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16.546 Computer Telecommunications:Modulation and Data Encoding

Professor Jay WeitzenElectrical & Computer Engineering Department

The University of Massachusetts Lowell

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Data Encoding at the PLData Encoding at the PL

Application

Presentation

Session

transport

Network

Data link

Physical

Application

Presentation

Session

transport

Network

Data link

Physical

Network

Data link

Physical

Source node Destination node

Intermediate node

Signals

Packets

Bits

Frames

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We Need to Encode PL FrameWe Need to Encode PL Frame

AL-Hdr Application Layer Msg

PL-Hdr Presentation Layer Msg

SL-Hdr Session Layer Msg

TL-Hdr Transport Layer Msg

NL-Hdr Network Layer Msg

DLL-Hdr Data Link Layer Msg

PL-Hdr Physical Layer Msg

Presentation

Session

Transport

Network

Data Link

Physical

Application7

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5

4

3

2

1

Network A Node

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Encoding TechniquesEncoding Techniques

Digital data, digital signalAnalog data, digital signalDigital data, analog signalAnalog data, analog signal

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Digital Data, Digital SignalDigital Data, Digital Signal

Digital signal– Discrete, discontinuous voltage pulses

– Each pulse is a signal element

– Binary data encoded into signal elements

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TerminologyTerminologyUnipolar

– All signal elements have same signPolar

– One logic state represented by positive voltage the other by negative voltageData rate

– Rate of data transmission in bits per secondDuration or length of a bit

– Time taken for transmitter to emit the bitModulation rate

– Rate at which the signal level changes– Measured in baud = signal elements per second

Mark and Space– Binary 1 and Binary 0 respectively

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Interpreting SignalsInterpreting Signals

Need to know– Timing of bits - when they start and end

– Signal levels

Factors affecting successful interpreting of signals– Signal to noise ratio

– Data rate

– Bandwidth

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Comparison of Encoding Schemes (1)Comparison of Encoding Schemes (1)

Signal Spectrum– Lack of high frequencies reduces required bandwidth

– Lack of dc component allows ac coupling via transformer, providing isolation

– Concentrate power in the middle of the bandwidth

Clocking– Synchronizing transmitter and receiver

– External clock

– Sync mechanism based on signal

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Comparison of Encoding Schemes (2)Comparison of Encoding Schemes (2)

Error detection– Can be built in to signal encoding

Signal interference and noise immunity– Some codes are better than others

Cost and complexity– Higher signal rate (& thus data rate) lead to higher

costs

– Some codes require signal rate greater than data rate

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Encoding SchemesEncoding Schemes

Nonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NRZI)Bipolar -AMIPseudoternaryManchesterDifferential ManchesterB8ZSHDB34B/5B, MLT-3, 8B/10 Schemes

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Nonreturn to Zero-Level (NRZ-L)Nonreturn to Zero-Level (NRZ-L)

Two different voltages for 0 and 1 bitsVoltage constant during bit interval

– no transition, i.e., no return to zero voltage

Absence of voltage for zero, constant positive voltage for one

More often, negative voltage for one value and positive for the other

This is NRZ-L

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Nonreturn to Zero InvertedNonreturn to Zero Inverted

Nonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal

transition at beginning of bit timeTransition (low to high or high to low) denotes a

binary 1No transition denotes binary 0An example of differential encoding

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NRZNRZ

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Differential EncodingDifferential Encoding

Data represented by changes rather than levelsMore reliable detection of transition rather than

levelIn complex transmission layouts it is easy to lose

sense of polarity

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NRZ pros and consNRZ pros and cons

Pros– Easy to engineer

– Make good use of bandwidth

Cons– dc component

– Lack of synchronization capability

Used for magnetic recordingNot often used for signal transmission

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Multilevel BinaryMultilevel Binary

Use more than two levelsBipolar-AMI (Alternate Mark Inversion)

– zero represented by no line signal– one represented by positive or negative pulse– one pulses alternate in polarity– No loss of sync if a long string of ones (zeros still a

problem)– No net dc component– Lower bandwidth– Easy error detection

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PseudoternaryPseudoternary

One represented by absence of line signalZero represented by alternating positive and

negativeNo advantage or disadvantage over bipolar-AMI

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Bipolar-AMI and PseudoternaryBipolar-AMI and Pseudoternary

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Trade Off for Multilevel BinaryTrade Off for Multilevel Binary

Not as efficient as NRZ– Each signal element only represents one bit

– In a 3 level system could represent log23 = 1.58 bits

– Receiver must distinguish between three levels (+A, -A, 0)

