CCNP BCMSN Notes - Wireless LAN Overview

Frame Transmission

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is used in 802.11 WLANs to avoid collisions.

The Distributed Coordination Function (DCF) handles the transmission of frames.

If one station is currently transmitting, a station wishing to transmit must wait for the current station to finish plus the length of the DCF Inter-frame Space (DIFS) and a random back-off timer before it may transmit.

Service Sets

Service Set Identifiers (SSIDs) are used to logically group related wireless clients.

Service set types:

            Independent Basic Service Set (IBSS) - An ad-hoc network where all clients communicate directly

            Basic Service Set (BSS) - Access is centralized on an access point

            Extended Service Set (ESS) - An access point bridged the wireless network to a wired network

An SSID can be mapped to a VLAN on an Ethernet network.

Radio Frequency

           2.4 GHz band = 2.412 - 2.484 GHz

           5 GHz band = 5.150 - 5.825 GHz

Types of interference:

           Reflection - Signal is reflected off an object

           Refraction - Bending of a signal as it passes through material of varying density

           Absorption - Signal strength weakens as it passes through an object

           Scattering - A signal is reflected in many different directions

           Diffraction - The bending of a signal around an object which partially blocks its path

           Fresnel zones - The elliptical sphere of space which must remain clear between two line-of-sight wireless transmitters to prevent diffraction

Measurements of signal strength:

           dB - Logarithmic ratio to a reference signal

           dBm - Reference to a 1.0 mW signal

          dBw - Reference to a 1.0 W signal

Receivers are generally rated in negative dBm, noting their sensitivity.

Antenna gain is expressed in dBi, referenced to a theoretical isotropic antenna which propagates a signal evenly in all directions.

Effective Isotropic Radiated Power (EIRP) = Tx power (dBm) + antenna gain (dBi) - cable loss (dB).

WLAN Antenna Categories

          Omnidirectional - Distributes signal in a donut-shaped area; low gain

          Semi-Directional - Produces an elongated but broad coverage area in one direction, includes Yagi antennas; medium gain

          Directional - Directs signal to a single point; high gain

WLAN Standards

Frame Types

         Management Frames - Used for service advertisement and membership management

                  Beacons

                  Client association

                  Client authentication

         Control Frames - Control traffic flow

                  Probe request/response

                  RTS/CTS messages

         Data Frames - Contain data payload

WLAN frames have a 32-byte header and 4-byte trailing checksum.

802.11b

Operates on the 14 channels within the 2.4GHz Industrial, Scientific, Medical (ISM) band.

Only channels 1, 6, and 11 are non-overlapping.

Direct Sequence Spread Spectrum (DSSS) modulation allows for varying speeds: 1.0, 2.0, 5.5, and 11.0 Mbps. Higher data rates require stronger signal strength.

DSSS speeds can be mixed among clients within an AP cell, allowing each client to transmit at its fastest potential.

802.11g

Expands upon 802.11b with greater speeds and more complex modulation.

802.11g operates on the same frequencies and channels as 802.11b.

Orthogonal Frequency Division Multiplexing (OFDM) allows for additional speeds of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.

802.11g is backward compatible with 802.11b, but if an 802.11b client joins an 802.11g cell, all clients must fall back to 802.11b.

802.11a

Shares the same data rates and modulation techniques as 802.11g, but is not compatible with it or 802.11b.

Operates on the 5 GHz Unlicensed National Information Infrastructure (U-NII) band.

The U-NII was divided by the FCC into three smaller bands:

         Lower band - 5.5 to 5.25 GHz; indoor use

         Middle band - 5.25 - 5.35 GHz; indoor and outdoor use

         Upper band - 5.725 - 5.825 GHz; outdoor use

Four non-overlapping channels are offered within each band (12 total).

Other Standards

         802.11e - QoS for WLANs

         802.11i - Security enhancements

         802.11n - Improvements for higher throughput