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Figure 6.1 Signalling for basic call set-up in the ISDN Figure 6.4aan ISDN SETUP message to the originating Local Exchange.. The Call Control process in the originating Local Exchange th

Alfred North Whitehead

We have seen in Chapter 3 that the Integrated Digital Network (IDN) andISDN evolved from the analogue PublicSwitched Telephone Network(PSTN) And we have seen as part of this evolution how the network has beenlogically segregated into a ‘switched information’ subnet (the user-plane orU-plane in ISDN parlance) and a ‘signalling’ subnet (the control-plane orC-plane) In terms of Whitehead’s dictum we can associate intelligence withthe C-plane and ability with the U-plane This separation of switching andsignalling arises naturally from the essentially different nature of thetechnologies used for switching and signalling As we will see, the two planescan evolve separately to exploit advances in their respective techniques andtechnologies

So far this book has focused mainly on the evolution of the user-plane—fromanalogue voice, through 64 kbit/s circuit switching, Frame Relay (as an ISDNbearer service), and in due course ATM Each of these switching techniquesand technologies provides additional flexibility in the range of services thatcan be offered to the user, and in the way that distance is perceived, orpreferably not perceived Remember, the overall aim in telecommunications

is to take the distance out of information—that is, Total Area Networking Inthis chapter we will look at the part the control-plane plays in Total AreaNetworking, and how it is evolving

Total Area Networking: ATM, IP, Frame Relay and SMDS Explained Second

Edition John Atkins and Mark Norris Copyright © 1995, 1999 John Wiley & Sons Ltd Print ISBN 0-471-98464-7 Online ISBN 0-470-84153-2

One of the increasing important factors shaping developments in munications networks and services is competition The long-heraldedliberalisation of telecommunications is now well under way almost everywhere,the traditional monopoly suppliers, the national PTOs, being forced to sharetheir market with newcomers, the ‘Other Licenced Operators’ or OLOs Inthis environment the customer is ‘king’, and if one service provider does notmeet his needs another will Those service providers will therefore prosperwho can respond most quickly to new customer demands We will see thatthe control-plane holds the key to such rapid response, and that it exercisesthis power by virtue of its intelligence.

telecom-Competition is, of course, not confined to telecommunications Theglobalisation of business and commerce that modern telecommunicationshas done so much to facilitate is itself bringing new opportunities to gaincompetitive advantage: indeed, that is its justification But the pace of change

is rapid and competitive advantage can quickly change hands as competitorsplay leap-frog in their search for success It is clear that the most successfulcompanies will be those with the ‘agility’ to respond quickly to theircompetitors’ activities and to the developing expectations of the customer.Compared with traditional private networks, which can quickly be overtaken

by advances in technology and which tend to divert a company’s resourcesaway from its core business, VPNs can make an important contribution to acompany’s agility As the platform for the implementation of VPNs therefore,the Intelligent Network may be expected to become an increasingly importantpart of every major company’s service infrastructure as public network INcapabilities develop In effect, VPNs will be a major step on the road to TotalArea Networking

This chapter is about the evolution of the IDN/ISDN to become theIntelligent Network or IN

6.1 SIGNALLING IN THE NETWORK—CCSS7

Before embarking on this story we will lay the foundations Since this chapterfocuses on the C-plane we must begin with a brief review of signalling, thelanguage of the C-plane In Chapter 3 we looked briefly at ISDN signallingbetween the user and the ISDN network and how a simple call would be set

up and cleared (Figures 3.1 and 3.2) Here we extend this to include signallingbetween the switches which uses a similar message-based signalling protocolknown as CCITT Common Channel Signalling System Number 7, or CCSS7(or even just C7) for short

In its full glory C7 is a very comprehensive and necessarily complexprotocol and justifies a book to itself much bigger than this one! We will limitourselves here to the essentials needed to develop our story Figure 6.1 shows

an example of the signalling involved in setting up and clearing a basic call,assuming ISDN terminals at both ends (see also Figures 3.1 and 3.2).The calling terminal, a digital telephone say, initiates call set-up by sending

