Global System Mobile, GSM, 2G
7.6 Mapping the Logical Channels onto Physical Channels
7.6.5 Signalling Procedures over the Air Interface .1 Synchronization to the Base Station
When a SIM card is inserted after switching the power on, the first step for the MS is to adjust to the local radio environment.
1) The MS starts the search for BCCH carriers. Normally, the station has a stored list of up to 32 carriers of the operator network. Signal level (RXLEV) measurements are performed on each of these frequencies. Alternatively, in case of roaming, if no list is available all GSM frequencies have to be measured to find best potential BCCH carrier. The MS uses path loss criteria and stored threshold data to create a preference list of potential carriers.
2) Locking on the preferred carriers the MS searches for a FCCH signal in order to perform frequency adjustment and to start time synchronization between the MS and the network.
3) The MS reads the SCH using an SCH training sequence of 64 bits for fine tuning of the frequency correction and time synchronization. After that the MS is able to read and decode synchronization data from the SCH, the BSIC and the Frame Number and
other information messages from BCCH and CCCH. Validating correct BSIC and path loss criteria, the MS can select the candidate cell.
4) The terminal then receives information on the BCCH. The BCCH continuously transmits information on the identity of the network (operator) and cell, and on the channel configuration for the cell as well as neighbour list with neighbour carriers.
5) The MS performs steps 1 to 4 in order to synchronize with the six neighbour cells with the strongest signal level (RXLEV) and read out their BCCH/SCH information.
6) Steps 1 to 5 are repeated at the time interval set by the network. When measured parameters significantly change (fall out of the range of defined thresholds), the MS starts the cell reselection process.
7.6.5.2 Registering With the Base Station
After locking on to the Broadcast Carrier in selected cell, the MS is able to register.
Through the registration, the network stores information about the location area in which the MS is camped. Therefore, the MS is known to the network so that it can be reached by calls initiated by the network. Registration takes place either when the terminal is powered up inside radio coverage area and when it enters a new location area. As part of the registration procedure, the GSM may check the status of terminal equipment by sending enquiry to the Equipment Identity Register (EIR).
The process of registration is as follows:
1) The MS pages the BS via RACCH.
2) The BS assign the registration channel SDCCH to the MS via message over AGCH.
3) The signalling over SDCCH includes authentication of the mobile user, location reg- istration. A successful registration results in the Visiting Location Register sending a message to the subscriber’s Home Location Register with details of the subscriber’s ID and where they should be paged for incoming calls.
4) The MS starts monitor paging channel, PCH.
If the terminal moves into another cell, synchronization to a new broadcast carrier takes place. If the cell belongs to a different location area, reregistration takes place so that the network can transfer paging signalling on the PCH to the new location area.
(The size of a location area is a trade-off between heavy registration signalling in small location areas and heavy paging signalling in large location areas containing many cells.) 7.6.5.3 Call Setup
Setting up calls to or from a terminal requires extensive signalling for transfer of address information and allocation of a radio channel/time slot. Initially, signalling is via the CCCH (Common Control CH) and, subsequently, the SDCCH. The outcome is that the call is assigned to a traffic channel. The procedure makes it possible for the network to determine in which cell within the location area the terminal is situated.
The setting up of a callto a terminalis initiated by the network paging the termi- nal over the PCH (Paging CH) in all cells belonging to the location area. The terminal acknowledges the call on the RACH (slotted Aloha). The procedure makes it possible to determine in which cell of the location areas the terminal is situated. Thereafter, the net- work sends a message to the terminal via the Access Grant Channel (AGCH), instructing it to switch over to a given SDCCH, which has an associated SACCH. The SDCCH is
92 Introduction to Mobile Network Engineering
RACH
AGCH
SDCCH
Channel request
Immediate assignment
Service request
Authentication request SDCCH
Authentication response SDCCH
Cypher mode commense
Cypher mode complete SDCCH
SDCCH
SDCCH
Call proceeding SDCCH
Setup
Alert SDCCH
Assign command
Assign complete SDCCH
FACCH
FACCH Connect
Connect acknowledgement FACCH
TCH
BSS MSC
MS
Data Data
Figure 7.25 Mobile-initiated call setup.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 ...
...
...
26-frame of broadcast carrier
Logical T1 channel
D - Stand-alone Dedicated Control Channel (SDCCH)
D
F S D D D D D D F S D
51 TDMA frame, BCCH + CCCH + SDCCH, downlink
Figure 7.26 Signalling on dedicated downlink control channel during the call setup.
used for transmission of the calling and called-party numbers, for authentication, for sending encryption keys and so on. Finally, a traffic channel is allocated to the terminal.
For setting up a callfrom a terminal, the terminal sends a call request via the RACH.
The network sends back details of the allocated SDCCH on the AGCH. Further sig- nalling takes place as described previously, the procedure is shown in Figure 7.25 and mapping of signalling frame to a broadcast carrier is shown in Figure 7.26.