WLAN FundamentalsBy Marc-André Léger

DESS, MASc, PHD(candidate)

Fall 2007

Save the forest

• Please do not print out more than one module at a time as it may evolve…

Module 3: WLAN Infrastructure devices part 2

Topology: the geometric configurationBasic Network Topologies

This is different than protocols!

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

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 the physical design.

Basic Network Topologies

The original topology – 1970’s

Workstation 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

Baseband or broadband

Baseband• The original band of frequencies of a

signal before it is modulated for transmission at a higher frequency.

• A type of data transmission in which digital or analog data is sent over a single unmultiplexed channel, such as an Ethernet LAN.

• Baseband transmission use TDM to send simultaneous bits of data along the full bandwidth of the transmission channel.

• Compare with broadband.

• 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 communications.

What is broadband?• The term is used to describe almost

any always on, high speed connection to the internet.

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 its infancy, 2.4kbps was the fastest dialup speed available. Current 53kbps dialup speeds would seem to be broadband compared to back then.

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

• Many always on, high speed services have lower upload and higher download capacities.

• In network terms, these are called asymmetrical services. This complicates things somewhat when rating a service as both are significant.

Back to topologies…

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 necessary signal conversions and protocols operations so that the workstation can send and receive data on the network.

Tap is a passive deviceDoes not alter the signal

Does not require electricity to operate

Bus/Tree Topology

Bus/Tree Topology

Bus Topology• Consists of a single wire to which

individual stations are attached• Each end has a terminator attached to it• Data travels the entire length of the cable• Transmission from any stations travels

entire medium (both directions)• Inexpensive and easy to install• Ethernet is the common form of a bus

topology system

Bus Topology

Baseband signals

Digital signals – 10 Mbps

Bidirectional and more outward in both directions from the workstation transmitting.

Easy to install and maintainFewer than 100 workstations

Buses can be split and joined, creating trees.

Bus Tree Topology

Bus Topology

Broadband signals

Usually uni-directional and transmit in only one direction.

Analogy and FDM for multiple channels (amplification necessary). Because of this, special wiring considerations are necessary.100 to 1000 workstations over larger distances due to easy amplification

Buses can be split and joined, creating trees.

Bus/Tree Topology

Twice the propagation delay of baseband

Bus/Tree Topology

Bus/Tree Topology

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

Stars versus a single line

Two types:

1. Star-wired bus

(often call the star topology)

2. Star-wired ring

Star-wired Topologies

Logically operates 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

Star-wired bus physical topology

Interconnection of hubs in star-wired bus topology

Modular connectors and twisted pair make installation 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

Logically operates as a ring but physically appears as a 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

Ring topology viewed logically

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

Three possible operations of the workstation repeater in a ring topology

Physical organization of a ring topologyLooks like a star!

The MAU replaces the hub for this topology

Multi-station access unit on a ring topology

Not really a specific topology since a workstation in a wireless 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

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

Speeds up to 11 Mbps with 802.11b standard.

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

Wireless Topology – different forms

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

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

Single-cell wireless LAN configuration

Multiple-cell wireless LAN configuration

Ad-hoc configuration for a wireless LAN

With directional antennae designed for point-to-point transmission (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

In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps

To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. (how good?)

What will Bluetooth’s impact be on 802.11b (Wi-Fi)?

Wireless Topology

•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

Other Wireless Standards

Summary of topologies

• Logical vs physical topologies• Bus and star-rings - old

technologies– Still some around

• You’ll probably use 1. Star-wired bus (star)

With bus or routers (now much easier to use)2. Wireless network (Wi-fi)

WLAN Performance

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 radio interfaces.

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

802.11g Establishes an additional modulation technique for 2.4 Ghz band. Intended to provide speeds up to 54 Mbps. Includes much greater security.

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

802.11i Address the current security weaknesses for both authentication and encryption protocols. The standard encompasses 802.1X, TKIP, and AES protocols.

WLAN Standards

802.11Range and speeds

Calculating bandwidth

Calculating bandwidth

• Data being transferred in bits per second• Network adds overhead (20%+)• Collisions and interferences add delays

(CNWA Ch7-p297)

Template

Other

20%1MB per min ++

Video

20%+2KB per page

Data

TotalOverheadFile sizeFile type

WLAN bandwidth

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End of session 3

Questions

Please note

• These slides are produced as presentation material for a technical college course, all references, sources and bibliographical information is available in the commentaries 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 material.