The OSI Model

Chapter 2

Topics

The OSI Model• Identify Organizations that set

standards for networking• Identify the seven layers of the

OSI• Explain specific functions

belonging to each OSI model layer

• Understand how nodes communicate through the layers

• Discuss the structure and purpose of data packets and frames

• Describe the two types of addressing

Networking Standards Organizations

• ANSI• EIA/TIA• IEEE• ISO• ITU• ISOC• IANA and ICANN

OSI Model

Application

SDP110

Professional Workstation 5000

Session

LAN 1 LAN 2Network

Physical

Presentation

Transport

Data Link

The OSI Process

Application

Presentation

Session

Transport

Network

Data-link

Physical

Application

Presentation

Session

Transport

Network

Data-link

Physical

Packet flow

OSI Layer Interaction

Presentation

Session

Transport

Network

Data link

Physical

Application

D N T S DATA

Presentation

Session

Transport

Network

Data link

Physical

Application

PP

NN

SS

PP

AA

SDP110

Professional Workstation 5000

SDP110

Professional Workstation 5000

SDP110

Professional Workstation 5000

SDP110

Professional Workstation 5000

A

DataData

Physical Physical

P

TT

DD

• Provides the user interface.• Connects the user to the network.• Provides file transfer service, mail

service, terminal emulation, and other services.

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

SDP110

Professional Workstation 5000

The Application Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

Presentation

• Encodes and decodes data.• Determines the format and structure

of data.• Compresses and decompresses data.• Encrypts and decrypts.

decoder ring

X A

BCD

The Presentation Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

Session

• Establishes and maintains connection.

• Manages upper layer errors.• Handles remote procedure calls.• Synchronizes communicating nodes.

The Session Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

Transport

• Takes action to correct faulty transmission.

• Controls the flow of data.• Acknowledges successful receipt of data.• Fragments and reassembles data.

Return receipt for

sender

The Transport Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

• Moves information to the correct address.

• Assembles and disassembles packets.• Addresses and routes data packets.• Determines best path for moving data

through the network.

LAN 1 LAN 2

The Network Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data Link

10010011 0000011 1010011 001011110010011 0000011 1010011 0010111

01

01

01

01

00

11

00

10

10

10

10

10

01

10

01

• Controls access to the communication channel.

• Controls the flow of data.• Organizes data into logical frames.• Identifies specific computer on the network.• Detects errors.

Data from upper layers

The Data Link Layer

Application

Presentation

Session

Transport

Network

Data Link

PhysicalPhysical

• Provides electrical and mechanical interfaces for a network.

• Specifies type of media used to connect network devices.

• Specifies how signals are transmitted on the network.

The Physical Layer

OSI Reference Model Layer Layer Name Header Protocols & Services Details

7Application

(All)Protocol Data Unit

Telnet, FTP, SMTP, HTTP, File and Print, E mail, (WWW), EDI, SMB, NCP

User requests network services here. Database and application services but not the applications

6Presentation

(People)Protocol Data Unit

ASCII, EBCDIC, TIFF, JPEG, GIF, PICT, MIDI, MPEG, QuickTime

Data representation and translation. Formats data for “presentation” to the layers above and below. Encryption, compression and translation.

5Session (Seem)

Protocol Data UnitRPC, ZIP, SCP, SQL, X Window, NetBIOS, NFS, ASP, DNA SCP

Establishes, maintains and manages communication sessions between computers. Think dialog control.

4Transport

(To)Segments

TCP, NBP, UDP, NCP, SPX, ADP, Windowing, flow control, synchronization

Reliable transmission of data segments. Sets the stage for disassembly and assembly of data before and after transmission. Remember: end-to-end connectivity.

3Network (Need)

Datagrams or Packets

IP, IPX, RARP, ARP, BootP, DHCP, ICMP, BGP, OSPF, RIP.

If it’s routing, it takes place here. Decides how data will be routed across the network, in addition to the structure and use of logical (IP) addressing. Routers operate here.

