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Transport layer

Overview

In the previous chapter, we looked at the application layer and saw that it is the interface between the human network and the data network. In this chapter, we will look at the transport layer and discuss how it aids in communication. At the end of this chapter, you should be able to describe the transport layer as well as its functions.

The transport layer in the OSI model, gets user data from the application layer and prepares it for transport over divergent devices. It also prepares the data for transfer over the network layer. The transport layer is responsible for the delivery of the application data from the source to the destination. In this layer, the PDU in use is the segment.

Purpose of transport layer

The transport layer is responsible for segmentation and control of the various communication streams, it is also responsible for assembly of the communication at the recipients end. The main functions defined at the OSI transport layer include:

Tracking of communication between the upper (application layer) and lower (network layer) from the source to the destination. This means that it separates the different applications from the packets it receives from the network layer.
Segmentation of application data from the sender. As mentioned earlier, the segment is the transport layer PDU.
Reassembly of the various segments into the various applications at the recipients.
Identification and differentiation of the different applications.

Segmentation is the splitting of the application data. If we use an analogy of sending a parcel, we can say it is like breaking up pieces of a big machine into small pieces which can then be sent easily as opposed to sending the whole machine. Reassembly would be the bringing together of the machine pieces together. This is necessary since most networks limit the amount of data that can be sent as a single PDU.

Conversation Multiplexing – you may have noticed that when using a network, such as the internet, you can be able to use many different applications simultaneously. For example, you may make a VOIP call while browsing a certain web page. This is made possible through multiplexing. Simply put, this is the interleaving of different communication streams over the same media so that many applications can be used concurrently. We will learn more on this in future chapters.

Transport layer protocols

In the TCP/IP protocol suite, the two main protocols are the TCP (Transmission Control Protocol) and the (UDP) User Datagram Protocol. Within these protocols there are various other protocols that are controlled by them. These protocols manage the different applications that communicate over the network. They differ in several ways as described below.

User Datagram Protocol (UDP)

UDP is used when the communication stream does not depend fully on the connection. This means that one end can be connected to the network while the other side is temporarily down. Communication does not have to be in on stream for it to work. This is unreliable transport. Some of the applications that are implemented using UDP include; DNS – Domain Name System which is how devices resolve ip addresses to names, Voice Over IP (VOIP) which is where calls can be made over a network and also streaming video

Transmission Control Protocol (TCP)

For TCP to work, the connection has to be established. This means that both ends of the communication; the sender and recipient must be online for it to work. This means that it uses reliable transport. Some of the applications that are implemented using TCP include; HTTP and HTTPS for websites, E-mail using SMTP and POP, and file transfers using FTP.

Port addressing

Consider the following scenario, a user is using his PC to send an email, at the same time he is browsing the web and listening to music online. How does the network infrastructure determine the appropriate destination for each of this data streams.

The various protocols implemented in TCP and UDP are used to identify and keep track of the different applications. Both these protocols have a header that identifies each communication stream using a different port number. For example, John wants to send an email to Mary and send a file to Luke, in this instance, the message sent by John may be identified as email:Mary and FTP:Luke. This would make sure that the recipient knows the application to use for the message that was sent. When receiving replies, the messages may be marked as Email:John and FTP:john. In the table below, the various ports for the different protocols in TCP and UDP are shown.

	Protocols	Port numbers
TCP	http	80
	https	443
	Smtp	25

UDP	Tftp	69
	Syslog	514
	Rip	520

NOTE: you are not expected to memorize these port numbers, however you are expected to know some of the common ones because they will be used in other topics.

Summary

In this chapter, we have looked at the transport layer in the OSI model. We have discussed some of its functions as well as some of the protocols. We have also discussed the role of ports in addressing different communication streams from the application layer. In the next chapter, we will look at the network layer.

Application layer

Overview

As I am sure, most of you use the internet in a number of ways; be it browsing the web, downloading movies, financial transactions, blogging, IM messaging among others. We are most likely oblivious of what it is that makes this communication possible.

In this chapter, we will look at the first layer of the TCP/IP and OSI models, which is the application layer.

In this chapter, you will learn to:

Describe the first 3 layers of the OSI model
Describe some of the application layer applications

Introduction

The application layer is the 7^th layer in the OSI model. When communicating, messages move from the application layer, down the other layers up to the physical layer, after transmission, the information moves from the physical layer up the model to the application layer again from where it is usable.

The application layer in the TCP/IP model defines 3 layers in the OSI model. These are; the application, presentation and session layers.

Presentation Layer

The presentation layer is the 6^th layer in the OSI model. It defines three main functions when it comes to communication over the network.

