This paper details the key features of models of wireless links. The goal is to help address the issue of how to modify TCP and link-layer protocols to improve performance over wireless links.
This paper points out the key problems in current wireless models: unrealistic assumptions, small parameter space, overly realistic and lack reproducibility. Next, they list characteristics of wireless links and how to model them:
- Error Losses: represent packet loss after link-layer retransmission attempts. Model by dropping packets with some probability.
- Delay Variation: represents delay due to link-layer recovery, etc. Model by randomly suspending transmission.
- Packet Reordering: caused by retransmission attempts. Model by randomly swapping packets or delaying single packets.
- Resource Allocation: some wireless links temporarily allocate bandwidth to streams. Model by introducing delay when a packet arrives to a queue that has been empty for longer than the channel holding time.
- Bandwidth Variation: may occur on CDMA and UMTS links. Model by occasionally reducing bandwidth of the link.
- Bandwidth Asymmetry: wireless links typically have asymmetric bandwidths.
Finally the paper makes some recommendations. They suggest that bit errors should be handled at the link layer using FEC and retransmissions. However, explicit corruption notification can be used to inform TCP when transmission fails after link-layer recovery attempts. TCP should be more resilient to reordering which would give link-layer protocols more flexibility in recovery. Finally, where possible, link layer protocols should reduce delay variation but delay tolerant TCP may be useful as well.
Thoughts
This paper seemed to state fairly obvious characteristics of wireless links and modeling techniques. The key contribution seems to be in pulling all of these characteristics together and stating where previous work has failed to do so.
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