Wireless LAN Technology

Class Index

Wireless LAN’sWireless LAN applicationsNomadic Access and Ad Hoc NetworkingWireless LAN requirementsWireless LAN Technology

Infrared LAN’sStrengths & Weaknesses

Class Index

Transmission Techniques Spread Spectrum LAN’s

ConfigurationTransmission Issues

Narrowband Microwave LAN’sLicensed Narrowband RFUnlicensed Narrowband RF

Wireless LANs

Wireless LANs are an indispensable adjunct to traditional wired LANs,

They satisfy requirements for: mobility, relocation, ad hoc networking, and coverage of locations difficult to wire.

Wireless LAN applications

Early wireless LAN products, were marketed as substitutes for traditional wired LANs

In a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN.

Wireless LAN applications

Buildings with large open areas (manufacturing plants, stock exchange trading floors, warehouses).

Historical buildings with insufficient twisted pair and where drilling holes for new wiring is prohibited.

Small offices where installation and maintenance of wired LANs are not economical.

Wireless LAN applications

In all of these cases, a wireless LAN provides an effective and more attractive alternative.

In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations.

Typical Wireless LAN Configuration – Single Cell Wireless LAN

There is a backbone wired LAN, such as Ethernet, that supports servers, workstations and one or more bridges or routers to link with other networks.

There is a control module (CM) that acts as an interface to a wireless LAN

Typical Wireless LAN Configuration – Single Cell Wireless LAN

Some of the end systems are standalone devices, such as a workstation or a server.

Hubs or other user modules (UMs) that control a number of stations off a wired LAN may also be part of the wireless LAN configuration.

Typical Wireless LAN Configuration – Multiple Cell Wireless LAN

Nomadic Access and Ad Hoc Network Nomadic access provides a wireless link

between a LAN hub and a mobile data terminal equipped with an antenna.

Users may move around with their portable computers and may wish access to the servers on a wired LAN from various locations.

An ad hoc network is a peer-to-peer network (not centralized server) set up temporarily to need some immediate need.

Nomadic Access and Ad Hoc Network

Nomadic Access Ad Hoc Network

Wireless LAN requirements

Wireless LAN must meet the same sort of requirements typical of any LAN, including:High capacity. Ability to cover short distances.Full connectivity among attached stations.Broadcast capacity

Wireless LAN specific requirements

Throughput: The MAC protocol should make as efficient use as possible of the wireless medium to maximize capacity.

Number of nodes: Wireless LANs may need to support hundreds of nodes across multiple cells.

Wireless LAN specific requirements

Connection to backbone LAN: In most cases, interconnection with stations on a wired backbone LAN is required.

Service area: A typical coverage area for a wireless LAN has a diameter of 100 to 300 m.

Battery power consumption: Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. Typical wireless LAN implementations have features to reduce

power consumption while not using the network, such as a sleep mode.

Wireless LAN specific requirements

Transmission robustness and security: Unless properly designed, a wireless LAN may be interference prone and easily eavesdropped. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.

Wireless LAN specific requirements

Collocated network operations: As wireless LANs become more popular, it is quite likely for two or more wireless LANs to operate in the same area or in some area where interference between the LANs is possible.

License-free operation: Users would prefer to buy an operate wireless LAN products without having to secure a license for the frequency band used by the LAN.

Wireless LAN specific requirements

Handoff/roaming: The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.

Dynamic configurations: The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion and relocation of end systems without disruption to other users.

Wireless LAN – Mobile Data Network – Wired LAN comparison

Kiviat Graphs provides a pictorial means of comparing systems among multiple variables.

Wireless LAN Technology Wireless LANs are generally categorized

according to the transmission technique that is used.

All current wireless LAN products fall into one of the following categories: Infrared (IR) LANs Spread spectrum LANs Narrowband microwave LANs

Wireless LAN Technologies – Key Characteristics

Infrared Spread Spectrum Radio

DiffusedInfrared

Direct BeamInfrared

FrequencyHopping

Direct Sequence NarrowbandMicrowave

Data Rate (Mbps)

1 to 4 1 to 10 1 to 3 2 to 20 10 to 20

Range (m) 15 to 60 25 30 to 100 30 to 250 10 to 40

Mobility Stationary/Mobile

StationaryWith LOS

Mobile Stationary / Mobile

Detectability Negligible Little Some

ModulationTechnique

ASK FSK QPSK FS/QPSK

RadiatedPower

- < 1 W 25 mW

Access Method

CSMA[1] Token Ring,CSMA

CSMA ReservationALOHA,CSMA

LicenseRequired

No No Yes unless ISM

WavelengthFrequency

: 800 to 900 nm(8000 to 9000 Å)

902 to 928 MHZ2.4 to 2.4835 GHZ5.725 to 5.85 GHz

902 to 928 MHz5.2 to 5.775 GHz

18.825 to 19.205 GHz

[1] CSMA: Carrier Sense Multiple Access. Check IEEE 802.11 Class 11

Infrared LANs – Strengths

The two competing transmission media for wireless LANs are microwave radio, and infrared (IR).

