Networking Basics: INTRODUCTION

1. INTRODUCTION

Data networks developed as a result of business applications that were written for microcomputers. The microcomputers were not connected so there was no efficient way to share data among them. It was not efficient or cost-effective for businesses to use floppy disks to share data.

Sneakernet created multiple copies of the data. Each time a file was modified it would have to be shared again with all other people who needed that file. If two people modified the file and then tried to share it, one of the sets of changes would be lost. Businesses needed a solution that would successfully address the following three problems:

How to avoid duplication of equipment and resources
How to communicate efficiently
How to set up and manage a network

Businesses realized that computer networking could increase productivity and save money. Networks were added and expanded almost as rapidly as new network technologies and products were introduced. The early development of networking was disorganized. However, a tremendous expansion occurred in the early 1980s.

In the mid-1980s, the network technologies that emerged were created with a variety of hardware and software implementations. Each company that created network hardware and software used its own company standards. These individual standards were developed because of competition with other companies. As a result, many of the network technologies were incompatible with each other. It became increasingly difficult for networks that used different specifications to communicate with each other. Network equipment often had to be replaced to implement new technologies.

One early solution was the creation of local-area network (LAN) standards. LAN standards provided an open set of guidelines that companies used to create network hardware and software. As a result, the equipment from different companies became compatible. This allowed for stability in LAN implementations.

In a LAN system, each department of the company is a kind of electronic island. As the use of computers in businesses grew, LANs became insufficient.

A new technology was necessary to share information efficiently and quickly within a company and between businesses. The solution was the creation of metropolitan-area networks (MANs) and wide-area networks (WANs). Because WANs could connect user networks over large geographic areas, it was possible for businesses to communicate with each other across great distances.

1.1 Network History

The history of computer networking is complex. It has involved many people from all over the world over the past 35 years. Presented here is a simplified view of how the Internet evolved. The processes of invention and commercialization are far more complicated, but it is helpful to look at the fundamental development.

In the 1940s computers were large electromechanical devices that were prone to failure. In 1947 the invention of a semiconductor transistor opened up many possibilities for making smaller, more reliable computers. In the 1950s large institutions began to use mainframe computers, which were run by punched card programs. In the late 1950s the integrated circuit that combined several, and now millions, of transistors on one small piece of semiconductor was invented. In the 1960s mainframes with terminals and integrated circuits were widely used.

In the late 1960s and 1970s smaller computers called minicomputers were created. However, these minicomputers were still very large by modern standards. In 1977 the Apple Computer Company introduced the microcomputer, which was also known as the Mac. In 1981 IBM introduced its first PC. The user-friendly Mac, the open-architecture IBM PC, and the further micro-miniaturization of integrated circuits led to widespread use of personal computers in homes and businesses.

In the mid-1980s PC users began to use modems to share files with other computers. This was referred to as point-to-point, or dial-up communication. This concept was expanded by the use of computers that were the central point of communication in a dial-up connection. These computers were called bulletin boards. Users would connect to the bulletin boards, leave and pick up messages, as well as upload and download files. The drawback to this type of system was that there was very little direct communication and then only with those who knew about the bulletin board. Another limitation was that the bulletin board computer required one modem per connection. If five people connected simultaneously it would require five modems connected to five separate phone lines. As the number of people who wanted to use the system grew, the system was not able to handle the demand. For example, imagine if 500 people wanted to connect at the same time.

From the 1960s to the 1990s the U.S. Department of Defense (DoD) developed large, reliable, wide-area networks (WANs) for military and scientific reasons. This technology was different from the point-to-point communication used in bulletin boards. It allowed multiple computers to be connected together through many different paths. The network itself would determine how to move data from one computer to another. One connection could be used to reach many computers at the same time. The WAN developed by the DoD eventually became the Internet.

1.2 Networking Devices

Equipment that connects directly to a network segment is referred to as a device. These devices are broken up into two classifications. The first classification is end-user devices. End-user devices include computers, printers, scanners, and other devices that provide services directly to the user. The second classification is network devices. Network devices include all the devices that connect the end-user devices together to allow them to communicate.

End-user devices that provide users with a connection to the network are also referred to as hosts. These devices allow users to share, create, and obtain information. The host devices can exist without a network, but without the network the host capabilities are greatly reduced. NICs are used to physically connect host devices to the network media. They use this connection to send e-mails, print reports, scan pictures, or access databases.

