CH 2 COMMUNICATING OVER THE NETWORK

Objectives
Upon completion of this chapter, you will able to
answer the following questions:
■ What is the structure of a network, including
devices and media necessary for communications?
■ What function do protocols perform in network
communications?
■ What are the advantages of using a layered
model to describe network functionality?
■ What is the role of each layer in the OSI
network model and the TCP/IP network model?
■ What is the importance of addressing and
naming schemes in network communications?

The Elements of Communication

People exchange ideas using many different communication methods. All of these methods have three elements in common:

■ Message source, or sender: Message sources are people, or electronic devices, that

need to send a message to other individuals or devices.

■ Destination, or receiver of the message: The destination receives the message and

interprets it.

■ Channel: A channel consists of the media that provides the pathway over which the

message can travel from source to destination.

This model of sending a message through a channel to the receiver is also the basis of network

communication between computers. The computers encode the message into binary

signals and transport them across a cable or through wireless media to the receiver, which

knows what rules to follow to understand the original message.

The basic model of communication between people and between computers is illustrated in Fig

Communicating the Messages

Computer networks carry messages large and small. Devices often exchange updates that

are small and require very little bandwidth, yet are very important. Other messages, for

example, high-quality photos, can be very large and consume a lot of network resources.

Sending a large photograph in one continuous stream of data might mean that a device

misses an important update or other communication that will need to be re-sent, using even more bandwidth.The answer to this problem is a process called segmentation, in which all messages are broken into smaller pieces that can be easily transported together across a medium. Segmenting messages has two primary benefits

■ Multiplexing

■ Increased efficiency of network communications

A second benefit of segmentation is that networks can more efficiently send the messagethrough different routes if necessary. This can happen because the Internet is always adjusting routes for efficiency. For example, consider what happens if someone in Las Vegas e-mails a picture of her new kitten to a friend in Boston. First, the picture of the kitten is segmented into small pieces and each piece is given, among other things, a destination address and a code telling where the piece belongs in the big picture. When the message is under way, the pieces might not travel along the same route. Traffic conditions on the Internet are constantly changing, and a large file with many segments can take a couple different routes. Depending on traffic conditions, the data containing the kitten’s ears might go through Chicago on the way to Boston, the paws might go through Denver, and the whiskers and tail might travel through Atlanta. It doesn’t matter which way the pieces trave as long as they all get to Boston and the destination computer can reassemble them into one photograph.

The downside to using segmentation and multiplexing to transmit messages across a network
is the level of complexity that is added to the process. Imagine if you had to send a 100-page letter, but each envelope would hold only one page. The process of addressing, labeling, sending, receiving, and opening the entire hundred envelopes would be time consuming for both the sender and the recipient. In network communications, each segment of the message must go through a similar process to ensure that it gets to the correct destination and can be reassembled into the content of the original message. Various types of devices throughout the network participate in ensuring that the pieces of the message arrive reliably at their destination.

End Devices and Their Role on the Network

An end device refers to a piece of equipment that is either the source or the destination of a

message on a network. Network users usually only see and touch an end device, which is most often a computer. Another generic term for an end device that sends or receives messages is a host. A host can be one of several pieces of equipment performing a wide variety of functions. Examples of hosts and end devices are as follows:

■ Computers, including workstations, laptops, and servers connected to a network

■ Network printers

■ Voice over Internet Protocol (VoIP) phones

■ Cameras on a network, including webcams and security cameras

■ Handheld devices such as PDAs and handheld scanners

■ Remote monitoring stations for weather observation

An end user is a person or group using an end device. Not all end devices are operated by

people all of the time, though. For example, file servers are end devices that are set up by people but perform their tasks on their own. Servers are hosts that are set up to store and share information with other hosts called clients. Clients request information and services, like e-mail and web pages, from servers, and servers reply with the requested information if they recognize the client.

 

When hosts communicate with each other, they use addresses to find each other. The host address is a unique physical address used by hosts inside a local-area network (LAN), and when a host sends a message to another host, it uses the physical address of the destination

device.

