Modeling Wireless Links for Transport Protocols

Summary

This paper puts forth a model to evaluate the performance of wireless links. The authors argue that this is especially useful when trying to decide if link layer protocols should have knowledge of the transport layer and use that knowledge when deciding what to do or if transport protocols should be redesigned for better performance on current wireless links.

This paper considers 3 main classes of wireless links: wireless LANs, wide-area cellular links and satellite links. The authors go into detail on the essential aspects of the model: types of wireless links in each class, topologies most common in each class and the traffic patterns in each class.

The performance metrics used for this experiment are throughput, delay, fairness, dynamics and goodput.

Then, the authors explain that current models are inadequate because they are either unrealistic, realistic but explore only a small part of the parameter space, overly realistic or lacking reproducibility.

The authors then choose several link characteristics and for each, explain what the current state is and how to model it:

• error losses and corruption: not a big concern because of FEC and link layer retransmissions. Can be modeled by dropping packets on a per-packet, per-bit or time-based loss probability
  
• delay variation: delay spikes can cause spurious timeouts, causing retransmissions. Can be modeled by suspending data transmission on the link.
  
• packet reordering: reordering is not widely enabled in practice. Can be modeled by swapping packets or delaying one packet for a given time.
  
• on-demand resource allocation
  
• bandwidth variation
• asymmetry in bandwidth and latency
• queue management
• effects of mobility
  

Criticism & Questions

I enjoyed this paper. It was well organized and easy to follow. I especially liked how they organized the various link characteristics to consider by explaining what the current state of each was as well as how to model that characteristic.

I am very curious to know if this model caught on or is being used by any networking researchers in the present time.