A NIC is a printed circuit board that fits into the expansion slot of a bus on a computer motherboard. It can also be a peripheral device. NICs are sometimes called network adapters. Laptop or notebook computer NICs are usually the size of a PCMCIA card. Each NIC is identified by a unique code called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network. More about the MAC address will be covered later. As the name implies, the NIC controls host access to the network.

There are no standardized symbols for end-user devices in the networking industry. They appear similar to the real devices to allow for quick recognition.

Network devices are used to extend cable connections, concentrate connections, convert data formats, and manage data transfers. Examples of devices that perform these functions are repeaters, hubs, bridges, switches, and routers. All of the network devices mentioned here are covered in depth later in the course. For now, a brief overview of networking devices will be provided.

A repeater is a network device used to regenerate a signal. Repeaters regenerate analog or digital signals that are distorted by transmission loss due to attenuation. A repeater does not make intelligent decision concerning forwarding packets like a router.

Hubs concentrate connections. In other words, they take a group of hosts and allow the network to see them as a single unit. This is done passively, without any other effect on the data transmission. Active hubs concentrate hosts and also regenerate signals.

Bridges convert network data formats and perform basic data transmission management. Bridges provide connections between LANs. They also check data to determine if it should cross the bridge. This makes each part of the network more efficient.

Workgroup switches add more intelligence to data transfer management. They can determine if data should remain on a LAN and transfer data only to the connection that needs it. Another difference between a bridge and switch is that a switch does not convert data transmission formats.

Routers have all the capabilities listed above. Routers can regenerate signals, concentrate multiple connections, convert data transmission formats, and manage data transfers. They can also connect to a WAN, which allows them to connect LANs that are separated by great distances. None of the other devices can provide this type of connection.

1.3 Networking Topology

Network topology defines the structure of the network. One part of the topology definition is the physical topology, which is the actual layout of the wire or media. The other part is the logical topology, which defines how the hosts access the media to send data. The physical topologies that are commonly used are as follows:

A bus topology uses a single backbone cable that is terminated at both ends. All the hosts connect directly to this backbone.
A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
A star topology connects all cables to a central point.
An extended star topology links individual stars together by connecting the hubs or switches.
A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.
A mesh topology is implemented to provide as much protection as possible from interruption of service. For example, a nuclear power plant might use a mesh topology in the networked control systems. As seen in the graphic, each host has its own connections to all other hosts. Although the Internet has multiple paths to any one location, it does not adopt the full mesh topology.

The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcast and token passing.

The use of a broadcast topology indicates that each host sends its data to all other hosts on the network medium. There is no order that the stations must follow to use the network. It is first come, first serve. Ethernet works this way as will be explained later in the course.

The second logical topology is token passing. In this type of topology, an electronic token is passed sequentially to each host. When a host receives the token, that host can send data on the network. If the host has no data to send, it passes the token to the next host and the process repeats itself. Two examples of networks that use token passing are Token Ring and Fiber Distributed Data Interface (FDDI). A variation of Token Ring and FDDI is Arcnet. Arcnet is token passing on a bus topology.

1.4 Network protocols

Protocol suites are collections of protocols that enable network communication between hosts. A protocol is a formal description of a set of rules and conventions that govern a particular aspect of how devices on a network communicate. Protocols determine the format, timing, sequencing, and error control in data communication. Without protocols, the computer cannot make or rebuild the stream of incoming bits from another computer into the original format.

Protocols control all aspects of data communication, which include the following:

How the physical network is built
How computers connect to the network
How the data is formatted for transmission
How that data is sent
How to deal with errors

1.5 Local Area Network

LANs consist of the following components:

Computers
Network interface cards
Peripheral devices
Networking media
Network devices

LANs allow businesses to locally share computer files and printers efficiently and make internal communications possible. A good example of this technology is e-mail. LANs manage data, local communications, and computing equipment.Some common LAN technologies include the following:

Ethernet
Token Ring
FDDI

1.6 Wide Area Network

WANs interconnect LANs, which then provide access to computers or file servers in other locations. WANs connect user networks over a large geographical area, so they make it possible for businesses to communicate across great distances. WANs allow computers, printers, and other devices on a LAN to be shared with distant locations. WANs provide instant communications across large geographic areas.

