Introduction To Layer 1 Physical Layer

Published Date: 02 Nov 2017

 ASSIGNMENT 1

 LAB REPORT

 PROJECT

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23 / July / 2010

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Introduction

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Finding and Analysis

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CSC 1209 – IT Principles

Assignment 1

Prepare an overview report with not less than THIRTY (30) pages, based on ONE (1) of the following topics.

Topics: (Choose only one)

The OSI 7 Layer – Application Layer

The OSI 7 Layer – Presentation Layer

The OSI 7 Layer – Session Layer

The OSI 7 Layer – Transport Layer

The OSI 7 Layer – Network Layer

The OSI 7 Layer – Data Link Layer

The OSI 7 Layer – Physical Layer

DOD Networking Model

TCP /IP Model

Marking Scheme

This assignment is 20% of coursework. Your assignment must include the following:

Assignment Cover Page

Assignment Marking Scheme

Assignment Question

Table of Content

Introduction

Findings and Analysis

Conclusion

References

Requirements

This is an individual assignment.

Font size: 12.

Font Style: Times New Roman.

All paragraphs must be fully justified with 1.5 line spacing.

All heading should have the "Title Case", bold and left-aligned, "Times New Roman", size 14, (do not capitalized and italic).

Use tab to start first sentence of a paragraph.

Between paragraphs – ENTER key 1x.

Remember to spell check and proofread your report.

All figures must be placed in the center.

All figures must contain name and number.

Please use the template that is provided.

Due date: Week 18 – (FRIDAY) before 5.00 pm.

Presentation: Week 19.

Prepare a PowerPoint presentation.

ACKNOWLEDGEMENT

First and foremost I would like to thank God. It would not be successful without God who guides me in my everyday life and activities. I thank God for the good health he has given to me, and for the success of my study.

Besides that, I would like to thank my beloved parent who gave me an opportunity to study in INTI International University undertaking foundation In Business IT (CFPI). They also help me in my financial problem. They also gave me a lot of support, help and strength when I am doing this assignment.

Then I would like to thank my Information Technology Principle lecturer, Ms. Hushalictmy A/P Paliyanny who helped me very much in my assignment. She guides me to do the assignment step by step. She did not refuse to help me when I was facing any problem. Without her guidance and persistent help this assignment would not been done smoothly. She also gave me a lot encouragement in carrying out this assignment work.

Next, I would like to thank my friends who helped me very much too. They gave me a lot of suggestion and support while I was doing this assignment. Without their moral support I do not think I could have finished my assignment and summit it in time.

Table of Contents

1. Introduction to OSI 7 layer 10

1.1. History of OSI Model 11

1.2. Introduction to Layer 1 Physical Layer 12

2. Function of Physical Layer 14

2.1. Transmission of Bit Stream 14

2.2. Data encoding and signaling 14

2.3. Multiplexing 22

3. Protocol of Physical Layer 26

3.1. RS-232 26

3.2. Ethernet 28

3.3. Asynchronous Transfer Mode (ATM) 29

4. Standards of Physical Layer 30

5. Hardware 31

5.1. Guided Media 31

5.2. Unguided Media 42

6. Conclusion 45

7. References 47

7.1. Websites 47

7.2. Books 47

8. Appendix 49

List of Figures

Figure 1.1: OSI Model 12

Figure 1.2: Way of transmission data in the OSI Model Layers 13

Figure 2.3: Transmission of Bit Stream 14

Figure 2.4: Data Encoding and Signaling 15

Figure 2.5: Data Encoding and Signaling 16

Figure 2.6: Amplitude, Frequency and Phase Signaling 17

Figure 2.7: Signaling Bits for Transmission Non Return to Zero 18

Figure 2.8: Signaling Bits for Transmission Manchester Encoding 19

Figure 2.9: Recognizing Frame Signal 21

Figure 2.10: Code Groups 22

Figure 2.11: Multiplexing 22

Figure 2.12: FDM 23

Figure 2.13: TDM 23

Figure 2.14: STDM 24

Figure 2.15: WDM 25

Figure 3.16: 25 pin RS-232 Connector 26

Figure 3.17: Way of Data Transmitted 28

Figure 5.18: Copper Wires 32

Figure 5.19: Coaxial Cable 33

Figure 5.20: Coaxial Cable Design and Coaxial Connectors 34

Figure 5.21: Unshielded Twisted Pair (UTP) Cable 36

Figure 5.22: T568A and T568B Standard Wires 37

Figure 5.23: Shielded Twisted Pair (STP) Cable 38

Figure 5.24: Sources of Interferences 39

Figure 5.25: Fiber Optic Cable 40

Figure 5.26: Fiber Optic Single Mode Cable 41

Figure 5.27: Fiber Optic Multimode Cable 41

Figure 5.28: Omndirectional Antenna 42

Figure 5.29: Wireless Media and Types 44

List of Tables

Table 5.1: Standard and the Application or UTP cables 37

Introduction to OSI 7 layer

OSI stand for Open System Interconnection Model. OSI Model describes how information from a software application in one computer moves through a network medium to a software application in another computer. OSI Model contain seven layers that are application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer.

