Early modes of Communication

2

Fire signalsSending messages by birds

Postal System

3

• Write address• Put stamp• Close envelop

Drop letter

Take letter to local post office

• Verify postage• “Route” letter

Take letter to destination’s postal office

Check destination address

Take letter to destination• Check address

• Open envelope

Protocols A protocol is a set of rules that govern data

communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese.

Key elements of a protocol are:-SyntaxSemanticsTiming

5

Examples of Protocols in Human Interactions

TELEPHONETELEPHONE

1. (Pick up / open up the phone.)

2. Listen for a dial tone / see that you have service.

3. Dial.

4. Should hear ringing …

5. Receiver: “Hello?”

6. Caller: “Hi, it’s Alice ….”Or: “Hi, it’s me” ( what’s that about?)

7. Caller: “Hey, do you think … blah blah blah …” pause

8. Receiver: “Yeah, blah blah blah …” pause

ASKING A QUESTIONASKING A QUESTION

1. Raise your hand.

2. Wait to be called on.

3. Or: wait for speaker to pause and vocalize

Communication Level• User-to-User Communication• Computer-to-Computer Communication• Computer-to-Network Communication

Data Communication Network

CommunicationSystem

Application Application

Computer BComputer A

User-to-User Communication

Computer-to-Computer Communication

Computer-to-Network Communication

CommunicationSystem

1.Simplex1.Simplex. This is a unidirectional connection, i.e., data can only travel in

one direction. Simplex connections are useful in situations where a device only receives or only sends data (e.g., a printer).

2. Half-duplex2. Half-duplex. This is a bidirectional connection, with the restriction that data

can travel in one direction at a time. (e.g., walkie talkie)

3. Full-duplex3. Full-duplex. This is a bidirectional connection in which data can travel in both directions at once. A full-duplex connection is equivalent to two

simplex connections in opposite directions. (e.g., Telephone)

1.Simplex1.Simplex. This is a unidirectional connection, i.e., data can only travel in

one direction. Simplex connections are useful in situations where a device only receives or only sends data (e.g., a printer).

2. Half-duplex2. Half-duplex. This is a bidirectional connection, with the restriction that data

can travel in one direction at a time. (e.g., walkie talkie)

3. Full-duplex3. Full-duplex. This is a bidirectional connection in which data can travel in both directions at once. A full-duplex connection is equivalent to two

simplex connections in opposite directions. (e.g., Telephone)

Connection TypesConnection Types

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

Physical characteristics of interfaces and mediumRepresentation of bitsData rateSynchronization of bitsLine configurationPhysical topologyTransmission mode

Physical characteristics of interfaces and mediumRepresentation of bitsData rateSynchronization of bitsLine configurationPhysical topologyTransmission mode

Physical Layer

Data link LayerFramingPhysical AddressingFlow controlError ControlAccess Control

FramingPhysical AddressingFlow controlError ControlAccess Control

Network Layer

Logical addressing Routing

Logical addressing Routing

Transport Layer

Service point addressingSegmentation and reassemblyConnection ControlFlow controlError Control

Service point addressingSegmentation and reassemblyConnection ControlFlow controlError Control

Session LayerDialog ControlSynchronization

Presentation LayerTranslationEncryptionCompression

Application Layer

Network Virtual TerminalFile Transfer, access and managementMail ServicesDirectory Services