RFGSM Terminology Explained: BS_AG_BLKS_RES, PLMN, Location & Routing Areas
This article explains essential GSM terms such as BS_AG_BLKS_RES, BS_PA_MFRMS, CCCH_CONF, BS_CC_CHANS, BS_CCCH_SDCCH_COMB, routing area, location area, Cell Global Identity, PLMN, and more.
BS_AG_BLKS_RES
BS_AG_BLKS_RES is transmitted in the BCCH SI-3 message. It’s a 3-bit field, ranging from 0 to 7. This field indicates how many of the CCCH blocks of a 51-frame multiframe (MF) on TS0 (Time Slot 0) are reserved for AGCHs (Access Grant Channels).
The number of PCH (Paging Channel) channels is reduced accordingly from the total CCCH blocks of 8 (in non-combined mode). In combined mode, CCCH blocks per TS are 4 rather than 8.
In non-combined mode, BS_AG_BLKS_RES = 3 means the first 3 CCCH blocks are used for AGCH, and the remaining 5 are used for PCH on time slot TS0.
BS_PA_MFRMS
BS_PA_MFRMS is transmitted over the BCCH SI-3 message. It’s a 3-bit field, ranging from 0 to 7. It indicates to all mobile stations (MS) within a cell after how many multiframes the content of the PCH has to be analyzed or “sniffed” for information relevant to them.
BS_PA_MFRMS = 3 means that after every 3 multiframes, the MS needs to look for information in the multiframe structure transmitted by the BTS (Base Transceiver Station).
BS_CC_CHANS
This indicates how many time slots (TSs) on the BCCH frequency are reserved for CCCHs (Common Control Channels). A value of 0 indicates that none are reserved for CCCH.
BS_CC_CHANS are not directly transmitted by the BTS, but are derived from CCCH_CONF.
BS_CCCH_SDCCH_COMB
This indicates whether the SDCCHs (Standalone Dedicated Control Channels) and CCCHs share a given time slot or not.
BS_CCCH_SDCCH_COMB is not transmitted by the BTS, but is derived from CCCH_CONF.
CCCH_CONF
CCCH_CONF is transmitted over the BCCH SI-3 message. It informs the mobile station about the actual CCCH configuration in a cell.
Relation Between CCCH_CONF, BS_CC_CHANS and BS_CCCH_SDCCH_COMB
| CCCH_CONF | BS_CC_CHANS | BS_CCCH_SDCCH_COMB |
|---|---|---|
| 0 | 1 | NO |
| 1 | 1 | YES |
| 2 | 2 | NO |
| 4 | 3 | NO |
| 6 | 4 | NO |
PLMN (Public Land Mobile Network)
The PLMN is considered the highest level of hierarchy in a GSM/GPRS network. It’s identified globally by a PLMN identifier. It is the network providing mobile cellular services.
The PLMN identifier is composed of MCC (Mobile Country Code) and MNC (Mobile Network Code). The MCC length is 3 digits, and the MNC length is 2 or 3 digits.
Location Area (LA)
A Location Area (LA) is an area in which a mobile station (MS) can move freely without updating its current location at the VLR (Visitor Location Register).
If the MS moves out of the LA, it informs the VLR of its current location using the LAU (Location Area Update) procedure. A Location Area (LA) includes one or more cells. This grouping helps in efficient location management.
Each LA is identified by an LAI (Location Area Identity).
LAI = MCC (3 digits) + MNC (2/3 digits) + LAC (2 octets)
Routing Area (RA)
The RA in GPRS is analogous to the LA in GSM. It is the area in which an MS can move freely without updating its current location with the SGSN (Serving GPRS Support Node).
If the MS moves out of a Routing Area, it informs the SGSN of its current location using the RAU (Routing Area Update) procedure. Like a Location Area (LA), a Routing Area (RA) can include one or more cells. This grouping helps in efficient location management.
Each RA is identified uniquely by RAI (Routing Area Identity).
RAI = LAI + RAC (1 octet), where RAC is the Routing Area Code.
An LA may contain one or more RAs.
Cell Global Identity (CGI)
The cell is considered the lowest level of hierarchy in a GSM/GPRS network, identified by the Cell Identity (CI). CI is unique within an LA.
To identify a cell across a PLMN, the Cell Global Identity (CGI) is defined.
CGI = MCC + MNC + LAC + CI (2 octets)
