Mạng và viễn thông P45

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Corporate Networks Although large multi-national companies can afford to employ specialist ‘telecommunications managers’, smaller companies individuals and are less privileged. For these latter groups, telecommunications are often just another subsidiary responsibility either a computer services for manager or even the general business manager. Nonetheless all telecommunications managers can affectthewayinwhichtheircompaniesoperate.Inthischapterweshalldiscusshow.

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  1. 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) 45 Corporate Networks Although large multi-national companies can afford to employ specialist ‘telecommunications managers’,smallercompanies individuals and are less privileged. For theselatter groups, telecommunications are often just another subsidiary responsibility either a computer services for manager or even the general business manager. Nonetheless all telecommunications managers can affectthewayinwhichtheircompaniesoperate.Inthischapterweshalldiscusshow.The following topics are covered 0 telecommunicationsperformanceandmanagement 0 cabling for telephonesandcomputers 0 officecomputernetworking(LANs,etc.) 0 privatenetworkdesign and operation 45.1 TELECOMMUNICATIONS MANAGEMENT The telecommunications manager should manage all aspects of telecommunications. He needs totake ownership of the full cost andfunctionalperformance of all information flows, by whatever means, throughout his company. The job should not consist only of designing and operating the company’s private networks. This loses sight ofthe‘strategichorizon’,theprofound effect that effective all-round commu- nication can have on a company’s operations. Telecommunications management involves the day-to-day operation and adminis- tration of resources, and it should include creative and strategic questioning. 0 What is our long term business goal? 0 What are the most important ‘information flows’ supporting it? 0 Why is ‘key’ information being carried on a particular medium? 0 What is the capability of new technology and how can it affect the core business processs to advantage? 815
  2. 816 CORPORATE 0 Why should I maintain a private network rather than use public services? Which gives better ‘added value’? 0 Why are we spending so much money on paper mail? 0 Why are we using facsimile rather than electronic mail? 0 How can we gain competitive advantage? 0 How can we lock-in our customers using telecommunications? 0 Which are the right standards for me to use? 0 How will legal regulation affect me? 0 Above all, are our board and seniormanagementproperlyclued up about tele- communications? If not, is that not my fault? Good telecommunications management keeps a company using the right technologies, the right suppliers, the right control procedures and the right monitoring measures, maintaining optimum performance at all points in time. From the suppliersthere is agreaterneedforproductswhichencompasswhole business solutions, tailored packages of services, products and support from a single supplier to meet the need of a business process neatly completely. No longer has the and telecommunications manager got the timeto bind together a rag-bagof bits, essentially the solution himself. Neither can he afford the time needed to resolve any subsequent problems or take on any necessary enhancements. What he has to do is to identify the need, understandit completely and find suitable suppliers. Thus he seeks a communications solution for ‘order collection’, ‘customer payment’, ‘customer account management’, etc. Having found solutions to the business communication needs, a framework neededis forperformanceand overallcostmanagement.Wecoveredthisinsomedetail in Chapters 34 and 35. Finally, the manager may turnto the technology and to thedesign andoperation of networks.Theremainder of thechapter, discussessome of the important current issues facing telecommunications managers in this particular area of responsibility 0 building cabling 0 computer networking 0 privatenetworkdesignandoperation 45.2 PREMISES CABLING SCHEMES Until the explosion in the number of office computer workstations in the early 1980s, buildings were designed and pre-wired with the needs only of electricity and telephone cabling in mind. However, the large number of personal computers, word processors, facsimile machines,mainframe computerterminals, printers and electronicmail terminals now in use means that the needs of data terminalcablingnowgenerally exceeds those of simple electricity and telephone.
