Roaming

The discussion so far has focused largely on the methods used to access the
network over the air interface. The air interface access is, of course,
extremely important. Other aspects, however, are necessary in order to make
a wireless communications network a mobile communications network.
Mobility implies that subscribers be able to move freely around the network
and from one network to another. This requires that the network
tracks the location of a subscriber to a certain accuracy so that calls destined
for the subscriber may be delivered. Furthermore, a subscriber should
be able to do so while engaged in a call.
The basic approach is as follows. First, when a subscriber initially
switches on his or her mobile phone, the device itself sends a registration
message to the local MSC. This message includes a unique identification for
the subscriber. Based on this identification, the MSC is able to identify the
HLR to which the subscriber belongs, and the MSC sends a registration message to the HLR to inform the HLR of the MSC that now serves the
subscriber. The HLR then sends a registration cancellation message to the
MSC that previously served the subscriber (if any) and then sends a confirmation
to the new serving MSC.
When mobile communications networks were initially introduced, only
the air interface specification was standardized. The exact protocol used
between the visited MSC and the HLR (or home MSC) was vendor-specific.
The immediate drawback was that the home system and visited system had
to be from the same vendor if roaming was to be supported. Therefore, a
given network operator needed to have a complete network from only one
vendor. Moreover, roaming between networks worked only if the two networks
used equipment from the same vendor. These limitations severely
curtailed roaming.
This problem was addressed in different ways on either side of the
Atlantic. In North America, the problem was recognized fairly early, and an
effort was undertaken to establish a standard protocol between home and
visited systems. The result of that effort was a standard known as IS-41.
This standard has been enhanced significantly over the years and the current
revision of the standard is revision D. IS-41 is used for roaming in
AMPS systems, IS-136 systems, and IS-95 systems.
Meanwhile, in Europe, nothing was done to address the roaming issue
for first-generation systems, but a major effort was applied to ensuring that
the problem was addressed in second-generation technology—specifically
GSM. Consequently, when GSM specifications were created, they addressed
far more than just the air interface. In fact, most aspects of the network
were specified in great detail, including the signaling interface between
home and visited systems. The protocol specified for GSM is known as the
GSM Mobile Application Part (MAP). Like IS-41, GSM MAP has also been
enhanced over the years.
Strictly speaking, the term MAP is not specific to GSM. In fact, the term
refers to any mobility-specific protocol that operates at layer 7 of the Open
Systems Interconnection (OSI) seven-layer stack. Given that IS-41 also
operates layer 7, the term MAP is also applicable to IS-41.