Chapter 7
Local Area Networks: The Basics
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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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Primary Function of a LAN
To provide access to hardware and software resources that
will allow users to perform one or more of the following
activities:
File serving - A large storage disk drive acts as a central
storage repository.
Print serving - Providing the authorization to access a
particular printer, accept and queue print jobs, and providing a
user access to the print queue to perform administrative
duties.
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Primary Function of a LAN (continued)
Video transfers - High speed LANs are capable of supporting
video image and live video transfers.
Manufacturing support - LANs can support manufacturing
and industrial environments.
Academic support – In classrooms, labs, and wireless.
E-mail support.
Interconnection between multiple systems.
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Advantages of Local Area Networks
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Ability to share hardware and software resources.
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Individual workstation might survive network failure.
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Component and system evolution are possible.
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Support for heterogeneous forms of hardware and software.
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Access to other LANs and WANs (Figure 7-1).
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Private ownership.
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Secure transfers at high speeds with low error rates.
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Disadvantages of Local Area Networks
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Equipment and support can be costly.
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Level of maintenance continues to grow.
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Private ownership possible.
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Some types of hardware may not interoperate.
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Just because a LAN can support two different kinds of
packages does not mean their data can interchange
easily.
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A LAN is only as strong as it weakest link, and there
are many links.
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Basic Local Area 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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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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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 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.
Like hubs, MAUs can be interconnected to increase network
size.
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Wireless LANs
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 and 802.11b 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.
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Wireless LANs
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.
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Wireless LANs
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.
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Wireless LANs
In actual tests, 11 Mbps 802.11b devices managed 5.5 Mbps
(from a July 2000 test by Network Computing).
To provide security, most systems use Wired Equivalent
Privacy (WEP), which provides either 40- or 128-bit key
protection.
What will Bluetooth’s impact be on 802.11b?
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Other Wireless Standards
• IEEE 802.11 (older 2 Mbps)
• IEEE 802.11b (11 Mbps, 2.4 GHz)
• 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)
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Peer-to-Peer LANs
Not as common as server-based LANs
Less, if any reliance on servers
Most peer-to-peer LANs still use one or more servers
Interesting collaborative-type applications (world-wide law
firm)
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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
Essentially 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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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 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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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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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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Ethernet
Originally, CSMA/CD was 10 Mbps.
Then 100 Mbps was introduced. Most NICs sold today are
10/100 Mbps.
Then 1000 Mbps (1 Gbps) was introduced.
10 Gbps is now beginning to appear.
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Ethernet
1000 Mbps introduces a few interesting wrinkles:
Transmission is full duplex (separate transmit and receive),
thus no collisions.
Prioritization is possible using 802.1p protocol.
Topology can be star or mesh (for trunks).
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Ethernet
A few more interesting wrinkles:
Cabling can be either UTP or optical (but 10 Gbps Ethernet
may not work over UTP due to radio frequency interference).
Where 10 Mbps Ethernet has less than 30% utilization due to
collisions, 1000 Mbps is limited only by traffic queueing.
Distance with 10 Mbps is limited by CSMA/CD propagation
time, whereas 1000 Mbps is limited only by media.
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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. May be extinct soon.
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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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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 quickly to CSMA/CD. Will be extinct soon.
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LANs In Action: A Small Office Solution
What type of system will interconnect twenty workstations in
one room and 15 workstations in another room to a central
server, which offers:
• Internal e-mail
• A database that contains all customer information
• High quality printer access
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LANs In Action: A Home Office
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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