  3. PREMISES CABLING SCHEMES 817 At first, companies installed dedicated wires for each terminal as they were needed; installation work and the costs associated with it mounted on a‘pay-as-you- up go’ basis. In time the wiring became a complicated spaghetti of leads strewn across false ceilings, knotted on cross-connection frames, and squeezed into congested conduits. Eachtimea new cable was needed,theceiling tiles wouldcomedownagain,the conduits would be opened up, and somehow another path would be found. The costs began to grow out of proportion.Each new cablewasharder to install thanthe previous one and each new installation led to faults being inflicted on existing wiring as cables were drawn into already congested ducts. Most companies now favour formally structured cabling schemes which are planned and installed at thetime of initialbuildingdevelopment orduringrefurbishment. Sufficient conduits, equipment cabinets and cables are provided to cater for wiring in excess of all foreseen needs. Figure 45.1 illustratestypical a structured cabling scheme. It comprises cables running out from a main trunking room or patch frame. First a number high-grade of backbone or riser cables run out (inbuildingconduitsoftencalled risers) to patch panels (passive components) or to cablinghubs (these are ‘active’ components which may convert electrical formats or interfaces as may be needed) located on a one-per- floor basis. From these secondary locations, a large number of lower grade cables run out to all possible locations withinthe office floor. It is common nowadays to find glass fibre cables used in the risers and unshielded twistedpair ( U T P ) or shielded twistedpair ( S T P ) copper cables used within a given building storey. Typically four four-pair category 5 (Cat 5 ) cables (i.e. eight wires per cable) are run from the floor patch panel to each single occupant office and terminated on eight-pin RJ-45 sockets, and four RJ-45 sockets per office allow for up to two telephone exten- sions and two data terminals per desk. Standard telephone guage unshielded twisted pair ( U T P ) is suitable for most LANs, data and telephone needs. For higher perform- ance needs, shielded twistedpair ( S T P ) ,coaxial cable or fibre optic cable may needed be (see Chapter 8). Thefloorpatchpanelprovidesalltheconnections onthat floor andforlater reconfiguration simply by changing the patch wire. Local area networks ( L A N s ) of logical star, bus and ring topologies can be created and reconfigured at will. In addition the floor patch panel is connected via riser cables to patch panels on other floors and back to the main patch frame. These allows for connection to public telecommunica- tions networks and the mainframe computer installations on other floors. The riser cables are usually either shielded twistedpair or fibre. Though structured cabling schemes require greater capital outlayfirst, they usually at reward their owners with much lower running costs, very fast and cheap terminal wir- ingleadtimes and muchreducedfrequency of faults.Thelargest single cause of wiring errors, tampering with the cables in the conduits, is eliminated by a structured scheme. Unstructured (ad hoc) wiring schemes should thus only be considered where very low terminal populations exists (less than about 20 terminals). Structuredwiringschemesgenerallyrequire very littlemaintenanceandoperate virtually trouble-free, but good management practice demands that a proper inventory be kept of the use of each wire, and that each be properly labelled. A number of regulatory stipulations need to be considered during the planning and installation phases cabling. First, it not good practice to run cables in air conditioning of is
  4. 818 CORPORATE Floor patch individual offices to Wires panel Plentifulsupplyofunshielded twisted cable pair Computer workstations - Floor patch in telephones and office locations panel Wires rl- Floor potch Wiring cabinet (one multiplexer other and per floor) equlpment t o individual offices - may hold Shieldedtwisted pair C orfibre r Main patch frame Company mainframe computer services U Figure 45.1 Structured cabling scheme ducts because of the smoke and fire risk arising from the combustion of the plastic insulation. (Actually, United States permits this but it illegal in the UK). Separate law is conduits for electrical and telecommunicationswiring are recommended to protect tele- communications users and equipment alike from the dangers of a mains electrical ‘shock’.