Intermediary Devices and Their Role on the Network

End devices are the hosts that initiate communications and are the ones that people are most

familiar with. But getting a message from the source to the destination can be a complex

task involving several intermediary devices along the way. Intermediary devices connect the

individual hosts to the network and can connect multiple individual networks to form an

internetwork.

Intermediary devices are not all the same. Some work inside the LAN performing switching

functions, and others help route messages between networks. Table 2-1 lists some intermediary

devices and their functions.

     

Network access                         devices Connect end users to their network. Examples are hubs,

                                                    Switches, and wireless access points.

Internet-work devices                Connect one network to one or more other networks.

                                                    Routers are the main example.

Communication servers             Route services such as IPTV and wireless broadband.

Modems                                      Connect users to servers and networks through

                                                    telephone or cable.

Security devices                          Secure the network with devices such as firewalls that

                                                    analyze traffic exiting and entering networks.

The management of data as it flows through the network is also a role of the intermediary

devices. These devices use the end host address, in conjunction with information about the network interconnections, to determine the path that messages should take through the network. Processes organization on the mediator network devices perform these functions:

■ Regenerate and retransmit data signals

■ Maintain information in relation to what pathways exist through the network and internetwork

■ Notify other devices of errors and communication failures

■ Direct figures along alternate pathways as there is a link failure.

Classify and direct messages according to quality of service (QoS) priorities
■ Permit or deny the flow of data, based on security settings
Figure 2-3 depicts two LANs with end devices connected by intermediary switches in the
LANs and routers between the LANs.

Lan conected with Router

 

Network Media

Communication across a network is carried on a medium. The medium provides the channel over which the message travels from source to destination. The three main types of media in use in a network are

■ Copper

■ Fiber-optic cable

■ Wireless

Each of these media has vastly different physical properties and uses different methods to

encode messages. Encoding messages refers to the way data is converted to patterns of

electrical, light, or electromagnetic energy and carried on the medium. Each  medium  is briefly described.

Media                     Example                                  Encoding

Copper                   Twisted-pair cable usually                           Electrical pulses

                                used as LAN media

Fiber-optics            Glass or plastic fibers in a vinyl coating        Light pulses

                                usually used for long runs in a LAN

                                and as a trunk

Wireless                  Connects local users through the air             Electromagnetic Waves

Fibre wire

                                                 Copper Wire
Network Protocols
For devices to communicate on a network, they must follow different protocols that perform
the many tasks to be completed. The protocols define the following:
■ The format of the message, such as how much data to put into each segment
■ The way intermediary devices share information about the path to the destination
■ The method to handle update messages between intermediary devices
■ The process to initiate and terminate communications between hosts.
The authors of the protocols might be writing them for a specific company that will own the
protocol. The protocol is treated like a copyright and can be licensed to other companies to
use. Protocols controlled by a company and not for public use are considered proprietary.
Other protocols are written for public use at no charge and are considered open source
protocols.

Protocol Suites and Industry Standards

In the early days of networking, each manufacturer had proprietary network equipment and protocols to support it. This worked well as long as the company that purchased the equipment did not need to share data outside its own network. As companies started to do business with other companies who were using different network systems, the need for a cross platform standard for network communication became apparent.

People from the telecommunications industry gathered to standardize the way network Communication works by writing common protocols. These standards are practices that are Endorsed by representatives from industry groups and are followed to ensure interoperability Between vendors. For example, Microsoft, Apple, and Linux operating systems each have a way to implement the TCP/IP protocol stack. This allows the users of different operating Systems to have common access to network communication. The organizations that standardize Networking protocols are the Institute of Electrical and Electronics Engineers

(IEEE) and the Internet Engineering Task Force (IETF).

                                                           

Interaction of Protocols

An example of the use of a protocol suite in network communications is the interaction

between a web server and a web browser. This interaction uses a amount of protocols and

standards in the process of exchanging information between them. The different protocols

work together to ensure that the messages are received and understood by both parties.

Examples of these protocols are as follows:

■ Hypertext Transfer Protocol (HTTP): HTTP is a common protocol that governs the

way that a web server and a web client interact. HTTP defines the pleased and formatting

of the requests and responses exchanged between the client and server. Both the

client and the web server software implement HTTP as part of the application. The

HTTP protocol relies on other protocols to govern how the messages are transported

between client and server.

