OSI Reference Model & Layered Communication

Sritrusta Sukaridhoto

OSI Layer

Open Systems Interconnection for Communications

There are 7 Layers Layers 7,6,5 are oriented more toward

services to the applications Layer 4,3,2,1 are oriented more toward

the flows data from end-to-end through network

7 Layer OSI

How to Remember (CISCO)

Please Physical Do Data Link Not Network Throw Transport Sousces Session Pizza Presentation Away Application

Application

Layer 7

Function & Description

An application that communicates with other computers is implementing OSI application layer concepts. Application layer refer to communications service to application.

Example

Telnet HTTP FTP WWW Browser NFS SMTP Gateway SNMP

Presentation

Layer 6

Function & Description

This layer main purpose is defining data format such as ASCII text, EBCDIC text, binary, BCD, JPEG.

Encryption is also defined by OSI as presentation layer service.

Example

JPEG ASCII TIFF GIF PICT MPEG MIDI Encryption

Session

Layer 5

Function & Description

This session layer defines how to start, control, and end conversation (called session).

This includes the control and management of multi bidirectional messages so that application can be notified if only some of a series of messages are completed.

Example

RPC SQL NFS Netbios Names AppleTalk ASP SCP DECnet

Transport

Layer 4

Function & Description

Layer 4 includes the choice of protocols that either do or do not provide error recovery. Multiplexing of incoming data for different flows to applications on the same host.

Example

TCP UDP SPX

Network

Layer 3

Function & Description

This layer defines end-to-end delivery packets. To accomplish this, the network layer defines logical addressing so that any endpoint can be identified. It also defines how routing works and how routes are learned so that the packets can be delivered

Network layer also defines how to fragment a packet into smaller packets to accommodate media with smaller maximum transmission unit size

Example

IP IPX AppleTalk DDP

Data Link

Layer 2

Function & Description

The data link specifications are concerned with getting data across one particular link or medium. The data link protocols define delivery across an individual link. These protocols are necessary concerned with the type of media in question.

Example

IEEE 802.3/802.2 HDLC Frame Relay PPP FDDI ATM

Physical

Layer 1

Function & Description

Deal with the physical characteristics of the transmission medium, Connection pins, use of pins, electrical currents, encoding and light modulation.

Example

Ethernet RJ-45 V.35 FDDI EIA/TIA-232

Some protocol define details of multiple layers.

Example: NFS

Benefit & Concept

Humans can discuss and learn about many details of a protocol specification easier

Standardized interfaces among layers Better environment for interoperability Reduce complexity, faster production Each layer can define header and trailers

around the user data One layer use the services of the layer

immediately below it.

Interaction Between OSI Layer

On Same Computer

Step 1 = Physical Layer, ensure bit synchronization

Step 2 = Data Link Layer, Frame Check Sequence (FCS) whether error occurred in transmission

Step 3 = Network Layer, Exam address Step 4 = Transport Layer, acknowleadment Step 5 = Session Layer, to ensure that a series

of messages is completed Step 6 = Presentation Layer, manipulate data

format Step 7 = Application Layer, processes the final

header and then can examine the true end-user data

Different Computer

Data Encapsulation

5 Step Data Encapsulation

Create the data Package the data for transport Add the destination network layer

address to the data Add the destination data link address

to the data Transmit the bit

Frames, Packets and Seqments

The layer 2 PDU (Protocol Data Unit) including the data link header and trailer = Frame

The Layer 3 PDU = packet The Layer 4 PDU = seqment

OSI, TCP/IP

OSI Transport Layer Function

Connection Oriented vs Connectionless Oriented

Error Recovery

Flow Control

Flow control is the process of controlling the rate at witch a computer sends data.

Implementation: Buffering Congestion Avoidance Windowing

Buffering

Buffering simply means that computers reserve enough buffer space that bursts of incoming data can be held until processed

Congestion Avoidance

Windowing

Data Link Function

MAC = Media Access Control

CSMA/CD = Carrier Sense Multiple Access/Collision Detect

OSI Network Layer Function

Routing Sending data to a nearby Router Routing data Across Network Delivering Data to End Destination

Addressing

Summary

Application (Layer 7) User Interface Telnet, HTTP Application (Layer 6) How data is presented JPEG,

ASCII Session (Layer 5) Keeping data separate from different

application OS, application access scheduling Transport (Layer 4) Reliable or unreliable delivery,

Multiplexing TCP, UDP, SPX Network (Layer 3) Logical addressing, Routing IP, IPX Data Link (Layer 2) MAC, error detection 802.3/802.2,

HDLC Physical (Layer 1) Devices Ethernet, V.35