WiFi Basics

WiFi is the marketing name for IEEE standard 802.11. It is a standard for both Level 1 (physical) and Level 2 (data link) of a wireless data transmission protocol.

802.11 defines many Level 1 variants. 802.11g is the new, high-speed Level 1 standard, versus 802.11b, the first generation WiFi. The radio frequency band is around 3 Gigahertz, same as a microwave oven. As you can see in your WiFi setup, there are channels within the band. These channels overlap so that at most 3 channels can be in use at the same time.

WiFi Concepts

  • There are two general types of WiFi transmission: DCF (Distributed Coordination Function) and PCF (Point Coordination Function). DCF is ethernet in the air. It employs a very similar packet structure, and many of the same concepts. There are two problems that make wireless different then wired. These problems demand that a DCF WiFI be a CSMA/CA network (Collision Avoidance) rather than a CSMA/CD network (Collision Detect). The result are the following protocol elements,

    As for PCF, it is a polling, token-ring type communcation system. We shall skip the details. It isn't much used.

    Network topologies, bridging

    A group of corresponding stations is called a BSS (Basic Service Set). The BSS can be organized in several ways.

    We will skip ad hoc and concentrate on Infrastructure.

    Since this is ethernet on the air, each transmitter/receiver has a 48 bit MAC consistent with the ethernet address. That is, same address space, OUI's, and so on. An AP is a bridge between wired and unwired ethernet, so it has two interfaces. As the leader of a BSS, it gives names the BSS by the ethernet address of its air interface. This is called the BSSID.

    An ESS is given a name, called the SSID (Service Set ID). This is the thing you type into your network configuration to join a WiFi network.

    A packet on the air will have three addresses, source, destination and BSSID (access point address, essentially). The AP takes traffic it receives off the air that has its address and drops it onto its wired interface, eliting its own address. That is, on the wired side, only the source and destination addresses are seen. The address of the AP is not used, either its wired or unwired addresses.

    When an AP sends a packet into the air, it uses the source and destination address of the packet it is bridging as found, and adds its own wireless address as the BSSID. A wired station sending to a wireless station uses the wireless stations's ethernet address just as if it were a wired station. The AP picks the packet off the wire, carries it across to its wireless interface, inserting its wireless address as the extra, third address, and sends it out to the destination.

    Association and so on


    Basic Service Set. A bunch of machines forming a cell.
    Extended Service Set. Using WiFi beyond a BSS, gluing together several BSS
    A 48 bit identifier for a BSS. If an infrastructure BSS, it is the MAC of the 802.11 side of the Acess Point. Else the local bit is set and a 48-bit identifier is randomly selected.
    Service set Identifier. An character string identifier for a ESS.
    Network Access Vector. A time slot reservation, in microseconds.
    Request To Send, Clear To Send. Reservation mechanism. Source,

    Quick description

    1. WiFi is standard 802.11, with various letters added. The standard includes a large number of physical variants.
    2. The link levels can either be an Independent BSS (IBSS) or an infrastructure BSS. An infra. BSS can be contention based or coordinated (Point Coordination Function).
    3. Infrastructure BSS uses AP (access points) and a distribution medium, e.g. ethernet (802.2), either the AP acting as bridges. In a simple example, the packet has three addresses, the two "transparent" endpoints and the BBSID, which is the way-point for the packet between air and wire.
    4. In an infra. BSS, the AP either Beacons or responds to a probe from a node. An association (after possible authentication) occurs, so that traffic from the host is bridged onto the distribution by the AP.
    5. Gratuitous ARPs open up switches to L2 forward traffic to associated AP.
    6. There are mobility issues, which are not part of the standard.
    7. It is a positive acknowledge system. Each packet is positively acknowledge (an ACK packet) before the next packet is sent.
    8. Packets carry NAV's which hold the channel clear for the time stated in the NAV.
    9. RTS/CTS pair with NAV covering up to the end of the ACK of the data packet are exchanged. The ACK has a NAV of 0.
    10. There are not NACKs.