24-11-2012, 05:48 PM
GSM INTERFACES
[attachment=41086]
Introduction
The GSM network is shown in the figure with the major interfaces indicated as Um, Abis and A
This is a BSS internal interface linking the BSC and a BTS, and it has not been standardized. The A bis interface allows control of the radio equipment and radio frequency allocation in the BTS.(2M link)
Um Interface
It is the air interface between mobile and BTS. There are uplink and downlink frequency bands. The carriers are spaced at 200KHz. Each carrier is providing 8 timeslots.
Physical Channel and Logical Channel Physical Channel
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.
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.
Common Control Channels (CCCH)
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
[attachment=41086]
Introduction
The GSM network is shown in the figure with the major interfaces indicated as Um, Abis and A
This is a BSS internal interface linking the BSC and a BTS, and it has not been standardized. The A bis interface allows control of the radio equipment and radio frequency allocation in the BTS.(2M link)
Um Interface
It is the air interface between mobile and BTS. There are uplink and downlink frequency bands. The carriers are spaced at 200KHz. Each carrier is providing 8 timeslots.
Physical Channel and Logical Channel Physical Channel
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.
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.
Common Control Channels (CCCH)
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