Components, architecture, and numbering in the Telephone network(PSTN),

1. Components, architecture, and numbering in the Telephone network(PSTN), Raimo Kantola raimo.kantola@hut.fi SG210, 4512471

2. Telecommunications studies in 38.xxx courses Services, Products Service Design, $£ 001 TF, L-courses, 105 Management and parametrization Networks and Network elements - exchanges - routers 118, 110, 188 Mathematical Theoretical Formalizing Architectures and Network topologies Mechanisms - signaling - routing - technologies ATM, TCP/IP, 802.xx - queuing methods - signal processing 122, 164, Traffic Theories, 188, 110

3. Components in a Wireline Telephone Network • access network cabling • the line card in an exchange • concentrator • data link to a main exchange • switching matrix in a main exchange • data links to a transit exchange • switching matrix in a transit exchange • data link to an international station • switching matrix in an international station By Ilkka Veuro/Hgin Puhelin

4. A Network Structure Building Street Distribution Responsibility boundary Concentrator House Distribution Local Exchange Building Concentrator International Exchange Transit exchange By Ilkka Veuro/Hgin Puhelin

5. Hypothetical Reference Connection in the Telecommunications Network 27 500 km National Network International Network Local LE PC SC TC ISC ISC ISC ISC ISC TC SC PC LE LE - Local Exchange PC - Primary Center TC -Tertiary Center ISC - International Switching Center Digital Exchange Digital link The basic rule: End to end -connection passes at most through 13 exchanges in telephone networks. There are exceptions due to private networks. End to end delay budget can be managed: -- delay over 150ms disturbs the quality of speech, delay over 20 ms demands echo suppression. -- A center poses delay << 1ms , the speed of light is the main limiting factor in data links. -- Satellite connections increase delay significantly.

6. Telecommunications Networks can be Reviewed in Layers OSI 7 3 2 1 Service Systems Components of intelligent networks , voice mail, ... Switching Systems Exchanges, Concentrators, Private Branch Exchanges (PBXs) Transmission Systems PDH, SDH, WDM, xDSL, BSS/GSM, radio link, cross-connection equipment ... Transmission Lines Copper cables, optical fibers, radio line, ...

7. Key Questions on different Layers: • Differentiation, fast development of services and deployment, new service architectures • Interoperability, billing Service systems • Dimensioning and design of networks, routing/routeing, • interworking (signaling), charging, mobility • circuit switching and packet switching Switching systems Transmission systems • Geographic coverage, large capacity (multiplexing), • Efficient use of the radio band, radio network design Transmission lines • Right of way, long operating life, more efficient use • of existing lines • Local market competition/natural monopoly

8. Interpretation of the System Hierarchy • Our comparison to the OSI template refers to the subscriber’s/user’s point of view. The systems on various layers include protocols on many layers of the OSI model. • The interface between the service layer and the switching layers is not well defined. The service layer is dependent on the switching layer (call control) concerning many supplementary services. • Three higher layers consist of computer managed equipment having plenty of software.

9. In the Enterprise viewpoint, telecommunications is decomposed into roles and stakeholders Service Provider + Service nodes PABX, private automatic branch exchange PBX, private branch exchange Subscriber: + Terminal equipment + PABX,PBX Network Provider/Network Operator: + transmission equipment, cross-connect systems + exchanges The figure above describes the target model. In reality, there are no service providers (if not the ISPs) independent of network operators in telecommunications networks. Also, teleoperators are fiercely trying to retaketheir positions lost to ISPs.

10. Topology describes the telecommunications network from the switching layer’s point of view • Mesh topology, every exchange has been coupled to others with a route orroutes consisting of circuit groups. • Star topology, two exchanges have been coupled to each other with a third exchange. Nodes are switching systems and connecting lines are transmission systems implementing the circuit groups.

11. The general structure of telecommunications networks consists of several star topology networks and tie lines. 524xxx 345xxx 411xxx 412xxx 544xxxx 602xxxx Initially the numbering is aligned with the topology:Whole Number blocks are allocated to exchanges =>Routing is easy to implement. a tie line

12. Network topology can be presented as a graph for processing it with software. G = (V, E), V - nodes (set of vertices or nodes, non-empty, finite set) E = {ej | j = 1, 2, … M} - data links(set of edges or links) ej = (vi, vk) = (i, k ) “Data structures and algorithm analysis” -course presents many ways to handle graphs with software. In the course s-38.122 some graph algorithms are applied in routing.Examples: - Find the shortest path from node a to node b. - Join two graphs.

13. Locality of Traffic Determines the Structure and Dimensioning of Telecommunications Networks • The structure is determined by the number of exchanges and traffic transmitted by them in different directions of the network. • High traffic --> mesh topology preferred • Low outgoing traffic --> star topology preferred • Networks are structured using all the preceding alternatives: • Star topology can be seen at the subscriber end where subscriber concentrators (1) and the local exchange (2) are located. • 1 and 2 are connected with a higher level exchange or directly to the nearest parallel exchanges. • Higher level city exchanges mostly implement mesh topology due to high traffic. • There is typically a restricted number of outgoing links from the cities. The outgoing links connect to the next level of hierarchy, called telecommunications area. • Correspondingly, telecommunications areas are connected with a restricted number of links. So, there is a clear hierarchy in the network.

