GSM is
widely used network system used in the World.It uses TDMA and FDMA as access
technologies.
Frequency Allocation and Physical Channels
From 890
MHz to 915 MHz is used for uplink (from MS to BTS) and 935 MHz to 960 MHz is
used for downlink (from BTS to MS). Both uplink and downlink has a bandwidth of
25 MHz each. That 25 MHz bandwidth is divided in to 124 carriers. Each carrier
is time divided in to TDMA frames which are consisted of 8 time slots. A one
time slot can be considered as a “channel”, each timeslot consists of a usable
part of 114 bits and a training sequence of 26 bits. This training sequence is
very important as it is used by the Mobile
In a GSM
Base Transceiver Station, up to 12 transceiver units can be used to serve for a
single cell. (That is for latest BTS). Each transceiver can handle one carrier.
(I.e.8 channels). That one channel can act as a traffic channel for a single
user. Normally time slot 0 and time slot 1 are used for signaling between the
mobile and the BTS. Since signaling doesn’t need data rate as that for traffic,
we can divide the signaling channels in to more logical channels.
Channel Concept in GSM
One carrier in GSM form 8 TDMA channels and these
channels can be used to transmit and receive signaling, voice or data. There
are special channels defined in GSM air interface for signaling purposes. These
are called logical channels. Sometimes they exist in the same physical channel
but logically separated. Some of the important logical channels are listed below.
·
Broadcast Channels (BCH) (Down-link
Only)
Ø Frequency Correction Channel (FCCH) - used for frequency correction of MS
Ø Synchronization
Channel (SCH) - carries information
on the TDMA frame number and the Base Station Identity Code (BSIC) of the BTS
Ø Broadcast Control CHannel (BCCH) - Broadcasts cell specific information to the MS
· Common control channels
Ø Paging Channel (PCH) - BTS Transmits a paging message to indicate an incoming
call or short message. The paging message contains the identity number of the
mobile subscriber that the network wishes to contact. At certain time intervals
the MS listens to the PCH. If it identifies its own mobile subscriber identity
number on the PCH, it will respond.
Ø Random Access Channel
(RACH) – used on uplink by MS to request a
SDCCH for call setup, locations update, SMS or answers paging message.
Ø
Access
Grant Channel (AGCH) – Used on downlink to allocate a
SDCCH (signaling) or a TCH to a MS.
·
Dedicated
Control Channels (DCCH)
Ø Stand alone Dedicated
Control Channel (SDCCH) - used for system signaling during
call set-up or registration. Uplink and downlink as well as transmission of
SMSs (in idle mode) are done in this logical channel.
Ø Slow Associated Control
Channel (SACCH)
– control channel associated with a TCH or an SDCCH. Measurement Reports from
the MS to the BTS are sent on the up-link. On the down-link the MS receives
information from the BTS what transmitting power to use. In addition, the SACCH
is used for the transmission of short text messages in call connected (busy)
mode.
Ø Fast Associated Control
Channel (FACCH)
– Control channel associated with a TCH. Also referred to as Fast Associated
Signaling (FAS), the FACCH works in stealing mode. That is, 20 ms of speech is
replaced by a control message. It is used during handover, as SACCH signaling
is not fast enough. Used on up-link and down-link
·
Traffic
Channels (TCH)
Traffic channels are used to transmit voice traffic.
They caries no signaling and MS tuned in to these TCHs according to the
information given through SDCCH.
·
Packet
Data Channels (PDCH)
PDCH are used transmit packet data traffic. They are
used in GPRS and EDGE.
These logical channels are mapped into the physical
channels in groups. For example BCH can be set at timeslot 0 of carrier 0.
Likewise others can be mapped to the physical channels in GSM air interface.Others
are defined as TCH channels and PDCHs are dynamically allocated according to
the request.
Data Coding, Encryption and Data Integrity
Techniques in GSM
As a digital communication system GSM has to convert
analog voice signals to digital data streams. Other than normal A/D conversion
GSM uses speech coding technique which will transform analog voice to digital
stream with bit rate 13 Kbits/s per subscriber.
After speech cording channel coding take place. Channel coding in GSM
uses the 260 bits from speech coding as input to channel coding and outputs 456
encoded bits. This will assure the data integrity on air interface. Channel
coding took place in hierarchical way such that its input is divided into three
blocks and do the coding to them. They are 50 very important bits, 132
important bits and 78 not so important bits. These blocks are coded in
different levels so that important bits may go through the air interface very
safely.
After channel coding , two stages of interleaving
takes place. Single step of interleaving divide a single information packet in
to four transmitting packets . Ultimately lost of one packet at air
interface will only affect 12.5% of information lost. Then the ciphering takes
place to encode the burst so that it cannot be interpreted by any other device
than the intended receiver. The ciphering algorithm in GSM is called the A5
algorithm. Then burst formatting include some extra information such as the 26
training sequence bits, 2 flag bits and 6 tail bits to the packet and modulate
the digital data with GMSK (gaussian-minimum-shift-keying) to transmit.
