Understanding GSM and GPRS RRC State Diagrams: States and Transitions

The Radio Resource Control (RRC) state diagram in GPRS and GSM illustrates how mobile devices transition between different states to establish, maintain, or terminate connections with the network. This diagram helps visualize the flow of control signaling between the mobile and the network.

This page explains the key RRC states in GSM and GPRS, including idle, dedicated, and handover states, and their roles in efficient resource management. RRC falls on layer-3 of a GSM mobile subscriber. On this page, we will explore the RRC state diagram along with the functions of all the states. There are two RRC states in a GSM mobile subscriber station: IDLE and Dedicated. We’ll examine what the mobile does during these modes. When you switch on your mobile, it will be in idle mode until you receive a call or you dial a number or initiate a GPRS data connection to browse the internet.

Radio Resource Control Procedure for a GSM mobile is outlined below.

GSM RRC layer is used for signaling between the GSM network and the Mobile Station (MS). In idle mode, the mobile does not involve itself in any form of communication. Also, dedicated resources are not assigned to it in idle mode. While in dedicated mode, the resource is assigned to the mobile to communicate with the Base Station Subsystem (BSS).

IDLE Mode

Once the appropriate best cell is selected by the mobile, the mobile is said to be camped on the respective Base Transceiver Station (BTS). After camping, the GSM mobile enters into the idle mode. In this mode, it monitors the BTS paging channel for the possibility of an incoming call.

The 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 idle mode, the GSM mobile receives the BCCH and CCCH channels from the BTS, transmits the RACH in case of a Mobile Originated (MO) call, does cell reselection, and most importantly performs measurements. The mobile performs measurements on any of the IDLE frames except on PCH/PPCH channels, FCCH, SCH, CBCH, neighbor cell BCCHs, serving cell PBCCH, etc.

Idle mode in a mobile is normally exited to switch to dedicated mode when Layer-1 (physical layer) is configured by the upper Layer for either TCH or SDCCH. During Idle mode, the GSM mobile will continue monitoring the downlink signal strength of neighbor cells to ensure it is camped on to the best available cell. As per the requirement, the mobile will monitor the received signal strength of 6 neighbor BCCH carriers other than the serving cell BCCH.

A mobile subscriber initially accesses a GSM BTS using a random access channel to perform a location update, to answer an incoming paging call, or to make an MO call. There are a total of eight time slots in all the frames, and there is no dedicated slot to be used by the mobile station. It can use any slot for sending the RACH. If a collision occurs, it is repeated a few times for establishing access to the network on the access burst.

It transmits a 5-bit number with 3 bits indicating the reason for network access. If access is granted to the mobile, it is indicated by the AGCH from the BTS on the downlink.

Dedicated Mode

As soon as the 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 dedicated mode, the mobile subscriber does the measurements from the other neighbor base stations (WCDMA, LTE, TD-SCDMA, etc.). These measurements are carried out in the idle slot of the GSM frame.

The mobile also does other GSM neighbor cell measurements mainly for handover and cell reselection purposes when the power from the serving/active cell becomes lower compared to the target cell towards which the mobile is moving.

CELL RE-SELECTION

In a GSM network, when a connected mobile moves to another GSM cell area, redirection disconnects the serving or active GSM network and reconnects to the target GSM cell. Cell reselection to other Radio Access Technologies (RATs) i.e., LTE or WCDMA is also possible when the serving cell has any issue. This is referred to as Cell reselection.

GPRS RRC State Diagram

GPRS uses a packet-switched-based architecture. Here, a 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, a location update needs to be carried out often to achieve this. But this consumes a lot of power, and the battery will drain fast. To avoid this, a GPRS RRC state machine has been developed for location management.

In GPRS mode, the mobile will have three states: idle, standby, and ready. The state of the mobile determines the frequency of the location update.

Idle State:

As mentioned, when a mobile is powered on, it will be in the idle state and will not be attached to the GPRS network. In this state, the GPRS-compatible mobile is not reachable, and a location update has not yet been performed.

Ready State:

After performing a GPRS attach, the mobile station enters the ready state. Here, either the 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 the ready state, the mobile keeps updating the Serving GPRS Support Node (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 a long period of time, this state is referred to as the standby state. This will cause the GSM ready timer to expire. Here, routing area updates are done when needed. A GSM Location Area (LA) is divided into several Routing Areas (RAs). A routing area is composed of several cells. When a mobile moves to a new routing area, then the SGSN will be informed of the same. Paging is performed by the network to determine the current cell of the mobile station in the standby state. The paging is performed within a GSM RA.

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

Conclusion

The GSM RRC state diagram provides a clear representation of the different states and transitions that manage mobile connections, ensuring the efficient use of network resources. Understanding the RRC state machine helps in troubleshooting and optimizing mobile communication, contributing to better service reliability and performance across GSM networks.