IFSM 310 – Stage 4 Assignment – Due Sunday

Chapter 9

The OSI Model

So far we have spent all of our time describing the four-layer

model used to design and implement the TCP/IP protocols and ap-

plications that make up the Internet. However, the TCP/IP model

is not the only model we can use to help us understand how net-

works work. The other model commonly used to make sense of

network design is called the Open System Interconnection (OSI)

model. While the TCP/IP model was designed and evolved as the

TCP/IP protocols were developed, deployed, and changed, the OSI

model was the result of a careful design process by many net-

working experts who worked to develop a general approach to

network models.

In today’s networked world, the OSI model and the TCP/IP model

serve two different purposes.1 The TCP/IP model is an implemen-

tation model, in that it provides the guidance for those who would

build TCP/IP-compatible network hardware or software. The OSI

model is more of an abstract model that can be used to under-

stand a wide range of network architectures.

While TCP/IP is the most widely used network technology today,

many different types of networks have been implemented and de-

ployed over the past 50 years. And as we continue to improve and

evolve networking, new implementation models may emerge.

The OSI model has seven layers instead of the four layers of the

TCP/IP model. Starting at the bottom (nearest the physical con-

nections) of the OSI model, the layers are: (1) Physical, (2) Data

Link, (3) Network, (4) Transport, (5) Session, (6) Presentation, and

1This, of course, is an oversimplification. Prior to 1990, there were opera-

tional network implementations based on ISO specifications that followed the

OSI network model very closely. But today, those ISO/OSI network implementa-

tions no longer are in broad use.

101

102 CHAPTER 9. THE OSI MODEL

(7) Application. We will look at each layer in the OSI model in turn,

starting with the Physical layer.

9.1 Physical (Layer 1)

The OSI Physical layer deals with the physical attributes of the

actual wired, wireless, fiber optic, or other connection that is used

to transport data across a single link. The Physical layer also

defines the shapes of the connectors and type of media which can

be used. Another problem solved at this layer is how to encode

the bits (0’s and 1’s) that make up the data being sent across the

medium.2 The “bit encoding” (or modulation) determines how

fast data can be sent across the link.

9.2 Data Link (Layer 2)

The OSI Data Link layer is concerned with how the systems using

a physical link cooperate with one another. When data is broken

into packets, the Data Link layer defines special sequences to indi-

cate the beginning and end of each packet. The stations commu-

nicating using the physical connection are assigned addresses to

allow for effective use of the media. Sometimes multiple stations

are sharing the same media (as on a wireless network) and the

Data Link layer defines how those stations will share the connec-

tions with the other systems connected to the network. Most Data

Link layers also have some form of checksum to detect and/or cor-

rect for errors in the transmitted data.

The design problems solved in the Physical and Data Link layers

of the OSI model are addressed by the Link layer of the TCP/IP

model.

9.3 Network (Layer 3)

Like the Internetwork Layer (IP) in the TCP/IP model, the OSI Net-

work layer deals with the global assignment of “routable” ad-

dresses to the various systems connected to the network. The

2“Manchester Encoding” is a common technique for encoding bits for trans-

mission across a wire.

9.4. TRANSPORT (LAYER 4) 103

Network layer governs how routers forward packets across mul-

tiple hops to get from their source to their destination. Like the

IP layer, The OSI Network layer does not attempt to be error free,

as it assumes that lost data will be detected and retransmitted at

the next layer up.

9.4 Transport (Layer 4)

The Transport layer in the OSI model manages packet loss and

retransmission as well as flow control and window size. The rest

of the functionality of the TCP/IP Transport layer is handled in the

Session layer in the OSI model.

9.5 Session (Layer 5)

The OSI Session layer handles establishing connections between

applications. The Session layer deals with “ports” so that a con-

necting client application can “find” the correct server application

on a particular system. Some aspects of secure transmission are

also handled in the OSI Session layer.

9.6 Presentation (Layer 6)

The Presentation layer focuses on how data is represented and

encoded for transmission across the network. As an example, the

Presentation layer would describe how to encode the pixels of an

image so that the receiving application can properly decode the

data. The Presentation layer also handles data encryption and

decryption.

9.7 Application (Layer 7)

The OSI Application Layer is very similar to the Application layer

in the TCP/IP model, in that it contains the applications them-

selves. Some applications are client applications that initiate

connections, and other applications are the server applications

that respond to those connection requests. The various pairs of

applications have protocol standards that define interoperability

104 CHAPTER 9. THE OSI MODEL

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Figure 9.1: Comparing the TCP and OSI Models

9.8 Comparing the OSI and TCP/IP Models

We can use the OSI model to provide an alternative “view” of the

TCP/IP model by comparing how the OSI model breaks network

functionality into its layers and how the TCP/IP model breaks its

functionality into layers.

