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Radio Link Control (RLC)

A new layer called Radio Link Control is added to the wireless node, between the LL and IFQgif. This required changes in  ns/tcl/lib/ns-mobilenode.tcl

The main features included are fragmentation and assembly, along with RLC retransmissions. The retransmit mechanism is a simplified form of Selective Retransmits. The fragmentation-reassembly and acknowledgements may be configured ON or OFF by the user and the RLC fragment size can be set. A new header, the RLC header is introduced. Though this was written as a part of GPRS stack implementation, it can be used independently with any other stack with minor modifications.

In case an RLC fragment is dropped by the MAC, in acknowledged mode, a duplicate acknowledgment for the last correctly received RLC fragment is sent back to the sender, which then retransmits the expected RLC fragment. In unacknowledged mode, if an RLC fragment is missing, the RLC does not pass any of the fragments to the LL. The LL in this case, would re-send the LL fragment (if in acknowledged mode) or will let the higher layers (ie TCP) handle it. In acked mode, currently only one MS can be supported.

In GPRS. the actual size of an RLC packet's payload depends on the coding scheme used. We take the average GPRS RLC payload size to be 200 bytes. In GPRS these 200 bytes are transmitted over four slots in consecutive TDMA frames, amounting to 50 bytes per slot. To model this, the RLC fragments are configured to be of 50 bytes and thus 50 bytes are transmitted in each TDMA slot.





Richa Jain (96D07007)
Tue Jul 24 22:29:53 IST 2001