Biblio5

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also other cities on the West Coast to be served and other cities on the East Coast. In addition, there are existing traffic nodes at intermediate points such as Chicago and St. Louis. An obvious approach would be to concentrate all traffic into one transcontinental route with drops and inserts at intermediate points.

Again, we must point out that switching enhances the transmission facilities. From an economic point of view, it would be desirable to make transmission facilities (carrier, radio, and cable systems) adaptive to traffic load. These facilities taken alone are inflexible. The property of adaptivity, even when the transmission potential for it has been predesigned through redundancy, cannot be exercised, except through the mechanism of switching in some form. It is switching that makes transmission adaptive.

The following requirements for switching ameliorate the weaknesses of transmission systems: concentrate light, discretely offered traffic from a multiplicity of sources and thus enhance the utilization factor of transmission trunks; select and make connections to a statistically described distribution of destinations per source; and restore connections interrupted by internal or external disturbances, thus improving reliabilities (and survivability) from the levels on the order of 90% to 99% to levels on the order of 99% to 99.9% or better. Switching cannot carry out this task alone. Constraints have to be iterated or fed back to the transmission systems, even to the local area. The transmission system must not excessively degrade the signal to be transported; it must meet a reliability constraint expressed in MTBF (mean time between failures) and availability and must have an alternative route scheme in case of facility loss, whether switching node or trunk route. This latter may be termed survivability and is only partially related to overflow (e.g., alternative routing).

The single transcontinental main traffic route in the United States suggested earlier has the drawback of being highly vulnerable. Its level of survivability is poor. At least one other route would be required. Then why not route that one south to pick up drops and inserts? Reducing the concentration in the one route would result in a savings. Capital, of course, would be required for the second route. We could examine third and fourth routes to improve reliability–survivability and reduce long feeders for concentration at the expense of less centralization. In fact, with overflow, one to the other, dimensioning can be reduced without reduction of overall grade of service.

8.3.3 Link Limitation

From a network design perspective a connectivity consists of one or more links in tandem.2 We define a link as the transmission facilities connecting two adjacent switches. CCITT in Rec. E.171 (Ref. 1) states that for an international connection there shall be no more than 12 links in tandem. This is apportioned as follows:

•4 links in the calling party’s country;

•4 links in the called party’s country; and

•4 international links.

This concept is illustated in Figure 8.3.

The PSTN network designer should comply with this CCITT criterion, in that for a national connection, there should be no more than four links in tandem. The reason

2It should be noted that there are connectivities with “no links in trandem.” This is an own-exchange connectivity, where the calling and called subscriber terminate their subscriber loops in the same exchange.

Complete international telephone connection

LE

PC

PC

LE

l

n

n

n

n

L

L

L

L

n

n

n

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Local

Four-wire chain

Local

system

system

			Subscriber's set		Four-wire switching exchange
			Local exchange		with terminating unit
			with two-wire		Four-wire switching exchange
			switching		International switching center
			Two-wire switching		(ISC) with virtual
			with terminating unit		international connecting

L International circuits n National ext. circuits LE Local exchange

l Subscriber's line PC Primary center

Figure 8.3 An international connection to illustrate the maximum number of links in tandem for such a connection. (From Figure 6/G.101 of Ref. 2.)

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CCITT/ ITU-T set this limit was to ensure transmission QoS. As we add links in tandem, transmission quality deteriorates. Delay increases and we include here processing delay because of the processing involved with a call passing through each switch. End-to- end bit error rate deteriorates and jitter and wander accumulate. Transcontinental calls in North America generally need no more than three links in tandem, except during periods of heavy congestion when a fourth link may be required for an alternate route.

8.3.4 Numbering Plan Areas

The geographical territory covered by the long-distance network will be broken up into numbering plan areas (NPAs). In North America, each NPA is assigned a three-digit area code. In other parts of the world, twoand even one-digit area codes are used. NPA size and shape are driven more by numbering capacity and future numbering requirements. Numbering plan administrators are encouraged to design an NPA such that it coincides with political and/ or administrative boundaries. For example, in the United States, an NPA should not cross a state boundary; in Canada, it should not cross a provincial boundary. NPAs are also important for establishing a rates and tariffs scheme.3

We know a priori that each NPA will have at least one long-distance exchange. It may be assigned more. This long-distance exchange may or may not colocate with the POP (point of presence).4 We now have made the first steps in determining exchange location. In other countries this exchange may be known as a toll-connecting exchange.

