The carrier number and the repetitive time slot number determine Physical Channel (e.g.) time slot 3 on carrier number 4. The duration of timeslot in GSM is 577 micro secs (15/26 ms). The transmission is digital and is in bursts and hence in GSM, physical channel, timeslot and burst refer to the same.
Logical Channel
The content or message of physical channel is logical channel. One or more logical channels can be mapped to one physical channel. For example, on one of the physical channel used for traffic,traffic is sent by using TCH (traffic channel) message while a handover message is transmitted using FACCH (Fast Associated Control Channel message).
- Traffic channels TCH
- Broadcast channels BCH
- Common Control Channels CCCH.
- Dedicated Control Channels DCCH.
GSM Interfaces-Traffic Channels
Traffic channels are used to send data or services. Once the call set up procedures are completed on the control channels, MS tunes to a traffic physical channel.It uses the Traffic Channel logical channel. Traffic channel carries speech or data traffic.
Speech is encoded into 13Kbps digital stream by using a special algorithm called Regular Pulse Excited- Linear Predictive Coder with Long Predictor Loop. Basically information from previous samples which does not change very quickly is used to predict the current sample. The coefficients of linear combinations of previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample represent the signal.
Traffic channels are classified into two types
- Full Rate TCH/F:It transmits full rate speech (13Kbps). A full rate TCH occupies one physical channel
- Half Rate TCH/H :It transmits half rate speech(6.5 Kbps). Two half rate TCHs can share one physical channel to double the capacity
There are enhanced full rate coders, (12.2Kbps) which improve the speech quality but still occupy one full rate TCH. Data rates supported by speech coders: A full rate traffic channel can support data rate of 9.6Kbps and a half rate channel 4.8 Kbps.
GSM Interfaces - Broadcast Channels BCH
Helps the MSs to orient themselves in the mobile radio network. They are Point to multi point channels ( BTS to MSs). They are defined for the down link direction only.
Types of Broad Cast Channels
- Broad Cast Control Channels-BCCH
- Frequency Correction Channel-FCCH
- Synchronization Channel-SCH
- Cell Broadcast Channel-CBCH
GSM Interfaces - Broadcast Control Channel (BCCH)
Through the BCCH logical channel MS is informed about the system configuration parameters like
- Local Area Identity
- Cell Identity
- Identity of BCCH carrier frequencies of Neighbor cells.
- Max trans power of MS to access
- Configuration of control channels
BCCH is also known as beacon. Each cell/sector will have one carrier frequency on which BCCH will be broadcasted.
GSM Interfaces
Frequency Correction Channel-FCCH
The logical Channel contains no information (continuous zeros) but pure carrier is transmitted so that the mobile can tune itself and synchronise with the frequency. FCCH will be transmitted in the same beacon carrier as BCCH.
Synchronisation Channel-SCH
This logical channel transmits information about TDMA frame structure in a cell /sector (frame number) and Base Station Identity Code which can be decoded only if the BTS belongs to the network for which the MS has subscribed. SCH will also be transmitted in the same carrier of FCCH and BCCH.
Cell Broadcast Channel- CBCH
This logical channel contains messages that can be received by all mobiles. These messages are sent by using dedicated channels and come under the category of short messages. However, they are considered as common channels since all mobiles can receive them.
They operate either in up link or down link directions and are point-to-point simplex channels
Types of Common Control Channels
- PCH-Paging Channel
- AGCH-Access GrantChannel
- RACH- Random Access Channel
Paging Channel-PCH
It is logical channel in the downlink direction and it is point-to-point type. It comprises of a paging message to indicate an incoming call or short message. The paging message consists of the identity of mobile the network wants to contact.
Access Grant CHannel-AGCH
It is a point-to-point logical channel in the downlink direction. It informs the mobile about the assignment of signaling channel for call set-up i.e. SDCCH.
Random Access CHannel (RACH)
It is a point-to-point logical channel in the uplink direction. It is sent by the mobile as a request for making a call or as a response to a paging to the mobile. It consists of the identity of the mobile and location area details as available in SIM.
GSM Interfaces
Dedicated Control CHannel (DCCH)
These are the logical channels, which are point-to-point, duplex and available in uplink and downlink directions.
Types of DCCH
- SDCCH-Standalone Dedicated Control CHannel
- SACCH-Slow Associated Control CHannel
- FACCH-Fast Associated Control CHannel
SDCCH-Standalone Dedicated Control CHannel
It is the logical channel for exchange of information between BTS and MS when no TCH is allotted. It also used to send SMS (Short Message Service).It is used for services like
- Subscriber authentication
- Ciphering Initiation
- Equipment validation
- Assignment of a TCH
- Call Setup
- Location Updates
GSM Interfaces
SACCH-Slow Associated Control CHannel
This logical channel is used for non-urgent information exchange between mobile and BTS. It can be associated either with SDCCH or TCH.
- In the uplink direction it is used to send averaged measurement of signal strength and quality of the present serving BTS and neighbouring BTSs.
- In the downlink direction it is used to transmit power control and timing advance data to the mobile.
SACCH is used even when calls are in progress
FACCH-Fast Associated CHannel.
This logical channel is used to exchange some urgent information like handover commands, call release instructions, etc.
While calls are in progress it is transmitted by stealing the bits from TCH, which is known as PRE-EMPTION.
While calls are in progress it is transmitted by stealing the bits from TCH, which is known as PRE-EMPTION.
GSM Interfaces
Logical Channel Summary
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