Page 31 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 2: Wireless System Architecture

If you have two devices with wireless network adapters- whether wireless enabled PDA’s, Notes
laptops or desktop computers- you can technically connect them in an ad hoc wireless network.
Essentially, any devices enabled with wireless network adapters within range of each other can
all communicate with each other over such an ad hoc network.
If you want to build a more traditional network- where multiple clients connect to a central point
which acts as a gateway to the rest of the world- you will also need a wireless router or wireless
access point to implement an infrastructure mode network.
A wireless router has a physical network connection which would be connected to a wired
network. It could be connected to a hub or switch on a network or for home users would
generally be connected to your cable or DSL modem. All of the wireless-enabled clients would
then connect to the wireless router. Typically the wireless router will act as the DNS server and
Internet gateway for the clients attached to it and it can also be configured to provide IP addresses
automatically using DHCP.
A wireless access point (AP) can be used to join wireless devices to a wired network, or to extend
the range of a wireless network. They don’t provide the DNS, DHCP, firewall or other functions
commonly found in wireless routers. They simply take a wired or wireless network input and
relay it to the wireless devices within its broadcast range.

2.1.2 Wireless System Networking Routers

The process of developing a wireless network becomes complex when you are deploying
multiple APs throughout a multi-story building, because in addition to your neighbors Access
Points (APs) you need to worry about location, channel selection, and interference from your old
APs with each other.

Selecting Channels

You have only three channels to work with in North America when dealing with 2.4 GHz
networking. Some people have suggested that you can get away setting up your channels in a
four-channel manner by using channels 1, 4, 8, and 11. This setup would give you overlap in the
shoulder area of the bands, but the overlap would be fairly small.
Cisco has done lab testing of the concept using four APs and four clients all transferring 50MB
files to and from a server. Even with this limited overlap, data ends up being lost, and stations or
computers wait their mandatory wait periods and retransmit data.
These problems slow down the overall throughput. The following table shows the final results.
So if you are told that you can use four channels when making your channel selections, this is
not advisable and you should stick to the reliable and time tested three channel configuration.
The simplest wireless network contains only one AP and that the issues that you need to deal
with for one AP are generally placement and signal loss. A centrally located AP in an office with
a few obstructions that may cause signal loss in the unshaded areas.

Notes In addition to the obstructions and construction material, there may be various
sources of external interference that can reduce the size of the coverage area.

In addition to the obstructions and construction material, there may be various sources of external
interference that can reduce the size of the coverage area.

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