WLAN Fundamentals
By 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
Basic Network Topologies
Topology: the geometric configuration
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
Basic Network Topologies
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.
Bus/Tree Topology
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.
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
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 bidirectional. 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…
Bus/Tree Topology
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 device
Does not alter the signal
Does not require electricity to operate
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
Bus Tree Topology
Baseband signals
Digital signals – 10 Mbps
Bidirectional and more outward in both directions
from the workstation transmitting.
Easy to install and maintain
Fewer than 100 workstations
Buses can be split and joined, creating trees.
Bus Topology
Bus/Tree 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
Advantages/disadvantages of a
bus
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.
Star-wired Topologies
Stars versus a single line
Two types:
1. Star-wired bus
(often call the star topology)
2. Star-wired ring
Star-wired Bus Topology
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 physical topology
Interconnection of hubs in starwired bus topology
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 Ring Topology
Logically operates as a ring but physically appears as
a star.
Star-wired ring topology is based on MAU (multistation 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.
Ring topology viewed logically
NIC Repeaters for Star-wired
Ring
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
Three possible operations of the
workstation repeater in a ring topology
Physical organization of a ring topology
Looks like a star!
Multi-station access unit on a ring
topology
The MAU replaces the hub for this topology
Wireless 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 – different
forms
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
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.
Single-cell wireless LAN
configuration
Multiple-cell wireless LAN
configuration
Ad-hoc configuration for a wireless
LAN
Wireless Topology
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 40or 128-bit key protection. (how good?)
What will Bluetooth’s impact be on 802.11b (Wi-Fi)?
Other Wireless Standards
•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
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
WLAN Standards
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.
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
File type
File size
Overhead
Data
2KB per
page
20%+
Video
1MB per
min ++
20%
Other
Total
WLAN bandwidth
Template
File type
File size
Overhead
Total
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.