8.3.5 Exchange Location

We have shown that the design of the long-distance network is closely related to the layout of numbering plan areas or simply numbering areas. These exchanges are ordinarily placed near a large city. The number of long-distance exchanges in a numbering area is dependent on exchange size and certain aspects of survivability. This is the idea of “not having all one’s eggs in one basket.” There may be other reasons to have a second or even a third exchange in a numbering area (NPA in the United States). Not only does it improve survivability aspects of the network, but it also may lead the designer to place a second exchange near another distant large city.

Depending on long-distance calling rates and holding times, and if we assume 0.004 erlangs per line during the busy hour, a 4000-line long-distance exchange could serve some 900,000 subscribers. The exchange capacity should be dimensioned to the forecast long-distance traffic load 10 years after installation. If the system goes through a 15% expansion in long-distance traffic volume per year, it will grow to over four times its present size in 10 years. Exchange location in the long-distance network is not very sensitive to traffic.

8.3.6 Hierarchy

Hierarchy is another essential aspect in long-distance (toll) network design. One important criterion is establishing the number of hierarchical levels in a national network. The United States has a two-level hierarchy: the local exchange carrier (or LATA [local

3This deals with how much a telephone company charges for a telephone call.

4POP, remember, is where the local exchange carrier interfaces with long-distance carriers. This whole concept of the POP is peculiar to the United States and occurred when the Bell System was divested. In other parts of the world it may be called a toll-connecting exchange.

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access and transport area]) and the interexchange carrier network. Our concern here is the interexchange carrier network, which is synonymous with the long-distance network. So the question remains: how many hierarchical levels in the long-distance or toll network?

There will be “trandem” exchanges in the network, which we will call transit exchanges. These switches may or may not be assigned a higher hierarchical level. Let us assume that we will have at least a two-level hierarchy.

Factors that may lead to more than two levels are:

•Geographical size;

•Telephone density, usually per 100 inhabitants;5

•Long-distance traffic trends; and

•Political factors (such as Bell System divestiture in the United States, privatization in other countries).

The trend toward greater use of direct HU (high-usage) routes tends to keep the number of hierarchical levels low (e.g., at two levels). The employment of dynamic routing can have a similar effect.

We now deal with fan-out. A higher-level exchange, in the hierarchical sense, fans out to the next lower level. This level, in turn, fans out to still lower levels in the hierarchy. It can be shown that 6- and 8-fan-outs are economic and efficient.

Look at this example. The highest level, one exchange, fans out to six exchanges in the next level. This level, in turn fans out to eight exchanges. Thus there is connectivity to 48 exchanges (8 × 6), and if the six exchanges in the higher level also serve as third-level exchanges, then we have the capability of 48 + 6, or 54 toll exchanges.

Suppose that instead of one exchange in the highest level, there were four interconnected in mesh for survivability and improved service. This would multiply the number of long-distance exchanges served to 48 × 4 c 192, and if we use the 56 value it would be 56 × 4 c 224 total exchanges. In large countries we deal with numbers like this. If we assign a long-distance exchange in each NPA, and assume all spare NPA capacity is used, there would be 792 NPAs in the United States, each with a toll exchange. Allow for a threeor four-level hierarchy and the importance of fan-out becomes evident.

Figure 8.4 shows one-quarter of a three-level hierarchy network, where the top level is mesh connected with four transit exchanges.

The fan-out concept assumes a pure hierarchy without high-usage routes. HU routes tend to defeat the fan-out concept and are really mandatory to reduce the number of links in tandem to a minimum.

8.3.7 Network Design Procedures

A national territory consists of a large group of contiguous local areas, each with a toll/ toll-connecting exchange. There will also be at least one international switching center (ISC). In larger, more populous countries there may be two or more such ISCs. Some may call these switching centers gateways. They need not necessarily be near a coastline. Chicago is an example in North America. So we now have established three bases to work from:

5The term telephone density should not mislead the reader. Realize that some “telephone lines” terminate in a modem in a computer or server, in a facsimile machine, and so on.

