Lecture 2 The OSI model Chapter 2, specifically pages 42-58 Dave Novak School of Business Administration, University of Vermont Sources: 1) Network+ Guide.

Lecture 2

The OSI model Chapter 2, specifically pages 42-58

Dave NovakSchool of Business Administration, University of Vermont

Sources: 1) Network+ Guide to Networks, Dean 2013 2) Comer, Computer Networks and Internets, 2004

Lecture Outline

Examine the seven layers of the OSI model in detail What the OSI model is and how it is

used in networking Understand how protocols at different

layers interact OSI’s relationship to networking

protocols Difference between MAC and IP

address Define encapsulation

OSI model and networking

OSI model provides a universal framework for network communication Predates popularity of TCP/IP Defines relationships between various

protocols, the specific services provided by protocols, and the layers of the model where the protocols operate

The OSI model and networking

Networked computers use many different protocols simultaneously

Protocols are responsible for providing different types of network services and functions

The OSI model and networking

Different layers of OSI are responsible for doing different things by providing different types of services and functions

The idea behind “layering” is to separate functionality and services by individual layer where there is no redundancy in the services / functions between layers Each layer of the OSI has a specific set

of functions and services that are handled at that layer

The OSI model and networking

The OSI model is a standardized framework for sub-dividing communications system functionality and services into separate layers

OSI model

The collection of networking protocols that operate at the various OSI layers are referred to as a protocol stack Protocols running on a networked computer

work together to provide all services required by a particular application

Services provided by the protocols are not redundant – if a protocol at one layer provides a particular service, the protocols at the other layers do not provide the same service

Protocols at different layers provide services to each other – allowing interaction between adjoining layers

OSI model

Promotes open system communication

OSI is a theoretical representation or framework for network services Does NOT prescribe hardware or software Does NOT describe how software programs

on different computers interact, or how they interact with humans

A Protocol Stack

The collection of protocols that operate at the various layers of the OSI model are referred to as a protocol stack

The protocols in the protocol stack work together to provide all services required by anapplication

Protocols at the different layers perform specificfunctions that are NOT duplicated by otherprotocols at other layers

Protocol Interaction

Services performed at a particular layer of the OSI model at the sending computer are also performed (or undone) at the corresponding layer of the receiving computer

The Session Layer (5) at the sending computer does not communicate directly with the Session Layer at the Receiving computer messages are passed down from the Session Layer (5) to the Transport Layer (4) and so on at the sending computer

Messages are passed up from the Transport Layer (4) to the Session Layer (5)at the receiving computer

The OSI Reference Model

7

6

5

4

3

2

1

The Application Layer (7) is thetop most layer – it is NOT the samething as an application

MS Word is an example of an application

MS Word is NOT a protocol that operatesat the Application Layer (7) of the OSI

FTP is an example of an Application Layer (7) protocol

The Physical Layer (1) is the bottommost layer of the OSI model – it addresses the transmission of bits overa particular medium

Interaction Between OSI Layers

OSI model

OSI model

Refer to the additional reading – Webopedia definition of OSI http://webopedia.internet.com/quick_re

f/OSI_Layers.asp

The OSI model defines a framework for implementing networking services via specific PROTOCOLS (depending on the protocol stack being used) in seven layers

Physical Layer (1)

Concerned with transmitting signals (representing raw bits) over a communication channel

Transmitting signals via energy of some form or another

Data are not organized into frames or packets at Layer 1

Physical Layer (1)

Addresses the nature of the medium and types of signals used Cable type: coaxial, twisted pair, fiber Signal type: Light pulses, electrical

voltage, radio waves Why would one be concerned about the

medium that is used? What difference does it make?

