802.11 IFS时间.docVIP

Now that the 802.11g standard has been finalized, comparisons with the other standards in the 802.11 family are inevitable. One conclusion that is frequently drawn is that 802.11g offers similar speeds to 802.11a. After all, both products are advertised as having a data rate of 54 Mbps. This article develops a simple model for the maximum TCP throughput of 802.11 networks so that a comparison can move beyond a simple comparison of nominal bit rates. According to the model, 802.11g is significantly faster than 802.11b. In a network consisting only of 802.11g clients, it is even slightly faster than 802.11a. However, protection mechanisms added to 802.11g to ensure backwards compatibility with legacy 802.11b clients can cut the throughput by 50 percent or more. Technology Transactions per second Mbps of TCP payload throughput Transactional speed relative to 802.11b 11b, 11 Mbps 479 5.6 1.0 11a, 54 Mbps 2,336 27.3 4.9 11g, 54 Mbps/no protection 2,336 27.3 4.9 11g, 54 Mbps/CTS-to-self protection 1,113 13.0 2.3 11g, 54 Mbps/RTS/CTS protection 750 8.8 1.6 The Transactional Model The basic transactional model assumed by this article is a maximum-length frame containing a single TCP segment followed by a TCP acknowledgement. To cope with the inherent unreliability of radio waves, the 802.11 MAC requires positive acknowledgement of every transmission. Each TCP packet must therefore be wrapped up in a frame exchange. The complete transaction consists of the two 802.11 data frames carrying TCP embedded in their respective exchanges: The TCP data segment: Distributed Interframe Space (DIFS): this interframe space indicates that an exchange has completed, and it is safe to access the medium again. The data frame containing the TCP segment. A Short Interframe Space (SIFS), which is a small gap between the data frame and its acknowledgement. The 802.11 ACK frame. The TCP ACK: DIFS The data frame containing the TCP ACK SIFS 802.11 ACK frame For this model, I will