Collaboration software provides access to real-time information and resources and allows meetings to be held remotely. WANs have created a new class of workers called telecommuters. These people never have to leave their homes to go to work.

WANs are designed to do the following:

Operate over a large and geographically separated area
Allow users to have real-time communication capabilities with other users
Provide full-time remote resources connected to local services
Provide e-mail, Internet, file transfer, and e-commerce services

Some common WAN technologies include the following:

Modems
Integrated Services Digital Network (ISDN)
Digital subscriber line (DSL)
Frame Relay
T1, E1, T3, and E3
Synchronous Optical Network (SONET)

1.7 Metropolitan Area Network

Wireless bridge technologies that send signals across public areas can also be used to create a MAN. A MAN usually consists of two or more LANs in a common geographic area. For example, a bank with multiple branches may utilize a MAN. Typically, a service provider is used to connect two or more LAN sites using private communication lines or optical services. A MAN can also be created using wireless bridge technology by beaming signals across public areas.

1.8 Storage Area Network (SAN)

A storage-area network (SAN) is a dedicated, high-performance network used to move data between servers and storage resources. Because it is a separate, dedicated network, it avoids any traffic conflict between clients and servers.

SAN technology allows high-speed server-to-storage, storage-to-storage, or server-to-server connectivity. This method uses a separate network infrastructure that relieves any problems associated with existing network connectivity.

SANs offer the following features:

Performance – SANs allow enhanced system performance.
Availability – SANs have built-in disaster tolerance.
Scalability – A SAN allows easy relocation of backup data, operations, file migration, and data replication between systems.

1.9 Virtual Private Network (VPN)

A vitual private network (VPN) is a private network that is constructed within a public network infrastructure such as the global Internet. Using VPN, a telecommuter can remotely access the network of the company headquarters.

Through the Internet, a secure tunnel can be built between the PC of the telecommuter and a VPN router at the company headquarters

1.10 Internet & Intranet

One common configuration of a LAN is an intranet. Intranet Web servers differ from public Web servers in that the public must have the proper permissions and passwords to access the intranet of an organization. Intranets are designed to permit users who have access privileges to the internal LAN of the organization. Within an intranet, Web servers are installed in the network. Browser technology is used as the common front end to access information on servers such as financial, graphical, or text-based data.

Extranets refer to applications and services that are Intranet based, and use extended, secure access to external users or enterprises. This access is usually accomplished through passwords, user IDs, and other application-level security. An extranet is the extension of two or more intranet strategies with a secure interaction between participant enterprises and their respective intranets.

1.11 Importance of Bandwidth

Bandwidth is defined as the amount of information that can flow through a network connection in a given period of time. It is important to understand the concept of bandwidth for the following reasons.

Bandwidth is finite. Regardless of the media used to build a network, there are limits on the network capacity to carry information. Bandwidth is limited by the laws of physics and by the technologies used to place information on the media. For example, the bandwidth of a conventional modem is limited to about 56 kbps by both the physical properties of twisted-pair phone wires and by modem technology. DSL uses the same twisted-pair phone wires. However, DSL provides much more bandwidth than conventional modems.Optical fiber has the physical potential to provide virtually limitless bandwidth. Even so, the bandwidth of optical fiber cannot be fully realized until technologies are developed to take full advantage of its potential.

Bandwidth is not free. It is possible to buy equipment for a LAN that will provide nearly unlimited bandwidth over a long period of time. For WAN connections, it is usually necessary to buy bandwidth from a service provider. In either case, individual users and businesses can save a lot of money if they understand bandwidth and how the demand will change over time. A network manager needs to make the right decisions about the kinds of equipment and services to buy.

Bandwidth is an important factor that is used to analyze network performance, design new networks, and understand the Internet. A networking professional must understand the tremendous impact of bandwidth and throughput on network performance and design. Information flows as a string of bits from computer to computer throughout the world. These bits represent massive amounts of information flowing back and forth across the globe in seconds or less.

The demand for bandwidth continues to grow. As soon as new network technologies and infrastructures are built to provide greater bandwidth, new applications are created to take advantage of the greater capacity. The delivery of rich media content such as streaming video and audio over a network requires tremendous amounts of bandwidth. IP telephony systems are now commonly installed in place of traditional voice systems, which further adds to the need for bandwidth. The successful networking professional must anticipate the need for increased bandwidth and act accordingly.

