A computer network is defined as the
interconnection of 2 or more independent computers
Need of Networks
– Communicate and collaborate
– Share information
– Share resources
– Sharing computer files and disk space
– Sharing high-quality printers
– Access to common fax machines
– Access to common modems
– Multiple access to the Internet
Classification of Networks
– Local Area Networks (LANs) - a computer
network covering a small geographic area, like
a home, office, or group of buildings. Typically
within 5-mile radius.
– Metropolitan Area Networks (MANs)- are
large computer networks usually spanning a
city. (within 30 miles)
– Wide-Area Networks (WANs) - any network
whose communications links cross
metropolitan, regional, or national boundaries.
The way in which the computers are
interconnected together is known as
Types of topologies
• Physical topology
• Logical topology
Types of physical topologies
Linear or bus topology
• Consists of a main cable, known as
backbone cable, with a terminator at
each end .
• All nodes (file server, workstations,
and peripherals) are connected to the
• Ethernet and LocalTalk networks use
Advantages of Bus Topology
• Easy to connect a computer or peripheral
to a linear bus.
• Requires less cable length.
• Easy to extend.
• If one node of the N/W is faulty, the N/W
can still remain working.
Disadvantages of Bus
• Entire network shuts down if there is a
break in the main cable.
• Terminators are required at both ends of
the backbone cable.
• Difficult to identify the problem if the entire
network shuts down.
• Not meant to be used as a stand-alone
solution in a large building.
• A star topology is designed with each node (file
server, workstations, and peripherals) connected
directly to a central network hub.
• Data on a star network passes through the hub
before continuing to its destination.
• The hub manages and controls all functions of
• It also acts as a repeater for the data flow.
Advantages of Star
• Easy to install.
• No disruptions to the network other than
connecting or removing devices.
• Easy to detect faults and to remove parts.
Disadvantages of Star
• Requires more cable length than a bus
• If the hub fails, nodes attached are
• More expensive than bus topology
because of the cost of the hub.
• A tree topology combines characteristics
of bus and star topologies.
• It consists of groups of star-configured
workstations connected to a bus
• Tree topologies allow for the expansion of
an existing network.
Advantages of a Tree
• Point-to-point wiring for individual
• Supported by several hardware and
Disadvantages of Tree
• Overall length of each segment is limited
by the type of cabling used.
• If the backbone line breaks, the entire
segment goes down.
• More difficult to configure than other
• Is a type of computer network configuration
where each network computer and device are
connect to each other forming A large circle.
• Data is divided into packets when transmitted.
• Packet is sent around the ring until it reaches
its final destination.
Advantages of ring
• Requires lesser amount of cable and there are
not much of installation problems
• All stations have equal access
Disadvantages of ring
• Failure of one computer may impact others
• Data transfer is slow
• It requires that every terminal should be
attached to each other.
• All the computers must have adequate
number of interfaces for the connections
to be made.
• Because of this requirement the
installations is somewhat difficult.
• The length of cable required is quite
higher as compared to other topologies.
Advantages of mesh
• Ease of troubleshooting.
• Data transfer is faster.
Disadvantages of mesh
• uses a lot of cabling.
• Most expensive topology
Choosing a Topology
• Money. A linear bus network may be the least
expensive way to install a network; you do not
have to purchase concentrators.
• Length of cable needed. The linear bus
network uses shorter lengths of cable.
• Future growth. With a star topology, expanding
a network is easily done by adding another
• Cable type. The most common cable in schools
is unshielded twisted pair, which is most often
used with star topologies.
Two types of transmission technologies are
2. Broadcast Network
Single communication channel is shared by all
the machines in the network.
1. Point to Point Network
Data is routed directly from source machine to
destination machine directly.
• When data is to be transmitted between two
remote machines using intermediate machines,
certain routing techniques have to be applied.
• The intermediate machines may be:
• A gateway is a network point that acts as an
entrance to another network.
• Acts as a bridge between two networks so that
data can be transferred between a number of
• In enterprises, the gateway is the computer that
routes the traffic from a workstation to the outside
network that is serving the Web pages.
• In homes, the gateway is the ISP that connects
the user to the internet.
• For example, when you send an e-mail to a friend
or when you log in to a Web site, there is a
gateway that allows the connection take place.
• A common connection point for devices in a
• Hubs are commonly used to connect
segments of a LAN.
