GSM: PHYSICAL AND LOGICAL CHANNELS
GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). FDMA parts involves the division by frequency of the 25 MHz bandwidth in to 124 carrier frequencies (Also called ARFCN) spaced 200 KHz for GSM-900. For GSM-1800 frequency spectrum of 75 MHz bandwidth is divided in to 374 carrier frequencies spaced 200 KHz. TDMA further divides each carrier frequencies in to 8 time slots such that each carrier frequency is shared by 8 users. So in GSM, the basic radio resource is a time slot with duration of 577 µs. 8 Time slots of 577 µs constitutes a 4.615 ms TDMA Frame. GSM uses Gaussian Minimum Shift Keying (GMSK) modulation scheme to transmit information (data and signalling) over Air Interface.
GSM uses number of channels to carry data over Air Interface, these channels are broadly divided in to following two categories:
- Physical Channels
- Logical Channels
A physical channel is determined by the carrier frequency or a number of carrier frequencies with defined hopping sequence and the Time Slot number.
8 Time Slots (1 Time Slot = 1 Physical Channel) of 577 µs constitutes a 4.615 ms TDMA Frame. In GSM standard data on a time slot transmitted in bursts, so time slot is often expressed in BP (Burst Period). 1 BP represents 1 TS. TDMA frame (4.615 ms of 8 TS) further structured in to multiframes. There are two types of multiframes in the system:
- 26 TDMA Multiframe: Consists 26 TDMA frames with duration of 120 ms and used to carry the Logical Channels TCH, SACCH, FACCH etc.
- 51 TDMA Multiframe: Consists 51 TDMA frames with duration of 234.5 ms and used to carry the Logical Channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH etc.
These multiframes further structured in to Superframe and Hyperframe.
SUPERFRAME: Superframe consists of 51*26 TDMA frames with duration of 6.12 sec. This is corresponding to the smallest cycle for which the organisation of all channels is repeated.
HYPERFRAME: Hyperframe consists 2048 superframes (2048*51*26 TDMA frames) with duration of 3 hrs, 28 min, 53 sec and 760 ms. It is in particular smallest cycle for frequency hopping, cyphering.
The frame hierarchy is used for synchronisation between BTS and MS.
Logical Channels are determined by the information carried within the physical channel. Logical channels used to carry data and signalling information. Different logical channels are mapped in either direction on physical channels.
Logical channels divided in to following two categories:
- Traffic Channels
- Signalling Channels
In GSM system two types of traffic channels used:
- Full Rate Traffic Channels (TCHF): This channel carries information at rate of 22.8 Kbps.
- Half Rate Traffic Channels (TCHH): This channels carries information at rate of 11.4 Kbps.
Signalling channel carries control information to enable the system to operate correctly. There are three main categories of signalling channels in GSM which are further divided in several categories:
1. BROADCAST CHANNELS (BCH)
- Broadcast Control Channel (BCCH)
- Frequency Correction Channel (FCCH)
- Synchronization Channel (SCH)
- Cell Broadcast Channel (CBCH)
2. COMMON CONTROL CHANNELS (CCCH)
- Paging Channel (PCH)
- Random Access Channel (RACH)
- Access Grant Channel (AGCH)
3. DEDICATED CONTROL CHANNELS (DCCH)
- Standalone Dedicated Control Channel (SDCCH)
- Fast Associated Control Channel (FACCH)
- Slow Associated Control Channel (SACCH)
BROADCAST CONTROL CHANNEL (BCCH) – DOWNLINK
- Broadcasts Network and Cell specific information required to identify the network and gain access.
- Broadcast parameters include Location Area Code (LAC), Mobile Network Code (MNC), Control Channel Structures, BCCH frequencies of neighbouring Cells and other access parameters.
FREQUENCY CORRECTION CHANNEL (FCCH) – DOWNLINK
- This channel contains frequency correction bursts, used by the mobiles for frequency correction.
- Bears information for frequency Synchronisation.
SYNCHRONIZATION CHANNEL (SCH) – DOWNLINK
- This channel is used by the MS to learn the Base Station Information Code (BSIC) as well as the TDMA frame number (FN).
- 6 bits of BSIC having two parts. 3 bits NCC and 3 bits BCC. NCC stands for Network Colour Code and used to identify the BTS for which measurement is made. BCC stands for Base-Station Colour Code and used for a better transmission in case of interference.
- BICS avoids ambiguity or interference which can arise when a Mobile Station can receive SCH from two cells using the same BCCH frequency.
CELL BROADCAST CONTROL CHANNEL (CBCH) – DOWNLINK
- This channel is used to broadcast specific information to network subscribers; such as weather, traffic, sports, stocks and other public services and announcement.
- This channel is assigned with SDCCH and usually occupies the second subslot of the SDCCH.
PAGING CHANNEL (PCH) – DOWNLINK
- This channel is used for alerting to Mobile Subscribers for incoming calls, SMS and other mobility services.
- Every MS in a cell periodically listen to this channel.
RANDOM ACCESS CHANNEL (RACCH) – UPLINK
- This channel is used by a MS seeking attention of the BTS. When MS wants to initiate dialogue with network, this channel is used to send request to the network for a dedicated resource.
- The real dialogue between MS and Network will take place on the dedicated channel.
- If the request is not granted within a specific time period by the network, the MS repeats the request on the RACCH.
ACCESS GRANT CHANNEL (AGCH) – DOWNLINK
- This channel is used by a BTS to notify the MS of the assignment of an initial SDCCH for initial signalling.
- In response to request from MS on RACH, the network allocates a specific dedicated signalling channel (SDCCH) for further communication. This response is sent on AGCH.
STAND-ALONE DEDICATED CONTROL CHANNEL (SDCCH) – UPLINK/DOWNLINK
- In response of RACCH, network allocates SDCCH over AGCH for further communication between MS and BTS.
- This channel is used for the Location Update, Voice Call Set up and SMS.
FAST ASSOCIATED CONTROL CHANNEL (FACCH) – UPLINK/DOWNLINK
- This channel is used to convey Handover information.
- There is no TS and frame allocation dedicated to this channel. This channel can be associated with SDCCH or TCH and works on the principle of stealing. The burst of TCH is replaced by FACCH signalling when required.
SLOW ASSOCIATED CONTROL CHANNEL (SACCH) – UPLINK/DOWNLINK
- This channel is always associated with TCH or SDDCH used for control and supervision of signals associated with the traffic channels.
- Used to convey the periodic carrier-signal strength measurements to the network, transmit power control and timing advance.