GSM PHYSICAL CHANNEL

GSM Bandwidth Allocation

GSM uses Frequency Division Duplexing(FDD) where the uplink and downlink of each channel operates on a different frequency . Therefore , two frequency band are allocated to GSM, separated by 20 MHz.

                 The following frequency band were initially allocated to GSM.

•    Uplink sub-band 890 MHz to 915 MHz
•    Downlink sub-band 935 MHz to 960 MHz

GSM FDMA Structure

Each band is divided into number of frequency channels, each channel carries having a 200 KHz bandwidth. Therefore 124 carriers have available within each of the up and down link bands, allowing for 2*100 khz guard bands. 

                                                    The up and downlink frequency channel pair allocation has been arranged such that the two frequencies comprising a channel pair are 45 MHz apart. Each of these frequency pairs are identified by an "Absolute Radio Frequency Carrier Number"(ARFCN) in the range 1-124 for GSM.   Up link and Downlink channel frequencies can be calculated as follows:

                   Up link frequencies : Fu(n)= 890+0.2n (1<=n<=124)

                   Downlink frequencies : Fd(n)=Fu(n)+45

GSM TDMA Structure

Each GSM carrier channel is subdivided by time into 8 timeslots. Timeslots are repeated in frames, each frame comprising 8 timeslots. The duration of single timeslot is 0.577ms. Therefore each TDMA frames repeat every 4.615ms. These timeslot are known as 'Physical channels'.     

Within each timeslot, a radio 'burst' is transmitted. This burst is comprises 8*156.25=1250 bit periods.If a frame is transmitted every 4.615ms , the row data rate over the carrier channel is (1\4.615ms)*1250=270kbps. The corresponding physical raw data rate is 270/8=33.75kbps.

Logical channels

The reason logical channels are used in because GSM has a number of control channels which do not require to operate at the full rate of a physical channel. Most of these control channels can therefore multiplexed onto single physical channel known as the 'control channel'. In a single carrier cell, this is always allocated to the physical channel 'timeslot 0'.

GSM Logical channel

• There  are two types of logical channel are defined: Traffic channel and Control channel.
• Each is further sub-divided as follows:

All the control and traffic channels are described as follow link GSM CHANNEL

GSM : GLOBAL SYSTEM FOR MOBILE COMMUNICATION

  GSM

         It is a set of recommendations and specification for a digital cellular telephone network.

         A GSM Network comprises of many function units.

• MOBILE STATION:- The Mobile station consists of physical equipment such as SIM card, Display and digital signal processor, Radio transceiver.It provides.

              1. Teleservices

              2. Data bearer services

              3. Air interface to the user

              4. The features' supplementary services

The MS also provides receptor for SMS messages, enabling the user to toggle between voice and user data.The mobile system facilitates access to voice messaging system. The MS also provides access to the various data service available in a GSM network.

* GSM system provides a greater subscriber capacity than analogue system, 

* GSM system allows 25KHz per user

SIM

 SIM(SUBSCRIBER IDENTIFICATION MODULE) :- By making a distinction between the subscriber identity and the mobile equipment identity, a GSM PLMN can route calls and perform billing based on the identity of the subscriber rather than the mobile equipment being used. This can be done using a removable Subscriber Information Module (SIM). A ”smart card” is one possible implementation of a SIM module

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MSISDN:-MSISDN means "mobile subscriber integrated services digital network". It is the telephone no of the MS. This is the no a calling party dials to reach the subscriber.It is used by the land network to route calls towards the MSC.

              MSISDN= CC+NDC+SN

CC-Country code (e.g for india , country code is 91)

NDC-National destination code

SN-Subscriber number

IMSI:- IMSI means "International mobile subscriber identity". It is a no that network identify unique to a SIM.It is the primary identity of the permenantly assigned to that subscriber.

               IMSI=MCC+MNC+MSIN

MCC-Mobile country code  (e.g for india, mobile country code is 404)

MNC-Mobile network code (it is a two digital code)

MSIN-Mobile subscriber identity number ( it is a ten digital code)

IMEI:- IMEI means "International mobile equipment identity".It is serial number unique to each mobile.This no  is permanently stored in the mobile equipment.IMEI can be used to identify MS, that are reported stolen or operating incorrectly.

