technology fundamentals chapter 2. knowledge checkpoints types of transmissions connections data...
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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?