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Introduction
Computer Networks
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Computer Networks
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“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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Business Applications (for Companies)
Home Applications (for people)
Mobile Users
Social Issues
Business Applications of
Networks
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A network with two clients and one server.
Business Applications of
Networks
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The client-server model involves requests
and replies.
Advantages:
Resource sharing
Money saving
Home Network Applications
 Access
to remote information
 Person-to-person
communication
 Interactive entertainment
 Electronic commerce
Home Network Applications
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In peer-to-peer system there are no fixed
clients and servers.
Home Network Applications
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Some forms of e-commerce.
Mobile Network Users
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Combinations of wireless networks and
mobile computing.
NETWORK HARDWARE
Transmission Technology
Broadcast networks
 Point-to-point networks
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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 fieldif it is for this m/c it processes the packet else
ignored
Broadcasting
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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
Multicasting
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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
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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 Hardware
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Local Area Networks
Metropolitan Area Networks
Wide Area Networks
Wireless Networks
Home Networks
Internetworks
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Classification of interconnected processors
by scale(by physical size)
Local Area Networks(LAN)
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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
Size:
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).
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Topologies
(a) Bus
(b) Ring
Bus
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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
Ring
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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
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A metropolitan area network based on cable
TV.
MAN
covers a city (Eg:cable TV n/w).
Wide Area Networks(WAN)
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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
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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.
Relation between Host and subnet
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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 packetswitched subnet.
Packet switching principle
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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.
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A stream of packets from sender to receiver.
Wireless Networks
Categories of wireless networks:
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System interconnection
Wireless LANs
Wireless WANs
System interconnection
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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
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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
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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 Networks
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(a) Bluetooth configuration
(b) Wireless LAN
Wireless Networks
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(a) Individual mobile computers
(b) A flying LAN
Home Network Categories
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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).
Internetworks
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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
The philosophy of connecting together
two entities. “Layering” is the key word.
Network Software
PROTOCOL
HIERARCHIES
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
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The philosopher-translator-secretary architecture.
Network Software
PROTOCOL
HIERARCHIES
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 Software
DESIGN ISSUES FOR THE LAYERS
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”:
Network Software
CONNECTION-ORIENTED /
CONNECTIONLESS SERVICES
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
Network Software
CONNECTION-ORIENTED /
CONNECTIONLESS SERVICES
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.
Network Software
CONNECTION-ORIENTED /
CONNECTIONLESS SERVICES
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.
Connectionless.
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
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Five service primitives for
implementing a simple connectionoriented service.
Service Primitives
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Packets sent in a simple client-server interaction on
a connection-oriented network.
Services to Protocols
Relationship
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The relationship between a service and a
protocol.
Services & Protocol
Service:
 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.
Protocol:
 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.
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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
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 Models
THE OSI REFERENCE MODEL
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.
OSI reference model
Reference Models
THE OSI
REFERENCE MODEL
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 Models
THE OSI REFERENCE
MODEL
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
THE OSI REFERENCE
MODEL
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 Models
THE OSI REFERENCE
MODEL
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 Models
THE OSI REFERENCE
MODEL
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.
THE OSI REFERENCE
MODEL
Presentation Layer  Concerned with the Syntax and
semantics of information transmitted.
 Understands the nature of the data being
transmitted.
 Converts ASCII/EBCDIC etc…
THE OSI REFERENCE
MODEL
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 Models
Data Transmission in the OSI Model -
THE OSI
REFERENCE MODEL
Reference Models
THE TCP/IP
REFERENCE MODEL
.
•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.
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It varies from host to host and n/w to n/w
Internet Layer –
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Is the linchpin that holds the whole architecture
together
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Its job is to inject packets into any n/w and have
them travel independently to the destination
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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
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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
UDP(UserDatagramProtocol)
 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
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It contains all the higher-level protocols.
HTTP,FTP,SMTP,DNS r some of them.
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.
Differences
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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
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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.
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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.
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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)
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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.
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TCP/IP model has only connectionless
communication in n/w layer but supports both
modes in the transport layer.
Reference Models
THE TCP/IP
REFERENCE MODEL
A CRITIQUE OF OSI:
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.
A CRITIQUE OF TCP/IP:
• Doesn't separate spec from implementation.
• Model is only good for describing TCP.
• Doesn't specify physical and data link layers.
THE HYBRID REFERENCE MODEL:
5 Application
4 Transport
3 Network
2 Data Link
1 Physical
Example Networks-Novell Netware
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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
Layer
Applicatn
Transport
SAP
NCP
Network
…..
FILE Server
SPX
IPX
DataLink
Ethernet
Physical
Ethernet
Token ring
Token ring
ARC Net
ARC Net
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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.
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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:
Bytes  2
2
1 1
12
Destn Adrs
Packet type
Transport control
Packet length
Check sum
12
Source Adrs
Data
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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 :
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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
1)X.25 Networks:
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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
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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.
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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
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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.
ATM
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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.
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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.
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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.
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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 Ref.Model
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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 system.ie, ATM has been designed to b
independent of the txn medium.
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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.
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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.
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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
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Phy. And AAL r each divided into 2 sublayers
Physical layer:PMD and TC
PhysicalMediumDependent:interfaces to the
actual cable.it 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.
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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)
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The ATM layers and sub layers and their functions.
Physical Layer
Transmission Media
Is the physical path betwn txer and rxer in a data txn system.
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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)
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Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
Transmission Media
MAGNETIC 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.
TWISTED PAIR:
 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
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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
(a) Category 3 UTP.
(b) Category 5 UTP.
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COAXIAL CABLE:
Are of 2 types:Baseband and Broadband
BASEBAND:50 Ohm cable
 Used for digital transmissions.
 Good noise immunity.
 Data rates as high as 1 Gbps 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
A coaxial cable.
Coaxial Cables