1.12 Throughput of Bandwidth

Bandwidth is the measure of the amount of information that can move through the network in a given period of time. Therefore, the amount of available bandwidth is a critical part of the specification of the network. A typical LAN might be built to provide 100 Mbps to every desktop workstation, but this does not mean that each user is actually able to move 100 megabits of data through the network for every second of use. This would be true only under the most ideal circumstances.

Throughput refers to actual measured bandwidth, at a specific time of day, using specific Internet routes, and while a specific set of data is transmitted on the network. Unfortunately, for many reasons, throughput is often far less than the maximum possible digital bandwidth of the medium that is being used. The following are some of the factors that determine throughput:

Internetworking devices
Type of data being transferred
Network topology
Number of users on the network
User computer
Server computer
Power conditions

The theoretical bandwidth of a network is an important consideration in network design, because the network bandwidth will never be greater than the limits imposed by the chosen media and networking technologies. However, it is just as important for a network designer and administrator to consider the factors that may affect actual throughput. By measuring throughput on a regular basis, a network administrator will be aware of changes in network performance and changes in the needs of network users. The network can then be adjusted accordingly.

1.13 Data Transfer Calculation

Network designers and administrators are often called upon to make decisions regarding bandwidth. One decision might be whether to increase the size of the WAN connection to accommodate a new database. Another decision might be whether the current LAN backbone is of sufficient bandwidth for a streaming-video training program. The answers to problems like these are not always easy to find, but one place to start is with a simple data transfer calculation.

Using the formula transfer time = size of file / bandwidth (T=S/BW) allows a network administrator to estimate several of the important components of network performance. If the typical file size for a given application is known, dividing the file size by the network bandwidth yields an estimate of the fastest time that the file can be transferred.

Two important points should be considered when doing this calculation.

The result is an estimate only, because the file size does not include any overhead added by encapsulation.
The result is likely to be a best-case transfer time, because available bandwidth is almost never at the theoretical maximum for the network type. A more accurate estimate can be attained if throughput is substituted for bandwidth in the equation.

Although the data transfer calculation is quite simple, one must be careful to use the same units throughout the equation. In other words, if the bandwidth is measured in megabits per second (Mbps), the file size must be in megabits (Mb), not megabytes (MB). Since file sizes are typically given in megabytes, it may be necessary to multiply the number of megabytes by eight to convert to megabits.

1.14 Introduction To OSI Models

The early development of networks was disorganized in many ways. The early 1980s saw tremendous increases in the number and size of networks. As companies realized the advantages of using networking technology, networks were added or expanded almost as rapidly as new network technologies were introduced.

By the mid-1980s, these companies began to experience problems from the rapid expansion.Just as people who do not speak the same language have difficulty communicating with each other, it was difficult for networks that used different specifications and implementations to exchange information. The same problem occurred with the companies that developed private or proprietary networking technologies.. Networking technologies strictly following proprietary rules could not communicate with technologies that followed different proprietary rules.

To address the problem of network incompatibility,ISO researched networking models like Digital Equipment Corporation net (DECnet), SNA, and TCP/IP in order to find a generally applicable set of rules for all networks. Using this research, the ISO created a network model that helps vendors create networks that are compatible with other networks.

The Open System Interconnection (OSI) reference model released in 1984 was the descriptive network model that the ISO created. It provided vendors with a set of standards that ensured greater compatibility and interoperability among various network technologies produced by companies around the world.

The OSI reference model has become the primary model for network communications. Although there are other models in existence, most network vendors relate their products to the OSI reference model. This is especially true when they want to educate users on the use of their products. It is considered the best tool available for teaching people about sending and receiving data on a network.

1.15 OSI Layers

The OSI reference model is a framework that is used to understand how information travels throughout a network. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media.

In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. Dividing the network into seven layers provides the following advantages:

It breaks network communication into smaller, more manageable parts.
It standardizes network components to allow multiple vendor development and support.
It allows different types of network hardware and software to communicate with each other.
It prevents changes in one layer from affecting other layers.
It divides network communication into smaller parts to make learning it easier to understand.

