GSM CONTROL CHANNELS
To manage the mobile network efficiently there is a need for control signalling to take place. Within GSM this falls into three groups:
· BCCH (Broadcast Control Channel) group
· CCCH (Common Control Channel) group
· DCCH (Dedicated Control Channel) group
BCCH (Broadcast Control Channel) Group
Mobile stations require a fair amount of information to act efficiently. An MS can receive, and potentially be received, by several cells and it has to choose between one of them. The information the MS needs to make this decision and to correctly synchronize with a cell to gain access to the network is conveyed by the BCCH group.
The BCCH group operates in a downlink direction only, from the BSS to the MS. The BCCH group consists of the following channels:
BCCH (Broadcast Control Channel)
The BCCH sends to the MS information about the present cell and all surrounding cells. It also sends relevant information about the network and when a MS is in idle mode it will listen to this channel. Information included within the BCCH is:
· L.A.I. (Local Area Identity)
· Nearby cells to be monitored in preparation for any handover
· Frequency lists
· Cell I.D.
· Power control levels
When the BCCH has no information to send, a dummy burst is transmitted instead to maintain integrity of the data structure.
SCH (Synchronizing Channel)
The SCH sends to the MS all information needed for frame synchronisation
FCCH (Frequency Control Channel)
The FCCH sends to the MS information known as the F burst. This allows the MS to find the SCH and gain synchronisation successfully.
Both the FCCH and SCH are provided to help the MS acquire synchronisation, and more precisely, the successful reception of an SCH burst will give the MS all the information needed for synchronisation. The problem is that the MS has to find the SCH burst amongst all the other channels that are being broadcast.
The SCH burst is transmitted exactly eight bit periods after an FCCH burst. The FCCH burst is by design easy to recognise, so for initial set-up, the MS must look for the FCCH only.
When the MS has found the FCCH it can then extract information from it. It is able to correct its own internal frequency to that of the received BTS, and look for the SCH burst on the same frequency. It can also now work out the starting point of the frame sequence, as the FCCH is always broadcast on timeslot 0.
CCCH (Common Control Channel) Group
The Common Control Channel Group is used to provide information in the initial set-up of calls, whether this is a call originated by the MS or by the network. The group consists of:
· RACH (Random Access Channel)
· PCH (Paging Channel)
· AGCH (Access Grant Channel)
· CBCH (Cell Broadcast Channel)
RACH (Random Access Channel)
The RACH is used to transmit an access request, from the MS, to the network. It is an uplink only transmission and, as its name implies, the MS chooses its own time to broadcast this request, so network management on receipt of a RACH request must be controlled.
PCH (Paging Channel)
PCH is used by the network to call the MS. The specifications do not lay down how many times a page maybe sent if no response is made by the MS, maybe in poor signal conditions, nor do they stipulate which network component is responsible for the repetition (MSC, BSC or BTS). Paging can be performed using either an IMSI or TMSI. A MS will though always respond to an IMSI page.
AGCH (Access Grant Channel)
On successful reception of a RACH by the MS. the BSS sends an AGCH, which allocates to the MS a SDCCH (Stand-alone Dedicated Control Channel). This is used purely for signalling purposes between the MS and the network.
CBCH (Cell Broadcast Channel)
Cell broadcast short messages require the means to transmit about one 80 octet message every two seconds, from the network to a MS which is in idle mode. The CBCH provides this function to any cell that supports this service.
As this service is “listened” to in parallel with paging requests and information on the BCCH, the allocated timeslots are limited.
DCCH (Dedicated Control Channel) Group
In some cases it is necessary to establish a link between the MS and the network purely for signalling purposes, whether it be for the user to set-up call forwarding, the transmission of a Short Message or call set-up. For network management needs, such as, location updating or authentication. The dedicated control channels perform these functions.
Within the DCCH group is the:
· SDCCH (Stand-alone Dedicated Control Channel)
· ACCH (Associated Control Channels)
· SACCH (Slow Associated Control Channel)
· FACCH (Fast Associated Control Channel)
SDCCH (Stand-alone Dedicated Control Channel)
A SDCCH is allocated to a MS during call set-up, whilst authentication processes are be carried out, network location updating requests, and the transmission or reception of Short Message Services.
ACCH (Associated Control Channels)
These channels can either be used with a SDCCH or a TCH, and are used to carry information relevant to the process being carried out.
SACCH (Slow Associated Control Channel)
A TCH always has a SACCH associated with it. The SACCH sends information to the MS about power levels and timing, and sends to the network information about link quality and receive level strengths from other cells.
