Understanding
`Telecommunications
`Networks
`
`Andy Valdar
`
`The Institution of Engineering and Technology
`
`Voip-Pal EX. 2049
`IPR2016—O1198
`
`Voip-Pal Ex. 2049
`IPR2016-01198
`
`

`

`Published by The Institution of Engineering and Technology, London, United Kingdom
`
`© 2006 The Institution of Engineering and Technology
`
`First published 2006
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`British Library Cataloguing in Publication Data
`Valdar, A.R.
`Understanding telecommunication networks
`I. Telecommunication systems
`I. Title II. Institution of Engineering and Technology
`621 .3'82
`
`ISBN (10 digit) 0 86341 362 5
`ISBN (13 digit) 918-086341-362-9
`
`Typeset in India by Newgen Imaging Systems (P) Ltd, Chennai
`Printed in the UK by MPG Books Ltd, Bodmin, Cornwall
`
`

`

`38
`
`Understanding telecommunications networks
`
`The necessary interconnection of all these types of networks within a country and to
`networks in other countries was described.
`
`We also introduced the concept of the Internet and how subscribers gain access to
`it via the PSTN, cable modems over Cable TV networks, ADSL broadband or over
`private circuits using optical fibre.
`
`
`
`Box 2.1 PBXS and Virtual Private Networks
`
`
`Businesses which have more than a few telephones use a private branch
`exchange system, known as a PBX, to provide call connections between each
`telephone (which become ‘extensions’) and li.nks into the PSTN {7]. The PBX
`is really a small version ofthe PSTN exchanges, typically ranging in sizes from
`10 up to 5,000 extensions. A private numbering scheme is required to enable
`extension to extension dialling, also special codes (e.g. ‘dial 9’) are required to
`enable calls to be made to the PSTN. Incoming calls from the PSTN have to be
`answered by a receptionist or operator at a manual console so that the appropri-
`ate (privately numbered) extension can be contacted. Alternatively, the exten-
`sion numbers can form part of the public numbering scheme (see Chapter 10)
`so that calls from the PSTN can be directly switch.ed by the PBX to the required
`extension (known as DDI), so avoiding the need for manual intervention where
`the caller knows the number ofthe wanted extension. Only the calls to the PSTN
`are charged. The corporate customer owns and pays for its PBX.
`In the case where a company extends over two or more sites (e.g. office
`or factory buildings) the PBXS on each site can be linked by private circuits,
`thus enabling calling between all the extensions. This is known. as a ‘private
`corporate network’ (or just ‘private network’). In this case the private num-
`bering scheme extends across all the PBXS and usually each PBX is linked to
`the PSTN. Charging only applies to calls leaving the private network for the
`PSTN, although, of course, a rental charge is made by the network operator for
`the lease of the private circuits.
`A virtual private network (VPN) provides an alternative to the use ofprivate
`circuits between each PBX, as shown in Fig. 2.l2(a). The VPN exchange
`switches calls between the PBXs connected to it (e.g. between PBXs ‘a’ and
`‘b’) as well as to trunk links to the other PBXS in the VPN corporate network.
`However, the VPN is provided over public exchanges, either as special business
`exchanges or as part of the PSTN. Each VPN exchange switches the private
`n.etwor.k calls of several private corporate networks, although each operates
`in isolation, using its own numbering scheme. Thus, each corporate network
`appears to have the benefits of a private set of links between their PBXS, even
`though connectivity is provided over public exchanges — hence the use of the
`word ‘virtual’ in VPN. The VPN customer is charged a subscription for the
`VPN service based on a certain level of inter-PBX calls (traffic); there are
`usually charges made when the level of calls between any two PBXS exceeds
`the agreed threshold.
`
`
`
`

`

`The many networks and how they link
`
`39
`
`Box 2.2 Centrex Service
`
`Centrex is the generic name of a service in which extension—to—extension calls
`within a customer’s site are switched by the public exchange, thus elimi-
`nating the need for a PBX [7]. This requires that each extension from the
`building be carried over the access network to the centrex exchange, which
`serves many centrex customers, each with their own private extension num-
`bering scheme. Again, call.s between the different centrex groups are kept
`isolated within the exchange, and charging only relates to calls that go out to
`the PSTN. The service can extend across several centrex exchanges (known
`as ‘networked centrex’), as required to serve the company’s private corpo-
`rate network, as shown in Fig. 2.l2(b).
`(Note that networked centrex is
`not the same as a VPN since the role of the PBXS is taken by the centrex
`exchanges.)
`
`Finally, the various specialised networks associated with a PSTN were introduced,
`namely:
`
`Operator services network;
`Business services network;
`
`Intelligent network;
`Private circuit-services network;
`
`Frame relay network;
`ATM network;
`
`IP network;
`
`MPLS network;
`Telex network.
`
`The chapter concluded with a simple model, which aimed to help position the
`specialised networks with the common access and core transmission networks.
`
`References
`
`1
`
`2
`
`3
`
`4
`
`SCHILLER, J.: ‘Mobile Communications’, Second edition, Addison-Wesley,
`Harlow, 2003, Chapter 4.
`FRANCE, P. W. and SPIRIT, D. M.: ‘An Introduction to the Access Network’,
`Chapter 1 of ‘Local Access Network Technologies’, edited by FRANCE, P.W.,
`IEE Telecommunications Series No. 47, Stevenage, 2004.
`BUCKLEY, 1.:
`‘Telecommunications Regulation’, IEE Telecommunications
`Series No. 50, Stevenage, 2003, Chapter 5.
`ANTTALAINEN, T.: ‘Introduction to Telecommunications Network Engineer-
`ing’, Artech House, Norwood, MA, 1999, Chapter 5.
`
`