UNIT
I
Network architecture – layers –
Physical links – Channel Access on links – Hybrid multiple access techniques –
Issues in the data link layer – Framing – Error correction and detection –
Link-level flow control
Introduction
Computer Network: Definition
Collection of autonomous computers interconnected
by single technology.
Connectivity
Connectivity occurs between two computers through physical
medium like coaxial cable or an optical fiber.
Physical Medium – Link
Computers - Nodes
When
a physical link occurs between a pair of nodes then it is referred as point-to-point.
When more than two nodes share a single physical
link then it is referred as Multiple
access.
Two common types of switched
network are
Ø
Circuit
switched – e.g. Telephone System
Ø
Packet
switched – e.g. Postal System
Packet
Switched Network
In this network nodes send discrete blocks of data to each
other. These blocks can be called as packet or message.
Store
and forward strategy:
This network follows this technique. It means
“Each node receives a complete packets over the link, stores in internal memory
and then forwards to next node”.
Circuit Switched Network
It first establishes a circuit across the links and allows
source node to send stream of bits across this circuit to the destination node
The representation of network is given by cloud
symbol
Cloud
represents the network
Nodes
inside the cloud (Switches) – Implement the network
Nodes
outside the cloud (host) - Use the network
Internetwork
Set of independent network are
interconnected to form inter network or internet.
Node that is connected to two or more network is called router or gateway. It is responsible
for forwarding data between the networks.
Addressing
The final requirement is that each node must be able to say
which of the other node it wants to communicate with.
This is done by assigning address to each node. when a
source node wants to deliver message to destination node, it specifies the
address of destination node.
Switches and Routers use this address to decide how to
forward the message. This process based on address is called Routing.
Unicast –
sending message to single node.
Broadcast –
Sending message to all the nodes on the network.
Multicast –
Sending message to some subnet not to all.
Resource
Sharing
Pblm: How do
several hosts share the same link when they all want to use it at the same
time?
Sol: Multiplexing – System
resources are shared among multiple users
Methods:
1. Synchronous Time Division
Multiplexing(STDM)
Divide time into
equal sized quanta
2. Frequency Division
Multiplexing(FDM)
Transmit each flow
at different frequency
3.
Statistical Multiplexing
First two methods are limited
in 2 ways
Ø
If
one flow does not have data to send then its time quantum remains idle, even
the other flow has data to transmit.
Ø
No
of flows are fixed and known ahead of time, it cannot be resized.
Statistical methods combine the ideas of both
STDM and FDM
Ø
Data
from each flow is transmitted on demand so no idle quantum
Ø
It
defines upper bound on size of data and it is referred as packet.
Common
communication patterns
Communication between a pair of processes is done
by request / reply basis. The process which sends request is referred as client
and the one which honors the request is referred as server.
This can be done using channels. Two types of channels are
Ø
Request
/ Reply channels
Ø
Message
stream Channels
Reliability
To get the reliable network, it is necessary to find how
network fails.
Three classes of failures
Ø
Bit
error
Ø
Packet
loss
Ø
Physical
link and node failure
Network
Architecture
Networks do not remain fixed at single point in time, but
it must evolve to accommodate changes based on the technologies on which they
are based and demands made by application programmer.
Network architecture guides the design and implementation
of network. Two commonly used architecture are
Ø
OSI
Architecture
Ø
Internet
or TCP/IP architecture
Layering
and Protocols
When the system gets complex, the system designer
introduces another level of abstraction. It defines unifying model with
important aspects of the system, encapsulated this model in interface objects
and hide it from users
In network, abstraction leads to layering. Layering provides two nice
features.
Ø
It
decomposes the problem of building a network into more manageable components.
Rather than implementing a monolithic piece of software that does everything
implement several layers, each of which solves one part of the problem.
Ø
It
provides more modular design. To add some new service, it is enough to modify
the functionality at one layer, reusing the functions provided at all the other
layers.
Protocols
A protocol is a set of rules that
governs data communication. It defines what is communicated, how it is
communicated, and when it is communicated. The key elements of a protocol are
syntax, semantics and timing.
Each protocol defines two different
interfaces.
Ø Service
interface - to the other
objects on the same computer that want to use its communication services. This
service interface defines the operations that local objects can perform on the
protocol.
Ø Peer
interface - to its
counterpart (peer) on another machine. It also defines the form and meaning of
messages exchanged between protocol peers to implement the communication
service.
Except
at the hardware level, peer to peer communication is indirect.
We can represent these
protocols as protocol graph. Nodes of the graph correspond to protocols, and
the edges represent a depends-on relation.
For example, the file access
program on host 1 wants to send a message to its peer on host 2 using the
communication service offered by protocol RRP. In this case, the file
application asks RRP to send the message on its behalf. To communicate with its
peer, RRP then invokes the services of HHP, which in turn transmits the message
to its peer on the other machine. Once the message has arrived at protocol HHP
on host 2, HHP passes the message up to RRP, which in turn delivers the message
to the file application. In this particular case, the application is said to
employ the services of the protocol stack RRP/HHP.
Encapsulation
Control information must be added with
the data to instruct the peer how to handle with the received message. It will
be added into the header or trailer.
Header - Small data
structure from few bytes to few kilobytes attached to the front of message.
Trailer –
Information will be added at the end of the message
Payload or message
body – Data send by the program
In
this case data is encapsulated with new message created by
protocol
at each level.In this example HHP
encapsulates RRP’s message by attaching a header of its own. Then
HHP sends the
message to its peer over some network, and then when the message arrives at the
destination host, it is processed in the opposite order.
Multiplexing
and De-Multiplexing
The fundamental idea of packet
switching is to multiplex multiple flows of data over a single physical link. This
can be achieved by adding identifier to the header message. It is known as demultiplexing or demux key. It gives
the address to which it has to communicate.
The messages are demultiplexed
at the destination side. In some cases same demux key is used on both sides and
in some cases different keys are used.
No comments:
Post a Comment