• A hub contains multiple ports.
• When a packet arrives at one port, it is
copied to the other ports so that all
segments of the LAN can see all packets
• A router is a device that forwards data packets
• A router is connected to at least two networks.
• Routers are located at gateways.
• Routers use headers and forwarding tables to
determine the best path for forwarding the
• They use protocols to communicate with each
other and configure the best route between any
• In networks, a device that filters and
forwards packets between LAN segments.
• LANs that use switches to join segments are
called switched LANs.
• Repeaters are used in transmission systems
to regenerate signals distorted by
• Analog repeaters frequently can only amplify
the signal while digital repeaters can
reconstruct a signal to near its original
• A device that connects two local-area
networks (LANs), or two segments of the
same LAN that use the same protocol is
known as bridge.
• Network protocols.
• Network/protocol architecture.
• A communication protocol is a set of rules
that specify the format and meaning of
messages exchanged between computers
across a network.
• A set of related protocols that are designed
for compatibility are called protocol suite.
Human and Computer
Human Protocol Computer Protocol
OK Web server
time? send me data
• Task of communication broken up into modules
• For example file transfer could use three
– File transfer application
– Communication service module
– Network access module
• Layers are the different components that
need to be designed/implemented when
• Each layer responsible for a set of
• Top layer relies on services provided by
• Layer makes it service available to higher
layer through an interface.
• Building complex systems is hard!
– Approach: “Divide and conquer”.
– Split job into smaller jobs, or layers.
• Analogy to other fields.
– Building a house: digging, foundation, framing, etc.
– Car assembly line…
• Basic idea: each step dependent on the previous
step but does not need to be aware of how the
previous step was done.
Analogy: Air Travel
• The problem: air travel.
• Decomposed into series of steps:
Arrival at airport Departure from airport
Check-in Baggage claim
More on the air travel analogy…
Check-in Baggage claim
intermediate air traffic sites
Airplane routing Airplane routing
A Three Layer Model
• Application Layer
• Transport Layer
• Network Access Layer
Network Access Layer
• Exchange of data between the computer
and the network
• Sending computer provides address of
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
• Reliable data exchange
• Independent of network being used
• Independent of application
• Support for different user applications
• e.g. e-mail, file transfer
Layered Protocol Design
• Layering model is a solution to the problem
of complexity in network protocols
• The model divides the network protocols into
layers, each of which solves part of the
network communication problem
– Each layer has its own protocol!
• Each layer implements a service to the layer
– Relying on services provided by the layers
• Set of layers, what their functions are, the
services each of them provide, and the
interfaces between them.
• A.k.a, protocol architecture or protocol
– ISO-OSI 7 layer architecture.
– TCP-IP architecture (Internet).
Protocol Data Units (PDU)
• At each layer, protocols are used to
• At the source, control information is added
to user data at each layer, a.k.a.,
• At the receiver, control information is
stripped off at each layer going up the
stack, a.k.a., decapsulation.
Example 1: ISO OSI
• ISO: International Standards Organization
• OSI: Open Systems Interconnection.
• Layer 7: Application
– Application-specific protocols (e.g. ftp, http, smtp)
• Layer 6: Session
− establish sessions between machines.
• Layer 5: Presentation
− performs proper co ordination between machines
• Layer 4: Transport
– Delivery of data between computers (end-to-end).
• Layer 3: Network
– Data routing across a network.
• Layer 2: Data Link
– Reliable transmission over physical medium.
• Layer 1: Physical
- Transmission of bits between two nodes.
Example 2: TCP/IP
• Model employed by the Internet.
TCP/IP Application ISO OSI
Access Data link
TCP/IP Protocol Architecture
• Computer Networks may use the
following routing technologies:
2. Circuit Switching
3. Packet Switching
4. Message Switching
• A physical circuit is established between
• Once the connection is established, the
data transfer takes place and then the
connection is released.
• The data transfer rate is high but error
• Data is divided into small, fixed size packets.
• Each packet contains the address of destination.
• All the intermediate machines inspect the address
and route it to the right machine.
• If any packet is missing, the destination machine can
request the source machine to retransmit it.
• Message as a whole is transmitted.
• Each message contains the address of the
• Message switching N/W is “Store and Forward” N/W.
• Once the message arrives at the intermediate
machine, it is stored in it completely and transmitted
when the line is free.