Embed Size (px)
Citation preview
1
Topic 4: Physical Layer- Chapter 8: Data Communication Fundamentals
Business Data Communications, 4e
2
Outline
Characteristics of Electromagnetic Signals
Data, Signal, and TransmissionAnalog Transmission of Digital DataDigital Transmission of Analog DataDigital Transmission of Digital Data
3
Electromagnetic Signals
Function of time Analog (varies smoothly over time) Digital (constant level over time,
followed by a change to another level)
Function of frequency (more important) Spectrum (range of frequencies) Bandwidth (width of the spectrum)
4
Periodic Signal Characteristics
S(t) = A sin(2ft + )
Amplitude (A): signal value, measured in volts
Frequency (f): repetition rate, cycles per second or Hertz
Period (T): amount of time it takes for one repetition, T=1/f
Phase (): relative position in time, measured in degrees
5
Bandwidth
Width of the spectrum of frequencies that can be transmitted if spectrum=300 to 3400Hz,
bandwidth=3100Hz
Greater bandwidth leads to greater costs
Limited bandwidth leads to distortion
6
Bandwidth on a Voice Circuit
Human hearing ranges from about 20 Hz to about 14,000 Hz (some up to 20,000 Hz). Human voice ranges from 20 Hz to about 14,000 Hz.
The bandwidth of a voice grade telephone circuit is 0 to 4000 Hz or 4000 Hz (4 KHz).
Guardbands prevent data transmissions from interfering with other transmission when these circuits are multiplexed using FDM.
7
Bandwidth on a Voice Circuit
8
Bandwidth on a Voice Circuit
It is important to note that the limit on bandwidth is imposed by the equipment used in the telephone network.
The actual capacity of bandwidth of the wires in the local loop depends on what exact type of wires were installed, and the number of miles in the local loop.
Actual bandwidth in North America varies from 300 KHz to 1 MHz depending on distance.
9
Data
Analog data Voice Images
Digital data Text Digitized voice or images
10
time(sec)
amp
litu
de
(vo
lts)
1 cycle
frequency (hertz)= cycles per second
phase difference
Analog Signaling
represented by sine waves
11
Phase
Frequency: 1 Period/Sec = 1 Hertz
Phase
12
Three Components of Data Communication
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
13
Data Transmissions
Analog Transmission of Analog Data Telephone networks (PSTN)
Digital Transmission of Digital Data A computer system
Analog Transmission of Digital Data Uses Modulation/Demodulation
(Modem)Digital Transmission of Analog Data
Uses Coder/Decoder (CODEC)
14
Digital Coding
Character: A symbol that has a common, constant meaning.
Characters in data communications, as in computer systems, are represented by groups of bits [1’s and 0’s].
The group of bits representing the set of characters in the “alphabet” of any given system are called a coding scheme, or simply a code.
15
Digital Coding
A byte consists of 8 bits that is treated as a unit or character. (Some Asian languages use 2 bytes for each of their characters, such as Chinese.)
(The length of a computer word could be 1, 2, 4 bytes.)
There are two predominant coding schemes in use today:
United States of America Standard Code for Information Interchange (USASCII or ASCII)
Extended Binary Coded Decimal Interchange Code (EBCDIC)
16
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
17
Why Use Analog Transmission?
Already in placeSignificantly less expensive Lower attenuation rates Fully sufficient for transmission of
voice signals
18
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
19
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)
Differential Phase Shift Keying (DPSK)
20
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 is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude
21
Amplitude Modulation and ASK
22
Frequency Shift Keying (FSK)
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
23
Frequency Modulation and FSK
24
Phase Modulation and PSK
25
Phase Shift Keying (PSK)
Also known as phase modulation (PM) Frequency and amplitude of the carrier
signal are kept constantThe carrier signal 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)
26
0 1 1
Differential Phase Shift Keying (DPSK)
0
27
Sending Multiple Bits Simultaneously
28
Sending Multiple Bits Simultaneously
3/2 11
10
/2 01
000
29
Sending Multiple Bits Simultaneously
In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques.
One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitude for a total of 16 different possible values.
30
Sending Multiple Bits Simultaneously
Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. It can transmits different numbers of bits on each symbol (6-10 bits per symbol).
The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.
31
Example
Use a drawing to show how the bit pattern 11100100 would be sent using a combination of 1-bit Amplitude Modulation and 1-bit Phase Modulation (1AM+1PM).
32
Modem
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
33
Modem Standards V.22
1200-2400 baud/bps (FM) V.32 and V.32bis
full duplex at 9600 bps (2400 baud at QAM) bis uses TCM to achieve 14,400 bps.
V.34 for phone networks using digital transmission beyond the local
loop. 59 combinations of symbol rate and modulation technique symbol rates 3429 baud. Its bit rate is up to 28,800 bps (TCM-
8.4) V.34+
up to 33.6 kbps with TCM-9.8
34
Modem Standards (Cont’d) V.42bis
data compression modems, accomplished by run length encoding, code book compression, Huffman encoding and adaptive Huffman encoding
MNP5 - uses Huffman encoding to attain 1.3:1 to 2:1 compression.
it uses Lempel-Ziv encoding and attains 3.5:1 to 4:1. V.42bis compression can be added to almost any modem
standard effectively tripling the data rate.
35
Voice Grade Modems
36
Data Compression
How fast if using V.42bis V.32 - 57.6kbps V.34 - 115.2 kbps V.34+ - 133.4 kbps V.90 ?
37
Data Compression
There are two drawbacks to the use of data compression:
Compressing already compressed data provides little gain.
