TDC 361 Basic Communications Systems Class 3

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TDC 361Basic Communications Systems Class 3

• Greg Brewster
• DePaul University

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Todays Class Topics

• Interfacing
• RS-232
• X.21
• Asynchronous vs. Synchronous
• Multiplexing
• Frequency Division Multiplexing
• Time Division Multiplexing
• Wavelength Division Multiplexing
• Errors
• Detecting Errors
• Correcting Errors

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Data Communications and Computer Networks
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Interfacing Connecting a device such as a modem
(or DCE - data circuit-terminating equipment or
data communicating equipment) to a computer (or
DTE - data terminal equipment). The connections
between the DTE and DCE are the interchange
circuits.
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Interface Standards Many different groups
contribute to interface standards International
Telecommunications Union (ITU) Electronics
Industries Association (EIA) Institute for
Electrical and Electronics Engineers
(IEEE) International Organization for Standards
(ISO) American National Standards Institute (ANSI)
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Interface Standards All interface standards
consist of four components 1. The electrical
component 2. The mechanical component 3. The
functional component 4. The procedural component
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Interface Standards The electrical component
deals with voltages, line capacitance, and other
electrical characteristics. The mechanical
component deals with items such as the connector
or plug description. A standard connector is the
ISO 2110 connector, also known as DB-25.
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Interface Standards The functional component
describes the function of each pin or circuit
that is used in a particular interface. The
procedural component describes how the particular
circuits are used to perform an operation. For
example, the functional component may describe
two circuits, Request to Send and Clear to Send.
The procedural component describes how those two
circuits are used so that the DTE can transfer
data to the DCE.
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RS-232 and EIA-232E An older interface standard
designed to connect a device such as a modem to a
computer or terminal. Originally RS-232 but has
gone through many revisions. The electrical
component is defined by V.28, the mechanical
component is defined by ISO 2110, and the
functional and procedural components are defined
by V.24.
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X.21 Another interface standard that was designed
to replace the aging RS-232. Currently popular in
Europe and with ISDN connections. Each circuit in
the X.21 standard can contain many different
signals. Since each circuit can transmit
different signals, the combination of signals on
the four circuits is much larger than if each
circuit performed only a single function.
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Interfacing a Computer and a Peripheral Firewire
- A bus that connects peripheral devices such as
wireless modems and high speed digital video
cameras to microcomputers. Designated as IEEE
1394. Firewire supports asynchronous connections
and isochronous connections (provides a
guaranteed data transport at a pre-determined
rate).
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Interfacing a Computer and a Peripheral Universal
Serial Bus (USB) - Modern standard for
interconnecting modems and other peripheral
devices to microcomputers. Support plug and
play. USB can daisychain multiple devices. Like
Firewire, USB is a high speed connection.
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Asynchronous Connections A type of connection
defined at the data link layer. To transmit data
from sender to receiver, an asynchronous
connection creates a one-character package called
a frame. Added to the front of the frame is a
Start bit, while a Stop bit is added to the end
of the frame. An optional parity bit can be added
to the frame which can be used to detect errors.
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Synchronous Connections A second type of
connection defined at the data link layer. A
synchronous connection creates a large package
(frame) that consists of header and trailer
flags, control information, optional address
information, error detection code (checksum), and
the data. A synchronous connection is more
elaborate but transfer data in a more efficient
manner.
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Half Duplex, Full Duplex, and Simplex
Connections A half duplex connection transmits
data in both directions but in only one direction
at a time. A full duplex connection transmits
data in both directions and at the same time. A
simplex connection can transmit data in only one
direction.
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Terminal-to-Mainframe Computer Connections A
point-to-point connection is a direct, unshared
connection between a terminal and a mainframe
computer. A multipoint connection is a shared
connection between multiple terminals and a
mainframe computer. The mainframe is called the
primary, and the terminals are called the
secondaries.
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Terminal-to-Mainframe Computer Connections To
allow a terminal to transmit data to a mainframe,
the mainframe must poll the terminal. Two basic
forms of polling include roll-call polling and
hub polling. In roll-call polling, the mainframe
polls each terminal in a round-robin fashion. In
hub polling, the mainframe polls the first
terminal, and this terminal passes the poll onto
the next terminal.
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Making Computer Connections In Action The back
panel of a personal computer has many different
types of connectors, or connections RS-232
connectors USB connectors Parallel printer
connectors Serial port connectors
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Data Communications and Computer Networks A
Business Users Approach

