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.
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,
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
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.
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.
- 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
- 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
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
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.
WiFi is standard 802.11, with various letters added.
The standard includes a large number of physical variants.
The link levels can either be an Independent BSS (IBSS) or an
An infra. BSS can be contention based or coordinated (Point
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.
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.
Gratuitous ARPs open up switches to L2 forward traffic to
There are mobility issues, which are not part of the standard.
It is a positive acknowledge system. Each packet is positively
acknowledge (an ACK packet) before the next packet is sent.
Packets carry NAV's which hold the channel clear for the
time stated in the NAV.
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.
There are not NACKs.