[ns] Mac/Tdma Fragmentation-related

[Yuci Gou]

Dear all

Now I use the Mac/Tdma protocol in ns2, there seems to be some problems with 
it. One of the problems is related to packet fragmentation. Every Tdma frame 
is divided into 3 time slots in my experiment. Let's just look at outbound 
packets at the base station. Packets type and their size in byte are listed 
below first:

Mobile IP Advertisement               120
Mobile IP Registration Reply           144
ARP                                           80
TCP control packets                     92
TCP data packets                         604

1. When I set slot_packet_len_ to 124, the time it takes for each packet 
except TCP data packets and Registration Replies is 3 time slots;
            TCP data packets      9 time slots
             Mobile IP Registration Replies   6 time slots

2. When I set slot_packet_len_ to 145, which is one byte bigger than the 
size of Registration Replies, the time it takes for each packet except TCP 
data packets is 3 time slots;

TCP data packets, 9 time slot.
...

It seems that TDMA does take fragmentation into account. But what I don't 
understand is 9 time slots are not enough for TCP data packets of 604 bytes. 
In both cases, the number of time slot should be 15, i.e. ceil(604 / 145) * 
3 = 15, ceil(604 / 125) * 3 = 15
(NB: y = ceil(x) rounds x to the nearest integer greater than or equal to 
x.)

Could you please explain why it happens like that?

Regards,
Yuci

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