Infrared LANs – Strengths

IR offer a number of significant advantages over the microwave radio approaches: The spectrum of IR is virtually unlimited, which

presents the possibility of achieving extremely high data rates.

The IR spectrum is unregulated worldwide, which is not true of some portions of the microwave spectrum.

IR shares some properties of visible light that make it attractive for certain types of LAN configurations.

Infrared LANs – Strengths

IR light is diffusely reflected by light-coloured objects; thus it is possible to use ceiling reflections to achieve coverage of an entire room.

IR light does not penetrate walls or other opaque objects. This has two advantages: IR communications can be more easily secured

against eavesdropping than microwave A separate IR installation can be operated in every

room in buildings without interference, enabling the construction of very large IR LANs.

Infrared LANs – Strengths

Equipment is relatively inexpensive and simple. (IR data transmissions usually use intensity modulation).

Infrared LANs - Weaknesses

Many indoor environments experience rather intense IR background radiation, from sunlight and indoor lighting.

This ambient radiation appears as noise in an IR receiver, requiring the use of transmitters of higher power than would otherwise be required and also limiting the range.

However, increases in transmitter power are limited by concerns of eye safety and excessive power consumption.

IR – Transmission Techniques

There are three alternative transmission techniques commonly used for IR data transmissions:Direct beam IROmnidirectionalDiffused

IR – Transmission Techniques

Directed-beam IR can be used to create point-to-point links.

In this mode, the range depends on the emitted power and on the degree of focusing.

A focused IR data link can have a range of kilometres.

An IR link can be used for cross-building interconnect between bridges or routers located in buildings within a LOS of each other

Token Ring LAN using Point-to-Point IR link

IR – Transmission Techniques

An omnidirectional configuration involves a single BS that is within LOS of all other stations on the LAN.

Typically, this station is mounted on the ceiling. And acts as a multi-port repeater.

The ceiling transmitter broadcasts an omnidirectional signal that can be received by all of the other IR transceivers in the area.

These other transceivers transmit a directional beam aimed at the ceiling base unit.

Omnidirectional IR Transmission using LOS

IR – Transmission Techniques

In a diffused network, all of the IR transmitters are focused and aimed at a point and a diffusely reflecting ceiling.

IR radiation striking the ceiling is reradiated omni-directionally and picked up by all of the receivers in the area.

Omnidirectional IR Transmission - Diffuse Networks

Spread Spectrum LANs

A SS wireless LAN makes use of a multiple cell arrangement.

Adjacent cells make use of different centre frequencies within the same band to avoid interference.

Spread Spectrum LANs

Within a given cell, the topology can be either hub or peer to peer.

In the former, the hub is typically mounted on the ceiling and connected to a backbone wired LAN to provide connectivity to stations attached to the wired LAN and to stations that are part of wireless LANs in other cells.

A peer to peer topology is one in which there is no hub. A MAC algorithm such as CSMA is used to control access. This topology is appropriate for ad hoc LANs.

Transmission Issues in Wireless LANs

A desirable, though not necessary, characteristicof a wireless LAN is that it be usable without having to go through a licensing procedure.

The licensing regulations differ from one country to another, which complicates this objective.

Narrowband Microwave LANs

The term “narrowband microwave” refers to the use of a microwave RF band for signal transmission, with a relatively narrow bandwidth (just wide enough to accommodate the signal).

Until recently, all narrowband microwave LAN products have used a licensed microwave band. More recently, at least one vendor has produced a LAN product in the ISM band.

Licensed Narrowband RF

Microwave radio frequencies usable for voice, data and video transmissions are licensed and coordinated within specific geographic areas to avoid potential interference between systems.

A narrowband scheme typically makes use of the cell configuration.

Adjacent cells use non-overlapping frequency bands within the overall band.

Licensed Narrowband RF

Licensed narrowband LAN guarantees interference-free communication.

Licensed spectrum gives the license holder a legal right to an interference free data communications channel.

Users of an ISM-band LAN are at risk of interference disrupting their communications, for which they may not have a legal remedy.

Unlicensed Narrowband RF

In 1995, “RadioLAN” became the first vendor to introduce a narrowband wirelss LAN using the unlicensed ISM spectrum.

This spectrum can be used for narrowband transmission at low power (<0.5 W).

The product operates at 10 Mbps in the 5.8 GHz band and has a range of 50 m in a semi-open office and 100 m in an open office.

Unlicensed Narrowband RF The product makes use of a peer-to-peer

configuration with an interesting feature: As a substitute for a stationary hub, it automatically

elects one node as the Dynamic Master, based on parameters such as location, interference, and signal strength.

The identity of the master can change automatically as conditions change.

The LAN also includes a dynamic relay functions, which allows each station to act as a repeater to move data between stations that are out of range of each other.