Executive Summary

WiFi is the marketing name for the 802.11-b variant of the IEEE standard 802.11. 802.11 is a wireless data communication protocol standard covering both the Physical and Datalink networking leves (levels 1 and 2), with the letter giving variants, especially in the Physical layer. At the time of writing, a new variant, 802.11-g is widely deployed. It differs from the -b varient in the Physical layer, giving higher transmission rates. I'm not sure if WiFi will refer to this variant as well. In these notes I will say WiFi to avoid starting a sentences with a number.

The 802.11 specifications cover many Datalink variations: either be Infrastructure, using Access Points and briding across to other technologies, or Ad Hoc, where hosts discover each other and can talk if they are nearby. The contention for the channel (the radio frequency) can be either distributed using the Distributed Coordination Function, DCF, or centralized using the Point Coordination Function PCF. To support wide integation of WiFi into existing network infrastructure, it is designed to be, as much as possible, ethernet in the air, at least when used in the Infrastructure mode, and using the distributed contention model of DCF.

There are two problems that make wireless different then wired.

• The hidden substation problem.
• High error rate.

• Positive Acknowledgement. Every packet sent is positively acknowledged by the receiver. The next packet is not sent until receiving a positive acknowledgement for the previous packet.
• Channel clearning. A transmission begins with a RTS (Request to Send) and the destination or receiver responds with a CTS (Clear to Send). Then the data packets flow. For the channel is cleared by these two messages. All that hear the CTS squelch. This helps with the hidden substation problem.
• Channel reservation. Each packet has a NAV (Network Allocation Vector) containing a number X. The channel is reserved to the correspondents (the sender and receiver of this packet) for an additonal X milliseconds after this packet. Once you have the channel, you can hold it with the NAV. The last ACK contains NAV zero, to immediately release the channel.
• Packet numbering/Fragment numbering. Each packet has a sequence number with 4 bits of fragment number and 12 bits of sequence number. Packets with duplicate sequence or fragment can be discarded by the receiver. They resulted from a retransmission, and if the receiver already valid frame, it only needs to forward the frame once. A large frame can be divided into fragments which are sent and acknowledged sequentially. These will have the same sequence number but consecutive fragment numbers. The receiver must reassembly these before resend or bridgeing.

As for PCF, it is a polling, token-ring type communcation system. It isn't used for the most popular WiFi configuration.

Physical Characteristics

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.

The allocated frequence band for -b is 2.415 GHz up to 2.484 GHz in 14 bands of 5 MHz each. Spread Spectrum communication is used, called DS for Direct Sequence, where the signal is modulated with a 11 bit Barker sequence, a pseudo random noise, to spread the information over a 22 MHz range (with additional signal outside the range being partially suppressed). Therefore, only every 5-th channel is usable, in order to separate WiFi transmissions. For example, in a single location, use only channels 1, 6 and 11, else there will be interference.

FYI Complementary Code Keying (CCK) encodes the bits, and the bits are impressed on the carrier using a phase shifting technology called orthogonal frequency-division multiplexing (OFDM). The result is communication in the range of 6 up to 54 Mbps, depending on sunspots and the like.

Network topologies, bridging

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

• Independent BSS, or ad hoc. The network is only the members of the BSS, they talk between themselves directly, they self-organize, there is not central autority.
• Infrastructure BSS. The BSS is organized around an Access Point which can bridge traffic out the BSS onto a distribution network. Members of the BSS talk to the AP only. You can often understand a domain by answering the question "who will hear a broadcast". A BSS (data link layer) is defined by who will hear a broadcast from the AP (but not by a station which is not an AP, because of the hidden station problem!).
• ESS (Extended Service Set). A bunch of BSS's connected by a distribution network. The distribution network connects the Access Points. WiFi doesn't specify the protocol that builds ESS's.

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

Definitions:

BSS

Basic Service Set. A bunch of machines forming a cell.

ESS

Extended Service Set. Using WiFi beyond a BSS, gluing together several BSS

BSSID

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.

SSID

Service set Identifier. An character string identifier for a ESS.

NAV

Network Access Vector. A time slot reservation, in microseconds.

RTS/CTS

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.