Cellular Telephone Networks

Cellular telephone networks extend the basic telephone service to mobile users with portable telephones. Unlike conventional telephone service where the call to a telephone number is directed to a specific line that is connected to a specific switch, in cellular telephony the telephone number specifies a specific subscriber's mobile station (telephone). Much of the complexity in cellular telephony results from the need to track the location of the mobile station. In this section we discuss how radio transmission systems and the telephone network infrastructure are organized to make this service possible.

Radio telephony was first demonstrated in 1915 when an analog voice signal was modulated onto a radio wave. Because electromagnetic waves propagate over a wide geographical area, they are ideally suited for a radio broadcasting service where information from a source or station is transmitted to a community of receivers that is within range of the signal. The economics of this type of communication dictate that the cost can be high for the station equipment but that the cost of the receivers must be low so that the service can become available to a large number of users. Commercial broadcast radio was introduced in the early 1920s, and within a few years the service was used by millions of homes.

The introduction of commercial radio resulted in intense competition for frequency bands. The signals from different stations that use the same frequency band will interfere with each other, and neither signal will be received clearly. A limited number of frequencies are available, so in the 1930s it became clear the regulation was needed to control the use of frequency bands. Governments established agencies responsible for determining the use and the allocation of frequency bands to various users.

Radio transmission makes communications possible to mobile users. Early mobile radio telephone systems used a radio antenna installed on a hill and equipped with a high-power multichannel transmitter. Transmission from the mobile users to the antenna made use of the power supplied by the car battery. These systems provided communications for police, taxis, and ambulance services. The limited amount of available bandwidth restricted the number of calls that could be supported and hence the number of subscribers that could use such systems was limited. For example, in the late 1940s, the mobile telephone service for the entire New York City could only support 543 users [CSTB 1997].

The scarcity of the radio frequency bands that are available and the high demand for their use make frequency spectrum a precious resource. Transmission of a radio signal at a certain power level results in a coverage area composed of the region where the signal power remains significant. By reducing the power level, the coverage area can be reduced and the frequency band can then be reused in adjacent areas. The frequency-reuse principle forms the basis for cellular radio communications, shown in Figure 4.51.

In cellular telephony, a region, for example, a city, is divided into a number of geographical areas called cells.10 Figure 4.51 shows how a region can be partitioned in a honeycomb pattern using hexagonal cells. Cell areas are established based on the density of subscribers. Large cells are used in rural areas, and

'"Unfortunately, the term cell appears in two separate and unrelated areas in networks. The geographic "cells" in a cellular network have nothing to do with the fixed-packet "cells" that are found in ATM networks. The context is usually sufficient to determine what kind of cell is being discussed.

FIGURE 4.51 Cellular network structure

small cells are used in urban areas. As shown in Figure 4.52, a base station is placed near the center of each cell. The base station has an antenna that is used to communicate with mobile users in its vicinity. Each base station has a number of forward channels available to transmit to its mobile users and an equal number of reverse channels to receive from its mobile users.11

Base stations are connected by a wireline transmission link or by point-to-point microwave radio to a telephone switch, called the mobile switching center (MSC), which is sometimes also called the mobile telephone switching office (MTSO). The MSC handles connections between cells as well as to the public switched telephone network. As a mobile user moves from one cell to another, a handoff procedure is carried out that transfers the connection from one base station to the other, allowing the call to continue without interruption.

In general, immediately adjacent cells cannot use the same set of frequency channels because doing so may result in interference in transmissions to users near their boundary.12 The set of available radio channels are reused following the frequency-reuse pattern. For example, Figure 4.51 shows a seven-cell reuse pattern in which seven disjoint sets of frequency channels are reused as shown. This pattern introduces a minimum distance of one cell between cells using the same frequency channels. Other reuse patterns have reuse factors of 4 and 12. As traffic demand grows, additional capacity can be provided by splitting a cell into several smaller cells.

As an example consider the Advanced Mobile Phone Service (AMPS), which is an analog cellular system in use in North America. In this system the frequency

"The channels are created by using frequency-division multiple access (FDMA), time-division multiple access (TDMA), or code-division multiple access (CDMA). These techniques are explained in Chapter 6.

12An exception is CDMA, which allows the same "code division" channel to be reused in adjacent cells.

AC = authentication center PSTN = public switched telephone network

BSS = base station subsystem STP = signal transfer point

EIR = equipment identity register VLR = visitor location register HLR = home location register MSC = mobile switching center

AC = authentication center PSTN = public switched telephone network

BSS = base station subsystem STP = signal transfer point

EIR = equipment identity register VLR = visitor location register HLR = home location register MSC = mobile switching center

FIGURE 4.52 Components of a cellular network band 824 to 849 MHz is allocated to transmissions from the mobile to the base station, and the band 869 to 894 MHz is allocated to transmissions from the base station to the mobile. AMPS uses a 30 kHz channels to carry one voice signal, so the total number of channels available in each direction is 25 MHz/30 kHz = 832 channels. The bands are divided equally between two independent service providers, so each cellular network has 416 bidirectional channels. Each forward and reverse channel pair has frequency assignments that are separated by 45 MHz. This separation between transmit and receive channels reduces the interference between the transmitted signal and the received signal.