Figure 6.1 Signalling for basic call set-up in the ISDN (Figure 6.4(a))

an ISDN SETUP message to the originating Local Exchange This SETUPmessage contains the calling and called party numbers and other informationneeded to establish an appropriate connection (such as whether a digitalconnection is needed from end-to-end or whether a partly analogue connectionwould do) The originating Local Exchange acknowledges receipt of thismessage by returning an ISDN CALL PROCEEDING message indicating thatthe network is attempting to set the call up

The Call Control process in the originating Local Exchange then translatesthe ISDN SETUP message into a corresponding CCSS7 message, which is anInitial Address Message or IAM This Initial Address Message is routedthrough the signalling subnet until it reaches the Local Exchange serving thecalled party (the destination Local Exchange), the routing decision at each

switch en route being based on the called party’s number and any other

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pertinent information contained in the IAM (such as whether a satellite link isacceptable).

The destination Local Exchange translates the Initial Address Message into

a corresponding ISDN SETUP message which it delivers to the called party.The called party accepts the call by returning an ISDN ALERTING message tothe destination Local Exchange The ALERTING message is translated into aCCSS7 Address Complete Message (ACM) which is passed back to the callingterminal as an ISDN ALERTING message as shown The ACM both indicates

to the other exchanges involved in the connection that the destinationLocalExchange has received enough address information to complete the call andpasses the alerting indication (i.e that the called party is being alerted) to theoriginating Local Exchange

The speech path is shown as switched through in the backward direction atthe originating Local Exchange on receipt of the SETUP message andswitched through in both directions at the Transit Exchange on receipt of theIAM This allows the caller to hear any ‘in-band’ signalling tones sent by thenetwork (for a variety of reasons, not all call attempts succeed)

The called telephone now rings and the originating Local Exchange sendsringing tone to the caller

When the call is answered (i.e the handset is lifted) the called telephonegenerates and sends an ISDN CONNECT message to the destination LocalExchange, which it translates into the corresponding CCSS7 Answer message(ANM) This is passed back to the calling Local Exchange where it istranslated back into an ISDN CONNECT message and passed to the calling

terminal At each switch en route any open switch points are operated to

complete the connection in both directions, giving an end-to-end connection,and the call enters the ‘conversation’ phase Billing for the call usually starts atthis point

Note that in the case of ISDN access there is a distinction between acceptingthe call and answering it The reason for this is that, unlike a PSTN access, theBasic Rate ISDN customer interface takes the form of a passive bus that cansupport simultaneously a number of different terminals (up to eight), ofdifferent types (such as fax machines, telephones, personal computers, and soon) The destination Local Exchange does not know until it receives theALERTING message from the called party whether he has an appropriateterminal connected to the interface that can take the call (amongst other thingsthe SETUP message may carry compatibility information that the terminalsmay use to ensure compatibility between calling and called terminals) Ifthere were not an appropriate terminal connected to the called access the callwould not be accepted

At some later time the calling party (say) clears the call This is signalled tothe originating LE by means of an ISDN DISCONNECT message, as shown inFigure 6.2 The originating Local Exchange then initiates release of the ISDNaccess circuit by returning an ISDN RELEASE message, acknowledged oncompletion by the calling terminal sending a ISDN RELEASE COMPLETEmessaage Release of the inter-exchange circuit is signalled to the TransitExchange by a CCSS7 Release (REL) message, completion of which is

Figure 6.2 Normal call clear sequence using CCSS7

Figure 6.3 The CCSS7 protocol stack for ISUP

signalled back by a CCSS7 Release Complete (RLC) message Successivecircuit segments are released in a similar way as shown A similar process inthe other direction is used if the call is cleared by the called party (thoughthere is also the option for the called party to suspend the call for a short time

by replacing the handset before resuming the call)

The basic CCSS7 protocol stack is shown in Figure 6.3 It is a layeredprotocol, but was defined before the publication of the OSI Reference Model(RM), and the CCSS7 levels of protocol, though similar, do not correspondexactly with the OSI layers The alignment of CCSS7 with the OSI ReferenceModel is a comparatively recent development as described below

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The Message Transfer Part, or MTP, provides for the reliable, error-freetransmission of signalling message from one point in the CCSS7 signallingsubnet (referred to as a Signalling Point) to another such point It is itselforganised as three distinct functional levels—similar to but not the same asthe lowest three OSI layers.