Physical Layer (1)

Hardware dealing with transmission of signals is defined at layer 1 of the OSI Cables, hubs, repeaters

Do not “understand” packets or frames only signals

The NIC provides a physical connection to the network and bridges layers 1 and 2

Physical Layer (1)

Networking technologies may use a variety of physical layer options Ethernet (the most popular wired networking

technology) supports a number of different physical layer options

Designation Cable Type Topology Speed Max Segment Length

10Base5 RG-8 coaxial Bus 10 Mbps 500 meters

10Base2 RG-58 coaxial Bus 10 Mbps 185 meters

10BaseT CAT 3 UTP Star 10 Mbps 100 meters

100BaseFX 62.5 / 125 multimode fiber

Star 100 Mbps 412 meters

100BaseTX CAT 5 UTP Star 100 Mbps 100 meters

Physical Layer (1)

Different types of media may be used to implement a given LAN technology For example, depending on the standard

being followed, an Ethernet LAN may require coaxial, fiber, or twisted pair wiring

Each standard has different topology and medium requirements

Physical Layer (1)

Max length of cable What happens if you exceed max cable

length standards?

Type of connectors Bit rate (data transmission rate) Monitor data error rates

Data Link Layer (2)

Converts signals and streams of bits into frames and vice versa Creates and recognizes frame

boundaries What is a frame and why is it

important?

Data Link Layer (2)

The frame format is different for various networking technologies Ethernet Token Ring ATM

Different Frame Formats

Ethernet frame (IEEE 802.3) v2 length >= 1536 B

Token Ring frame (IEEE 802.5) length > 4500 B

Dest.MAC

SourceMAC

Length DATA FCSPreamble SFD

8 bytes 1 6 6 2 46 – 1500 bytes 4

StartDel.

AccessControl

FrameControl

Dest.MAC

SourceMAC

DATA FCS EndDel.

FrameStatus

1 1 1 6 6 4500 >= 0 4 1 1

Data Link Layer (2)

Access control technique of various technologies defined at Layer 2 The way in which networked devices

“gain access to the medium”, communicate with other devices, and transfer data differs based on the technology being used

For example, the process two devices use to communicate over wireless is different from Ethernet

Data Link Layer (2)

Error detection in the bit to frame conversion process Identifies and corrects frame errors Errors related to LAN communication

Between 2 hosts

The physical address or MAC address is contained in the frame header

Data Link Layer (2)

Provides conduit or link between the hardware and software on the computer and the physical network medium This is done via the NIC

Data Link Layer (2) Sublayers

LLC – Interface to layer 3. Controls frame synchronization, flow control, and error checking

MAC – Interface to layer 1. Controls how PC accesses and transmits dataspecifies the Media Access Technique used

Data Link Layer (2)

In terms of network design – Data Link Layer (2) is single most important layer in determining what hardware is used LAN technology and topology requires

certain physical layer options and vice versa

Implies access technique used

Data Link Layer (2)

Data link layer protocols designed to work locally (LAN or subnet centric) Other higher-layer protocols (at layers

3 and 4) are required for error detection and flow control in communicating remotely and over larger distances

Hardware that recognizes frames operates at layer 2 of the OSI Switch, bridge

Network Layer (3)

Packages data/frames into IP datagrams

Higher level, routable network addresses (like the IP address) are recognized and managed

Hardware that recognizes network addresses (like IP addresses) works at Layer 3 Routers

Network Layer (3)

Responsible for end-to-end communication

Network Layer (3)

Responsible for end-to-end communication

How is end-to-end functionality different from functionality provided by Data Link Layer (2) protocols?

Network Layer (3)

Example protocol: IP Example services: network

addressing, fragmentation/reassembly, routing What does routing mean?

Frames, Packets, and IP Datagrams

MAC and IP Address Comparison

MAC address (physical address)

IP address (network address)

Transport Layer (4)

Network layer (3) and Transport layer (4) protocols designed to work together as a pair TCP/IP – Internet protocol stack SPX/IPX

Layer 4 protocols provide services to complement Layer (3) TCP provides very specific services that

IP does not – designed to work together with each protocol providing unique services

Transport Layer (4)

Provides messaging service for Session layer (5) and hides the underlying network from the upper layers Example protocols: TCP, UDP Example of Transport Layer services:

flow control, multiplexing, retransmission, message sequencing

Transport Layer (4)

Provides end-to-end error control on the network How is this different from Layer 2 error

control?

