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Other configurations combine these two parts into one physical, multipurpose device. For further discussions concerning different handset configurations, please see Chapter 10, “Mobile Internet Devices.” In some systems, such as Global System for Mobile (GSM) communication, General Packet Radio Services (GPRS), Enhanced Datarates for GSM and TDMA Evolution (EDGE), and Wideband Code Division Multiple Access (WCDMA), the subscriber data is stored separately on a Subscriber Identity Card (SIM). This feature enables a user to change SIM cards when leaving work in order to convert his or her
Terminal Mobile Terminal Equipment
Figure 1.2 The TE is a Palm Pilot, and the MT is an R520.
phone into a private phone that has a private number. The SIM card can also host additional services through the use of SIM Toolkit technology. SIM Toolkit is beyond the scope of this book, however, so we will not describe this concept further.
The Base Station Subsystem
Although the architecture varies a bit between different systems, there is always an antenna that receives signals from the handsets and transports it to the mobile systems. The antennas can be found at various high-level places in order to obtain the best possible coverage. Connected to each antenna is usually a base station that processes the call setups and routes the calls to the network. In Figure 1.3 and throughout this book the base station is depicted as an antenna tower— although the core of the functionality lies in a small shed that is usually located at the bottom of the tower.
A cell is the basic geographical unit of a cellular system and is defined as the area of radio coverage that one base station antenna system provides. Each cell is assigned a unique number called a Cell Global Identity (CGI). The coverage area of a mobile system consists of a huge number of these cells, hence the words cellular system and cellular phones.
One cell sometimes sends information in all directions from the base station, and sometimes there are three sectors surrounding the antenna. The first configuration is common in rural areas, where it is crucial to obtain as high coverage as possible. The latter configuration, on the other hand, is especially suited
Figure 1.3 Cells and antennas.
for high-traffic areas, and the cells can be directed in clever ways in order to cope with the high traffic. One common example is a stadium, where the load on the network can be incredibly high at times. In these cases, one cell is usually aimed directly at that spot so that it does not deal with any other traffic. So, a base station has an antenna that enables an air interface connection with the MS. When setting up a call, there are commonly some resources (transceiver, power, and so on) allocated to the user in question. One major difference between second-generation (2G) and third-generation (3G) systems is that the allocation in the base station is much more flexible in 3G. In 2G, there is commonly one kind of resource that is dedicated to a certain kind of service, and this kind of limitation would make a multiservice 3G system very inefficient.
A number of base stations are then connected to a controller (a BSC) for GSM and to a Radio Network Controller (RNC) for WCDMA. Much of the intelligence of the mobile system exists here. The BSC/RNC manages all advanced radio -related functions, handover (going from one cell to another), radio channel assignments, Quality of Service (QoS), and the collection of cell configuration data. Advanced load balancing and admission control functionality also exists in the BSC/RNC. The controllers and the base stations together are called the base station subsystems.
The Core Network
The core network has traditionally been equipped with switches and subscriber-handling functionalities. These features include subscriber handling, authentication, security, and system maintenance. As more and more advanced services are introduced, the core network becomes more and more of a data network in which circuit-switched and packet-switched services share the same network. As we explain the GPRS architecture in Chapter 3, “GPRS—Wireless Packet Data”, this migration will become more obvious. The main task of the traditional core network is to route traffic that enters a mobile network from other networks to the right base station and to route calls from an MS within the system to the right destination network, as shown in Figure 1.4.
The destination network for data services might be another mobile network, a land-line phone network, or the Internet. The advent of advanced data services changes this situation, however, and creates a need for items such as SMS centers, WAP gateways, and so on. We describe these concepts in more detail in Chapter 9, “Application Architectures.”
After our call is routed from the MS via the base station, the BSC, and the core network, it now finds the right destination network. The core network switches determine whether the call should be sent to a land-line phone network, to