The Mobile Equipment and SIM are the only parts of the GSM network which the subscriber will really see. Let’s see what inside it.
GSM Mobile Equipment
There are three main types of ME, these are listed below:
These devices are mounted in a vehicle and the antenna is physically mounted on the outside of the vehicle.
Portable Mobile Unit
This equipment can be handheld when in operation, but the antenna is not connected to the handset of the unit.
Hand portable Unit
This equipment comprises of a small telephone handset not much bigger than a calculator. The antenna is being connected to the handset.
The ME is capable of operating at a certain maximum power output dependent on its type and use. These mobile types have distinct features which must be known by the network, for example their maximum transmission power and the services they support. The ME is therefore identified by means of a classmark. The classmark is sent by the ME in its initial message.
The following pieces of information are held in the classmark:
Identifies the phase of the GSM specifications that the mobile complies with.
RF Power Capability
The maximum power the MS is able to transmit, used for power control and handover preparation. This information is held in the mobile power class number.
Indicates which ciphering algorithm is implemented in the MS. There is only one algorithm (A5) in GSM phase 1, but GSM phase 2 specifies different algorithms (A5/0–A5/7).
Indicates the frequency bands the MS can receive and transmit on. Currently all GSM MSs use one frequency band, in the future this band will be extended but not all MSs will be capable of using it.
Short Message Capability
Indicates whether the MS is able to receive short messages.
ME power class as below,
GSM SIM – Subscriber Identity Module
The SIM is a “smart card” which plugs into the ME and contains information about the MS subscriber hence the name Subscriber Identity Module.
The SIM contains several pieces of information:
International Mobile Subscriber Identity (IMSI)
This number identifies the MS subscriber. It is only transmitted over the air during initialization.
Temporary Mobile Subscriber Identity (TMSI)
This number identifies the subscriber, it is periodically changed by the system management to protect the subscriber from being identified by someone attempting to monitor the radio interface.
Location Area Identity (LAI)
Identifies the current location of the subscriber.
Subscriber Authentication Key (Ki)
This is used to authenticate the SIM card.
Mobile Station International Services Digital Network (MSISDN)
This is the telephone number of the mobile subscriber. It is comprised of a country code, a network code and a subscriber number.
Most of the data contained within the SIM is protected against reading (Ki) or alterations (IMSI). Some of the parameters (LAI) will be continuously updated to reflect the current location of the subscriber. The SIM card and the high degree of inbuilt system security, provide protection of the subscriber’s information and protection of networks against fraudulent access.
SIM cards are designed to be difficult to duplicate. The SIM can be protected by use of Personal Identity Number (PIN) password, similar to bank/credit charge cards, to prevent unauthorized use of the card.
The SIM is capable of storing additional information such as accumulated call charges.
This information will be accessible to the customer via handset/keyboard key entry. The SIM also executes the Authentication Algorithm.