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WLAN Fundamentals

By Marc-Andr LgerDESS, MASc, PHD(candidate)

Fall 2007

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Save the forest

Please do not print out more than one

module at a time as it may evolve

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Module 3: WLAN Infrastructure

devices part 2

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Topology: the geometric configuration

Basic Network Topologies

This is different than protocols!

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Four basic topologies

Local area networks are interconnected

using one of four basic configurations:

1. Bus/tree

2. Star-wired bus

3. Star-wired ring

4. Wireless

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Logical vs physical design (topologies)

1. Logical design: how the data moves around

the network from workstation to workstation

2. Physical design: how the network physically

appears if drawn on a sheet of paper (laid out)

Names of topologies usually refer to thephysical design.

Basic Network Topologies

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The original topology 1970sWorkstation has a network interface card (NIC)

provides a physical connection to a network

Data can be transferred using either Baseband: digital signals

Broadband: analog signals.

Bus/Tree Topology

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Baseband or broadband

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Baseband

The original band of frequencies of asignal before it is modulated fortransmission at a higher frequency.

A type of data transmission in which digitalor analog data is sent over a singleunmultiplexed channel, such as anEthernet LAN.

Baseband transmission use TDM to sendsimultaneous bits of data along the fullbandwidth of the transmission channel.

Compare with broadband.

http://www.webopedia.com/TERM/B/modulate.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/multiplexing.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/TDM.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/broadband.htmlhttp://www.webopedia.com/TERM/B/broadband.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/TDM.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/multiplexing.htmlhttp://www.webopedia.com/TERM/B/baseband.htmlhttp://www.webopedia.com/TERM/B/modulate.html

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A type of data transmission in which a

single medium (wire) can carry several

channels at once.

Cable TV, for example, uses broadband

transmission.

In contrast, baseband transmission allows

only one signal at a time. Most

communications between computers,

including the majority of

local-area networks, use baseband

http://www.webopedia.com/TERM/B/media.htmlhttp://www.webopedia.com/TERM/B/channel.htmlhttp://www.webopedia.com/TERM/B/baseband_transmission.htmlhttp://www.webopedia.com/TERM/B/baseband_transmission.htmlhttp://www.webopedia.com/TERM/B/communications.htmlhttp://www.webopedia.com/TERM/B/computer.htmlhttp://www.webopedia.com/TERM/B/local_area_network_LAN.htmlhttp://www.webopedia.com/TERM/B/local_area_network_LAN.htmlhttp://www.webopedia.com/TERM/B/computer.htmlhttp://www.webopedia.com/TERM/B/communications.htmlhttp://www.webopedia.com/TERM/B/baseband_transmission.htmlhttp://www.webopedia.com/TERM/B/baseband_transmission.htmlhttp://www.webopedia.com/TERM/B/channel.htmlhttp://www.webopedia.com/TERM/B/media.html

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What is broadband?

The term is used to describe almostany always on, high speedconnection to the internet.

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More on broadband

Broadband is a broad bandwidth connection. It allows a large amount of data to travel through a

medium at the same time. There are many opinions about what a large amount of

data entails. It is all relative. When the internet was in itsinfancy, 2.4kbps was the fastest dialup speed available.Current 53kbps dialup speeds would seem to bebroadband compared to back then.

The other consideration is that network traffic is bi-directional. In internet terms, there are download andupload paths for a connection.

Many always on, high speed services have lower uploadand higher download capacities.

In network terms, these are called asymmetricalservices. This complicates things somewhat when ratinga service as both are significant.

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Back to topologies

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Workstation has a network interface card (NIC)

provides a physical connection to a network

Attaches to the bus (a coaxial cable) via a tap.

NIC is an electronic device that performs the necessarysignal conversions and protocols operations so that the

workstation can send and receive data on the network.

Tap is a passive device

Does not alter the signal

Does not require electricity to operate

Bus/Tree Topology

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Bus/Tree Topology

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Bus Topology

Consists of a single wire to whichindividual stations are attached

Each end has a terminatorattached to it

Data travels the entire length of the cable Transmission from any stations travelsentire medium (both directions)

Inexpensive and easy to install

Ethernet is the common form of a bustopology system

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Bus Topology

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Baseband signals

Digital signals 10 Mbps

Bidirectional and more outward in both directionsfrom the workstation transmitting.

Easy to install and maintain

Fewer than 100 workstations

Buses can be split and joined, creating trees.

Bus Tree Topology

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Bus Topology

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Broadband signals

Usually uni-directional and transmit in only one direction.

Analogy and FDM for multiple channels (amplification

necessary). Because of this, special wiring considerationsare necessary.

100 to 1000 workstations over larger distances due to easy amplification

Buses can be split and joined, creating trees.

Bus/Tree Topology

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Twice the propagation delay of baseband

Bus/Tree Topology

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Bus/Tree Topology

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Difficult to add new devices if no tap exists.

No tap existing means cutting into the line

As such, this topology is loosing popularity

Plenty still around; Ethernet uses this.

Advantages/disadvantages of a

bus

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Stars versus a single line

Two types:

1. Star-wired bus

(often call the star topology)

2. Star-wired ring

Star-wired Topologies

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Logicallyoperates as a bus, but physically looks like a star.Star design is based on hub. All workstations attach to hub.

Hub is an unintelligent device that immediately transmits whatever

data it receives to all connections

Unshielded twisted pair usually used to connect workstation

to hub.

Hub takes incoming signal and immediately broadcasts it

out all connected links.

Hubs can be interconnected to extend size of network.

