Wi-Fi (802.11abgn) - 3

In last articles we had an introduction to Wi-Fi network and Channels. Refer

Introduction to Wi-Fi

Wi-Fi Channels

Let us start digging deep the BSS.

Address:

Yes, the addresses are the most important factors in any form of communication. It will sound indeed ugly if we are sending a telegram or post without an address is it not. In 802.11 four address are defined, and they are

a. Destination Address - This address is the MAC of the device which is the intended receptor of the packet.
  
    Receiver Address - This is the address of the immediate next station, if the data packet needs to pass through multiple nodes to reach the actual destination.

b. Source Address - It is the MAC of the device which is transmitting the packet.
  
    Transmitter Address - This is the address of the node which actually transmitted the packet to the air. This is the case similar to receiver address, when the data packet needs to pass through multiple nodes to reach the actual destination. In this scenario, the source address of the packet will also be changed no when it passes through the intermediate person.

c. BSS Address - This is to identify to which BSS the packet actually belongs to. This is basically for filtering out the unwanted packets. We will discuss this further on this very shortly.

d. Actual Source Address - This address is optional and we will discuss this when we proceed for Wireless Distribution System (WDS). We will still dig this when we move for Ad-hoc network or IBSS. For now we will have this alone in our mind: "This is an optional address which will have the MAC ID of the actual Source which have sent the packet to the Air".

All these addresses are 6 byte size and they are unique. No two devices will have same physical address. We will see further on this when we proceed for 802.11 MAC header.

Beacon:

This will serve almost like a backbone for the BSS. As we saw earlier, Every BSS will have an infrastructure element which can be either routers, access points or gateways. All of these devices have similar implementations on the layer 2 and 1.

Beacons are management frames which delivers the complete information about the infrastructure element. The major elements are

a. SSID - It is the name of the service set, it can be any alphanumeric string and some device manufacturers may be having some restrictions.
b. Channel - The channel in which the service set is operating.
c. Security and authentication scheme related parameters.
d. Data rates supported
e. Power save related informations for station.
f. Quality of service parameters for stations.
e. Informations related to 11n, 11g, 11b standards.
g. Several vendor specific informations.

and lot more.

Every devices associated to the infrastructure element should listen for the beacons periodically. The infrastructure will be transmitting beacons to the air periodically with the interval specified by "Beacon Interval" which is usually 100 ms and can be increased or decreased.

In the next article we will take a sample beacon frame and try to understand how the protocols are implemented.