28 Network Routing, Znterconnection and Znterworking The control of the routing of calls and connections (so-called ‘traffic’) across telecommunications networks is the most difficult but most important responsibility of a network operator. Only by careful planning and management of appropriate call and traffic routing plans can the network operator ensure successful connection of calls and the efficient use of network resources. In this chapter we discuss the techniques used by network operators in establishing efficient call routing patterns and the special problems caused by network interconnection and interworking, when calls or connections originated in one network have to be passed to another operator’s network for completion. 27.1 THE NEED FOR A NETWORK ROUTING PLAN We might choose, laudable as it may seem, to attempt to run our telecommunications network by completing as many calls as possible or delivering the greatest proportion of data messages. The problem is that if we attempt to do so, we are bound adversely to affect the intelligibility of messages and the time it takes to deliver them. To attempt the ‘complete if you can’ routing philosophy, we simply programme the exchanges to route all messages ‘any way possible’, rather than ever fail anything when a circuit is free. Unfortunately, the network will perish of congestion and suffer appalling signal quality. Studying the scenario, however, is highly instructive and we shall look at an example or two shortly. The rational and rewarding alternative to the ‘any way possible’ regime is to have network routing plan, together with a supporting numbering plan. The appropriate routing algorithms laid out by the routing and numbering plans are selected to control network traffic and to comply with the overall constraints which the transmission plan and engineering guidelines impose on end-to-end connections made across the network. To work within these various constraints, and still to achieve a network that is reason- ably cheap as well as highly efficient is an arduous test of planning and administration; 491 Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 492 NETWORK ROUTING, INTERCONNECTION AND INTERWORKING 1 A C I I L- 1-q l I I - - - Busy direct circuits Connection established Figure 28.1 Uncontrolled circular routing but it is well worth while when we consider the alternative of uncontrolled network routing and its disastrous effect on network congestion and the quality of connections, which the examples in Figure 28.1 and 28.2 will illustrate. In the example shown in Figure 28.1, a circuit-switched connection (such as a telephone call) is desired from exchange A to exchange B. Exchange A has direct circuits to B, but these are currently busy, so exchange A has made a connection to exchange C and passed the call on, intending to transit this exchange and connect via the direct circuits from C to B. Unfortunately these circuits are also busy, and by taking no cognisance of the call’s previous history, exchange C extends the connection back to exchange A using a similar logic. (‘My direct circuits are busy, but I know there is also a route via A’) The process continues in a circular fashion until either all the circuits between A and C also become busy, so that the call eventually fails, or finally a circuit becomes available on either of the routes A-B or C-B, in which case the call eventually completes. In either eventuality the circular routing between A and C ties up network resources, restricting communication between customers on exchanges A and C. Furthermore, even if the call does eventually complete, the transmission quality is likely to be appalling or the delay in packet data delivery may be intolerably long, due to the large number of links in the connection. A second effect of uncontrolled routing is shown in Figure 28.2, where a com- munication path has finally been completed over 9 individual links, transmitting 8 intermediate exchanges. As in the circular routing example of Figure 28.1, even though the call has completed, an undue quantity of network resources have been tied up, causing possible congestion for other traffic. In addition, the large number of links again lead to poor transmission quality or intolerable delay. The quality will be particularly poor if one or more of the nine links passes over a satellite connection. In this case the THE NEED FOR A ROUTING PLAN 493 I L- - - - Busy direct circuits Connection established Figure 28.2 Uncontrolled transit routing end-to-end propagation time may be several seconds. On a packet mode data connec- tion, the data throughput rate is likely to be severely limited by such long propagation delays, particularly if the protocol requires acknowledgement of individual packets. In practice, cases as extreme as those illustrated in Figures 28.1 and 28.2 are unlikely to arise. Nonetheless, the problems of circular routing and the need to set a maximum number ofhops are real. Circular routing does not typically take place between only two 494 NETWORK ROUTING, INTERCONNECTION AND INTERWORKING exchanges as shown in Figure 28.1, more likely in a ring of three, four or five transit exchanges. Engineering guidelines governing connection quality typically set maximum number of hops to values like 3, 5,7 or 9 depending upon the type of network and the use to which it is being put. 28.2 NETWORK ROUTING OBJECTIVES AND CONSTRAINTS To maintain high transmission quality and to ensure the minimization of time delays both on call set up and