Data from the application layer is converted and coded into a form that can be understood by the recipient’s device.
Since application data may be uncompressed, the presentation layer compresses the data and this is in a manner that can be interpreted and decompressed by the destination device.
To ensure security of the data which may be transmitted over insecure networks, the presentation layer also encrypts the data.

In this course you are not expected to fully understand the functionality at this layer, however, you are supposed to be familiar with the processes that are involved as listed above.

The Session Layer

Communication over networks can be likened to moderated debates, as such, it is the work of the session layer to make sure that the communication is kept alive, some of the functions include; starting dialogs, keeping them active, and where necessary restart them if they are inactive or idle.

NOTE: Most applications, like web browsers or e-mail clients, incorporate functionality of the OSI layers 5, 6 and 7.

TCP/IP application layer

The application layer as defined by the TCP/IP protocol suite defines the user data. Some of the most common applications that you may know of include the following:

Hypertext Transfer Protocol (HTTP) and Secure HTTP (HTTPS) are used to transfer web pages from the web. Whenever you browse or access web pages, these are the protocols involved.
Email is an important component in communication in not only enterprises but also individuals. The two most common protocols that are used in the transfer of email messages are the Simple Mail Transfer Protocol (SMTP), and the Post Office Protocol (POP).
Transferring of files between different devices is also important, for this we use the FTP (File Transfer Protocol).

Functions

The application layer is the interface between the end user and the hidden or underlying network devices. The protocols defined here are used by both the sender and recipient and they must match for communication to work. For example, when sending an email, both the sender and the recipient must have SMTP and POP.

There are several protocols at this layer, however, they all define the following:

Processes on both ends of a communication stream.
They define the message type that is being sent.
They define the format or the syntax of the messages being sent and received.
They have fields meant for control and additional information on the message.
They specify the type of expected response to a message that has been sent
They specify how the application layer will interact with the transport and other lower layers.

There are many application layer protocols that have been implemented on networks. We will explore how we control these protocols in future chapters, however, you may want to explore further the various common protocols for both the CCNA exams as well as practical use in the field.

Summary

In this chapter, we have looked at the application layer and how it plays a critical role in communication. In the next chapter, we will discuss the transport layer and its role in communication. There is much more information on the application layer not encompassed in this chapter, therefore, further reading is recommended. The concepts learnt in this chapter, will be applied throughout the course as well as in the certification examinations.

In the next chapter, we will look at the transport layer and discuss how it is crucial in communication.

TCP/IP and the OSI Model

Overview

In the previous chapter we were introduced to computer networks and we got to know their importance in everyday lives. In this chapter, we will look at the layered models that are used in communication over networks.

We will discuss the TCP/ip model and the OSI model to better understand how communication works. This will lead into detailed discussion on each of the layers so as to better understand them.

Use of layered models

To better understand the various protocols and how they work with each other, we use layered models. A layered model shows how the protocols work at each layer as well as how the layers interact with the layers either above them or below them. The layered models that are used in modern computer networks are two; the OSI model (Open Systems Interconnection) and the TCP/IP model (Transmission Control Protocol/ Internet Protocol)

There are several benefits that to using the OSI and TCP/IP models in explaining how network protocols work and these include the following.

Each layer in a model defines different protocols, therefore by using a layered model, network engineers can be able to define and design protocols which conform to the specific layer.
Competitions between different vendors is increased, this is because the models define standards and therefore product superiority is not based on the use of certain protocols since all products support them.
The layered model is useful since it allows for independence between other layers. This means that if a change in technology or capabilities is made in one layer, it will not affect another layer either above it or below it.
Since the layered model is an open standard, it provides for conformity and interoperability between different networking devices.

As mentioned earlier, the two models that we will be discussing in this course are the OSI and the TCP/IP models. However, these two models are different.

The TCP/IP model is protocol model and it describes the functions at each of its layers. The protocols that are defined in this model are usually based on vendor specifications. However, the TCP/IP model is an open standard and therefore no company has control of the definitions. This protocols and definitions in TCP/IP, are maintained in public records maintained by the IETF (Internet Engineering Task Force) and they are known as RFCs (Requests for Comments). The TCP/IP model was introduced in the 1970’s by the US’s DOD (Department Of Defense).

NOTE: although you are not required to know all the background information, you will be expected to understand some of the RFC’s for the course as well as the CCNA examinations.