2

Data Link Sublayers are MAC and LLC

(Data)

Frames

MAC, LLC, Frame Relay, LAPB, PPP, calculating CRC or FCS, controls access to the physical medium

Deals with the links and mechanisms to move data. Topology (Ethernet or Token Ring) is defined here. Switches and Bridges operate here. Remember: Framing.

1Physical

(Processing) Bits (1s and 0s)

Ethernet, Token Ring, HSSI, 802.3, bit synchronization, physical connector specifications.

The electrical and physical specifications for the network media that carry data bits across a network. Hubs and repeaters operate here.

OSI Layers and Components

Device OSI Model Layer

Hub Layer 1 (physical)

Switches Layer 2 (Data Link)

Bridges Layer 2 (Data Link)

Routers and some switches

Layer 3 (Network)

NICs Layer 1 (physical)

WAPs Layer 1 (physical) andLayer 2 (Data Link)

Physical Layer Topologies

• Bus• Star• Ring• Mesh

A Physical Bus Topology

Terminator TerminatorT-connector T-connectorT-connector

A Physical Star Topology

Hub

Node

A Physical Ring Topology

A Physical Mesh Topology

Physical Layer Devices

• NIC• Transceivers• Repeaters• Hubs• MAUs

The Network Interface Card

• Basic component of a network infrastructure

A Transceiver on a Cable Modem Card

Transceiver chipTransceiver chip

The Repeater

• Repeaters are used to extend the maximum length of a network segment.

• There is a limit to the number of 10Mbps repeaters that can be used in serial on a network without separating them by at least a layer 2 device.

• The 5-4-3 Rule dictates how many repeaters can be used on a network and where they can be placed.

The Hubs

• Passive Hubs– A passive hub is typically unpowered and

makes only physical, electrical connections.

• Active Hubs– An active hub is usually powered and it

actually regenerates and cleans up the signal.

NetTech Solutions

The Multistation Access Unit MAU

• A MAU is a Physical layer device that is unique to Token Ring networks.

The Data Link Layer

• The Data Link layer is actually made up of two sublayers:– The Media Access Control

(MAC) sublayer– The Logical Link Control (LLC)

sublayer

Physical

Data Link MAC

LLC

Data Link Layer Concepts

• Frames– A frame is a unit of data

transmission.

Physical

Data Link

Network

Transport

Bits

Frames

Packets

SegmentsApplication

Messages, Datagrams, or user data

The Hardware MAC Address

• A 48-bit value, commonly represented as a 12-digit hexadecimal number.– Each two-digit set is separated by colons,

like so:– 07:57:AC:1F:B2:76

• The first three sets of digits (07-57-AC) are known as the Organizationally Unique Identifier (OUI)

• The last three sets (1F-B2-76) is referred to as the device ID.

A MAC Address

Operating SystemOperating System MethodMethod

Win9x winipcfg

WinNT/2000/2003/XP ipconfig /all

Linux ifconfig –a

Novell config

Cisco IOS sh int interface_name

Six bytes longSix bytes long

OUI or Company identifier

OUI or Company identifier

Universal LAN MAC address

Universal LAN MAC address

Physical Address. . . . . . . . . : 07-57-AC-1F-B2-76

Finding the MAC address

Logical Topology

• The logical topology dictates the way the information flows.

• The Data Link layer can describe the method of media access.

• Three terms:– Contention– Polling– Token Passing

Media Access Methods

• Common Media Access Methods include:– Media Access

• Contention-based or competitive• Controlled or deterministic

– Multiplexed Media Access– Polling– Token-Based Media Access– CSMA/CD– CSMA/CA– Contention Domains

CSMA/CD

3. Transmit3. Transmit

4. Collision4. Collision

5. Wait5. Wait

1. Data to transmit1. Data to transmit

2. Check network2. Check network

CSMA/CA

5. Transmit5. Transmit6. Monitor for jam signal6. Monitor for jam signal

1. Data to transmit1. Data to transmit

2. Check network2. Check network 3. Jam signal3. Jam signal

4. Wait4. Wait

Contention Domains

Contention Domain A Contention Domain B

Project 802

• One of the major components of the Data Link layer is the result of the Institute of Electrical and Electronics Engineers’s (IEEE’s) 802 subcommittees and their work on standards for local area and metropolitan area networks (LANs/MANs).

