Business Data Communications & Networking Lecture 4 More Data Transmission

Business Data Communications & Networking

Lecture 4

More Data Transmission


Telecommunications Standards

Where do they come from? Standard setting bodies Governments

Two types Market-driven and voluntary Government-regulated and mandatory


Advantages

Assures a large market, which encourages mass production and often lowers costs

Encourages vendors to enter market because investment is protected

Allows products from multiple vendors to communicate, providing consumers with wider selection


Disadvantages

Standards process can freeze technology too early, due to the length of the standards-setting process and the speed with which technology changes

Current process allows for multiple standards for the same thing


Institute of Electrical and Electronics Engineers (IEEE)

the largest professional society in the world

develops standards in the area of electrical engineering and computing

publishes scores of journals and runs numerous conferences each year

e.g. IEEE 802.x network standards


American National Standards Institute (ANSI)non-gov’t and nonprofit organizationmembers are U.S. manufacturers and

other interest groupssets a variety of a standards, not just

computer-related ANSI proposals are usually approved by

ISO as international standardse.g. 802.x, created by IEEE, approved by

ANSI, passed on and approved by ISO


National Institute of Standards and Technology (NIST)

formerly known as the National Bureau of Standards (NBS)

an agency of the U.S. Dept.. of Commerce

issues standards that are mandatory for purchases made by the U.S. Government except the Department of Defense


Industry Associations

Electronic Industries Association (EIA)

Telecommunication Industry Association (TIA)

e.g. EIA-232 (formerly RS-232-C)


Int’l Telecommunications Union (ITU)

formerly known as Consultative Committee on International Telegraph and Telephone (CCITT)

standardize techniques and operations in the telecommunications field

e.g. CCITT Group 3 Fax CCITT V.x modem standards


ISO (International Standards Organization)

a member of ITU-T founded in 1946issues standards on a vast number of

subjects, ranging from nuts and bolts to telephone pole coatings

has almost 200 Technical Committees


Internet Engineering Task Force (IETF)

Part of the Internet Architecture Board (IAB)

IETF proposes and published Internet RFCs

IAB determines which RFCs become standards, based on IETF recommendations


RFCInternet Standard

Stable and well-understoodTechnically competentNumerous independent and interoperable

implementations in operation Significant public supportRecognizably usefulDiffers from other standards processes

because of the emphasis on operational experience


Data Communication Basics

Analog or DigitalThree Components

Data Signal Transmission


Codec

Codec: coder and decoder

telephone

analog voice

analog line

digital line

analog voice

0 1 1 0 1 0 0 0 1 1 0

digitized voice

Analog Data Choices


DSU

DSU: data service unit

analog line

digital line

moduated data

0 1 1 0 1 0 0 0 1 1 0

data

data

modem

Digital Data Choices


Transmission Choices

Analog transmission only transmits analog signals, without

regard for data content attenuation overcome with amplifiers

Digital transmission transmits analog or digital signals uses repeaters rather than amplifiers


Data, Signals, and TransmissionData, Signals, and Transmission

DataData

SignalSignal

TransmissionTransmissionSystemSystem

AA

DDDD

DDAA

AA


Advantages of Digital Transmission

The signal is exactSignals can be checked for errorsNoise/interference are easily filtered

outA variety of services can be offered

over one lineHigher bandwidth is possible with

data compression


Analog Encoding of Digital Data

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


Modem

an acronym for modulator-demodulatoruses a constant-frequency signal

known as a carrier signalconverts 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


Methods of Modulation

amplitude modulation (AM) or amplitude shift keying (ASK)

frequency modulation (FM) or frequency shift keying (FSK)

phase modulation or phase shift keying (PSK)


Amplitude Shift Keying (ASK)

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 telephone lines are very susceptible to

variations in transmission quality that affect amplitude


1 0 0 1

ASK Illustration


Frequency Shift Keying (FSK)

in radio transmission, known as frequency modulation (FM)

the frequency of the carrier wave varies in accordance with the signal to be sent

signal is transmitted at constant amplitude

more immune to noise than ASKrequires more analog bandwidth than ASK


1 1 0 1

FSK Illustration


Phase Shift Keying (PSK)

also known as phase modulation (PM)frequency and amplitude of the carrier

signal are kept constantthe carrier is shifted in phase

according to the input data streameach phase can have a constant value,

or value can be based on whether or not phase changes (differential keying)


0 0 1 1

PSK Illustration


0 1 1

Differential Phase Shift Keying (DPSK)

0


Complex Modulations

Combining modulation techniques allows us to transmit multiple bit values per signal change (baud)

Increases information-carrying capacity of a channel without increasing bandwidth