– Requires approx. 3dB more signal power for same probability of bit error

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BiphaseBiphase

Manchester– Transition in middle of each bit period– Transition serves as clock and data– Low to high represents one– High to low represents zero– Used by IEEE 802.3

Differential Manchester– Midbit transition is clocking only– Transition at start of a bit period represents zero– No transition at start of a bit period represents one– Note: this is a differential encoding scheme– Used by IEEE 802.5

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Biphase Pros and ConsBiphase Pros and Cons

Con– At least one transition per bit time and possibly two

– Maximum modulation rate is twice NRZ

– Requires more bandwidth

Pros– Synchronization on mid bit transition (self clocking)

– No dc component

– Error detection

• Absence of expected transitionAbsence of expected transition

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Modulation RateModulation Rate

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ScramblingScrambling

Use scrambling to replace sequences that would produce constant voltage

Filling sequence – Must produce enough transitions to sync

– Must be recognized by receiver and replace with original

– Same length as original

No dc componentNo long sequences of zero level line signalNo reduction in data rateError detection capability

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B8ZSB8ZS

Bipolar With 8 Zeros SubstitutionBased on bipolar-AMI If octet of all zeros and last voltage pulse preceding was

positive encode as 000+-0-+ If octet of all zeros and last voltage pulse preceding was

negative encode as 000-+0+-Causes two violations of AMI codeUnlikely to occur as a result of noiseReceiver detects and interprets as octet of all zeros

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HDB3HDB3

High Density Bipolar 3 ZerosBased on bipolar-AMIString of four zeros replaced with one or two

pulses

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B8ZS and HDB3B8ZS and HDB3

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Digital Signal Encoding For LANsDigital Signal Encoding For LANs4B/5B-NRZI

– Used for 100BASE-X and FDDI LANs

– Four Data Bits Encoded into Five Code Bits, 80%

MLT-3– 100BASE-TX & FDDI Over Twisted Pair

8B/6T– Uses Ternary Signaling (Pos, Neg, Zero Voltages)

– Eight Data Bits Encoded into 6 Ternary Symbols

8B/10B– Used for Fibre Channel & Gigabit Ethernet

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10 Gigabit Ethernet (1 of 2)10 Gigabit Ethernet (1 of 2)• IEEE 802.3ae• MAC: it’s just Ethernet

– Maintains 802.3 frame format and size– Full duplex operation only– Throttled to 10.0 for LAN PHY or 9.58464 Gb/s for WAN PHY

• PHY: LAN and WAN phys– LAN PHY uses simple encoding mechanisms to transmit data on dark fiber and

dark wavelengths– WAN PHY adds a SONET framing sublayer to utilize SONET/SDH as layer 1

transport

• PMD: optical media only– 850 nm on MMF to 65m– 1310 nm, 4 lambda, WDM to 300 m on MMF; 10 km on SMF– 1310 nm on SMF to 10 km– 1550 nm on SMF to 40 km

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10 Gigabit Ethernet (2 of 2)10 Gigabit Ethernet (2 of 2)

• Supports dark wavelength and SONET/TDM with unlimited reach

• Several Coding Schemes (64b/66b; 8B/10B; Scramblers)

• Three optional interfaces: XGMII; XAUI; XSBI• Extension of MDIO interface• Continues Ethernet’s reputation for cost effectiveness

and simplicity (goal 10X performance for 3X cost)• Expected target for ratification in Spring 2002

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802.3ae to 802.3z Comparison802.3ae to 802.3z Comparison

1 Gigabit Ethernet• CSMA/CD + Full

Duplex• Carrier Extension• Optical/Copper Media• Leverage Fibre Channel

PMD’s• Reuse 8B/10B Coding• Support LAN to 5 km

10 Gigabit Ethernet• Full Duplex Only• Throttle MAC Speed• Optical Media Only• Create New Optical

PMD’s From Scratch• New Coding Schemes• Support LAN to 40

km; Use SONET/SDH as Layer 1 Transport

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Converting From Analog To DigitalConverting From Analog To Digital

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Pulse Code Modulation: a digital Pulse Code Modulation: a digital encoding scheme used in TDMencoding scheme used in TDM

In this modulation technique, an analog signal is digitized, and interleaved with other digitized voice signal to create a single bit stream

At the receiving end, the bit stream is decomposed into separate digital streams of lower frequencies, each stream is then converted back into what resembles the original voice signal.

