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This paper presents our analysis and simulation of the IEEE 802.15.3 MAC's
channel time allocation methods and acknowledgment policies. We show their
impact on a TCP flow's behavior and performance, with particular attention
on round trip time, congestion window growth, and packet recovery. In
addition, we present improvements to the delayed acknowledgment policy
that facilitate quick congestion window growth. The outcomes of our studies
are as follows. (1) Channel time allocations (CTAs) should have durations
determined by TCP's maximum congestion window. It is shown this allocation
provides a simple solution to the well known limitation of TCP's low
default maximum congestion window and slow-start threshold when operating
over high-speed links. (2) Data and acknowledgment CTAs must be ordered
to ensure fast congestion window growth. (3) The position of the data
and acknowledgment CTA pair has no impact on TCP's performance because
current TCP implementations use coarse clock granularity and have large
minimums for round-trip time and retransmission timeout. (4) The delayed
acknowledgment policy needs to be augmented with an adaptive algorithm
that adjusts its burst size based on bit error rate (BER). When the BER
is low, large bursts are desirable since this minimizes the overhead of MAC
acknowledgements. However, when the BER is high, short bursts are preferred
since this will allow the MAC to quickly identify and recover any dropped
frames. Furthermore, our results also show that an adaptive burst size
algorithm will allow the receiver's buffer to be reduced to only 50-60\%
of the maximum burst size without impacting performance.
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