           IMEI=TAC+FAC+SNR+SP

TAC-Type approval code (it is six digit code)

FAC-Final assembly code(it is two digit code)

SNR- Serial no( it is six digit code)

SP-   Spare( it is single digit code)

TMSI:-The GSM system can also assign a Temporary Mobile Subscriber Identity (TMSI). After the subscriber’s IMSI has been initialized on the system, the TMSI can be used for sending backward and forward across the network to identify the subscriber. The system automatically changes the TMSI at regular intervals, thus protecting the subscriber from being identified by someone attempting to monitor the radio channels. The TMSI is a local number and is always transmitted with the Local numbers and is always transmitted with the Location Area Identification (LAI) to avoid ambiguities.

EIR:- EIR means" Equipment identity register". A listing of the allowed IMEI is maintained by the PLMN’s in the Equipment Identity Register (EIR) to validate the mobile equipment.

GSM CHANNEL

GSM Frequency band

•          900 Band :- Uplink 890-915MHz, Downlink 935-960MHz, Channel spacing 200 KHz, Carrier Frequency 124, Duplex Spacing 45 MHz
•         1800 Band:- Uplink 1710-1785 MHz, Downlink 1805-1855MHz, channel spacing 200 KHz, Carrier Frequency 374, Duplex Spacing 95 MHz
•     1900 Band:- Uplink 1850-1910 MHz, Downlink 1930-1990 MHz, Channel spacing 200 KHz, Carrier Frequency 299, Duplex Spacing 80 MHz

Uplink:-      The communication going from a satellite to ground.

Downlink:- The communication going from ground to satellite.

GSM bandwidth allocation 

GSM CHANNELS

CHANNEL:- Channel is the medium which through we receive and transmits signals or information from one place to another place.

TRAFFIC CHANNEL(TCH):- It carry digitally encoded user speech or user data and have identical function and formats on both the forward and reverse link.

•   When transmitted as full-rate, user data is contained within one Ts per frame.
•  When transmitted as half-rate, user data is mapped onto the same time slot, but is sent in       alternate frames.

CONTROL CHANNEL:- It carry signalling and synchronizing commands between the base station and the mobile station.

BROADCAST CHANNELS(BCH):- Broadcast channel operates on the forward link of a specific ARFCN within each cell, and transmits data only in the first timeslot(TS0) OF Certain GSM frames

1. BROADCAST CONTROL CHANNEL(BCCH):- The BCCH is a forward information such as cell and network identity and operating characteristics of the cell .  The BCCH also broadcast a list of channels that are currently in use within the cell.
2. FREQUENCY CORRECTION CHANNEL(FCCH):- The FCCH is a special data between which occupies TS0 for the very first GSM frame and is repeated every ten frames within a control channel multi-frames.  The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.
3. SYNCHRONIZATION CHANNELS(SCH):- It is used to identify the serving base station while allowing each mobile to frame synchronize with the base station .  The SCH is transmitted once every ten frames within the control channel multi-frame.

COMMON CONTROL CHANNELS(CCCH):- CCCH are the most commonly used control channels and are used to page specific users and receive mobile request for service.

1. PAGING CHANNEL(PCH):- It is forward link channel . The PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from the PSTN. The PCH transmit the IMSI of the target subscriber along with a request for acknowledgement from the mobile unit on the RACH.
2. RANDOM ACCESS CHANNEL(RACH):-It is a reverse link channel , used by a subscriber unit to acknowledge a page from the PCH, and is also used by mobile to originate a call.
3. ACCESS GRANT CHANNEL(AGCH):- The AGCH is used by the base station to provide forward link communication to the mobile and carrier data which instructs the mobile to operate in a particular physical channel with a particular dedicated control channel.

DEDICATED CONTROL CHANNEL(DCCH):- DCCH are bi-directional and have the same format and function on the both the forward and reverse link.

1. STAND-ALONE DEDICATED CONTROL CHANNEL(SDCCH):- The SDCCH carries signalling data following the connection of the mobile with the base station and just before a TCH assignment is issued by the base station. The SDCCH ensures that the mobile station and the base station remain connected while the base station and MSC verify the subscriber unit and allocate resources for the mobile. The SDCCH is used to send authentication and a list messages as the mobile synchronize itself with the frame structure and waits for  a TCH.
2. SLOW ASSOCIATED CONTROL CHANNEL(SACCH):- SACCH carries general information between the MS and BTS.  On the forward link, the SACCH IS used to send slow but regularly changing control information to the mobile , such as transmit power level instruction and specific timing advance instructions for each user on the ARFCN. On the reverse SACCH carries information about the received signal strength and quality of the TCH ,as well as BCH measurement results from neighbouring cells.
3. FAST ASSOCIATED CONTROL CHANNEL(FACCH):- FACCH carries urgent message and contains essentially the same type of information as the SDCCH.