Lecture 8, Wireless LANs

Categorized according to the transmission technique that is used. All current wireless LANs fall into one of the following categories:
- Infrared (IR) LAN - an individual cell of an IR LAN is limited to a single room, since infrared light does not penetrate opaque walls
- Spread spectrum LANs - make use of spread spectrum technology. In most cases these LANs operate in the ISM (institutional, scientific, medical) bands, so no FCC licensing is required
- Narrowband microwave - operate in microwave frequencies but do no use spread spectrum

Common in most homes - used for remote control devices
Two competing transmission media for wireless LANs: microwave radio, using either spread spectrum or narrowband transmission, and infrared
Advantages of IR:
- unlimited spectrum (high data rate)
- unregulated world-wide
- could use various light-colored objects for reflection (wide coverage)
- does not penetrate through walls (secure, separate installations can operate in every room)
- inexpensive simple equipment
- susceptible to ambient radiation (e.g., sunlight and indoor lighting)
Radio allows roaming, propagate through walls - more popular

Three technologies used for IR data transmission:

Direct beam infrared
- create point-to-point links
- focuses IR data link can have a range of kilometers - cross building interconnection
- Can set up to operate in ring configuration
Omnidirectional
- Single base station within a line of sight of all other stations on the LAN (repeater)
- Ceiling transmitter broadcasts an omnidirectional signal that can be received by all of the other IR transceivers in the area
- Transceivers transmit a directional beam aimed at the ceiling base unit
Diffused
- All IR transmitters are focused and aimed at a point on a diffusely reflecting ceiling
- IR radiation strikes the ceiling, reradiates omnidirectionally and picked up by all of the receivers in the area
- Ceiling station (base) is wired to a service

Most popular type of wireless LAN
Idea: spread the information signal over a wider bandwidth in order to make jamming and interruption more difficult
There are two types - frequency hopping and direct-sequence spread spectrum
Techniques developed by military in 1970 to help secure transmission

Signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at split-second intervals
Receives hops between frequencies in sync with the transmitter

Each bit of the original signal is represented by multiple bits in the transmitting signal, known as chipping code
Chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used. 10-bit chipping code spreads the signal across a frequency band that is 10 times greater then a 1-bit chipping code. 10 bits is FCC regulation.
Combine the digital information stream with the pseudorandom bit streaming using an exclusive OR

Narrowband microwave refers to the use of a microwave radio frequency band for signal transmission

Microwave frequencies usable for voice, data and video are licensed and coordinated within specific geographic areas to avoid interference between systems
Motorola holds 600 licenses (1200 frequencies) in the 18 GHz range that cover all metropolitan areas with population > 30,000
Non-overlapping frequency bands
Guarantees interference free communication

Industrial/Scientific/Medical spectrum
no government license needed
world-wide band 2.400 - 2.484 GHz
in US and Canada 902-928 MHz and 5.725-5.850 GHz these bands are used for cordless telephones, garage door openers, hi-fi speakers
The 900 MHz band works best but is crowded and equipment using it only works in N. America.

802.11
ESS consists of two or more basic service sets interconnected by a distributed system
Three types of stations based on mobility
- No transition - stationary or moves within the range of the communicating stations
- BSS transition - moves from one BSS to another BSS within a single ESS. Need addressing capabilities to be able to recognize the new location of the station
- ESS transition - movement from a BSS in one ESS to a BSS within another ESS

Services needed to provide functionality equivalent to wired LANs

Association - establishes an initial association between a station and an access point (before any transmission)
Reassociation - enables an established association to be transferred from one access point to another, allowing a mobile station to move from one BSS to another
Disassociation - a notification from either a station or an access point that an existing association is terminated. Before leaving BSS or shutdown
Authentication - establishes identity of a station.
Privacy - used to prevent the contents of messages from being read by the than the intended recipient.

In addition 802.11 includes the following:

Three physical medium defined in 802.11

Infrared defined at 1 and 2 Mbps operating at wavelength 850-950 mm
Direct sequence spread spectrum operating in 2.4 GHz ISM band. Up to seven channels each with 1 or 2 Mbps data rate
Frequency hopping spread spectrum operating in 2.4 GHz ISM

802.11 supports operation in two separate modes, a distributed coordination DCF and a centralized point-coordination mode PCF
802.11 MAC is called Distributed Foundation Wireless MAC (DFWMAC), access mechanism based on CSMA/CA (collision avoidance)
Stations continuously monitor the medium
A station that is ready to transmit will sense the medium
If the medium is busy, the transmitter starts a contention procedure
First the transmitter waits for a predetermined time period, known as the DCF Interframe Space (DIFS) length
Then , based upon a random calculation, the transmitter picks a future time slot from the next 31 slots (contention window)
The transmitter will send the frame in that timeslot unless another station starts a transmission
If there was a transmission, the station will freeze a counter of the number of slots left until the picked slot and will continue decrementing the counter after another station finishes its transmission
Backoff used
Collisions can occur only when two or more stations select the same time slot to transmits