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How wireless devices can distinguish between signals

Now, the final question of this article is how devices in a wireless network are able to differentiate between different signals coming from other wireless devices like wireless routers, laptops and mobile phones.

MAC address

The MAC address or hardware address or physical address is a unique code that is assigned to each piece of network equipment, like network adapters and wireless access points. In network communications (wireless or cabled) the network packets that are sent from any device contain the MAC address of the sender and of the MAC address of the intended receiver. This is how any device in the network can decide whether a certain packet is addressed to that device or not.

In Windows 7, you can see the MAC address of your own network card by following these steps.

  1. Go to the start menu and type cmdin the search programs and files search box (in Windows XP you must first select Run... from the Start menu).
  2. A command line window will appear. Type the command ipconfig /all in the command line.

You will now see a list of network devices in your computer.

wireless network card hardware address

The MAC address of my wireless network card.

Your wireless network card will see packets from other networks, but it will discard the packets that were not intended for it.

SSID, the service set identifier

When a wireless device connects to a wireless access point it does so by sending out an association request. This request contains the SSID, which is network's public name.

ssid list

The list of available wireless network SSIDs in Windows.

When a connection is established the client (your laptop, pc, smart phone or tablet) and the access point maintain the connection by exchanging management packets. These management packets contain the SSID and more information about the network.

Your device therefore knows to which network it is connected and to which access point it should "listen" for packets.

Frequency bands

Wireless access points can operate in different frequency bands, also called channels.

wireless channels

Channel selection in the Linksys WRT54G wireless router.

 

When a client connects to a wireless access point in a certain channel it does see packets from other networks in that channel, but it discards them, because they originate from an access point with an SSID and MAC address that doesn't belong to the session that the client has with the access point is is connected with.