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Introduction. Computer Networks. Computer Networks. “Computer network” is an interconnected collection of autonomous computers. A system of interconnected computers Two computers are said to be interconnected if they are able to exchange information. Uses of Computer Networks.

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Computer Networks

computer networks
Computer Networks
  • “Computer network” is an interconnectedcollection of autonomous computers.
  • A system of interconnected computers
  • Two computers are said to be interconnected if they are able to exchange information
uses of computer networks
Uses of Computer Networks
  • Business Applications (for Companies)
  • Home Applications (for people)
  • Mobile Users
  • Social Issues
business applications of networks
Business Applications of Networks
  • A network with two clients and one server.
business applications of networks1
Business Applications of Networks
  • The client-server model involves requests and replies.


Resource sharing

Money saving

home network applications
Home Network Applications
  • Access to remote information
  • Person-to-person communication
  • Interactive entertainment
  • Electronic commerce
home network applications1
Home Network Applications
  • In peer-to-peer system there are no fixed clients and servers.
home network applications2
Home Network Applications
  • Some forms of e-commerce.
mobile network users
Mobile Network Users
  • Combinations of wireless networks and mobile computing.
transmission technology
Transmission Technology
  • Broadcast networks
  • Point-to-point networks
broadcast networks
Broadcast Networks

Single communication channel shared by all the m/c s.

Short msgs(packets)sent by any m/c are received by all other m/cs

An address field within the packet specifies for whom it is intended for.

After receiving, a m/c checks the address field-if it is for this m/c it processes the packet else ignored

  • Broadcast systems allow the possibility of addressing a packet to all destinations by using a spl. code in the address field
  • When a packet with this code is transmitted, it is received and processed by every m/c on the network.
  • This mode of opertn is called BROADCASTING
  • Some broadcast systems support txn to a sub- set of m/c s known as MULTICASTING.
  • For this 1 bit of the address filed is reserved.
  • When a packet is sent to a certain group it is delivered to all m/cs subscribing to that group
point to point
  • Consist of many connections between individual pairs of m/cs.
  • From src to detsn a packet on this type of n/w may have to visit one or more intermediate m/cs.
  • Multiple routes of diffrnt lengths r possible-so routing algrthm play a vital role in p2p n/w.

Smaller geographically localized n/ws tend to use brodcasting.

Larger n/ws are point-to-point.

network hardware1
Network Hardware
  • Local Area Networks
  • Metropolitan Area Networks
  • Wide Area Networks
  • Wireless Networks
  • Home Networks
  • Internetworks
local area networks lan
Local Area Networks(LAN)
  • Privately owned n/ws within a single buildng or campus of upto a few kms size.
  • Widely used to connect PCs and workstations in companies & factories to share resources and exchange infrmn.
  • LANs r distinguished from other n/ws by 3 characteristics:
  • Size,Transmission Technology &Topology

Worst-case txn time is bounded and known in advance.So design is easy

  • Txn Technology
  • Often use a txn technology consisting of a single cable to which all the m/cs are attached.
  • Traditional LANs run at a speed of 10 to 100 Mbps(Megabits/Second-1,000,000 bits/s).

(a) Bus

(b) Ring

  • In this at any instant 1 m/c is the master and is allowed to transmit.
  • All other m/cs are refrain from sending
  • An arbitration mechanism is needed to solve conflicts if 2 or more m/cs want to transmit simultaneously(may b centralized or distributed)
  • IEEE 802.3(Ethernet) is a bus based broadcast n/w with decentralized control operating at 10 to 100 Mbps.
  • m/cs on ethernet can transmit at any time ;if 2 or more packets collide, each computer waits a random time and tries again later
  • In this each bit propagates around on its own,not wait for the rest of the packet to which it belongs.
  • Each bit circumnavigates the entire ring in the time it takes to transmit a few bits,often before the complete packet has been transmitted.
  • IEEE 802.5(the IBM token ring) is a popular ring-based LAN operating at 4 and 16 Mbps.
metropolitan area networks
Metropolitan Area Networks
  • A metropolitan area network based on cable TV.
  • MAN covers a city (Eg:cable TV n/w).

Wide Area Networks(WAN)

  • Spans a large geographical area(a country /continent).
  • It contains collection of m/cs intended for running user pgms-known as HOSTS.
  • Hosts are connected by a communication SUBNET.
  • Job of the subnet is to carry msgs from host to host
In most WAN the subnet consists of 2 distinct components:
  • Transmission lines and switching elements.
  • Transmission lines (circuits,channesl or trunks) moves bits betwn m/c s
  • Switching elements are specialized computers used to connect 2 or more txn lines.
  • When data arrive on an incoming line the switching element must choose an outgoing line to forward them.
  • These switching computers r known as router.
In abov fig.Each host is connected to a LAN on which a router is present.
  • The collection of communication lines and routers form the subnet.
  • When a packet is sent from one router to another via one or more intermediate routers,the packet is received at each intermediate router ,stored there until the required o/p line is free and then forwarded.
  • A subnet organized according to this principle is called store-and-forward or packet-switched subnet.
packet switching principle
Packet switching principle
  • When a process on some host has a msg to send to a process on some other host ,the sending host first cuts the msgs into packets, each one having its number in sequence.
  • These packets are then injected into the n/w one at a time in quick succession.
  • The packets are transported individually over the n/w and deposited at the receiving host, where they r reassembled into the original msg and delivered to the receiving process.
wireless networks
Wireless Networks

