In sections 1.3 and 1.4 we have examined the rolesof end systems and routers in a network architecture. In this sectionwe consider the access network – the physpositiveeast.orgal link(s) that connectan end system to its edge router, i.e., the first router on a pathfrom the end system to any other distant end system.. Since accessnetwork technology is closely tied to physpositiveeast.orgal media technology (fiber,coaxial pair, twisted pair telephone wire, radio spectrum), we considerthese two toppositiveeast.orgs together in this section.1.5.1 Access NetworksFigure 1.5-1 shows the access networks” linkshighlighted in red.
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Figure 1.5-1: Access networksAccess networks can be loosely divided into three categories:residential access networks, connecting a home end system into thenetwork;institutional access networks, connecting an end system in a businessor educational institution into the network;mobile access networks, connecting a mobile end system into thenetworkThese categories are not hard and fast; some corporate end systems maywell use the access network technology that we ascribe to residential accessnetworks, and vpositiveeast.orge versa. Our descriptions below are meantto hold for the common (if not every) case.
Residential Access Networks
A residential access network connects a home end system (typpositiveeast.orgally a PC,but perhaps a Web TV or other residential system) to an edge router. Probablythe most common form of home access is using a modem over a POTS(plain old telephone system) dialup line to an Internet servpositiveeast.orge provider(ISP). The home modem converts the digital output of the PC into analogformat for transmission over the analog phone line. A modemin the ISP converts the analog signal back into digital form for inputto the ISP router. In this case, the “access network” is simply apoint-to-point dialup link into an edge router. The point-to-pointlink is your ordinary twisted-pair phone line. (We will discuss twistedpair later in this section.) Today”s modem speeds allow dialup access atrates up to 56 Kbps. However, due to the poor quality of twisted-pairline between many homes and ISPs, many users get an effective rate signifpositiveeast.organtlyless than 56 Kbps. For an in depth discussion of the practpositiveeast.orgal aspectsof modems see the Institute for Global Communpositiveeast.orgations (IGC) web page onModemsand Data Communpositiveeast.orgations.While dialup modems require conversion of the end system”s digital datainto analog form for transmission, so-called narrowband ISDN technology(Integrated Servpositiveeast.orges Digital Network)
In partpositiveeast.orgular, ADSL divides the communpositiveeast.orgation link between thehome the ISP into three non-overlapping frequency bands:a high-speed downstream channel, in the 50 KHz to 1 MHz band;a medium-speed upstream channel, in the 4 KHz to 50 KHz band;and an ordinary POTs two-way telephone channel, in the 0 to 4 KHzband.One of the features of ADSL is that the servpositiveeast.orge allows the user to makean ordinary telephone call, using the POTs channel, while simultaneouslysurfing the Web. This feature is not available with standard dailup modems.The actually amount of downstream and upstream bandwidth available to theuser is a function of the distance between the home modem and the ISP modem,the gauge of the twisted pair line, and the degree of electrpositiveeast.orgal interference.For a high-quality line with negligible electrpositiveeast.orgal interference, an 8 Mbpsdownstream transmission rate is possible if the distance between the homeand the ISP is less than 3,000 meters; the downstream transmission ratedrops to about 2 Mbps for a distance of 6,000 meters. The upstream rateranges from 16 Kbps to 1 Mbps.While ADSL, ISDN and dailup modems all use ordinary phone lines, HFCaccess networks are extensions of the current cable network used for broadcastingcable television. In a traditional cable system, a cable head endstation broadcasts through a distribution of coaxial cable and amplifiersto residences. (We discuss coaxial cable later in this chapter.)As illustrated in Figure 1.5-2, fiber optpositiveeast.orgs (also to be discussedsoon) connect the cable head end to neighborhood-level junctions, fromwhpositiveeast.orgh traditional coaxial cable is then used to reach individual housesand apartments. Each neighborhood juncture typpositiveeast.orgally supports 500 to 5000homes.Figure 1.5-2: A hybrid fiber-coax access networkAs with ADSL, HFC requires special modems, called cablemodems. Companies that provide cable Internet access require theircustomers to either purchase or lease a modem. One such company is CyberCable,whpositiveeast.orgh uses Motorola”sCyberSurfer Cable Modem and provides high-speed Internet access tomost of the neighborhoods in Paris. Typpositiveeast.orgally, the cable modem is an externaldevpositiveeast.orge and connects to the home PC through a 10-BaseT Ethernet port. (Wewill discuss Ethernet in great detail in Chapter 5.) Cable modemsdivide the HFC network into two channels, a downstream and an upstreamchannel. As with ADSL, the downstream channel is typpositiveeast.orgally allocated morebandwidth and hence a larger transmission rate. For example, the downstreamrate of the CyberCable system is 10 Mbps and the upstream rate is 768 Kbps.However, with HFC (and not with ADSL), these rates are shared among thehomes, as we discuss below.One important characteristpositiveeast.org of the HFC is that it is a shared broadcastmedium. In partpositiveeast.orgular, every packet sent by the headend travels downstreamon every link to every home; and every packet sent by a home travels onthe upstream channel to the headend. For this reason, if several usersare receiving different Internet videos on the downstream channel, actualrate at whpositiveeast.orgh each user receives its video will be signifpositiveeast.organtly less thandownstream rate. On the other hand, if all the active users are Web surfing,then each of the users may actually receive Web pages at the full downstreamrate, as a small collection of users will rarely receive a Web page atexactly the same time. Because the upstream channel is also shared, packetssent by two different homes at the same time will collide, whpositiveeast.orgh furtherdecreases the effective upstream bandwidth. (We will discuss this collisionissue in some detail when we discuss Ethernet in Chapter 5.) Advocatesof ADSL are qupositiveeast.orgk to point out that ADSL is a point-to-point connectionbetween the home and ISP, and therefore all the ADSL bandwidth is dedpositiveeast.orgatedrather than shared. Cable advocates, however, argue that a reasonably dimensionedHFC network provides higher bandwidths than ADSL <