Page 162 - DCAP311_DCAP607_WIRELESS_NETWORKS

Wireless Networks

Notes IEEE 802.11 wireless LANs use a media access control protocol called Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA). While the name is similar to Ethernet.s Carrier
Sense Multiple Access with Collision Detection (CSMA/CD), the operating concept is totally
different.
Wi-Fi systems are half duplex shared media configurations where all stations transmit and
receive on the same radio channel. The fundamental problem this creates in a radio system is
that a station cannot hear while it is sending, and hence it impossible to detect a collision. Because
of this, the developers of the 802.11 specifications came up with a collision avoidance mechanism
called the Distributed Control Function (DCF).
According to DCF, A Wi-Fi station will transmit only if it thinks the channel is clear. All
transmissions are acknowledged, so if a station does not receive an acknowledgement, it assumes
a collision occurred and retries after a random waiting interval.
The incidence of collisions will increase as the traffic increases or in situations where mobile
stations cannot hear each other.

There are plans to incorporate quality of service (QoS) capabilities in Wi-Fi with the adoption
of the IEEE 802.11e standard. The 802.11e standard will include two operating modes, either of
which can be used to improve service for voice:
1. Wi-Fi Multimedia Extensions (WME): This uses a protocol called Enhanced Multimedia
Distributed Control Access (EDCA), which is Extensions an enhanced version of the
Distributed Control Function (DCF) defined in the original 802.11 MAC.
The enhanced part is that EDCA will define eight levels of access priority to the shared
wireless channel. Like the original DCF, the EDCA access is a contention-based protocol
that employs a set of waiting intervals and back-off timers designed to avoid collisions.
However, with DCF, all stations use the same values and hence have the same priority for
transmitting on the channel.
With EDCA, each of the different access priorities is assigned a different range of waiting
intervals and back-off counters. Transmissions with higher access priority are assigned
shorter intervals. The standard also includes a packet-bursting mode that allows an access
point or a mobile station to reserve the channel and send 3- to 5-packets in sequence.

2. Wi-Fi Scheduled Multimedia (WSM): True consistent delay services can be provided with
the optional Wi-Fi Scheduled Multimedia (WSM). WSM operates like the little used Point
Control Function (PCF) defined with the original 802.11 MAC.

In WSM, the access point periodically broadcasts a control message that forces all stations
to treat the channel as busy and not attempt to transmit. During that period, the access
point polls each station that is defined for time sensitive service.
To use the WSM option, devices must first send a traffic profile describing bandwidth,
latency, and jitter requirements. If the access point does not have sufficient resources to
meet the traffic profile, it will return a busy signal.
Security has been one of the major deficiencies in Wi-Fi, though better encryption systems
are now becoming available. Encryption is optional in Wi-Fi, and three different techniques
have been defined. These techniques are given here:
(a) Wired Equivalent Privacy (WEP):
An RC4-based 40-or 104-bit encryption with a static key.
(b) Wi-Fi Protected Access (WPA):

This is a new standard from the Wi-Fi Alliance that uses the 40 or 104-bit WEP key,
but it changes the key on each packet. That changing key functionality is called the
Temporal Key Integrity Protocol (TKIP).

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