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Channels

The physical air interface provided by ns is used. The number of frequency channels to be created for the Uplink and Downlink can be set by the user through max_num_freq in the OTcl script. We differentiate between the uplink and downlink channels by creating separate timers to clock the TDMA on each. A hard-coded skew of 3 time-slots is maintained between the uplink and downlink TDMA frames. On each frequency (uplink as well as downlink), slot 0 is reserved for signalling and broadcasts. The user can decide the number of slots to reserve for GPRS traffic on each frequency through gprs_slots_per_frame in the OTcl script. The remaining slots are left for GSM mobiles. The frequency channel a packet is to be transmitted on is stamped onto a new field called chan_ in the common header of the packet.

The logical channels are mapped such that:

PBCH - slot0 on uplink/downlink frequency 0.

PRACH - slot0 on uplink frequency 0.

PAGCH - slot0 on downlink frequency 0.

PDTCH - all other slots/frequencies.



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