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Steps Required to Generate PCMSteps Required to Generate PCMStreamsStreams

Sampling: periodic measurement of the analog signals at regular intervals

Quantizing: assigning discrete values to samplesCoding: assigned binary codes to samples using

what is known as the PCM code word

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SamplingSampling

Figure 2.2 : creating a PAM wave for a single sinusoid.(a) is a sinusoid signal, (b) a pulse train, (c) the result of

passing (a) and (b) through a point by point multiplier.

(a)

(b)

(c)

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SamplingSampling

Sampling rate: how often should we take measurements of the analog signal

at least at twice the rate of its highest frequency component

For a voice channel with a frequency range between 300 Hz and 3400 Hz (bandwidth of 3100 Hz) we need to take a sample at least at a rate of 2 X 3100 = 6200 Hz or every 1/6200 second

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SamplingSampling

In practical system, we sample multiple channel, we combine the samples of all channels into a single signal called the PAM signal (Pulse Amplitude Modulation signal)

In American systems we sample 24 channelsIn the European systems 30 channels are sampled

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QuantizationQuantization

To represent samples by a fixed number of bitsFor example if the amplitude of the PAM signal

range between -1 and +1 there can be infinite number of values. For instance one value can be -0.2768987653598364834634

For practicality, we may use 20 different discrete values between -1 and +1 volts

Each value at a 0.1 increment

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Quantization: the binary worldQuantization: the binary world

Because we live in a binary world, we select the total number of discrete values to be binary number multiple (i.e., 2, 4, 8, 16, 32, 64, 128, 256, and so on)

This facilitate binary codingFor instance, if there were 4 values they would be

as follows: 00, 01, 10, 11This is a 2-bit code

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Quantization:Quantization:16 coded quantum steps16 coded quantum steps

Between -1 and + 1 volts signal16 discrete stepseach step at 0.125 volts increment or decrement

from the adjacent step0 0000 0v 3 0011 0.375v1 0001 0.125v 4 0100 0.500v2 0010 0.25v 5 0101 0.625v

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Quantization: 16 quantum stepsQuantization: 16 quantum steps(-1 to + 1 volts)(-1 to + 1 volts)

+1

-1

0

Range of standardvalues (V)

15 : 111114: 111013: 110112: 110011: 101110: 10109: 10018: 10007: 01116: 01105: 01014: 01003: 00112: 00101: 00010: 0000

Coded values

Figure 2.4: quantization and resulting codingusing 16 quantizing steps

8 9 10 1112 13 12 11 10.. 6 .........

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Quantization DistortionQuantization Distortion

Quantization error is the different between the quantum value and the true value

More steps reduce quantizing distortion in linear quantization

This will require higher bandwidth, since we need more bits for each code word

Voice represent a problem because of the wide dynamic range, the level from the loudest syllable of the loudest talker to the lowest syllable of the quietest talker

S/D = 6n + 1.8 dB EX: 7 bit PCM cod 6.7 + 1.8 = 43.8practical system S/D = 30 - 33 dB

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CompandingCompanding

Compression/ExpandingNon-linearThe voltage level between the loudest and the

lowest is segmented in non-linear manorThe voltage range of each segment varies

according to the level of the voltage

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Non-linear QuantizationNon-linear Quantization

0

0.5

1.5

3.0

5.0

Voltage levelsSegment #

1

2

3

4

5

6

7

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Figure2_5: Nonlinear quantization using 8 segments with eachsegment assigned two steps (two coded words)

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Non-linear QuantizationNon-linear Quantization

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Segment 1

Segment 2Segment 3

Segment 4

Input Voltage

Compressed OutputVoltage

Figure 2.6: The relationship between the input voltage (-5 to+5) and the compressed output voltage

-5.0

Segment 2 has 3 steps like allof the other segments

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Coding for Modern PCM systemsCoding for Modern PCM systems

Non-linearLogarithmicA-Lawu-Law

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A-LawA-Law

A

Vvfor

A

AXY

0

log1

VvA

Vfor

A

AXY

log_1

log(_1

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U-LawU-Law

)1log(

|)|1log(||

u

XuY

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Coding for Modern PCM systemsCoding for Modern PCM systems

Where = instantaneous input voltage V = maximum input voltage for which peak limitation is

absent i = number of quantization steps starting from the

center of the range B = number of quantization steps on each side of the

center of the range.

