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Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 7
Local Area Networks: The Basics
Data Communications and Computer Networks
Chapter 7
Lesson Objectives
•State the definition of a local area network
•List the functions and application areas of a local area
network
•Cite the advantages and disadvantages of local area network
topologies
•Identify the physical and logical local area network
topologies
•Specify the different medium access control techniques
•Describe the common local area network systems, such as
Ethernet, token ring and FDDI
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Introduction
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.
Since the local area network first appeared in the 1970s, its
use has become widespread in commercial and academic
environments.
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Functions of a Local Area Network
File server - A large storage disk drive that acts as a central
storage repository.
Printer server - Provides the authorization to access a
particular printer, accept and queue print jobs, and provides a
user access to the print queue to perform administrative
duties.
Interconnection - A LAN can provide an interconnection to
other LANs and to wide area networks.
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Functions of a Local Area Network
Video transfers - High speed LANs are capable of supporting
video image and live video transfers.
Manufacturing support - LANs can support manufacturing
and industrial environments.
Distributed processing - LANs can support network operating
systems which perform the operations of distributed
processing.
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Advantages of Local Area Networks
• 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.
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Data Communications and Computer Networks
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Disadvantages of Local Area Networks
• Equipment and support can be costly.
• Level of maintenance continues to grow.
• Private ownership.
• 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, and there are
many links.
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Basic Network 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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Bus/Tree Topology
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.
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Bus/Tree Topology
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.
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
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.
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
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Star-wired Ring Topology
Logically operates as a ring but logically appears as a star.
Star 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.
Like hubs, MAUs can be interconnected to increase network
size.
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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Data Communications and Computer Networks
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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.
Newer IEEE 802.11 standard defines various forms of
wireless LAN connections.
Speeds up to 20 Mbps possible.
Workstations reside within a basic service set, while multiple
basic service sets create an extended service set.
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Data Communications and Computer Networks
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Medium Access Control Protocols
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:
1. Contention-based protocols
2. Round robin protocols
3. Reservation protocols
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Contention-Based Protocols
First come first served.
Most common example is carrier sense multiple access with
collision detection (CSMA/CD).
If no one is transmitting, a workstation can transmit.
If someone else is transmitting, the workstation “backs off”
and waits.
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Contention-Based Protocols
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.
Hopefully, both workstations do not try again at the exact
same time.
CSMA/CD is an example of a non-deterministic protocol.
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Data Communications and Computer Networks
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Round Robin Protocols
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 deterministic protocol.
Token ring is more complex than CSMA/CD. What happens
if token is lost? Duplicated? Hogged?
Token ring LANs are losing the battle with CSMA/CD LANs.
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Data Communications and Computer Networks
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Reservation Protocols
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.
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Medium Access Control Sublayer
To better support local area networks, the data link layer of
the OSI model was broken into two sublayers:
1. Logical link control sublayer
2. Medium access control sublayer
Medium access control sublayer defines the frame layout and
is more closely tied to a specific medium at the physical layer.
Thus, when people refer to LANs they often refer to its MAC
sublayer name, such as 10BaseT.
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Data Communications and Computer Networks
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IEEE 802 Frame Formats
The IEEE 802 suite of protocols defines the frame formats for
CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5).
Each frame format describes how the data package is formed.
Note how the two frames are different. If a CSMA/CD
network connects to a token ring network, the frames have to
be converted from one to another.
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Local Area Network Systems
Ethernet or CSMA/CD
• Most common form of LAN today.
• Star-wired bus is most common topology, but bus topology also
available.
• Ethernet comes in many forms depending upon medium used and
transmission speed and technology.
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Data Communications and Computer Networks
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Local Area Network Systems
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.
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Local Area Network Systems
FDDI (Fiber Distributed Data 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.
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Data Communications and Computer Networks
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Local Area Network Systems
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 to CSMA/CD.
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Data Communications and Computer Networks
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LANs In Action : A Small Office Solution
What type of system will interconnect twenty workstations to a
central server, which offers:
• Electronic mail
• A database that contains all customer information
• Internet access
• High quality printer access
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Data Communications and Computer Networks
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LANs In Action: A Home Office Local Area
Network Solution
What if you have two computers at home and want both to share a
printer and a connection to the Internet.
Some type of “network-in-a-box” solution might solve this problem.
Essentially a LAN with a 2- or 3-port hub, connecting cables, and
software.
In some models, the hub also acts as a router to the Internet.
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