Isdn Vs Cable Essay Research Paper
ISDN VS. cABLE MODEMS1.0 Introduction The Internet is a network of networks that interconnects computersaroundthe world supporting both business and residential users. In 1994 amultimedia Internet application known as the World Wide Web becamepopular. The higher bandwidth needs of this application havehighlightedthe limited Internet access speeds available to residential users. Evenat 28.8Kilobits per second (Kbps) the fastest residential access commonlyavailable at the time of this writing the transfer of graphical imagescan befrustratingly slow. This report examines two enhancements to existing residentialcommunications infrastructure: Integrated Services Digital Network(ISDN) and cable television networks upgraded to pass bi-directional digitaltraffic(Cable Modems). It analyzes the potential of each enhancement todeliverInternet access to residential users. It validates the hypothesis thatupgradedcable networks can deliver residential Internet access morecost-effectively while offering a broader range of services. The research for this report consisted of case studies of two commercial deployments of residential Internet access each introduced in thespring of1994: + Continental Cablevision and Performance Systems International(PSI)jointly developed PSICable an Internet access service deployed overupgraded cable plant in Cambridge Massachusetts; + Internex Inc. began selling Internet access over ISDN telephone circuits available from Pacific Bell. Internex’s customers areresidences andsmall businesses in the “Silicon Valley” area south of San Francisco California. 2.0 The Internet When a home is connected to the Internet residential communicationsinfrastructure serves as the “last mile” of the connection between thehome computer and the rest of the computers on the Internet. Thissection describes the Internet technology involved in that connection.This section does not discuss other aspects of Internet technology indetail; that is well done elsewhere. Rather it focuses on the services that need to be provided for home computer users to connect to theInternet. 2.1ISDN and upgraded cable networks will each provide differentfunctionality(e.g. type and speed of access) and cost profiles for Internetconnections. Itmight seem simple enough to figure out which option can provide theneededlevel of service for the least cost and declare that option “better.”A keyproblem with this approach is that it is difficult to define exactly theneededlevel of service for an Internet connection. The requirements depend on the applications being run over the connection but these applicationsareconstantly changing. As a result so are the costs of meeting theapplications’requirements. Until about twenty years ago human conversation was by far the dominant application running on the telephone network. The network wasconsequently optimized to provide the type and quality of service neededforconversation. Telephone traffic engineers measured aggregatestatisticalconversational patterns and sized telephone networks accordingly.Telephony’s well-defined and stable service requirements are reflectedin the”3-3-3″ rule of thumb relied on by traffic engineers: the average voicecalllasts three minutes the user makes an average of three call attemptsduringthe peak busy hour and the call travels over a bidirectional 3 KHzchannel. In contrast data communications are far more difficult tocharacterize. Datatransmissions are generated by computer applications. Not only doexistingapplications change frequently (e.g. because of software upgrades) butentirely new categories such as Web browsers come into being quickly adding different levels and patterns of load to existing networks.Researchers can barely measure these patterns as quickly as they aregenerated let alone plan future network capacity based on them. The one generalization that does emerge from studies of both local andwide-area data traffic over the years is that computer traffic is bursty. Itdoes notflow in constant streams; rather “the level of traffic varies widelyoveralmost any measurement time scale” (Fowler and Leland 1991). Dynamicbandwidth allocations are therefore preferred for data traffic sincestaticallocations waste unused resources and limit the flexibility to absorbburstsof traffic. This requirement addresses traffic patterns but it says nothing abouttheabsolute level of load. How can we evaluate a system when we never know how much capacity is enough? In the personal computing industry thisproblem is solved by defining “enough” to be “however much I can affordtoday ” and relying on continuous price-performance improvements indigitaltechnology to increase that level in the near future. Since both of the infrastructure upgrade options rely heavily on digital technology anothercriteria for evaluation is the extent to which rapidly advancingtechnologycan be immediately reflected in improved service offerings. Cable networks satisfy these evaluation criteria more effectively thantelephone networks because: + Coaxial cable is a higher quality transmission medium thantwistedcopper wire pairs of the same length. Therefore fewer wires andconsequently fewer pieces of associated equipment need to beinstalled and maintained to provide the same level of aggregatebandwidth to a neighborhood. The result should be cost savings andeasier upgrades. + Cable’s shared bandwidth approach is more flexible at allocatinganyparticular level of bandwidth among a group of subscribers. Since itdoes not need to rely as much on forecasts of which subscribers willsign up for the service the cable architecture can adapt more readilyto the actual demand that materializes. + Telephony’s dedication of bandwidth to individual customerslimitsthe peak (i.e. burst) data rate that can be provided cost-effectively.In contrast the dynamic sharing enabled by cable’s bus architecturecan