Data rates over 100 Kbps place considerable pressure on the traditional microcomputer serial port controller that controls the communications between the serial port and the modem.
38
Analog Channel Capacity: BPS vs. Baud Baud=# of signal changes per second. ITU-T now recommends
the term baud rate be replaced by the term symbol rate. BPS=bits per second In early modems only, baud=BPS. The bit rate and the symbol
rate (or baud rate) are the same only when one bit is sent on each symbol.
Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase
Increases information-carrying capacity of a channel without increasing bandwidth
Increased combinations also leads to increased likelihood of errors
39
Digital Transmission of Analog Data
Codec = Coder/Decoder Converts analog signals into a digital
form and converts it back to analog signals
Where do we find codecs? Sound cards Scanners Voice mail Video capture/conferencing
40
Codec vs. Modem
Codec is for coding analog data into digital form and decoding it back. The digital data coded by Codec are samples of analog waves.
Modem is for modulating digital data into analog form and demodulating it back. The analog symbols carry digital data.
41
Digital Encoding of Analog Data
Primarily used in retransmission devices 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.
Pulse-code modulation (PCM) 8000 samples/sec sufficient for 4000hz
42
Pulse Code Modulation (PCM) Analog voice data must be translated into
a series of binary digits before they can be transmitted.
With Pulse Code Modulation (PCM), the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.
The difference between the original analog signal and the translated digital signal is called quantizing error.
43
PCM
44
PCM
45
PCM
46
PCM
PCM uses a sampling rate of 8000 samples per second.
Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).
47
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
48
Analog/Digital Modems (56k Modems)
The basic idea behind 56K modems (V.90) is simple. 56K modems take the basic concepts of PCM and turn them backwards. They are designed to recognize an 8-bit digital signal 8000 times per second.
It is impractical to use all 256 discrete codes, because the corresponding DAC output voltage levels near zero are just too closely spaced to accurately represent data on a noisy loop. Therefore, the V.90 encoder uses various subsets of the 256 codes that eliminate DAC output signals most susceptible to noise. For example, the most robust 128 levels are used for 56 Kbps, 92 levels to send 52 Kbps, and so on. Using fewer levels provides more robust operation, but at a lower data rate.
49
Downstream vs. Upstream
50
Downstream vs. Upstream
51
Analog/Digital Modems (56k Modems)
Noise is a critical issue. Recent tests found 56K modems to connect at less than 40 Kbps 18% of the time, 40-50 Kbps 80% of the time, and 50+ Kbps only 2 % of the time.
It is easier to control noise in the channel transmitting from the server to the client than in the opposite direction.
Because the current 56K technology is based on the PCM standard, it cannot be used on services that do not use this standard.
52
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
53
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
Used for low speed (64Kbps) ISDN
54
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
55
Biphase Alternatives to NRZ
E.g. Manchester coding and Differential Manchester coding
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
56
Manchester Code
Transition in the middle of each bit period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0Used in Ethernet
57
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
58
Digital Encoding Illustration
59
Transmission Timing - Asynchronous vs. Synchronous
Sampling timing – How to make the clocks in a transmitter and a receiver consistent?
Asynchronous transmission – sending shorter bit streams and timing is maintained for each small data block.
Synchronous transmission – To prevent timing draft between transmitter and receiver, their clocks are synchronized. For digital signal, this can be accomplished with Manchester encoding or differential Manchester encoding.
60
Digital Interfaces
The point at which one device connects to another
Standards define what signals are sent, and how
Some standards also define physical connector to be used
61
Generic CommunicationsInterface Illustration
62
DTE and DCE
DTE DTE
host com puter term inal
in terface in terface
m odem m odem
DCE
63
RS-232C (EIA 232C)
EIA’s “Recommended Standard” (RS)
Specifies mechanical, electrical, functional, and procedural aspects of the interface
Used for connections between DTEs and voice-grade modems, and many other applications
64
*EIA-232-D
new version of RS-232-C adopted in 1987
improvements in grounding shield, test and loop-back signals
the prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace
65
*RS-449
EIA standard improving on capabilities of RS-232-C
provides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps
covers functional/procedural portions of R-232-C electrical/mechanical specs covered by
RS-422 & RS-423
66
*Functional Specifications
Specifies the role of the individual circuits
Data circuits in both directions allow full-duplex communication
Timing signals allow for synchronous transmission (although asynchronous transmission is more common)
67
*Procedural Specifications
Multiple procedures are specifiedSimple example: exchange of
asynchronous data on private line Provides means of attachment between
computer and modem Specifies method of transmitting
asynchronous data between devices Specifies method of cooperation for
exchange of data between devices
68
*Mechanical Specifications
25-pin connector with a specific arrangement of leads
DTE devices usually have male DB25 connectors while DCE devices have female
In practice, fewer than 25 wires are generally used in applications
69
DB-25 Female
DB-25 Male
*RS-232 DB-25 Connectors
70
*RS-232 DB-25 Pinouts
71
*RS-232 DB-9 Connectors
Limited RS-232
72
*RS-422 DIN-8
Found on Macs
DIN-8 Male DIN-8 Female
73
*Electrical Specifications
Specifies signaling between DTE and DCE
Uses NRZ-L encoding Voltage < -3V = binary 1 Voltage > +3V = binary 0
Rated for <20Kbps and <15M greater distances and rates are
theoretically possible, but not necessarily wise
74
*RS-232 Signals (Asynch)
Odd Parity
Even Parity
No Parity