on message or speech propagation, it is desirable to minimize the overall number of links and exchanges making up a connection. In addition it is desirable to limit the number of concatenated links of certain transmission types (e.g. satellite links, or links using low rate speech), since the tandem connection of such devices can lead to unaccept- able transmission impairment, as we shall see in Chapter 33. Historically networks were designed in a hierarchical fashion. This enabled the number of links required on a maximally adverse connection between any two endpoints to be limited. The maximum hop limit is set by the number of layers in the network hier- archy. Network topology rules ensure full interconnectivity of all exchanges in the top layer of the hierarchy and connection of each lower level exchange with at least one exchange in the next higher layer. Thus a hierarchical network structure consisting of n layers needs, at most, only (2n - 1) links to interconnect any two exchanges. For example, in a network comprising a three layer hierarchy, any exchange may be connected to any other without the need to use more than (3 X 2 - 1) = 5 links, as Figure 28.3 shows. In more modern networks, the strict hierarchical method of network design is becoming less common, in favour of simpler and more flexible network topologies and routing schemes. Less rigidly structured networks prevail in which each exchange recognizes the need to select an economical routing conforming with engineering guidelines and the requirements of the transmission plan (Chapter 33) the need to contain the likelihood of rapidly escalated network congestion the need to charge for calls in line with the incurred costs the need for flexibility of routing, in order that network operators may take advantage of the non-coincidence of busy periods (so-called route busy hours) via transit exchanges the need to minimize the overall number of links in a connection, and in particular to limit the number of satellite links or bandwidth compression equipments which may be used in tandem the policy of preferred transmission media, say when alternative satellite and cable links are available to the same destination the need to avoid circular routings NETWORK ROUTING OBJECTIVES AND CONSTRAINTS 495 1 I Top layer Figure 28.3 Five-link connection in a three-tier network hierarchy Careful network design and a shrewd call routing programme at each exchange will ensure conformance to the routing plan, but this may require considerable adminis- trative effort in establishing appropriate routing tables at each switch or exchange within the network as we discuss later. Signals which accompany the call or connection setup message are intended to help to convey the previous history of the call or packet (for example, the existence of a previous satellite link) and make appropriate routing choice easier. In the example of Figure 28.4, caller 1, connected to exchange A and wishing to call B, has reached exchange C by means of a satellite link. Although both cable and satellite links are available from exchange C to exchange B, the call is only allowed to mature if a circuit is available on the cable link. If instead the call were to be permitted to overflow to the satellite link, then the connection would not meet the required transmission quality standard. (If, however, the connection was only possible by the use of a double satellite link, then the call could have been permitted to mature). By contrast, caller 2, (on exchange C, may be connected either over the satellite or the cable link. Two alternative routing policies are available to the owner and operator of exchange C to ensure optimum routing of both caller’s calls. In the one shown, the operator has chosen to make the satellite first choice for caller 2’s calls. This inflicts 496 NETWORK ROUTING, INTERCONNECTION AND INTERWORKING Exchange Caller 1 Satellite Satellite Exchange , Cable Exchange c (only choice for B caller 1 ) Destination I -* Caller 2 Figure 28.4 Routing based on call history the propagation delays associated with satellite links on a large proportion of caller 2’s calls to exchange B, but has the advantageous effect of maximizing the availability of cable circuits for connection of caller l’s calls to exchange B, so preventing the failure of calls in the instance when otherwise only an unacceptable ‘double satellite path’ were available. In the alternative scheme the operator of exchange C could have chosen to make the cable link to exchange B first choice, even for caller 2’s calls. This would have the effect of minimizing the propagation time of caller 2’s calls, and may be desirable when caller 1 is a customer of a different network operator. A common feature of all good routing schemes is their simplicity. Complicated routing schemes can lead to administrative difficulties and oversights. Apart from network congestion and poor transmission quality, slow call set-up and a burden of exchange data maintenance can also result. All routing schemes rely upon the exchanges to analyse the dialled number or network address (i.e. OS1 layer 3 address) to determine the destination of the call. Additionally, signalling information about required supplementary services (e.g. closed user group or intelligent network services) and the call’s previous history (e.g. ‘previous satellite link’) help to determine the selection of an appropriate route to the destination and an appropriate charge. Closed user group (CUG) information carried at