Wireless LANs

802.11b Wireless LANs

Infrastructure



There are two basic infrastructure models for wireless set-ups. The first is what is known as an ad-hoc network (pictured to the left). Every station has a wireless connection to provide a self contained network.





The second type of infrastructure involves what is known as an "Access Point" to provide connectivity to an existing distribution network. An access point is typically a simple bridge between the wireless "Basic Servic Sets (BSS)" and the wired network (labeled "Distribution System" in the picture to the right).

Each BSS (the blue circles in the picture to the right) acts as a single ethernet collision domain. A BSS can overlap with another BSS to provide a greater coverage area. Each BSS can be on it's own channel (see below) to provide separate collision domains.

Technology

The 802.11 spec uses spread spectrum techologies (which were invented in World War II to make it tougher for the enemy to jam and intercept friendly signals). There are two type of spread spectrum technologies - Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS).

Before 802.11, most of the wireless LAN products were DSSS and ran in the 902 - 928MHz range. However since that band wasn't available world-wide, the focus was shifted to the 2.4000 - 2.4835GHz range.

DSSS (used in 802.11b)
Direct Sequence Spread Spectrum takes the normal narrow-band transmission and spreads it over a larger range, thus preventing interference from blocking out the entire signal. 802.11 defines 11 separate channels. For North America, they start at 2412 MHz and are spaced 5 MHz apart. However, since channel bandwidth is about 20 MHz in use, only 3 channels can be used at the same time (refer to drawing on the right).

DSSS can support data rates of 1, 2, 5.5, and 11 Mbps. (All 11 Mbps 802.11 devices are DSSS). Most products will automatically transmit at the highest data rate possible based on the node's current signal to noise ratio (SNR).

FHSS (used in 802.11)
Frequency Hopping Spread Spectrum works by jumping between various defined channels during a single transmission. In the United States, the FCC requires a minimum of 75 frequencies and a maximum dwell time on each channel of 400ms. The 802.11 spec defines 79 channels and 22 hop patterns.

802.11 FHSS only supports data rates of 1 or 2 Mbps.

"Hidden Node" Phenomenon
Normal 802.3 ethernet uses CSMA/CD (Carrier Sense Media Access/Collision Detection) to make sure that only one node is talking on the shared medium at a time. However, in a wireless network, it's possible that not all nodes are able to see each other, so reliance on CSMA/CD can result in a 40% or more disruption of communications. Say, for instance, Station A and Station B are within range of an Access Point, but not within range of each other.


When Station A is transmitting, Station B has no way of knowing this and may transmit at the same time, resulting in a collision and corrupted packets at the Access Point. 802.11 solves this by using CSMA/CA (Carrier Sense Media Access/Collision Avoidance). When Station A wants to transmit, it first sends a RTS (Request To Send) packet to the Access Point with information including how long it wants to transmit. The Access Point, upon receiving the RTS, transmits a CTS (Clear To Send) signal that is "heard" by all the stations that communication to it. Station B then knows to back off a specified time before trying to transmit it's own data.

Security
There are two types of security: authentication and encryption. Authentication is usually implemented in two fashions. Some vendors use the wireless network name as a primitive type of authentication - if your node's (wireless) network name doesn't match the access point's network name, you aren't permitted access. (However other vendors don't use network names as a "security" feature - you're able to connect to any access point, regardless of it's network name). The second type of authentication involves access lists based on the wireless NICs MAC (Media Access Control) address. This is the proper way to implement authentication security.

Encryption is handled through WEP (Wired Equivalent Privacy). It's a RC4 PRNG algorithm from RSA Data Security. Typically, vendors offer 40-bit and 128-bit encryption.

Roaming
Roaming is when your wireless node communicates to one access point, then as you move the node around, it seemlessly transfers communications to another access point. 802.11 only defines basic message formats to support roaming - everything else is left up to the vendors. However IAPP (Inter-Acces Point Protocol) was jointly developed by Aironet, Lucent, and Digital Ocean to provide interoperability between different vendors.