MAC Layers

The media access control (MAC) layer is powerful, with enough features
to support sequence control as well as Retry fields that support “MAC
layer acknowledge,” which reduces interference and increases the usage
of available bandwidth on a given wireless channel.
In order to ensure reliable communications when other stations are
present, you need the following MAC fields:


Type

Subtype

Duration

WEP (wired equivalent privacy)

Sequence control

Frag

WEP fields permit data security that is analogous (in some respects)
to the physical security characteristics of a wired Ethernet. Both
sequence controls and Frag fields deal with “fragmentation,” which permits
a WLAN to function in tandem with devices that cause signal fading
or interference patterns.
MAC works very easily with normal wired Ethernetworks in combination
with either an access point or a wireless router. The idea is to
make certain that wired and wireless nodes on your LAN can function
seamlessly with each other.

CSMA
WLANs use a standard referred to as carrier sense multiple access with
collision avoidance (CSMA/CA) as a MAC method. However, normal
Ethernetworks use a carrier sense multiple access with collision detection
(CSMA/CD) method.

Roaming
Regardless of what equipment you use, 802.11 permits a wireless client
to roam across multiple access points (Figure 10.2). These access points
can function on either the same or different channels. After a certain
interval has elapsed, an access point may transmit a beacon signal (with
time stamp) to execute the following tasks:


Synchronize wireless client

Indicate supported data rates

Indicate other parameters

Provide a traffic indication map

When a client roams, it uses the transmitted beacon to determine the
strength of its existing connection to the access point. Should the connection
appear to be weak, then the roaming station can try to link up
with another access point to sustain its connection to the network.