Very popular!

Star-wired Bus Topology

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Star-wired bus physical topology

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Interconnection of hubs in star-

wired bus topology

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Modular connectors and twisted pair makeinstallation and maintenance of star-wired bus

better than standard bus.

Hubs can be interconnected with twisted pair,coaxial cable, or fiber optic cable.

Biggest disadvantage: when one station talks,

everyone hears it. This is called a shared network.All devices are sharing the network medium.

Star-wired Bus Topology

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Logically operates as a ring but physically appears asa star.

Star-wired ring topology is based on MAU (multi-

station access unit) which functions similarly to a hub.Where a hub immediately broadcasts all incoming

signals onto all connected links, the MAU passes the

signal around in a ring fashion in one direction.

Like hubs, MAUs can be interconnected to increase

network size.

Star-wired Ring Topology

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Ring topology viewed logically

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All data must pass through a NIC repeater

Performs basic functions:

Bypass: data does not copy to the

workstation, used for inactive devices

Copy: data from ring copied to workstation

Write: data from the workstation copied to

the ring

Regenerates: recopies data back onto ring

NIC Repeaters for Star-wired

Ring

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Three possible operations of the

workstation repeater in a ring topology

Ph i l i ti f i t l

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Physical organization of a ring topology

Looks like a star!

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The MAU replaces the hub for this topology

Multi-station access unit on a ring

topology

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Not really a specific topology since a workstation in awireless LAN can be anywhere as long as it is within

transmitting distance to an access point.

Range varies from 50 to 800 ft with speeds of 2 to 11 Mbps

Most wireless LANs include a wired LAN backbone

Usually requires a wireless NIC

Wireless Topology

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Newer IEEE 802.11 and 802.11b (Wi-Fi) standard definesvarious forms of wireless LAN connections.

Speeds up to 11 Mbps with 802.11b standard.

Workstations reside within a basic service set, while multiplebasic service sets create an extended service set.

Wireless Topology different

forms

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Acceptable transmission ranges broken up into areas:

2. Basic service set that surrounding an access point

3. Extended service set collection of basic service sets

Workstations reside within a basic service set, while multiple

basic service sets create an extended service set.

Wireless Topology

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Two basic components necessary: the client radio,usually a PC card with an integrated antenna, and

the access point(AP), which is an Ethernet port plus

a transceiver.

The AP acts as a bridge between the wired and

wireless networks and can perform basic routing

functions.

Workstations with client radio cards reside within a

basic service set, while multiple basic service sets

create an extended service set.

Wireless Topology

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Single-cell wireless LAN

configuration

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Multiple-cell wireless LAN

configuration

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Ad-hoc configuration for a wireless

LAN

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With directional antennae designed for point-to-pointtransmission (rare), 802.11b can work for more than 10

miles.

With an omni-directional antenna on a typical AP,range may drop to as little as 100 feet.

Distance is inversely proportional to transmission

speed - as speed goes up, distance goes down.

Wireless Topology

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In actual tests, 11 Mbps 802.11b devices managed5.5 Mbps

To provide security, most systems use Wired

Equivalent Privacy (WEP), which provides either 40-or 128-bit key protection. (how good?)

What will Bluetooths impact be on 802.11b (Wi-Fi)?

Wireless Topology

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IEEE 802.11 (older 2 Mbps)IEEE 802.11b (11 Mbps, 2.4 GHz)

Also called Wi-Fi (wireless fidelity)

IEEE 802.11a (54 Mbps, 5 GHz, in 2002)IEEE 802.11g (54 Mbps, 2.4 GHz, in 2002)

HiperLAN/2 (European standard, 54 Mbps in

5 GHz band)

Other Wireless Standards

O S

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Other Wireless Standards

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Summary of topologies

Logical vs physical topologies

Bus and star-rings - old

technologies

Still some around

Youll probably use

1. Star-wired bus (star)

With bus or routers (now much easier to use)

2. Wireless network (Wi-fi)

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WLAN Performance

S

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802.11 Standards

802.11 The original WLAN Standard. Supports 1 Mbps to 2 Mbps.

802.11a High speed WLAN standard for 5 Ghz band. Supports 54 Mbps.

802.11b WLAN standard for 2.4 Ghz band. Supports 11 Mbps.

802.11e Address quality of service requirements for all IEEE WLAN radiointerfaces.

802.11f Defines inter-access point communications to facilitate multiplevendor-distributed WLAN networks.

802.11g Establishes an additional modulation technique for 2.4 Ghzband. Intended to provide speeds up to 54 Mbps. Includesmuch greater security.

802.11h Defines the spectrum management of the 5 Ghz band for use inEurope and in Asia Pacific.

802.11i Address the current security weaknesses for both authenticationand encryption protocols. The standard encompasses 802.1X,

TKIP, and AES protocols.

WLAN Standards

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802.11Range and speeds

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Calculating bandwidth

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Calculating bandwidth

Data being transferred in bits per second

Network adds overhead (20%+)

Collisions and interferences add delays

(CNWA Ch7-p297)

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Template

Other

20%1MB per

min ++

Video

20%+2KB per

page

Data

TotalOverheadFile sizeFile type

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WLAN bandwidth

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Template

TotalOverheadFile sizeFile type

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End of session 3

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Questions

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Please note

These slides are produced as presentation

material for a technical college course, allreferences, sources and bibliographical

information is available in thecommentaries section of the PowerPoint

presentation and may not be visible to

viewers of PDF versions. The course instructor has no pretensions

to be the original author of any of the