PCU and GP

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THE GCH INTERFACE

The GCH interface provides a synchronous connection between the MFS and the BTS, using 1-->5 16kbps time slots. The GCH links pass transparently through the BSC. Its have 3 functions are as follows:

1. Transfer of PDUs between the MFS and the BTS, thus packet data is not directly handled by the BSC but passes transparently through it on the GCH interface.
2. Synchronization with the radio interface at GCH Link establishment.
3. Correction of clock drift between Abis and BTS clocks.

The protocol for GCH interface uses the 2 layers described follow:

1. L1-GCH Layer: L1-GCH Layer is the physical layer based on ITU-T. The L1-GCH Layer uses digital transmission at a rate of 2Mbps with a frame of 32*62 kbit/s time slots. An L1-GCH channel has a transmission rate of 16 kbps.
2. L2-GCH Layer : is the data link. This layer is in charge of the data transfer of the GCH frames between the MGS and the BTS. The L2-GCH layer offers a service of data transport for the RLC/MAC layers located in the MFS.It main have 3 functions are : * GCH link establishment and release. * Synchronization with radio interface. * RLC/MAX PDUs transfer.

On GSM transmission, refer to call setup.A new link, the M-EGCH is available. The M-EGCH is a link  established between the MFS and the BTS and is defined per TRE. An M-EGCH is made up of one to 36 GCHs. The M-EGCH link of TRE is used five to carry: 

1. Uplink signaling message after one-block allocation, on UL two phase access.
2. MFS-BTS control message.
3. TBF signaling message on the TBF PACCH
4. Signaling message when MPDCHs are defined on the TRE.
5. TBF traffic when TBFs are established on the PDCHs of the TRE.

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