FACCH (Fast Associated Control Channel)
The FACCH is transmitted as a TCH but with the frame carrying flags to distinguish it from one that carries traffic data. It is used to pass messages such as the progress of call establishment, authentication and handover sequences.
Mobility Management
Mobility Management entails the GSM network keeping track of the MS whilst on the move. Basically, there are two different situations: MS idle and MS busy.
These two cases together lead to all the relevant cases we need to consider, namely:
· MS is turned off.
· MS is turned on but is in the idle state.
· MS is busy in the conversation mode.
MS turned off is the case when the network cannot reach the MS because it does not answer a paging message. It does inform the system about possible changes of LA, as it is simply inoperative. In this case, the MS is simply considered to be detached from the system (IMSI detached).
When the MS is first switched on, it must choose one cell above all others that it considers suitable for its use. This is termed cell selection, and enables the MS to affiliate onto a particular cell; if the MS makes a call, or if a call is received for the MS, or if the MS crosses a LAC border as above then this will be the cell that handles the communication. If the MS moves into a new cell in idle mode, it carries out cell reselection. Cell selection, reselection and location updating are all completely automatic and require no user action.
In order to select or reselect a cell, the MS needs to know the downlink signal strength of all the cells in its immediate vicinity, and some configuration parameters, which are transmitted in the BCCH by the BTS. This information is then factored into a formula, the outcome of which indicates to the MS the relative “quality” of each cell, and the MS then simply selects the best on offer (see C1 and C2 selection). Unusually therefore, the MS selects or reselects its own cells without any instructions or commands from the BTS (although it does use information from the BTS). This is one of only a very few examples of the MS working autonomously in GSM.
When the MS is turned on but in idle state, the system can page the MS successfully. This is the situation when MS is considered attached (IMSI attached).
A MS is therefore considered to be in idle mode whenever it is switched on but is not actually engaged in a call. During idle mode the MS does not transmit anything at all for most of the time, although if it moves across the border into a different location area of the network it contacts the VLR via its current cell to inform it of the change.
In the case that the MS is busy, the network has traffic channels allocated for the data flow to/from the MS. Whilst moving, the MS must also be able to change to a new traffic channel as the signal on the traffic channel drops below an unacceptable level.
How does the mobile know the BCCH frequencies of the cells in its vicinity when it switches on? It doesn’t!
If this is the first time that the mobile has been powered up with this particular SIM card inserted, it scans the entire GSM downlink frequency range, and performs its calculations to determine six strongest BCCH frequencies it can detect. It will also detect any non-BCCH carriers mixed in by the fact that they do not contain the FCH, SCH or BCCH.
If the mobile has previously been powered up with the same SIM card, then it will have remembered the last known BCCH frequencies from an instant before it was powered down and will scan these first, since it can save some time (and therefore battery power). If it cannot detect the last known BCCH it will then perform a frequency scan.
Detecting the BCCH frequency of a cell, as opposed to any other carriers, is vital as the mobile uses various pieces of information from the BCCH as it calculates which is the best cell within range. The actual calculation is known as C1 on C2 depending on the Phase of manufacture of the MS. The mobile calculates a C values for every cell for which it has decoded a BCCH, and simply then picks the cell with the highest value.
C1 and C2 calculationThe C1 formula contains elements related to the uplink and to the downlink of the air interface, this way the mobile takes into account both sides of the air interface. It would though be much simpler to select the cell based on the downlink signal strength, but if the cell radius were 35Km from a high-powered BTS, a lot of power would be needed in the mobile handset for the uplink; creating a real network design conundrum. It is for this reason that the uplink is also taken into account, so that if a particularly strong downlink is received, the C1 formula prevents the mobile from selecting a cell with an enormous uplink transmit power requirement.
For C2, which is based upon the C1 criteria but and includes an “attract value” to boost the resultant C2 value, effectively making the cell look good to the MS. However depending upon the time the mobile has been in the cell, a “repel value”, which could be greater than the attract value, may be subtracted to reduce the value of C2 and therefore prevent the mobile from reselecting the cell.
Once a mobile has selected a cell, it may of course move around and perhaps cross into a new cell. When this happens there is no signal from the BTS, indeed the BTS is unaware which cell the mobile is in at any time in idle, so the mobile must again think for itself and realise when it has entered a new cell.
The
method used to do this depends upon the type of MS and also the configuration
of the cell in question. If the mobile is a first generation type (Phase 1),
then reselection will use exactly the same C1 formula as before. Phase 2 MS open
up the possibility of using a more sophisticated approach known as the C2
formula, but only if the cell has been configured to broadcast the appropriate
information on the BCCH; if not, then the Phase 2 MS too will use C1 for
reselection.