connection setup time can be used to ensure that only certain customer lines or ports may be connected together. This might help, for example, to prevent unauthorized dial-in to a computer centre. Only members of the CUG may be connected to the centre. Intelligent network services include, among others, freephone, in which the charges for the call are invoiced to the receiver rather than the call originator. Finally, the connection history (e.g ‘previous satellite link’) or required quality attributes of the connection (e.g. for frame relay the committed information rate (CZR) may also affect call setup or connection routing. THE ADMINISTRATION OF ROUTING TABLES 497 The switches in all types of networks therefore need to analyse the network address to determine the intended destination of a connection and other service parameters and quality information to assess any constraints on the path to the destination. Ideally, only the minimum amount of information is analysed at any particular switch or exchange, to minimize time and effort required to determine the next step in the path. Thus, for example, at an outgoing international telephone exchange at least the country code indicator digits of the dialled number need to be inspected to select the appropriate route to the country concerned. A trunk telephone exchange must inspect only the area code to determine the onward route selection. Finally, a destination local exchange needs to examine all the digits of the destination customer’s local number to select the exact line required. 28.3 THE ADMINISTRATION OF ROUTING TABLES Historically, network routing plans were administered by means of routing tables in each of the individual switches or exchanges. Each exchange thus had a ‘look-up’ table of permissible address code (e.g. telephone area codes), and alongside each code, a list of the alternative routes available for completion of relevant calls. Thus in the example of Figure 28.5, we illustrate the network topology of six interconnected nodes, and the routing tables resident in exchanges A and B to reach the various telephone number blocks, OOlXX (at A), 012XX (at B), 034XX (at D), 053XX (at C), 069XX (at E) and 091XX (at F). The example of Figure 28.5 illustrates the complexity of setting up and administering the routing plan, as well as the problems of circular routing and maximum hop count already discussed. The first observation is that each of the exchanges requires a separate routing table. There is little or no commonality between the routing tables (the other four for exchanges C, D, E and F are not shown), so that considerable manual effort is required first to work out the tables and second to type them in to the configuration data of each of the individual exchanges. There is a very high probability in complex networks of errors in the routing plan design and further potential for errors during the typing-in stage. If we now examine closely the routing commands given to exchanges A and B in Figure 28.5 for the handling of codes 053XX (to exchange C) we can see the potential for a circular route being set up, for if exchange D is told to use ‘via A’ as a third choice route to exchange C (code 053XX) then at times when the links B-C and D-C are overloaded or out-of-service due to network failure calls to code 053XX may be passed in endless loop A-RD-A, etc. We also see the problem of minimizing the maximum hop count. The intention of the designer of the network in Figure 28.5 is that the maximum hop count shall be three. Thus, for example, the third choice route from A-to-C is A-E-D-C. However, the third choice route from E-to-C might also be via three hops (E-A-RC), so how do we prevent the circuitous routing (exceeding the maximum hop count) A-E-A-B-C? The answer is that the routing tables need also to take account of the origin of the call as well as the intended destination. Thus calls arising at exchange E but origin- ated by exchanges other than exchange E should not be allowed the third option to 498 NETWORK ROUTING, INTERCONNECTION AND INTERWORKING a m 'S - a (c 'S - Y X m d .M ROUTING PROTOCOLS USED IN MODERN NETWORKS 499 exchange C. Similarly, calls appearing at exchange E directly from exchange A should not be passed directly back again. Further increasing the problem, the dialled digit train may need to be altered. Historically, this was necessary because the switching action ofelectromagnetic exchanges was triggered by the pulsed digit train, so that the digits were literally ‘used’ to activate the switching. As a result each subsequent exchange sent fewer digits to the next along the chain of the connection. So that, for example, an electromagnetic exchange at point A in Figure 28.5, might expect only to receive the digits XX when accepting calls to the digit range OOlXX, the ‘001’ having been used or deleted by previous exchanges in the connection. Modern computer controlled exchanges generally relay the entire dialled number, but when signalling to older electromechanical exchanges they may have to adapt the train to the routing digits required to activate the switching (Chapter 6). The problems of call origin and call history dependent routing described above make for complicated signalling between the exchanges and complicated routing tables (based on the route origin) within the exchanges. Worse still, every time further capacity or new trunks are added to the network topology, all the routing tables in each of the exchanges may need to be amended. Routing table administration remains one of the major operational burdens of telephone and ISDN network operators. 