The University of Bolton

School of Business & Creative Technologies

Wireless Networks

Introduction

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Overview

1. Why use wireless?2. Types of wireless networks3. Wireless LAN Standards4. Wireless LAN standards organisations5. Wireless infrastructure components6. Wireless LAN 802.11 topologies7. Wireless LAN operation

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1. Why use wireless networks?

• Increased mobility

• Greater freedom

• Happier employees

• Low installation costs compared to a wired infrastructure

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2. Wireless network types

3. Wireless LAN Standards

• The IEEE 802.11 wireless LAN standard defines how radio frequency (RF) in the unlicensed industrial, scientific, and medical (ISM) frequency bands is used for the Physical layer and the MAC sub-layer of wireless links.

• Wireless LAN standards have continuously improved with the release of IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, and draft 802.11n.

• Data rates is a key factor in choosing a wireless LAN standard.

• Data rates of wireless LAN standards are affected by the modulation technique used. Modulation is the technique by which user data is carried by the transmitter’s signal.

• 3 modulation techniques are available...– Direct Sequence Spread Spectrum (DSSS)– Orthogonal Frequency Division Multiplexing (OFDM)– Multiple Input / Multiple Output (MIMO) 5

3. Wireless LAN standards continued...

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3. Wireless LAN standards continued...

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3. Wireless LAN standards continued...

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4. WLAN standards organisations

• Standards ensure interoperability between devices made by different manufacturers.

• Internationally, the three key organizations influencing WLAN standards are:– ITU-R , regulates the allocation of the RF spectrum and satellite

orbits– IEEE , developed and maintains LAN / MAN family of standards

( 802.3 Ethernet, 802.5 Token Ring, 802.11 Wireless LANs)– Wi-Fi Alliance

• Although the IEEE has specified standards for RF modulation devices, it has not specified manufacturing standards, so interpretations of the 802.11 standards by different vendors can cause interoperability problems between their devices.

• The Wi-Fi Alliance is an association of vendors whose objective is to improve the interoperability of products that are based on the 802.11 standard by certifying vendors for conformance to industry norms and adherence to standards.

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5. Wireless LAN networks components

• WLANs use radio frequencies (RF) instead of cables for the transmission of network traffic.

• WLANs connect clients to the network through a wireless access point (AP) instead of an Ethernet switch.

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5. Wireless LAN networks components cont...

Wireless Network Interface Card (Wireless NIC)• The wireless NIC, using the modulation technique it is configured to

use, encodes a data stream onto an RF signal to be transmitted.• Unlike 802.3 Ethernet interfaces built into PCs, the wireless NIC is

often not visible because there is no requirement to connect a cable to it.

Wireless Access Points• An access point connects wireless clients (or stations) to the wired

LAN. • Client devices do not typically communicate directly with each other;

they communicate with the AP. • An access point converts the TCP/IP data packets from their 802.11

frame encapsulation format in the air to the 802.3 Ethernet frame format on the wired Ethernet network.

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6. Wireless LAN networks components cont...

Wireless Access Points continued.... • An access point is a Layer 2 device that functions like an 802.3

Ethernet hub. • RF is a shared medium and access points hear all radio traffic. • Just as with 802.3 Ethernet, the devices that want to use the medium

contend for it. • Unlike Ethernet NICs, though, it is expensive to make wireless NICs

that can transmit and receive at the same time, so radio devices do not detect collisions.

• Instead, WLAN devices are designed to avoid collisions.• Devices on a WLAN must sense the medium for energy (RF

stimulation above a certain threshold) and wait until the medium is free before sending.

• Wired Ethernet LANs use Carrier Sense Multiple Access with Collision Detection (CSMA/CD) but wireless LANs use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). 12

5. Wireless LAN networks components cont...

Wireless Routers• Wireless routers perform the role of access point, Ethernet switch, and

router. • For example, the Linksys WRT300N is really three devices in one box.• First, there is the wireless access point, which performs the typical

functions of an access point. • A built-in four-port, full-duplex, 10/100 switch provides connectivity to

wired devices. • Finally, the router function provides a gateway for connecting to other

network infrastructures.• The WRT300N is most commonly used as a small business or

residential wireless access device. • The expected load on the device is low enough that it should be able to

manage the provision of WLAN, 802.3 Ethernet, and connect to an ISP.

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6. Wireless LAN 802.11 Topologies

• Wireless LANs can accommodate various network topologies. • When describing these topologies, the fundamental building block of

the IEEE 802.11 WLAN architecture is the basic service set (BSS). • The standard defines a BSS as a group of stations that communicate

with each other. • Ad hoc wireless network

– Wireless networks can operate without access points; this is called an ad hoc topology. Client stations which are configured to operate in ad hoc mode configure the wireless parameters between themselves.

• Basic Service Sets– Access points provide an infrastructure that adds services and improves

the range for clients.

• Extended Service Sets– When a single BSS provides insufficient RF coverage, one or more can be

joined through a common distribution system into an extended service set (ESS).

– In an ESS, one BSS is differentiated from another by the BSS identifier (BSSID), which is the MAC address of the access point serving the BSS.

– The coverage area is the extended service area (ESA).

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6. Wireless LAN 802.11 Topologies continued...

• Ad-hoc wireless topology (also known as an Independent Basic Service Set – IBSS). The diagram show radio waves connecting the computers.

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6. Wireless LAN 802.11 Topologies continued...

• Basic Service Set (BSS) wireless topology.

• The figure below show an integrated wired/wireless network.

• The links between the switches and router are trunks.

• The wireless access point is hardwired to the switch S2.

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6. Wireless LAN 802.11 Topologies continued...

• Extended Service Set (ESS) wireless topology.

• Uses more than one wireless access point.

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7 Wireless LAN operation - Client and AP Association

• A key part of the 802.11 process is discovering a WLAN and subsequently connecting to it.

• The primary components of this process are as follows:– Beacons - Frames used by the WLAN network to advertise its

presence.– Probes - Frames used by WLAN clients to find their networks.– Authentication - A process which is an artefact from the original

802.11 standard, but still required by the standard.– Association - The process for establishing the data link between

an access point and a WLAN client.

• Before an 802.11 client can send data over a WLAN network, it goes through the following three-stage process: – 802.11 probing– 802.11 authentication– 802.11 association

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