  5. RKING OFFICE COMPUTER 819 In Germany,fire regulations require sealing up the inside of the riser conduits at each floor level with cement. This makes the pre-installation of many cables in a structured cabling scheme attractive, as the effort and cost associated with drilling through each floor, pulling a single cable and then re-sealing the cement would be very expensive and time-consuming. However, a regulatory restriction applies in the United Kingdom which may cause companies to considerseparatecabling schemes fordataand telephone uses. The restriction is that all cables and patch frames carrying telephone wiring which may possibly be connected to thepublic telephone network(via the PBX) must maintained be by the registered PBX maintainer. Furthermore, any data or other equipment connected to wires in the same cable (irrespective of whether these wires are connected to thePBX) must either be approved for connection to the telephone network (with ‘green dot’), or they must be isolated by means of a barrier box (intended to prevent high voltages leaking onto the public telephone network). These restrictions are easy to avoid by installing separate structured cabling schemes for telephone and datawiring. Thus cables and patching equipment are duplicated, but conduits and cabinets may shared. be 45.3 OFFICE COMPUTER NETWORKING Issues in the design and management of office computer networks are 0 meeting capacity and response time needs 0 flexibility for reconfiguration 0 compatibility of application and network management software We discussed network dimensioning to meet data network capacity and user response needs in Chapter 30. Typical response times needed from a real computer network are shown in Table 45.1. Achieving such targets takes a considerable amount of skill; there are far too manydetailed factors to be considered which we have not the time for here, but many excellent books,consultancyorganizations and suppliers are available to Table 45.1 Typicalresponsetimeexpectations ‘Maximum’ Function response time Computer activation 3.0 S Feedback of error 2-4 S Respond to identify code 2.0 S Keyboard entry 0.1 S Respond to simple enquiry 2.0 S Request for next page 0.5 S Respond to ‘execute problem’ 15.0 S
  6. 820 CORPORATE NETWORKS assist with complex data networking problems. Basic guidance is easier: ‘keep it as simple as possible’. Careful design of relevant software, together with the use of the formulae given in Chapter 30 will go a long way! The flexibility to reconfigure networks to data or meet a number of needs simultaneously is derived from the use of a number of tools and techniques, e.g. - e structured cabling e localarea networks ( L A N s ) (Chapter 19) e open computer architectures (e.g. OS1 and TCPIIP, see Chapters 9 and 19). The use of L A N s or other computer network architectures (e.g. X.25, framerelay, ATM, TCP/IP, APPN, SNA), needs to be tempered by careful preparation. None of the tools are as flexible as they may seem, and lack of appreciation of this fact can lead to equipment incompatibilities. Local area networks ( L A N s ) should be designed and run using rigorous ‘mini-data centre’ procedures. Out of hand, they canbe very difficult to manage and problems can be hard to diagnose. Theselection of aparticular LAN (e.g. ethernet or tokenring) oraparticular computer architecture (e.g. TCPIIP, OSI, S N A or A P P N ) needs to take cognizance of the applications to which itwill be put (e.g.$le transfer or printer sharing or mainframe database repository) and also of the computerhardware software and (e.g. PC operating system software:DOS, Windows,Windows95,WindowsNT, OS/2, UNI, M V S , etc.)whichit is expected toserve.Equipmentfromaparticularcomputer manufacturer tendsto have been developed one with architecture mind. in For example, the IBM architecture is SNA or A P P N and the preferred LAN technology is token ring. Meanwhile Digital Electric Comporation’s( D E C ) proprietary architecture is called D E C N E T and the preferred LAN technology is ethernet. In the longer term, internetprotocols (e.g. T C P j I P ) and open systeminterconnection ( O S I ) standards allow greater inter-changeability of computer equipment and network components. 45.4 PRIVATE NETWORKS Public telecommunications networks are usually large and complex, requiring massive capital and manpower resources. a result, network changes are usually conducted in As a careful and steady manner because the cost of mistakesis magnified by the vast scale. Also, because public telecommunications operators (PTOs) are normally required by terms of licence to provide services of uniform availability over widegeographic areas, the development and rollout of innovative and technologically advanced services can be held up, and prices to main business customers can prejudiced by the subsidisation of be uneconomic rural areas. Because of this, and driven by the desire to adopt the latest techniques, some companies have developed extensive private or corporate networks. The advantage to be gained depends on the service offerings and tariffs of the public telecommunications operator(s) and also on the legal constraints placed locally on such a network.