Each of these layers represents their particular function that each of the layers will perform a special and independent task. OSI Model is divided into two parts of layers that are upper layer and lower layer. Layer 1, layer 2, layer 3 and layer 4 are the lower layers where else layer 5, layer 6 and layer 7 are the upper layers.

Layer 1 is Physical layer. This layer is the lowest layer in the OSI model .This layer responsible for the transmission of bits over the physical medium. This layer describes the physical characteristic of the various communication media, which consists of mechanical, electronic and functional, interface. In this layer the modification of the simple digital signal 1s and 0s to a better accommodates the characteristics of the physical medium. RS-232 and ATM are the example of protocol with physical layer components.

Layer 2 is Data Link layer. This layer is concerned with the transmission of characters. This later is responsible for ensuring error-free and reliable transmission of data. Data link layer able to use protocol to request retransmission o correction of any error It also establishes and controls the physical communication path to the next network node.

Layer 3 is Network layer. This layer is responsible for setting up the appropriate routing of massage through the network. Network layer concerned with the type of switching networks used for routing data between the networks. The network layer split up long massage from transport layer into smaller groups of bits, add destination addresses and routing information to the packets, and moves the packets between nodes on a network.

Layer 4 is Transport layer. This responsible to identify the address of receiver computer, monitoring the quality of the communication channel and selecting the most cost efficient communication service based on the reliability for a particular transmission. Transport layer also defines error recovery procedures.

Layer 5 is Session layer. The function of this layer is to establish and maintain and also defined the rules for the connection and communication between two communicating computer. The session layer also sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end.

Layer 6 is Presentation layer. This layer defines the format data uses as it is transmitted on a communication lines. Presentation layer may perform data compression, protocol conversion and file conversion. This layer is responsible for converting the data generated by an application program into a form compatible with terminals on the network.

Layer 7 is Physical layer. This is the top layer in OSI model. This network provides a network for the end user. Physical layer is defines by the computer system sending massage, server as the interface between user ant he communication system. It does also perform the actual information processing.

History of OSI Model

In late 1970s, two projects we began independently but carry the same goal. The purpose of the project is to unifying standard for architecture of networking systems. One of the projects was administered by the International Organization for Standardization (ISO), while the other was undertaken by the International Telegraph and Telephone Consultative Committee (CCITT).

In 1983, both of the documents were merged together and form a standard called The Reference Model for Open Systems Interconnection Reference Model or even just Open System Interconnection (OSI) model.

Open system Interconnection (OSI) was developed by the International Organization for Standardization (ISO) in 1984. OSI is an attempt to provide standard to the way networking should work. It is as a conceptual framework of standards for communication in the network across different equipment and applications by different vendors.

OSI model was designed to establish the data communication standard that will promote multi-vendor interoperability. It consists of seven layers, with a specific set of network functional allocated to each layer, and guidelines for implementation of the interfaces between layers.

OSI Model

Layer 7

Application

Layer 6

Presentation

Layer 5

Session

Layer 4

Transport

Layer 3

Network

Layer 2

Data link

Layer 1

Physical

Figure 1.1: OSI Model

Introduction to Layer 1 Physical Layer

Physical layer is the first layers in the Open System Interconnection (OSI) Model but is the last layer that receives and processes the data from the sending device while it is first layer to receive the data at the destination end.

The OSI Physical layer provides the means to transport across the network media the bits that make up a Data Link layer frame. This layer accepts a complete frame from the Data Link layer and encodes it as a series of signals that are transmitted onto the local media. The encoded bits that comprise a frame are received by either an end device or an intermediate device.

At this stage of the communication process, the user data has been segmented by the Transport layer, placed into packets by the Network layer, and further encapsulated as frames by the Data Link layer. The purpose of the Physical layer is to create the electrical, optical, or microwave signal that represents the bits in each frame. These signals are then sent on the media one at a time.

It is also the job of the Physical layer to retrieve these individual signals from the media, restore them to their bit representations, and pass the bits up to the Data Link layer as a complete frame.

