[ns] Still problem with mac 802.11

[Alper Kamil Demir]

IEEE 802.11 uses CSMA/CA algorithm to mediate the access to the shared medium. data (DATA), access control (RTS, CTS), and reliability control (ACK) are different types of frames used in CSMA/CA, consequently in 802.11. RTS/CTS is one of access mechanisms used in 802.11. There is also a basic access mechanism. Mainly, access mechanisms are aimed to solve problems occuring because of limited range of radio communications and phisically distributed nature of radio nodes such as hidden terminal, exposed terminal, etc... On the other hand, different types of IFS (inter frame space) time control (such as DIFS, SIFS, etc..) are used to provide a mechanism for priority access. Albeit 802.11 is not intended for service differentation (802.11e is used for this), however, all these mechanisms also provides a means of bandwith utilization and simple quailty of service. 
RTS/CTS does not necessarily mean that we are reserving the shared medium. However, it may be considered as a very simple impilicit reservation mechanism if properly applied. You can both reserve and share resources. They are not mutually exclusive. If it is very implicit then you may need to contend. I am not sure which contention window you are refering to. Hope this helps.

Alper K. Demir




From: glitch74
Sent: Wed 4/20/2005 4:59
To: ns-users
Subject: [ns] Still problem with mac 802.11


Hi ns-users! 
I've still problem with mac 802.11 implemented in ns2. 
 
I can't understand why mac 802.11 implementation use a rts/cts/data/ack 
pattern. 
In 802.11 packets would not need rts/cts/data/ack to be sent! Isn't it? 
If a packet waits a difs+backoff to be sent, why it is needed the pattern 
above?  
Using rts/cts means that we're doing reservation of the shared medium. 
But if we reserve it....it isn't shared!!! We not need to contend it, infact 
i've observed that the value of the contention window is always 31 (the lower 
value) ns2 never doubles it, so it never reaches the value 1032 (upper level) 
Thank's a lot. 
Please HEEEEEEEELP ME!!!!!!!!!!!!!!!! I'm going crazy! 
Bye 
 



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