Open System Interconnection (OSI) Model

Objectives• Explain the OSI reference model, which

sets standards for LAN and WAN communications

• Discuss communication between OSI stacks when two computers are linked through a network

• Apply the OSI model to realistic networking situations

OSI Reference Model• Foundation that brings continuity to LAN

and WAN communications• Product of two standards organizations:

• ISO• ANSI

• Developed in 1974• Set of communication guidelines for

hardware and software design

OSI Guidelines Specify…• How network devices contact each other;

how devices using different protocols communicate

• How a network device knows when to transmit and not transmit data

• How physical network network devices are arranged and connected

OSI Guidelines Specify…• Methods to ensure that network

transmissions are received correctly• How network devices maintain a consistent

rate of data flow• How electronic data is represented on

network media

OSI Layers

OSI Layers• Bottom layers

• Support for physical connectivity, frame formation, encoding, and signal transmission

• Middle layers• Establish and maintain a communication session

between two network nodes• Monitor for error conditions

• Uppermost layers• Application/software support for encrypting data and

assuring interpretation/presentation of data

Physical Layer Functions• Provides transfer medium (eg, cable)• Translates data into a transmission signal• Sends signal along the transfer medium• Includes physical layout of network• Monitors for transmission errors• Determines voltage levels for data signal

transmissions and to synchronize transmissions• Determines signal type (eg, digital or analog)

Analog Signals

Digital Signals

Data Link Layer Functions• Constructs data frames• Creates CRC information; checks for errors• Retransmits data if there is an error• Initiates communications link; makes sure it

is not interrupted (ensures node-to-node physical reliability)

• Examines device addresses• Acknowledges receipt of a frame

Data Link Layer• Data link frame contains fields consisting of

address and control information• Two important sublayers

• Logical link control (LLC)• Media access control (MAC)

• Connectionless service versus connection-oriented service

Network Layer Functions• Determines network path for routing

packets• Helps reduce network congestion• Establishes virtual circuits• Routes packets to other networks,

resequencing packet transmissions when needed

• Translates between protocols

Transport Layer Functions• Ensures reliability of packet transmissions• Ensures data is sent and received in the same order• Sends acknowledgement when packet is received• Monitors for packet transmission errors and

resends bad packets• Breaks large data units into smaller ones and

reconstructs them at the receiving end for networks using different protocols

Session Layer Functions• Establishes and maintains communications

link• Determines which node transmits at any

point in time• Disconnects when communication session

is over• Translates node addresses

Presentation Layer Functions• Translates data to a format the receiving

node understands (eg, from EBCDIC to ASCII)

• Performs data encryption• Performs data compression

Application Layer Functions• Enables sharing remote drivers and printers• Handles e-mail messages• Provides file transfer services• Provides file management services• Provides terminal emulation services

Communicating Between Stacks

• OSI model provides standards for:• Communicating on a LAN• Communicating between LANs• Internetworking between LANs and WANs and

between WANs and WANs

Peer Protocols

Applying the OSI Model