data and computer networks

8/2/2019 Data and Computer Networks

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6th Semester, BEIT 1

Data and Computer Networks:An Overview

Tuhina Samanta


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Outline: Computer Network

Classification of Networks

Circuit/Packet Switching

ISDN

ADSL SONET


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Computer Network Acomputer networkis a system for

communicating between two or morecomputers and associated devices. It is an

interconnection of computers for the purposesof sharing information and resources.

Computer networks can be classified accordingto their size: Personal area network (PAN)

Local area network (LAN)

Metropolitan area network (MAN)

Wide area network (WAN)


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Router

An example of a network

Internet

SegmentNode

Hub

Hub

Bridge


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PANAPANis a network that is used for

communicating among computers and

computer devices (includingtelephones) in close proximity of arounda few meters within a room

It can be used for communicatingbetween the devices themselves, or forconnecting to a larger network such asthe internet


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A LAN is a network that is used for communicatingamong computer devices, usually within an officebuilding or home

LANs enable the sharing of resources such as files orhardware devices that may be needed by multiple users

Is limited in size, typically spanning a few hundredmeters, and no more than a mile

Is fast, with speeds from 10 Mbps to 10 Gbps

Requires little wiring, typically a single cable connectingto each device

Has lower cost compared to MANs or WANs

Local Area Network


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MANs and WANs MANSIntermediate area coverage between

LAN and WAN

Point-to-point switched network is used in MANs Higher data rate than LAN and WAN

WANs area coverage 1000m to 1000Km.

Telephone network subscribers avail 64kbps data

rate. High speed WANs are capable of availing 100Gbps

with ATM service.



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Network Topology The physical topology of a network

refers to the configuration of cables,

computers, and other peripherals.Physical topology should not beconfused with logical topology which

is the method used to pass informationbetween workstations.


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Types of Physical Topologies Linear bus

Star

Ring

Tree


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Linear BusAll nodes (file server, workstations, andperipherals) are connected to the linear

cable.


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Linear Bus Advantages

Easy to connect a computer or peripheral to a linearbus.

Requires less cable length than a star topology. Disadvantages

Entire network shuts down if there is a break in themain cable.

Terminators are required at both ends of thebackbone cable. Difficult to identify the problem if the entire network

shuts down. Not meant to be used as a stand-alone solution in a

large building.


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StarA star topology is designed with each

node connected directly to a central

networkhub, switch, or concentrator
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Star Advantages

Easy to install and wire.

No disruptions to the network when connecting or

removing devices. Easy to detect faults and to remove parts.

Disadvantages Requires more cable length than a linear topology.

If the hub, switch, or concentrator fails, nodesattached are disabled.

More expensive than linear bus topologies because ofthe cost of the hubs, etc.


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Ring The ring network is like a bus network, but the

end of the network is connected to the first node

Nodes in the network use tokens to communicatewith each other

Backbone


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Tree or Expanded StarA tree topology combines characteristics of

linear bus and star topologies. It consists of

groups of star-configured workstationsconnected to a linear bus backbone cable.

Tree topologies allow for the expansion of

an existing network, and enable schools toconfigure a network to meet their needs.


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Tree or Expanded Star


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Tree or Expanded Star

Advantages Point-to-point wiring for individual segments.

Supported by several hardware and software vendors. Disadvantages

Overall length of each segment is limited by the type ofcabling used.

If the backbone line breaks, the entire segment goesdown.

More difficult to configure and wire than othertopologies.


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Choice of Configuration

PhysicalTopology

CommonCable

CommonProtocol

Linear BusTwisted PairCoaxialFiber

Ethernet

StarTwisted PairFiber

Ethernet

TreeTwisted PairCoaxialFiber

Ethernet


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Types of LANs Protocols

The three most popular types of LANs

Token ring

Ethernet FDDI (Fiber Distributed Data Interface)


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Operation of Token Ring

At the start, a freeToken is circulating onthe ring, this is a data

frame which to allintents and purposes isan empty vessel fortransporting data. To

use the network, amachine first has tocapture the free Tokenand replace the datawith its own message.


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Operation of Token Ring


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Operation of token ring

Machine 1 wants to send some data to machine 4, so it first has tocapture the free Token. It then writes its data and the recipient'saddress onto the Token.

The packet of data is then sent to machine 2 who reads the address,

realizes it is not its own, so passes it on to machine 3. Machine 3 doesthe same and passes the Token on to machine 4. 4 retrieves data from the Token. It cannot, however, release a free

Token on to the ring, it must first send the message back to number 1with an acknowledgement to say that it has received the data

The receipt is then sent to machine 5 who checks the address, realizes

that it is not its own and so forwards it on to the next machine in thering, number 6 who does the same task. Ultimately when the receipt reaches machine 1, machine 1 recognizes

the address, reads the acknowledgement from number 4, and thenreleases the free Token back on to the ring ready for the next machineto use.


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Circuit Switching

Method used by the telephone network

TDM of FDM is used for data transfer

Steps: Circuit establishment

Information transfer

Circuit disconnect

Once path is establishes, communication isfull duplex


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Circuit Switching


Networknodes

Message

SubscriberB

Message

Message

Message

SubscriberA


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Packet Switching

Used in the Internet Data is sent in Packets (header contains control

info, e.g., source and destination addresses)

Per-packet routingAt each node the entire packet is received,

stored, and then forwarded No capacity is allocated

Header Data


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Packet Switching

Packet 1

Packet 2

Packet 3

Packet 1Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

processingdelay ofPacket 1

at Node 2

propagationdelaybetweenHost 1 &Node 1transmission

time ofPacket 1

at Host 1

Host 1 Host 2

Node 1 Node 2


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Networknodes

Message

SubscriberB

SubscriberA

Message

Message

Message

Figure 7.13

Connectionless Packet Switching

Virtual circuit

message has header with source

& destination address

CRC check bit are used to

detect errors

Each switch checks error, if yes, ask

retransmission, if not find next hop.

