SIM cards can be removed and used in different phones, which
is why there is no link between the IMEI and the subscriber. This allows subscribers
within a GSM network to move from device to device but maintain their identity within
the GSM network. This is an important concept because it is where the IMS concept of
identity and mobility was born.
Credentials stored in the SIM card are used to authenticate the subscriber. These
credentials include a cipher key for encryption across-the-air interface. One of the major
concerns of wireless operators is eavesdropping on the air interface. This is easy to
accomplish with a device programmed to intercept wireless transmissions. It is for this
reason that the transmissions over the air interface are encrypted.
An IMSI is not for public consumption and is sent as infrequently as possible. Usually
the IMSI is sent during registration with the network, but if a subscriber is roaming,
this number may be replaced by a Temporary Mobile Subscriber Identity (TMSI). We
won??™t go into detail here as to when that is the case.
The IMSI is comprised of a Mobile Country Code (MCC), which is usually three digits
long, followed by two or three digits identifying the Mobile Network Code (MNC), and
finally the unique Mobile Subscriber Identity Number (MSIN). The Mobile Subscriber
International Services Digital Network (MSISDN) is the telephone number that gets
assigned to the subscriber and becomes the subscription identity for the subscription
and the public address, while the IMSI remains known only to the network.
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