Computer Data Communications

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Overview of Wireless LANsKey Application AreasWireless LAN requirementsIEEE 802.11 ArchitectureIEEE 802.11 TerminologyIEEE 802.11 ServicesIEEE 802.11 Medium Access Control (MAC)IEEE 802.11 Protocol ArchitectureIEEE 802.11 Physical LayersWireless LAN Access PointWireless LAN Access Point/ Client FeaturesWireless LAN Applications

Introduction

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A wireless local area network (LAN) is a flexible data communications system implemented as an extension to, or an alternative for, a wired LAN

Using radio frequency (RF) technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections.

Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed

Key application areas:LAN extensioncross-building interconnectnomadic accessad hoc networking

Overview of Wireless LANs

LAN extensionOriginally targeted to reduce cost of wiring, but new

buildings now have sufficient wiring in place

Still useful in buildings where wiring is problematicbuildings with large open areas, historical buildings with insufficient twisted pair small offices wired LANs are not economical

Typically, a wireless LAN will be linked into a wired LAN on the same premises

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Single-Cell LAN extensionIn addition, there is a control module (CM) that acts

as an interface to a wireless LAN. The control module (CM) includes either bridge or router functionality to link the wireless LAN to the backbone. It includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems. Figure 1 illustrates the Single-Cell LAN extension

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Single-Cell LAN extensionHubs or other user modules (UMs) that control a

number of stations off a wired LAN may also be part of the wireless LAN configuration. This configuration can be referred to as a single-cell wireless LAN; all of the wireless end systems are within range of a single control module.

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Single-Cell LAN Extension

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Figure 1: Single-Cell LAN Extension

Multi-Cell LAN extensionA multiple-cell wireless LAN, there are multiple control

modules interconnected by a wired LAN.

Each control module supports a number of wireless end systems within its transmission range.

For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support.

Figure 2 illustrates the Multi-Cell LAN extension

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Multi-Cell LAN Extension

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Figure 2: Multi-Cell LAN Extension

Cross-building interconnect

Connect LANs in nearby buildings, be they wired or wireless LANs

Point-to-point wireless link is used between two buildings (e.g. two microwave or infrared transmitter/receiver units can be placed on the rooftops of two buildings within the line of sight of each other)

Devices are typically bridges or routers.

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Nomadic AccessProvides a wireless link between a LAN hub and a

mobile data terminal (e.g. laptop computer) Figure 3 illustrates the Infrastructure Wireless LAN

ExamplesEnable an employee returning from a trip to transfer data from

a personal portable computer to a server in the office. Access in an extended environment such as a campus or a

business operating out of a cluster of buildings. In both of these cases, users may wish access to the servers on

a wired LAN from various locations.

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Nomadic Access :-

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Infrastructure Wireless LAN

Figure 3: Infrastructure Wireless LAN

Ad hoc networks

A peer-to-peer network (no centralized server) set up temporarily to meet some immediate need

For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting.

Figure 4 illustrates the Adhoc Networks

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Ad hoc networks

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Figure 4: Adhoc Networks

Wireless LAN RequirementsEfficient throughputSupport for multiple nodesConnection to backbone LANBroad service area (~ 100-300m)Allows for reduced power consumption while not

using the network (e.g. sleep mode)Transmission robustness and securityCo-located network operationLicense-free operationHandoff/roamingDynamic and automated addition, deletion, and

relocation

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Wireless LAN TechnologyInfrared (IR) LANs

Individual cells are limited to a single room, because infrared light does not penetrate opaque walls

Spread spectrum LANsIn most cases, these LANs operate in the ISM (Industrial,

Scientific, and Medical) bands so that no FCC licensing is required for their use in the U.S.

Narrowband microwaveDo not use spread spectrum. Some of these products operate

at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands

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IEEE 802.11 Architecture

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Figure 5: IEEE 802.11 Architecture

IEEE 802.11 ArchitectureIEEE has defined the specifications for a wireless

LAN, called IEEE 802.11, which covers the physical and data link layers.

The standard defines two kinds of services; the basic set (BSS) and the extended service set (ESS)

Figure 5 illustrates the IEEE 802.11 Architecture.

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IEEE 802.11 Architecture - BSSIEEE 802.11 defines the basic service set (BSS) as the building

block of a wireless LAN.

