic.

back down and allow the other call to proceed. A ground start interface is shown in Figure 7.8.

Terminology in signaling often refers back to manual switchboards or, specifically, to the plug used with these boards and its corresponding jack as illustrated in Figure 7.9. Thus we have tip (T), ring (R), and sleeve (S). Often only the tip and ring are used, and the sleeve is grounded and has no real electrical function.

Figure 7.7 Fully disassociated channel signaling. This signaling may be used with CCITT Signaling System No. 7, described in Chapter 13.

Figure 7.8 Ground-start interface block diagram. (From Figure 2-7 of Ref. 8, reprinted with permission.)

7.9 METALLIC TRUNK SIGNALING

7.9.1 Basic Loop Signaling

As mentioned earlier, many trunks serving the local area are metallic-pair trunks. They are actually loops much like the subscriber loop. Some still use dial pulses for address signaling along with some form of supervisory signaling.

Loop signaling is commonly used for supervision. As we would expect, it provides two signaling states: one when the circuit is opened and one when the circuit is closed. A third signaling state is obtained by reversing the direction or changing the magnitude of the current in the circuit. Combinations of (1) open/ close, (2) polarity reversal, and (3) high/ low current are used for distinguishing signals intended for one direction of signaling (e.g., dial-pulse signals) from those intended for the opposite direction (e.g., answer signals). We describe the most popular method of supervision on metallic pair trunks below, namely, reverse-battery signaling.

Figure 7.9 Switchboard plug with corresponding jack (R, S, and T are ring, sleeve, and tip, respectively).

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Figure 7.10 Reverse-battery signaling (From Figure 6-27 of Ref. 7, reprinted with permission.)

7.9.2 Reverse-Battery Signaling

Reverse-battery signaling employs basic methods (1) and (2) just mentioned, and takes its name from the fact that battery and ground are reversed on the tip and ring to change the signal toward the calling end from on-hook to off-hook. Figure 7.10 shows a typical application of reverse-battery signaling in a common-control path.

In the idle or on-hook condition, all relays are unoperated and the switch (SW) contacts are open. Upon seizure of the outgoing trunk by the calling switch (exchange) (trunk group selection based on the switch or exchange code dialed by the calling subscriber), the following occur:

•SW1 and SW2 contacts close, thereby closing loop to called office (exchange) and causing the A relay to operate.

•Operation of the A relay signals off-hook (connect) indication to the called switch (exchange).

•Upon completion of pulsing between swtiches, SW3 contacts close and the called subscriber is alerted. When the called subscriber answers, the S2 relay is operated.

•Operation of the S2 relay operates the T relay, which reverses the voltage polarity on the loop to the calling end.

•The voltage polarity causes the CS relay to operate, transmitting an off-hook (answer) signal to the calling end.

When the calling subscriber hangs up, disconnect timing starts (between 150 ms and 400 ms). After the timing is completed, SW1 and SW2 contacts are released in the calling switch. This opens the loop to the A relay in the called switch and releases the calling subscriber. The disconnect timing (150 –400 ms) is started in the called switch as soon as the A relay releases. When the disconnect timing is completed,the following occur:

•If the called subscriber has returned to on hook, SW3 contacts release. The called subscriber is now free to place another call.

•If the called subscriber is still off-hook, disconnect timing is started in the called switch. On the completion of the timing interval, SW3 contacts open. The called subscriber is then returned to dial tone. If the circuit is seized again from the calling switch during the disconnect timing, the disconnect timing is terminated and the called subscriber is returned to dial tone. The new call will be completed without interference from the previous call.

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When the called subscriber hangs up, the CS relay in the calling switch releases. Then the following occur:

•If the calling subscriber has also hung up, disconnection takes place as previously described.

•If the calling subscriber is still off-hook, disconnect timing is started. On the completion of the disconnect timing, SW1 and SW2 contacts are opened. This returns the calling subscriber to dial tone and releases the A relay in the called switch. The calling subscriber is free to place a new call at this time. After the disconnect timing, the SW3 contacts are released, which releases the called subscriber. The called subscriber can place a new call at this time.

REVIEW EXERCISES

REFERENCES

1. IEEE Standard Dictionary of Electrical and Electronic Terms, 6th ed., IEEE Std 100-1996, IEEE, New York, 1996.

2. R. L. Freeman, Telecommunication Transmission Handbook, 4th ed., Wiley, New York, 1998.

3. National Networks for the Automatic Service, CCITT-ITU Geneva, 1968.

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4. Specifications of Signaling Systems 4 and 5, CCITT Recommendations, Fascicle VI.4, IXth Plenary Assembly, Melbourne, 1988.