  7. PRIVATE NETWORKS 821 Site 2 Customerpremises equipment ( C P E ) Site 3 Figure 45.2 A privatetelephonenetwork A typical private network is shown in Figure 45.2. The example shows threeprivate branch exchanges (PBXs) in different office buildings, interconnected by circuits between the buildings. The PBXs and other customer premises equipment ( C P E )may be owned by the private company, or leased from the PTO. The wiring in each office normally belongs to the private company. However, the circuits interconnecting the different PBXs in different buildings almost certainly are leased from the PTO. Thus a private network usually comprises customer premises equipment (or customer apparatus) and leased circuits which interconnect the different locations. The private network illustrated in Figure 45.2 is capable of switching telephone calls between any two telephones in any of the three different offices.If desired, a single numbering plan could cover all three offices, each telephone having a unique three- or four-digit extension number. In this way users benefit from a short dialling procedure, and the company can savemoney by transportinginter-officecallsoverthe leased circuits rather than having to pay the full public telephone tariff for each call. Provided that the telephone trajic between different offices exceeds a given threshold value (the exact value of which depends on the relative leased circuit and PSTN call tariffs) it will almostcertainly be cheaper to use direct leased circuits this in manner, asthe calculation in Table 45.2 shows. The cost benefit of the private network (compared with using the public network) is prone to changes in PTO tariffs. As an example, in the United Kingdom the balance of public service tariffs and leased line rental charges has changed so radically since the early 1980s that many companies large are now considering the abolition of the private networks they established at that time. Another factor encouraging companies to give up their private networks is the desire to reduce the in-house manpower needed for the day-to-day responsibilitiesnetwork of operation. So the pressure is for the public network operators to assume this greater responsibility!
  8. 822 CORPORATE NETWORKS Table 45.2 Threshold for leased circuit consideration Leased circuit tariff = EL per quarter Public telephone tariff per minute = & p Telephone usage minutes per quarter = m Public telephone bill per quarter = mp therefore if mp > L or minutes per quarter m > L / p a leased circuit is worth consideration The calculations shown Table 45.2 are oversimplified. The full analysis should take in account of traffic profile effects (see Chapters 30 and 31). Only the point-to-point traffic should be considered, and the numberof leased circuits costed needs to take account of the grade of service (following the Erlang formula). Private networks are common in medium and large companies where the economics of large scale helps them to ‘pay-in’, particularly for intra-company communication between main offices. For communication with suppliers or customers, or even to staff in small remote offices, private networks are often connected to the public switched telephone network (PSTN). In Figure 45.2 each of the PBXs is illustrated with a direct link to the PSTN. Apart from cost or the capabilities of new technology, another motive for a company to establish a private network could be the network security afforded. Take a small pri- vate packet-switched or other data network which connects a computer holding sensitive data to a number of remote workstations. Unauthorized users with workstations which are notconnected to the private network cannot gain access to thesensitive information. When it is not possible or economic to establish a private network to ensure the security of information, scramblers or encryption devices may be employed in conjunc- tionwith the public network. Scramblers work pairs, in at eachend of acon- nection, converting speech into a coded, but unintelligible signal for transmission on the public line. Equivalent devices when used to scramble sensitive data are usually called encryption devices. 45.5 ARCHITECTURE OF PRIVATE NETWORKS Public and private networks are based on similar technical principles, but differ in scale and complexity. Privatenetworks generally cater for much lighter traffic, but more specialized service needs. An example of the difference in technical standards is illustrated by making a comparison between the signalling systems used for conveying telephone calls between PBXs over a private network with those used between the exchanges of a public network. Inter-PBX signalling systems often only carry short digit strings (equivalent to a four-digit extension number), but in addition they need to carry information necessary for invoking special features such as ring back when free, etc. As a result, the speed at which individual digits of the dialled number are sent