MTP Level 1—the physical level—is usually referred to as the SignallingData Link It provides a physical transmission path (usually a 64 kbit/stime-slot in a higher-order multiplex) between adjacent Signalling Points.MTP Level 2, usually known as Signalling Link Control, deals with theformation and sending of Message Signal Units (MSUs) over the SignallingData Link, checking for errors in transmission using a cyclic RedundancyCode added to the MSU before transmission (in effect a form of parity check),and correcting any such errors by retransmitting the MSU In this way MTPlevel 2 ensures that signalling messages get neither lost nor duplicated It alsooperates a flow control procedure for message units passed over thesignalling link Like level 1, level 2 operates only between adjacent SignallingPoints So a signalling ‘connection’ between an originating and destinationLocal Exchange involved in setting up a call actually involves a number ofindependent signalling links in tandem

MTP Level 3 is concerned with routing signalling messages to theappropriate point in the CCSS7 signalling subnet based on unique 14-bitaddresses, known as Signalling Point Codes, assigned to each such point inthe signalling subnet Routing is predetermined with alternative routesspecified for use if the primary route becomes unavailable So at eachSignalling Point reached by a signalling message a decision is made as towhether the message is addressed to that Signalling Point or is to be routedonward to another When used to route signalling messages in this way aSignalling Point is operating as a Signalling Transfer Point or STP

The ISDN User Part, or ISUP, uses the services provided by the MTP It isconcerned with the procedures needed to provide ISDN switched servicesand embraces the functions, format, content and sequence of the signallingmessages passed between the signalling points An example of ISUP at work

is shown in Figure 6.1

Whilst the focus here is on the ISDN it should be realised that the firstversion of CCSS7, published in 1980, did not cover ISDN services, which werenot defined until 1984 The 1980 CCSS7 standard defined the Telephony UserPart, or TUP, which does for analogue telephone services what ISUP does forISDN services In practice the two ISDN and Telephony User Parts willco-exist, perhaps for many years, before TUP is entirely supplanted by ISUP.But for clarity and brevity here, and because we are looking to the future, wefocus on ISUP

One of the shortcomings of the 1980 version of CCSS7 was that signallingwas defined in terms of the messages that passed between adjacent exchanges.This was fine for analogue telephony services But the ISDN, with itspowerful signalling between user and network, brought a much wider range

of services into prospect Many of these services require signalling messages

to be passed between the originating and destination Local Exchanges

without the intervention of intervening exchanges en route Indeed, in some

cases signalling is required between the Local Exchanges even in the absence

of a connection being established between them

This requirement found the MTP wanting and in 1984 the SignallingConnection Control Part, or SCCP, was added to CCSS7 to provide greaterflexibility in signalling message routing Whilst the Telephony User Part(TUP) uses only the services of the MTP, the ISDN User Part (ISUP) alsomakes use of the SCCP as shown in Figure 6.3 The SCCP was designed toprovide the (by then) standard OSI network layer service, supporting bothconnectionless and connection-oriented methods of message transfer Ineffect, it created a packet-switched network within the signalling subnet bymeans of which any Signalling Points can send signalling messages to anyother Signalling Point, independent of switched connection in the switchedinformation subnet We will see below that even this is not the completepicture for ISUP, but we break the story here in order to renew it later when

we have introduced the idea of the Intelligent Network

6.2 THE TRANSITION TO THE INTELLIGENT NETWORK

In principle the IDN and ISDN are sufficiently flexible to provide servicestailored to each company’s specific requirements Providing such customisedservices means making (part of) the public network behave as though it werethe company’s own private network, i.e a Virtual Private Network Inpractice, however, this flexibility has not been achieved with the IDN/ISDN.The potential flexibility of stored program control—that is, software control

of switches—has not been realised because of the way the call controlsoftware and its associated data has been implemented in the exchanges.The problem stems from the fact that the service information relating to acustomer’s lines is stored in the serving Local Exchange the companies withthe greatest needs—those with the most to gain from customised services—arelarge and spread over many sites, indeed often over a number of countries Sothe service information relating to such companies is distributed over apotentially large number of Local Exchanges, perhaps hundreds Indeed,when looking at the collective requirements of corporate customers theinformation is distributed over all Local Exchanges, perhaps thousands Theproblem of managing such a large distributed database and the associatedco-ordination of customised call control has provided prohibitive

