WLAN basics

The “Must Knows”

Presented by Keith Charles.

Corporate Trainer.

Lesson One

What is a WLAN?

A WLAN is a network where clients connect to the network using the atmosphere (RF) as the transmission media.

Define transmission media.

In simple terms, the transmission media is the path along which data travels. Unlike in a wired network where we use wire or “bounded” transmission media, the WLAN client receives and transmits data across the atmosphere.

The client.

Similar to wired environments, the client is a receiver of services. A WLAN client is sometimes described as a mobile client.

When you plug in your internet cable or hardwire a PC for internet access you are creating a wired interface. The interface must be present in your client for wireless connectivity.e.g. centrino or a radio card (antenna).

Most users are aware of Linksys; Netgear; Apple etc.

WLAN transmissions.

Today most laptops are sold with wireless capability, meaning that they are equipped with the interface, a radio, that allows them to send and receive data through the air.

Question: how do you think your laptop finds your neighbor’s network as soon as it is powered up?

Answer: They scan or search. More on this later.

WLAN designs.

Two very basic designs are available:-

1. peer to peer OR adhoc. In this design also called an independent basic service set (IBSS), there is no central provider of network services i.e. an access point or AP.

WLAN Designs

2. The infrastructure network in which there is at least 1 access point (AP) which acts like a server in a wired network.

Therefore in an infrastructure design you have an access point with at least one client. In the case where there is 1 AP it is called a Basic Service Set or BSS.

IBSS

EBSS

WLAN Design

3. In an infrastructure design where there is more than one AP you now have an Extended Basic Service Set.

Each AP has a coverage area or cell or RF footprint. The reception degrades as you move out of the cell. This is normally shown with bars at the right bottom of your PC.

Think Verizon, the ad “can you hear me now?”

The Access Point

An access point is a powerful little device. Remember it is connected to the wired network. That means it has to take data from the wired side and “convert” them to something transmittable on the air.

At this time let us remember the term “frame”.

An access point receives frames from the wired side and spits out frames on the wireless side.

IEEE 802.3 frames>>>>>>>>IEEE 802.11 frames. Ethernet frames>>>>>>>>>>WLAN frames.

Question.

If I have one AP I will therefore have one cell or footprint. 1.How many footprints/cells would I have with 2 APs ?

2. If I have two APs and I want to be able to have a non-interrupted connection when moving from cell to cell, what do you think I must do?

Note cell overlap

Scanning.

A client finds a network using a scan. A scan can be active or passive.

Think of the times when you heard about a search for a black box after a plane crash. That box sends out a beacon that can be picked up by the searcher.

Note:- beacons are also sent from client to client in a design with no AP. Can you remember the name of that design?

An access point sends out a similar beacon, more or less exclaiming “I am here”. In an active scan, the client sends out a probe looking for an AP.

The client sends a “probe request” and the AP may or may not send a “probe response”.

Scanning

A passive scan is not really directed, if you will. The client just “listens” for beacons on every channel.

Note: these explanations are very basic. There is a lot more but this is enough for building an understanding.

Finding An AP.

If you have ever set up a wireless network you will recall setting up an SSID. Or if you have never, you may recall the name of your neighbor’s wireless. That is the SSID or Service Set Identifier.

I’ve seen some funny SSIDs in my searches. This is basically a network name and is helpful for WLAN management. Some APs allow us to hide or not broadcast our SSIDs.

Note: this provides very little security, since the SSID can be found elsewhere. More on that later.

Notice SSIDs &signal strength

Terminology to remember

Client Access point Infrastructure mode Peer to peer IBSS Extended BSS Scanning Passive scan Active scan SSID Beacon Frame 802.11 802.3 Radio frequency

Looking Ahead….Lesson Two

Associate Reassociate Disassociate Authenticate Deauthenticate Probe Data rate Modulation Encoding

Info.

I am very much interested in continuing this once weekly lesson on wireless technology. If you are in favor of this please send your vote of support to

kc.ittrainer@gmail.com

Note: I propose to cover all topics. It will get tougher as we move

along so I intend to develop a few podcasts and video tutorials.

Questions and suggestions are welcomed!