The 802.11 standard specifies a common medium access control (MAC) Layer, which
provides a variety of functions that support the operation of 802.11-based wireless
LANs. In general, the MAC Layer manages and maintains communications between
802.11 stations (radio network cards and access points) by coordinating access
to a shared radio channel and utilizing protocols that enhance communications
over a wireless medium. Often viewed as the "brains" of the network,
the 802.11 MAC Layer uses an 802.11 Physical (PHY) Layer, such as 802.11b or
802.11a, to perform the tasks of carrier sensing, transmission, and receiving
of 802.11 frames.
Medium access basics
Before transmitting frames, a station must first gain access
to the medium, which is a radio channel that stations share. The 802.11 standard
defines two forms of medium access, distributed coordination function (DCF)
and point coordination function (PCF). DCF is mandatory and based on the CSMA/CA
(carrier sense multiple access with collision avoidance) protocol. With DCF,
802.11 stations contend for access and attempt to send frames when there is
no other station transmitting. If another station is sending a frame, stations
are polite and wait until the channel is free.
As a condition to accessing the medium, the MAC Layer checks
the value of its network allocation vector (NAV), which is a counter resident
at each station that represents the amount of time that the previous frame needs
to send its frame. The NAV must be zero before a station can attempt to send
a frame. Prior to transmitting a frame, a station calculates the amount of time
necessary to send the frame based on the frame’s length and data rate. The station
places a value representing this time in the duration field in the header of
the frame. When stations receive the frame, they examine this duration field
value and use it as the basis for setting their corresponding NAVs. This process
reserves the medium for the sending station.
An important aspect of the DCF is a random back off timer
that a station uses if it detects a busy medium. If the channel is in use, the
station must wait a random period of time before attempting to access
the medium again. This ensures that multiple stations wanting to send data don’t
transmit at the same time. The random delay causes stations to wait different
periods of time and avoids all of them sensing the medium at exactly the same
time, finding the channel idle, transmitting, and colliding with each other.
The back off timer significantly reduces the number of collisions and corresponding
retransmissions, especially when the number of active users increases.
With radio-based LANs, a transmitting station can’t listen
for collisions while sending data, mainly because the station can’t have it’s
receiver on while transmitting the frame. As a result, the receiving station
needs to send an acknowledgement (ACK) if it detects no errors in the received
frame. If the sending station doesn’t receive an ACK after a specified period
of time, the sending station will assume that there was a collision (or RF interference)
and retransmit the frame.
For supporting time-bounded delivery of data frames, the 802.11 standard defines
the optional point coordination function (PCF) where the access point grants
access to an individual station to the medium by polling the station during
the contention free period. Stations can’t transmit frames unless the access
point polls them first. The period of time for PCF-based data traffic (if enabled)
occurs alternately between contention (DCF) periods.
The access point polls stations according to a polling list, then switches
to a contention period when stations use DCF. This process enables support for
both synchronous (i.e., video applications) and asynchronous (i.e., e-mail and
Web browsing applications) modes of operation.
No known wireless NICs or access points on the market today, however, implement
PCF.
802.11 MAC Layer Functions
The following summarizes primary 802.11 MAC functions, especially as they relate
to infrastructure wireless LANs:
- Scanning: The 802.11 standard defines both passive and active scanning;
whereby, a radio NIC searches for access points. Passive scanning is mandatory
where each NIC scans individual channels to find the best access point signal.
Periodically, access points broadcast a beacon, and the radio NIC receives
these beacons while scanning and takes note of the corresponding signal strengths.
The beacons contain information about the access point, including service
set identifier (SSID),
supported data rates, etc. The radio NIC can use this information along with
the signal strength to compare access points and decide upon which one to
use.Optional active scanning is similar, except the radio NIC initiates the
process by broadcasting a probe frame, and all access points within range
respond with a probe response. Active scanning enables a radio NIC to receive
immediate response from access points, without waiting for a beacon transmission.
The issue, however, is that active scanning imposes additional overhead
on the network because of the transmission of probe and corresponding response
frames. - Authentication: Authentication is the process of proving identity,
and the 802.11 standard specifies two forms: Open system authentication and
shared key authentication. Open system authentication is mandatory, and it’s
a two step process. A radio NIC first initiates the process by sending an
authentication request frame to the access point. The access point replies
with an authentication response frame containing approval or disapproval of
authentication indicated in the Status Code field in the frame body.Shared key authentication is an optional four step process that bases authentication
on whether the authenticating device has the correct WEP
(wired equivalent privacy) key. The radio NIC starts by sending an authentication
request frame to the access point. The access point then places challenge
text into the frame body of a response frame and sends it to the radio NIC.
