4.5 Frames and Multiframes

4.5.1 LOGICAL CHANNEL CONCEPT

Logical Channels

• Multiframes provide a way of mapping the logical channels on
to the physical channels (timeslots)
• A logical channel is a series of consecutive instances of a
particular timeslot

• A multiframe is a repeating combination of logical channels

The term ‘logical’ channel is used because the traffic and signalling channels do not have
exclusive use of a physical resource i.e. the carrier frequency (unlike 1st Generation cellular
systems).
Multiframes allow one timeslot allocation (physical channel) to be used for a variety of
purposes (logical channels) by multiplexing the logical channels onto the timeslot.
 

4.5.2 MULTIFRAME CONCEPT

The term ‘multiframe ’ is used to describe a repeating pattern of TDMA frame timeslots
transmitted in sequence on a physical channel.
When a user is allocated a timeslot within a TDMA frame for a voice traffic call, he generally
has exclusive use of that timeslot (full rate traffic) for the duration of the call. Therefore, every
8th timeslot (one TS per TDMA frame) the user transmits a burst of in data. However, after 12
bursts have been sent, a SACCH logical channel burst is inserted and after another 12 traffic
burst have been sent an idle burst is inserted. This pattern then repeats itself over the next 26
bursts and continues to do so until the call is terminated

4.5.3 TRAFFIC CHANNEL(TCH) MULTIFRAMES

This repeating pattern of 26 traffic channel bursts is known as a traffic channel ‘multiframe’ as
illustrated below:

Traffic Channel Multiframe

• The TCH multiframe consists of 26 timeslots.
• This multiframe maps the following logical channels:

•TCH
•SACCH
•FACCH

• TCH Multiframe structure:

4.5.3.1 TCH Multiframe Time Duration

Notice that a multiframe always refers to a set of instances of the same timeslot within a
TDMA frame, therefore each ov the above timeslots occur 4.615mS apart (the duration of a
TDMA frame). Therefore, the time duration for a multiframe is calculated as the length of the
multiframe (in timeslots) x 4.615mS.
In addition to TCH and SACCH timeslots, the traffic channel can also carry FACCH
information. The FACCH is unique amongst logical channels in that it does not have a
dedicated timeslot for transmission.

4.5.3.2 TCH Multiframe SACCH Function

During a call the MS is continually monitoring power levels from neighbouring base stations.
It does this in the time intervals between its allocated transmission timeslot. Once, during
each traffic channel multiframe, there is an uplink SACCH burst which is used to send a
report on these measurements to the current serving base station.
The BTS uses the downlink SACCH burst to send power control and other call-control signals
to the mobile.

4.5.3.3 TCH Multiframe Idle Channel Function

The idle slot (TS 25 in the multiframe) occurs to allow for half rate TCH/H operation in which
two mobiles would share the multiframe and sets of reports would need to be sent to the base
station. Slot 25 would then be a second SACCH burst.

4.5.3.4 TCH Multiframe FACCH Function

The FACCH is used for purposes that require instant access such as a handover command
message from the base station. When this is needed, FACCH uses a TCH burst and sets a
‘stealing flag’ in the burst to show that it is not a traffic channel burst.

4.5.4 CONTROL CHANNEL MULTIFRAMES

4.5.4.1 BCCH Carriers

Although a cell can contain multiple carriers (frequencies), every cell must have at least one
TS of one of its carriers dedicated to control functions. This physical control channel
transports a number of logical signalling channels multiplexed together. The most important
of these logical signalling channels is the BCCH as it carries network configuration
information. It is for this reason that the carrier containing this control channel (and therefore
the BCCH logical channel) is often referred to as the ‘BCCH Carrier’.

4.5.4.2 Control Channel Multiframe Structures

Control Channel Multiframe
• The control channel multiframe is formed of 51 timeslots
• CCH multiframe maps the following logical channels:
Downlink:
• FCCH
• SCH
• BCCH
• CCCH (combination of PCH and AGCH)

Control channel multiframes are inherently more complex than TCH multiframes as they
carry multiple logical signalling channels multiplexed onto a single physical channel (TS0).
The logical channels that are transported on the control, channel are organised such that the
control channel sequence repeats every 51 occurrences of TS0 in consecutive TDMA frames as
shown above.

4.5.4.3 Combined and Non-Combined CC Multiframes

The CCCH are general signalling blocks (each of 4 TSs) that are allocated to specific signalling
channels depending on the signalling capacity requirements. This includes SDCCH, SACCH
AGCH and PCH allocation.
When the signalling capacity requirements are calculated, it may be determined that the
capacity available on a single control channel is not sufficient. In such cases, additional
physical channels are allocated to signalling.
When multiple physical signalling channels are required they are always allocated on the
BCCH carrier using TS 2 4 or 6 (in addition to TS0) . Where this is the case, the multiple
signalling channels are arranged in either combined or non -combined formats as shown
below. Therefore the structure of the signalling channel multiframe may vary depending on
the signalling capacity requirements.

4.5.5 GSM MULTIFRAME HIERARCHICAL STRUCTURE

4.5.5.1 Superframes

The primary purpose of the superframe layer is to provide a point at which both TCH and CC
multiframes are synchronised. Therefore, 51 TCH multiframes of 26 TS each are grouped
together and 25 CC multiframes of 51 TS are grouped together. IN both cases the time
duration of a superframe is 26x51 TS = 6.12 seconds.

4.5.5.2 Hyperframes

The synchronisation channel (SCH) transmits a TDMA frame number (FN) which enables a
mobile to synchronise with the base station at TDMA-frame level. The FN is a 22 bit number
which resets after each hyperframe, i.e. after 2048 x 26 x 51 = 2715648 TDMA frames.