UMTS Networks : Architecture, Mobility and Services

86 UMTS Networks
GSM radio has to perform many tasks, and these are tied to further timing structures
and channelisation. The channelisation system in GSM is very straightforward and
consists of two types of channels to begin with. The radio channels implementing
traffic in the radio interface are called physical channels.
Physical channels exist from the radio interface up to the BSC and their allocation to
traffic or signalling use is controlled by the BSC. There are three types of physical
channels:
. BCCH (Broadcast Control Channel): BCCH always has a constant location, which is
TDMA frame timeslot 0 (slots are numbered from 0 to 7). This physical channel
carries the information that Mobile Stations (MSs) listen to in order to synchronise
themselves and access the network.
. SDCCH (Stand-alone Dedicated Control Channel): when a transaction is to be set
up, the MS moves to the SDCCH and carries out transaction-related signalling
activities. SDCCH could be physically located to the same timeslot as BCCH. If
so, the combination is called a ‘‘combined BCCH/SDCCH’’. In larger cells having
more than one TRX, the BCCH and SDCCH are normally separated from each
other. In this case the SDCCH is in the BCCH TRX timeslot 1.
. TCH (Traffic Channel): when signalling is complete and the actual transaction with
user traffic starts, the MS is transited or forced to a TCH for user traffic. Note that all
transactions do not require the TCH. For example, sending an SMS only requires
SDCCH resources, the same applies to MM-related registration procedures.
Both the MS and the network have logical issues that need to be carried out, and mostly
(but not always) these are tied to timing structures. Such logical issues are called logical
channels.
Logical channels use physical channel resources to undertake their tasks, and the
structure used to pack logical channels onto physical channels is predefined with some
options. In standard GSM, the following logical channels exist:
. SCH (Synchronisation Channel): the MS uses this to get into synchronisation with
the network. Place: physical BCCH.
. FCCH (Frequency Correction Channel): this channel updates MSs about frequency
information. Place: physical BCCH.
. BCCH (logical Broadcast Control Channel): this logical channel contains system
information. The MS first opens the SCH and the FCH so that it can read the
BCCH information and, thus, camp on the cell. Place: physical BCCH.
. RACH (Random Access Channel): the MS uses this channel to perform an initial
access procedure or service (e.g., location update, voice call, etc.). Place: physical
BCCH.
. AGCH (Access Grant Channel): the network uses this logical channel for commanding
the mobile station to a logical SDCCH. Place: physical BCCH.
. PCH (Paging Channel): the network uses this logical channel to send paging information
to an MS in the case of a mobile-terminated transaction. Place: physical BCCH.
. SDCCH (logical Stand-alone Dedicated Control Channel): one physical SDCCH
may contain either four or eight logical SDCCH blocks depending on the selected
channel configuration. Transaction signalling always requires one logical SDCCH