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Figure 8.4 A three-level hierarchy with initial fan-out of six and subsequent fan-out of eight. The highest level consists of four transit exchanges, but only one is shown.

1. There are existing local areas, each with a long-distance exchange.

2. There is one or more ISCs placed at the top of the network hierarchy.

3. There will be no more than four links in tandem on any connection to reach an ISC.

As mentioned previously, Point 1 may be redefined as a long-distance network consisting of a grouping of local areas probably coinciding with a numbering (plan) area. This is illustrated in a very simplified manner in Figure 8.5, where T, in CCITT terminology, is a higher-level center, a “Level 1” or “Level 2 center.” Center T, of course, is a long-distance transit exchange with a fan-out of four; these are four local exchanges (A,

Figure 8.5 (a) Areas and (b) exchange relationships.

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Table 8.1 Traffic Matrix Example—Long-Distance Service (in erlangs)

To Exchange

From

Exchange	1	2	3	4	5	6	7	8	9	10
1		57	39	73	23	60	17	21	23	5
2	62		19	30	18	26	25	2	9	6
3	42	18		28	17	31	19	8	10	12
4	70	31	23		6	7	5	8	4	3
5	25	19	32	5		22	19	31	13	50
6	62	23	19	8	20		30	27	19	27
7	21	30	17	40	16	32		15	16	17
8	21	5	12	3	25	19	17		18	29
9	25	10	9	1	16	22	18	19		19
10	7	8	7	2	47	25	13	30	17

B, C, and D) connect to T.6 The entire national geographic area is made up of such small segments as shown in Figure 8.5, and each may be represented by a single exchange T, which has some higher level or rank.

The next step is to examine traffic flows to and from (originating and terminating) each T. This information is organized and tabulated on a traffic matrix. A simplified example is illustrated in Table 8.1. Care must be taken in the preparation and subsequent use of such a table. The convention used here is that values (in erlangs or ccs) are read from the exchange in the left-hand column to the exchange in the top row. For example, traffic from exchange 1 to exchange 5 is 23 erlangs, and traffic from exchange 5 to exchange 1 is 25 erlangs. It is often useful to set up a companion matrix of distances between exchange pairs. The matrix (Table 8.1) immediately offers candidates for HU routes. Nonetheless, this step is carried out after a basic hierarchical structure is established.

We recommend that a hierarchical structure be established at the outset, being fully aware that the structure may be modified or even done away with entirely in the future as dynamic routing disciplines are incorporated (see Section 8.4). At the top of a country’s hierarchy is (are) the international switching center(s). The next level down, as a minimum, would be the long-distance network, then down to a local network consisting of local serving exchanges and tandem exchanges. The long-distance network itself, as a minimum, might be divided into a two-layer hierarchy.

Suppose, for example, that a country had four major population centers and could be divided into four areas around each center. Each of the four major population centers would have a Level 1 switching center assigned. One of these four would be the ISC. Each Level 1 center would have one or several Level 2 or secondary centers homing on it.7 Level 3 or tertiary centers home on a Level 2 switching center. This procedure is illustrated in Figure 8.6. Its hierarchical representation is illustrated in Figure 8.7 setting out the final route. One of the Level 1 switching centers is assigned as the ISC. We define a final route as a route from which no traffic can overflow to an alternative route. It is a route that connects an exchange immediately above or below it in the network hierarchy and there is also a connection of the two exchanges at the top hierarchical level of the network. Final routes are said to make up the “backbone” of a network. Calls that are offered to the backbone but cannot be completed are lost calls.8

6Of course, in the United States, T would be the POP (point of presence). 7Homing on meaning subsidiary to in a hierarchical sense. It “reports to.”

8Completed calls are those where a full connectivity is carried out indicated by both calling and called subscriber in the off-hook condition.

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Figure 8.6 A sample network design.