V

vX

B

iY

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13-segment A-Law Curve13-segment A-Law Curve

6

5

4

3

2

1

0

0 1 1 1 X X X

0 1 1 0 X X X

0 1 0 1 X X X

0 1 0 0 X X X

0 0 1 1 X X X

0 0 1 0 X X X

0 0 0 1 X X X

0 0 0 0 X X X

NEGATIVE

1 0 0 0 X X X

1 0 0 1 X X X

1 0 1 0 X X X

1 0 1 1 X X X

1 1 0 0 X X X

1 1 0 1 X X X

1 1 1 0 X X X

1 1 1 1 X X X

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64

80

96

112

Segment(Chord)

Code

0 1/4 2/4 3/4 1 (V)

POSITIVE

Figure 2.7: 13-segment approximation of the A-lawcurve used with E1 PCM equipment

1/64

1/32

1/16

1/8

1/4

1/2

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PCM Code WordPCM Code Word

S DCBA

SignSegmentNumber

LevelValue

Figure 2.8: PCM Code Example

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S/D for A-law & u-LawS/D for A-law & u-Law

For A = 87.6: S/D = 37.5 dBu = 255: S/D = 37

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Modems: Modulator/DemodulatorModems: Modulator/Demodulator

Used to Package bits for transport over broadband media– 3 ways to encode information on a carrier

    PhasePhase    FrequencyFrequency    AmplitudeAmplitude

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Definition of ModulationDefinition of Modulation

Let m(t) be an arbitrary modulating (information) waveform. (could be either analog or digital)

Let c(t)=cos(ct +t) be the carrier

 The argument of the sinusoid is the instantaneous phase

(ct + t)

 

 The instantaneous frequency (2fi)is given by d/dt (ct

+ t) = c +d/dt(tfi

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Types of ModulationTypes of Modulation

If c(t)=m(t) cos(ct +), the information is

transported in the amplitude of the carrier. We call this Amplitude Modulation (AM)

If fi(t)=km(t), the information is transported in

the instantaneous frequency. We call this frequency modulation (FM).

If t=km(t) the information is carried in the instantaneous phase, and we call this phase modulation (PM).

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Modulation TechniquesModulation Techniques

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Amplitude Shift KeyingAmplitude Shift Keying

Values represented by different amplitudes of carrier

Usually, one amplitude is zero– i.e. presence and absence of carrier is used

Susceptible to sudden gain changesInefficientUp to 1200bps on voice grade linesUsed over optical fiber

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Frequency Shift KeyingFrequency Shift Keying

Values represented by different frequencies (near carrier)

Less susceptible to error than ASKUp to 1200bps on voice grade linesHigh frequency radioEven higher frequency on LANs using co-ax

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Frequency ModulationFrequency Modulation

FM Used for high fidelity audio broadcast and digital transmission. Uses Shannon concept of bandwidth expansion.     

 

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FSK on Voice Grade LineFSK on Voice Grade Line

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Phase Shift KeyingPhase Shift Keying

Phase of carrier signal is shifted to represent dataDifferential PSK

– Phase shifted relative to previous transmission rather than some reference signal

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Phase ModulationPhase Modulation

Generally used for digital modulation 

 

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Quadrature PSKQuadrature PSK

More efficient use by each signal element representing more than one bit– e.g. shifts of /2 (90o)

– Each element represents two bits

– Can use 8 phase angles and have more than one amplitude

– 9600bps modem use 12 angles , four of which have two amplitudes

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Constellation SpaceConstellation Space

Create 2-axis (e.g. sine and cosine) actually it could be a n-dimensional hyper-plane

Express digital modulation alphabet as points in the hyper-plane. The farther apart the points are in the space, the more immunity there is against noise and interference.

More distance, better error performance. Keep this in mind. 

The maximum power is the length of the longest vector. The average transmitter power is the average distance squared of all the points.

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Case Study 1: ASKCase Study 1: ASK

• If m(t) = {0,1} and we amplitude modulate a carrier with m(t) then the modulation is called on/off keying (OOK) or 2-amplitude shift keying (2-ASK) • 2-ASK, (points are at (0,0), and (0,1), in the 2 dimensional (sine, cosine plane). Minimum distance between points is 1 for 1 unit of power, and 1 bit per symbol. • Distance between points corresponds to error performance

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Case Study 2: Multi-Level ASKCase Study 2: Multi-Level ASK

•If maximum power is normalize to 1 then points are at (0,0), (0,1/3), (0,2/3), (0,1). Distance is reduced from 2-ASK and performance is worse. Requires 3x or 9x power to maintain 1 unit of distance. • From Shannon, as we add more information in a fixed bandwidth, it becomes increasingly expensive in terms of SNR to add more data. 

