GSM RRC layer state diagram

RRC falls on layer-3 of GSM mobile subscriber. On this page we will see RRC state diagram along with functions of all the states.

There are two RRC states in GSM mobile subscriber station viz. IDLE and Dedicated. We will see what mobile does during these modes. When we switch the mobile it will be in idle mode until we receive a call or we ourselves dial a number or initiate GPRS data connection to browse the internet. Radio Resource Control Procedure for the GSM mobile is outlined below.

GSM RRC layer is used for signaling between the GSM network and MS. In the idle mode mobile does not involve in any form of communication. Also dedicated resource is also not assigned to it in the idle mode. While in the dedicated mode, the resource is assigned to mobile to communicate with the BSS.

IDLE Mode

Once the appropriate best cell is selected by mobile then and mobile is said to be camped to the respective BTS. After camping GSM mobile enters into the idle mode. In this mode, it monitors the BTS paging channel for posibility of incoming call. Mobile runs a procedure periodically to check, whether it has been camped to the most suitable cell or not i.e. it checks the signal strength and quality from the incoming broadcast channels from the camped on cell. This procedure is called cell reselection.

In the idle mode, GSM mobile receives the BCCH and CCCH channels from BTS,transmits RACH in case of MO call, does cell reselection and also the most important is measurements. Mobile does measurements on any of the IDLE frame except on PCH/PPCH channels,FCCH,SCH,CBCH, neighbour cell BCCHs, serving cell PBCCH etc. Idle mode in mobile is normally exited to switch to dedicated mode when the Layer-1(physical layer) is configured by the upper Layer for either TCH or SDCCH.

During Idle mode, GSM mobile will continue monitoring downlink signal strength of neighbor cells to ensure it is camped on to the best available cell. As per requirement mobile will monitor received signal strength of 6 neighbor BCCH carriers other than serving cell BCCH.

Mobile subscriber initially accesses a GSM BTS using an random access channel to perform location update, to answer incoming paging call or to make a MO call.

There are total eight time slots in all the frames and there is no dedicated slot to be used by the mobile station. It can use any slots for sending RACH. If collision occurs it is repeated for few times for establishing access to the network on access burst. It transmits 5 bit number with 3 bit indicating reason for the network access. If access is granted to the mobile then it is indicated by AGCH from BTS on the downlink.

Dedicated Mode

As soon as RRC connection is established the GSM mobile moves to dedicated mode from the idle mode state.

If the mobile is supporting multi-RATs or multi modes then during the dedicated mode, mobile subscriber does the measurements from the other neighbor base stations (WCDMA, LTE, TD-SCDMA etc.). These measurements are carried in idle slot of the GSM frame.

Also mobile does other GSM neighbor cell measurements mainly for handover and cell reselection purpose when the power from serving/active cell becomes lower compare to target cell where mobile is moving towards.

CELL RE-SELECTION

In GSM network, when a connected mobile moves to another GSM cell area, re-direction disconnects the serving or active GSM network and re-connects to the target GSM cell. Cell reselection to other RAT i.e. LTE or WCDMA is also possible when the serving cell will have any issue. This is referred as Cell reselection.

This section has covered GSM RRC states. For protocol stack involving other layers such as physical layer,LAPD,LAPDm,RRM,MM,CM,SCCP,BSSMAP and BTSM refer GSM protocol stack.

GPRS RRC layer states

GPRS uses packet switched based architecture. Here connection is established when we want to send/receive data using FTP/HTTP protocols. It is released once we have carried out our goal of internet browsing or file transfer. Hence location update need to be carried out often to achieve this. But this consumes lot of power and battery will drain fast. To avoid this GPRS RRC state machine has been developed for location management. In GPRS mode, mobile will have three states viz. idle, standby, and ready. The state of the mobile determines frequency of the location update.

Idle State: As mentioned when mobile is powered on it will be in idle state and will not be attached to the GPRS network. In this state the GPRS compatible mobile is not reachable and location update is not yet performed.

Ready State: After performing GPRS attach, mobile station enters into the ready state. Here either mobile will be in packet transfer mode or it might have just finished the transfer. By GPRS detach the mobile will get disconnected from the network and it will go back to the idle state. All the PDP contexts will be deleted after disconnection.During ready state mobile keep updating SGSN about its whereabouts.

Standby State: when the mobile is powered on and will be attached to the GPRS network but packet transfer has not been initiated for long period of time. This state is referred as standby state. This will cause GSM ready timer to expire. Here routing area updates are done when needed. GSM LA (location area) is divided into several RAs(routing areas). A routing area composed of several cells.

When mobile moves to a new routing area then SGSN will be informed of the same. Paging is performed by the network to determine the current cell of mobile station in standby state. The paging is performed within a GSM RA.

This section has covered GPRS RRC states. For protocol stack involving other layers such as Physical layer,LLC,RLC,MAC,SM,GMM and SNDCP refer GPRS protocol stack.

Summary of functions of RRC layer in GSM network

•  Channel assignment
•  channel release
•  channel change and handover
•  change of channel frequencies
•  hopping
•  sequences (algorithms) and frequency tables
•  measurement reports from the MS
•  power control
•  discontinuous transmission reception
•  time advance
•  modification of channel modes (speech and data)
•  cipher mode setting

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