if the statistical aggregation properties of neighborhood trafficcooperate give a customer access to a faster peak data rate than theexpected average data rate. 2.2 Why focus on Internet access?Internet access has several desirable properties as an application toconsider for exercising residential infrastructure. Internet technologyisbased on a peer-to-peer model of communications. Internet usageencompasses a wide mix of applications including low- and high-bandwidth as well as asynchronous and real-time communications.Different Internet applications may create varying degrees ofsymmetrical (both to and from the home) and asymmetrical trafficflows. Supporting all of these properties poses a challenge forexistingresidential communications infrastructures. Internet access differs from the future services modeled by otherstudiesdescribed below in that it is a real application today with growingdemand. Aside from creating pragmatic interest in the topic thisfactoralso makes it possible to perform case studies of real deployments. Finally the Internet’s organization as an “Open Data Network” (in thelanguage of (Computer Science and Telecommunications Board of theNational Research Council 1994)) makes it a service worthy of studyfrom a policy perspective. The Internet culture’s expectation ofinterconnection and cooperation among competing organizations mayclash with the monopoly-oriented cultures of traditional infrastructureorganizations exposing policy issues. In addition the Internet’sstatusas a public data network may make Internet access a service worthencouraging for the public good. Therefore analysis of costs toprovidethis service may provide useful input to future policy debates. 3.0 TechnologiesThis chapter reviews the present state and technical evolution ofresidential cable network infrastructure. It then discusses a topic not covered much in the literature namely how this infrastructure can beused to provide Internet access. It concludes with a qualitativeevaluation of the advantages and disadvantages of cable-based Internetaccess. While ISDN is extensively described in the literature its useasan Internet access medium is less well-documented. This chapterbriefly reviews local telephone network technology including ISDNand future evolutionary technologies. It concludes with a qualitativeevaluation of the advantages and disadvantages of ISDN-based Internetaccess. 3.1 Cable TechnologyResidential cable TV networks follow the tree and branch architecture.In each community a head end is installed to receive satellite andtraditional over-the-air broadcast television signals. These signalsarethen carried to subscriber’s homes over coaxial cable that runs from the head end throughout the community Figure 3.1: Coaxial cable tree-and-branch topology To achieve geographical coverage of the community the cablesemanating from the head end are split (or “branched”) into multiplecables. When the cable is physically split a portion of the signalpoweris split off to send down the branch. The signal content however isnotsplit: the same set of TV channels reach every subscriber in thecommunity. The network thus follows a logical bus architecture. Withthis architecture all channels reach every subscriber all the time
whether or not the subscriber’s TV is on. Just as an ordinarytelevisionincludes a tuner to select the over-the-air channel the viewer wishes to watch the subscriber’s cable equipment includes a tuner to selectamong all the channels received over the cable. 3.1.1. Technological evolutionThe development of fiber-optic transmission technology has led cablenetwork developers to shift from the purely coaxial tree-and-brancharchitecture to an approach referred to as Hybrid Fiber and Coax(HFC)networks. Transmission over fiber-optic cable has two main advantagesover coaxial cable: + A wider range of frequencies can be sent over the fiber increasingthe bandwidth available for transmission; + Signals can be transmitted greater distances withoutamplification. The main disadvantage of fiber is that the optical components requiredto send and receive data over it are expensive. Because lasers arestilltoo expensive to deploy to each subscriber network developers haveadopted an intermediate Fiber to the Neighborhood (FTTN)approach. Figure 3.3: Fiber to the Neighborhood (FTTN) architecture Various locations along the existing cable are selected as sites forneighborhood nodes. One or more fiber-optic cables are then run fromthe head end to each neighborhood node. At the head end the signal isconverted from electrical to optical form and transmitted via laser over the fiber. At the neighborhood node the signal is received via laser converted back from optical to electronic form and transmitted to thesubscriber over the neighborhood’s coaxial tree and branch network. FTTN has proved to be an appealing architecture for telephonecompanies as well as cable operators. Not only ContinentalCablevision and Time Warner but also Pacific Bell and Southern NewEngland Telephone have announced plans to build FTTN networks.Fiber to the neighborhood is one stage in a longer-range evolution ofthe cable plant. These longer-term changes are not necessary to provide Internet service today but they might affect aspects of how Internetservice is provided in the future. 