  9. ARCHITECTURE OF PRIVATE NETWORKS 823 between the PBXs may not be the critical factor, because the small number of digits constrainthecallsetuptimeanyway,butthesignalvocabularymust be wide to support all the various special functions. By contrast, inter-exchange signalling systems need to carry longer digit strings.A full international number and its prefix could be up to 18 digits long, and must pass quickly between exchanges so that the set-up delay is reasonable.Exchangesinapublicnetworkmay need to transferinformationon charging or routing the call, as well as any customer-requested supplementary services (e.g. calling line identity (CLZ) or ring back when free). Finally, the inter- exchange signallingsystemmay be used to conveyinformationfornetworkadministration, including network management information control and signals, or for remote operation and maintenance commands. Public network standards tend to be robust, designed to withstand and control the severe and diverse demands of the wide range of uses to which public networks are exposed. They need to be resistant both to failure and fraud. Private network standards are nearly always derivatives of public network standards, and are often developed from the interface used to connect customer premises equipment to public network exchanges. Figure 45.3 illustrates an example pertinent in the UK, where the primary rate ISDN PBX-to-network interface, called D A S S 2 (digital access signalling system No. 2 ) is similar to the digitalprivate network signalling system ( D P N S S ) which may be used on private networks between PBXs to support ISDN-like services (the ITU-T’s signalling systems for the similar purposes are called DSS-l (digital signalling system I ) and Q-SIC. These are defined in the Q.33X series of recommendations). The advantage of using similar standards for PBX-to-exchange and PBX-to-PBX interfaces is that it minimizes the complexityof PBXs which are required to operate in both modes. Similar benefits can be gained in other typesof networks (e.g. packet networks, etc.) aligning by the technical standards used in private and public network variants. Minor differences are inevitable, have always existed and are bound to persist. They are the results of the contrasting network circumstances and demands on public and private networks. Thus a private packet-switched networkis likely to be based on ITU-T’s X.25 standard, but it may include a number of customized software features. Likewise the message handling system, as we saw in Chapter 23, recognizes separate public and private management domains. c) Public Private network ISDN Figure 45.3 PBX-to exchange and PBX-to-PBX ISDN signalling
  10. 824 CORPORATE NETWORKS 45.6 PLANNING PRIVATE NETWORKS The smaller traffic scale of private networks, coupled with their relative freedom from licence constraints,meansthat their structurecan differfrom that of publicones. Indeed, the optimum topology is not only independent of the real cost of equipment, but can be highly distorted by the PTO’s relative tariffs for leased circuits and public network services. Besides the economy measures of Chapter 38, three particular network routing tech- niques which offer considerable scope for cost reduction to private network planners are 0 establishmentoftelecommunicationshubs 0 publicnetworkoverflow 0 publicnetworkbypass 45.6.1 Telecommunications Hubs The small traffic scale of private networks tends to encourage the use of a star net- work topology in which one, or a small number exchanges at the hub of the network of provide a transit switching point between all other exchanges. The configuration is illustrated in Figure45.4, where all calls between any pair exchanges are switched via of the hub exchange A. By employingsuchatopologyahandful of circuitsmaybe sufficient tocarry alltraffic (let us say 3 circuitsfromeachoutlyingexchange to exchange A, a total of 21 circuits), as against a more meshed or fully interconnected networkofonly 1 circuitbetweeneachpair of exchanges which wouldrequire 29 circuits (as shown in the inset of Figure 45.4). PBX or other exchange 29 circuits Figure 45.4 ‘Hub’ or ‘star network’ topology for private networks
