Wireless System Configuration
When deploying a wireless LAN, determine which configuration makes most sense to install. The
802.11 standard refers to two different wireless system configurations, infrastructure mode and ad
hoc mode, which vary in terms of usage and operation. This section explores each configuration.
Table G-2
802.11 Standards Comparison
Standard
RF
Spectrum
Max
Speed Compatibility
RF Interference
Impacts
Date
Ratified
802.11a
5 GHz 54 Mbps Does not work with
802.11b or 802.11g
Slight 1999
802.11b
2.4 GHz 11 Mbps Works with 802.11g Moderate 1999
802.11g
2.4 GHz 54 Mbps Works with 802.11b Moderate 2004
792
Appendix G: IEEE 802.11 Fundamentals
Infrastructure Mode Configuration
An
infrastructure wireless LAN
(sometimes referred to as
infrastructure mode
) is what most
companies, public hotspots, and home users implement. An infrastructure wireless LAN, as
depicted in Figure G-1, offers a means to extend a wired network. In this configuration, one or
more access points interface wireless mobile devices to the distribution system. Each access point
forms a radio cell, also called a
basic service set (BSS)
, which enables wireless users located
within the cell to have connectivity to the access point. This allows users to communicate with
other wireless users and with servers and network applications connecting to the distribution
system. A company, for example, might use this configuration to enable employees to wirelessly
access corporate applications and the Internet from anywhere within the facility.
Figure G-1
Infrastructure Wireless LAN
Each access point in the infrastructure wireless LAN creates a radio cell, with a coverage area that
depends on the following variables:
■
The construction of the facility
■
The physical layer
■
Transmit power
■
Antenna type
The range of the coverage area is typically 150 feet in most enterprise facilities. The desired level
of performance, however, can impact the effective range of the access points. Chapter 23,
“Wireless LAN Solutions,” covers this in more detail.
With the infrastructure configuration, data traffic going from one wireless user to another user must
travel through the access point. The access point switches the data traffic going from user A to user B,
Access Points
Access Points
Access Points
Ethernet Switch
Distribution
System
Wireless LAN
Radio Signals Ethernet Cable
Wireless System Configuration
793
for example, and retransmits the data to user B. In infrastructure mode, data transmissions do not
occur directly between the wireless users. As a result, significant data traffic between wireless users
decreases throughput because the access point must relay the data to the destination user. In this
application, you can think of the access point as a Layer 2 switch. It provides access to a common
medium that two hosts can use to communicate with each other, but there is no direct connection
between the hosts. If the source wireless user is sending data to a node on the distribution system,
then the access point does not need to retransmit the data to other wireless users.
Figure G-2 illustrates three different 802.11 radio cell configurations, which include partial
overlap, collocated, and disjointed cells. If the company installs access points with overlapping
radio cells, as shown in part A of Figure G-2, then users are able to roam throughout the facility.
The radio card within the user’s mobile device will automatically reassociate with access points
having stronger signals. For example, a user might begin downloading a file when associated with
access point A. As the user walks out of range of access point A and within range of access point
B, the wireless LAN automatically reassociates the user to access point B and the user’s file
download continues through access point B. The user generally does not experience any
noticeable delay, but voice-over-WLAN phones might drop connections if the roaming delay
exceeds 100 milliseconds.
Figure G-2
Various 802.11 Radio Cell Configurations
The 802.11 standard also supports collocated (part B of Figure G-2) and disjointed (part C) radio
cells. A company might install disjointed access points if complete coverage throughout the facility
is not necessary. For example, the company might install an access point in each conference room
but not in the rest of the building. If the radio cells are disjointed, then users will temporarily lose the
network connection and then reassociate when coming within range of another access point. An
802.11 network, though, supports this form of network in a manner similar to a wireless network that
supports roaming with the overlapping radio cells. The reassociation delay is a function of the time
it takes the user to move into range of the next access point. The wireless application in use, however,
might or might not be able to tolerate this longer roaming delay.
BSS 1 BSS 2
BSS 1 BSS 2
A – Partial Overlap
C – Disjointed
B – Collocated
BSS 1 BSS 2
794
Appendix G: IEEE 802.11 Fundamentals
The co-located radio cell configuration is useful if a company needs greater capacity than what a
single access point can deliver. In this scenario, two or more access points are set up so that their
radio cells overlap significantly. This works well, assuming that the access points are set to
nonconflicting radio channels. A portion of the users in the area, for example, associate with access
point A, and the other users associate with access point B. This boosts the wireless LAN capacity
in that particular area. For example, with 802.11g, you could install up to three co-located cells,
each using a non-overlapping channel, to have three times 54 Mbps of available bandwidth.