FIBER OPTICS:
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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
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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.
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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
(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
(a) Side view of a single fiber.
(b) End view of a sheath with three
fibers.
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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 Cables
A comparison of semiconductor diodes and
LEDs as light sources.
Fiber Optic Networks
A fiber optic ring with active repeaters.
Transmission Media
Comparison of Fiber Optics and Copper Wire
Bandwidth
Distance between repeaters
Interference
Physical
Flow
Fiber
Copper
Higher
30 Km
Low
Smaller/Lighter
Uni-directional
Lower
5 Km
High
Bi-directional
Unguided Txn Media (Wireless transmission)
•Radio transmission
•Microwave transmission
•IR and millimeter waves
•Light wave transmission
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for
communication.
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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:
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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
(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
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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.
THE TELEPHONE SYSTEM
Public Switched Telephone System
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Structure of the Telephone System
The Local Loop
Trunks and Multiplexing
Switching
Structure of the Telephone
System
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
A typical circuit route for a medium-distance call.
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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

Local loops
 Analog twisted pairs going to houses and
businesses
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Trunks
 Digital fiber optics connecting the switching
offices
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Switching offices
 Where calls are moved from one trunk to
another
Analog & Digital Txns
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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 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.
Modems
Modems
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
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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
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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)
Transmit(2)
Receive (3)
Rqst to send(4)
Computer
or
Terminal
Clear to send(5)
Data Set Ready(6)
Common Return(7)
Carrier Detect(8)
DataTerminalReady(20)
Modem
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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.
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Procedural:
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.
MULTIPLEXING:
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)
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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.
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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.
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The interleaving can b at the bit level or in blocks of bytes.
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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).
SONET/SDH
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.
A SONET Path
Source Muxer
Repeater
Muxer
Repeater
Destn Muxer
section
Line
path
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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 called a path.
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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.
SWITCHING
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:
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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
COMPARISON OF CIRCUIT SWITCHED AND
PACKET SWITCHED NETWORKS
ISDN
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.
• NARROW BAND
• BROAD BAND
NARROWBAND
ISDN
Integrated Services Digital Network:
A completely digital
circuit-switched phone system. Integrates voice and non-voice
services.
ISDN Services:
Voice:
Intercom
Caller-id
Connecting telephone to a computer caller’s database can b
displayed on the screen.
Nonvoice:
Remote electricity meter reading, online medical, burglar alarms
N-ISDN- 64-Kbps channel
ISDN SYSTEM ARCHITECTURE:
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 INTERFACE:
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.
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Cellular Radio

Paging s/ms
Cordless telephones
Analog cellular phones
Digital cellular phones
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SUMMARY
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.