Increased combinations also leads to increased likelihood of errors

Typically, amplitude and phase modulation are combined


Quadrature Amplitude Modulation (QAM)

the most common method for quadbit transfer

combination of 8 different angles in phase modulation and two amplitudes of signal

provides 16 different signals, each of which can represent 4 bits


90

45

0

135

180

225

270

315

amplitude 1

amplitude 2

Quadrature Amplitude Modulation Illustration


Quadrature Amplitude Modulation Uses

CCITT V.22 bis modem the "bis" qualifier is a French term for

"duo" or "twice" supports transmission of full-duplex

2400 bps synchronous or asynchronous data over a switched, 2-Wire, voice circuit

the modulation rate is 600 baud, with each baud representing four data bits


Trellis Coded Modulation (TCM)

sophisticated mathematics are used to predict the best fit between the incoming signal and a large set of possible combinations of amplitude and phase changes

a Forward Error Correcting (FEC)used in the V.32 modem (9600 bps)

and all the higher speed modems


CCITT V-Series Modem Recommendations

V.22: 1200 bps duplex modem standardized for use in the PSTN and on leased circuits

V.29: 9600 bps modem standardized for use on point-to-point 4-wire leased telephone circuits

V. 32: 2-wire, duplex modems operating at data rate of up to 9600 bps for use on the PSTN and on leased circuits


V.32 bis Modems

allows transport of asynchronous or synchronous data up to 14400 bps

the modulation rate is 2400 bauduses the Trellis coding with QAM uses groupings of seven bits

only six of these bits contain actual user data, the remaining bit is the convolutional coded, redundant bit generated from the previous bits


V.34 Modems

capable of supporting full-duplex synchronous or asynchronous data over 4-Wire leased lines or 2-Wire circuits up to 28.8 kbps

the modulation rate (baud rate) and carrier frequency can vary

multi-dimensional Trellis-coding is employed


V.34+ Modems

data rate up to 33.6 kbps over dial-up circuits

can achieve the above data rate only over extremely clean lines

use a range of adaptive techniques that enable a modem to learn and adjust to line conditions.


56kbps Modems

asymmetrical; can download at 56kbps but upload at 33.6kbps only

requires digital T-1 or ISDN PRI connection at central site or ISP

no official standard yettwo incompatible systems

U.S. Robotics (56K x2) Rockwell (56K flex)


Digital Encoding of Analog Data

Primarily used in retransmission devicesUses 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.

8000 samples/sec sufficient for 4000hz


Converting Samples to Bits

QuantizingSimilar concept to pixelizationBreaks wave into pieces, assigns a

value in a particular range8-bit range allows for 256 possible

sample levelsMore bits means greater detail, fewer

bits means less detail


Codec

Coder/Decoderconverts analog signals into a digital

form and converts it back to analog signals

e.g., hi-fi music, television pictures, the output of copying machine, videoconferencing


Digital Encodingof Digital Data

Most common, easiest method is different voltage levels for the two binary digits

Typically, negative=1 and positive=0Known as NRZ-L, or nonreturn-to-

zero level, because signal never returns to zero, and the voltage during a bit transmission is level


Differential NRZ

Differential version is NRZI (NRZ, invert on ones)

Change=1, no change=0Advantage of differential encoding is

that it is more reliable to detect a change in polarity than it is to accurately detect a specific level


Problems With NRZ

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


Biphase Alternatives to NRZ

Require at least one transition per bit time, and may even have two

Modulation rate is greater, so bandwidth requirements are higher

Advantages Synchronization due to predictable

transitions Error detection based on absence of a

transition


Manchester Code

Transition in the middle of each bit period

Transition provides clocking and dataLow-to-high=1 , high-to-low=0Used in Ethernet


Differential Manchester

Midbit transition is only for clockingTransition at beginning of bit

period=0Transition absent at beginning=1Has added advantage of differential

encodingUsed in token-ring


Digital Encoding Schemes


Asynchronous & Synchronous Transmission

Concerned with timing issuesHow does the receiver know when

the bit period begins and ends?Small timing difference become

more significant over time if no synchronization takes place between sender and receiver


Asynchronous Transmission

Data transmitted 1 character at a time

Character format is 1 start & 1+ stop bit, plus data of 5-8 bits

Character may include parity bit

Timing needed only within each character

Resynchronization each start bit

Uses simple, cheap technology

Wastes 20-30% of bandwidth


Synchronous Transmission

Large blocks of bits transmitted without start/stop codes

Synchronized by clock signal or clocking data

Data framed by preamble and postamble bit patterns

More efficient than asynchronous

Overhead typically below 5%

Used at higher speeds than asynchronous

Requires error checking, usually provided by HDLC

Documents

Business Data Communications & Networking Lecture 4 More Data Transmission