Categories of wireless networks:

  • System interconnection
  • Wireless LANs
  • Wireless WANs
system interconnection
System interconnection
  • Interconnecting the components of a computer using short-range radio.
  • Bluetooth is an Eg for this.
  • It allows different components like mobiles, digital cameras etc to connect to a computer by merely being brought within range.
wireless lan
Wireless LAN
  • s/ms in which every computer has a radio modem and antenna with which it can communicate with other s/ms.
  • There is a standard for wireless LANS called IEEE 802.11,which most s/ms implement and which is becoming more widespread.
wireless wans
Wireless WANs
  • The radio n/w used for cellular phones is an eg for low-b/w wireless s/ms.
  • Wireless LANs can operate at rates up to about 50 Mbps over distance of 10 mtrs.
wireless networks1
Wireless Networks
  • (a) Bluetooth configuration
  • (b) Wireless LAN
wireless networks2
Wireless Networks
  • (a) Individual mobile computers
  • (b) A flying LAN
home network categories
Home Network Categories
  • Computers (desktop PC, PDA, shared peripherals
  • Entertainment (TV, DVD, VCR, camera, stereo, MP3)
  • Telecomm (telephone, cell phone, intercom, fax)
  • Appliances (microwave, fridge, clock, furnace)
  • Telemetry (utility meter, burglar alarm, babycam).
  • A collection of interconnected networks is called internetwork or just internet
  • Two different and frequently incompatible networks are connected using machines called gateways
network software
Network Software

The philosophy of connecting together two entities. “Layering” is the key word.

network software1
Network Software


Layers -

The concept that network software is organized functionally into levels. A level on one host talks to the same level on another host (its peer).

Protocol -

The protocol is the convention or standard that a layer uses to talk to the other layer. An agreement or standard on the conversation.

Physical Medium -

Underneath the layers is the wire or fiber or whatever.

Interface -

Defines the services that one layer offers another (either up or down.)

Important that each layer perform specific actions.

protocol hierarchies
Protocol Hierarchies
  • The philosopher-translator-secretary architecture.
network software2
Network Software


  • Network architecture -
  • A set of layers and protocols. It contains details on what happens in the layer and what the layers says to its peer.
  • Functional interfaces and implementation details are not part of the spec, since that's not visible outside the machine.
  • Protocol stack -
  • A list of protocols used by a system, one protocol per layer.
network software3
Network Software


o Addressing

o Number of logical channels per connection (for priority purposes)

o Error control. (garbled or missing.)

o Preservation of message ordering.

o Flow control.

o Breaking up messages into a smaller chunks (and reassembly.)

o Multiplexing messages on same connection.

o Routing - how to get from one host to another.

That word “Multiplexing”:

connection oriented connectionless services

Network Software

  • Connection oriented service -
  • Like the phone system. The system establishes a connection, uses it, and closes it. Acts like a tube. Data comes out the other end in the same order as it goes in.
          • Connection Setup
          • Data Transfer
          • Connection Termination
  • Connectionless service -
  • Like the post office. Each message has the entire address on it. Each
      • message may follow a different route to its destination. Ordering not
      • maintained.
          • Data Transfer
connection oriented connectionless services1

Network Software

Quality of service -

Will the message arrive??

A reliable connection-oriented service guarantees success.

It is implemented by having the rxer acknowledge the receipt of each msg .

It is appropriate for FileTransfer( the owner of the file wants to b sure that all the bits arrive correctly and in the same order)

Has 2 variations:

o Message sequence - message boundaries and order are maintained.

o Byte streams - messages are broken up or combined; flow is bytes.

connection oriented connectionless services2

Network Software

Datagram Service –(Unreliable Connectionless)

Like junk mail. It's not worth the cost to determine if it actually arrived. Needs a high probability of arrival, but 100% not required.


No acknowledgment.

Acknowledged datagram service -

As above, but improved reliability via acknowledgment.

Request-reply service -

The sender transmits a single datagram containing a request;

The reply contains the answer

Summarized in this Table.

service primitives
Service Primitives
  • Five service primitives for implementing a simple connection-oriented service.
service primitives1
Service Primitives
  • Packets sent in a simple client-server interaction on a connection-oriented network.
services to protocols relationship
Services to Protocols Relationship
  • The relationship between a service and a protocol.
services protocol
Services & Protocol


  • Is a set of primitives (operations) that a layer provides to the layer above it.
  • It defines what operations the layer is prepared to perform.
  • It say nothing about how these operations are implemented.
  • It relates to an i/f between 2 layers with the lower layer being the service provider and the upper layer being the service user.
  • Is a set of rules governing the format and meaning of the packets.
  • Entities use protocols to implement their service definitions.
  • They r free to change their protocol at will, provided they do not change the service visible to their users.
In general:
  • A service is like an object. It defines operations that can b performed on an object but does not specify how these operations are implemented.
  • A protocol relates to the implementation of the service .
reference models
Reference Models

There are two competing models for how the software is layered.

These are the OSI and the TCP models.

The protocols associated with the OSI model r rarely used ,but the model itself is quite general and still valid.

The model itself is not of much use but the protocols are widely used.

reference models1
Reference Models


This model is based on a proposal developed by the ISO as a first step towards the standardization of the protocols used in the various layers.

