Chapter 7
Local Area Networks:
The Basics
Topics
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Definition of LAN
Primary function, advantage and disadvantage
Difference between Client/Server network and
peer-to-peer network
Topologies of LANs
MAC techniques
Applications
Definition of LAN
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A local area network is a communication
network that interconnects a variety of
data communicating devices within a small
geographic area and broadcasts data at
high data transfer rates with very low error
rates.
Primary Function
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To provide
access to hardware and
software resources that will allow users to
perform one or more of the following activities:
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Manufacturing
Education (lab, office)
Interconnectivity with other networks
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Advantages
 Ability
to share hardware and software resources.
 Individual workstation might survive network failure.
 Component and system evolution are possible.
 Support for heterogeneous forms of hardware and
software.
 Access to other LANs and WANs (Figure 7-1).
 Private ownership.
 Secure transfers at high speeds with low error rates.
Disadvantages
 Equipment
and support can be costly.
 Level of maintenance continues to grow.
 Some types of hardware may not
interoperate.
 Just because a LAN can support two different
kinds of packages does not mean their data
can interchange easily.
 A LAN is only as strong as it weakest link
LAN Topologies
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Four basic ones:
Bus/Tree Topology
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The original topology.
Workstation has a network
interface card (NIC) that
attaches to the bus (a
coaxial cable) via a tap.
Data can be transferred
using either baseband
digital signals or broadband
analog signals.
Bus/Tree Topology
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Baseband signals are bidirectional and more
outward in both directions from the workstation
transmitting.
Broadband signals are usually uni-directional
and transmit in only one direction. Because of
this, special wiring considerations are necessary.
Buses can be split and joined, creating trees.
Star-wired Bus Topology
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Logically operates as a bus, but physically looks
like a star.
Star design is based on hub. All workstations
attach to hub.
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.
Single Hub
Multiple-Hub
Star-wired Bus Topology
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Advantage:
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Disadvantage
Star-wired Ring Topology
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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
Like hubs, MAUs can be interconnected to
increase network size
Logical View
Physical View
Wireless LANs
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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
Two basic components
 Wireless
NIC card
 Access point (AP)
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Acts as a bridge between the wired and wireless networks
and can perform basic routing functions
Wireless Standards
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IEEE 802.11 (older 2 Mbps)
IEEE 802.11b (11 Mbps, 2.4 GHz, 200-300 feet)
 Also
named Wi-Fi (Wireless Fidelity)
IEEE 802.11g (54 Mbps, 2.4 GHz, in 2002)
 IEEE 802.11a (54 Mbps, 5 GHz, in 2002, 90
feet)
 HiperLAN/2 (European standard, 54 Mbps in 5
GHz band, 90 feet)
Distance is inversely proportional to transmission
speed - as speed goes up, distance goes down
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Peer-to-Peer Networks
Different from client-server architecture
 May not possess servers
 Focus on applications
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 Groupware/collaboration
 Content-management
 Distributed
file-sharing
 Distributed processing
tools
Medium Access Control Protocols
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How does a workstation get its data onto the LAN
medium?
A medium access control protocol is the software that
allows workstations to “take turns” at transmitting data.
Three basic categories:
Contention-Based Protocols
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Essentially first come, first served
Carrier sense multiple access with collision detection
(CSMA/CD)
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If no one is transmitting, a workstation can transmit
If someone else is transmitting, the workstation “backs off” and
waits
If two workstations transmit at the same time, a collision occurs
When the two workstations hear the collision, they stop
transmitting immediately
Each workstation backs off a random amount of time and tries
again
CSMA/CD is an example of a
Round Robin Protocols
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Each workstation takes a turn transmitting and the turn is
passed around the network from workstation to
workstation.
Most common example is token ring LAN in which a
software token is passed from workstation to
workstation.
Token ring is an example of a
Token ring more complex than CSMA/CD. What
happens if token is lost? Duplicated? Hogged?
Token ring LANs are losing the battle with CSMA/CD
LANs.
Reservation Protocols
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Workstation places a reservation with central server.
Workstation cannot transmit until reservation comes up.
Under light loads, this acts similar to CSMA/CD.
Under heavy loads, this acts similar to token ring.
Powerful access method but again losing out to
CSMA/CD.
Most common example of reservation protocol is
demand priority protocol.
Local Area Network Systems
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Ethernet
 Most
common form of LAN today
 Star-wired bus or bus as the topology
 CSMA/CD as the medium access protocol
 Ethernet comes in many forms depending
upon medium used and transmission speed
and technology.
Local Area Network Systems
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IBM Token Ring
 Deterministic
LAN offered at speeds of 4, 16
and 100 Mbps.
 Very good throughput under heavy loads.
 More expensive components than CSMA/CD.
 Losing ground quickly to CSMA/CD. May be
extinct soon.
Local Area Network Systems
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FDDI (Fiber Data Distributed Interface)
 Based
on the token ring design using 100 Mbps fiber
connections.
 Allows for two concentric rings - inner ring can
support data travel in opposite direction or work as
backup.
 Token is attached to the outgoing packet, rather than
waiting for the outgoing packet to circle the entire
ring.
 Losing the battle to Ethernet?
Local Area Network Systems
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100VG-AnyLAN
Deterministic LAN based on demand priority
access method.
Similar to hub topology (star design).
Two levels of priority - normal and high.
Supports a wide-variety of media types.
Losing ground quickly to CSMA/CD. Will be
extinct soon
A Small Office Solution
What about home networking?