Transmission of Data

Figure 1.2: Way of transmission data in the OSI Model Layers

Function of Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

Transmission of Bit Stream

Figure 2.3: Transmission of Bit Stream

Data encoding and signaling

The responsible of physical layer is to various the encoding data and signaling data that function to transform the data from the bits that reside within a computer or other devices into signal that can be sent through the overall network. Such as modify the simple digital signals that are 0s and 1s to a better accommodate the characteristics of the physical medium.

Diagram of Data Encoding and Signaling

Figure 2.4: Data Encoding and Signaling

Signaling

The Physical layer must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media. The method of representing the bits is called the signaling method. The Physical layer standards must define what type of signal represents a "1" and a "0". This can be as simple as a change in the level of an electrical signal or optical pulse or a more complex signaling method.

Diagram of Data Encoding and Signaling

Figure 2.5: Data Encoding and Signaling

Signaling Methods

Bits are represented on the medium by changing one or more of the following characteristics of a signal:

Amplitude

Frequency

Phase

The nature of the actual signals representing the bits on the media will depend on the signaling method in use. Some methods may use one attribute of signal to represent a single 0 and use another attribute of signal to represent a single 1.

The signaling method used must be compatible with a standard so that the receiver can detect the signals and decode them. The standard contains an agreement between the transmitter and the receiver on how to represent 1s and 0s. If there is no signaling agreement - that is, if different standards are used at each end of the transmission - communication across the physical medium will fail.

Ways to Represent the Signal on the Medium

Figure 2.6: Amplitude, Frequency and Phase Signaling

Non Return to Zero (NRZ) Signaling Method

A simple signaling method, Non Return to Zero (NRZ) will be examined at first. In NRZ, the bit stream is transmitted as a series of voltage values. A low voltage value represents a logical 0 and a high voltage value represents a logical 1. The voltage range depends on the particular Physical layer standard in use.

This simple method of signaling is only suited for slow speed data links. NRZ signaling uses bandwidth inefficiently and is susceptible to electromagnetic interference. Additionally, the boundaries between individual bits can be lost when long strings of 1s or 0s are transmitted consecutively. In that case, no voltage transitions are detectable on the media. Therefore, the receiving nodes do not have a transition to use in resynchrosnizing bit times with the transmitting node.

Diagram of Non Return to Zero (NRZ) Signaling

Figure 2.7: Signaling Bits for Transmission Non Return to Zero

Manchester Encoding Method

The Manchester Encoding scheme, bit values are represented as voltage transitions. A transition from a low voltage to a high voltage represents a bit value of 1. A transition from a high voltage to a low voltage represents a bit value of 0. One voltage transition must occur in the middle of each bit time. This transition can be used to ensure that the bit times in the receiving nodes are synchronized with the transmitting node.

Diagram of Manchester Encoding Signaling

Figure 2.8: Signaling Bits for Transmission Manchester Encoding

Encoding

Encoding is a method of converting a stream of data bits into a predefined code. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the received. Using predictable patterns helps to distinguish data bits from control bits and provide better media error detection.

In addition to creating codes for data, encoding methods at the Physical layer may also provide codes for control purposes such as identifying the beginning and end of a frame. The transmitting host will transmit the specific pattern of bits or a code to identify the beginning and end of the frame.

Group Bits

Grouping Bits use the word encoding to represent the symbolic grouping of bits prior to being presented to the media. By using an encoding step before the signals are placed on the media, we improve the efficiency at higher speed data transmission.

As the use of higher speeds on the media, the possibility that data will be corrupted is there. By using the coding groups, the detection errors more efficiently. Additionally, as the demand for data speeds increase, we seek ways to represent more data across the media, by transmitting fewer bits. Coding groups provide a method of making this data representation.

The Physical layer of a network device needs to be able to detect legitimate data signals and ignore random non-data signals that may also be on the physical medium. The stream of signals being transmitted needs to start in such a way that the receiver recognizes the beginning and end of the frame.

Signal Patterns

Signal patterns is one of the way to provide frame detection is to begin each frame with a pattern of signals representing bits that the Physical layer recognizes as denoting the start of a frame. Another pattern of bits will signal the end of the frame. Signal bits not framed in this manner are ignored by the Physical layer standard being used.

Valid data bits need to be grouped into a frame if not, the data bits will be received without any context to give them meaning to the upper layers of the networking model. This framing method can be provided by the Data Link layer, the Physical layer, or by both.

