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GSM - Global System for Mobile Communications


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 to identify the timeslot number.
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.



Access Networks- Mobile


The Access Network is defined as the setting up the communication between end user terminal and the transmission Network. 
The bandwidth allocated for a mobile telecom operator is a very expensive resource. Therefore the company must occupy the frequencies very efficiently.
The widely used moble technology is GSM(Global System for Mobile Communications).GSM networks operate in a number of different carrier frequency ranges.GSM 900MHZ Band is used, when increasing the coverage. Typically GSM 900MHz can cover wide area compared with 1800MHz band.When the frequency getting higher and higher air lose increases. Sometimes, 900 Band will not enough to deliver high customer demand ,in that case we have to use 1800 band to increase the capacity.

We are using different access technologies to address each user  privately with the limited resource frequencies.TDMA,FDMA and CDMA are the access technologies which allow to share the limited bandwidth with subscribers.
    •        FDMA (Frequency Division Multiple Access)          In FDMA, signals from various users are assigned to various frequencies. (Just as in an analog system) Cross talk between adjacent channel users is avoided by maintaining a frequency guard band between them.  Advantages of FDMA are that the capacity increases can be done by reducing the information rate and efficient digital codes. Also the technological advances required for implementing the system are simple
    •     TDMA (Time Division Multiple Access)         This is the access method used in GSM. In a TDMA system data from each user is conveyed in time intervals called slots. Several slots make up a frame. Each slot is made up of a preamble plus information bits addressed to various (mobile) stations. The functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver.     However it is a disadvantage that TDMA requires a large amount of signal processing for matched filtering and correlation detection for synchronization with a time slot.
    •  CDMA (Code Division Multiple Access)   The primary advantage of CDMA is its ability to tolerate a fair amount of interfering signals compared to FDMA and TDMA. So the problems of frequency band assignment and adjacent channel interference is greatly reduced. With CDMA, adjacent micro cells share the same frequencies whereas with FDMA/TDMA it is not feasible for adjacent micro cells to share the same frequencies because of interference. 

I will discuss about these access technologies in later posts.