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With diagrams compare and contrast the relative advantages and disadvantages of digital transmission over analogue transmission in modern communication systems. There are main types of transmission that are used by two different forms of signals from one computer to another computer and they are:  Analogue Transmission  Digital Transmission Analogue Transmission An analogue transmission has no break in the signal and the movement along the sine wave will never stop. So therefore the analogue transmission is continuous.

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The waves fluctuate (rise or fall) a certain amount of times per unit of time this type of fluctuation over time is called frequency. Frequency is measured in hertz, this means the large amount of fluctuations the higher the frequency. The frequency will then translate into higher notes. The low frequency will produce low deep notes. This signal also fluctuates in its loudness. The loudness is called amplitude. A low volume has low amplitude. In analogue transmission the physical quantity of the signal carrying the information through (amplitude, frequency, and phase) varies in a direct relationship to the information to be transmitted.

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Advantage of Analogue Transmission The advantage of analogue transmission that it has no break in the signal and it’s always continuous. Disadvantage of Analogue Transmission The disadvantage of analogue transmission is that the noise enters when the wave travels. This can cause a loss of quality at the other end of transmission this is because the analogue signal can’t be transmitted over a long distance. The signal becomes weaker the further the wave has to travel. This means the signal has to be boosted or amplified at regular intervals if it has to be transmitted very far.

Digital Transmission Computers are digital machines because at their most basic level they can tell apart between just two values, that are 0 and 1, or off and on. This has been describes any system based on discontinuous data or events. There is no simple way to represent all the values in between, such as the value of 0. 25. All data that a computer processes must be encoded digitally, as a series of zeroes and ones. The opposite of a digital is an analogue. Advantages of Digital Transmission.

The advantage of a digital transmission is that the signal is always exact, and also can be checked if it has any errors, the noise or the interference can be easily filtered out. There are a variety of services can be offered over one line, also Digital signal can be processed very fast. Disadvantages of Digital Transmission Digital transmission system uses a different in Frequency, Amplitude and how it represents a binary data analogue transmission. This can’t transfer the analogue signals because of the speed that release on a form of a message.

This line of the diagram below represents one specific point on an analogue wave. The measurements have been taken continuously and very rapidly from the analogue signal. A computer uses only digital signals. In order to be able to transfer digital data over conventional analogue transmission media such as phone lines, all signals must be converted to analogue. The process of converting digital to analogue is called modulation. Of course, at the other end, the signal must be changed back to digital. This is called demodulation. A device called a Modem is used to do this. Modem is short for Modulator/Demodulator.

Name and describe two examples of information, which are commonly transmitted in digital form, but are presented to users in analogue form. Use the diagram to show how this information is transformed and transmitted as a digital signal. Example One: Most of the computers are digital devices. Therefore, any signals and data from an analogue device must be passed through a suitable analogue-to-digital converter before they can be received and processed by computer. The monitors are analogue, so the data or the information before they can be presented to the user they converted to analogue.

This is an example of the diagram shown below: Example Two: Another example is mobile phone, which is used to transmit and receive voice signals this is before the voice signals can be transmitted, it must be changed into a form that is suitable for transmissions. It must first undergo a process of conversion from analogue to digital form. It then must be compressed so that it can be transmitted efficiently. Before the signal is finally sent into the air, it must be tailored so that it travels effortlessly through the air medium.

At the receiver similar manipulations of the signal in reverse order needs to be applied so that the original voice signal can be recovered. All the manipulations of the signal use techniques and technology from DSP. I have inserted an image of the mobile phone with some of annotations; I have also inserted a diagram showing voice signals being transferred into digital form whilst speaking through the telephone: Image One: Diagram One: A small microphone in the mouthpiece of the telephone handset captures the speaker’s voice through a vibrating diaphragm; this creates variations in the electrical current flowing through the microphone.

These electrical signals are transmitted through the telephone lines, and played on the vibrating diaphragm of the receiver part of the listener’s handset. Describe briefly with a diagram what a modem is. Explain the role of a modem in the transmission of computer data over analogue sections of the Public Switch Telephone Network (PSTN). Briefly explain and suggest organizations and application which would be likely to use: > Leased lines > Dial-up connections To access external networks. Justify your choice in each case. What is a modem?

Modulator-demodulator – a device or program that enables a computer to transmit data over analogue telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analogue waves. A modem converts between these two forms. The role of the modem to transmit and receive digital data over a communications line normally used for analogue signals. A modem attached to a computer converts digital data to an analogue signal that it uses to modulate a carrier frequency.

This frequency is transmitted over a line, frequently as an audio signal over a telephone line, to another modem that converts it back into a copy of the original data. Types of Modem There are two main types of modems and they are:  External Modem Internal Modem External Modem This is the simplest type of modem device to install because you don’t have to open the computer. External modems have their own power supply and connect with a cable to a computer’s serial port. The telephone line plugs into a socket on the rear panel of the modem. The diagram shown below: Internal Modem.

Most internal modems come installed in the computer you buy. Internal modems are more integrated into the computer system and you do not need any special attention. Internal modems are activated when you run a communications program and are turned off when you exit the program. This convenience is especially useful for novice users. These types of modems usually cost less. This is a diagram I have shown below of an internal modem. The PSTN stands for Public Switch Telephone Network, this was designed for voice transmission only, and using analogue electrical signals to represent what is spoken and heard at each end of a link.

A telephone mouthpiece contains a diaphragm, which vibrates whenever it is struck by sound waves, and these vibrations are converted to electrical signals (digital signals) by a modem, which have direct link to what is said. Leased Lines A leased line connection is a wide area network that offers fast, reliable connection for all data traffic. Leased line is has a permanent and good connection to the Internet that brings enhanced Internet performance and reliability.