Purpose of signal Patterns

Figure 2.9: Recognizing Frame Signal

Code Groups

Encoding techniques use bit patterns called symbols. The Physical layer may use a set of encoded symbols - called code groups - to represent encoded data or control information. A code group is a consecutive sequence of code bits that are interpreted and mapped as data bit patterns. For example, code bits 10101 could represent the data bits 0011.

Code Group Diagram

Figure 2.10: Code Groups

Multiplexing

Multiplexing means to break one high speed physical communication circuit into several lower speed logical circuit so that many different devices can simultaneously use it but still they have their own circuits.

Figure 2.11: Multiplexing

Frequency Division Multiplexing (FDM)

Frequency Division Multiplexing (FDM) can be describe as dividing the circuit horizontally so that many signal can travel in a single communication circuit simultaneously. The circuit is divided into a series of separation channels, each transmitting circuit on a different frequency, much like series of different radio of television stations. All signals exist in the media at the same time, but because they are on different frequencies, they do not interfere with each other.

Use of FDM is to divide one circuit into four channels. Each channel is a separate logical circuit and the device connected to them is unaware that their circuit is multiplexed. The guard bands are the unused portion of the circuit that separates the frequencies from each other. FDM contain internal modem in it.

Figure 2.12: FDM

Time Division Multiplexing (TDM)

The Time Division Multiplexing (TDM) shares a communication circuit among two or more terminals by having them takes turns, dividing the circuit vertically, so to speak. Time on the circuit is allocated even when data cannot be transmitted, so that some capacity was wasted when terminals are idle.TDM is more efficiency than FDM because it does not required a guard bands. Guard bands use space on the circuit that otherwise could be used to transmit data.

Figure 2.13: TDM

Statistical Time Division Multiplexing (STDM)

Statistical Time Division Multiplexing (STDM) is the exception to the rule that the capacity of the multiplexed circuit is combines. STDM allows more terminals or computers to be connected to a circuit than does RDM or TDM. Not all computers will be transmitting continuously at their maximum transmission speed. STDM is called statistical because selection of transmission speed for the multiplexed circuit is based on a statically analysis of the usage requirements of the circuit to be multiplexed.

Figure 2.14: STDM

Wavelength Division Multiplexing (WDM)

A technique of sending signals of several different wavelengths of Light into the Fiber simultaneously. In fiber optic communications, wavelength-division Multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single Optical Fiber by using different wavelengths (colors) of Laser light to carry different signals. This allows for a multiplication in capacity, in addition to making it possible to perform Bidirectional communications over one strand of fiber.

Figure 2.15: WDM

Protocol of Physical Layer

Protocol is a set of rule that defined how two devices communicate each other over the network. In this physical layer there are a few of protocol used such as RS-232, Ethernet, and Asynchronous Transfer Mode (ATM).

RS-232

RS-232 standard is an asynchronous serial communication method used. Serial means the information is sent one bit per time and the asynchronous means that the information is not sent in a predefined times slots. Data transfer can start at any time given ant the task of the receiver is to detect it when the particular massage starts or ends.

This RS-232 standard describes the method of communication where information is sent bit by bit on a physical channel. RS-232 is recommended standard that the most commonly used to respectively connect Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). A simple example is connecting a computer to a modem. Revisions of the RS-233 protocol have included EIA-232 (Electronic Industries Alliance), and recently, EIA/TIA-232 (Telecommunications Industry Association).

Figure 3.16: 25 pin RS-232 Connector

Start Bits

RS-232 is an asynchronous type of communication. This means that the sending of a data word can start on each movement. Each data word is started with an attention bit. This attention bit also known as the start bit which is always identified by the space line level. This is because the line is in mark state when idle ant the start bit is easily recognized by the receiver.

Data Bits

Data bits are directly following the start bit and the data bits are sent. A bit value 1 causes the line to go to the mark state mark bit value 0 is represent by a space. The least signification bit is always the first bit sent through the physical medium.

Parity Bits

Parity bit function is for error detecting. It is possible to add an extra bit to the data word automatically. The transmitter function is to calculate the value of the bits depending on the information sent. Then the receiver will perform the same calculation and check if the actual parity bit value corresponds to the calculated value.

Stop Bits

When the receiver had missed the start bit causes of the noise on the transmission line. It will start on the first following data bits with a space value that cause garbled date to reach to the receiver. To resynchronize the communication a mechanism is required. In order to do this a framing is introduced. Framing means, that the data bits and parity bit are contained in a frame of start and stop bits. The period of the time lying between start and stop bits are constant and are defined by baud rate and number of data and parity bits. The stop bit is identifying the end of a data for valid start and stop bits pairs.

