Business Data Communications, 6e

• Data
– Analog: Continuous value data (sound, light, temperature)
– Digital: Discrete value (text, integers, symbols)
• Signal
– Analog: Continuously varying electromagnetic wave
– Digital: Series of voltage pulses (square wave)
• Transmission
– Analog: Works the same for analog or digital signals
– Digital: Used only with digital signals
2


• Analog data to analog signal
– Inexpensive, easy conversion (e.g., telephone)
– Data may be shifted to a different part of the available spectrum (multiplexing)
– Used in traditional analog telephony
• Analog data to digital signal
– Requires a codec (encoder/decoder)
– Allows use of digital telephony, voice mail
3


• Digital data to analog signal
– Requires modem ( mo dulator/ dem odulator)
– Allows use of PSTN to send data
– Necessary when analog transmission is used
• Digital data to digital signal
– Requires CSU/DSU (channel service unit/data service unit)
– Less expensive when large amounts of data are involved
– More reliable because no conversion is involved
4

5

• Analog transmission
– only transmits analog signals, without regard for data content
– attenuation overcome with amplifiers
– signal is not evaluated or regenerated
• Digital transmission
– transmits analog or digital signals
– uses repeaters rather than amplifiers
– switching equipment evaluates and regenerates signal
6

Two alternatives:
(1) signal occupies the same spectrum as the analog data
(2) Analog data are encoded to occupy a different spectrum.
Digital data are encoded using a modem to produce analog signal.
Analog data are encoded using a codec to produce a digital bit stream.
Two alternatives:
(1) signal consists of two voltage levels to represent two binary values
(2) digital data are encoded to produce a digital signal with desired properties.
7

Analog
Signal
Digital
Signal
Analog
Transmission
Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data.
Not used.
Digital
Transmission
Assumes that the analog signal represents digital data. Signal is propagated through repeaters; at each repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal.
Digital signal represents a stream of 1s and 0s which may represent digital data or may be an encoding of analog data.
Signal is propagated though repeaters; at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.

• Cost – large scale and very large scale integration has caused continuing drop in cost
•
Data Integrity
– effect of noise and other impairments is reduced
•
Capacity Utilization
– high capacity is more easily and cheaply achieved with time division rather than frequency division
• Security & Privacy – Encryption possible
•
Integration
– All signals (Voice. Video, image, data) treated the same
9

• Data encoding and decoding technique to represent data using the properties of analog waves
• Modulation: the conversion of digital signals to analog form
• Demodulation: the conversion of analog data signals back to digital form
10

• An acronym for modulator-demodulator
• Uses a constant-frequency signal known as a carrier signal
• Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal
• The receiving modem translates the analog signal back into digital data
11

• Amplitude modulation (AM) or amplitude shift keying (ASK)
• Frequency modulation (FM) or frequency shift keying (FSK)
• Phase modulation or phase shift keying
(PSK)
12

13

• In radio transmission, known as amplitude modulation (AM)
• The amplitude (or height) of the sine wave varies to transmit the ones and zeros
• Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude
14 Business Data
Communications, 5e

1 0 0
1
15

• In radio transmission, known as frequency modulation (FM)
• Frequency of the carrier wave varies in accordance with the signal to be sent
• Signal transmitted at constant amplitude
• More resistant to noise than ASK
• Less attractive because it requires more analog bandwidth than ASK
16

1
1
0 1
17

• Also known as phase modulation (PM)
• Frequency and amplitude of the carrier signal are kept constant
• The carrier signal is shifted in phase according to the input data stream
• Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)
18

0 0 1 1
19

• Designed for digital transmission over ordinary phone lines
• Uses 4-kHz bandwidth
• Adheres to ITU-T standards
20

• Permits Internet access over cable television networks.
• ISP is at or linked by high-speed line to central cable office
• Cables used for television delivery can also be used to deliver data between subscriber and central location
• Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique
• Splitter is used to direct TV signals to a TV and the data channel to a cable modem
21

22

• New modem technology for high-speed digital transmission over ordinary telephone wire.
• At central office, a combined data/voice signal is transmitted over a subscriber line
• At subscriber’s site, twisted pair is split and routed to both a PC and a telephone
– At the PC, an ADSL modem demodulates the data signal for the PC.
– At the telephone, a microfilter passes the 4-kHz voice signal.
• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques.
23

24

• Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form
• Best-known technique for voice digitization is pulse-code modulation (PCM)
• The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.
• Good-quality voice transmission can be achieved with a data rate of 8 kbps
• Some videoconference products support data rates as low as 64 kbps
25

26

• Most common, easiest method is different voltage levels for the two binary digits
• Typically, negative=1 and positive=0
• Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level
27

• Differential version is NRZI (NRZ, invert on ones)
• Change=1, no change=0
• Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level
28

• Difficult to determine where one bit ends and the next begins
• In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
• Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted
29

• Require at least one transition per bit time, and may even have two
• Modulation rate is greater, so bandwidth requirements are higher; maximum modulation rate is twice NRZ
• Advantages
– Synchronization due to predictable transitions
– Error detection based on absence of a transition
30

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high=1 , high-to-low=0
• Used in Ethernet and other LANs
31

• Midbit transition is only for clocking
• Transition at beginning of bit period=0
• Transition absent at beginning=1
• Has added advantage of differential encoding
• Used in token-ring
32

33

• Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line.
• Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band
– A higher frequency may be needed for effective transmission
– Analog-to-analog modulation permits frequencydivision multiplexing
34

35

• Avoids timing problem by not sending long, uninterrupted streams of bits
• Data 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.
• Simple and cheap but requires an overhead of 2 to 3 bits per character
36

37

• Block of bits transmitted in a steady stream without start and stop codes.
• Clocks of transmitter and receiver must somehow be synchronized
– Provide a separate clock line between transmitter and receiver; works well over short distances,
– Embed the clocking information in the data signal.
• Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern
• The data plus preamble, postamble, and control information are called a frame
38

• More efficient than asynchronous transmission
• Preamble, postamble and control information are typically < 100 bits
• Introduces the need for error checking
39

• All transmission media have potential for introduction of errors
• All data link layer protocols must provide method for controlling errors
• Error control process has two components
– Error detection : redundancy introduced so that the occurrence of an error will be detected
– Error correction : receiver and transmitter cooperate to retransmit frames that were in error
40

• Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit character=12.5%)
• Noise impulses are often long enough to destroy more than one bit
41

• Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33bit divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2 -N
• Overhead is low (1-3%)
42

43