Sharing of Hardware
– Computing Resources
Sharing of Software
– Multiple Single User Licenses are more
expensive than a single multiple user license
– Maintaining of Systems becomes easier
• Sharing of Information
- easy accessibility from anywhere
- search capability
– internet telephony
– audio conferencing
– video conferencing
Types of Networks
• Local Area Network (LAN):
– Network within a floor/building/campus
– At most a few kilometers distance
• Wide Area Network (WAN):
– Country-wide or global network
– Distance: 100s or 1000s of kilometers
– Based on a computer network.
– Network is transparent to the user.
– Users don't have to remember computer names, their
architectures, addresses, etc.
– The application may be running on multiple
• Networks are organized as
layers or levels.
• Reduces design complexity.
• Each network architecture has
– number of layers
– names of layers
– functions of each layer
• Between adjacent layers there is an interface.
• Reference models exist to architect a network.
• Rules and conventions used by two communicating
entities are collectively known as protocol.
• Each layer of network architecture
has its own protocol.
• Two communicating entities at the
same layer are called peers.
• A list of protocols used by a system is called protocol
• A set of layers and protocols is referred to as
OSI Reference Model
• Open Systems Interconnection (OSI) model has
been developed by ISO.
• It defines seven layers for communication.
• It is called a Reference Model since different
networks may use less or more layers, have
somewhat different division of services amongst the
• Functions for each layer are defined.
• Peer layers communicate as per defined protocol.
• Each layer provides services to the upper layer.
• Concerned with transmitting raw bits
over a channel.
• Makes sure that when one side sends a
1 bit,the other side receives it as a 1.
• Covers all aspects for communication,
including,mechanical, electrical, functional and procedural.
• Mechanical interface defines the connectors used, number
of pins, their placement, size, material used.
• Electrical interface defines the voltage/current levels of
• Functional interface attaches meaning to each signal.
• Procedural interface specifies the sequence of events.
Data Link Layer
• Data link layer attempts to make
physical link reliable.
• May divide upper layer packet into
• Ensure that peer entity will recognize
• May introduce sequence numbers and acknowledgments.
• Some mechanism for error control and flow control.
• How to share the link, if it is half-duplex or multipoint.
• Can be connection-oriented or connectionless.
• Network layer is designed to facilitate
communication between systems across a
• Implements network routing and message delivery
through networks to the correct destination.
• Congestion control can also be done at network
• Internet protocol or IP is one example.
Implements end-to-end protocol.
Implements required quality-of-service.
May detect erroneous packets.
Reorders packets which arrive out-of-sequence.
Ensures that there is no loss or duplication of
May provide connection-less or connectionoriented type of service.
Provides for the connection management.
Multiplexing and demultiplexing.
Packetization, flow control, etc.
• Allows users on different computers to establish a
• This layer requests for a logical connection to
be established based on an end-user's request.
• Any necessary log-on or password validation
is also handled by this layer.
• This layer provides services like dialogue discipline
which can be full duplex or half duplex.
• Session layer can also provide check pointing
(or synchronization) mechanism.
• If a failure occurs between checkpoints, all data
can be retransmitted from the last checkpoint.
Presentation and Application
Presentation layer defines the data format to be
exchanged between the programs.
Manages abstract data structures, and converts
them host-representation to networkrepresentation and vice versa.
Application layer is the highest level of OSI
May define common standards like Network
Examples at this layer are file transfer, electronic mail,
remote login, etc. www.ebooksadda.in
TCP/IP Reference Model
• Carries information in its raw form.
• Electrical, optical or radio signals.
• Guided Media
Twisted pair (shielded, unshielded)
• Unguided Media (wireless)
• Point-to-point link
- Two nodes are connected to each other.
• Multi-point link
- Shared media
- Any two nodes can communicate with each other.
* Information is sent to all nodes at the same
* Information is sent to a group of nodes.
Types of Links
Information can be sent only in one direction.
Information can be sent in both directions.
but only in one direction at any given time.
• Information can be sent in both directions
Multiplexing of Several
• Frequency Division Multiplexing (FDM)
• Frequency spectrum is divided among the logical
• Each user has exclusive control of of one
• Time Division Multiplexing (TDM)
• Users take turn in round-robin fashion.
• Code Division Multiplexing (CDM)
• Asynchronous TDM or Statistical Multiplexing
• If two nodes are not directly connected, the information
may need to travel through intermediate nodes, called,
• A channel for data transfer is setup before communication
starts. All resources are reserved.
• Message is sent to next-hop, where it is stored, and
forwarded to next node.
• Also known as ``store-and-forward'' network
• Message is divided into smaller packets.
• Packets are sent independently through ``store-andwww.ebooksadda.in
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