3.2 ISDN TechnologyUnlike cable TV networks which were built to provide only localredistribution of television programming telephone networks provideswitched global connectivity: any telephone subscriber can call anyother telephone subscriber anywhere else in the world. A call placedfrom a home travels first to the closest telephone company CentralOffice (CO) switch. The CO switch routes the call to the destinationsubscriber who may be served by the same CO switch another COswitch in the same local area or a CO switch reached through a long-distance network. Figure 4.1: The telephone network The portion of the telephone network that connects the subscriber tothe closest CO switch is referred to as the local loop. Since all calls enter and exit the network via the local loop the nature of the localconnection directly affects the type of service a user gets from theglobal telephone network. With a separate pair of wires to serve each subscriber the localtelephone network follows a logical star architecture. Since a CentralOffice typically serves thousands of subscribers it would be unwieldyto string wires individually to each home. Instead the wire pairs areaggregated into groups the largest of which are feeder cables. Atintervals along the feeder portion of the loop junction boxes areplaced.In a junction box wire pairs from feeder cables are spliced to wirepairsin distribution cables that run into neighborhoods. At each subscriberlocation a drop wire pair (or pairs if the subscriber has more thanoneline) is spliced into the distribution cable. Since distribution cables are either buried or aerial they aredisruptiveand expensive to change. Consequently a distribution cable usuallycontains as many wire pairs as a neighborhood might ever need inadvance of actual demand. Implementation of ISDN is hampered by the irregularity of the localloop plant. Referring back to Figure 4.3 it is apparent that loops areofdifferent lengths depending on the subscriber’s distance from theCentral Office. ISDN cannot be provided over loops with loading coilsor loops longer than 18 000 feet (5.5 km). 4.0 Internet Access This section will outline the contrasts of access via the cable plantwithrespect to access via the local telephon network. 4.1 Internet Access Via CableThe key question in providing residential Internet access is what kindofnetwork technology to use to connect the customer to the Internet Forresidential Internet delivered over the cable plant the answer isbroadband LAN technology. This technology allows transmission ofdigital data over one or more of the 6 MHz channels of a CATV cable.Since video and audio signals can also be transmitted over otherchannels of the same cable broadband LAN technology can co-existwith currently existing services. BandwidthThe speed of a cable LAN is described by the bit rate of the modemsused to send data over it. As this technology improves cable LANspeeds may change but at the time of this writing cable modems rangein speed from 500 Kbps to 10 Mbps or roughly 17 to 340 times the bitrate of the familiar 28.8 Kbps telephone modem. This speed representsthe peak rate at which a subscriber can send and receive data duringthe periods of time when the medium is allocated to that subscriber. It does not imply that every subscriber can transfer data at that ratesimultaneously. The effective average bandwidth seen by eachsubscriber depends on how busy the LAN is. Therefore a cable LANwill appear to provide a variable bandwidth connection to the Internet Full-time connectionsCable LAN bandwidth is allocated dynamically to a subscriber onlywhen he has traffic to send. When he is not transferring traffic hedoesnot consume transmission resources. Consequently he can always beconnected to the Internet Point of Presence without requiring anexpensive dedication of transmission resources. 4.2 Internet Access Via Telephone Company In contrast to the shared-bus architecture of a cable LAN the telephone network requires the residential Internet provider to maintain multipleconnection ports in order to serve multiple customers simultaneously.Thus the residential Internet provider faces problems of multiplexingand concentration of individual subscriber lines very similar to thosefaced in telephone Central Offices. The point-to-point telephone network gives the residential Internetprovider an architecture to work with that is fundamentally differentfrom the cable plant. Instead of multiplexing the use of LANtransmission bandwidth as it is needed subscribers multiplex the use of dedicated connections to the Internet provider over much longer timeintervals. As with ordinary phone calls subscribers are allocatedfixedamounts of bandwidth for the duration of the connection. Eachsubscriber that succeeds in becoming active (i.e. getting connected tothe residential Internet provider instead of getting a busy signal) isguaranteed a particular level of bandwidth until hanging up the call. BandwidthAlthough the predictability of this connection-oriented approach isappealing its major disadvantage is the limited level of bandwidth that can be economically dedicated to each customer. At most an ISDNline can deliver 144 Kbps to a subscriber roughly four times thebandwidth available with POTS. This rate is both the average and thepeak data rate. A subscriber needing to burst data quickly for example to transfer a large file or engage in a video conference may prefer ashared-bandwidth architecture such as a cable LAN that allows ahigher peak data rate for each individual subscriber. A subscriber whoneeds a full-time connection requires a dedicated port on a terminalserver. This is an expensive waste of resources when the subscriber isconnected but not transferring data. 5.0 Cost Cable-based Internet access can provide the same average bandwidthand higher peak bandwidth more economically than ISDN. Forexample 500 Kbps Internet access over cable can provide the sameaverage bandwidth and four times the peak bandwidth of ISDN accessfor less than half the cost per subscriber. In the technology reference model of the case study the 4 Mbps cable service is targeted atorganizations. According to recent benchmarks the 4 Mbps cableservice can provide the same average bandwidth and thirty-two timesthe peak bandwidth of ISDN for only 20% more cost per subscriber.When this reference model is altered to target 4 Mbps service toindividuals instead of organizations 4 Mbps cable access costs 40%less per subscriber than ISDN.