How is this different from end-to-end communications provided at Layer 3?

Encapsulation

Each protocol adds headers to information it receives from the layer above it

When a datagram is encapsulated, the entire datagram is placed into the payload area of a specific frame format

Transport Layer (4) adds its header to message and passes down to Network Layer (3) Network Layer adds its header in front of Transport Layer header and so on

Encapsulation

4

3

2

Upper Layers

Lower Layers (1 – 4)

Layers 1 – 4 are called the LOWER layers

Lower layers are concerned with proper transmission of data across the network

Upper Layers (5 – 7)

Layers 5 – 7 called the UPPER layers Upper layers are concerned with how

different application communicate between different hosts Difficult to identify and separate upper layer

protocols in some cases – many applications bundle services provided at layers 5, 6, and 7

Example: WS FTP not only uses Layer 7 protocol FTP, but manages syntax and compression issues at Layer 6, and session management issues at Layer 5

Upper layer protocols/services know nothing about, or understand networking or addressing

Session Layer (5)

Responsible for establishing, maintaining, and ending communication There are no separate Session Layer

protocols Session Layer functions are integrated

into other protocols that also include Presentation and Application Layer functions

Set up, manage, and tear down “sessions” or “connections” between Presentation Layer (6) entities

Session Layer (5)

Coordinates communication and organizes into one of three categories:

1) Simplex – only one node can transmit

2) Half duplex – both nodes can transmit, but only one at a time. Once one node is finished transmitting data, the other node can transmit

3) Full duplex – both nodes can transmit simultaneously without disrupting the other node

Session Layer (5)

Traffic cop for communications between two nodes on a network

For Internet applications – mapping between logical ports and sessions

Presentation Layer (6)

Primary role is to preserve meaning of information transmitted between systems Computers communicating on a network

often use different syntax

Ensures communication between entities is of a form both can understand

Computers must negotiate a common syntax so they can choose a transfer syntax that they both have in common and both understand

Semantics?

Presentation Layer (6)

Different applications and programming languages use different data types and different syntax Text-based languages based on sequence

while visual languages are based on spatial layout and relationships between symbols

Which statements in a program are acceptable to the compiler?

Presentation Layer (6)

Network perspective: applications send messages to each other

Application perspective: messages contain specific types of data Many types of data use very standard

(universal) formats MPEG for video JPEG for still images ASCII for text

Not all data types have universal formats Not all computers format data types the same

way

Presentation Layer (6)

If needed, systems can select transfer syntax that provides additional services such as: Data compression and decompression Encryption and decryption

Application Layer (7)

Entrance point for applications to access the OSI model (structured networking framework) and use network resources NOT an application, but access to

protocols that provide network services Coordinates network services

Identify parties and make sure each can be reached

Ensures communication resources exist (For example, is there are modem at the sender’s computer?)

Application Layer (7)

Most Application Layer protocols provide services (such as mail, print, network management services) that applications (such as WS FTP, Internet Explorer, Outlook) use to access the network Application Layer protocols often include

Session and Presentation Layer functions Typical protocol stack consists of 4 separate

protocols that run at the application, transport, network, and data-link layers

FTP, TCP, IP, specific Ethernet protocols

Application Layer (7)

FTP, TCP, IP, specific Ethernet protocols Example: WS_FTP (application software) uses FTP (a

layer 7 protocol that ALSO performs layer 6 and 5 functions) running over TCP/IP (the layer 4 and 3 protocols – Internet

communication) and Ethernet LAN technology (layer 2 protocols specifying frame formats, error detection, addressing on the LAN)

The upper layers using FTP are unaware of what type of transport protocol (4), what type of network protocol (3), and what type of LAN technology or data-link protocol (2) is being used

Different lower layer protocols CAN be used – the application is not aware of this, and doesn’t need to deal with any lower layer functionality

Lecture Summary

Details of the OSI model KNOW the layers (A, P, S, T, N, D, P)

and be able to describe basic services provided at each layer

Using TCP/IP stack as example, on what layer does IP operate? What about TCP?