Technology Fundamentals

Chapter 2

Knowledge Checkpoints

Types of transmissions Connections Data conversions Modem concepts Hardware associated with

modems Analog-to-digital transmission Phone service and modems

In the Beginning

There was data and it needed to move.

Data was converted into a signal andthe signal moved on a pathway.

Important Terms Circuit Port Two-wire, four-wire Analog Local loop PSTN Physical, logical Amplifier Attenuated Channel Line Trunk CPE PBX Local Exchange CO VC, SVC

Interference Frequency Switched Dedicated Leased Line Communication band Hertz Wavelength Amplitude Phase Twisted pair Narrowband, broadband,

wideband Codecs Modems Multiplexors Standards

Data and Signals are Building Blocks of a Network

A signal is the transmission of data Signals are the electro-magnetic

encoding of data Data moves as a signal To transfer data a physical

wire/radio wave is used to convert into a signal

Data and signals can be either analog or digital

Data and Signals Can Assume 2 Forms

Analog Digital

Analog versus Digital

Analog is a continuous waveform (variable by amplitude and frequency), with examples such as music and video.

Voice conversations are carried on a POTS network as analog data.

Telephone switches connect calls at a Central Office (CO)

Voice Concepts

Voice Traffic

•Voice traffic is either inter-LATA or Intra-LATA

•Competing phone companies maintain a POP for long distance calls

•Analog data is transmitted as an analog signal onlywithin the local loop

Analog versus Digital

Digital is a discrete or non-continuous waveform with examples such as computer 1s and 0s.

All Signals Have Three Components

Amplitude

Frequency

Phase

AmplitudeThe amplitude of a signal is the height of the wave above or below a given reference point.

Frequency The frequency is the number of times a

signal makes a complete cycle within a given time frame.

Spectrum - The range of frequencies that a signal spans from minimum to maximum.

Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal.

Attenuation - Loss of signal strength.

Phase The phase of a signal is the position of

the waveform relative to a given moment of time or relative to time zero.

A change in phase can be any number of angles between 0 and 360 degrees.

Phase changes often occur on common angles, such as 45, 90, 135, etc.

Converting Digital Data into Digital Signals

There are numerous techniques available to convert digital data into digital signals.

Four techniques:

• NRZ-L

• NRZ-I

• Manchester

• Differential Manchester

Analog to Digital Analog signals work poorly over long

distances on copper wire Signals require a boost which can

distort the signal and introduce noise (it accumulates)

Digitization converts audio signal to digital and avoids noise

An A-to-D Converter makes an audio signal into a digital stream of numbers.

Pulse Code Modulation

The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.

Pulse Code Modulation

When the binary values are later converted to an analog signal, a waveform similar to the original results.

The more snapshots taken in the same amount of time, the better the resolution

PBX Concepts Provides shared use of voice

circuits within a business organization

Major players are telco-related Features are important in decisions

to purchase

Signals on a Pathway

Transmitting a Signal

Conducted (physical) Radiated (wireless)

The Physical Layer Responsible for establishing, maintaining

and termination of physical connections between connecting devices

Point-to-point links Transmits and receives a stream of bits--

no data recognition Controlled by protocols defined by

electrical, mechanical, and procedural specifications

Wire Propagation Effects

Propagation Effects Signal changes as it travels If change is too great, receiver may not be

able to recognize it

Distance

OriginalSignal

FinalSignal

Wire Propagation Effects: AttenuationAttenuation: Signal Gets Weaker as it

Propagates May become too weak for receiver to

recognizeSignalStrength

Distance

Wire Propagation Effects: Distortion

Distortion: Signal changes shape as it propagates Adjacent bits may overlap May make recognition impossible for

receiver

Distance

Wire Propagation Effects: Noise

Noise: Thermal Energy in Wire Adds to Signal Noise floor is average noise energy Random energy, so noise spikes sometimes

occurSignalStrength

Time

Signal

Noise

Spike

Noise Floor

Error

Wire Propagation Effects

Noise and Attenuation As signal attenuates, noise increases Noise errors increase with distance

SignalStrength

Distance

Signal

Noise Floor

Wire Propagation Effects: SNR

Signal-to-Noise Ratio (SNR) Signal strength divided by average noise

strength As SNR falls, errors increase

SignalStrength

Distance

Signal

Noise FloorSNR

Wire Propagation Effects: Noise & Speed

Noise and Speed As speed increases, each bit is briefer Noise errors increase as speed increases

One BitNoiseSpike

Average NoiseDuring Bit

Low Speed(Long

Duration)

One BitNoiseSpike

Average NoiseDuring Bit

High Speed(Short

Duration)

OK Error

Wire Propagation Effects: Interference

Interference Energy from outside the wire Adds to signal, like noise Often intermittent (comes and goes), so hard

to diagnoseSignal

Strength

Time

Signal

Interference

ISDN Modem(Codec) Translates analog device signal into digital

signal for propagation Constantly samples the intensity of the

voice analog signal

Sample

1/8,000 sec

0110010

Codec Divides each second into 8,000

sampling periods Only measures intensity of voice signal

in each

Sample

1/8,000 sec Sampling Period

IntensityValue

0110010

Types of Transmission Lines Circuits (2 & 4-wire) Channel

Difference between digital & analog conversations? Lines & Trunks

What is different about the conversation? Switches

CPE (within organizations) Connect trunks outside (part of a network) Connect exchanges

Virtual Circuits Logical connections that act real (take different routes)

