GPRS / EGPRS

1. Introduction
After a GPRS/EGPRS MS is powered on it has to determine which cell to use. The initial cell selection that occurs when the GPRS/EGPRS MS first accesses the network is identical for GPRS/EGPRS MSs and circuit switched (CS) MSs. However, initial PLMN selection differs between a single RAT and a multi-RAT GPRS MS. PLMN selection and Inter System cell re-selection between GPRS/EGPRS and UMTS networks for a multi-RAT GPRS MS is described in the USER DESCRIPTION, GSM - UMTS, CELL RESELECTION AND HANDOVER .

The BSC governs the CS MSs' cell re-selection behaviour in idle and dedicated mode by different methods. Idle mode CS MSs perform the cell re-selection by using the C1 and C2 criteria (see User Description, Idle Mode Behaviour ). This autonomous cell selection is governed by parameters broadcast in the System Information SI1, SI2, SI3 and SI4 on the BCCH. CS MSs in dedicated mode are not autonomous. Instead they are being steered by the Locating functionality implemented in the BSC

As opposed to the CS MSs, the GPRS/EGPRS MSs perform an autonomous cell re-selection in both packet idle and packet transfer mode. Consequently, no measurement reports will be sent from the GPRS/EGPRS MS to the BSC. Two different cell re-selection algorithms are used depending on whether or not a PBCCH is configured in the cell (see GPRS/EGPRS CHANNEL ADMINSTRATION

If no PBCCH is allocated in the serving cell, the GPRS/EGPRS MS will read the SI broadcast on BCCH and use the C1/C2 criteria for autonomous cell selection and re-selection. This procedure is described in User Description, Idle Mode Behaviour .

In a cell with configured PBCCH, the GPRS/EGPRS MSs use the more advanced GPRS/EGPRS specific re-selection criteria C31 and C32. The BSC controls the cell borders with special GPRS/EGPRS cell selection parameters broadcast in the PSI3 on the PBCCH

Note: All GPRS/EGPRS cell re-selection parameters (plain capital text) described in this document are broadcast to the GPRS/EGPRS MS (e.g. as SI or PSI). They are mapped on the corresponding BSC command parameters for the CS Idle mode behaviour and CS Locating features (bold capital text), if not explicitly stated otherwise.

2. Glossary

2.1 Concepts
BA(GPRS) list :
The BA (GPRS) list includes neighbour cells with GPRS/EGPRS support and the Absolute Radio Frequency Channel Number (ARFCN) of the BCCH frequencies in these cells.

Cell Selection :
The procedure for deciding which cell to camp on after attaching the GPRS/EGPRS MS to the network.

Cell Re-Selection :
The procedure to chose the best cell after the initial cell selection while moving the GPRS/EGPRS MS in the network.

Class A, B or C mode of operation :
* Class A- The GPRS/EGPRS MS may have a CS connection and a GPRS/EGPRS transfer at the same time.
* Class B- The GPRS/EGPRS MS may be GPRS and IMSI attached simultaneously but it cannot use both services at the same time.

* Class C - The GPRS/EGPRS MS is only allowed to be attached to one service at a time. A GPRS/EGPRS MS that only supports GPRS/EGPRS and not CS traffic will always work in class C mode of operation.

GPRS MM states :
* Idle state - The GPRS/EGPRS MS is turned on but not GPRS attached. The GPRS/EGPRS MS is "invisible" to GPRS/EGPRS, i.e. outside the GPRS/EGPRS coverage area.
* Stand by state - The GPRS/EGPRS MS is attached to the GPRS MM and sends RA updates to the SGSN and performs GPRS/EGPRS cell selection and re-selection.
* Ready state - A packet transfer is ongoing or has recently ended. A READY timer with fixed value defines how long time the GPRS/EGPRS MS shall remain in ready state after transfer. The GPRS/EGPRS MS performs cell update when changing cell in ready state.

GPRS MS Modes Packet :
* Idle mode - The GPRS/EGPRS MS is prepared to transfer LLC PDUs on GPRS/EGPRS physical channels. The GPRS/EGPRS MS is not allocated any radio resources on a GPRS/EGPRS physical channel. Instead it listens to the PBCCH and PCCCH or, if those are not provided by the network, to the BCCH and the CCCH.
* Packet Transfer mode - The GPRS/EGPRS MS is prepared to transfer LLC PDUs on GPRS/EGPRS physical channels. The GPRS/EGPRS MS is allocated radio resources on one or more GPRS/EGPRS physical channels for the transfer of LLC PDUs.

Multi-RAT GPRS MS GPRS/EGPRS MS that can obtain service from GPRS/EGPRS/GSM and one or more different RATs such as UMTS..

RA/LA The RA is identical to the LA.


2.2 Abbreviations and acronyms
ARFCN Absolute Radio Frequency Channel Number

BA BCCH Allocation

BCCH Broadcast Control Channel

BCCHNO Broadcast Control Channel Number

BSC Base Station Controller

BSIC Base Station Identity Code

BSS Base Station Systems

BTS Base Transceiver Station

CCCH Common Control Channel

CGI Cell Global Identity

CS Circuit Switched

EGPRS Enhanced General Packet Radio Service

GPRS General Packet Radio Service

GSM Global System for Mobile communication

HCS Hierarchical Cell Structures

HSN Hopping Sequence Number

IMSI International Mobile Subscriber Identity

LA Location Area

LLC Logical Link Control

MAC Medium Access Control

MM Mobility Management

MS Mobile Station

PBCCH Packet Broadcast Control Channel

PCCCH Packet Common Control Channel

PCU Packet Control Unit

PDU Packet Data Unit

PLMN Public Land Mobile Network

PSI Packet System Information

RA Routing Area

RAT Radio Access Technology

RLA_C Received Level Average (circuit switched)

RLA_P Received Level Average (packet switched)

RLC Radio Link Control

SGSN Serving GPRS Support Node

SI System Information

TBF Temporary Block Flow

TDMA Time Division Multiple Access

TS Time Slot

UL Uplink

UMTS Universal Mobile Telecommunications System

VLR Visitor Location Register

3. Capabilities
When a GPRS/EGPRS MS is powered on there is a mechanism that always selects the best cell in terms of signal strength on which to camp. If the best cell does not support GPRS/EGPRS, the GPRS/EGPRS MS will not attach to the GPRS/EGPRS network even if other sufficiently strong cells (but weaker than this cell) with GPRS/EGPRS support exist. Depending on the class of the GPRS/EGPRS MS, it may still attach to the CS domain (i.e. location update, type normal or IMSI attach).

To be able to access GPRS/EGPRS from anywhere in the network, regardless of where the GPRS/EGPRS MS is powered on, it has to be able to select a cell. Initial cell selection for a GPRS/EGPRS MS is done the same way as for a CS MS.

The GPRS/EGPRS MS must also be able to register its current location to the network so that the network knows where to route incoming data transfers. This is done in the VLR functionality in the SGSN.

The cell selection and re-selection algorithms used for both Packet Idle and Packet Transfer mode are the cores of the GPRS/EGPRS MS cell selection behaviour. They support the GPRS/EGPRS MS with information about the structure of the network, and provide the network with a mechanism to indirectly steer the GPRS/EGPRS MS to the desired cells.

The C1/C2 (i.e. serving cells without a PBCCH) criteria, or the more advanced C31/C32 criteria enable the GPRS/EGPRS MS whether in Packet Idle or Packet Transfer mode to choose the most suitable cell to camp on, based on signal strength. Camping on the most suitable cell provides the GPRS/EGPRS MS with the highest probability of good communication with the system.