Message enter into a QUEUE to

wait for line free to transmitIncreased utilization of line is at the

expense of queuing delay

Loss of message may occur because

of insufficient buffer.

End-to-end error recovery is needed


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Packet 2

Packet 1

Packet 1

Packet 2

Packet 2

Figure 7.15

Datagram Packet witching

Operations:1. Address information is included in

header

2. CRC for error recovery

3. Switches inspect destination addressin header to determine next hop

4. Packets are put in QUEUE to wait

for line becoming available

5. Sharing lines among multiple

packets, high utilization is at the

expense of queue delay6. Packets travel independently and

may along different paths

7. Route may be detoured,thusbypassing failure and congestion

8. Packets may arrive out of order,

reordering may be required


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Differences Between Circuit & Packet Switching

Circuit-switching Packet-Switching

Guaranteed capacity No guarantees (best effort)

Capacity is wasted if datais bursty

More efficient

Before sending dataestablishes a path

Send data immediately

All data in a single flowfollow one path

Different packets mightfollow different paths

No reordering; constantdelay; no packet drops

Packets may be reordered,delayed, or dropped


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OSI Layer

Application

Presentation

Session

Transport

Network

Data LinkPhysical


Provides for reliable transfer ofinformation across the physical link Sends blocks (frames) with necessarysynchronization

Error control, flow control

Concerned with transmission of

bit stream Deal with mechanical, electricalfunctionality Procedural characteristics toaccess physical medium


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Digital Service: ISDN

ISDN, which stands for Integrated ServicesDigital Network, is a system of digital phoneconnections.

Put a bridge "the last mile" between the CentralOffice (CO) and the premise connection (home). With ISDN, voice and data are carried by bearer

channels (B channels) occupying a bandwidthof 64 kbps.

A data channel (D channel) handles signalingat 16 kbps or 64 kbps, depending on the servicetype.


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ISDN

H channels provide a way to aggregate Bchannels. They are implemented as:

H0=384 kb/s (6 B channels) H10=1472 kb/s (23 B channels)

H11=1536 kb/s (24 B channels)

H12=1920 kb/s (30 B channels) -International (E1) only


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ISDN Services

There are two basic types of ISDN service:

Basic Rate Interface (BRI)

BRI consists oftwo 64 kbps B channelsand one 16kbps D channelfor a total of 144 kbps. D channel isfor handshaking and control

Primary Rate Interface (PRI)

PRI is intended for users with greater capacityrequirements. Typically the channel structure is 64kbps23 B channelsplus one 64 kbps D channelfor atotal of 1536 kbps.


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ISDN Services

ISDN lines can be dedicated lines that arealways up and connected or they can be dialon demand (DOD) lines .

The connection time for an ISDN line is veryquick, in the order of 0.5 second or so.

This can result in a substantial cost saving ifused over long distance or paying by theminute.

The line charges are only for when data isbeing transferred and not when it is sittingidle .


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ISDN - OSI Layers

OSI Model ISDN

Network D Channel

Data Link B Channel

Physical


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ISDN Line Encoding

Standard telecommunication 2B1Q which standsfor 2 Binary elements encoded in 1 quaternary.

A dibit (digital bit) represents two binaryelements for each voltage change

Dibit Voltage

10 +3

11 +1

01 -1

00 -3


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ISDN Premise Connection

Central

Office

NT-1Network

Termination1

ISDNDevice#1

ISDNDevice#8

. . . TerminalAdapter

POTS(TE2)

S/T Interface (4 wire, linear bus topology)

BRILocal loop(2 wire)

Terminal Equipments

PRI uses NT2100 W

Plain old telephone set


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Asymmetrical Digital Subscriber Line

Asymmetrical Digital Subscriber Line (ADSL) is amethod to use the existing analog local loop linesfor digital data transfer to and from the home.

It is asymmetrical in that the upstream transferrate is slower than the downstream data rate. The data transfer rate is adaptive. The range for upstream data transfer is 64 kbps

to 768 kbps. The range for downstream datatransfers is 1.5 Mbps to 8 Mbps. ADSL is a Physical layer protocolwhich covers the

transmission of data, and cabling requirements.


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ADSL Premise Equipment

DSLAMATU-C

Modem orCSU/DSU

POTSswitch

ATU-R

Ethernet

to connectNIC

Splitter SM

ServiceModule

ADSL Modem

POTS

Localloop

Splitter

Central Office

RJ45

RJ11


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ADSL Premise Equipment ADSL shares the bandwidth of the local loop with the

existing phone system.

It does not require modification to the central officeswitch.

At Central office (CO) a splitter combines the ADSL information with the POTS switch's

analog information.

At the central office end, the ADSL signal is sent to the DigitalSubscriber Line Access Module (DSLAM) and then to a

communication server.

At the premise end, another splitter (ATU-R device) separates the ADSL information

from the analog information.

The Service Module translates the information to Ethernet.


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Broadband DigitalCommunications: SONET

Synchronous Optical Network (SONET)is a fiber-optic transmission protocol

that provides synchronous transport Used primarily for network transport of

broadband communications between

switching nodes in telephone network. Data rate 51.84Mbps or greater


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Books:

1. Willium Stallings, Data and ComputerCommunications , Sixth Edition

2. Tanenbaum, Computer Networks,Fourth Edition

3. B. Forouzan, Data Communication

and Networking, Fourth Edition, TheMcGraw-Hill Companies, Inc.

data and computer networks

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