Developed by ISO == International Standards Organization

The model is called ISO OSI == ISO Open Systems Interconnection Ref. model because it deals with connecting with open systems (systems that are open for communication with other s/ms)

Principles used to develop OSI Layering:

1. Need a layer for each different level of abstraction.

2. Each layer performs a well defined function.

3. Each layer should be standardizable.

4. Layer boundaries should minimize data flow across those boundaries.

5. The right number of layers - don't put too many functions together, but not too many layers either.

reference models2
Reference Models


Physical Layer -

Purpose -- Transmits raw bits over a communication channel.

Design issues:

making sure that when one side sends 1 bit it is received by the other side as a 1 bit, not as a 0 bit.

Other issues r how many volts should b used to represent a 1s and 0s,whether txn may proceed simultaneously in both directions,how the initial connection is established, and how its is stopped when both sides are finished.

Here design issues largely deal with mechanical, electrical and timing interfaces, and the physical txn medium.

reference models3
Reference Models


Data Link Layer -

Framing -- Breaks apart messages into frames. Reassembles frames into messages.

Error handling -- solves damaged, lost, and duplicate frames.

Flow control -- keeps a fast transmitter from flooding a slow receiver.

Gaining Access -- if many hosts have usage of the medium, how is access arbitrated.



Network Layer

It controls the operation of the subnet

Routing -- Which path is followed by packets from source to destination. Can be based on a static table, when the connection is created, or when each packet is sent.

Congestion -- Controls the number packets in the subnet.

Accounting -- Counts packets/bytes for billing purposes.

Heterogeneity -- Interfacing so one type of network can talk to another.

reference models4
Reference Models


Transport Layer

Basic function of the transport layer is to accept data from above, split it into

smaller units pass these to the n/w layer and ensure that the pieces all arrive

at the other end correctly.

Reliability -- Ensures that packets arrive at their destination. Reassembles out of order messages.

Service Decisions -- What type of service to provide; error-free point to point, datagram, etc.

Mapping -- Determines which messages belong to which connections.

Naming -- "Send to node xyzzy" must be translated into an internal address and route.

Flow control -- keeps a fast transmitter from flooding a slow receiver.

reference models5
Reference Models



Session Layer –

Allows users on different m/cs to establish sessions between them.

A session might b used to allow a user to logon into a remote time sharing

s/m or to transfer file betwn 2 m/cs.

It allows dialog control-keeping track of whose turn is to transmit.

Token management-preventing 2 parties from attempting the same critical

operation in the same time

Synchronization-check pointing long txns to allow them to continue from

were they where after crash.




Presentation Layer -

  • Concerned with the Syntax and semantics of information transmitted.
  • Understands the nature of the data being transmitted.
  • Converts ASCII/EBCDIC etc…



Application Layer -

  • Contains a variety of protocols commonly needed by users.
  • One widely used application layer protocol is HTTP.
  • Other protocols r FTP,mail etc…
reference models6
Reference Models


Data Transmission in the OSI Model -

reference models7
Reference Models



  • Protocol used in ARPANET (a research n/w by DoD) and in Internet .Common mechanism that is gaining on/surpassing the OSI Model
Host to Network Layer -
  • This lowest level is not defined in this model.
  • It varies from host to host and n/w to n/w

Internet Layer –

  • Is the linchpin that holds the whole architecture together
  • Its job is to inject packets into any n/w and have them travel independently to the destination
  • It defines an official packet format and protocol called IP.

So the job of this layer is to deliver IP packets where they r

supposed to go (ROUTING) and congestion control

Transport layer
  • It is designed to allow peer entities on the source and destination hosts to carry on a conversation.
  • 2 end-to-end protocols are defined here-TCP and UDP
  • TCP-is a reliable connection-oriented protocol that allows a byte stream originating on one m/c to b delivered without error on any other m/c on the internet.
  • It fragments the incoming byte stream into discrete msgs and passes each one into the internet layer.
  • At the destn the rxing TCP process reassembles the rxd msgs into the o/p stream.
  • TCP also handles flow control
  • is an unreliable connection less protocol for applicatn that do not want TCP’s sequencing or flow control.
  • It is widely used for one-shot,client-server type request-reply queries and applications in which prompt delivery is more important than accurate delivery-txing speech or video.
application layer
Application Layer
  • It contains all the higher-level protocols.
  • HTTP,FTP,SMTP,DNS r some of them.
comparison of osi tcp ip
Comparison of OSI & TCP/IP

Similarities r

  • Both r based on the concept of stack of independent protocol and also functionalities of layers is roughly similar.
  • 3 concepts r central to OSI
  • Services,Interfaces,Protocols
  • OSI model can make the distinction betwn these 3 concepts explicitly.
  • The service defn tells what the layer does,Not how entities above it access it or how the layer works.
  • It defines the layers semantics
A layers Interface tells the process above it how to access it.
  • It defines what the parameters are and what results to expect.
  • It says nothing about how the layer works inside.
The peer protocol used in a layer r layers own business .
  • It can use any protocol it wants to ,as long as it gets the job done.
  • It can also change them at will without affecting s/w in higher layers
  • With TCP/IP reverse is true-Protocol came first and the model is a description of the existing protocol.
  • There was no problem with the protocol fitting the model but the model did not fit any other protocol stack.
  • So it was not especially useful for describing other, non TCP/IP networks.
  • OSI has 7 layers but TCP has 4 layers.
The TCP/IP did not originally clearly distinguish betwn service, interface and protocol.
  • So the protocols in OSI model r better hidden than in the TCP/IP model and can b replaced relatively easily as the technology changes.
  • The OSI ref. model was devised before the protocol were invented.
  • This ordering means that the model was not biased toward one particular set of protocols (quite general)
The OSI model supports both connection oriented and connection less communicatn in the n/w layer ,but only connection oriented service in the transport layer.
  • TCP/IP model has only connectionless communication in n/w layer but supports both modes in the transport layer.
reference models8
Reference Models