The solution to this co-ordination problem has been to separate the

‘advanced’ service logic and the associated customer information from the

‘basic’ call control logic and switches Basic call control continues to reside inthe Local Exchange But the advanced service logic defining the customer’srequirements is centralised in what is an intelligent database as shown inFigure 6.4 Adding this centralised network intelligence to the IDN/ISDNcreates what has become known as the Intelligent Network, or IN As we willsee, with this arrangement it becomes comparatively straightforward to

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Figure 6.4 The IDN/ISDN + centralised network intelligence = IN

manage a comprehensive, up-to-date picture of a corporate customer’s

‘private’ network requirements and to co-ordinate switching operationsthroughout the network in order to implement these requirements CCSS7continues to be the signalling system of choice for IN operations

6.3 IN ARCHITECTURE AND TERMINOLOGY

The main building blocks of the IN are the Service Switching Point (SSP) andthe Service Control Point (SCP) as shown in Figure 6.5 The SSP is (usually)part of the Local Exchange whose call control software has been restructured

to separate basic call control from the more advanced call control needed forIntelligent Network Services (this terminology is somewhat circular—IntelligentNetwork Services are simply those services which need the IntelligentNetwork capability)

Basic call control looks after the basic switching operations that take place

in an exchange It has been restructured to incorporate what are known asPoints In Call (PICs) and Detection Points (DPs) as defined points in the basiccall control state machine At these points trigger events may be detected andcall processing temporarily suspended whilst reference is made to thecentralised Service Control Point (SCP) to find out how the call should behandled from that point Typical trigger events include such things asrecognition of the Calling Line Identity (CLI) and recognition of dialled digitstrings

The Service Control Point (SCP) is a general-purpose computing platform

on which the advanced service logic needed for Intelligent Network Services

is implemented together with the information that defines each corporatecustomer’s network services It must be fast to provide the rapid responseneeded and to handle the potentially very high trafficlevels arising from itscentral location And of course it has to be reliable To meet these stringent

Figure 6.5 IN architecture and terminology

Figure 6.6 The Service Switching Point (SSP)

requirements more than one SCP is normally provided In practice there may

be a dozen or more

Figure 6.6 introduces more jargon The Service Switching Point softwarewithin the exchange consists of a Call Control Function (CCF) and a ServiceSwitching Function (SSF) The Call Control Function looks after the basic callcontrol needed for simple telephony switching operations The Service

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Figure 6.7 The Service Control Point (SCP)

Switching Function provides the control itnerface with the Service ControlPoint (and with another IN network element known as the IntelligentPeripheral (IP) that we will look at shortly)

And there is yet more jargon! The Service Control Point (SCP) contains theadvanced service logic needed to implement Intelligent Network Services, asshown in Figure 6.7 Each such service, such as 0800 Freefone (which we willlook at in more detail below), requires a Service Logic Programme (SLP)which is built from Service Independent Building-blocks (SIBs) together withthe service information defining the corporate customer’s detailed requirementswhich is held in the associated Service Data Point (SDP) Service IndependentBuilding-blocks would typically include such operations as numer translation,connecting announcements, charging, and so on Strictly speaking the ServiceData Point need not be co-located with the Service Control Point But itusually is and we will assume here that the Service Data Point resides withinthe Service Control Point

The Service Logic Execution Environment (SLEE) is the generic softwarethat controls the execution of the Service Logic Programmes It interworkswith the basic call control process and the simple switching functions in theService Switching Point and screens the Service Logic Programmes from thelow-level SCP–SSP interactions and controls the impact of new Service LogicProgrammes on existing IN Services

CCSS7 needed to be extended to support IN Services In particular aTransaction Capabilities (TC) Application Part has been added to support

Figure 6.8 The CCSS7 protocol stack for IN

signalling that is not related to switched connections, as shown in Figure 6.8.And here we can complete the picture for ISUP Since some ISDNsupplementary services involve signalling that is not related to switchedconnections, ISUP may also use the services of the Transaction CapabilitiesApplication Part as shown

