Introduction A Communications Model pot

INTRODUCTION
INTRODUCTION


A Communications Model
A Communications Model
•
Source
– generates data to be transmitted
•
Transmitter
–
Converts data into transmittable signals
•
Transmission System
–
Carries data
•
Receiver
– Converts received signal into data
•
Destination
–
Takes incoming data


Simplified Communications
Simplified Communications
Model - Diagram
Model - Diagram

Simplified Data
Simplified Data
Communications Model
Communications Model


Networking
Networking
•
Point to point communication not
usually practical
–
Devices are too far apart
–
Large set of devices would need
impractical number of connections
•
Solution is a communications
network


Simplified Network Model
Simplified Network Model


Wide Area Networks
Wide Area Networks
•

Large geographical area
•
Crossing public rights of way
•
Rely in part on common carrier
circuits
•
Alternative technologies
–
Circuit switching
–
Packet switching
–
Frame relay
–
Asynchronous Transfer Mode (ATM)


Network Models
Network Models


Multi-layer Network Models
Multi-layer Network Models
•
The process of transferring a message
between sender and receiver is more
easily implemented by breaking it down
into simpler components.
•

Instead of a single layer, a group of
layers are used, dividing up the tasks
required for network communications.
•
The two most important such network
models are the OSI and Internet
models.


The OSI Reference Model
The OSI Reference Model
•
Stands for Open Systems
Interconnection
•
Created by the International
Standards Organization (ISO) as a
framework for computer network
standards
•
Released in 1984, the model has 7
layers


The OSI 7-layer Model
The OSI 7-layer Model


Application: provides a set of utilities used by
application programs

Presentation: formats data for presentation to the
user, provides data interfaces, data compression and
translation between different data formats
Session: responsible for initiating, maintaining and
terminating each logical session between sender and
receiver
Transport: deals with end-to-end issues such as
segmenting the message for network transport, and
maintaining the logical connections between sender
and receiver
Network: responsible for making routing decisions
Data Link: deals with message delineation, error
control and network medium access control
Physical: defines how individual bits are formatted
to be transmitted through the network


The OSI Environment
The OSI Environment


OSI as Framework for
OSI as Framework for
Standardization
Standardization


Layer Specific Standards
Layer Specific Standards

Elements of Standardization
Elements of Standardization
•
Protocol specification
–
Operates between the same layer on two
systems
– May involve different operating system
–
Protocol specification must be precise
•
Format of data units
•
Semantics of all fields
•
allowable sequence of PCUs
•
Service definition
– Functional description of what is provided
•
Addressing
–
Referenced by SAPs


The Internet (TCP/IP) Protocol
The Internet (TCP/IP) Protocol
•
Stands for Transmission Control

Protocol/ Internet Protocol. Used on the
Internet.
•
TCP/IP’s 5 layer suite was developed to
solve to the problem of internetworking
•
Network layers can also be placed in
three groups:
–
application layer (includes the application
layer),
–
internetwork layer (includes the transport
and network layers)
–
hardware layer (includes the data link and
physical layers).


The Internet’s 5-Layer Model
The Internet’s 5-Layer Model
Application: used by application program
Transport: responsible for establishing
end-to-end connections, translates domain
names into numeric addresses and
segments messages
Network*: responsible for end-to-end
addressing and routing, determines
destination address if unknown
Data Link*: deals with message

delineation, error control & network access
Physical*: defines how information will be
transmitted through the network
*same as corresponding layer in OSI model

Network Models
Network Models


OSI v TCP/IP
OSI v TCP/IP


Message Transmission Using
Message Transmission Using
Layers
Layers
•
Network model layers use protocols, i.e., sets
of rules to define how to communicate at each
layer and how to interface with adjacent
layers.
•
Generally, messages travel down all network
layers.
• When a message is sent to the next layer, that
layer places it in an envelope and adds
addressing information related to that layer.
•
At the receiving end, messages travels up

through the network layers, each layer
removing the envelopes added when the
message was sent.

Fig. 1-4 Message transmission
Fig. 1-4 Message transmission
using layers
using layers

NETWORK STANDARDS
NETWORK STANDARDS


The Importance of
The Importance of
Standards
Standards
•
Standards are necessary in almost every
business and public service entity.
•
The primary reason for standards is to
ensure that hardware and software
produced by different vendors can work
together.
•
The use of standards makes it much easier
to develop software and hardware that link
different networks because software and
hardware can be developed one layer at a

time.


The Standards Making
The Standards Making
Process
Process
Two types of standards:
–
Formal standards are developed by an
official industry or government body.
–
Defacto standards emerge in the
marketplace and supported by several
vendors, but have no official standing.


The Standards Making
The Standards Making
Process
Process
Formal standardization process has three
stages
1. Specification stage: developing a
nomenclature and identifying the problems to
be addressed.
2. Identification of choices stage: those
working on the standard identify the various
solutions and choose the optimum solution
from among the alternatives.

3. Acceptance, the most difficult stage:
defining the solution and getting recognized
industry leaders to agree on a single, uniform
solution