28.4 ROUTING PROTOCOLS USED IN MODERN NETWORKS In contrast to telephone networks, where typically the individual switches (exchanges) are supplied by different equipment manufacturers, data networks have often been built from switches all supplied by a single manufacturer, with a common network manuge- ment system. The common manufacturer and network management system shared by all the switches enables the use of proprietary signalling and control mechanisms to be applied to traffic routing within the network. Thus most data network management systems require only the association of groups of destination network addresses to particular switches. The routing tables for all other switches are then generated according to the network management system’s knowledge of the current network topology, using a set of automated routing design rules and routing algorithms (e.g. preference for high capacity routes over low capacity routes, preference for low hop count path, etc.). The human task of administering routing tables in modern data networks is thus far more straightforward than telephone network routing table administration. Once the route is set-up for a particular connection (i.e. in a connection-oriented network such as X.25 packet switching, frame relay or ATM), it is not usually altered during the duration of the call (i.e. the period of communication). Leaving the routing of the connection unaltered (path oriented routing) means that the transmission propagation time across the network between the two devices is not subject to any unnecessary jitter (variability of delay). In addition, there is much reduced risk of cells which might otherwise have taken different paths from getting out of order. It is also much easier to determine and manage a network loading scheme, because nominal bandwidth allocations may be made to each of the connections which must statistically share a given physical transmission path. 500 NETWORK ROUTING, INTERCONNECTION AND INTERWORKING In the most modern of networks (e.g. router and ATM networks), the entire routing administration is automated, so that switches within the network are programmed to ‘learn’ about the topology of the network the ideal route to a given destination (network address). A routing protocol is employed by such networks so that the individual nodes can discover the network topology automatically and keep themselves abreast of changes. Examples of routing protocols are used in the Internet are 0 routing information protocol (RIP) 0 open shortest path first (OSPF) 0 border gateway protocol (BGP) 0 exterior gateway protocol (EGP) Routing protocols are used widely in the Znternet to pass information between routers about the various sub-networks making up the network. One of the first protocols developed was the exterior gateway protocol (EGP) defined by RFCs 827,888 and 904). This was a protocol intended to be used between router on a sub-network (say university campus) and an inter-site network (internet) so that internal UNIX computers on the sub-network could locate and establish connections to exterior ones in bordering networks. EGP has subsequently been largely replaced by the border gateway protocol (BGP) defined by RFC 1267. Within most router networks (e.g. Cisco, Wellfleet, 3Com, etc.) it is common to use proprietary routing protocols (interior routing protocols, ZRP), but the RIP (routing information protocol) defined by RFC 1058 set the initial standard for transfer of routing topology information, so that a routing table could be maintained by a source router. The table enables the router (near the source of a message) to determine the best path across the Internet. RIP complements the hello protocol of RFC 89 1 which is used to register and synchronise new connections in the network. The OSPF (open shortest path Jirst) protocol is a newer, more complex and more sophisticated protocol than RIP but intended to bring about a simplification of the topology of the Internet by introducing a structured hierarchy of routing nodes. It has become the accepted ‘standard’ routing protocol in router networks, Intranets (corporate router networks) and the Internet. As an example of the way in which switches within a modern network may be pro- grammed automatically to discover the network topology and keep abreast of all changes made to it, thus enabling optimal routing of calls, connection and traffic at all times, we discuss next how the hello state machine defined in the ATM network standards (see also Chapter 26) enables constant updating of the ATM network topology state. 28.5 NETWORK TOPOLOGY STATE AND THE ‘HELLO STATE MACHINE’ The ATM forum is developing, as part of its PNNZ (private network-node interface, based on the ATM UN1 v3.1) specification, a sophisticated source routing control mechanism, in many ways similar to the techniques used in the Internet. [...]... this will result in a change of telephone number or network address, since this is associated with considerable cost, upheaval and effort To change a telephone number, letterheads,product packaging and documentation all need to be changed and customers and suppliers must all be informed To change data network addresses, large numbers of computers and network devices may need to be re-programmed Number . effort. To change a telephone number, letterheads, product packaging and documentation all need to be changed and customers and suppliers must all be