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GPRS (GMM) - 15

GPRS Mobility Management (GMM) [Under GPRS]
» GMM - 24.007/8 «

From 24.008,

" The main function of the Mobility Management sublayer is to support the mobility of user terminals, such as informing the network of its present location and providing user identity confidentiality.

A further function of the MM sublayer is to provide connection management services to the different entities of the upper Connection Management (CM) sublayer (see 3GPP TS 24.007). "

In class A and B mobiles, MM sublayer consists of GMM (for GPRS) and MM (for GSM), both need to coordinate with each other for "combined" operations.

GMM states
» GPRS description - 23.060 «

Based on GMM activities, MS will be in one of the following states:

1) IDLE
2) STANDBY
3) READY

Initial GMM state for MS is of course IDLE when it is not attached to network.

STANDBY state signifies that MS is attached to network. In this state, MS does RA update once RA is changed or after certain time interval. PDP context can be activated or deactivated from this state. Paging is needed if network would like to identify MS's location.

READY state mean MS is attached to network and MS is either doing TBF or a TBF has been completed. In READY state, MS does cell or RA update when cell or RA is changed. That mean network do not have to page the MS as cell is known.

Following diagram shows MS GMM states on MS and network side.

READY time value is negotiated with network in ATTACH request/accept.

Cell reselection
» Cell selection criteria - 5.08 «

MS by itself may choose to select a different cell based on "certain criteria". Short name for this procedure is "cell reselection". In GSM, for idle state (no call in progress, no dedicated channel present), MS can go ahead and do the cell re-selections. But when call is in progress, what MS is required to do is handover for cell reselection - this is with the help of network and ordered by network using HO Command.

In GPRS, distinction is made between packet idle and packet transfer conditions (MS is attached that mean GMM is either in standby or ready state). In packet idle condition (no TBF present, no PDU being transmitted over air), MS can go ahead with cell reselection. During packet transfer condition (TBF in progress, LLC PDUs are being transmitted), cell reselection is based on NCO (Network Control Order). MS can do its own cell selection (NCO0/NCO1) or wait for cell change order from network (NCO2). Difference between NCO0 and NCO1 is: in NCO1, MS need to inform measurement results based on which it has done cell reselection. NCO2 is then similar to GSM HO.

During packet transfer condition, when layer 1 does cell reselection (NCO0/1), on going PDU transfer i.e. TBF is abandoned. GRR layer (RLC/MAC) informs this to GMM.

- If new cell is in same RA, GMM "triggers" LLC to send next LLC PDU to network (from new cell). This (indirectly) indicates cell update to network. If LLC is in Ack mode, it knows where it has left, and can restart sending of SN PDUs or just send Receive Ready PDU. In Unack mode, UI frame can be sent.

- If new cell is in different RA, little more signaling is required as new RA could be in new SGSN. GMM suspends SN PDU transfer by sending "suspend request" to LLC. GMM now does RA update procedure. Once RA update is complete, GMM send "resume request" to LLC to resume SN PDU transfer.

References: GPRS in Practice by McGuiggan

Copyright © Samir Amberkar 2010§ §

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