MOB Subject 6 - IEEE 802.11 standards Wifi operation

Master of Computer Science 1 - MOB Mobile Internet and Surrounding

Fladenmuller Baey - ”IEEE 802.11standards” 1/8

MOB Subject 6 - IEEE 802.11 standards

Wifi operation

1. Generality

1. One wishes to install a wireless local network in a closed environment (office). In order to

cover all the desired zone, it proves necessary to install several Wifi access points (802.11b).

One wishes that whatever the place or the users is, they do not have to configure differently

their Wifi card? How is that possible?

A set of basic services (BSS) is the set form a acces point and cover stations in its area. A system

Distribution (Distribution System-DS) can connect several BSS between them to form a set of

services outstretched (Extended Service Set -ESS).

2. Would you advise to use the same frequency channel or to choose different for each access

point? Why?

By using frequency channels different, we reduce risks interference and we increases the

bandwidth.

3. What is the interest to use the RTS/CTS mechanism?

The aim is to avoid the problem of hidden terminals.

If two wireless terminals do not see, they are likely to issue in the same time and I do not

know that there is a collision. This is the hidden terminal problem. This problem was solved by

the exchange of packets RTS / CTS (Request To Send / Clear To Send), which gives the control

sentence a base station, which normally sees all the terminals that are in communication

her.

A map will emit sends an RTS message to request authorization the base station. The application

is parameter by a time value that indicates given the quantity of e sueh send. The base station

with a message repond CTS parameter with time value. During the time indicated PERIOD,

only the map that has authorization may be issued. In principle, all cards see the message CTS.

The use of RTS / CTS signaling induced overload. If there is no terminal or hidden messages to

send are of small size, this overhead is not profitable. 802.11b provides a threshold for the

implementation of mechanically RTS / CTS. If message size does not pass of the threshold, the

sender tries to transmit directly without seeking authorization from RTS.

4. In 802.11 standards, the flow of MAC frames is determined partly by its type. The control

frames such that RTS and CTS do not use systematically the maximum flow. Why?

For reasons of compatibility with earlier versions, the flow of frames control should be the

maximum flow rates authorized by the standards used by the Connected Equipment.

5. What makes it possible to increase the flows between the different families of protocol

802.11?

The data modulation: BPSK to 802.11, CCK (Complementary Code Keying) for 802.11b and

OFDM for 802.11a and 802.11g.