Chapter 2 Transmission Basics

Data transmission The transfer of any kind of information from one location to another. Communication means sending a stream of bits or bytes from one location to another via transmission medium such as copper wire, fiber optics, laser, radio,infra red light, Bluetooth.

Data transmission Bit – most basic information unit used in computing & information theory. Single bit is ‘1’/’0’, ‘true/false’,’on/off’. Byte – a series of 8 bits, also called a character. Computer reads & writes information in bits and bytes.

Binary table Binary Number Name Example 0 or 1 Bit (binary digit) 4 bits Nibble 1101 8 bits Byte 11100101 1024 bytes =210 bytes 1 Kilobytes 1KB 1024 kilobytes = 2 20 bytes 1 Megabytes 1MB 1024 megabytes = 2 30 bytes 1 Gigabytes 1GB

Types of signals Signals – electric or electromagnetic representation of data. Can be coded for security purposes. Transmitter – converts a message into a signal, which is carried to a receiver by the communication channel. 2 types of signals ; Analog Signal Digital Signal

Analog signal Def : are continuous electrical signals that vary with time & represented by a waveform. All natural signal around us is of this type. Waveform is characterized by 4 properties; Amplitude – measure of wavelength at any given point of time. (Measured in Volt [V]) Frequency – number of times waveform repeats itself in a second. (Measured in Hertz[Hz] Wavelength – distance between two corresponding points on a waveform. (Measured in Meter [m]) Phase – progress of a wave over time in relationship to a fixed point or other wave with same frequency / amplitude. (Measured in degree [degree] in relation to starting point.

Analog signal Voltage 5 4 3 2 1 Amplitude 1s 2s 3s -5V

Digital signal Def : non-continuous & they change in individual steps. Consists of pulse & digits with discrete levels or values. Value of pulse constant, but there is an abrupt change from one digit to another. Have 2 amplitude levels which represents two voltage pulses. (1/0, true/false, high/low etc) Positive voltage pulse = 1, no pulse = 0 Computer is a digital device & they work on digital signals. 1 amplitude time

Comparison of digital & analog signals Digital Signal Analog Signal New technology Natural Two discrete values; 0/1, high/low, true/false, on/off Has all possible values Digital devices less affected by noise interruption & produces less error during transmission. Affected by noise, higher chances of error during data transmission. Digital devices are easy to design, implement & cheap. Analog devices are harder to design, implement & costly. Digital data easy to store, retrieve & modify Analog data difficult to store & modify Digital devices can easily interface with other digital devices, like PC Analog devices are difficult to interface & communicate.

Transmission flaws Def : signal which is transmitted at input & signal received at output does not always match due to transmission flaws over the transmission channel. Cause of transmission flaws : Noise Attenuation Latency

noise Def : unwanted signal which can disturb or harm the communication system. Common source of noise is electromagnetic interference (EMI) or wave that is emitted from electric carrying cable or electric devices. Source of EMI – electric motor, power lines, copier, television, lights & machines. There are 4 types of noises : Thermal Noise Induced Noise Impulse Noise Crosstalk

Noise Thermal Noise – created by the movement / colliasion of the electrons due to heat in a conductor (wire). Induced Noise – caused by varying magnetic field originated from external circuit or equipment. (engines may also cause this). Impulse Noise – pulse(s) appearing at the output point of a data line, which is not transmitted at the input point. (Ex; lightning). Crosstalk – refers to signal affecting another nearby signal

NOISE How to eliminate noise ? Use repeater & amplifier Repeater – device that regenerates digital signal. Amplifier – device with amplify or regenerate analog signal.

attenuation Def : a loss of power. A signal sent via a transmission media, will lose a portion of its power in form of heat or other form of energy. How to eliminate attenuation?? Use amplifier

latency Also known as delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. Together latency & bandwidth defines speed & capacity of network.

modulation Def : is the addition of information/signal to an electronic or optical signal carrier. A device that performs modulation is a modulator. A device that performs the inverse operation which is demodulation is a demodulator. A device that can do both modulation & demodulation is a modem.

modulation Modulation methods: Amplitude modulation (AM) – the amplitude of voltage applied to the carrier is varied over time. Frequency modulation (FM) – the frequency of the career waveform in varied in small but meaningful amounts. Phase modulation (PM) – the natural flow of the alternating current waveform is delayed temporarily.

multiplexing Def : transmitting multiple signals over a single communication line or computer channel. 2 common multiplexing techniques: Frequency Division Multiplexing (FDM) – separates signals by modulating the data onto different carrier frequencies. Time Division Multiplexing (TDM) – separates signals by interleaving bits one after the other.

Types of data transmission Serial transmission – transmission of several bits one after another through a single wire. Parallel transmission – multiple wires are used for transmitting bits simultaneously.

Comparison between serial & parallel transmission Bits are transmitted over single wire Bits are transmitted over multiple wires simultaneously. Slower than parallel transmission Faster than serial transmission Used over long distance Used for short distance Ex: connection between computer & modem. Ex: connection between motherboard & hard disk

Communication modes 3 modes of communication; Simplex Half-Duplex Full-Duplex one way communication Receiver can only receive but can’t reply / resend. Flow of information is unidirectional, hence rarely used for data communication. Ex : television, radio

Communication modes Half-duplex Full-duplex Two way communication but not at the same time. Ex : walkie-talkies Full-duplex Simultaneous two way communication. Improves the efficiency of communication Ex : telephone line

Transmission modes 2 types of transmission modes: Asynchronous transmission Synchronous transmission A technique widely used in personal computers to provide connectivity to printers, modems, fax machines etc. Codes are sent in untimely manner with uneven intervals between characters. Sending & receiving data is controlled by start & stop bit.

Transmission modes Synchronous transmission Technique of transmission in which the transmitter & receiver are synchronized. Codes are sent from transmitting station to receiving station to establish the synchronization. Data is transmitted in continuous stream.

Comparison between asynchronous & synchronous transmission. Suited for low speed serial transmission Suited for higher speed transmission Does not include error checking feature Has error checking feature Each character is contained in an envelope of start & stop bit It groups character together into blocks Less efficient More efficient