Although 802.11 can be configured as an Ad Hoc peer-to-peer network consisting
of only a few nodes, the primary business application will utilize WLANs as an extension
of the traditional wired LAN, using a concentrator called an Access Point (AP).
A traditional wired distribution system brings network access to each WLAN in a
network, terminating in Access Points (APs). The APs bridge the wired network
with the WLANs and act as the concentrator for WLAN traffic. End devices such
as laptops or workstations are outfitted with standard PCMCIA or PCI expansion
cards to provide access to the WLAN.
This configuration, if properly designed, will allow users to roam in-between
APs, and allow IT managers to load balance between overlapping AP coverage
areas on-the-fly based on traffic or noise, and grow their network easily and
at minimal cost. Best of all, all of these advantages are gained transparently
to the user.
Another configuration option that can be employed by IT managers is the Point
Co-ordination Function (PCF). Normally the WLAN will function like an Ethernet
in that intelligence is distributed among each device, this is called
Distributed Co-ordination Function (DCF). However, for time-constrained
applications that need a deterministic delivery system, PCF can be employed and
the WLAN will function more like a Token Ring, where intelligence will come
from one device who polls each station to determine if they are allowed to
transmit / receive data. This guarantees a maximum latency.
Most WLAN devices are manageable through a Telnet, SNMP or web browser
interface, or some combination of the three. In addition to traffic statistics,
the access point also includes management features such as mapping of access
points and their associated clients, and controlling access and traffic flow.
802.11b Brief Technical Summary
The 802.11b standard encompasses the physical and Data Link layers of the OSI
model. It uses the 802.2 LLC and 802.3 MAC addressing system, using the same
48-bit addressing scheme. This makes interoperability with a pre-existing
Ethernet network extremely convenient.
As far as the actual media access mechanism, 802.11b again uses a very similar
method to CSMA/CD, in fact it's just a modified version of this protocol to
account for the different medium. Carrier Sense Multiple Access / Collision
Avoidance (CSMA/CA) follows this algorithm: Listen to the medium for a free moment to
transmit, when it's clear wait for a random time interval, and then if it's
still clear transmit. Since the stations cannot listen for a collision while
transmitting, the receiving station sends an explicit ACK for each frame
received. This adds to the overall overhead but makes the network necessarily
robust for wireless use.
The physical layer uses a technique called Direct Sequence Spread Spectrum
and operates in the unlicensed 2.4GHz radio band. Quadrature Phase-Shift
Keying (QPSK) is used to squeeze 8bits into every symbol (with a base signaling
rate of 1.375 MSps (Million Symbols per Second) that makes 8 * 1.375 = 11Mbps).