  11. PLANNING 825 Thestar topologyshown in Figure45.4 is similar tothe hierarchicalnetwork structure described for larger scale (public) networks in Chapter 32. The smaller traffic on privatenetworks means that the use of the star topology (or hierarchical structure) is nearlyalwaysmorecost-effective thana moremeshednetwork of inter-exchange connections. It not only minimizes the total number circuits required, but also tends of to minimize the total number of circuit miles. Minimizing the number of circuit miles is most important, because it is this value that relates to the leased circuit charges levied by the public telecommunications operator (PTO). Careful siting of the hub exchange is also important. In international private networks, the choice of the hub site may be especially important, because the price of leased circuits may not be the same in each of the countries. Let us return to Figure 45.4. It might be the case that exchange A is the geographical hub of the eight sites, that the prices of leased circuits into and out of the country in which exchangeF is situated are cheaper. In such an instance, it may be more economic to use exchange F as the network hub. Similar anomalies of leased circuit prices can arise even within a single country, caused either by a banded (rather than directly distance proportioned) price structure or because of the differential price structure of competing PTOs. Circuit numbers are calculated in the normal way for the type of network and the circuit lengths are measured as radial distances. The costs can then be worked out accordingly.Repeating theprocedurefordifferentoptionalnetworksbasedonthe various available hub sites helps to determine the cheapest configuration. 45.6.2 Public Network Overflow The traffic between any two exchanges of a network has a ‘peaked’ profile. However, although public networks are mandated to carry this peak of demand, there is no similar mandate for private networks. In particular, there is no need for the private network to be able to carry the peak demand using its own resources alone. Instead it is permissible, and may be far more economic, to overflow the peak demand to the public network. Figure 45.5 illustrates this technique. Two PBXs (or other type of private exchange) called A and B, and located at different offices of the same company, are connected together as a private network. For most of the day the traffic demand between exchanges A and B is between 6 and 8 Erlangs, but between three and four in the afternoon it peaks at 12 Erlangs. The private network planner has done some cost calculations. He has established a network of eight directleased circuitson the private network between exchangesA and B, but has elected to overflow the excess 7 Erlangs of peak traffic via the public network. The total cost of the configuration is difficult to calculate, because the cost of calls overflowed via the public network mustbe measured or estimated from the complicated teletraffic formulae presentedin Chapters 30 and 32. For each hour of the day thetraffic that is carried on the eight circuits direct route, and the traffic overflowing from it, is determined using the Erlang formula, by inputting the traffic demand for that hour. Thus, until 5.00 a.m. there is no traffic and no overflow. From 9.00 a.m. to 3.00 p.m. the traffic is 7 Erlangs, and the overflow from eight circuitsis 1.25 Erlangs so 75 call minutes per hour are sent via the public network. Similarly, during the hour of peak activity (3.00p.m. to 4.00p.m.) the offered traffic is 12 Erlangs, the overflow from the eight
  12. 826 CORPORATE Public network /---- l I Overflow Exchange I I A I * m B I I , B leased c i r c u y I I I Traffic demand 12. Traffic overflowed via public network A to8 10. 8- 6. carried over 4. 'private' network 2. 0 12 6 9 12 3 6 12 Figure 45.5 Public network overflow circuits is 5.07 Erlangs, and 304 minutes route via the public network during this period. Over the whole day (adding also the overflowed minutes in the 4.00p.m. to 6.00p.m.) a total of 904 minutes overflow dailyvia the public network. At a cost &P minute of per and &Lper quarter tariff for each leased circuit, this configuration an approximate gives overall quarterly charge of Estimated quarterly charge = ( L + 3 X 22 days* X 904 X P) (There are approximately 22 business days per month.) As in the last example, the optimum number of leased circuits (for minimum cost) is determined in a trial-and- error fashion by calculating two costs for various configurations. 45.6.3 Public Network Bypass Wherepermitted by law,publicnetworkbypassmay be anadditionalmethod of A deriving additional telecommunications savings. cost saving is made on long distance calls over the public network routing thecall as far as by possible within the private net- work before handing it over to the public network for completion. By so doing, the public network is bypassed as far aspossible, the shorter geographical length the pub- of lic network connection reducing the chargeable tariff. Figure 45.6 illustrates the method
