Transmission Delay in ns2

My observation is that packets are retransmitted 4 times until they are dropped on the MAC layer. This is the default value for MAC_LongRetryLimit in ns2.
The Transmission Delay in my simulation consisting of two nodes has only a minor randomness. It highly depends on the current data rate, and graphs are as angular as the troughput graphs.

What could be the reason?

  1. No retransmissions due to bit errors as there are just 2 nodes and no interference by noise.
  2. No retransmissions induced by multipath fading, as it is not simulated.
  3. Retransmissions only occur for queued packets after the nodes move out of transmission range. These retransmissions always fail and therefore do not affect the transmission delay.
  4. The interface queue behaves strange. It does not do FIFO. The interface queue size does not affect the graph shape much anyway.