9.9 Link Layer (TCP/IP)

The TCP/IP Link layer combines the Physical and Data Link layers

from the OSI model. The Physical and Data Link layers are usually

implemented in hardware. Products like Ethernet, WiFi, satellite,

or fiber optic often are implemented in a network driver card that

plugs into the back of a computer or router. The network driver

card generally implements both the physical and the data link

9.10. INTERNETWORK LAYER (TCP/IP) 105

aspects of the connection in the hardware on the card. In most

cases, the data link layers are tuned to the limitations and require-

ments of their corresponding physical layers. So in real systems,

it is somewhat rare for a particular data link layer to be arbitrarily

paired with any number of physical layers. Since it can be hard

to separate the physical and data link aspects for a particular link

technology, the TCP model combines them into a single layer for

simplicity.

9.10 Internetwork Layer (TCP/IP)

One place that maps pretty cleanly between the two models is

the OSI Network and TCP/IP Internetwork layers. They perform

the same functions of creating a globally routable address space

and building routers to insure that packets properly find their way

from the source to the destination across multiple hops.

9.11 Transport Layer (TCP/IP)

The features of the Transport layer in TCP/IP are spread across the

Transport and Session layers of the OSI model. The OSI Transport

layer deals with flow control and packet retransmission, while the

OSI Presentation layer deals with multiple applications running on

multiple ports as well as session establishment and teardown.

The Secure Sockets Layer (SSL) in the TCP/IP model corresponds

to parts of the Session and Presentation layers in the OSI model.

9.12 Application Layer (TCP/IP)

The TCP/IP Application Layer combines the non-security aspects

of the OSI Presentation layer and the OSI Application layer. While

many TCP/IP applications deal with issues like encoding and de-

coding various types of data, the TCP/IP model does not see data

formatting as a separate “layer”. Various data encoding and de-

coding technologies are used in TCP/IP applications, but TCP/IP

tends to treat these capabilities as library code that applications

make use of as needed for the application.

106 CHAPTER 9. THE OSI MODEL

9.13 Conclusion

While the TCP/IP model described in this book is widely used to

guide the implementation of TCP/IP networks, hardware, and soft-

ware, the OSI model can help us look at and compare a wide

range of network architectures ranging from openly developed

networks to proprietary vendor-specific networks.

9.14 Glossary

abstract model: A model and set of terminology that is used to

generally understand a problem area and guide the development

of standards and implementations to solve problems.

implementation model: A model and set of terminology that is

used to guide the development of standards and an implementa-

tion to solve a particular problem.

ISO: International Organization for Standardization. A worldwide

body that develops standards in computing, networking, and

many other areas.

OSI: Open System Interconnection. A seven-layer model used to

help organize the design of various approaches to network archi-

tecture.

9.15 Questions

You can take this quiz online at http://www.net-intro.com/quiz/

1. What is the primary value of the OSI network model?

a) OSI networks are used in the southern hemisphere

b) The OSI approach can be use to analyze many different net-

work models

c) OSI networks make better use of limited bandwidth

d) OSI networks are more secure

2. How many layers does the OSI model have?

a) Four

9.15. QUESTIONS 107

b) Six

c) Seven

d) Nine

3. Which of the OSI layers deals with the shape of connectors

for network connections?

a) Physical

b) Data Link

c) Network

d) Transport

4. Which of the layers is most similar between the OSI and TCP

network models?

a) TCP Link Layer and OSI Data Link Layer

b) TCP Internetwork Layer and OSI Network Layer

c) TCP Transport Layer and OSI Transport Layer

d) TCP Application Layer and OSI Session Layer

5. What layer does the TCP/IP Secure Sockets Layer map to in

the OSI network model?

a) Secure Data Link Layer (SDLL)

b) Secure Network Layer (SNL)

c) Secure Transport Layer (STL)

d) Session and Presentation Layers

6. Why does the TCP model combine the OSI Data Link and

Physical layers into a single Link layer?

a) Because the TCP model does not worry about the Physical

layer

b) Because the TCP model designers were ignored at the 1981

OSI meeting in Utrect, Netherlands

c) Because quite often the design of Data Link and Physical

layers are tightly connected for a particular technology

d) To make the TCP model easier to understand by end users