Ch 2. Network Models

1. LAYERED TASKS

• Concept of layers– Consider two friends who communicate through mail– What happens when one sends a letter to the other?– Or when one calls to the other?

Tasks Involved in Sending a Letter

2. The OSI Model

• International Standards Organization (ISO)– Established in 1947 – Multinational body dedicated to worldwide

agreement on international standards– An ISO standard that covers all aspects of network

communications is the Open Systems Interconnection (OSI) model

– The model was first introduced in the late 1970s

OSI 7 Layer Model OSI model is NOT a

protocol. It aims to “show” how to facilitate communications between systems

Layered architecture

Well-defined interface between each pair of adjacent layers provide “modularity” to network system

Use

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Link

Interaction between Layers

Message Exchange in OSI Model

Encapsulations

Physical (PHY) Layer• Responsible for movements of individual

bits from one node to the corresponding other(s)– Physical characteristics of interfaces and

medium– Representation of bits (encoding)– Data rate– Synchronization– Line configuration (point-to-point, multipoint)– Topology (mesh, star, ring, bus, …)– Transmission mode (simplex, half-duplex, …)

Application

Presentation

Session

Transport

Network

Data Link

Physical

Physical (PHY) Layer

At sender At receiver

Data Link Layer• Responsible for moving frames from one

node to the corresponding other(s) – Framing (Divides the strep into data units)– Physical addressing (of the sender/receiver)– Flow control (avoid overwhelming)– Error control (detect damaged/lost frames)– Access control

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data Link Layer

At sender At receiver

Hop-to-hop delivery

• PHY and data link layers responsible for one-hop (or hop-to-hop) delivery

Network Layer• Responsible for the delivery of

individual packets from the source node to the destination node(s) (but, it may not guarantee the delivery!)– Logical addressing (of the sender/receiver)– Routing

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network Layer

At sender At receiver

Allow End-to-end Delivery

Transport Layer• Responsible for reliable delivery from

one process to another– Service-point (port) addressing– Segmentation and reassembly – Connection control

• Connectionless, or connection-oriented

– End-to-end flow control– End-to-end error control

Application

Presentation

Session

Transport

Network

Data Link

Physical

Transport Layer

At sender At receiver

Processes in a Host Node

• Process-to-process delivery– Multiplex (mux) and de-multiplex (demux)

Session Layer

• Responsible for dialog control and synchronization

Application

Presentation

Session

Transport

Network

Data Link

Physical

At sender At receiver

Presentation Layer• Responsible for translation,

compression and encryption

Application

Presentation

Session

Transport

Network

Data Link

Physical

At sender At receiver

Application Layer• Responsible for providing service to

user

Application

Presentation

Session

Transport

Network

Data Link

Physical

At sender At receiver

Summary