Reference model

Goal:

2

message exchange between application processes

What we should do?

message

Electrical signal

message

Where is the destination station?How to reach the station?What if error happens?What if congestion happens?How to make electrical signal?………..

Reference model

message

Electrical signal

message

Reference model

• Divide and conquer; divide the whole work into separate small tasks – layers

• Each layer is independent of other layers.

• Each layer in a station is communicating with the same layer in the corresponding station.

Introduction 1-6

analogy: overseas air travel

• Each task is working independently.

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Introduction 1-7

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Reference model for the travel

layers: each layer has its own mission.– Each layer works independently.– Lower layers provide services to the very

higher layer.

Reference model

application

transport

network

link

physical

application

presentation

session

transport

network

data link

physical

OSI 7 layersInternet layers

Application layer

• Application process has its own task to provide some services.– Web client/server, email, ftp, DNS etc.

• Then how can processes exchange messages to provide some services.

Transport Layer• Transport layer delivers all messages from

several application processes to the corresponding transport layer. – Then, how they can differentiate all messages?

networkaccess 1

IP

Transport

networkAccess 2

IP

transport

network networkaccess1 access2

IP

subnet 1

end-to-end

subnet 2

AP1 AP2 AP3 AP1 AP2 AP3

Network layernode-to-node delivery

• How to find the path to reach the destination.

Link layer

• Link layer takes the responsibility of delivering data between two nodes which are connected by the same physical link.

4.12

Link layer

• The data unit to be delivered at the link layer is called a frame.

• goal– How to know where the frame begins and

ends?• In addition,

– Is there any error during transmission?– Is there any frame loss because of the

processing speed difference between a sender and a receiver?

– And others.

Physical layer

• How to make a data frame into an electrical signal? (coding)

Physical layer and link layer

• How is the data unit (PDU) at each layer delivered to its lower or higher layer at the boundary?– encapsulation

• PDU consists of data part and header part.

• The header has the information with which each layer can do its own functions.

encapsulation

Introduction 1-17

source

application

transportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

message M

Ht M

Hn

frame

encapsulation

Communication protocol

Common protocol

TCP connection req

TCP connectionresponse

<file>

Hi

Hi

Got thetime?

2:00

time

How to specify in protocol

• Procedure to exchange messages• What kind of information should be

exchanged to do jobs.• How these information is contained

in the header.