A small number of channels within each cell have been designated to function as setup channels. For example, the AMPS system allocates 21 channels for this purpose. These channels are used in the setting up and handing off of calls as follows. When a mobile user turns on his or her unit, the unit scans the setup channels and selects the one with the strongest signal. It then monitors this setup channel as long as the signal remains above a certain threshold. To establish a call from the public telephone network or from another mobile user to a mobile user, the MSC sends the call request to all of its base stations, which in turn broadcast the request in all the forward setup channels, specifying the mobile user's telephone number. When the desired mobile station receives the request message, it replies by identifying itself on a reverse setup channel. The corresponding base station forwards the reply to the MSC and assigns a forward and reverse voice channel. The base station instructs the mobile station to begin using these channels, and the mobile telephone is rung.

To initiate a call, the mobile station sends a request in the reverse setup channel. In addition to its phone number and the destination phone number, the mobile station also transmits a serial number and possible password information that is used by the MSC to validate the request. This call setup involves consulting the home location register, which is a database that contains information about subscribers for which this is the home area. The validation involves the authentication center, which contains authentication information about subscribers. The MSC then establishes the call to the public telephone network by using conventional telephone signaling, and the base station and mobile station are moved to the assigned forward and reverse voice channels.

As the call proceeds, the signal level is monitored by the base station. If the signal level falls below a specified threshold, the MSC is notified and the mobile station is instructed to transmit on the setup channel. All base stations in the vicinity are instructed to monitor the strength of the signal level in the prescribed setup channel. The MSC uses this information to determine the best cell to which the call should be handed off. The current base station and the mobile station are instructed to prepare for a handoff. The MSC then releases its connection to the first base station and establishes a connection to the new base station. The mobile station changes its channels to those selected in the new cell. The connection is interrupted for the brief period that is required to execute the hand-off.13

When roaming users enter an area outside their home region, special procedures are required to provide the cellular phone service. First, business arrangements must be in place between the home and visited cellular service providers. To automatically provide roaming service, a series of interactions is required between the home network and the visited network, using the telephone signaling system. When the roamer enters a new area, the roamer registers in the area by using the setup channels. The MSC in the new area uses the information provided by the roamer to request authorization from the roamer's home location register. The visitor location register contains information about visiting subscribers. After registering, the roamer can receive and place calls inside the new area.

Two sets of standards have been developed for the signaling required to support cellular telephone service. The Global System for Mobile Communications (GSM) signaling was developed as part of a pan-European public land mobile system. The Interim Standard 41 (IS-41) was developed later in North America, using much of the GSM framework. In the following section we describe the protocol layered architecture of GSM.

In the GSM system the base station subsystem (BSS) consists of the base transceiver station (BTS) and the base station controller (BSC). The BTS consists of the antenna and transceiver to communicate with the mobile telephone.

13Again CDMA systems differ in that they carry out a "soft" handoff that uses a "make before break" connection approach.

The BTS is also concerned with the measurement of signal strength. The BSC manages the radio resources of one or more BTSs. The BSC is concerned with the setup of frequency channels as well as with the handling of handoffs. Each BTS communicates with the mobile switching center through the BSC, which provides an interface between the radio segment and the switching segment as shown in Figure 4.53.

The GSM signaling protocol stack has three layers as shown in Figure 4.53. Layer 1 corresponds to the physical layer, and layer 2 to the data link layer. GSM layer 3 corresponds to the application layer and is divided into three sublayers: radio resources management (RRM), mobility management (MM), and call management (CM). Different subsets of these layers/sublayers are present in different elements in the GSM network. We discuss these proceeding from the mobile station to the MSC in Figure 4.53.

The radio air interface between the mobile station and the BTS is denoted as Um. The physical layer across the Um interface is provided by the radio transmission system. the LAPD protocol is a data link protocol that is part of the ISDN protocol stack and is similar to asynchronous balanced mode in HDLC discussed in Chapter 5. LAPDm denotes a "mobile" version of LAPD. The radio resources management sublayer between the mobile station and the BTS deals with setting up the radio channels and with handover (the GSM term for handoff).

The interface between the BTS and its BSC is denoted as the Abis interface. The physical layer consists of a 64 kbps link with LAPD providing the data link layer. A BSC can handle a handover if the handover involves two cells under its control. This approach relieves the MSC of some processing load. The interface

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