INTRODUCTIONINTRODUCTION

A Communications ModelA 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 - DiagramModel - Diagram

Simplified Data Simplified Data Communications ModelCommunications Model

NetworkingNetworking• 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 ModelSimplified Network Model

Wide Area NetworksWide 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 ModelsNetwork Models

Multi-layer Network ModelsMulti-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 ModelThe 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 ModelThe 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 decisionsData Link: deals with message delineation, error

control and network medium access controlPhysical: defines how individual bits are formatted to

be transmitted through the network

The OSI EnvironmentThe OSI Environment

OSI as Framework for OSI as Framework for StandardizationStandardization

Layer Specific StandardsLayer Specific Standards

Elements of StandardizationElements 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) ProtocolThe 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 ModelThe 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 ModelsNetwork Models

OSI v TCP/IPOSI v TCP/IP

Message Transmission Using Message Transmission Using LayersLayers

• 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 layersusing layers

NETWORK STANDARDSNETWORK STANDARDS

The Importance of The Importance of StandardsStandards

• 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 ProcessProcess

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

Telecommunications Telecommunications Standards OrganizationsStandards Organizations

• International Organization for Standards (ISO)– Member of the ITU, makes technical

recommendations about data communications interfaces.

Telecommunications Telecommunications Standards OrganizationsStandards Organizations

• International Telecommunications Union - Telecommunication Standardization Sector (ITU-TSS)– Technical standard setting organization of

the UN ITU. Formerly called the Consultative Committee on International Telegraph and Telephone (CCITT)

– Comprised of representatives of over 150 Postal Telephone and Telegraphs (PTTs), like AT&T, RBOCs, or common carriers.

TC Standards OrganizationsTC Standards Organizations– American National Standards Institute (ANSI)– Institute of Electrical and Electronics Engineers (IEEE)– Electronic Industries Association (EIA)– National Institute of Standards and Technology (NIST)– National Exchange Carriers Association (NECA)– Corporation for Open Systems (COS)– Electronic Data Interchange -(EDI) of Electronic Data

Interchange for Administration Commerce and Transport (EDIFACT).