Relation to the OSI Model

802 Network standards

• 802.1 LAN/MAN Management (and Media Access Control Bridges)• 802.2 Logical Link Control• 802.3 CSMA/CD• 802.4 Token Bus• 802.5 Token Ring• 802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)• 802.7 Broadband Local Area Networks• 802.8 Fiber-Optic LANs and MANs• 802.9 Isochronous LANs• 802.10 LAN/MAN Security• 802.11 Wireless LAN• 802.12 Demand Priority Access Method• 802.15 Wireless Personal Area Network• 802.16 Wireless Metropolitain Area Network• 802.17 Resilient Packet Ring• 802.18 LAN/MAN Standards Committee

Data Link Layer Devices

• Three main devices manipulate data at the Data Link layer:– Bridges– Switches– Wireless access points (WAPs)

Various Network Technologies

Technology Speed(s) Access Method Topologies Media

IEEE 802.3 10, 100, CSMA/CD Logical bus Coax or1000, UTP10,000Mbps

IEEE 802.5 4, 16, Token passing Physical star STP or 100Mbps, logical ring UTP

IEEE 802.11 1, 11, CSMA/CA Cellular RF 54Mbps through air

FDDI 100Mbps Token passing Physical star, Fiber optic logical ring (UTP

implemented as TP-PMD)

Layer Two Devices

• Bridge• Switch

A Bridge Routing Table

3B-07-81-15-AC-10

3B-07-81-72-12-A1

3B-07-81-29-81-83

3B-07-81-82-46-AC

3B-07-81-A3-29-37

3B-07-81-91-76-80

Bridge

Local table for Int 13B-07-81-15-AC-103B-07-81-72-12-A13B-07-81-29-81-83

Local table for Int 13B-07-81-15-AC-103B-07-81-72-12-A13B-07-81-29-81-83

Local table for Int 23B-07-81-82-46-AC3B-07-81-A3-29-373B-07-81-91-76-80

Local table for Int 23B-07-81-82-46-AC3B-07-81-A3-29-373B-07-81-91-76-80

Bridge Broadcasting

Broadcast

Unicast

Types of Bridges

• Transparent: – Connects networks of the same type

(Ethernet and Ethernet)

• Source-route: – Connects Token Ring networks

• Translational: – Connects dissimilar network types

(Ethernet and Token Ring)

• Remote: – Connects different networks across

a remote connection

Routed Network vs. Switched Network

ROUTED vs. SWITCHED

Reads protocol address Reads only MAC address

Sends unknown data to a single gateway Sends unknown data to all interfaces

Blocks broadcasts Forwards broadcasts to all interfaces

Removes MAC addresses and attaches new Does not change address at all

MAC

Leaves the protocol address unchanged

Connections Within a SwitchEight-port switch

Client 1 Server 1 Client 2Server 2

MAC Address Table

Port Address

2 A3:24:A3:3A:DF:23

4 A3:4F:A3:3A:DF:24

5 A3:24:AA:B3:CE:00

7 23:6A:65:AF:AF:01

The Wireless Access Point

• The WAP contains at least one radio antenna that it uses to communicate with its clients via radio frequency (RF) signals.

The OSI Model’s Middle Layers

• The Network Layer• The Transport Layer

Physical

Data Link

Network

Transport

Session

Presentation

Application

Network Layer Concepts

• There are two key concepts in this layer– Logical network addressing– Routing

Logical Network Addressing

• Each device needs two addresses to communicate over a network– Physical address– Logical address

Protocols

• The language of communication for computers.

• IPX/SPX– A 32-bit value represented as an

eight-digit hexadecimal number for the network portion.

Network Address Node Address

00004567:006A7C11FB56

TCP/IP

• A 32-bit value represented as a dotted decimal.– Four Octets separated by a dot.

192.168.0.100

255.255.255.0

IP Address

Subnet Mask

Packets

• The Network Layer divides the information into logical chunks called packets.

