NS2 simple Simulation

the simulation requires the following : Build an NS2 network simulator that will simulate a system consisting of two nodes each one is transmitting to the other one and receiving from it in full-duplex manner. The channel between the two nodes has a AWGN with SNR (Eb/No) equal to 6 dB. The transmitters are using BPSK (this will allow you to find the probability of bit errors from the BPSK BER curves). The transmitter and receiver DLC layers have infinite buffers (queues) and the data arrival rate from the upper layers follows a Poisson distribution with average arrival rate of 140 packets per second. Packets have exponentially distributed length with average packet size of 500 bytes (including the TCP header information). The transmission speed is 1 Mbps. The transmitter and receiver are using Stop and Wait ARQ and the ACK packet is of length 4 bytes. The distance between the transmitter and receiver is 200m and assuming that the packets are traveling at the speed of light. service time was found and BER I think it's wrong but anyway I can change it no problem in that. the problem is that the packet length does not change and I belive the problem is in sendpacket procedure in the attached tcl fie. I would be very grateful if you found time to look into this problem and please see attached tcl file.