Examples of non-connection-related signalling include Operation istration & Maintenance (OA&M) messages, customer-to-customer datatransfer (via the signalling subnet), IN applications such as signallingbetween the SSP and SCP, and signalling for cellular mobile telephonenetworks (where roaming may be thought of as a particular example of an INservice tailored to a specific situation)

Admin-A new protocol has been developed specifically for the Intelligent Network,the so-called Intelligent Network Application Part (INAP), which may beconsidered part of the CCSS7 protocol suite Looking upwards, INAPinterfaces directly with the Service Logic Execution Environment of the SCP.The Transaction Capabilities (TC) Application Part supports TC users such asINAP (and MAP, the Mobile Application Part) It provides the OSI Sessionlayer service together with dialogue control and is responsible for managingcommunications with remote TC users

INAP defines the CCSS7 signalling messages relating to IN services and thefunctions and interactions they cause (in the form of finite state machines).INAP is in turn defined in terms of Abstract Syntax Notation 1 (ASN.1)making it independent of the computing platform and porotable to anyprocessing environment This is an important consideration in the quest for

IN products that will actually interwork and in providing network operatorswith a means of enhancing their systems ‘in-house’ rather than beingcontinually dependent on the manufacturers INAP, like SCCP, is closelyaligned with OSI standards It is based on the OSI Remote Operations ServiceElement (ROSE)

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It is worth noting here that the Signalling Connection Control Part (SCCP)can ensure that IN messages destined for a failed SCP are automaticallyre-routed to an operational one.

6.4 EXAMPLES OF IN SERVICES

0800 Freefone

Probably the best known example of an IN Service is 0800 Freefone, which wewill use here to illustrate the main ideas of the Intelligent Network and tointroduce another IN network element, the Intelligent Peripheral (IP) mentionedabove The Freefone example is illustrated here with reference to a hypotheticalcase of a large insurance company with branches in high streets up and downthe country, six area offices each dealing with the administration of the highstreet branches within their respective geographic areas, and a nationalheadquarters office in the capital

A typical requirement of such a company, illustrated for clarity in Figure6.9, would be to have a single, unique telephone number covering the wholecountry, such that:

• during normal office hours calls to that number would be routed to thehigh street branch nearest to the caller;

• out of normal office hours calls are routed to the area office covering thecaller’s location;

• when the area offices are closed calls should be routed to the headquartersoffice where they would be handled by the company’s automated callhandling system;

• the calls should be free (to the caller)

This service requirement can be satisfied by using the 8000 Freefone servicewhereby the company has an easily remembered number, say 0800 123abc.The branch and area offices will naturally have a variety of unco-ordinatedtelephone numbers the list of translations from 0800 1234abcd to theappropriate branch or area office telephone number is stored in the central INdatabase, i.e the SCP, together with the time of day, day of week, and day ofyear routing schedule

It is 09: 31 on a normal Monday morning and a customer (or potentialcustomer) of the company dials the company’s national number, 0800 123abc.Though it is not necessary, we will assume in what follows that both callerand company are ISDN-based, so ISDN access signalling (Q.931) is used atboth ends of the call as shown in Figure 6.10 The caller’s telephone number is

01234 567abc

The basic call control process in the serving SSP, by reference to its Trigger

Figure 6.9 Typical service requirement of large insurance company

Table, recognises the 0800 code as involving an IN service Basic call control isthen suspended and the SSP sends a CCSS7 signalling message to its SCPgiving the dialled number, 0800 123abc, and the caller’s telephone number,

01234 567abc (the CLI) By reference to its routing schedule for that particular

0800 number (123abc) the SCP knows that for that time of day, day of week,and day of year the call should be routed to the high street branch nearest tothe caller And by reference to the CLI the SCP knows that the telephonenumber of the nearest such branch office is 01234 654cde (the caller and hisnearest branch office may not have the same area code; it depends on thegeography)

The SCP then returns a CCSS7 signalling message to the SSP advising thatthe actual destination number for the call is 01234 654cde and that the allershould not be charged for the call On receipt of this message the SSP resumesbasic call processing, the call is routed through the network to 01234 654cde inthe usual way, and the call is charged to the insurance company

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