  13. KS PLANNING PRIVATE 827 Final destination Private network London (company A ) ..... Alternative network public route Figure 45.6 Public network bypass ofbypass. Intheexampleshown,a company privatenetwork is establishedinthe United Kingdom, linking exchanges in LondonandEdinburgh.A private caller connected to the private exchange in London wishes to make a call to an office in Edinburgh.Unfortunatelythedesireddestination is not connecteddirectly tothe desired network. However, routing the call over the private network from London by to Edinburgh, and then into the public network for the final short hop, the public network (and itsdistancedependenttariff)has been bypassedforthegreater part of the connection. Only the tariff of a local call is payable, rather than the much higher tariff that would have been due if the call had been routed over the public network a trunk as call for the entire distance between London and Edinburgh. Whether or not the overall economics favour such a bypass may change from day-to-day, depending on public network tariffs and on the existing topology of the private network. You should not assume that the example used is economic in all cases; specific evaluation is necessary on each occasion. Furthermore, the transmission quality of the end-to-end connection may preclude certain configurations. Particularly in thedata networking case, the speed of data propogation is often critical. Anothermeans of achievingpublicnetwork bypass isby the use of out-ofarea exchange lines. Thus our Edinburgh office could have purchased London out-of-area exchangelines from the PTO. In effect, thismakesthe office appear to the public telephone user as if it were in London. Itwill have a London telephone number and call charges will assume that London is the endpoint. Thus calls from the Edinburgh to London office, or vice versa, count as 'local calls'. There is a rental charge, however, for the out of area line, similar to a leased circuit charge for a London/Edinburgh circuit.
  14. 828 CORPORATE 45.7 AWORDOF WARNING Havingdescribedanumber of clever routing methodologies to reduce the costs of private networks, it is worth recalling, before we end the discussion, that the end-to-end connection must be properly serviceable. The transmission planning and routing ‘rules’, set out in Chapters 28 and 33, are applicable here. For example, it is no use creating a super-cheap end-to-end telephone connection which comprises so many component links that speech is unintelligibleoverit.However, the high performance of digital switching and transmission makes this much less likely. If the private network is to be connected to the public network there may be legal restrictions on the type and technical characteristicsof the signal that may be conveyed. In some countries telephone network bypass is illegal. In nearly all countries there will be signal power and technical line limitations. 45.8 PTO LEASED CIRCUIT OFFERINGS Generally, PTOs offer a comprehensive range of different leased (leaselines) or private circuit types,each attunedto one oranumber of particulartypes,suchasdata, telephone or packetswitching.Withcare,theplanning of privatenetworks can be relatively straightforward. The five most common types of leased circuit conform to ITU-T recommendations M. 1020, or M. 1025, M. 1040, their national equivalents, or they are digital circuits. leased Recommendation M.1020 defines the conditioning line-up line (i.e. limits, see Chapter 36) of a high grade four-wire analogue circuit, of 3.1 kHz bandwidth, suitable for high speed data orspeech use. A circuit lined up according to M.1020 has a very flat frequency response (i.e. shows very little attenuation distortion), very good group delay response, and good circuit stability. Such a circuit is suitable for most of the analogue data modems described in Chapter 9 and can be used for point-to-point or networked (e.g. packet network) applications. Recommendation M.1040 defines the line conditioning of a four-wire leased circuit, intended primarily for voice conversation. Its