The radio NIC uses its WEP key to encrypt the challenge text and then sends
it back to the access point in another authentication frame. The access
point decrypts the challenge text and compares it to the initial text. If
the text is equivalent, then the access point assumes that the radio NIC
has the correct key. The access point finishes the sequence by sending an
authentication frame to the radio NIC with the approval or disapproval. - Association: Once authenticated, the radio NIC must associate with
the access point before sending data frames. Association is necessary to synchronize
the radio NIC and access point with important information, such as supported
data rates. The radio NIC initiates the association by sending an association
request frame containing elements such as SSID and supported data rates. The
access point responds by sending an association response frame containing
an association ID along with other information regarding the access point.
Once the radio NIC and access point complete the association process, they
can send data frames to each other.
- WEP: With the optional WEP enabled, the wireless NIC will encrypt
the body (not header) of each frame before transmission using a common key,
and the receiving station will decrypt the frame upon receipt using the common
key. The 802.11 standard specifies a 40-bit key and no key distribution method,
which makes 802.11 wireless LANs vulnerable to eavesdroppers. The 802.11i
committee, however, is improving 802.11 security by incorporating 802.1X
and stronger encryption into the standard.
- RTS/CTS: The optional request-to send and clear-to-send (RTS/CTS)
function allows the access point to control use of the medium for stations
activating RTS/CTS. With most radio NICs, users can set a maximum frame length
threshold whereby the radio NIC will activate RTS/CTS. For example, a frame
length of 1,000 bytes will trigger RTS/CTS for all frames larger than 1,000
bytes. The use of RTS/CTS alleviates hidden node problems, that is, where
two or more radio NICs can’t hear each other and they are associated with
the same access point.If the radio NIC activates RTS/CTS, it will first send a RTS frame to access
point before sending a data frame. The access point will then respond with
a CTS frame, indicating that the radio NIC can send the data frame. With
the CTS frame, the access point will provide a value in the duration field
of the frame header that holds off other stations from transmitting until
after the radio NIC initiating the RTS can send its data frame. This avoids
collisions between hidden nodes. The RTS/CTS handshake continues for each
frame, as long as the frame size exceeds the threshold set in the corresponding
radio NIC. - Power Save Mode: The optional power
save mode that a user can turn on or off enables the radio NIC to conserve
battery power when there is no need to send data. With power save mode on,
the radio NIC indicates its desire to enter "sleep" state to the
access point via a status bit located in the header of each frame. The access
point takes note of each radio NIC wishing to enter power save mode, and buffers
packets corresponding to the sleeping station.In order to still receive data frames, the sleeping NIC must wake up periodically
(at the right time) to receive regular beacon transmissions coming from
the access point. These beacons identify whether sleeping stations have
frames buffered at the access point and waiting for delivery to their respective
destinations. The radio NICs having awaiting frames will request them from
the access point. After receiving the frames, the radio NIC can go back
to sleep. - Fragmentation: The optional fragmentation function enables an 802.11
station to divide data packets into smaller frames. This is done to avoid
needing to retransmit large frames in the presence of RF interference. The
bits errors resulting from RF interference are likely to affect a single frame,
and it requires less overhead to retransmit a smaller frame rather than a
larger one. As with RTS/CTS, users can generally set a maximum frame length
threshold whereby the radio NIC will activate fragmentation. If the frame
size is larger than the threshold, the radio NIC will break the packet into
multiple frames, with each frame no larger than the threshold value.
This tutorial is meant to provide an overview of the 802.11 MAC functions.
In future articles, we’ll discuss each function in more detail and show practical
configuration settings.
Jim Geier provides independent consulting
services to companies developing and deploying wireless network solutions.
He is the author of the book, Wireless
LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.
Don’t miss Jim Geier as one of the featured speakers at the 802.11 Planet Conference
and Expo next week. He’ll be giving a workshop on RF Site Survey Basics,
and speaking on panels discussing wireless data and home networking.
