It is the scarcity of spectrum that led to thecellular networkdesign shown inFig. 1-30 that is now used for mobile phone networks.To manage the radiointerference between users, the coverage area is divided into cells. Within a cell,users are assigned channels that do not interfere with each other and do not causetoo much interference for adjacent cells. This allows for good reuse of the spec-trum, orfrequency reuse, in the neighboring cells, which increases the capacityof the network. In 1G systems, which carried each voice call on a specific fre-quency band, the frequencies were carefully chosen so that they did not conflictwith neighboring cells. In this way, a given frequency might only be reused once
66INTRODUCTIONCHAP. 1in several cells. Modern 3G systems allow each cell to use all frequencies, but ina way that results in a tolerable level of interference to the neighboring cells.There are variations on the cellular design, including the use of directional or sec-tored antennas on cell towers to further reduce interference, but the basic idea isthe same.CellsBase stationFigure 1-30.Cellular design of mobile phone networks.The architecture of the mobile phone network is very different than that of theInternet. It has several parts, as shown in the simplified version of the UMTS ar-chitecture in Fig. 1-31. First, there is theair interface. This term is a fancy namefor the radio communication protocol that is used over the air between the mobiledevice (e.g., the cell phone) and thecellular base station. Advances in the air in-terface over the past decades have greatly increased wireless data rates. TheUMTS air interface is based onCode Division Multiple Access(CDMA), a tech-nique that we will study in Chap. 2.The cellular base station together with its controller forms theradio accessnetwork. This part is the wireless side of the mobile phone network. The con-troller node orRNC(Radio Network Controller) controls how the spectrum isused. The base station implements the air interface. It is calledNode B, a tem-porary label that stuck.The rest of the mobile phone network carries the traffic for the radio accessnetwork. It is called thecore network. The UMTS core network evolved fromthe core network used for the 2G GSM system that came before it. However,something surprising is happening in the UMTS core network.Since the beginning of networking, a war has been going on between the peo-ple who support packet networks (i.e., connectionless subnets) and the people whosupport circuit networks (i.e., connection-oriented subnets). The main proponentsof packets come from the Internet community. In a connectionless design, everypacket is routed independently of every other packet. As a consequence, if somerouters go down during a session, no harm will be done as long as the system can
SEC. 1.5EXAMPLE NETWORKS67RNCRNCMSC /MGWGMSC/ MGWSGSNGGSNRadio access networkCore networkAirinterface(“Uu”)Node BPSTNInternetPacketsCircuits(“Iu-CS”)Access/ Coreinterface(“Iu”)
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