Data Communication Networks

prunelimitNetworking and Communications

Oct 23, 2013 (3 years and 5 months ago)

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Data Communication Networks

Computer Architecture

Department of Computer Science

Communication Level


User
-
to
-
User Communication


Computer
-
to
-
Computer Communication


Computer
-
to
-
Network Communication

Data Communication Network

Communication

System

Application

Application

Computer B

Computer A

User
-
to
-
User

Communication

Computer
-
to
-
Computer


Communication

Computer
-
to
-
Network


Communication

Communication

System

ISO Reference Model


A communication systems is
complex piece of hardware and
software.


ISO has adopted a layer
approach for a reference model


Network Environment


Data communication protocols
and standards.


OSI
-
Environment


adds application oriented
protocols and standards.


Real System Environment


Proprietary Software and
Services



Data Communication Network

Network
-
dependent

functions

Application

Application

Computer
-
to
-
Network


Communication

Real System Environment

Computer A

User
-
to
-
User

Communication

Computer B

Application
-
oriented

functions

ISO
-

Environment

Network Environment

Application
-
oriented

functions

Network
-
dependent

functions

ISO Model Structure


Network
-
dependent and
application oriented components
are


Implemented as number of layers


Layer boundaries defined based on
experience


Each layer performs well
-
defined
functions


Layer operates according to a defined
protocols by exchanging messages.


Each layer communication with its
peer, similar layer in a remote
system.


Each layer is aware of the layer
structure and may depend on other
layers.

The ISO Layer


Up to seven protocol layers


Three layer (1
-
3) are network
dependent.


Data communication protocols


Three layers (5
-
7) are application
dependent


End application user process


Services offered by the operating
system


The intermediate transport layer


Masks the application oriented
layers from network dependent
layers.

The application Layers


The application layer


The presentation Layer


The session layer


The transport layer


Network
-
Dependent Layers


The network layer


The link layer


The physical layer

The application layer


Provides an interface to a range of network
distributed information services.


File transfer and access management


Web
-
browsing


Electronic mail Services


The application access a set of primitives


Supported by operating system (system calls).


The operating system use a communication subsystem
(software and hardware).


The application layer Services


Identification of the intended communication partner


Determining the current availability of an intended partner


Establishing authority of communicate


Agreement on privacy mechanism


Authentication of intended communication partner


Selection of the dialog discipline, including the initiation and
release procedure


Agreement of responsibility of error recovery


Identification of constrains on data syntax


Character Set, Data Structure



The Presentation Layer


Manage the representation of data during the
transfer between two communicating applications.


Negotiates and selects the appropriate transfer
syntax during a transaction.


It performs all necessary conversions.


This layer is also concerns with data security.

The session Layer


Interaction management


Synchronization


Exception Reporting


Allows two application layer protocol entities to
organize and synchronize their dialog and manage
their data exchange


The transport Layer


Provide services to compensate for the varying
quality of services provided by the network layers.


Full error control


Flow control


Acts as an interface between the higher application
-
oriented layers and the network
-
dependent protocol
layers.


Network
-
Dependent Layers


The network layer


Responsible for establishing and clearing a network
-
wide
connection between two transport layer protocol entities.


Network routing, flow control across computer
-
to
-
network interface,
etc.


The link layer


Responsible for providing reliable information transfer facility.


Error detection in transmission errors and retransmission of messages.


Services


Connectionless
, which treats each information frame as self
-
contained and transferred using a best
-
try approach.


Connection oriented, which aims to provide error
-
free
information transfer facility.

Network
-
Dependent Layers


The physical layer


Responsible for the physical and electrical interfaces
between the user equipment and network termination
equipment.


It provides the link layer with a means of transmitting a
serial bit
-
stream between two equipments.



Error Detection Methods


Forward error control


Each transmitted character or frame contains additional
redundant information which aims


To detect when errors are present


Determine where in the received bit stream the errors are


Backward (feedback) error control


Each transmitted character or frame include sufficient
information to enable the receiver to when errors are
present.


Can not determine the error
laocation
.




Error Detection and Recovery


Parity


The transmitter adds additional bit to each
character prior to transmission.


Block Sum Check


A set of parity bits computed from a complete
block of data.


Cyclic Redundancy Check


A single set of check digits is generated for each
frame transmitted.

Parity


The parity bit of a character
is computers as follow


The number of 1 bits in the
code are added together
modulo 2.


The parity bit is chosen so that
the number of 1 bits is


Even Parity


Odd Parity


Parity can detect an error in
on bit, but not two

Hamming Distance


Codeword

is defines as the
combined messages unit that
include


The useful data bits


The error control and detection
bets


The
Hamming distance

is
defined as the minimum
number of bit positions in
which two valid
codewords

differ.

Example:

If we assume even parity

0000000 0

0000001 1

0000010 1

0000011 0

Block Sum Check (BSC)


The resulting set of
parity bit for each
column is the
Block
Sum Check

character.

Cyclic Redundancy Check


A single set of check digits is generated for each
frame transmitted based on


The content of the frame


Appended to the tail of the frame


The check digits is selected based on


The type of anticipated transmission errors


16 and 32 are the most common


The check bits are computed based on the frame
check sequence or the
cyclic redundancy
check.