A basic service set is made of stationary or mobile wireless stations and optional central base station, known as the access point (AP).

The BSS without an AP is a stand-alone network and cannot send data to other BSSs. It is called an ad hoc architecture.

A BSS with an AP is sometimes referred to an infrastructure network.

Figure 6 illustrates the BSS.

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IEEE 802.11 Architecture - BSS

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Figure 6: Basic service sets (BSSs)

IEEE 802.11 Architecture - ESSAn extended service set (ESS) is made up of two or more BSSs

with APs.

In this case, the BSSs are connected through a distribution system, which is usually a wired LAN. The distribution system connects the APs in the BSSs.

IEEE 802.11 does not restrict the distribution system; it can be any IEE LAN such as an Ethernet.

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IEEE 802.11 Architecture - ESSThe extended service set (ESS) uses two types of stations:

mobile and stationary.

The mobile stations are normal stations inside a BSS.

The stationary stations are AP stations that are part of a wired LAN.

When BSSs are connected, the stations within reach of one another can communicate without the use of an AP.

However, communication between two stations in two different BSSs usually occurs via two APs. Figure 7 illustrates an ESS

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IEEE 802.11 Architecture - ESS

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Figure 7: Extended service sets (ESSs)

IEEE 802.11 Terminology

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Access point (AP) Any entity that has station functionality and providesaccess to the distribution system via the wirelessmedium for associated stations

Basic service set(BSS)

A set of stations controlled by a single coordinationfunction

Coordination function The logical function that determines when a stationoperating within a BSS is permitted to transmit andmay be able to receive PDUs

Distribution system(DS)

A system used to interconnect a set of BSSs andintegrated LANs to create an ESS

Extended service set(ESS)

A set of one or more interconnected BSSs andintegrated LANs that appear as a single BSS to the LLClayer at any station associated with one of these BSSs

MAC protocol dataunit (MPDU)

The unit of data exchanged between two peer MACentites using the services of the physical layer

MAC service data unit(MSDU)

Information that is delivered as a unit between MACusers

Station Any device that contains an IEEE 802.11 conformant MACand physical layer

IEEE 802.11 ServicesThe IEEE 802.11 services are listed below:

AssociationReassociationDisassociationAuthenticationDeauthenticationPrivacy

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IEEE 802.11 ServicesAssociation:

Establishes an initial association between a station and an AP. Before a station can transmit or receive frames on a wireless LAN, its identity and address must be known.

For this purpose, a station must establish an association within an AP within a particular BSS.

The AP can then communicate this information to other Aps within the ESS to facilitate routing and delivery of frames.

Reassociation: Enables an established association to be transferred from one AP

to another, allowing a mobile station to move from one BSS to another.

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IEEE 802.11 ServicesDisassociation:

A notification from either a station or an AP that an existing association is terminated.

A station should give this notification before leaving an ESS or shutting down.

Authentication: Used to establish the identity of a stations to each other. For a wireless LAN, in which connectivity is achieved simply by

having an attached antenna that is properly tuned. The authentication service is used by stations to establish their

identity with stations they wish to communicate with.

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IEEE 802.11 ServicesDeauthentication:

This service is invoked whenever an existing authentication is to be terminated.

Privacy:Used to prevent the contents of messages from being read

by other than the intended recipient.The standard provides for the optional use of encryption to

assure privacy.

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IEEE 802.11 Medium Access Control (MAC)

MAC (Medium Access Control) layer covers three functional areas:

Reliable data deliveryAccess controlSecurity

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IEEE 802.11 Medium Access ControlReliable Data Delivery

Basic data transfer mechanism involves an exchange of two or four frames (data, ACK, and optional CTS/RTS)

Access ControlUsed DFWMAC (distributed foundation wireless MAC)

that provides a distributed access control mechanism with an optional centralised control built on top of that. Figure 8 illustrates the IEEE 802.11 Protocol Architecture.

SecurityThe prevention of unauthorised access using wireless

network.