A high-usage (HU) route is defined as any route that is not a final route; it may connect exchanges at a level of the network hierarchy other than the top level, such as between 11 and 12 in Figure 8.7. It may also be a route between exchanges on different hierarchical levels when the lower-level exchange (higher level number) does home on a higher level. A direct route is a special type of HU route connecting exchanges in the local area. Figure 8.8 shows a hierarchical network with alternative routing. Note that it employs CCITT nomenclature.

Before final dimensioning can be carried out of network switches and trunks, a grade

Figure 8.7 Hierarchical representation showing final routes.

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Figure 8.8 A hierarchical network showing alternative (alternate) routing. Note the CCITT nomenclature.

of service criterion must be established.9 If we were to establish a grade of service as p c 0.01 per link on a final route, and there were four links in tandem, then the grade of service end-to-end would be 4 × 0.01 or 0.04. In other words, for calls traversing this final route, one in 25 would meet congestion during the busy hour. The use of HU connections reduces tandem operation and tends to improve overall grade of service.

The next step in the network design is to lay out HU routes. This is done with the aid of a traffic matrix. A typical traffic matrix is shown in Table 8.1. Some guidelines may be found in Section 4.2.4. Remember that larger trunk groups are more efficient. As a starting point (Section 4.2.4) for those traffic relations where the busy hour traffic intensity was > 20 erlangs, establish a HU route; for those relations < 20 erlangs, the normal hierarchical routing should remain in place.

National network design as described herein lends itself well to computer-based design techniques. The traffic intensity values used in traffic matrices, such as Table 8.1, should be taken from a 10-year forecast.

9Grade of service is the probability of meeting congestion (blockage) during the busy hour (BH).

188 LOCAL AND LONG-DISTANCE NETWORKS

Figure 8.9 A simplified network with circuit groups connecting pairs of nodes with one-way and both-way working.

8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK

8.4.1 New Routing Techniques

8.4.1.1 Objective of Routing. The objective of routing is to establish a successful connection between any two exchanges in the network. The function of traffic routing is the selection of a particular circuit group, for a given call attempt or traffic stream, at an exchange in the network. The choice of a circuit group may be affected by information on the availability of downstream elements of the network on a quasi-real-time basis.

8.4.1.2 Network Topology. A network comprises a number of nodes (i.e., switching centers) interconnected by circuit groups. There may be several direct circuit groups between a pair of nodes and these may be one-way or both-way (two-way). A simplified illustration of this idea is shown in Figure 8.9.

Remember that a direct route consists of one or more circuit groups connecting adjacent nodes. We define an indirect route as a series of circuit groups connecting two nodes providing end-to-end connection via other nodes.

An ISC is a node in a national network, which in all probability will have some sort of hierarchial structure as previously discussed. An ISC is also a node on the international network that has no hierarchical structure. It consists entirely of HU direct routes.

8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK

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Figure 8.10 Hierarchical routing in a nonhierarchical network of exchanges.

8.4.2 Logic of Routing

8.4.2.1 Routing Structure. Conceptually, hierarchical routing need not be directly related to a concept of a hierarchy of switching centers, as just described. A routing structure is hierarchical if, for all traffic streams, all calls offered to a given route, at a specific node, overflow to the same set of routes irrespective of the routes already tested.10 The routes in the set will always be tested in the same sequence, although some routes may not be available for certain types of calls. The last choice route is final (i.e., the final route), in the sense that no traffic streams using this route may overflow further.

A routing structure is nonhierarchical if it violates the previously mentioned definition (e.g., mutual overflow between circuit groups originating at the same exchange). An example of hierarchical routing in a nonhierarchical network of exchanges is shown in Figure 8.10.

8.4.2.2 Routing Scheme. A routing scheme defines how a set of routes is made available for calls between pairs of nodes. The scheme may be fixed or dynamic. For a fixed scheme the set of routes in the routing pattern is always the same. In the case of a dynamic scheme, the set of routes in the pattern varies.

8.4.2.2.1 Fixed Routing Scheme. Here routing patterns in the network are fixed, in that changes to the route choice for a given type of call attempt require manual intervention. If there is a change it represents a “permanent change” to the routing scheme. Such changes may be the introduction of new routes.

10Tested means that at least one free circuit is available to make a connectivity. This “testing” is part and parcel of CCITT Signaling System No. 7, which is discussed in Chapter 13.