14. The trend is towards bigger capacity exchanges • For instance, the number of exchanges in Sonera’s wireline telephone network dropped from ca. 500 to 40 in the 1990’s. • Local exchanges have been replaced with concentrators in order to reduce costs. Concentrator can be: - static: compresses PCMs with low usage into PCMs with almost full usage The time slot allocation for subscribers is independent of traffic. - dynamic: the time slot to/from the exchange is reserved only for the purpose of a call In both cases: - a call between two subscribers of the concentrator is always switched via the exchange - an exception may be direct support of emergency calls in dynamic concentrators This might be necessary in big concentrators having over 1000 subscribers. - the amount of software in concentrators is just a fraction compared with exchanges

15. Exchanges switch calls independently or under the control of intelligent networks • Functions in exchanges: • call control: number analysis, routing/ routeing, call manage-ment, handling of supplementary services, subscriber database • accounting, statistics and charging • signaling, connectivity with a wide set of different interfaces • operation and maintenance • The number analysis function of an exchange is able to deal with all kind of E.164 numbers. However, for administrative reasons number translations are made at control points of the Intelligent Network. As a result, the software of services can be concentrated IN nodes.

16. The number of exchange types is decreasing in the wireline network • Subscribers are connected to local exchanges. • In general, transit exchanges have no subscribers ( => no subscriber signaling and no subscriber database). Transit exchanges are vanishing due to growing capacity of other exchanges. • International switching stations understand international signaling and also take care of special characteristic of international inter-administrative accounting. • Mobile networks mostly have their own exchanges with the characteristic functions of mobility management and mobile network signaling.

17. Intelligent networks concentrate software of services • SSP= Service Switching Point = an exchange having additionally the SSF • SSF - service switching function = ability to transfer call processing to SCP on demand • SCP = Service Control Point = a control point having the service logic • SDP = service data point = database • SMS = management, SCE = Service Creation Environment SCE SMS SDP SCP SSP call control exchange

18. Numbering (basend on E.164) • The primary function of exchanges is to switch the call in accordance with the dialled number to an appropriate outgoing link to the next exchange and finally to a subscriber= number analysis => route • Sub-areas in local telecommunications area network have their own area numbers (destination number prefix) • For instance: 09-325xxxx Mellunmäki & Vartiokylä • length of numbers varies between 4…8 numbers (09-region) • Access to Finland with code 358 and + international prefix (00 or an operator specific prefix) 1 44 34 358 ... 00 990 994 + …. 9 50 40 ... Syntax approximately: _ _ _ ABCdefgh 0

19. Semantics of phone numbers (E.164) • A directory number can represent a subscriber or a service • A directory number indicating a subscriber is both a routeing number and a “logical” number (in a phone book) • Portability of numbers breaks this conjunction • Service numbers are always “logical” and must be translated into routeing numbers • A caller should be able to deduce costs of a call based on the directory number. Therefore allocation of numbers is aligned with geography and the topology of the network.

20. Numbering in mobile networks • A MSISDN number (e.g. 040-7501636) has been allocated in some HLR (home location register) based on a prefix part of the number. • GMSC (gateway MSC) exchanges know this allocation. • When a number is called, the call is transferred to some GMSC that knows which one of the HLRs has information about the location of the subscriber.

21. Properties of number analysis in exchanges • Analysis can be affected by • dialled number • direction of incoming call (a set of junction lines defines the direction), origin or category (e.g. an operator assisted call) • Analysis can return • a set of routing options • instruction for translating the number (e.g. 0800-number):In this case it might be necessary to make the analysis again. • The operator uses MML-commands to build the Analysis trees based on route plans.

22. A B d e f g The number analysis tree of an exchange associates routeing with signaling information From call signaling we get: ABC - area ABCd - the shortest subscriber number ABCdefgh - the longest subscriber number Buckets/ route alternatives C Bucket-file describes routing options of which one is selected taking usage of the links into account. In addition, the incoming direction can impact on the choice of the start point for the analysis. It is also possible to make number translations before choosing the route. Nodes d,e,f,g are needed depending both on the number length and the father node. h

23. x Memory requirements for the analysis tree Assumptions: - the analysis is made for the first 4 numbers of max. 8 - 800 000 subscribers (e.g. Helsinki telecom area). Calculation: - 8 numbers give max. 10^8 - 1 subscriber number space - usage of this number space 0.8/100% = 0.8% - average usage of numbers = n ==> n8 = 800 000 => n = 5.47. - analyzing 4 numbers demands on the average ca. 1+6+36+216=259 nodes- the size of a node e.g. 16 bit x 16 = 32 bytes. - space needed for nodes is 32 x 259 = 8 288 bytes, of which for real is used ignoring characters * and # 5.47 x 2 x 259 = 2834 bytes = ca. 34%. By continuing the calculation it’s easy to show that analyzing all the 8 numbers causes no problem concerning memory requirements (instead, it is a management problem).