The high-priced call charges of ISDN and the insufficiencies of the standard dial-up connection mean that leased line service is the cost-effective alternative, supplying your business with a high-speed connection at a fixed annual price. With no call or usage charges, you pay upfront for unlimited use. All types of organisations are serious about their Internet access use Leased Lines. In today’s business environment the Internet has become an essential communication, information, and marketing tool, this may needs to be available all of the time.

Many companies might lose money if they could not receive orders or e-mails so they need the guarantee of a Service Level Agreement (SLA). This means that should their connection be down longer than the time allowed in the SLA they would receive service credits as compensation. A leased line will give the customer a complete direct connection from their network straight into one of Onyx Internet’s Points of Presence (POP), this type of service provides a fast link out onto the Internet. Leased Lines are suitable for heavy Internet users and they can easily be upgraded as Internet usage or traffic flow increases.

This diagram below shows how a leased line will provide your network with a fast link out to the Internet: Dial-up connections A dial-up connection uses the public telephone network and the user only pays for the time taken for transmission. Alternatively, other nodes may require the link for most of the working day, in which case a permanent leased line is appropriate. Leased lines provide a more reliable transmission medium and also allow higher speeds of data transmission. Dial-up connections are cheaper, but support lower transmission rates than leased lines.

They are more cost effective than leased lines for low-volume work and allow the operator to choose the destination of transmissions. Describe the main functions of a communication protocol. Communication protocol Protocols are basically sets of rules that enable how two of the devices should communicate. Protocols are used for modem communications, usually between a home and a business user and Internet service provider’s server (ISP). Computers must use the same modem protocols and setting if they are to communicate reliable, when sending and receiving data. For example if a modem is set to use 8 data bits, and the other one 1-stop bit.

The modem in this case will not be able to understand the data unless it uses the same setting and also both modems must have the same speed operating. Therefore when connecting to the Internet, the connect speed could be different. The connect speed is depending on the modem speed at the Internet Service Provider’s site, once the dial-up connecting is been made. For communication between two devices, the protocol must set up as a minimum:  Data transmission rate must be measured in bits per seconds, if the devices have different maximum transmission rates. The rate will allow dictating the slower of the two devices.

The protocol must decide which device is currently has control of the communication link. If the two devices may transmit to each other at the same time or one direction at a time.  If the synchronous, (which send data in a stream with sender and receiver synchronised to identify when one character ends and another stars) or asynchronous (which characters are sent at regularly timed intervals, with star and stop bits to separate characters) transmission is to be used. The Protocols specify the form of handshaking, the method of data flow control and mechanisms for error control and data compression.

Aloha is a protocol for satellite and terrestrial radio transmissions. In pure Aloha, a user can transmit at any time but risks collisions with other users’ messages. “Slotted Aloha” reduces the chance of collisions by dividing the channel into time slots and requiring that the user send only at the beginning of a time slot. Aloha was the basis for Ethernet, a local area network protocol. Aloha is also called the Aloha method this refers to a simple communications scheme in which each source (transmitter) in a network sends data whenever there is a frame to send.

If the frame successfully reaches the destination (receiver), the next frame is sent. If the frame fails to be received at the destination, it is sent again. This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific. In a wireless broadcast system or a half-duplex two-way link, Aloha works perfectly. But as networks become more complex, for example in an Ethernet system involving multiple sources and destinations that share a common data path, trouble occurs because data frames collide (conflict).

The heavier the communications volume, the worse the collision problems become. The result is degradation of system efficiency, because when two frames collide, the data contained in both frames is lost. Diagram is shown below: With a diagram briefly explain the concept of handshaking and flow control and state why they are a necessary part of data communication. Handshaking means that communicating devices must have a common method of determining their presence and readiness in order to communicate. Although the details vary, a handshake can be illustrated simply by the following example.

Suppose that device A wishes to communicate with device B. Device A signals “Hello Device B, are you there? ” to which device B repliers, “Yes, I am here A. ” Finally, device A responds,” I see that you are there B. ” The handshake should also define when each device has finished sending or receiving a message. A flow control is a mechanism that is needed to ensure that data transmission flows smoothly, that the communications channel is not overloaded and that the transmitting device does not send data more quickly than the receiver handle.

Identify two methods of error checking in data transmission and explain how they help to maintain the integrity of a data transmitted. The two methods of error checking in data transmission I will analyse will be the following: > CRC (Cyclical Redundancy Checking) > Parity Bit Method One: CRC stands for Cyclical Redundancy Checking this is an error checking technique that is used to ensure the accuracy of transmitting digital data. The transmitted messages are divided into predetermined lengths that are used as dividends, are divided by a fixed divisor.

The remainder of the calculation is appended onto and sent with the message. At the receiving end, the computer recalculates the remainder. If it does not match the transmitted remainder, an error is detected. Method Two: In computers, Parity (from the Latin parities: equal or equivalent) refers to a technique of checking whether the data has been lost or it has written over when it’s been moved from one place in storage to another or when transmitted between the computers. The method works by an additional binary digit, the parity bit, is added to a group of bits that are moved together.

This bit is used only for the purpose of identifying whether the bits being moved arrived successfully. Before the bits are sent, they are counted and if the total number of data bits is even, the parity bit is set to one so that the total number of bits transmitted will form an odd number. If the total number of data bits is already an odd number, the parity bit remains or is set to 0. At the receiving end, each group of incoming bits is checked to see if the group totals to an odd number.

If the total is even, a transmission error has occurred and either the transmission is retried or the system halts and an error message is sent to the user. Distinguish between packet, circuited and message switching. Packet-switched are describes the type of network in which relatively are in small units of data called packets they are routed through a network based on the destination address which contained within each packet. Breaking communication down into packets allows the same data path to be shared among many users in the network.

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