WDFXW文档下载网
首页     文档分类     最新文档    

 
注册 | 登录 点击查看的信息 | 退出
 

80211 IFS时间

文档名称:80211 IFS时间
分享到:

文档名称:80211 IFS时间
文档关注次数:1
文档格式:纸质版或者PDF电子版(用Acrobat Reader打开)或Word版本doc格式
PDF格式 80211 IFS时间 免费下载 Word格式 80211 IFS时间 免费下载免费下载文档
文档大小:388KB
上传者:lidong602167
添加时间:2019/05/14
内容摘要:
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 assume that the host operating system limits the outgoing frame size to 1,500 bytes. 802.11 permits much larger frame sizes, but this flexibility has not traditionally been used by client products. Most access points connect to existing networks with Ethernet, and therefore limit the payload size to the maximum Ethernet payload size. (In fact, this simple precaution is required to obtain Wi-Fi certification.)
Encapsulation
In addition to the payload data, there are 36 additional bytes of data added in the encapsulation process. The 802.11 MAC header adds 28 bytes of data for various control and management functions, error detection, and addressing. A further eight bytes are added by the SNAP encapsulation header to identify the network layer protocol. (For further encapsulation details, see Figures 3-9 and 3-13 of 802.11 Wireless Networks.) The total size of the MAC payload for the TCP data segment is 1,536 bytes (12,288 bits). For the TCP ACK, the 40 bytes of TCP/IP header information also gets a further 36 bytes of SNAP header for a total of 76 bytes (608 bits).
Throughput
This article measures the throughput of the network at the TCP payload layer. Each transaction delivers one data frame. There are 1,500 bytes of Ethernet payload, but 40 bytes must be subtracted for header
评论
序号
评论内容
用户名
日期
发表新评论 或 请提供可参考的全文下载链接
用户名
 
相关免费下载文档
 
 
文档下载排行
 
 
最新免费下载文档
 
WDFXW文档分享网 |联系我们
版权所有:www.WDFXW.net  | 鲁ICP备14035066号-3