  • Bad Timing -
  • TCP already in use by the time OSI came along.
  • Bad Technology -
  • Layers don't match reality . Chosen because IBM's SNA has seven layers.
  • Dominated by phone company mentality.
  • Bad Implementation -
  • Huge, unwieldy, slow.
  • Doesn't separate spec from implementation.
  • Model is only good for describing TCP.
  • Doesn't specify physical and data link layers.
  • 5 Application
  • 4 Transport
  • 3 Network
  • 2 Data Link
  • 1 Physical
example networks novell netware
Example Networks-Novell Netware
  • Is the most popular n/w in the PC world.
  • Designed to b used by companies downsizing from a mainframe to a n/w of PCs.
  • In such a s/m each user has a desktop pc functioning as a client.
  • This is based on a client-server model.
  • It uses a proprietary protocol stack shown below
novell netware ref model
Novell Netware Ref. model








FILE Server







Token ring


Token ring


This stack is based on the old XeroxNetworkSystem(XNS).
  • It looks more like TCP/IP than OSI
  • The n/w layer runs an unreliable connection less internetwork protocol called IPX.
  • It passes packets transparently from source to destn.
  • Its functionality is similar to IP except that it uses

12-byte addresses instead of 4 byte adrs.

Above IPX comes a connection oriented protocol NCP (Network Core Protocol).
  • It is the core of NetWare.
  • SPX is also available but only provides transport
  • IPX format:
  • 2 2 1 1 12 12


Source Adrs


Destn Adrs

Packet type

Transport control

Packet length

Check sum

Transport control: counts how many networks the packet has traversed-when this exceeds a max.the packet is discarded.
  • Packet type: is used to mark various control packets.
  • 2 addresses each contain a 32 bit network number ,a 48 bit m/c number(802 LAN adrs) and 16 bit local adrs (socket)
Working :
  • About once a min. each server broadcasts a packet giving its address and telling what services it offers.
  • These broadcasts use SAP protocol
  • The packets are seen collected by a spl agent running on the router m/cs.
  • The agent use this information contained in them to construct db of which servers are running where.
  • When a client m/c is booted, it broadcasts a request asking where the nearest server is.
  • The agent on the local router m/c sees this rqst,looks in its db of servers and matches up the request with the best server.
  • The choice of server to use is then sent back to the client.
  • The client can now establish an NCP connectn with the server.
  • Using this connectn,the client and sever negotiate a max. packet size.
  • From this point on ,the client can acces the file s/m and other services using this connection.
example datacommunication services
Example DataCommunication Services

1)X.25 Networks:

  • Many older n/ws especially outside the US follow a standard called X.25.
  • Developed during 1970s by CCITT (Comite Consultatif International Telegraphique et Telephonique).
  • It provide an i/f betwn public packet-switched n/ws and their customers.
  • The phy.layer protocol called X.21 specifies the physical,electrical and procedural i/f betwn the host and the n/w.
  • Only few public n/ws support this std becos it requires digital rather than analog signaling on the telephone line
The DLL std has a number of variations.
  • They all r designed to deal with txn errors on the telephone line betwn the user’s equipment (hosts) and the public n/w (router)
  • The n/w layer protocol deals with addressing, flow control, delivery confirmation etc.
  • It allows the user to establish virtual circuits and then send packets of up to 128 bytes on them.
  • Most X.25 n/ws work at a speed up to 64 kbps, which make them obsolete for many purposes.
X.25 is connection oriented and supports both switched virtual circuits an permanent ones.
  • A switched VC is created when one computer sends a packet to the n/w asking to make a call to remote computer.
  • Once established packets can b sent over the connection, always arriving in order.
  • A permanent VC is used the same way as switched one but it is set up in advance by agreement betwn the customer and the career.
  • It is always present and no call setup is required to use it and similar to a leased line.
2 broadband isdn and atm
2)Broadband ISDN and ATM
  • B-ISDN is a new n/w for the future that will replace the entire telephone s/m and all the specialized n/ws with a single integrated n/w for all kinds of infrmn transfer.
  • This hav a huge data rate compared to all existing n/ws.
  • It will offer video on demand ,live TV from many sources, full motion multimedia email,CD-quality music, LAN interconnection etc over the telephone line.
  • The underlying tech. that makes it posible is called ATM (AsynchronousTranferMode).
  • ATM Virtual Circuits
  • ATM networks r connection oriented.
  • Sending data requires first sending a packet to set up the connection.
  • As the setup packet travels its way thru the subnet all the routers on the path make an entry in their internal tables noting the existence of the connection and reserving whatever resources r needed for it.
Connections r often called Virtual circuits.
  • It also support permanent virtual circuits, which r permanent connectn betwn 2 hosts(like leased lines)
  • Each connection has a unique connectn identifier.
The basic idea of ATM is to transmit all infrmn in small, fixed-size packets called cells.
  • The cells are of 53 bytes long-5 bytes r header and 48 bytes r payload.
  • Part of the header is the connectn identifier.
  • So the sending & rxing hosts and all intermediate routers can tell which cells belongs to which connectns.
  • Cell routing is done in hardware at high speed.
  • The main advtg of fixed size cells is that it is easy to build h/w routers to handle short,fixed-length cells.
  • Another plus of ATM is that the h/w can b set up to copy one incoming cell to multiple o/p lines,a property that is required for handling a TV pgm that is being broadcast to many rxers.
  • Small cells donot block any line for very long which makes guaranteeing quality of service easier.
ATM n/ws r organized like WANs with lines and routers.
  • Most common speed for ATM n/ws r 155 Mbps nd 622 Mbps.
  • 155 is chosen for txing HDTV.
  • 622 is for four 155 Mbps channels
ATM has its own ref.model other than OSI nd TCP/IP.
  • It consists of 3 layers,Physical,ATM and ATM Adaptation Layer plus whatever users want to put on top of that.
  • Phy.Layer:deals with phy.medium :voltages, bit timing etc.
  • No prescribed rules, but says that ATM cells can b sent on a wire of fibre by themselves or packaged in other carrier, ATM has been designed to b independent of the txn medium.
ATM Layer:deals with the cells and cell transport.
  • It defines the layout of a cell and tells what the header fields mean.
  • It also deals with the establishment and release of VCs.
  • Congestion control is also done here.
AAL:Most aplications donot want to work directly with cells .A layer above the ATM layer has been defined to allow users to send packets larger than a cell.
  • The ATM interface segments these packets, transmits the cells individually, and reassembles them at the other end.