Transmission of Data

1 1 1 1 1 0 1 1 0 0 0 0 1 0 1 1 1 1

Data Flow

Character A

Leading Ideal

Bits

Start Bit

Parity Bit

Trailing Idle Bits

Stop Bit

Figure 3.17: Way of Data Transmitted

Ethernet

Ethernet is the most popular Local Area Network (LAN) architecture. Ethernet consists of certain specification, standards, hardware devices and components. Ethernet most common used in physical layer.

This Ethernet protocol allows for bus topology, star topology and tree topology depends on the cables and other factor.

Ethernet current standard at the 10Mbps level is 10BaseT. ‘10’ means the speed of transmission 10 megabits per second, the’ base’ mean baseband which it has the full control of the wire on a single frequency and the ‘T’ mean twisted pair cable. For the older standards like 10Base2 and 10Base5 it is used in the coaxial cable and for fiber cable it used at the level of 10BaseFL.

Fast Ethernet

Fast Ethernet is the protocol that can support the transmission up to 100 Mbps. In this Fast Ethernet it requires the use of different, more expensive network concentrator or hub ant also network interface cards. Besides that, It also required category 5 twisted pair cable or fiber optic cable. There are several types of Fast Ethernet Standards:-

100BaseT - 100 Mbps over 2-pair category 5 or better UTP cable.

100BaseFX - 100 Mbps over fiber cable.

100BaseSX -100 Mbps over multimode fiber cable.

100BaseBX - 100 Mbps over single mode fiber cable.

Gigabit Ethernet

Gigabit Ethernet standard is the protocol that has a transmission speed of 1Gbps that equal t 1000 Mbps. Gigabit Ethernet can be used in both copper and fiber optic cabling. The 1000BaseT, copper cable used in the Gigabit Ethernet. There are several types of Gigabit Ethernet Standards:-

1000BaseT - 1000 Mbps over 2-pair category 5 or better UTP cable.

1000BaseTX - 1000 Mbps over 2-pair category 6 or better UTP cable.

1000BaseFX - 1000 Mbps over fiber cable.

1000BaseSX -1000 Mbps over multimode fiber cable.

1000BaseBX - 1000 Mbps over single mode fiber cable.

Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM) is one of the Physical Layer protocols. Types of physical medium used in ATM included shielded and unshielded twisted pair, coaxial cable and fiber optic cable which can provide cell transport capabilities.

This ATM physical layer is divided into two parts. The first part is physical medium sub layer that is responsible for sending and receiving a continuous flow of bits with associated timing information to synchronize transmission and reception.

The second part is the transmission convergence sub layer. The transmission convergence sub layer is responsible to header error control sequence generation and verification. I t is to generate a and check the header error control code to ensure it is a valid data. Besides that, it responsible to transmission frame adaptation which packages ATM cell into frames acceptable to the particular physical layer implementation. It is also to transmission frame generation and recoveries which can generates and maintain the appropriate physical layer frame structure.

Standards of Physical Layer

The Physical layer consists of hardware, developed by engineers, in the form of electronic circuitry, media, and connectors. Therefore, it is appropriate that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.

By comparison, the protocols and operations of the upper OSI layers are performed by software and are designed by software engineers and computer scientists. As we saw in a previous chapter, the services and protocols in the TCP/IP suite are defined by the Internet Engineering Task Force (IETF) in RFCs.

Similar to technologies associated with the Data Link layer, the Physical layer technologies are defined by organizations such as:

The International Organization for Standardization (ISO)

The Institute of Electrical and Electronics Engineers (IEEE)

The American National Standards Institute (ANSI)

The International Telecommunication Union (ITU)

The Electronics Industry Alliance/Telecommunications Industry Association (EIA/TIA)

National telecommunications authorities such as the Federal Communication Commission (FCC) in the USA.

Hardware

The Physical layer is concerned with network media and signaling. This layer produces the representation and groupings of bits as voltages, radio frequencies, or light pulses. Various standards organizations have contributed to the definition of the physical, electrical, and mechanical properties of the media available for different data communications.

Guided Media

Copper Wires

The most commonly used media for data communications is cabling that uses copper wires to signal data and control bits between network devices. Cabling used for data communications usually consists of a series of individual copper wires that form circuits dedicated to specific signaling purposes.

Other types of copper cabling, known as coaxial cable, have a single conductor that runs through the center of the cable that is encased by, but insulated from, the other shield. The copper media type chosen is specified by the Physical layer standard required to link the Data Link layers of two or more network devices.

