1 1 Requirements for the internet connection




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2.3.6 TSP/IP model

The historical and technical standard of the Internet is the 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.

Figure illustrates 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.

A comparison of the OSI model and the TCP/IP models will point out some similarities and differences.

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:

  • TCP as an OSI Layer 4 protocol

  • IP as an OSI Layer 3 protocol

  • Ethernet as a Layer 2 and Layer 1 technology

Remember that there is a difference between a model and an actual protocol that is used in networking. The OSI model will be used to describe TCP/IP protocols.

2.3.7 Detailed encapsulation process

All communications on a network originate at a source, and are sent to a destination. The information sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged through a process called encapsulation.

Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information.

To see how encapsulation occurs, examine the manner in which data travels through the layers as illustrated in Figure . Once the data is sent from the source, it travels through the application layer down through the other layers. The packaging and flow of the data that is exchanged goes through changes as the layers perform their services for end users. As illustrated in Figure , networks must perform the following five conversion steps in order to encapsulate data:

Build the data.
As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.

Package the data for end-to-end transport.
The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.

Add the network IP address to the header.
The data is put into a packet or datagram that contains a packet header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.

Add the data link layer header and trailer.
Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.

Convert to bits for transmission.
The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium. A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN.

Summary

An understanding of the following key points should have been achieved:

  • Understanding bandwidth is essential when studying networking

  • Bandwidth is finite, costs money, and the demand for it increases daily

  • Using analogies like the flow of water and flow of traffic can help explain bandwidth

  • Bandwidth is measured in bits per second, bps, kpbs, Mbps, or Gbps

  • Limitations on bandwidth include type of media used, LAN and WAN technologies, and network equipment

  • Throughput refers to actual measured bandwidth, which is affected by factors that include number of users on network, networking devices, type of data, user’s computer and the server

  • The formula T=S/BW (transfer time = size of file / bandwidth) can be used to calculate data transfer time

  • Comparison of analog and digital bandwidth

  • A layered approach is effective in analyzing problems

  • Network communication is described by layered models

  • The OSI and TCP/IP are the two most important models of network communication

  • The International Organization for Standardization developed the OSI model to address the problems of network incompatibility

  • The seven layers of the OSI are application, presentation, session, transport, network, data link, and physical

  • The four layers of the TCP/IP are application, transport, internet, and network access

  • The TCP/IP application layer is equivalent to the OSI application, presentation, and session layers

  • LANs and WANs developed in response to business and government computing needs

  • Fundamental networking devices are hubs, bridges, switches, and routers

  • The physical topology layouts include the bus, ring, star, extended star, hierarchical, and mesh

  • A WAN consists of two or more LANs spanning a common geographic area

  • A SAN provides enhanced system performance, is scalable, and has disaster tolerance built in

  • A VPN is a private network that is constructed within a public network infrastructure

  • Three main types of VPNs are access, Intranet, and Extranet VPNs

  • Intranets are designed to be available to users who have access privileges to the internal network of an organization

  • Extranets are designed to deliver applications and services that are Intranet based, using extended, secured access to external users or enterprises

Module 3 Networking Media

Copper cable is used in almost every LAN. Many different types of copper cable are available, with each type having advantages and disadvantages. Proper selection of cabling is key to efficient network operation. Because copper carries information using electrical current, it is important to understand some basics of electricity when planning and installing a network.

Optical fiber is the most frequently used medium for the longer, high bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Using optical media, light is used to transmit data through thin glass or plastic fiber. Electrical signals cause a fiber-optic transmitter to generate the light signals sent down the fiber. The receiving host receives the light signals and converts them to electrical signals at the far end of the fiber. However, there is no electricity in the fiber-optic cable itself. In fact, the glass used in fiber-optic cable is a very good electrical insulator.

Physical connectivity allowed an increase in productivity by allowing the sharing of printers, servers, and software. Traditional networked systems require that the workstation remains stationary permitting moves only within the limits of the media and office area.

The introduction of wireless technology removes these restraints and brings true portability to the computing world. Currently, wireless technology does not provide the high-speed transfers, security, or uptime reliability of cabled networks. However, flexibility of wireless has justified the trade off.

Administrators often consider wireless when installing a new network or when upgrading an existing network. A simple wireless network could be working just a few minutes after the workstations are turned on. Connectivity to the Internet is provided through a wired connection, router, cable or DSL modem and a wireless access point that acts as a hub for the wireless nodes. In a residential or small office environment these devices may be combined into a single unit.

Students completing this module should be able to:

  • Discuss the electrical properties of matter.

  • Define voltage, resistance, impedance, current, and circuits.

  • Describe the specifications and performances of different types of cable.

  • Describe coaxial cable and its advantages and disadvantages over other types of cable.

  • Describe shielded twisted-pair (STP) cable and its uses.

  • Describe unshielded twisted-pair cable (UTP) and its uses.

  • Discuss the characteristics of straight-through, crossover, and rollover cables and where each is used.

  • Explain the basics of fiber-optic cable.

  • Describe how fibers can guide light for long distances.

  • Describe multimode and single-mode fiber.

  • Describe how fiber is installed.

  • Describe the type of connectors and equipment used with fiber-optic cable.

  • Explain how fiber is tested to ensure that it will function properly.

  • Discuss safety issues dealing with fiber-optics.

3.1 Copper Media

3.1.1 Atoms and electrons

All matter is composed of atoms. The Periodic Table of Elements lists all known types of atoms and their properties. The atom is comprised of:

  • Electrons – Particles with a negative charge that orbit the nucleus

  • Nucleus – The center part of the atom, composed of protons and neutrons

  • Protons – Particles with a positive charge

  • Neutrons – Particles with no charge (neutral)

To help explain the electrical properties of elements/materials, locate helium (He) on the periodic table. Helium has an atomic number of 2, which means that helium has 2 protons and 2 electrons. It has an atomic weight of 4. By subtracting the atomic number (2) from the atomic weight (4), it is learned that helium also has 2 neutrons.

The Danish physicist, Niels Bohr, developed a simplified model to illustrate the atom. This illustration shows the model for a helium atom. If the protons and neutrons of an atom were the size of an adult (#5) soccer ball in the middle of a soccer field, the only thing smaller than the ball would be the electrons. The electrons would be the size of cherries and would be orbiting near the outer-most seats of the stadium. In other words, the overall volume of this atom, including the electron path, would be about the size of the stadium. The nucleus of the atom where the protons and neutrons exist would be the size of the soccer ball.

One of the laws of nature, called Coulomb's Electric Force Law, states that opposite charges react to each other with a force that causes them to be attracted to each other. Like charges react to each other with a force that causes them to repel each other. In the case of opposite and like charges, the force increases as the charges move closer to each other. The force is inversely proportional to the square of the separation distance. When particles get extremely close together, nuclear force overrides the repulsive electrical force and keeps the nucleus together. That is why a nucleus does not fly apart.

Examine Bohr's model of the helium atom. If Coulomb's law is true, and if Bohr's model describes helium atoms as stable, then there must be other laws of nature at work. How can they both be true?

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