This is the function of AAL.

ATM model is defined as being 3D .
  • The user plane deals with data transport, flow control, error correction, and other user functions.
  • Control plane is concerned with connection mgmt.
  • The layer and plane mgmt functions relate to resource mgmt and inter layer coordination
Phy. And AAL r each divided into 2 sublayers
  • Physical layer:PMD and TC
  • PhysicalMediumDependent:interfaces to the actual moves the bits ON and OFF and handles the bit timing.
  • TC(TxnConvergence):when cells r txed the TC layer sends them as string of bits to the PMD layer .
  • At the other end the TC sublayer gets a pure incomng bit stream frm the PMD layer.
  • Its job is to convert this bit stream into a cell stream for the ATM layer.
  • It handles all the issues related to telling where cells begin and end in the bit stream.
The AAL layer split into a SAR and CS
  • SAR(segmentatn and Reassebly):it breaks up packets into cells on the txn side and puts them back together again at the destn.
  • CS(ConvergenceSublayer):makes it posible to have ATM s/ms offers different kinds of services to different applicatns.(eg:file handling and VOD have different requirements concerning error handling, timing etc)
transmission media
Transmission Media

Is the physical path betwn txer and rxer in a data txn system.

  • Guided Media
  • In this waves r guided along a solid medium such as copper twisted pair,coaxial cable and OFC etc-Known as Wired txn.
  • B/W is important
  • Unguided Media
  • Atmosphere and outer space r eg,that provides a means of txing EM signals but donot guide them-known as wireless txn
guided transmission media wired
Guided Transmission Media (Wired)
  • Magnetic Media
  • Twisted Pair
  • Coaxial Cable
  • Fiber Optics
transmission media1
Transmission Media
      • Most common way to transport data from one computer to another is to write them onto magnetic tape or removable media, physically transport the disk or tape to the destn m/c and read them back in again.
      • Simply two insulated copper wires of thickness 1mm of thickness twisted together in a helical form- the twisting cuts down on electrical interference.
      • Heavily used in the phone system –
      • TPs can run several Kms without amplification, but for longer distance repeaters r needed.
      • are of 2 types :STP and UTP
      • STP: is covered with a metallic braid or sheathing that reduces interference-better performance than UTP
      • UTP :ordinary telephone wire. No shield, highly prone to interference
Categories of UTP:
  • Category 3:Consist of 2 insulated wires gently twisted together.
  • 4 such pairs r grouped in a plastic sheath
  • b/w:16 MHz
  • Category 5:similar to category 3 but more twists/cms,which results in less crosstalk, and a better quality signal over a long range, more suitable for high-speed computer communication
  • b/w: 100MHz
twisted pair
Twisted Pair

(a) Category 3 UTP.

(b) Category 5 UTP.

  • Are of 2 types:Baseband and Broadband
  • BASEBAND:50 Ohm cable
      • Used for digital transmissions.
      • Good noise immunity.
      • Data rates as high as 1Gbps for short distances.
      • Now being replaced by fiber.

BROADBAND:75 Ohm cable

      • Used for analog transmissions (called broadband.)
      • Can run 300 MHz for long distances.
      • Analog signaling has better S/N than digital signaling.
      • Interfaces must convert digital signals to analog and vice versa.
      • Designed for long distances - can use amplifiers.
      • One difference betwn baseband and broadband is that broadband s/m can cover a large area and therefore need analog amplifiers to strengthen the signal periodically
coaxial cable
Coaxial Cable

A coaxial cable.