PVCs and SVCs Private Virtual Circuits are

permanently available Routes may differ to destinations

Switched Virtual Circuits Routes vary but connection is “on-

demand”

Three Types of Connections Switched (dial-up) Leased Line (private) Dedicated (owned)

Spectrum Influences Services

Wideband >64Kbps

Broadband >2Mbps (optical)

Narrowband <64Kbps

Media influences frequency & spectrum

Wire = 1MhZ Cable = 1GhZ

Mutliplexors Reduces number of links between

2 points Condensing multiple conversations

to travel on a single channel Methods can be used to divide up

the multiple conversations to travel on a single line: FDM, TDM, STDM, WDM

Modems

Computers are digital & the telephone line is analog The translation device called a

modem

DigitalSignal

Modem

AnalogSignal

Modems

Devices are digital;Transmission line is analog The line is analog the short distance

to the CO (Central Office)

DigitalDevice

AnalogTransmission Line

ModulationA Modem is a Modulator and Demodulator

Modulation is converting outgoing digital device signals into analog transmission line signals

Demodulation is converting incoming analog transmission line signals into digital device signals

DigitalSignal

Modem

AnalogSignal

Modem Types Internal Modem

On printed circuit board inside PC Does not take up desktop space

Internal modems do not require a serial cable but instead require an IRQ to be assigned.

Home Connections

Local PC

modem modem

Remote PC

Phone network

1000001 1000001

Input Digital data

Processing Transform digital data input to analog data output (modulation)

Output - Input Analog

Processing Transform analog data input to digital data output (demodulation)

Output Digital data

PSTN

GOLDMAN & RAWLES: ADC 3e FIG: 03-14

Modem TypesExternal Modem

Easy to install (just plug into serial port)

Takes up desk space Needs wire to serial port (adds to

wiring clutter)

Modem Types

PC Card Modems Fit in PC Card slots on notebooks Easy to install Expensive

Modem Connection for Internal Modem

Phone Line toTelephone

Phone Line toWall Jack

Modem Connection for External Modem

PhoneLine to

Telephone

Wall Power:usually uses

“brick” transformer

Serial Cableto Serial

Port

Phone Line toWall Jack

Modem Standards

Two Modems Must Follow Same Standards Speed standards (modulation method) Error correction and compression

standards Facsimile standards Modern modem standards are created by

the ITU-T

Modem Standards

Training Period When two modems first start talking,

they negotiate standards to use; settle on highest common standards

Modem Speed Standards (ITU-T)

V.90 Receive at 56 kbps but send at only 33.6

kbps Fall back to 33.6 kbps if cannot support

V.34 33.6 kbps send and receive

V.32 bis 14.4 kbps send and receive bis means second (version of the

standard)

V.90 Modems Telephone Bandwidth is Limited

Bandwidth limits speed Limits speed to about 35 kbps When you transmit, limited to 35 kbps

35 kbps

V.90 ModemsTelephone Bandwidth is Limited

Limits occur when you transmit in analog to the phone company

Analog-to-digital converter (ADC) at the telephone company limits you to about 35 kbps

TelephoneNetwork

TelephoneNetworkADCADCPCPC

V.34modem

33.6 kbps

V.90 Modems Receive at 56 kbps

ISP connects with digital line to the Telco

Sends at 56 kbps No analog-to-digital converter to limit

speed

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

DigitalLink

NoADC!

56 kbps

V.90 ModemsReceive at 56 kbps

56 kbps digital channels inside phone network

Only local loop is analog today Trunk lines and switches support 56

kbps transmission

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

Digital Switches and Trunk Lines

56 kbps

V.90 ModemsReceive at 56 kbps

Telephone company transmits in analog to subscriber at 56 kbps

Digital to audio converter (DAC) does not limit speed to less than 56 kbps

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

DigitalLink

56 kbps

V.90 ModemTelephone Company Does Not Have to Do

Anything Differently ADCs and DACs are already in place for

ordinary voice service Lack of change in phone system

allowed fast implementation

TelephoneNetwork

TelephoneNetwork

ADCADC

PCPC56 kbpsmodem ISPISP

DigitalLink

DACDAC

V.90 ModemISP Does Have to Do Things Differently

Digital line to telephone network Special equipment at ISP Does not use a V.90 modem If used V.90 modem, could only send at

33.6 kbps

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

DigitalLink

NoV.90

Modem!

•Modern modems use combinations of amplitude, frequency, and phase modulation to achieve high data rates.

•The fastest dial-up modem at the moment is 56 Kbps.

•Modems can support auto answer, auto dial, auto disconnect, and auto redial.

Modems

•Connection negotiation is the ability of a modem to automatically fall forward or fallback to faster or slower speeds, respectively.

•Modems can perform data compression and error correction and support the MNP 1-5 protocols.

•Most modern modems can support the fax standards.

Modem Characteristics

Internal and External Modems

Why Your Dial up is Slow

Modem Pools

•A relatively inexpensive technique that allows multiple workstations to access a modem without placing a separate modem on each workstations.

•Modem pools can also be used to allow external users to dial into a business or corporate network via a modem in the modem pool.

The Reason Your ISP is Slow

Important Figures & Tables to Study Figure 2.8, p. 33 Table 2, p. 37 Table 2.1, p. 38 Figure 2.11, p. 38 Figure 2.13, p. 42

Knowledge Checkpoints Describe current voice technologies How is a PBX used? Give an example of a business

application of a voice technology What problems can data encounter

during transmission? How does a modem work?