These cables can be used to connect nodes on a LAN to intermediate devices, such as routers and switches. Cables are also used to connect WAN devices to a data services provider such as a telephone company. Each type of connection and the accompanying devices have cabling requirements stipulated by Physical layer standards.

Networking media generally make use of modular jacks and plugs, which provide easy connection and disconnection. Also, a single type of physical connector may be used for multiple types of connections. For example, the RJ-45 connector is used widely in LANs with one type of media and in some WANs with another media type.

Types of Copper Wires

Copper Wires

Figure 5.18: Copper Wires

Copper Cable

Coaxial cable consists of a copper conductor surrounded by a layer of flexible insulation, as shown in the figure.

Over this insulating material is a woven copper braid, or metallic foil, that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, also reduces the amount of outside electromagnetic interference. Covering the shield is the cable jacket.

All the elements of the coaxial cable encircle the center conductor. Because they all share the same axis, this construction is called coaxial, or coax for short.

Uses of Coaxial Cable

The coaxial cable design has been adapted for different purposes. Coax is an important type of cable that is used in wireless and cable access technologies. Coax cables are used to attach antennas to wireless devices. The coaxial cable carries radio frequency (RF) energy between the antennas and the radio equipment.

Coax is also the most widely used media for transporting high radio frequency signals over wire, especially cable television signals. Traditional cable television, exclusively transmitting in one direction, was composed completely of coax cable.

Cable service providers are currently converting their one-way systems to two-way systems to provide Internet connectivity to their customers. To provide these services, portions of the coaxial cable and supporting amplification elements are replaced with multi-fiber-optic cable. However, the final connection to the customer's location and the wiring inside the customer's premises is still coax cable. This combined use of fiber and coax is referred to as hybrid fiber coax (HFC).

In the past, coaxial cable was used in Ethernet installations. Today UTP offers lower costs and higher bandwidth than coaxial and has replaced it as the standard for all Ethernet installations.

Types of Connectors with Coaxial Cables

Figure 5.19: Coaxial Cable

Figure 5.20: Coaxial Cable Design and Coaxial Connectors

Unshielded Twisted Pair (UTP) Cable

Unshielded twisted-pair (UTP) cabling, as it is used in Ethernet LANs, consists of four pairs of color-coded wires that have been twisted together and then encased in a flexible plastic sheath. As seen in the figure, the color codes identify the individual pairs and wires in the pairs and aid in cable termination.

The twisting has the effect of canceling unwanted signals. When two wires in an electrical circuit are placed close together, external electromagnetic fields create the same interference in each wire. The pairs are twisted to keep the wires in as close proximity as is physically possible. When this common interference is present on the wires in a twisted pair, the receiver processes it in equal yet opposite ways. As a result, the signals caused by electromagnetic interference from external sources are effectively cancelled.

This cancellation effect also helps avoid interference from internal sources called crosstalk. Crosstalk is the interference caused by the magnetic field around the adjacent pairs of wires in the cable. When electrical current flows through a wire, it creates a circular magnetic field around the wire. With the current flowing in opposite directions in the two wires in a pair, the magnetic fields - as equal but opposite forces - have a cancellation effect on each other. Additionally, the different pairs of wires that are twisted in the cable use a different number of twists per meter to help protect the cable from crosstalk between pairs.

Unshielded Twisted Pair (UTP) Cabling Standard

The UTP cabling commonly found in workplaces, schools, and homes conforms to the standards established jointly by the Telecommunications Industry Association (TIA) and the Electronics Industries Alliance (EIA). TIA/EIA-568A stipulates the commercial cabling standards for LAN installations and is the standard most commonly used in LAN cabling environments.

Some of the elements defined are:-

Cable types

Cable lengths

Connectors

Cable termination

Methods of testing cable

Unshielded Twisted Pair (UTP) Cable Model

Figure 5.21: Unshielded Twisted Pair (UTP) Cable

Types of Unshielded Twisted pair (UTP) Cable

UTP cabling, terminated with RJ-45 connectors, is a common copper-based medium for interconnecting network devices, such as computers, with intermediate devices, such as routers and network switches.

Different situations may require UTP cables to be wired according to different wiring conventions. This means that the individual wires in the cable have to be connected in different orders to different sets of pins in the RJ-45 connectors. The following are main cable types that are obtained by using specific wiring conventions:

Ethernet Straight-through

Ethernet Crossover

Rollover

Ethernet Straight-Through, Ethernet Crossover, and Rollover Cables.

Using a crossover or straight-through cable incorrectly between devices may not damage the devices, but connectivity and communication between the devices will not take place. This is a common error in the lab and checking that the device connections are correct should be the first troubleshooting action if connectivity is not achieved.