      • An optical txn s/m has 3 components: the light source, the txn medium and the detector .
      • A pulse of light indicates a 1 bit and the absence of light indicates a zero bit.
      • The txn medium is an ultra-thin fibre of glass.
      • The detector generated an electrical pulse when when light falls on it.
      • Attaching a light source to one end of the OFC and a detector to the other ,we hav a unidirectional data txn s/m that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the o/p into electrical signal at the rxing end.
working principle
Working principle
  • When light passes from one medium to another (from fused silica to air) the ray is refracted (bent) at the silica/air boundary.
  • The amount of refraction depends on the properties of 2 media.
  • For angles of incidence above a certain critical value, the light is refracted back into the silica completely.
  • Thus a light ray incident at or above the critical angle is trapped inside the fiber and can propagate for many Kms with virtually no loss.
In OFC many different rays will b bouncing around at different angles.
  • Each ray is said to hav a different mode.
  • So fiber having this property is called multimode fiber
  • If the fiber’s diameter is reduced to a few wavelenghts of light ,the fiber acts like a wave guide, and the light can only propagate in a straight line without bouncing, yielding a single mode fiber
  • This r more expensive but can b used for longer distance.
fiber optics
Fiber Optics

(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

(b) Light trapped by total internal reflection.

fiber cables
Fiber Cables

(a) Side view of a single fiber.

(b) End view of a sheath with three fibers.

Similar to coax but no braid.
  • In multimode fibers the core is 50 microns in diameter about the thickness of human hair.
  • In single mode the core is 8 to 10 microns
  • The core is surrounded by a cladding with a lower index of refraction than the core to keep all the light in the core.
  • Next comes a thin plastic jacket to protect the cladding.
fiber cables1
Fiber Cables

A comparison of semiconductor diodes and LEDs as light sources.

fiber optic networks
Fiber Optic Networks

A fiber optic ring with active repeaters.


Unguided Txn Media (Wireless transmission)

  • Radio transmission
  • Microwave transmission
  • IR and millimeter waves
  • Light wave transmission
the electromagnetic spectrum
The Electromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.

The number of oscillations per second is called frequency (f)-unit Hz
  • The distance betwn 2 consecutive max and min is called wavelength λ (lambda)
  • In vacuum em waves travel at the speed of light (c=3*108 m/sec)
  • The fundamental relation betwn λ f and c in vacuum is λ f =c.
Radio Transmission:
  • Radio waves r easy to generate can travel long distances, and can penetrate building easily, so they r widely used for communication both indoors and outdoors
  • Radio waves r also omni directional-they travel in all directions from the source, so the txer and rxer do not have to b aligned physically.
  • Properties of radio waves r frequency dependent
  • At low freq. radio waves pass through obstacles well, but the power falls off sharply with distance from the source.
  • At high freq. radio waves tend to travel in straight lines and bounce off obstacles .
  • They r also absorbed by rain.
  • At all frequencies radio waves r subject to interference from motors and other electrical equipments
radio transmission
Radio Transmission

(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.

(b) In the HF band, they bounce off the ionosphere.

Microwave Transmission
  • Above 100MHz ,the waves travel nearly in a straight lines and can there fore b narrowly focused.
  • Concentrating all the energy into a small beam by means of a parabolic antenna gives a much higher SNR.
  • Here the txing and rxing antenna s must b accurately aligned with each other.
  • Before fiber optics ,these microwaves formed the heart of long-distance telephone txn s/m.
  • Mw communicatn is so widely used for long-distance telephone communicatn, mobile phones, TV distribution etc.
  • Its main advtg is that no right of way is needed,and by buying a small plot of ground every 50 km and putting a mw tower on it ,one can bypass the telephone s/m and communicate directly
  • Mw is relatively inexpensive.
IR and Millimeter waves
  • Unguided IR and millimeter waves r widely used for short-range communicatn.
  • Remote controls used on TV,VCR all use IR communication
  • They r relatively directional, cheap and easy to build
  • One drawback is they cannot pass through solid objects.
  • It is a plus also, bcos IR s/m in one room of a building will not interfere with a similar s/m in adjacent rooms.
  • Security of IR s/m against eavesdropping is better than that of radio s/m bcos of this.
  • So no govt. license is needed to operate an IR s/m.
Light wave transmission.
  • We can connect 2 LANs in 2 buildings va lasers mounted on their roof tops.
  • Optical signaling using lasers is inherently unidirectional.
  • So each building needs its own laser and photo detector.
  • This scheme offers very high b/w and very low cost.
  • It is relatively easy to install and require no license.

Convection currents can interfere with laser communication systems.

A bidirectional system with two lasers is pictured here.

public switched telephone system
Public Switched Telephone System
  • Structure of the Telephone System
  • The Local Loop
  • Trunks and Multiplexing
  • Switching
structure of the telephone system
Structure of the Telephone System

(a) Fully-interconnected network.

(b) Centralized switch.

(c) Two-level hierarchy.

Each telephone has 2 Copper wires coming out of it that go directly to the telephone companies nearest end office-this is a local loop
  • Each end office has a number of outgoing lines to one or more nearby switching centers-called toll office.(and lines r called toll connecting trunks).
  • If the caller and callee donot hav a toll office in common ,the path will have to b established somewhere higher in the hierarchy.
  • Primary, sectional and regional offices form a network by which the toll offices r connected.
  • these exchanges communicate with each other via high-b/w intertoll trunks
major components of the telephone system
Major Components of the Telephone System
  • Local loops
    • Analog twisted pairs going to houses and businesses
  • Trunks
    • Digital fiber optics connecting the switching offices
  • Switching offices
    • Where calls are moved from one trunk to another
analog digital txns
Analog & Digital Txns
  • In Analog Signal being txed as continuous electrical voltage
  • In digital s/m only 2 voltages r allowed.
  • Analog signals always suffer infrmn losses when amplified but digital signals have low error rate
  • Voice,data ,music and images transmisn r easy and much data rate is possible.
  • Digital txn is much cheaper .
  • Maintenance of a digital s/m is easier than maintenance of analog one.
the local loop
The Local Loop

The use of both analog and digital transmissions for a computer to computer call.