1.16 Peer to Peer communication

In order for data to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is referred to as peer-to-peer. During this process, the protocols of each layer exchange information, called protocol data units (PDUs). Each layer of communication on the source computer communicates with a layer-specific PDU, and with its peer layer on the destination computer

Data packets on a network originate at a source and then travel to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field. Then it adds whatever headers and trailers the layer needs to perform its function. Next, as the data moves down through the layers of the OSI model, additional headers and trailers are added. After Layers 7, 6, and 5 have added their information, Layer 4 adds more information. This grouping of data, the Layer 4 PDU, is called a segment.

The network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving the data through the internetwork. It accomplishes this task by encapsulating the data and attaching a header creating a packet (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses.

The data link layer provides a service to the network layer. It encapsulates the network layer information in a frame (the Layer 2 PDU). The frame header contains information (for example, physical addresses) required to complete the data link functions. The data link layer provides a service to the network layer by encapsulating the network layer information in a frame.

The physical layer also provides a service to the data link layer. The physical layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.

1.17 TCP/IP Model

The U.S. Department of Defense (DoD) created the TCP/IP reference model, because it wanted to design a network that could survive any conditions, including a nuclear war. In a world connected by different types of communication media such as copper wires, microwaves, optical fibers and satellite links, the DoD wanted transmission of packets every time and under any conditions. This very difficult design problem brought about the creation of the TCP/IP model.

Unlike the proprietary networking technologies mentioned earlier, TCP/IP was developed as an open standard. This meant that anyone was free to use TCP/IP. This helped speed up the development of TCP/IP as a standard.

The TCP/IP model has the following four layers:

Application layer
Transport layer
Internet layer
Network access layer.

Although some of the layers in the TCP/IP model have the same name as layers in the OSI model, the layers of the two models do not correspond exactly. Most notably, the application layer has different functions in each model.

The designers of TCP/IP felt that the application layer should include the OSI session and presentation layer details. They created an application layer that handles issues of representation, encoding, and dialog control.

The transport layer deals with the quality of service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications.

TCP is a connection-oriented protocol. It maintains a dialogue between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a circuit exists between the communicating computers. It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period.

The purpose of the Internet layer is to divide TCP segments into packets and send them from any network. The packets arrive at the destination network independent of the path they took to get there. The specific protocol that governs this layer is called the Internet Protocol (IP). Best path determination and packet switching occur at this layer.

The relationship between IP and TCP is an important one. IP can be thought to point the way for the packets, while TCP provides a reliable transport.

The name of the network access layer is very broad and somewhat confusing. It is also known as the host-to-network layer. This layer is concerned with all of the components, both physical and logical, that are required to make a physical link. It includes the networking technology details, including all the details in the OSI physical and data link layers.

some of the common protocols specified by the TCP/IP reference model layers. Some of the most commonly used application layer protocols include the following:

File Transfer Protocol (FTP)
Hypertext Transfer Protocol (HTTP)
Simple Mail Transfer Protocol (SMTP)
Domain Name System (DNS)
Trivial File Transfer Protocol (TFTP)

The common transport layer protocols include:

Transport Control Protocol (TCP)
User Datagram Protocol (UDP)

The primary protocol of the Internet layer is:

Internet Protocol (IP)

The network access layer refers to any particular technology used on a specific network.

Regardless of which network application services are provided and which transport protocol is used, there is only one Internet protocol, IP. This is a deliberate design decision. IP serves as a universal protocol that allows any computer anywhere to communicate at any time.

Similarities include:

Both have layers.
Both have application layers, though they include very different services.
Both have comparable transport and network layers.
Both models need to be known by networking professionals.
Both assume packets are switched. This means that individual packets may take different paths to reach the same destination. This is contrasted with circuit-switched networks where all the packets take the same path.

Differences include:

TCP/IP combines the presentation and session layer issues into its application layer.
TCP/IP combines the OSI data link and physical layers into the network access layer.
TCP/IP appears simpler because it has fewer layers.

TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, networks are not usually built on the OSI protocol, even though the OSI model is used as a guide.

Although TCP/IP protocols are the standards with which the Internet has grown, this curriculum will use the OSI model for the following reasons:

It is a generic, protocol-independent standard.
It has more details, which make it more helpful for teaching and learning.
It has more details, which can be helpful when troubleshooting.

Networking professionals differ in their opinions on which model to use. Due to the nature of the industry it is necessary to become familiar with both. Both the OSI and TCP/IP models will be referred to throughout the curriculum. The focus will be on the following: