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LAN / WAN / Extranet
and Network Topology
1
Motivation

Local Area Networks (LAN) were
motivated by:
– Decreasing computer size
– Decreasing computer cost
– Realizing computers could help with many
tasks
2
Interchangeable Media

The first data transfers:
– Used:
• Magnetic tapes
• Disks

Data transferred between computers
in a method similar to using floppy
disks.
3
LAN Generations

First
– CSMA/CD and token ring
– Terminal to host and client server
– Moderate data rates

Second
– FDDI
– Backbone
– High performance workstations

Third
– ATM
– Aggregate throughput and real time support for
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multimedia applications
Third Generation LANs

Support for multiple guaranteed classes of
service
– Live video may need 2Mbps
– File transfer can use background class

Scalable throughput
– Both aggregate and per host

Facilitate LAN/WAN internetworking
5
LAN technologies



MAC protocols used in LANs, to control access to the
channel
Token Rings: IEEE 802.5 (IBM token ring), for computer
room, or Department connectivity, up to 16Mbps; FDDI
(Fiber Distributed Data Interface), for Campus and Metro
connectivity, up to 200 stations, at 100Mbps.
Ethernets: employ the CSMA/CD protocol; 10Mbps (IEEE
802.3), Fast E-net (100Mbps), Giga E-net (1,000 Mbps); by
far the most popular LAN technology
6
A Computer Consists Of
Circuit Boards

Inside a computer are electronic components
on circuit boards.
– Containing electronic components
– Containing wires

Computers having different circuit boards
for external devices.
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Circuit Boards Plug Into
A Computer

Computers are built so it contains a
set of sockets.
– Using wires to connect sockets together
– Using wires to carry power and data
– Plugging circuit boards into sockets to
control external devices
8
Illustrations of the components visible in a computer
when the cover has been removed. A circuit board
can plug into each socket; wires connect the sockets
to other components.
9
Connecting Computers In Early
Systems

Transferring data between two
computers consisted of two circuit
boards connected by a cable.
Figure 7.2 Illustration of
an early computer
communication system
formed using two circuit
boards plugged into
sockets in two
computers.
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Early Systems

The computers use cables to transfer
data electronically.
– Operating like an I/O device
– Writing data to circuit board
Figure 7.3 Two pairs
of interface boards
connecting three
computers. Each new
computer added to
the set requires a
new pair of interface
boards and an 11
additional cable.
Early Systems


Advantage of early LANs were speed.
Disadvantages of early LANs were
inconvenience and cost. Requiring effort
to:
– Add a new computer
– Connect incompatible hardware
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Connecting A Computer
to A LAN

A computer needs additional hardware
to connect it to a LAN.

The speed of the LAN does not depend
on the speed of the computer attached
to it.
– Communication by heterogeneous computers
13
• In many LAN systems, a cable
connects each computer to a hub.
Computers connected to a LAN. Each computer attaches to the hub with a cable; the computers can
14
then communicate directly.
NIC


A computer needs network interface
hardware and a cable that connects to
the LAN.
A computer uses the network interface
to send and receive data.
15
The Importance Of LAN
Technology

LANs changed the way people
used computer networks.
– Sharing resources
– Connecting machines within a
building
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Relationship To The Internet

Xerox gave universities a prototype
of a new LAN technology.
– Beginning of Ethernet
– Developing the idea of inexpensive
and widely available LANs
17
Many Independent Networks

By late 1970s, many organizations began
installing Local Area Networks because
they:
– Were inexpensive.
– Were easy to install
– Could operate them independently of a
central administration.
18
The Proliferation of LANs

Advantages

Disadvantages
– An organization can: – Independent groups can:
• budget funds
• decide who has access
• Encourage proliferation of
different LAN technologies
• devise policies for use
19
Facts About LANs

Engineers have devised many LAN
technologies

LAN performance determines cost.

LAN technology may only work with
specific computers.
20
LANs Are Incompatible

Various LAN technologies are completely
incompatible.
– Connecting multiple LANs is not possible
• Engineered to operate over limited distance
• May be electrically incompatible
• Encoding information may not make sense to another
LAN
21
IEEE802.3 Medium Access
Control

Random Access
– Stations access medium randomly

Contention
– Stations content for time on medium
22
IEEE 802.3 Frame Format
23
10Mbps Specification
(Ethernet)

<data rate><Signaling method><Max segment length>

10Base5
10Base2
10Base-T
10Base-FP

Medium
Coaxial
Coaxial
UTP
850nm fiber

Signaling
Baseband
Baseband
Baseband
Manchester
Manchester Manchester
Manchester
On/Off


Topology
Bus
Bus
Star
Star

Nodes
100
30
-
33
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100Mbps (Fast Ethernet)

100Base-TX
100Base-FX
100Base-T4

2 pair, STP 2 pair, Cat 5UTP
2 optical fiber
4 pair, cat 3,4,5

MLT-3
4B5B,NRZI
8B6T,NRZ
MLT-3
25
Gigabit Ethernet Configuration
26
Gigabit Ethernet - Differences

Carrier extension

At least 4096 bit-times long (512 for
10/100)

Frame bursting
27
Gigabit Ethernet - Physical

1000Base-SX
– Short wavelength, multimode fiber

1000Base-LX
– Long wavelength, Multi or single mode fiber

1000Base-CX
– Copper jumpers <25m, shielded twisted pair

1000Base-T
– 4 pairs, cat 5 UTP

Signaling - 8B/10B
28
Wide Area Technologies Exist

WAN technology includes an
additional special-purpose computer
at each site that:
– Connects to the transmission lines
– Keeps communication independent of the
computer
29
Few WANs, Many WANs

WANs cost much more than LANs.
– Require more planning
– Require more hardware

Only a few companies build their
own WAN.
30
WANs And LANs Are
Incompatible

Many Wide Area Networks and Local
Area Networks exist.
– Cannot connect a WAN to a LAN
– Cannot interconnect the wires from
two different networks
31
WANs for Voice

Requires very small and nonvariable delays for
natural conversation--difficult to provide this with
packet-switching

As a result, the preferred method for voice
transmission is circuit-switching

Most businesses use public telephone networks,
but a few organizations have implemented private
voice networks
32
WANs for Data

Public packet-switched networks (X.25)

Private packet-switched networks

Leased lines between sites (non-switched)

Public circuit-switched networks

Private circuit-switched networks (interconnected
digital PBXs)

ISDN (integrated X.25 and traditional circuitswitching)
33
WAN Considerations

Nature of traffic
– stream generally works best with dedicated
circuits
– bursty better suited to packet-switching

Strategic and growth control--limited with
public networks

Reliability--greater with packet-switching

Security--greater with private networks
34
Wireless LANs

IEEE 802.11

Basic service set (cell)
– Set of stations using same MAC protocol
– Competing to access shared medium
– May be isolated
– May connect to backbone via access point
(bridge)

Extended service set
– Two or more BSS connected by distributed
system
– Appears as single logic LAN to LLC level
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








Wireless LAN—links clients within the vicinity of each
other.
A network adapter card that is connected to a transmitter,
called an access point, via a cable.
The transmitter located on a wall gives the signal an
uninterrupted path to a wall-mounted receiver on the far
side of the room.
Data packets are transmitted over the airwaves to the
receiver, which is also connected to network clients by a
cable.
Wireless Extended LAN—connections to clients a
couple of miles away.
Similar connectivity is an Extended LAN.
Transmitter and receiver are typically located outside the
buildings.
Forms an electronic data communication bridge called a
wireless bridge.
Data packets up to 25 miles away from the transmitter use
spread spectrum radio technology.
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Wireless LAN - Physical

Infrared
– 1Mbps and 2Mbps
– Wavelength 850-950nm

Direct sequence spread spectrum
– 2.4GHz ISM band
– Up to 7 channels
– Each 1Mbps or 2Mbps

Frequency hopping spread spectrum
– 2.4GHz ISM band
– 1Mbps or 2Mbps

Others under development
37
Wireless LANs





Mobility
Flexibility
Hard to wire areas
Reduced cost of wireless systems
Improved performance of wireless
systems
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LAN Extension

Buildings with large open areas
– Manufacturing plants
– Warehouses



Historical buildings
Small offices
May be mixed with fixed wiring
system
39
Single Cell Wireless LAN
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Multi Cell Wireless LAN
41
Client/Server Architecture

Combines advantages of distributed and
centralized computing

Cost-effective, achieves economies of scale

Flexible, scalable approach
42
Intranets

Uses Internet-based standards & TCP/IP

Content is accessible only to internal users

A specialized form of client/server
architecture
43
Extranets

Similar to intranet, but provides
access to controlled number of
outside users
– Vendors/suppliers
– Customers
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Every computer network has the same basic components:
• Cables or wireless connection
• Network adapter cards that transmit and receive
information
• Client software that makes all these components
work together
Topology: The way in which components are assembled.
There are six topologies used in the design
of a computer network:
• Bus
• Star
• Ring
• Token Passing
• Hubs
• Hybrid
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Topologies


Tree
Bus
– Special case of tree
• One trunk, no branches


Ring
Star
46
LAN Topologies
47
A bus topology requires:
• Clients are connected to the same cable known as a trunk, segment, or backbone.
• All data packets are received by every client regardless of whether the data packet is
addressed.
• Data packets not addressed to the client are ignored.
• Data packets addressed to the client are accepted and processed by the client.
• Data packets travel the complete length of the cable, then bounce back.
• This is called signal bounce and continues until the signal loses energy and dissipates.
• The network operating system has the responsibility to keep the transmission moving along
the cable.
• A client can malfunction and the network continues to operate, which is called a passive
topology.
• A passive topology is easy to construct, highly reliable, and susceptible to slow performance
during heavy network traffic.
• A break in the cable is commonly caused by an improper network connection.
• Breaks are hard to track down because every client and device on the network could be
suspect.
• A network outage does not shut down a client's operation. Clients work as a stand-alone.
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Frame Transmission - Bus LAN
49
Bus and Tree

Multipoint medium

Transmission propagates throughout medium

Heard by all stations
– Need to identify target station
• Each station has unique address

Full duplex connection between station and tap
– Allows for transmission and reception

Need to regulate transmission
– To avoid collisions
– To avoid hogging
• Data in small blocks - frames

Terminator absorbs frames at end of medium
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• Clients connect to a central device called a concentrator or hub.
• Clients transmit and receive data packets to and from the concentrator.
• It is the job of the concentrator to redirect data packets to the appropriate client.
• Clients only receive data packets addressed to them.
• Star topology reduces the network traffic clients must handle.
• The concentrator detects if a client is not connected to the network and returns data
packets to the sender.
• Failure of one client does not disable the entire network.
• Network services become unavailable if the concentrator malfunctions.
• Clients use 10BaseT network adapter cards that automatically detect trouble with the
concentrator.
• The client then stops any transmission of data packets and operates as a stand-alone
computer until
the concentrator becomes operational.
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Ring Topology

Repeaters joined by point to point links in closed
loop
– Receive data on one link and retransmit on another
– Links unidirectional
– Stations attach to repeaters

Data in frames
– Circulate past all stations
– Destination recognizes address and copies frame
– Frame circulates back to source where it is removed

Media access control determines when station can
insert frame
52
Frame
Transmission
Ring LAN
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• Each client is connected consecutively to the single cable.
• There are no ends to the ring network that must be sealed with a terminator.
• Data packets pass clockwise from one client to the next.
• If the data packet isn't addressed to the client, the client resends the data
packet to the next client.
• The client strengthens the signal, allowing the data packet to travel a further
distance.
• Clients connect to a hub. Within the hub is the ring.
• If one client is not working properly, there is a good chance the entire
network will fail.
• This depends on the network operating system.
• IBM token ring automatically ignores inactive clients.
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• A token ring is a ring-like topology.
• Clients are connected together to form a ring.
• Each data packet is transmitted to each client on the network.
• A special packet, called a token, is used to control transmissions on the network.
• The token packet is delivered to a client clockwise along the network.
• The client can accept the token and transmit a data packet.
• Addresses, data, and other necessary information are added to the token packet.
• The modified token packet becomes the new data packet and is sent to the hub
for delivery to the destination.
• The destination client acknowledges the data packet.
• The client that sent the data packet creates a new token packet and passes it to
the next client.
• The client can ignore the token packet, which is then passed along to the next
client on the ring.
• This is called token passing.
• Token packets can travel more than 10,000 times around the network per second.
55
Star Topology

Each station connected directly to
central node
– Usually via two point to point links

Central node can broadcast
– Physical star, logical bus
– Only one station can transmit at a time

Central node can act as frame switch
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Hubs
• A hub is a central processing device on a network.
• A hub is used with the star and ring topologies as a concentrator for network traffic.
• Hub topology is used to divide large network requirements into smaller, serviceable LANs
called segments.
• A hub enables a network administrator to monitor and manage network traffic.
• A hub is used to link segments together; cabling used by various clients can be mixed and
matched.
Types of Hubs
• Networks use one of two kinds of hubs:
• Passive hubs
• Active hubs
Passive Hubs
• A passive hub acts as a connector box known as a wiring panel.
• A passive hub connects cables from all clients.
• A passive hub only provides connectivity among network clients. It does not interrupt
transmissions on the network.
Active Hubs
• An active hub ID known as a multi-port repeater.
• An active hub joins together clients.
• An active hub boosts the signal along the network like a repeater.
• An active hub can, in some cases, redirect data packets to the appropriate client.
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Hybrid Topology
• The disadvantage of each topology is reduced by combining topologies.
• The combined topology is called a hybrid network.
• There are two common hybrid networks: star bus and star ring topology.
Star Bus Topology
• The star bus topology combines the star topology and the bus topology.
• Clients of the smaller networks use the star topology to connect to its network
concentrator.
• Network concentrators are linked together in a bus topology.
• The star bus topology controls traffic flow on the network.
• Traffic between concentrators is less demanding than traffic among clients.
• The bus topology is a more direct and efficient design for connecting concentrators.
• The star topology avoids transmission conflicts among clients.
• The star bus topology enables network expansion.
• If network response time is slow because of an increase in traffic, clients can be
transferred to a different
segment with its own concentrator.
58
• The star ring topology combines the best of the star and ring topologies.
• The star ring topology is referred to as the star wired ring topology.
• The star ring topology divides the network into smaller networks each having a
concentrator or hub used to connect clients.
• Concentrators are joined together using a master concentrator (sometimes
called the main hub) in the form of a ring.
• The advantage is that network traffic is handled by a topology that best suits
the volume of traffic.
• The network is scalable.
• A hybrid network reduces the chance that a complete network failure will
occur.
• Only clients directly linked to a concentrator will lose access to network
services if the concentrator malfunctions.
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Question ???
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