Conversion is done by the modems and codecs.


Transmission Impairments:

  • Txn lines suffer from 3 major problems:
      • Attenuation - the loss of energy as the signal propagates outward

On guided media the signal falls logarithmically with distance.

      • Delay Distortion - different frequencies travel at different speeds so the wave form spreads out.
      • Noise - unwanted energy that combines with the signal - difficult to tell the signal from the noise.


A device that converts digital data to and from an analog signal for transmission over phone lines.

Because attenuation is frequency dependent, modems use a sine wave carrier of a particular frequency, and then modulate that frequency. Various modulations include:

  • Amplitude modulation: Two different amplitudes of sine wave are used to represent 1's and 0's.
  • Frequency modulation(FSK): Two (or more) different frequencies, close to the carrier frequency, are used.
  • Phase modulation: The phase of the sine wave is changed by some fixed amount .Each phase shift transmits 2 bits of infrmn

Binary Signal

rs 232c
  • Is an i/f between the computer/terminal and modem.
  • Ie,It connects DTE(DataTerminalEqupmnt ) and DCE(DataCircuit-TerminatingEqupmnt)
  • It is an Eg for a phy.layer protocol
  • It must specify in detail the mechanical, electrical, functional and procedural i/fs
Mechanical specficatn:
  • Is a 25 pin connector,top row has pins numbered 1 to 13 and bottom row 14 to 25
  • Electrical:
  • A voltg more negative than -3 V is a binary 1 and a voltg more + than +4V is a binary 0.
  • Data rates upto 20Kbps r permitted upto 15 meters.
  • Functional:
  • Tells which circuits r connected to each of the 25 pins and what they mean.

Some of the principle RS-232C Circuits

Protective Ground(1)






Receive (3)

Rqst to send(4)

Clear to send(5)

Data Set Ready(6)

Common Return(7)

Carrier Detect(8)


When the terminal or computer is powered up,it sets the DataTerminalReady 1.
  • When the modem is powered up it sets DataSetReady 1
  • When the modem detects a carrier on the telephone line it asserts carrier detect pin.
  • RequestToSend indicates that the terminal wants to send data.
  • ClearToSend means that the modem is prepared to accept data.
  • Data r transmitted on the Transmit circuit(2) pin and receive on the Receive(3) pin.
  • It is the protocol, that is the legal sequence of events
  • The protocol is based on action-reaction pairs
  • When the terminal asserts RTS the modem replies with CTS, if it is able to accept data
  • Similar action reaction pairs exists for other circuits also.


To make efficient use of high-speed telecommunications lines, some form of multiplexing is used. Multiplexing allows several transmission sources to share a larger txn capacity.

Frequency Division Multiplexing:

Can b used with analog signals

A number of signals r carried simultaneously on the same medium by allocating to each signal a different freq band

FDM is posibl when the useful b/w of the channel exceeds the required b/w of the signals to b txed.


Wavelength Division Multiplexing:

  • The same as FDM, but applied to fibers.
  • There's great potential for fibers since the bandwidth is so huge (25,000 GHz).
pulsecodemodulation pcm
  • The analog signals r digitized at the end-office by a device

called codec, producing a 7 or 8 bit number.

  • The codec makes 8000 samples/sec (125microsec/sample).This technique is called PCM
  • PCM forms the heart of modern telephone s/m.
Time Division Multiplexing:
  • In TDM, the users take turns, each one having exclusive use of the medium in a round robin fashion.
  • TDM can be all digital.
  • It is possible when the achievable data rate of the medium exceeds the data rate of the digital signals to b txed.
  • Multiple digital signals can b carried on a single txn path by interleaving portions of each signal in time.
  • The interleaving can b at the bit level or in blocks of bytes.
T1 carrier:
  • Is a std for PCM and is in widespread use in North America and Japan.
  • It consists of 24 voice channels multiplexed together.
  • The analog signals r sampled on a round-robin basis with the resulting analog stream being fed to the codec.
  • Each of the 24 channels, in turn, gets to insert 8 bits in to the o/p stream.
  • Seven bits r data and 1 is for control yielding 7*8000=56kbps of data and 1*8000=8kbps of signaling infrmn/channel.
  • A frame consists of 24*8=192 bits plus 1 extra bit for framing yielding 193 bits every 125 micro sec.
  • This gives a gross data of 1.544Mbps

The T1 carrier (1.544 Mbps).



Synchronous Optical NETwork/SynchronousDigitalHierarchy

Early days of fiber optics ,every telephone company has its own proprietary optical TDM s/m.

This is a standard for optical TDM s/m

Most long distance traffic in the US uses trunks running SONET in its physical layer.

Design goals include:

1. Common among different carriers - requires frequency, timing standards.

2. Common among different countries - needed to supersede previous national standards.

3. Multiplexed multiple digital channels together in a standard fashion.

4.Had to provide support for operations, administrations, and maintenance (OAM).

SONET is a traditional TDM s/m- it is SYNCHRONOUS also

It uses a highly accurate master clock.

Data is transmitted SYNCHRONOUSLY.

Bits on a SONET line r sent out at extremely precise intervals,controlled by the master clock.



Source Muxer




Destn Muxer

  • A SONET s/m consist of switches, muxers,and repeaters.
  • In SONET terminology a fiber going directly from any device to any other device ,with nothing in betwn is called a section
  • A run betwn 2 muxers (one or more muxers in betwn)is called a line.
  • The connectn betwn the src and destn is calleda path.





