Procedure Sequences

For all cellular systems, each transaction (incoming or outgoing call, data session,
text message, etc.) carried out by the user requires a number of separate procedures
to occur in a set sequence. Each transaction will have its own unique requirements,
but the general sequence will usually be common to all transactions.

The general sequence used for the GSM family of technologies is shown in
At this stage, this sequence ignores the complex requirement for setting
up the radio resources. In reality, the radio resource procedures would occur
within the sequence shown at any point where there is a requirement to allocate,
change, or release radio connections.

Note that the standard sequence illustrates the required procedures in six identifiable
steps. This sequence is referred to as the elementary procedure. Before any
communication with the core network, there must be a radio connection in place.
The mobile must also identify the user to the network prior to any service request
being granted.

The first step in the sequence is the assignment of an appropriate channel, which
may be after a paging message has been received from the network in case of an
incoming call. This is followed by the mobile handset requesting the service that is
required, as well as the security procedures (authentication of the user, and ciphering
of the radio channel). Both security procedures are optional; and although
available in most networks, there are exceptions.

These mobility management procedures are used for establishing a mobility
management (MM) connection with the core network, including the authentication
of the user. Once complete, connection management (CM) procedures are used
to set up and manage the transaction itself, including establishing outgoing or
incoming calls, setting up and managing data sessions, or sending or receiving a
text message. Once complete, all relevant channels will be released.

The elementary procedures will be initiated while a handset is in idle mode,
requiring it to move into dedicated mode for the duration of the sequence, before
falling back to idle mode once the sequence is complete. If another transaction
is required while one is in progress, the mobile uses the existing MM procedures
already in place as the basis for the new transaction. For example, no further security
procedures, or even a paging message, will be required for a mobile that receives
a text message while in the middle of a voice call. For GPRS and UMTS packetdata
operation, similar considerations apply.