Routing

• Routing is the process of moving data throughout an internetwork, passing through several network segments using devices called routers, which select the path the data takes.

• Two ways data can get the information as to where to go.– Static routing table– Dynamic routing table

Static Routing Tables

• Manually configured• Useful on small networks• Lower cost routers• No route discovery traffic on

network

Dynamic Routing Tables

• Uses a routing protocol like:– RIP– OSPF– NLSP– IGRP– EIGRP

The Routing Process

Source100.1.10.19

Destination200.6.42.7

MAC address changes on

each hop

MAC address changes on

each hop

Subnet 1 Subnet 3

Destination IP address does not change

Destination IP address does not change

Subnet 2

Routing Table Information

Routing table on router A

Destination Next Hop Type Cost Interface

140.240.0.0 140.240.20.254 Direct 1 1

192.233.66.0 192.233.66.253 Direct 1 2

135.54.0.0 192.233.66.254 Remote 2 2

126.0.0.0 192.233.66.254 Remote 3 2

140.240.20.254 192.233.66.253 192.233.66.254 135.54.20.254 135.54.15.254 126.0.0.254

140.240.0.0 192.233.66.0 135.54.0.0 126.0.0.0

11 22 11 22 11 22

A B C

Routable and Non-routable

Protocol Routing Protocol Routable

IPX RIP YesIPX NLSP YesNetBEUI None NoTCP RIP YesTCP OSPF YesXNS RIP Yes

Routing Entry Components

Local port address

Local port address

Route cost (in hops)

Route cost (in hops)

Network ID of destinationNetwork ID

of destinationMask for this entryMask for this entry

Address of first hop

Address of first hop

To See the Routing Table

• To view a tcp/ip routing table, at the command prompt, type:c:\route print

• In Cisco IOS type:show ip route

Typical Default Routing Table Entries

Local broadcastLocal broadcast

Internetwork broadcast

Internetwork broadcast

MulticastMulticast

Default gatewayDefault gateway

Local loopbackLocal loopback

Local subnetLocal subnet

Network cardNetwork card

Network Layer Devices

• Routers,• Bridges, and • Switches

Physical

Data Link

Network

Transport

Session

Presentation

Application

The Difference

• Routers block broadcast• Switches allow broadcast• Router work on the Network

layer• Switches work on the Data

Link layer.

Layer 3 Switches

• Performs the multiport, virtual LAN, data-pipelining functions of a standard Layer 2 Switch, but it can also perform basic routing functions between virtual LANs.

Transport Layer

• Transport Layer Concepts– The Connection Type

• Connection-oriented• Connectionless

Physical

Data Link

Network

Transport

Session

Presentation

Application

Transport Layer Implementations

• The IPX/SPX Protocol

Synchronize

Negotiate connection

Synchronize

Acknowledge

Connection established

Actual data transfer(segements)

IPX/SPX Protocol Model

Ethernet IEEE802.3

Token Ring/ IEEE

802.5FDDI ARCnet PPP

IPX

NetBIOSemulator

Physical

Data Link

Network

Transport

Session

Presentation

Application

NetWare shell

(client)

Application NetWare core

protocol (NCP) RPC

RPC based

applicationLU 6.2 support

SPX

The OSI Model’s Upper Layers

• Closest to the User• Controls the way data is

packaged• Responsible for starting,

maintaining, and closing

Physical

Data Link

Network

Transport

Session

Presentation

Application

Network Protocols

• TCP/IP• IPX/SPX• NetBEUI• AppleTalk/AppleTalk over IP

Routing

• The IP address of the sender• The IP address of the destination• The IP address of the next router

to which the packet should be sent

AppleTalk/AppleTalk over IP

• Take note of the table on page 86

AppleTalk Addressing

• Uses an address that is 24 bits long.

• Sixteen of those bits are given to the network, and each network can support 254 nodes.

• First releases were not routable.

• Version 2 is with RTMP

Summary

• Summary and Exam Essentials Page 46

• Written Lab Page 47• Review Questions page

48