GSM Operational Modes . . . .5.1 Cell Selection . . . .5.2 SI (System Information) . . . .5.3 Attach . . . .5.4 Authentication . . . .5.5 Cell Reselection . . . .5.6 Location Updates . . . .5.7 Periodic Type and IMSI Attach . . . .5.8 Location Update Signalling Procedures . . . .5.9 Dedicated Mode Procedures . . . .5.10 Adaptive Power Control . . . .5.11 DTX (Discontinuous Transmission) . . . .5.12 Ciphering . . . .5.13 Mobile-Originated Call . . . .5.14 Call Routing to a Mobile . . . .5.15 Mobile-Terminated Call. . . .5.16 Mobile-Terminated SMS . . . .5.17 Handover . . . .5.18 Types of Handover . . . .5.19 Inter-System Roaming . . . .5.20 International Roaming Process . . . .5.21 Detach . . . .5.22 GPRS Mobile State Changes. . . .5.23 GPRS Attach . . . .5.24 PDP Context Activation . . . .5.25
GSM and UMTS Core Network
At the end of this section you will be able to:
OBJECTIVES
Procedures
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■ outline the functionality of the GSM and GPRS mobile station modes and states
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■ describe the causes and effects of moving from one state/mode to another ■
■ list the actions performed by the mobile at switch-on and in idle mode
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■ describe the cell selection and reselection processes ■
■ explain the function and process of dedicated mode procedures including power control,
handover, and DTX (Discontinuous Transmission)
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■ list typical information received in System Information messages ■
■ describe the location update procedure
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■ describe the basic call sequences for both mobile-originated and mobile-terminated calls ■
■ outline the authentication and ciphering procedures ■
■ outline the procedures involved when a GPRS mobile wishes to access the network and
subsequently pass traffic to an external network
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■ describe the purpose of a GPRS attach ■
■ state what information a PDP context contains, and explain why a PDP context activation is
required before data transits the GPRS network
■
GSM and UMTS Core Network
Procedures
GSM Operational Modes
A GSM MS has a number of operational modes: no service mode, limited service mode, idle mode and dedicated mode.
‘No Service’ represents an MS that can find no GSM coverage.
A GSM mobile that cannot find an acceptable cell may acquire any cell and offer a limited service (emergency calls only). This may result from failure to find a cell on the selected PLMN, a missing SIM card, the PLMN not allowed at location update, or illegal MS/ME response to update.
Idle mode represents a mobile that has camped on a cell from the selected PLMN (this selection may be either automatic or manual). An idle mode mobile will register its existence with the system (if appropriate), and will listen to the BCCH (Broadband Control Channel) and CCCH (Common Control Channel) data. This camping results from PLMN selection, cell selection/reselection and location updating.
When an MS is camped on a cell it continually ascertains whether any adjacent cell is better than the current serving cell.
An MS is in dedicated mode when it has a two-way communication link with a BTS. It has accessed a cell and is operating on either an SDCCH (Stand-alone Dedicated Control Channel) or a TCH (Traffic Channel). When in this mode the MS must be time-aligned, have its power controlled, and provide measurement reports to assist in handover.
Idle Mode Limited Service Mode No Service Mode
Dedicated Mode
(example) (example)
5.2
GSM and UMTS Core Network
Cell Selection
At power-on, the mobile needs to arrive at a state whereby it is ready to make or receive calls. To do this, it must identify a suitable BTS and ‘camp on’ to that cell.
When it is first switched on, the MS will be able to see many channels from many networks. It will scan these channels and take signal strength measurements, and will identify which are BCCHs. It synchronizes to the BCCHs and reads system information. On assessing the system information, the MS assesses whether or not it has tuned into a channel on a valid network. The MS assesses quality for cells belonging to the valid network and selects the best from those that meet the quality criteria.
Once the MS has camped on, the IMSI attach or location update procedure can take place.
Further Reading: 3GPP TS 23.122, 45.008 (C1 functionality)
Power-on
Scan for available radio channels
Tune to each available channel
Read system information
Assess quality on cells from wanted network
Camp on
Possible IMSI attach or location update BCCH? N Next Y N Y N Y Valid network? Select best?
Procedures
SI (System Information)
The BCCH function is essential in the process of cell selection and subsequent network access.
The BCCH has a relatively low capacity, being able to transmit a single 23-octet message every 235 ms. To overcome the scarcity of BCCH availability, a series of SI (System Information) messages has been defined. These messages are transmitted with differing periodicity and contain data necessary for selection and access.
There is an overlap of information elements between these messages to ensure that important data is transmitted as frequently as possible.
The cell allocation parameter identifies the set of frequencies allocated to the cell.
The access control parameter includes the SIM access class. This parameter allows operators to apply cell barring to a particular access class, group of access classes or all access classes. Classes 0–9 are allocated to the general public, 11–15 are allocated to the emergency services, utilities and PLMN operators. Additionally the access control parameter may be used to indicate to the mobile how many times it may repeat a RACH (Random Access Channel) procedure in the event that an initial attempt does not receive a response from the network.
The control channel description parameter indicates whether signalling resources are implemented in the cell using combined or non-combined multiframe structures.
The GPRS information element indicates whether or not the cell supports GPRS services.
Further Reading: 3GPP TS 44.018 (Radio Resource Control)