Transcript pulse communication

UNIT-III
DATA AND PULSE
COMMUNICATION
History of Data Communication
1837- Telegraph was invented by Samuel
Morse
1849- First slow speed telegraph printer
1876- Telephone was invented
1949- All electronic diode based computer
1969- Internet began to evolve
Network Standards
Why Standards?
• Standards provide a fixed way for
hardware and/or software systems to
communicate.
• For example, USB enables two pieces of
equipment to interface even though they
are manufactured by different companies.
• By allowing hardware and software from
different companies to interconnect,
standards help promote competition.
Types of Standards
• There are two main types of standards:
• Formal: a standard developed by an
industry or government standardsmaking body
• De facto: standards that emerge in the
marketplace and are widely used, but
lack official backing by a standardsmaking body
The Standardization Processes Three
Steps
• Specification: developing the
nomenclature and identifying the
problems to be addressed.
• Identification of choices: identify
solutions to the problems and choose
the “optimum” solution.
• Acceptance: defining the solution,
getting it recognized by industry so that
a uniform solution is accepted.
Some Major Standards Making
Bodies
• ISO: International Organization for
Standardization (www.iso.ch)
• ITU-T: International Telecommunications Union –
Telecom Group (www.itu.int)
• ANSI: American National Standards Institute
(www.ansi.org)
• IEEE: Institute of Electrical and Electronic
Engineers (see standards.ieee.org)
• IETF: Internet Engineering Task Force
(www.ietf.org)
Digital Data Transmission
• The transmission of binary data across a link can be accomplished
either in parallel mode or serial mode
• In parallel mode, multiple bits are sent with each clock pulse
• In serial mode, one bit is sent with each clock pulse
• There are 2 subclasses of serial transmission: synchronous and
asynchronous
Parallel Transmission
• Binary data may be organized into groups of n bits each
• By grouping, we can send data n bits at a time instead of
one. This is called parallel transmission
• The advantage of parallel transmission is speed but its
advantage is cost
FIGURE 3-8: PARALLEL DATA
TRANSMISSION
Used most often for communication with
Serial Transmission
• In serial transmission, one bit follows another, so we need only one
communicating channel to transmit data between 2 communicating
devices
• The advantage of serial transmission is the reduction of the cost of
transmission over the parallel transmission
• Serial transmission occurs in one of 2 ways: asynchronous or
synchronous
FIGURE 3-9: SERIAL DATA
TRANSMISSION
Asynchronous Transmission
• In asynchronous transmission, the timing
of a signal is unimportant
• Information is received and translated by
agreed-upon patterns
• Patterns are based on grouping the bit
stream into bytes
• The sending system handles each group
independently, relaying it to the link
whenever ready, without regard to a timer
Asynchronous Transmission (cont.)
• To alert the receiver to the arrival of a new
group, an extra bit called start bit is added to the
beginning of each byte
• To let the receiver know that the byte is finished,
one or more additional bits called stop bits are
appended to the end of the byte
• This mechanism is called asynchronous
because at the byte level, sender and receiver
do not have to be synchronized
• But within each byte, the receiver must still be
synchronized with the incoming bit stream
Asynchronous Transmission (cont.)
• When the receiver detects a start bit, it sets a
timer and begins counting bits as they come in
• After n bits, the receiver looks for a stop bit and
after the stop bit is detected, it ignores any
received pulses until the next start bit
Synchronous Transmission
• In synchronous transmission, the bit
stream is combined into longer frames
which may contains multiple bytes
• Each byte is introduced onto the
transmission link without a gap between it
and the next one
• It is the responsibility of the receiver to
reconstruct the information
Synchronous Transmission (cont.)
• Without gaps and start/stop bits, timing becomes
very important therefore the accuracy of the
received information is completely dependent on
the ability of the receiver to keep an accurate
count of the bits as they come in
Simplex, Half-Duplex, and FullDuplex Transmission
• A communications channel is classified as one of three types:
(depending on the direction of transfer)
– Simplex
– Full-Duplex
– Half-Duplex
•
Simplex: a simplex mechanism can only transfer data in a single
direction
– It is analogous to broadcast radio or television
– Figure 9.8a illustrates simplex communication
•
Full-Duplex: allows transmission in two directions simultaneously
– It is analogous to a voice telephone conversation
• in which a participant can speak even if they are able to hear background
music at the other end
– Figure 9.8b illustrates the concept
DTE-DCE Interface
• There are usually four basic functional units involved in the
communication of data: a DTE and DCE on both end of transmission
• The DTE generates the data and pass them to a DCE. The DCE
converts the signal to a format appropriate to the transmission
medium
• When the signal arrives at the receiving end, this process is
reversed
Data Terminal Equipment (DTE)
• DTE includes any unit that functions either as a
source of or as a destination for binary digital
data
• It can be a terminal, microcomputer, printer, fax
machine and etc.
Fax
Data Circuit-Terminating
Equipment (DCE)
• DCE includes any functional unit that transmits
or receives data in the form of an analog or
digital signal through a network
• Commonly used DCEs include modems
Modem
DTE-DCE Interface Standard
• Many standards have been developed to define
the connection between a DTE and a DCE
• Each standard provides a model for the
mechanical, electrical, and functional
characteristics of the connection
• The most active organizations defining the
interface standard are the Electronic Industries
Association (EIA) and the International
Telecommunication Union-Telecommunication
Standards Committee (ITU-T)
DTE-DCE Interface Standard
(cont.)