The C2 formula allows the network operator to configure his cells to allow idle MS into each particular cell only under the circumstances of his choosing. To minimise signalling in the event of a call being made, the network operator wants to keep the fast moving MS in the macrocells and only allow MS into the microcell if they have actually stopped in that area. This is because fast moving MS would only be in the microcell for a few seconds, so if they made a call in those few seconds an immediate handover would be required; this is inefficient use of signalling resources and may even happen so quickly that the network may not be able to react fast enough to prevent the call from dropping.
The Base Station Identity Code (BSIC) comprises of the network colour code (NCC) and the Base station Colour Code (BCC). It is transmitted by all cells in the synchronisation channel (SCH) in timeslot 0 of the BCCH carrier. The mobile needs to decode the BSIC so that it can include it in the measurement report to the serving cell, as a way of ensuring that only valid neighbour cells are being detected and measured, rather than interference sources or competitors’ networks in the same frequency band.
In order to decode the BSIC then, the mobile must first locate timeslot 0 of the neighbour cell’s BCCH carrier, then locate the SCH and extract the BSIC. This is not as easy as it sounds, as the timing structure of different cells is not synchronised at all. In the normal course of events when the mobile is taking a measurement of the signal strength of the carrier, however, it does not have time to hang around and do this, as it must tune back to its serving cell’s downlink frequency within a couple of milliseconds.
Therefore, the mobile uses the idle frame of the 26-frame traffic channel multiframe. In this period it is not communicating with its serving cell, and so has a whole 4.615 ms to remain tuned to the neighbour’s BCCH frequency. In the periods when no valid BSIC is decoded for a particular neighbour, the measurement of that neighbour is still included in the measurement report but is ignored by the BTS.
Local Area Registration Initialisation
Prior to establishing any communication links to other parties, the MS must first acquire synchronisation with the GSM system. This begins after the MS is turned on in the network. The first step is for the MS to search for and acquire a FCCH burst on a BCCH frequency. The MS will scan all RF channels and obtain the average signal strength of each channel. During the scanning process, several readings of the RF level are taken so that the MS achieves an accurate estimate of the channel power. Thus, the scanning may take several seconds.
For each of the channels, starting with the one of highest signal strength level, the MS searches the frequency correction burst, on the Frequency Correction Channel (FCCH). This is the first step of the process known as frequency synchronisation. This information is present in the slot TO. The FCCH burst is a long sine wave that is offset by 67.7 kHz from the carrier frequency. The cell transmits all zeros for the frequency correction burst. The MS has to take out this offset before an estimate of the carrier frequency can be established. If no frequency burst is detected, the MS then seeks another a channel with the next highest signal strength level.
After the frequency correction burst is detected, the MS will try to synchronise with the time synchronisation burst (SCH). The SCH always occurs in the next frame in the same timeslot as the FCCH. The occurrence of this is eight Burst Periods (BPs) later than the FCCH. The SCH contains precise timing information on the timeslot boundaries in order to permit refining of the received slot timing. The SCH message also contains the current frame number, to which the MS is also synchronised. This time synchronisation is generally carried out in two steps - coarse and fine. Here the internally stored synchronising pattern is correlated, and at the peak of correlation the channel is considered to be synchronised. If synchronisation does not occur, the process of frequency synchronisation with the next highest power channel in the list will start. If the synchronisation is successful, the MS will read the TDMA frame number and the BSIC, which is part of the SCH transmission. Assuming that the MS is synchronised, it decodes the information on a Broadcast Control Channel (BCCH). The BCCH information contains such items like adjacent cell list, BCCH location, minimum received signal strength, and LAI and provides beacon frequencies of surrounding BTS cells. All BCCH transmissions are at a standard power level, which permits the MS to determine received power from that BTS as well as from adjoining BTS’s. Therefore, when the BCCH information is correctly decoded, the MS will follow one of the following two paths.
If the BCCH information includes the present BCCH channel number then the MS will simply stay on the Channel. If the current channel is not included in the BCCH information list or the received signal strength level is below the desired level, then the MS will continue searching for the next control channel.
After the MS has successfully synchronised to a valid BCCH, it must maintain the link and monitor the Paging Channel (PCH). The MS unit is also required to maintain information on the neighbouring BCCHs. The information includes synchronisation information and the average measured RE level of at least six adjacent cell channels. This information is important for the hand-off process.
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