5. “Signaling” from Telecommunications Planning Documents, ITT Laboratories, Madrid, November, 1974.

6. Specifications for Signaling Systems R1 and R2, CCITT Recommendations, Fascicle VI.4, IXth Plenary Assembly, Melbourne, 1988.

7. BOC Notes on the LEC Networks—1994, Special Report SR-TSV-002275 Issue 2, Bellcore, Piscataway, NJ, 1994.

8. W. D. Reeve, Subscriber Loop Signaling and Transmission Handbook—Analog, IEEE Press, New York, 1992.

176 LOCAL AND LONG-DISTANCE NETWORKS

Figure 8.1 General structure of a PSTN.

We will incorporate a new, generic device in the subscriber plant. It is called a remote subscriber unit (RSU). It distributes service to the subscriber plant customer. The basic service it provides is plain old telephone service (POTS), using a typical subscriber line interface card (SLIC). The RSU has a large group of optional capabilities, listed below:

•It may or may not have a local switching capability.

•In most cases the RSU will carry out a concentrator function.

•It may be a fiber-to-wire interface point for a hybrid fiber-wire pair system.

•It may be a node for wireless local loop employing point-to-multipoint radio.

•It may provide an add-drop multiplex (ADM) capability on a synchronous optical network of 16.32, or 48 km (SONET) or SDH self-healing ring (SHR).

Several of these RSU concepts are illustrated in Figure 8.2. Figure 8.2a shows the conventional subscriber distribution plant and identifies its various functional parts. Figure 8.2b illustrates a digital subscriber line (DSL) feeding an RSU. The DSL might consist of one or several DS1 or E-1 bit streams. In this case the DSL would be on two wire pair, one for downstream and one for upstream. Figure 8.2c is the same as Figure 8.2b, but in this case the DSL is carried on two fibers plus a spare in a fiber optic cable configuration. Figure 8.2d illustrates the use of a fiber optic or wire-pair bus to feed several RSUs. Figure 8.2e shows a simple self-healing ring architecture employing either SONET or SDH (see Chapter 17).

In sparsely populated rural areas, point-to-multipoint full-duplex radio systems are particularly applicable. Such systems would operate in the 2 GHz or 4 GHz band with 10-, 20-, or 30-mile links. Access to the system would probably be time division multiple access. However, in densely populated urban areas, the 23-, 25-, or 38-GHz frequency bands should be considered to keep link length well under 3.5 miles (5.5 km). Because of expected high interference levels from nearby users, code division multiple access should be considered for this application. The TDMA and CDMA access schemes are described in Chapter 16.

8.2.2 What Affects Local Network Design?

There are a number of important factors that will influence the design of a local network. Among these factors are:

•Subscriber density and density distribution;

•Breakdown between residential and business subscribers;

Figure 8.2a Conventional subscriber distribution plant.

•Further breakdown among types business subscribers as a function of white-collar and blue-collar workers. For example, a large insurance company will have much greater calling activity than a steel mill with the same number of workers; and

•Cultural factors.

Figure 8.2b A digital subscriber line carrying a DS1 or E-1 configuration feeds an RSU. The RSU provides the necessary functions for subscriber loop interface.

Figure 8.2e A fiber optic cable self-healing ring feeds RSUs.

8.3 DESIGN OF LONG-DISTANCE NETWORKS

8.3.1 Introduction

A long-distance network connects an aggregate of local networks. Any subscriber in the nation should be able to reach any other subscriber. Likewise, any subscriber in the nation should be able to connect with any other subscriber in the world. So we have need not only of local and long-distance (toll) exchanges, but international exchanges as well.

8.3.2 Three Design Steps

The design of a long-distance network basically involves three considerations:

1. Routing scheme given inlet and outlet points and their traffic intensities; 2. Switching scheme and associated signaling; and

3. Transmission plan.

In the design each design step will interact with the other two. In addition, the system designer must specify the type of traffic, lost-call criterion or grade of service, a survivability criterion, forecast growth, and quality of service (QoS). The trade-off of these factors with economy is probably the most vital part of initial planning and downstream system design.

Consider transcontinental communications in the United States. Service is now available for people in New York to talk to people in San Francisco. From the history of this service, we have some idea of how many people wish to talk, how often, and for how long. These factors are embodied in traffic intensity and calling rate. There are