• The EIA standards are called EIA-232, EIA-422,
EIA-449, and so on
• The ITU-T standards are called the V series and
the X series
EIA-232 Interface
• Originally issued in 1962 as the RS-232
standard (recommended standard)
• The most recent version, EIA-232-D,
defines not only the type of connectors to
be used but also the specific cable and
plugs and the functionality of each pin
EIA-232 Mechanical specification
• The EIA-232 defines the interfaces as a 25-wire cable with
a male and a female DB-25 pin connection attached to
either end. The length of the cable may not exceed 15
meters
• A DB-25 connector is a plug with 25 pins, each of which is
attached to a single wire with a specific function. However,
fewer are actually used in current practice
• Another implementation of EIA-232 uses a 9-wire cable
with a mail and a female DB-9 pin connector attached to
either end
• 9-pin connector is more commonly found in PCs but it
covers signals for asynchronous serial communication only
• Male connector is used on DTE and female connector is
used on DCE
Electrical Specification
• EIA-232 states that all data must be transmitted as
logical 1s and 0s (called mark and space) using NRZ-L
encoding, with 0 defined as a positive voltage and 1
defined as a negative voltage
• EIA-232 defines 2 distinct ranges, one for positive
voltages and one for negative
• To be recognized as data, the amplitude of a signal must
fall between 3 and 15 volts or between -3 and -15 volts
• EIA-232 allows for a maximum bit rate of 20 kbps,
although in practice this often is exceeded
Functional Specification
DB-25 Implementation
DB-9 Implementation
Functioning of EIA-232 in Synchronous
Full-Duplex Transmission
Flow Control
• Means to ask the transmitter to stop/resume
sending in data
• Required when:
– DTE to DCE speed > DCE to DCE speed
(e.g. terminal speed = 115.2kbps and line speed =
33.6kbps, in order to benefit from modem’s data
compression protocol)
• without flow control, the buffer within modem will overflow –
sooner or later
– the receiving end takes time to process the data and
thus cannot be always ready to receive
Modem
• The most familiar type of DCE is a modem
• Modem is derived from the words Modulator and
Demodulator
• It is an electronics device used to transmit information of
a computer to the destination through the telephone
channel
Telephone Bandwidth
• Traditional telephone line can carry frequencies between 300 Hz
and 3300 Hz (note: some say that 3400 Hz)
• All of this range is used for transmitting voice, where a great deal of
interference and distortion can be accepted without loss of
intelligibility
• However, the edges of this range cannot support the transmission of
base-band digital data
Bell Modems
• The first commercial modems were produced by
the Bell Telephone Company in the early 1970
ITU-T
Bell
Baud Rate
Bit Rate
Modulation
V.21
103
300
300
FSK
V.22
212
600
1200
4-PSK
V.23
202
1200
1200
FSK
V.26
201
1200
2400
4-PSK
V.27
208
1600
4800
8-PSK
V.29
209
2400
9600
16-QAM
DATA COMMUNICATION CODES
MORSE CODE
• First character code developed
• For transmitting data over telegraph wires
– telegrams (remember Western Union)
• Used dots (short beep) and dashes (long
beeps) instead of 1’s and 0’s
• More frequent the character, the fewer the
beeps
• Problems:
– variable “length” character representation
FIGURE 3-2: MORSE CODE
BAUDOT CODE
• One of first codes developed for machine
to machine communication
• Uses 1’s and 0’s instead of dots and
dashes
• For transmitting telex messages (punch
tape)
• Fixed character length (5-bits)
– 32 different codes
– increased capacity by using two codes for
shifting
BAUDOT CODE (cont.)
• Problems:
– required shift code to switch between
character sets
– no lower case, few special characters
– no error detection mechanism
– characters not ordered by binary value
– designed for transmitting data, not for data
processing
• International Baudot
– Added a 6th bit for parity
FIGURE 3-3: BAUDOT CODE
EBCDIC
• Extended Binary Coded Decimal Interchange
Code
• 8-bit character code developed by IBM
–
–
–
–
used for data communication, processing and storage
extended earlier proprietary 6-bit BCD code
designed for backward compatibility or marketing?
still in use today on some mainframes and legacy
systems.
• Allows for 256 different character
representations (28)
– includes upper and lower case
ASCII CODE
• American Standard Code for Information
Interchange
• 7-bit code developed by the American National
Standards Institute (ANSI)
– most popular data communication character code
today
• Allows for 128 different character
representations (27)
– includes upper and lower case
– lots of special characters (non-printable)
– generally used with an added parity bit
FIGURE 3-5: 7-BIT ASCII
CODE
Error detection
Error detection means to decide whether the
received data is correct or not without having
a copy of the original message.
Error detection uses the concept of
redundancy, which means adding extra bits
for detecting errors at the destination.
Types of Errors
Single-bit error
Burst error
Redundancy
Four types of redundancy checks are used
in data communications
Vertical Redundancy Check
VRC
Longitudinal Redundancy Check
LRC
VRC and LRC
Checksum
Cyclic Redundancy Check
CRC
Cyclic Redundancy Check
• Given a k-bit frame or message, the
transmitter generates an n-bit
sequence, known as a frame check
sequence (FCS), so that the resulting
frame, consisting of (k+n) bits, is exactly
divisible by some predetermined
number.
• The receiver then divides the incoming
frame by the same number and, if there
is no remainder, assumes that there
was no error.
Binary Division
Error Correction
It can be handled in two ways:
1) receiver can have the sender retransmit
the entire data unit.
2) The receiver can use an error-correcting
code, which automatically corrects
certain errors.
Single-bit error correction
To correct an error, the receiver reverses the
value of the altered bit. To do so, it must
know which bit is in error.
Number of redundancy bits needed
• Let data bits = m
• Redundancy bits = r
Total message sent = m+r
The value of r must satisfy the following
relation:
2r ≥ m+r+1
Error Correction
Hamming Code
Hamming Code