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IEEE 802.11 Protocol Architecture

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Figure 8: IEEE 802.11 Protocol Architecture

IEEE 802.11 Physical Layer

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802.11 802.11a 802.11b 802.11g

Availablebandwidth

83.5 MHz 300 MHz 83.5 MHz 83.5 MHz

Unlicensedfrequency ofoperation

2.4 - 2.4835 GHz

DSSS, FHSS

5.15 - 5.35 GHzOFDM

5.725 - 5.825GHz OFDM

2.4 - 2.4835 GHz

DSSS

2.4 - 2.4835 GHz

DSSS, OFDM

Number of non-overlappingchannels

3(indoor/outdoor)

4 indoor

4(indoor/outdoor)

4 outdoor

3(indoor/outdoor)

3(indoor/outdoor)

Data rate perchannel

1, 2 Mbps6, 9, 12, 18,24, 36, 48, 54

Mbps

1, 2, 5.5, 11Mbps

1, 2, 5.5, 6, 9,11, 12, 18, 24,36, 48, 54 Mbps

Compatibility 802.11 Wi-Fi5 Wi-Fi Wi-Fi at 11 Mbpsand below

Figure 9: IEEE 802.11 Physical Layer

IEEE 802.11 Physical LayerThe physical layer for IEEE 802.11 has been issued in four

stages: -802.11 (1997)

MAC layer and three physical layer specifications; two 2.4-GHz band, one infrared, all operating at 1 and 2 Mbps

IEEE 802.11a (1999) operates in the 5-GHz band at up to 54 Mbps

IEEE 802.11b (1999) operates in the 2.4-Ghz band at 5.5 and 11 Mbps.

IEEE 802.g (2002) extends IEEE 802.11b to higher data rates

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Original 802.11 Physical Media DefinitionsDirect-sequence spread spectrum (DSSS) operating in the

2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps

Frequency-hopping spread spectrum (FHSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps

Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm

All of the original 802.11 products were of limited utility because of the low data rates

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IEEE 802.11bExtension of the IEEE 802.11 DSSS scheme,

providing data rates of 5.5 and 11 Mbps (higher data rate is achieved with more complex modulation)

Apple Computer was first, with AirPort wireless networking, followed by other vendors

Wireless Ethernet Compatibility Alliance created to certify interoperability for 802.11b products

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Problems with 802.11 and 802.11b

Original 802.11 and 802.11b may interfere with other systems that operate in the 2.4-GHz bandBluetoothHomeRFother devices--including baby monitors and garage door

openers

Limited data rate results in limited appeal

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Higher-Speed 802.11 Options802.11a

Uses 5-GHz band. Uses orthogonal frequency division multiplexing (OFDM)

rather than spread spectrum Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps

802.11gHigher-speed extension to IEEE 802.11b. Combines physical layer encoding techniques used in

802.11a and 802.11b to provide service at a variety of data rates

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Wireless LAN Access PointAccess Point is a transceiver device which

connects to the wired network from a fixed location.It receives, buffers and transmit data between

the WLAN and the wired network infrastructure.It supports 15-50 client devices and can function

within a range of less than a hundred to several hundred feet

End users access the wireless LAN through PC cards.

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Wireless LAN Access Point /Client Features

RoamingProvides mobility within a subnet, across

subnets or across SSID subnetsProtect against AP failure, client PCs will

detect link loss and roam to alternate AP.

Best AP selectionPC Clients should scan all channels for the best

AP and rescan periodically and move to a better AP if possible.

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Wireless LAN Access Point /Client Features

Load balancingLoad balancing provides the capability for PC

clients to improve network performance by switching to the least utilized Access Point

PC client driver should periodically look for a better AP during network idle

The AP needs to monitor utilization and signal clients to discourage association and encourage roaming during sustained periods of high utilization

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Wireless LAN Access Point /Client Features

Network traffic filteringUsed in AP to eliminate unnecessary traffic to

improve network performance.

ManagementTo provide capability to centrally monitor and

manage an enterprise wide wireless network. (SNMP protocol support)

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Wireless LAN – ApplicationsHome

Home networking of computers and internet applicances

Small Office Home Office (SOHO)Ease of installations and re-location, eliminate

wiring cost, scalability

Public Hot-spotsHigh speed wireless internet access at hot

spots such as hotels, airports, conference centre and other areas while travelling

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Wireless LAN - ApplicationsEnterprise

Extension to existing infrastructure network, provide constant connectivity to corporate facilities while supporting users mobility. E.g. University, wireless internet and intranet access across the campus.

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