Straight-through, Ethernet Crossover, and Rollover Cable Types:

Cable Type

Standard

Application

Ethernet Straight-through

Both end T568A or both end T568B

Connecting a network host to a network device such as a switch or hub

Ethernet Crossover

One end T568A other end T568B

Connecting two network hosts

Connecting two network intermediary devices (switch to switch or router to router)

Rollover

Close Propriety

Connect a workstation serial port to a router console port, using an adapter

Table 5.1: Standard and the Application or UTP cables

Figure 5.22: T568A and T568B Standard Wires

Shielded twisted Pair (STP) Cable

Another type of cabling used in networking is shielded twisted-pair (STP). As shown in the figure, STP uses two pairs of wires that are wrapped in an overall metallic braid or foil.

STP cable shields the entire bundle of wires within the cable as well as the individual wire pairs. STP provides better noise protection than UTP cabling, however at a significantly higher price.

For many years, STP was the cabling structure specified for use in Token Ring network installations. With the use of Token Ring declining, the demand for shielded twisted-pair cabling has also waned. The new 10 GB standard for Ethernet has a provision for the use of STP cabling. This may provide a renewed interest in shielded twisted-pair cabling.

Shielded Twisted Pair (STP) Cable Model

Figure 5.23: Shielded Twisted Pair (STP) Cable

External Signal Interference

Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent.

The timing and voltage values of these signals are susceptible to interference or "noise" from outside the communications system. These unwanted signals can distort and corrupt the data signals being carried by copper media. Radio waves and electromagnetic devices such as fluorescent lights, electric motors, and other devices are potential sources of noise.

Cable types with shielding or twisting of the pairs of wires are designed to minimize signal degradation due to electronic noise.

Sources of Interferences

Figure 5.24: Sources of Interferences

Fiber Optic Cable

Fiber-optic cabling uses either glass or plastic fibers to guide light impulses from source to destination. The bits are encoded on the fiber as light impulses. Optical fiber cabling is capable of very large raw data bandwidth rates. Most current transmission standards have yet to approach the potential bandwidth of this media.

Fiber Optic Cable Model

Figure 5.25: Fiber Optic Cable

Single Mode Fiber Optic Cables

Single-mode optical fiber carries a single ray of light, usually emitted from a laser. Because the laser light is uni-directional and travels down the center of the fiber, this type of fiber can transmit optical pulses for very long distances.

Fiber Optic Single Mode Cable Model

Figure 5.26: Fiber Optic Single Mode Cable

Fiber Optic Multimode Cables

Multimode fiber typically uses LED emitters that do not create a single coherent light wave. Instead, light from an LED enters the multimode fiber at different angles. Because light entering the fiber at different angles takes different amounts of time to travel down the fiber, long fiber runs may result in the pulses becoming blurred on reception at the receiving end. This effect, known as modal dispersion, limits the length of multimode fiber segments.

Multimode fiber, and the LED light source used with it, is cheaper than single-mode fiber and its laser-based emitter technology.

Fiber Optic Multimode Model

Figure 5.27: Fiber Optic Multimode Cable

Unguided Media

Broadcast Radio

Broadcast radio is an omni-directional transmission of radio frequency waves from a single point. Radio transmission is the most commonly used form of wireless media is radio transmission. Radio data transmission uses the same basic principles as standard radio transmission. Each device or computer on the network has a radio receiver or transmitter that uses a specific frequency range that does not interfere with commercial radio station.

The transmitter are very slow power, designed to transmit a signal only a few miles, and are often built into portable computers or handheld devices such as phone and personal digital assistants. Wireless technologies for LAN environment such as, Bluetooth and IEEE802.11b.

Figure 5.28: Omndirectional Antenna

Infrared

Infrared transmission uses low frequency light waves to carry the data through the air on a direct line of sight path between two points. This technology is similar to the technology used in infrared television remote controls.

It is prone to interference, particularly heavy rain, smoke and fog that obscure the light transmission. Infrared transmitters are quite small but are seldom used for regular communication among portable and handheld computers because of their line of signal transmission requirements. Infrared is not very common, but it is sometimes used to transmit data from building to building.

Terrestrial Microwave Transmission

Terrestrial microwaves system are primarily for long distance telephone and video transmission. These use line of sight transmission in the 3000 MHz to single digit GHz frequency range. Data transmission speeds often reach 45 Mbps.