■ Transport protocol: Transmission Control Protocol (TCP) is the transport protocol

that manages the individual conversations between web servers and web clients. TCP

divides the HTTP messages into smaller pieces, called segments, to be sent to the destination client. It is also responsible for controlling the size and rate at which messages

are exchanged between the server and the client.

Internetwork protocol: The most common internetwork protocol is Internet Protocol (IP). IP is responsible for taking the formatted segments from TCP, encapsulating them into packets, assigning the appropriate addresses, and selecting the best path to the destination host.

■ Network access protocols: Network access protocols describe two primary functions: data-link management and the physical transmission of data on the media. Data-link management protocols take the packets from IP and format them to be transmitted over the media. The standards and protocols for the physical media govern how the signals are sent over the media and how they are interpreted by the receiving clients. Transceivers on the network interface cards implement the appropriate standards for the media that is being used.

Technology-Independent Protocols

Protocols that guide the network communication process are not dependent on any specific technology to carry out the task. Protocols explain what must be done to communicate, not how the task is to be completed. For example, in a classroom, the protocol for asking a question might be to raise a hand for attention. The protocol instructs students to raise their hands, but it does not specify how high to raise them or specify whether the right hand or left hand is better or whether waving the hand is helpful. Each student can raise his or her hand in a slightly different way, but if the hand is raised, the teacher will likely give attention to the student.

So network communication protocols state what tasks must be completed, not how to complete them. This is what enables different types of devices, such as telephones and computers, to use the same network infrastructure to communicate. Each device has its own technology, but it is able to interact with different devices at the network level. In the previous example of Apple, Microsoft, and Linux, the operating systems must find a way to present data to others using TCP/IP, but each operating system will have its own way to do it.

Using Layered Models

The IT industry uses layered models to describe the complex process of network communication. Protocols for specific functions in the process are grouped by purpose into well-defined layers.

The Benefits of a Layered Model

By breaking the network communication process into manageable layers, the industry can benefit in the following ways:

■ Defines common terms that describe the network functions to those working in the industry and allows greater understanding and cooperation.

■ Segments the process to allow technologies performing one function to evolve independently of technologies performing other functions. For example, advancing technologies of wireless media is not dependent on advances in routers.

■ Fosters competition because products from different vendors can work together.

■ Provides a common language to describe networking functions and capabilities.

■ Assists in protocol design, because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below. As an IT student, you will benefit from the layered approach as you build your understanding of the network communication process.

Protocol and Reference Models

Networking professionals use two networking models to communicate within the industry:protocol models and reference models. Both were created in the 1970s when network communication was in its infancy.

A protocol model provide a model that directly matches the organization of a particular protocol suite. The hierarchical set of related protocols in a suite typically represents all the functionality required to interface the human network with the data network. The TCP/IP model is a protocol model because it describe the functions that arise at each layer of protocols in the TCP/IP suite.

A reference model provides a common reference for maintaining consistency within all types of network protocols and services. A reference model is not intended to be an implementation arrangement or to provide a sufficient level of detail to define precisely the services of the network architecture. The primary purpose of a reference model is to aid in clearer understanding of the functions and process involved. The Open Systems Interconnection (OSI) model is the most widely known internetwork reference model.

The OSI model describes the entire communication process in detail, and the TCP/IP model describes the communication process in terms of the TCP/IP protocol suite and the way it functions. It is important to know details of the OSI model to understand the entire network communication process and to know the TCP/IP model to understand how the process is implemented in current networks.

The OSI model is used to reference the process of communication, not to regulate it. Many Protocols in use today apply to more than one layer of the OSI model. This is why some of the layers of the OSI model are combined in the TCP/IP model. Some manufacturers use Variations on these models to demonstrate the functions of their products within the industry. Shows both OSI and TCP/IP models.