The OSI model is a reference model and it is used to provide or maintain consistency and standardization among the various protocols and services that are used in networks. It does not give strict regulations on how or what protocols and services should work at each layer, rather, it works as a guideline. A good analogy would be a city’s building regulations committee, this body would be in charge of the regulations on where a person can put up a structure, the strength specifications, the maximum height and other regulations, however, it cannot prescribe the specific design of the building to be put up. The reference model is aimed at giving clear understanding of what happens – the functions and processes at each layer, however, it does not say exactly how they should be implemented.

The OSI and TCP/IP models are shown below, however, the OSI model is the most commonly used model in today’s networks. Network engineers use it in designing, operating and troubleshooting networking equipment where necessary. This does not mean that the TCP/IP model is not relevant as we will see as we proceed in this course.

TCP/IP model

This model was first introduced in the 1970’s. There are four categories as you can see from the output above. Network communications were first defined using this model and for successful communication to occur, the functions of each layer must be in place in a network.

From the output below, you can see the function of each and every layer of this model. The application layer, is the component that interfaces with the user, when you are using a web browser, this is a component of the application layer.

The transport layer defines the various ports and helps differentiate the different types of communication from a single user. You may be sending an email, browsing and listening to internet radio on one computer. It is the work of the transport layer to differentiate the different types of communications. The transport layer also helps in interoperability between different network devices such as a PDA and a computer.

The internet layer is meant to provide the best path to remote networks, this differentiates the different devices on a network. If a message is to be sent from one computer to another on a remote network, it is the work of the internet layer to make sure the message gets to the intended recipient. You may compare the internet layer to an address you use when you want to send a letter.

The network access layer acts as an interface between the hardware and software components in the network. The application, transport and internet layers are all implemented by software, however, the network access translates the messages from these layers to a form that can be transmitted over various media such as fiber optic cables, copper wire and wirelessly.

The protocols that are defined in the TCP/IP model describe the various functions and processes at each layer. This means that the protocols at each layer have to have specific functions as described by the TCP/IP model.

If we want to send an email, for example, the steps shown below would be the functions of each layer for this communication.

At the application layer, we would create the email and this would be the data that would be communicated over the network.
The transport layer would then break this data into segments and add information in a process known as encapsulation.

NOTE: encapsulation is a process of adding protocol specific information as well as converting a protocol data unit (PDU) into a form that conforms into the layer it is in. in this case, the PDU at the transport layer is the segment.

The segments would then be passed down to the internet layer and encapsulated into packets, in this layer, logical addressing would be added. (more on logical addressing will be discussed later)
The packets would then be passed to the network access layer, the network access layer would then prepare the packets for transmission over the physical media such as fiber optic cable by converting the data to light signals.
When the data is received at the destination, the reverse process would happen, i.e., removal of protocol specific information – decapsulation as well as reassembly into the application data would be carried out.
The data would then be passed to the user. This process is illustrated below.

As mentioned above, each form that the data takes at each layer of the TCP/IP model is known as a PDU, we have already discussed the transport and internet layer PDUs. The list below shows each of the PDUs at the different layers of the TCP/IP model.

Data – the end user information, this may include, email content, website information among others. This is the information presented to the user.
Segment – as mentioned earlier, this is the PDU at the transport layer.
Packet – in the internetwork layer, the packets are the PDUs and they include the logical addressing for remote delivery.
Frame – this is the form that data at the network access layer takes, there is also addressing at this layer which is physical addressing such as the MAC address.
Bits – the form that is carried over the physical media form is Bits, these may be in many forms such as electrical signals, light signals and others.

OSI model

The OSI model provides an extensive list of functions and services that can occur at each layer. It also describes the interaction of each layer with the layers directly above and below it.

Comparison of TCP/IP and OSI models

The protocols that are discussed in the TCP/IP model, can be clearly defined and described in the OSI mode. In this model, as shown in the diagram above, the application and network access layer are broken down to define more functions. The application layer is broken into the application, presentation and session layers, while the network access layer is broken up into the data link and the physical layers.

The OSI model shows more detailed functions of each of these layers unlike the TCP/IP model. At the data link and physical layer, the OSI model only describes how data is handled from the network layer to the physical layer. It does not state what exactly happens at each of them.

The network and transport layers of the OSI model are very similar to those of the TCP/IP model. The functions at these layers are the same.

The transport layer defines how communication between different user machines happens. At this layer, the rules define how messages are acknowledged, how the communication recovers from errors, and how the segments are arranged. In the OSI model, the protocols that mainly work here are the UDP and TCP protocols. We will look at these in future topics.

