Chapter 3. Data Transmission

Transmission Terminology

: Data transmission occurs between transmitter and receiver over some transmission medium.

Communication is in the form of electromagnetic waves.

• Guided media
• Twisted pair
• Coaxial cable
• Optical fiber
• Unguided media (wireless)
• Propagation through air, vacuum, and seawater

Direct link

• The term direct link is used to refer to the transmission path between two devices in which signals propagate directly from transmitter to receiver with no intermediate devices.
• Amplifiers or repeaters are not used to increase signal strength
• Note that this term can apply to both guied and unguided media

Point-to-Point

A guided transmission medium is point to point if it provides a direct link between two devices and those are the only two devices sharing the medium

Multi-point

In a multipoint guided configuration, more than two devices share the same medium.

Simplex (단방향)

• Signals are transmitted in only one direction
• One station is transmitter and the other is receiver
• E.g., TV

Half duplex (반이중)

• Both stations transmit, but only one at a time
• E.g., Radio

Full duplex (전이중)

• Both stations may transmit simultaneously
• The medium is carrying signals in both directions at the same time.
• E.g., Telephone

Analog

An analog signal is a continuous wave form that changes over time

Digital

• A digital siganl is discrete
• It can have only a limited number of defined values (0 and 1)

Analog vs. Digital

	Analog	Digital
Advantage	Less bandwith	High noise immunity
	More accurate	High level of security
Disadvantage	Low noise immunity	Large bandwith
	Low level of security	Less accurate

Signals

• Electric or electromagnetic or light representations of data
• Means of propagating data

Analog signal

Represent data with continuously varying sine wave

Digital signal

Represent data with sequence of voltage pulses

Modem

coverts digital data from computers to analog for phone line transmission

• On the receiving end, the analog is converted back to digital
• On the path between a pair of modems, the signal is analog

Codec

• A codec is a device, or computer program, capable of encoding and/or decoding a digital data stream or signal
• The name is contraction of COder-DECoder
• Codecs are used in video conferencing, streaming media and video editing applications
• A video camera's analog-to-digital converter (ADC) converts its analog signals into digital signals, which are then passed through a video compressor for digital transmission or storage
• A receiving device then runs the signal through a video decompressor, then a digital-to-analog converter (DAC) for analog display

• A data transfer device that converts an analog signal to a digital signal or converts a digital signal to an analog signal
• An audio codec converts analog audio signals into digital signals for transmission or storage
• A receiving device then converts the digital signals back to analog using an audio decompressor, for playback
• An example of this is the codecs used in the sound cards of personal computers
• A video codec accomplishes the same task for video signals

Amplifier

An electronic device that can increase the power of a signal

Repeater

• An electronic device that receives a signal and retransmits it.
• Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side.

Digital technology

• VLSI (very large-scale integration) is the current level of computer microchip miniaturization and refers to microchips containing in the hundreds of thousands of transistors
• LSI (large-scale integration) meant microchips containing thousands of transistors
• MSI (medium-scale integration) meant a microchip containing hundreds of transistors
• SSI (small-scale integration) meant transistors in the tens

Data integrity

• In the context of networking, refers to the overall completeness, accuracy and consistency of data.
• Data integrity must be imposed when sending data through a network.
• This can be achieved by using error checking and correction protocols.
• The use of repeaters has made it possible to transmit data longer distances over lower quality lines while maintaining the integrity of the data

Capacity utilization

• Capacity utilization or capacity utilization is the extent to which an enterprise or a nation uses its installed productive capacity.
• It is the relationship between output that is produced with the installed equipment
• It has become economical to build transmission links of very high bandwidth, including satellite channels and optical fiber
• A high degree of multiplexing is needed to utilize such capacity effectively
• Capacity utilization is the measure of how much of the available capacity is used

Security and privacy

Encryption techniques can be applied to digital data and to analog data that have been digitized.

Sine Wave

• Sine wave is the fundamental periodic signal
• It can be represented by three parameters
• Amplitude (A)
• Maximum value or strength of the signal over time
• Typically measured in volts
• Frequency (f)
• Rate at which the signal repeats
• Hertz (Hz) or cycles per second
• Period (T)
• The amount of time for one repetition
• T = 1 / f

Bandwidth and Data Rate

• Bandwidth
• Bandwidth is defined as the maximum amount of data that can be transmitted in a fixed number of time
• Bandwidth is the maximum "data rate" across a given path
• For digital devices, the bandwidth is usually expressed in bits per second(bps) or bytes per second
• For analog devices, the bandwidth is expressed in cycles per second, or Hertz (Hz)

• Data rate
• Data rate is the amount of data being transferred through a connection within a second
• Data rate cannot be higher than the bandwidth of the connection
• Data rate is measured in "bits per second" or bps

Asynchronous and Synchronous Transmission

• Asynchronous
• Asynchronous transmission is the transmission of data in which each character is a self-contained unit with its own start and stop bits and an uneven interval between them.
• Strategy is to avoid the timing problem by not sending long, uninterrupted streams of bits
• Data are transmitted one character at a time, where each character is 5 to 8 bits in length
• Timing or synchronization must only be maintained within each character
• The receiver has the opportunity to resynchronize at the beginning of each new character

• Synchronous
• Synchronous transmission is a data transfer method which is characterized by a continuous stream of data
• A block of bits is transmitted in  steady stream without start and stop codes
• Block may be many bits in length
• To prevent timing drift between transmitter and receiver, their clocks must somehow be synchronized
• Provide a separate clock line between transmitter and receiver
• Embed the clocking information in the data signal
• Frame
• Data plus preamble, postamble, and control information

Transmission Impairments

• Signal received may differ from signal transmitting causing:
• Analog - degradation of signal quality
• Digital - bit errors
• Attenuation (decrease, or reduction)
• Distortion (misrepresentation)
• Noise

Attenuation

• The strength of a signal falls off over any transmission medium
• A reduction commonly occurs while transmitting analog or digital signals over long distances
• Attenuation is historically measured in dB but it can also be measured in terms of voltage

Delay Distortion

• Delay distortion is caused by the fact that the velocity of propagation of a signal through a cable is different for different frequencies
• Propagation velocity varies with frequency
• Different frequency components will reach the destination at different time even they were transmitted at the same time
• For a signal with a given bandwidth, the velocity tends to be highest near the center frequency of the signal and to fall off toward the two edges of the band

Noise

• Noise is a term for unwanted modifications that a signal may suffer during transmission
• Noise may be divided into four categories:
• Thermal noise
• Intermodulation noise
• Crosstalk
• Impulse noise

Thermal noise

• When the temperature increases, the movement of electrons will increase
• It creates noise voltage
• Noise voltage is influenced by the temperature and therefore it is called thermal noise

Intermodulation noise

• Intermodulation is a process which occurs when multiple signals pass through a system or medium
• When the signals at different frequencies share the same transmission medium, then the result is intermodulation noise

Cross talk

• Cross talk is a noise that is caused by the inductive coupling between two wires that are closed to each other
• Sometime when talking on the telephone, you can hear another conversation in the background

Impulse Noise

• It is caused by external electromagnetic interferences
• it describes random occurrences of spikes or irregular pulses