A basic SONET frame is a block of 810 bytes transmitted every 125 Micro Sec

  • Because it's Synchronous, the frame is sent whether there's data to be carried or not.
  • Data rate is 51.84 Mbps.
  • This basic channel is called STS-1(Sync.TransportSignal-1).
  • Multiple channels can be multiplexed to get higher bandwidth.


  • This is what happens inside the phone exchange - the various wires or fibers interconnect the switching centers. Methods of switching include:
  • Circuit Switching
  • Message switching
  • Packet switching
  • Circuit Switching:
  • When a computer or a phone places a call, the switching equipment within the telephone system seeks out a physical “copper” path all the way from calling phone to receivers phone .This is the technique
  • One important property of this is the need to establish an end-to-end path before any data can b sent.
  • The elapsed time between end of dialing and the start of ringing is 10 sec.
  • During this time interval, the telephone s/m is hunting for a copper path.
  • Ie, Before actual data transmission, the call request signal must propagate all the way to the destination and b acknowledged.
  • Benefits of established path:
  • Once the setup has been completed ,the only delay for data is the propagation time for the electromagnetic signal about 5 msec/1000km.
  • No congestion

(a)Circuit switching.

(b) Packet switching.

Message Switching:
  • No physical copper path is established in advance between the sender and the receiver.
  • When the sender has a block of data to b sent ,it is stored in the first switching office (router) and then forwarded later.
  • Each block is received in its entirety, inspected for errors and then re-transmitted.
  • This method is called store-and-forward.
  • No limit on block size-ie routers must have disks to buffer long blocks
  • This method may tie up routers for long periods of time - not good for interactive traffic.
  • There is no limit on block size ,means routers must have disks to buffer long blocks.
  • Ie, a single block may tie up router-router line for minutes-so it is not good for interactive traffic.
Packet Switching:
  • Divides the message into blocks (packets).
  • Therefore packets use the transmission lines for only a short time period - allows for interactive traffic.
  • Packets r buffered in routers main memory instead of on disk.
  • In this the first packet of a multi packet message can b forwarded before the second one has fully arrived reducing delay and improving throughput.

Timing of events in

(a) Circuit switching (b) Message switching (c) Packet switching

  • This is a method of combining Voice and Data over a single wire. Used heavily by the phone system in a number of applications.
  • Ie, Integration of voice and non-voice services.


Integrated Services Digital Network: A completely digital circuit-switched phone system. Integrates voice and non-voice services.

ISDN Services:




Connecting telephone to a computer caller’s database can b displayed on the screen.


Remote electricity meter reading, online medical, burglar alarms

n isdn 64 kbps channel
N-ISDN- 64-Kbps channel


Digital bit pipe: a conceptual pipe between the customer and carrier through which bits flow- in both directions.

The digital bit pipe supports multiple independent channels by TDM of the bit stream.

In fig (a) :

The carrier places a n/w terminating device NT1 on the customers premises and connects it to the ISDN exchange in the carrier’s office using TP.

The NT1 box has a connector on into which a passive bus cable can b inserted.

Up to 8 ISDN telephones, terminals ,alarms etc can b connected to the cable.

From the customers point of view the n/w boundary is the connector NT1

n isdn


The ISDN bit pipe supports multiple channels interleaved by TDM.

Different standardized channels r:

A :4 kHz analog telephone channel

B :64 kbps digital PCM channel for voice or data

C :8/16 kbps digital channel

D :16 kbps digital channel for out-of-band signaling

E :64 kbps digital channel for internal ISDN signaling

H :384,1536, or 1920 kbps digital channel

Typically a number of channels are combined together. In USA, Primary Rate ISDN contains 23 channels (each 64 kbps carrying voice or data) + 1 channel for signaling and control (16 kbps digital channel.) In Europe, instead of 23 channels, 30 are used.

The primary Rate is designed to connect to a business with a PBX(PrivateBranchXchg).

As it turns out, most companies now need far more capacity than 64 kbps for the many uses beyond voice. So this is less than adequate.

N-ISDN may have a life as a connection to homes for people wanting to download images etc. But it's not useful for serious business applications.

b isdn
  • This is a digital virtual circuit capable of 155 Mbps. Characteristics include:
    • ATM Packet Switched Technology.

The service offered is connection oriented (from the customer's point of view) but is implemented internally with packet switching.

2 kinds of connections r offered: Permanent VCs & Switched VCs


Comparing Virtual Circuits and Circuit Switching

  • In circuit switching n/w for making a connection a physical path is established from the source to the destination.
  • In a VC n/w like ATM when a circuit is established ,the route from source to destination is chosen and all the switches (routers) along the way make table entries so they can route any packets on the VC
  • They also have the opportunity to reserve resources for the new circuit.

When a packet comes along, the switch inspects the packet’s header to find out which VC it belongs to.

Then it looks up that VC in its tables to determine which communication line to send on.

Cellular Radio
  • Paging s/ms
  • Cordless telephones
  • Analog cellular phones
  • Digital cellular phones

2.1 Theoretical Basis For Data Communication

What every sophomore EE knows !!! How much data can be put on a wire? What are the limits imposed by a medium?

2.2 Transmission Media

Wires and fibers.

2.3 Wireless Transmission

Radio, microwave, infrared, unguided by a medium.

2.4 The Telephone System

The system invented 100 years ago to carry voice.

2.5 Narrowband ISDN

Mechanisms that can carry voice and data.