Modeling Wireless Link

The loss characteristics that a wireless link offers to TCP (or any transport protocol) depends on whether it has a data link protocol.

Raw wireless link

A wireless link, compared to a wired link, has high noise, low bandwidth, and low propogation delay. Typical values are:
  • Packet loss probability (for 1 Kbyte packet) ranging from 0.15 to 0.001.
  • Bandwidth ranging from 50Kbit/s to 1.5 Mbit/s.
  • A link between two stationary points in a stationary environment does not have time varying statistics. So this can be modeled by a single state with some loss prob and bandwidth in the above ranges.

    But a link between mobile points has time-varying statistics, eg, it can alternate between a GOOD state and a BAD (or "fade") state (the rate of alternation depends on the speed of mobility, extent of interfering buildings, etc). The average BAD state duration can be taken to be 1/100 of the average GOOD state duration. For a slow mobility environment, a typical GOOD duration is 0.12 sec (or about 128 packet transmission times). For a fast mobility environment, a typical GOOD duration is 0.06 sec (about 6 packet transmission times).

    So this can be modeled by a two-state Markov chain, with one GOOD state and one BAD state. For a slow mobility environment, the GOOD to BAD transition rate would be (1/0.12) and the BAD to GOOD transition rate would be (1/0.0012). The rates can be similarly obtained for a fast mobility environment. The GOOD state has a low packet loss probability, say 10-2. The BAD state has a high packet loss probability, say 1.

    Wireless link with link-level protocol

    A wireless link can have a link-level protocol (eg, LAPB, HDLC, ...), that offers an improved link to TCP. Such a link-level protocol would typically
  • break up TCP packets into smaller link packets, eg 128 bytes, to counter the effects of noise.
  • do link packet retransmissions, perhaps upto some MAX-RETRY-COUNT, after which it discards the packet.
  • The link packet retransmission provides a trade-off between packet loss probability and bandwidth. For example, if MAX-RETRY-COUNT is very high (practically infinity), then the link offered to TCP has no loss but the average time to send a packet is

    If MAX-RETRY-COUNT is 0, then the link offered to TCP has the same bandwidth and loss probability as the wireless link, but as modified by the smaller link packets. For example, if a link packet is 1/10 the size of a TCP packet, then link packet loss prob is 1/10 that of TCP packet loss prob; the bandwidth would be the same (assuming link protocol header is small compared to link packet payload, and assuming low delay and low bandwidth (reasonable for wireless link)). The mean durations (measured in link packet transmission times) of GOOD and BAD states for the link protocol is also scaled by the same factor.

    Thus, for the TCP wireless example described above, a typical link protocol packet size would be 128 bytes, or 11 link packets per TCP packet. For the fast mobility environment, the link offered to TCP can be described by the GOOD-BAD Markov chain with mean BAD state duration of about 2 or 3 link packets.