 

 

TCP/IP Model

The TCP/IP model defines the four communication functions that protocols perform. TCP/IP is an open standard, which means that one company does not control it. The rules and implementations of the TCP/IP model were cooperatively developed by members of the industry using Request for Comments (RFC) documents. RFC documents are publicly accessible documents that define specifications and policies of the protocols and of the Internet in general. Solicitation and maintenance of RFCs are the responsibility of the IETF .describes the functions of each layer of the TCP/IP model.

Layers                                                     Description

Application                   Represent application data to the user for example the HTTP present data to the user in a web browser application like internet explorer. 

Transport                      Supports communication between devices and performs error correction.

Internet                         Find the best path through the network.

Network  Access                   Control Hardware device and media.

 

Communication Process

The TCP/IP model describes the functionality of the protocols that make up the TCP/IP protocol suite. These protocols, which are implemented on both the sending and receiving hosts, interact to provide end-to-end delivery of applications over a network.

A complete communication process includes these steps:

1. Creation of data at the application layer of the originating source end device.

2. Segmentation and encapsulation of data as it passes down the protocol stack in the source end device.

3. Generation of the data onto the media at the network access layer of the stack.

4. Transportation of the data through the internetwork, which consists of media and any intermediary devices.

5. Reception of the data at the network access layer of the destination end device.

6. Decapsulation and reassembly of the data as it passes up the stack in the destination device. You learn more about the encapsulation and decapsulation processes in the next section.

7. Passing this data to the destination application at the application layer of the destination end device.

Protocol Data Units and Encapsulation

For application data to travel uncorrupted from one host to another, header (or control data), which contains control and addressing information, is added to the data as it moves down the layers. The process of adding control information as it passes through the layered model is called encapsulation. Decapsulation is the process of removing the extra information and sending only the original application data up to the destination application layer.

Each layer adds control information at each step. The generic term for data at each level is protocol data unit (PDU), but a PDU is different at each layer. For example, a PDU at the internetwork layer is different from the PDU at the transport layer, because internetwork layer data has been added to the transport layer data. The different names for PDUs at each layer are listed.

ENCAPSULATION

Sending and Receiving Process

The common task of sending an e-mail has many steps in the process. Using the proper terms for PDUs and the TCP/IP model, the process of sending the e-mail is as follows:

1. An end user, using an e-mail application, creates data. The application layer codes the data as e-mail and sends the data to the transport layer.

2. The message is segmented, or broken into pieces, for transport. The transport layer adds control information in a header so that it can be assigned to the correct process and all segments put into proper order at the destination. The segment is sent down to the internetwork layer.

3. The internetwork layer adds IP addressing information in an IP header. The segment is now an addressed packet that can be handled by routers en route to the destination. The internetwork layer sends the packet down to the network access layer.

4. The network access layer creates an Ethernet frame with local network physical address information in the header. This enables the packet to get to the local router and out to the web. The frame also contains a trailer with error-checking information. After the frame is created, it is encoded into bits and sent onto the media to the destination.

5. At the destination host, the process is reversed. The frame is decapsulated to a packet, then to a segment, and then the transport layer puts all segments into the proper order.

6. When all data has arrived and is ready, it is sent to the application layer, and then the original application data goes to the receiver’s e-mail application. The message is successful.

 Steps in Communication Process

OSI Model

The Open Systems Interconnection (OSI) model, known as the OSI model, provides an

abstract description of the network communication process. Developed by the International Organization for Standardization (ISO) to provide a road map for nonproprietary protocol development, the OSI model did not evolve as readily as the TCP/IP model. Many of the OSI protocols are no longer in use, but knowledge of the model as a reference is a basic expectation for networking professionals. Many professionals refer to the layers by number rather than name, so it is important to know both. The OSI model is just a reference model, so manufacturers have been free to create protocols and products that combine functions of one or more layers. New protocols might not exactly match the functions described at each layer but might fit into parts of two different layers. As designed, the communication process begins at the application layer of the source, and data is passed down to each lower layer to be encapsulated with supporting data until it reaches the physical layer and is put out on the media. When the data arrives at the destination, it is passed back up through layers and decapsulated by each layer. Each layer provides data services to the layer directly above by preparing information coming down the model or going up.

Layers