PDUs and communication over a layered model

The OSI model defines how messages are encoded, formatted, encapsulated, and segmented so that they can be transmitted over networks. As we mentioned earlier, the data is usually broken down into different PDUs and the layers in the OSI model define how each PDU is controlled so as to make communication successful.

Addresses are one of the ways that communication is made successful in the network. If we can use the post office analogy, you can imagine how difficult it would be if not impossible to send letters without a destination address or how difficult it would be if the recipient would not know who to reply to. The diagram below shows the various addresses that are used in communication over the network.

The use of each type of address, will be discussed in subsequent chapters. The PDUs define the type of data at each layer of this model.

NOTE: the process of transmitting data on the network is guided by the TCP/IP and OSI reference models therefore you should not ignore these concepts since they are crucial to understanding communication over the network.

Summary

In this chapter, we have discussed how communication works over the layered model. We have looked at the TCP/IP and OSI reference models and how they define communication at each layer. We have also looked at the protocol data units and compared the two models. In the next chapter, we will look at the application layer.

The world of networks

Introduction

Communication has evolved greatly. Traditional communication methods such as mail have been overtaken by more sophisticated forms. Electronic Communication methods offer higher speeds; more efficiency, reliability, integrity, security; scale across larger geographical areas and require less resources to use. Tools and services such as e-mail, blogs, podcasts, instant messaging, and social networks among other multimedia methods have changed the way we communicate to a great extent.

In this chapter, we give an overview of the world of networks: we will discuss the following:

The role of networks in our lives
Explain the qualities and elements of a network
Define key terms and diagrams used in this course
Explain what a converged network is
Give a brief history of computer networks

This chapter is crucial since some of the concepts learned in it will be useful in understanding the rest of the course.

The role of networks.

The need to communicate is human nature. Communication has evolved over the years, from carvings on rocks to more complex forms. The diagram below is an apt description of this evolution.

Modern communication methods are as a result of the evolution of computer networks. Since the first e-mail was sent back in 1965, communication has come a long way.

We are no longer restricted with face to face communication. And with the internet the world has become more interconnected than ever before.

Computer networks are not just responsible for communication, but our entire way of life has evolved. The way we work, the way we interact, among other ways have changed significantly.

Some of the ways we use computer networks include:

Business – online banking, e-mail, e-commerce.
Entertainment – online movies, online games.
Social interactions – instant messaging, social networking sites.

Clearly, therefore, networks play a central role in our lives.

Elements of a network

In order to understand networks we need to understand the elements of a network. There are four main elements that define how communication over networks works.

Rules or agreements to govern how the messages are sent, directed, received and interpreted – examples include protocols.
The messages or units of information that travel from one device to another – these may be packets, frames among others.
A means of interconnecting these devices – a medium that can transport the messages from one device to another – such as copper cables, fibre optic among others.
Devices on the network that exchange messages with each other – these may include ip phones, computers, servers, routers among others.

In this course, we will use several network devices, from these, we will create simple and complex topologies, based on the requirements. Keep these in mind because they will be used throughout the course as well as the exams. These are shown below.

Device symbol	Device name

	Branch office
	Cloud, ISP, internet
	CSU/DSU
	Server
	Firewall
	Head office
	Layer 3 switch
	Ip phone
	Modem
	Router
	Hub
	Wireless router
	Switch
	LAN media
	WAN media

These are some of the icons that will be used in this course.

Converged network

The world of networks has grown over the years. We are finding more and more ways to use computer networks. Communication over networks today is not just limited to transmitting static data, rather new applications such as teleconferencing, video, news and other multimedia applications can be found on computer networks.

In yester years, communication was segmented. There were different networks for each application.

For voice, there were telephone networks,
For video, there were TV and cable service providers
For data there were internet service providers.

However, all these services can be provided over the same network and this has been attributed to the growth of computer networks. Therefore, a converged network is one in which all these services; data, voice, and audio, are provided over the same network.

This essentially eliminates the need for multiple service providers for each of these services.

Other concepts.

Fault tolerance and scalability – this means that networks should be able to recover from down time or other faults and should be able to expand as needed.

Quality of service – this is a measurement instituted in networks so as to ensure that the services provided in a converged network work optimally. For example communication would be poor, if a network had only video and not audio when a business executive is in a video conference call. To ensure quality of service, different services are given different priorities.

Security – the network should be safe from intrusions from unauthorized access. This would ensure that communication is effective and confidentiality, integrity and availability of information is maintained.

Summary

This introductory chapter is meant to help you understand the importance of communication over networks. These concepts, will be crucial in understanding this course. We will learn the fundamentals of communications over networks. In the next chapter, we will look at how communication works using the layered models and learn some more concepts.