lab 13 Performance of anomaly in multi-rate wireless

Objective

• What is performance anomaly?
• How to deal with performance anomaly and enhance throughput
  

background

in 802.11b, node A and node B both want to send data to the server. Node A start to move away form the the server. The transfer rate of node A adjust from 11M to 1M. Node B will change the rate that affected by node A. It called " performance anomaly".


simulation steps

we observe the data collect from node C

generate 11m_sd and 1m_sd and 11m_rd and 1m_rd for node A and node B

11m_rd and 1m_rd are the record from node C, received form node B, A .




observe the throughput. 0~15 sec. node C recv. 2Mbps form node A and node C
when A's data rate decreased to 1Mbps, node B's throughput also deceased to 750kbps-->
performance anomaly. After 30 secs, node B's data rate is recovered.

• Method 1
  change frame size. if rate is 11Mbps, CBR packets size 1000bytes
  rate is 1Mbps, CBR packets size change into 256 bytes.
  

This method slow down the rapid decrease of node B.

• Method 2
  change the contention window
  data rate 11Mbps contention window set to 32
  data rate 1Mbps contention window set to 352
  all node packet size are the same which is 1000 bytes

The rapid decreasing of node A (red line)will not bother node B(blue line)

The performance anomaly can be solved.

lab 12 Thoughput limit of IEEE 802.11b

Objectives

• Thoughput limit of IEEE 802.11b
• understand the issue may affect Thoughput

Background

• Physical Layer-physical layer converage protocol(PLCP) and physical medium dependent(PMD)
  MAC
  |
  PLCP
  |
  PMD
• 802.11b MAC layer
  MAC header|DATA|CRC
  30bytes 4 bytes
• in basic mode, some proof is shown on the text book

ns2 simulation

using long preamble with packet size 2000bytes


calculate value 6.8Mbps

using short preamble with packet size 2000 bytes


calculate value 7.4Mbps


The simulation is very close to the calculate value, according to the text book.

作業(5/5)：NS2技能自我檢視

問題：在做lab11的實驗，我要怎麼模擬兩個不同傳輸協定互傳？
描述：在lab11的實驗測試802.11b 傳給802.11b和 802.11e傳給802.11e，但是在實際的環境中，環境通常是有不同的兩個不同的協定方式互傳，如：實驗室所使用的ap是802.11a/b/g/n，而筆電的傳輸協定是802.11a/b/g，中間的傳輸如何用ns2模擬呢？







A:原來在下個實驗13就有。

lab 11 The comparison of 802.11b DCF and 802.11e EDCA

understand the mechanism of 802.11b DCF and 802.11e EDCA

• CSMA/CA
  802.3 use the carrier sense multiple access/collision detectionthe packets need to detect the channel is idle or busy.If the collision occurs, the jan signal will send.It not easy to the detect
  the collision in wireless networks. The signal gets weak and the hidden terminal problem.
• Distribution coordination function(DCF)
  Definition in the IEEE 802.11b protocol
  point coordination function (PCF)
  -contention free period : special node to control the time use in the channel.
  -contention period : each node in contention.
• DIFS: Distributed Inter-Frame Space, the time need to wait if the channel is idle.
• RTS/CTS is not regulated in the IEEE standard.
  
  
• Enhanced Distribution Channel Access(EDCA)
• To get better QoS. The priority of the data is defined, such as audio, video, best effort...etc.
  

ns2 simulation

three wireless node n0-->video---> n1, n1--->FTP flow--->n2, n2--->exponential traffic--->n0
priority1-video
priority2-ftp flow
priority3-exponetial traffic

802.11b results










802.11e results









Observation of two results.

average psnr 32.8(802.11b)<34.8(802.11e)
average delay 1.04(802.11b)>0.08(802.11e)

作業(4/5)：NS2技能自我檢視

問題： awk程式是做什麼用的？
描述：在課本的練習當中，會用awk指令去分析trace file，以得到Throughput、Delay、Jitter等，但是為什麼要用這個指令去分析？這個指令特別的地方是在哪裡？他似乎是以一種程式語言，可寫成.awk的檔，再利用＄awk file.awk out.fr。

在老師提供的資訊，看到了awk這個指令介紹了。

作業(3/5)：NS2技能自我檢視

問題：想要改增加修改自己的MAC?
描述：在課本有提到，自己增加802.11e的檔案，參閱自己所安裝的檔案，我在C:\cygwin\home\Sky\ns-allinone-2.32\ns-2.32\mac找到許多類似可以修改的檔案，如tdma可以看到3個相關的檔案，mac-tdma.o，mac-timers.cc，mac-timers.h，原本以為mac-tdma.o應該是某檔案的原始檔，但是用記事本打不開。那我又看到兩個副檔名為.cc,.h，這就是C++的code和標頭檔，除了mac-timers.cc，mac-timers.h ，.o如何新增修改。

作業(2/5)：NS2技能自我檢視

問題： DropTail queue?
描述：在ns by sample裡，我看到了DropTail 這個term，maximum size 10，在計算機網路有點忘記有沒有提過，在基本的OTcl對於node的描述如下
NAM display

$ns duplex-link $n(0) $n(1) 5Mb 2ms DropTail
$ns duplex-link $n(1) $n(2) 5Mb 2ms DropTail
bandwidth / 20 ms of delay/droptail

還有RED queue...有點忘記 還要繼續補起來