A microwave is an extremely high frequency radio communication beam that s transmitted over a direct line of sight path between any two points. As it is named implies, a microwaves signals is an extremely short wavelength. Microwaves radio transmission performs same as cables. As with visible light waves, microwaves signals can be focused into narrow, powerful beams that can be projected over a long distances. As the distance between communication points increases, towers are used to elevate the radio antennas to account for the earth’s curvature and maintain a clear line of sigh path between the two parabolic reflectors.

The transmission media typically used for a long distance data or voice transmission. It does not require the laying of any cable, because long distance antenna with microwaves repeater station can be placed approximately 25 to 50 miles apart. A typically long distance antenna might feet wide, although over short distances in the inner cities, the dish antennas can be less than 2 feet in diameter. The airwaves in larger cities are becoming congested because so many microwaves dish antenna have been installed that the interfere with one and another.

Satellite Microwaves

Transmission via satellite is a transmission that involves the transmission nearby microwaves dish antenna ant also the satellite many miles up in the space. Satellite communications provide data network connectivity for locations without another means of connection. Protocols including GPRS enable data to be transferred between earth stations and satellite links.

Satellite microwaves is essentially terrestrial microwave except that the signal travels from earth to a satellite and back to the earth, thus achieving much greater distances than lone of sight transmission. A satellite geosynchronous orbit can transmit signals approximately one third the distance around the earth. Frequencies are typically in the 3 GHz to 30 GHz range with a data transfer rate of 100 Mbps.

Wireless Media and Types

Figure 5.29: Wireless Media and Types

Conclusion

This is the top layer in OSI model. This network provides a network for the end user. Physical layer is defines by the computer system sending massage, server as the interface between user ant he communication system. It does also perform the actual information processing.

The world data communication would not exist if there were neither conducted media nor radiated media by which to transfer data. The conducted media consists of twisted pair cable, coaxial cables, and fiber optic. The radiated media consists of satellite and radio transmission. Each of the media has their advantages, disadvantages and design limitation.

All the data transmitted over a communication medium is either digital or analog. It is transmitted with a signal which like data can be either in digital or analog. All signals consist of three basic component amplitude, frequency and phase. Two important factors affecting the transfer of signal over a medium are attenuation and the signal to noise ratio.

Since both data and signal can be either digital or analog, for combination of data and signals can be produced. Digital data with digital signal are represented by digital encoding. For digital data with analog signal to be transmitted the digital data must be modulated onto analog signal. There are three basic forms of modulation amplitude, frequency and phase modulation.

For multiple signals to share one medium, the medium must be divided into multiple channels. Presently, there are two basic ways to divide a medium by dividing frequencies or by dividing time. Frequency division involves assigning no overlapping frequency range to the different signals. Time division of a medium required that the available time of the medium be divided among the user. Frequency division multiplexing, synchronous time division multiplexing and statistical time division multiplexing are the three most commonly found forms of multiplexing.

All terminals possess several basic characteristics and application areas, with a distinction made between dumb, smart, and intelligent terminals. Most terminals have one of two different types interface the standard RS-232 interface, the coaxial interface.

References

Websites

Article: The OSI Model's Seven Layers Defined and Functions Explained

Website: http://support.microsoft.com/kb/103884

Article: The 7 Layers

Website: http://www.infocellar.com/networks/osi-model.htm

Article: OSI 7 Layers Reference Model For Network Communication

Website: http://www.javvin.com/osimodel.html

Article: History of the OSI Reference Model

Website: http://www.tcpipguide.com/free/t_HistoryoftheOSIReferenceModel.htm

Article: Physical Layer Sublayers

Website: http://www.tcpipguide.com/free/t_PhysicalLayerLayer1-2.htm

Article: The physical layer

Website: http://homepages.uel.ac.uk/u0118336/GroupEmeka/Web2/Physical%20Layer.htm

Article: What is asynchronous transfer mode (ATM)?

Website: http://www.wisegeek.com/what-is-asynchronous-transfer-mode-atm.htm

Article: ATM Asynchronous Transfer Mode

Website: http://compnetworking.about.com/od/networkprotocols/g/bldef_atm.htm

Article: RS232 Specifications and standard

Website: http://www.lammertbies.nl/comm/info/RS-232_specs.html

Article: Ethernet (Physical/Data Link Layers)

Website: http://www.fcit.usf.edu/network/chap2/chap2.htm

Books

Author: Jerry Fitzgerald, Alan Dennis

Year: 2002

Book Title: Business Data Communication and Networking

Place: United state of America

Publication Company: John Wiley & Sons, INC.

Author: Cure White

Year: 1995

Book Title: Data Communication And Network An OSI Framework

Publisher Company: Boy & Fraser