Solved: 802.11 RTS/CTS and hidden node problem

kfarrington · ‎08-20-2008

Guys,

A little confused here.

The hidden node problem is if two nodes within a cell can hear the AP but not each other. OK.

But, when we talk about 802.11b and 802.11g backwards compatibility causing reduced throughtput in terms of bandwidth, it seems that this is always blamed on 802.11g stations having to use RTS/CTS.

But,

Even in an 802.11g only cell, dont stations still have to use RTS/CTS mechanisms for the hidden node problem?

I'm confused.com!

Thx

Ken

didyap · ‎08-26-2008

When 802.11b clients are associated to an 802.11g access point, the access point will turn on a protection mechanism called Request to Send/Clear to Send (RTS/CTS). Originally a mechanism for addressing the "hidden node problem" , RTS/CTS adds a degree of determinism to the otherwise multiple access network. When RTS/CTS is invoked, clients must first request access to the medium from the access point with an RTS message. Until the access point replies to the client with a CTS message, the client will refrain from accessing the medium and transmitting its data packets. When received by clients other than the one that sent the original RTS, the CTS command is interpreted as a "do not send" command, causing them to refrain from accessing the medium. One can see that this mechanism will preclude 802.11b clients from transmitting simultaneously with an 802.11g client, thereby avoiding collisions that decrease throughput due to retries. One can see that this additional RTS/CTS process adds a significant amount of protocol overhead that also results in a decrease in network throughput.

In addition to RTS/CTS, the 802.11g standard adds one other significant requirement to allow for 802.11b compatibility. In the event that a collision occurs due to simultaneous transmissions (the likelihood of which is greatly reduced due to RTS/CTS), client devices "back off" the network for a random period of time before attempting to access the medium again. The client arrives at this random period of time by selecting from a number of slots, each of which has a fixed duration. For 802.11b, there are 31 slots, each of which are 20 microseconds long. For 802.11a, there are 15 slots, each of which are nine microseconds long. 802.11a generally provides shorter backoff times than does 802.11b, which provides for better performance than 802.11a, particularly as the number of clients in a cell increases. When operating in mixed mode (operating with 802.11b clients associated) the 802.11g network will adopt 802.11b backoff times. When operating without 802.11b clients associated, the 802.11g network will adopt the higher-performance 802.11a backoff times.

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didyap · ‎08-26-2008

When 802.11b clients are associated to an 802.11g access point, the access point will turn on a protection mechanism called Request to Send/Clear to Send (RTS/CTS). Originally a mechanism for addressing the "hidden node problem" , RTS/CTS adds a degree of determinism to the otherwise multiple access network. When RTS/CTS is invoked, clients must first request access to the medium from the access point with an RTS message. Until the access point replies to the client with a CTS message, the client will refrain from accessing the medium and transmitting its data packets. When received by clients other than the one that sent the original RTS, the CTS command is interpreted as a "do not send" command, causing them to refrain from accessing the medium. One can see that this mechanism will preclude 802.11b clients from transmitting simultaneously with an 802.11g client, thereby avoiding collisions that decrease throughput due to retries. One can see that this additional RTS/CTS process adds a significant amount of protocol overhead that also results in a decrease in network throughput.

In addition to RTS/CTS, the 802.11g standard adds one other significant requirement to allow for 802.11b compatibility. In the event that a collision occurs due to simultaneous transmissions (the likelihood of which is greatly reduced due to RTS/CTS), client devices "back off" the network for a random period of time before attempting to access the medium again. The client arrives at this random period of time by selecting from a number of slots, each of which has a fixed duration. For 802.11b, there are 31 slots, each of which are 20 microseconds long. For 802.11a, there are 15 slots, each of which are nine microseconds long. 802.11a generally provides shorter backoff times than does 802.11b, which provides for better performance than 802.11a, particularly as the number of clients in a cell increases. When operating in mixed mode (operating with 802.11b clients associated) the 802.11g network will adopt 802.11b backoff times. When operating without 802.11b clients associated, the 802.11g network will adopt the higher-performance 802.11a backoff times.

kfarrington · ‎01-15-2009

This is most helpful. Excellent and many thx mate :)

sorry for the delay in responding

Kind regards,

Ken