Data Communications and
Computer Networks: A
Business User’s Approach
Chapter 7
Local Area Networks : The Basics
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Last time
• Types of errors and their prevention
• Error detection
– Parity – 50%
– CRC – can detect nearly all errors
• Error correction
– 1. Do nothing
– 2. Return an error message to the transmitter
– 3. Fix the error with no further help from the transmitter
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Three Major Types of Networks
 Local Area Network (LAN)
 Serves users within a confined geographical area
(usually within a mile).
 Metropolitan Area Network (MAN)
 Covers a geographic area the size of a city or
suburb. The purpose of a MAN is often to bypass
local telephone companies when accessing longdistance service.
 Wide Area Network (WAN)
 Covers a wide geographical area, such as a state or
a country. Examples: Tyment, Telenet, Uninet, and
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Accunet.
Emphasis
Definition of a Local Area Network (LAN)
• Physical and Logical LAN Topologies
• Different Medium Access Control Protocols
• Common LAN Systems
– Ethernet (1st commercially available LAN)
– Token ring
– FDDI (fiber data distributed interface) ring
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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
•very low error rates.
(WANs now do this too)
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.
Print 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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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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Data Communications and Computer Networks
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Basic Network Topologies
Topology: the geometric configuration
Separate from protocols!
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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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.
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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
1. baseband digital signals
2. broadband analog signals.
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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
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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.
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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.
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Twice the propagation
delay of baseband
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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.
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Star-wired Topologies
Stars versus a single line
Two types:
1. Star-wired bus
(often call the star topology)
2. Star-wired ring
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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.
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!
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Star-wired bus physical topology for a LAN
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Interconnection of two hubs in a star-wired bus LAN
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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 in one direction.
Like hubs, MAUs can be interconnected to increase network
size.
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Ring topology viewed logically
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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
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Three possible operations of the workstation repeater
on a ring topology
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Physical organization of a ring topology
Looks like a star!
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Multi-station access unit on a ring topology
The MAU replaces the hub for this topology
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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.
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
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Stanford’s wireless network
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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.
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Wireless Topology
Acceptable transmission ranges broken up into areas:
1. Basic service set – that surrounding an access point
2. 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.
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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.
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Single-cell wireless LAN configuration
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Multiple-cell wireless LAN configuration
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Ad-hoc configuration for a wireless LAN
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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.
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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)?
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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)
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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)
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Protocols
• Last time topologies
• Now how the data actually flows
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Data Communications and Computer Networks
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Medium Access Control (MAC) 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.
Since a LAN is a broadcast network, it’s imperative that
only one workstation at a time be allowed to transmit data
onto the network. (for multiple channels, one channel per
workstation)
Switches have changed that, but we’ll take about that in Ch8.
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Medium Access Control Protocols
Three basic categories:
1. Contention-based protocols (1st come; 1st serve)
2. Round robin protocols (take your turn!!)
3. Reservation protocols (requests please!)
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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.
Half duplex protocol.
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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. Workstations use a persistence algorithm to decide when to
resubmit.
Hopefully, both workstations do not try again at the exact
same time.
CSMA/CD is an example of a non-deterministic protocol
(cannot calculate time at which a workstation will transmit)
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Collisions
Collision window is the interval during which a workstation’s
signal can propagate down the bus and back. During this
period a workstation might falsely hear no one transmitting
and retransmit a signal.
Changes for collisions go up when every workstation wants
to send something – heavy traffic!
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Collisions
• Busy CSMA/CD networks can waste over
half their time dealing with collisions.
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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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Token Passing Protocols
• Before workstation can transmit it must
possess the one and only token
• Two types:
– Token ring
– Token bus
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Data Transmission on a token ring LAN
Station A has just released the token
Station M copies the data meant for it from B
B removes the data it sent and releases the token 58
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Why CSMA/CD more popular than
Token Ring
1st LAN MAC method, more installations and equipment.
Token rings have almost always lagged behind in
transmission speed.
CSMA/CD less expensive to implement because of
widespread marketing and acceptance.
CSMA/CD is a simpler protocol.
Wireless CSMA/CD now exists
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Token Bus
Just like the token ring, but a bus instead
Each workstation maintains a list of neighbors that one passes
the token to (logical not physical) neighbor
Designed primarily for manufacturing plants since a nondeterministic protocol is not acceptable there. Why?
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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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Problems with the OSI model for LANs
Problems with broadcast networks!
• Routing (no decisions have to be made)
• Data link and physical link tightly coupled
– Need to specify hardware! Ugh!
• Broadcast networks need to be have a
medium access control to decide who talks
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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 (MAC) 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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Modification of OSI model to split data link layer into
two sublayers
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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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Frame format for IEEE 802.3 CSMA/CD Protocol
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Frame Format for IEEE 802.5 Token Ring Protocol
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Local Area Network Systems
The entire system with protocol and topology
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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 around.
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 available.
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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
Cabling can be either UTP (unshielded twisted pair) 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 limited only by media.
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Ethernet Standards
Key
• XXXBase or XXX Broad
• XXX 10 means 10Mbps
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Local Area Network Systems
IBM Token Ring
Deterministic LAN offered at speeds of 4, 16 and 100 Mbps
using star-ring topology.
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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FDDI dual ring topology
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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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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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What we covered
• Topologies
– Star-bus
– Wireless
• Protocols
– CSMA/CD
• Systems
– Ethernet
Next time: wireless networks?
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