performance is not as good as that of an M.1020 circuit, but because time and equipment is needed for line conditioning, the less cost to the customer is lower. M.1025 provides a quality midway between M.1040 and M.1020. Digital leased circuits are usually defined in terms of their bit rate (e.g. 64 kbit/s, 2 Mbit/s (El), 1.5 Mbit/s (Tl), 34Mbit/s(E3), 45 Mbit/s (T3), 52Mbit/s (STS-1) or 155 Mbit/s (STM-l)), their biterrorratio ( B E R ) (typically 10-5 or 10-9) and their overall availability (typicallyaround 99.5%). They are usually presented with oneof the followinginterfaces:V.24(RS232),X.21V.35,V.36(RS449),G.703 or an ISDN-like interface. Provided that these parameters suit the given application, the circuit may be used for any type of service (e.g. telephone, data). Other types of leased circuits that are sometimes available are wideband analogue (e.g. FDM group, supergroup) or high speed digital circuits. In addition some PTOs are willing toprovidejustthe physicalmedium, and allowtheuser to configurethe bandwidthinthemanner desired.Examplesinclude the wholesale of opticalfibre
  15. MAKING USE OF MOBILE RADIO TECHNOLOGY 829 capacity (so-called dark fibre) and the leasing and sale of satellite dishes for direct customer access to very high bandwidth satellite circuits (e.g. International Business Service ( I B S ) , which is a satellite service of INTELSAT allowing companies direct access for rates up to 2 Mbit/s). Finally, other specialized leased circuit services may be available from particular PTOs. 45.9 MAKING USE OF MOBILE RADIOTECHNOLOGY The companiesthat propose to mobile communications must careful attention to use give the coordination and managementthe system. Proper control must established over of be 0 equipment purchasing 0 air-time subscription 0 cost management 0 user-justification rules Urgency of contactneeded - Radio pager Cellular phone recommended recommended High @ Company chairman @ Hospital doctor @ Salesman - Ordinaryfixed Telepoint cordless telephone recommended handset ( C T 2 ) recommended Low @ Office typist @ Gordener Low High Mobility ( W i t h i n 5 minutes (More thon 5 minutes of telephone) from telephone) Figure 45.7 ‘Quantification’ chart for mobile communication users
  16. 830 CORPORATE NETWORKS By dealing with a single supplier for carphone and hand-held units, and also for ‘air- time’ supply, the administration and control of costs is made considerably easier, but the ‘buying-power’ leverage over the supplier also increased. The decision to purchase is cellular or other mobiletelephones for useby individual departments needs to be tempered by a central coordinating unit. One of the problems is that the falling cost of handsets is bringing many of them withinthe financial authorization of the lowest levels of management. To get around the problem of integrating new and old generation equipment in this fast-changing environment one tactic could be to rent the handsets. However, because much of the involved cost is made up of usagecharges(subscription and call charges)usersshould not betempted toattachtoomuchimportancetowhatthe secondary issue of handset cost. Usage costs of cellular radio technology are typically around &25 per month per handset, plus a per minute rate for calls of up to three timesthe ordinary telephonycharges.Cordlesstechnology(e.g. DECT) is slightly cheaper but nonetheless can represent a substantial extra cost over normal telephones or rudiopugers. For this reason, companies are recommended to employ some means of assessing their users’ needfor service. A simple technique is shown diagrammatically in Figure 45.7. Figure 45.7 classifies users into four broad categories according to their ‘mobility’ (how long they are likely to be away from the telephone) and their ‘need for urgent communication’ (i.e. whether they are likely to need to contact or be contacted by others urgently). Examples of typical users in each the fourcategories of mobile com- of A munications are shown. hospital doctorwith low mobility but high urgencyof contact is recommended to have a radiopager, whereas for a company chairman or salesman with high mobility and high contact urgency, a cellular telephone is recommended. However, if this service is heavily congested a public cordless network (e.g. DECT if available) might provide some degree of alternative. For an office typist we recommend no mobile telephone at all, and our recommendation for the DECT handset is the humble gardener, assuming of course he does not intend to‘lie low’.
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