 

 

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Case 3: Orthogonal FSKCase 3: Orthogonal FSK

• Points are at (0,1) and (1,0) for 2-FSK. Distance is sqrt(2). Error performance better than 2-ASK but not as good as others.

•Frequencies are chosen so that the waveforms are orthogonal over the period of the bit T.

 

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Case 4: QPSK and PSKCase 4: QPSK and PSK

y(t)

x(t)A-A

A

-A

y(t)

x(t)A-A

A

-A

y(t)

x(t)A-A

Example signal constellationdiagram for BPSK signal.

y(t)

x(t)A-A

Example signal constellationdiagram for BPSK signal.

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Higher Order Modulations Very Higher Order Modulations Very Inefficient in terms of PowerInefficient in terms of Power

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Case 6: QAMCase 6: QAM

Beyond 3 bits/symbol, PSK too power inefficient. Must use Beyond 3 bits/symbol, PSK too power inefficient. Must use hybrid amplitude and phase modulation called QAMhybrid amplitude and phase modulation called QAM

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Example V.32 ConstellationExample V.32 Constellation

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Performance of Digital to Analog Performance of Digital to Analog Modulation SchemesModulation Schemes

Bandwidth– ASK and PSK bandwidth directly related to bit rate

– FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies

– (See Stallings for math)

In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK

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Coherent vs. Non-Coherent Coherent vs. Non-Coherent DetectionDetection

Coherent detection requires a copy of the carrier to be recovered from the received signal for use in the detection process. It is more efficient because it uses all phase information, but requires added complexity

Non-coherent detection using an envelope detector is much easier to implement, but less efficient because it uses only the envelope information and not the phase information.

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Digital Data, Analog SignalDigital Data, Analog Signal

Public telephone system– 300Hz to 3400Hz

– Use modem (modulator-demodulator)

Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PK)

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Analog Data, Digital SignalAnalog Data, Digital Signal

Digitization– Conversion of analog data into digital data

– Digital data can then be transmitted using NRZ-L

– Digital data can then be transmitted using code other than NRZ-L

– Digital data can then be converted to analog signal

– Analog to digital conversion done using a codec

– Pulse code modulation

– Delta modulation

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Pulse Code Modulation(PCM) (1)Pulse Code Modulation(PCM) (1)

If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal– (Proof - Stallings appendix 4A)

Voice data limited to below 4000HzRequire 8000 sample per secondAnalog samples (Pulse Amplitude Modulation, PAM)Each sample assigned digital value

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Pulse Code Modulation(PCM) (2)Pulse Code Modulation(PCM) (2)

4 bit system gives 16 levelsQuantized

– Quantizing error or noise– Approximations mean it is impossible to recover

original exactly8 bit sample gives 256 levelsQuality comparable with analog transmission8000 samples per second of 8 bits each gives

64kbps

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Nonlinear EncodingNonlinear Encoding

Quantization levels not evenly spacedReduces overall signal distortionCan also be done by companding

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Delta ModulationDelta Modulation

Analog input is approximated by a staircase function

Move up or down one level () at each sample interval

Binary behavior– Function moves up or down at each sample interval

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Delta Modulation - exampleDelta Modulation - example

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Delta Modulation - OperationDelta Modulation - Operation

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Delta Modulation - PerformanceDelta Modulation - Performance

Good voice reproduction – PCM - 128 levels (7 bit)

– Voice bandwidth 4khz

– Should be 8000 x 7 = 56kbps for PCM

Data compression can improve on this– e.g. Interframe coding techniques for video

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Analog Data, Analog SignalsAnalog Data, Analog Signals

Why modulate analog signals?– Higher frequency can give more efficient transmission

– Permits frequency division multiplexing (chapter 8)

Types of modulation– Amplitude

– Frequency

– Phase

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Analog Analog ModulationModulation

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Spread SpectrumSpread Spectrum

Analog or digital dataAnalog signalSpread data over wide bandwidthMakes jamming and interception harderFrequency hoping

– Signal broadcast over seemingly random series of frequencies

Direct Sequence– Each bit is represented by multiple bits in transmitted signal

– Chipping code

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Encoding Schemes - WAN TechniquesEncoding Schemes - WAN Techniques

1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0

0 0 0 0 V B 0 V B

AMI

B8ZS

HDB3

0 0 0 V B 0 0 V B 0 0 V

Both are well suited to characteristics of WAN channels

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Encoding Schemes - Encoding Schemes - SpectralSpectral Density Density

