Basic Network Architecture

Figure 1-6 shows a typical (although very basic) mobile communications
network. A number of base stations are connected to a Base Station Controller
(BSC). The BSC contains logic to control each of the base stations.
Among other tasks, the BSC manages the handoff of calls from one base
station to another as subscribers move from cell to cell. Note that in certain
implementations, the BSC may be physically and logically combined with
the MSC.
Connected to the BSC is the Mobile Switching Center (MSC). The MSC,
also known in some circles as the Mobile Telephone Switching Office
(MTSO), is the switch that manages the setup and teardown of calls to and from mobile subscribers. The MSC contains many of the features and functions
found in a standard PSTN switch. It also contains, however, a number
of functions that are specific to mobile communications. For example, the
BSC functionality may be contained with the MSC in certain systems, particularly
in first-generation systems. Even if the BSC functionality is not
contained within the MSC, the MSC must still interact with a number of
BSCs over an interface that is not found in other types of networks. Furthermore,
the MSC must contain a logic of its own to deal with the fact that
the subscribers are mobile. Part of this logic involves an interface to one or
more HLRs, where subscriber-specific data is held.
The HLR contains subscription information related to a number of subscribers.
It is effectively a subscriber database and is usually depicted in
diagrams as a database. The HLR does, however, do more that just hold
subscriber data; it also plays a critical role in mobility management—that
is, the tracking of a subscriber as he or she moves around the network. In
particular, as a subscriber moves from one MSC to another, each MSC in
turn notifies the HLR. When a call is received from the PSTN, the MSC that
receives the call queries the HLR for the latest information regarding the
subscriber’s location so that the call can be correctly routed to the subscriber.
Note that, in some implementations, HLR functionality is incorporated
within the MSC, which leads to the concept of a “home MSC” for a
given subscriber.
The network depicted in Figure 1-6 can be considered to represent the
bare minimum needed to provide a mobile telephony service. These days, a
range of different features’ services are offered in addition to just the capability
to make and receive calls. Therefore, most of today’s mobile communications
networks are much more sophisticated than the network depicted
in Figure 1-6. As we progress through this book, we will introduce many
other network elements and interfaces as we build from the fundamentals
to the sophisticated technologies of third-generation networks.