.2

.4

.6

.8

1.0

1.2

.2 .4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0

NRZ-LNRZI

B8ZS,HDB3

AMI, Pseudoternary

Manchester,Diff. Manchester

Normalized Frequency (f/R)

Mean SquareVoltage per UnitBandwidth

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Communications InterfaceCommunications Interface

TransmissionOr

Network

Information Exchange

•Content Material•Acquisition•Conversion•Compression•Buffering•Media Access•Protocol•Segmentation•Streaming

•Packet Routing•Node Switching

•Buffering(Network Delay & Transmission Jitter)

•Content Material•Acquisition•Conversion•Compression•Buffering•Media Access•Protocol•Reassembly•Synchronization

SourceWS Destination WS

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Asynchronous and Synchronous Asynchronous and Synchronous TransmissionTransmission

Timing problems require a mechanism to synchronize the transmitter and receiver

Two solutions– Asynchronous

– Synchronous

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AsynchronousAsynchronous

Data transmitted one character at a time– 5 to 8 bits

Timing only needs maintaining within each character

Resync with each character

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Asynchronous (diagram)Asynchronous (diagram)

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Asynchronous - BehaviorAsynchronous - Behavior

In a steady stream, interval between characters is uniform (length of stop element)

In idle state, receiver looks for transition 1 to 0Then samples next seven intervals (char length)Then looks for next 1 to 0 for next char

SimpleCheapOverhead of 2 or 3 bits per char (~20%)Good for data with large gaps (keyboard)

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Synchronous - Bit LevelSynchronous - Bit Level

Block of data transmitted without start or stop bits

Clocks must be synchronizedCan use separate clock line

– Good over short distances– Subject to impairments

Embed clock signal in data– Manchester encoding– Carrier frequency (analog)

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Synchronous - Block LevelSynchronous - Block Level

Need to indicate start and end of blockUse preamble and postamble

– e.g. series of SYN (hex 16) characters

– e.g. block of 11111111 patterns ending in 11111110

More efficient (lower overhead) than async

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Synchronous (diagram)Synchronous (diagram)

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Line ConfigurationLine Configuration

Topology– Physical arrangement of stations on medium– Point to point– Multi point

• Computer and terminals, local area networkComputer and terminals, local area networkHalf duplex

– Only one station may transmit at a time– Requires one data path

Full duplex– Simultaneous transmission and reception between two stations– Requires two data paths (or echo canceling)

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Traditional ConfigurationsTraditional Configurations

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InterfacingInterfacing

Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities

Need an interface called data circuit terminating equipment (DCE)– e.g. modem, NIC

DCE transmits bits on mediumDCE communicates data and control info with DTE

– Done over interchange circuits

– Clear interface standards required

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Characteristics of InterfaceCharacteristics of Interface

Mechanical– Connection plugs

Electrical– Voltage, timing, encoding

Functional– Data, control, timing, grounding

Procedural– Sequence of events

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V.24/EIA-232-FV.24/EIA-232-F

ITU-T v.24Only specifies functional and procedural

– References other standards for electrical and mechanical

EIA-232-F (USA)– RS-232

– Mechanical ISO 2110

– Electrical v.28

– Functional v.24

– Procedural v.24

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Mechanical SpecificationMechanical Specification

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Electrical SpecificationElectrical Specification

Digital signalsValues interpreted as data or control, depending

on circuitMore than -3v is binary 1, more than +3v is

binary 0 (NRZ-L)Signal rate < 20kbpsDistance <15mFor control, more than-3v is off, +3v is on

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Local and Remote LoopbackLocal and Remote Loopback

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Procedural SpecificationProcedural Specification

E.g. Asynchronous private line modemWhen turned on and ready, modem (DCE) asserts DCE

readyWhen DTE ready to send data, it asserts Request to Send

– Also inhibits receive mode in half duplex

Modem responds when ready by asserting Clear to sendDTE sends dataWhen data arrives, local modem asserts Receive Line

Signal Detector and delivers data

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Dial Up Operation (1)Dial Up Operation (1)

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Dial Up Operation (2)Dial Up Operation (2)

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Dial Up Operation (3)Dial Up Operation (3)

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Null ModemNull Modem

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ISDN Physical Interface DiagramISDN Physical Interface Diagram

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ISDN Physical InterfaceISDN Physical Interface

Connection between terminal equipment (c.f. DTE) and network terminating equipment (c.f. DCE)

ISO 8877Cables terminate in matching connectors with 8

contactsTransmit/receive carry both data and control