• Chapter 5
• Multiplexing Sharing a Medium

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• Introduction
• Under the simplest conditions, a medium can carry
  only one signal at any moment in time.
• For multiple signals to share one medium, either
• the medium must somehow be divided, giving each
  signal a portion of the total bandwidth, OR
• Multiple signals must be combined into a single
  signal
• The current techniques that can accomplish this
  include frequency division multiplexing, time
  division multiplexing, and wavelength division
  multiplexing.

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Frequency Division Multiplexing (FDM) Assignment
of non-overlapping frequency ranges to each
user or signal on a medium. A multiplexor
accepts multiple analog inputs inputs and assigns
frequencies to each device. Modulation is used
to move input signals into the assigned frequency
ranges. The multiplexor is attached to a
high-speed communications line. A corresponding
demultiplexor on the other end of the line
separates the multiplexed signals. FDM is only
used with analog signals
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Frequency Division Multiplexing Analog signaling
is used to transmit the signals. Broadcast radio
and television, cable television, and the AMPS
cellular phone systems use frequency division
multiplexing. This technique is the oldest
multiplexing technique. Since it involves analog
signaling, it is more susceptible to noise.
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Time Division Multiplexing Sharing of the signal
is accomplished by dividing available
transmission time on a medium among
users. Digital signaling is used
exclusively. Time division multiplexing comes in
two basic forms Synchronous time division
multiplexing, and Statistical, or asynchronous
time division multiplexing.
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Synchronous Time Division Multiplexing The
original time division multiplexing. Assigns a
static fixed bandwidth (in bps) to each input
device. The multiplexor accepts input from
attached devices in a round-robin fashion and
transmits the data in a never ending pattern. T1
and ISDN telephone lines are common examples of
synchronous time division multiplexing.
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Synchronous Time Division Multiplexing If one
device generates data at a faster rate than other
devices, then the multiplexor must sample the
incoming data stream from that device more often
than it samples the other devices. If a device
has nothing to transmit, the multiplexor will
still insert a piece of data (typically just 0
bits) from that device into the multiplexed
stream as a placeholder.
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So that the receiver may stay synchronized with
the incoming data stream, the transmitting
multiplexor can insert 1s and 0s into the data
stream that act as synchronization bits.
Receiver looks for synch bits to make sure it
stays on track
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Digital Carrier Systems

• Carrier Systems
• Standard multiplexed digital trunks developed by
  Bell System starting in early 1960s.
• All use Synchronous Time Division Multiplexing to
  derive multiple digital voice channels (64 Kbps)
  on a high-speed digital circuit.

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T-Carrier Systems
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The T1 System

• T1 was the first T-carrier system deployed by the
  Bell System (in 1962)
• Carries 24 digital signals of 64 Kbps each.
• Total Bit Rate 1.544 Mbps
• Digital Information 1.536 Mbps
• Framing Bits 8 Kbps
• T1 multiplexor is called a channel bank.

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T1 uses TDM

• T1 uses TDM to share the circuit among 24
  channels
• T1 multiplexor allocates exactly one byte per
  frame to each time slot
• Each time slot carries exactly 88000 64 Kbps
  of bandwidth
• Bandwidth is pre-reserved even if no data is
  currently being sent

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Channel Banks

• T-1 Channel Bank converts between 24 analog voice
  channels and a single T-1 trunk.
• Digitizes 24 voices (codec function)
• Combines 24 DS0 signals into a DS1 signal
  (multiplexing function)

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T1 Details

• Bipolar Representation
• T1 uses Bipolar Coding to represent 1 and 0 bits
• 1 bit represented by alternating 3 volt, -3
  volt pulses
• 0 bits represented by no voltage
• Framed Format
• T1 transmits 8000 frames per second, 193 bits per
  frame (8000 193 1,544,000).

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Bipolar Representation
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T1 Frame Format

• Each DS0 called a time slot
• 8000 frames/sec 8 bits/slot 64 Kbps
• 24 8 1 193 bits/frame
• 8000 frames/sec 193 bits/frame 1.544 Mbps
• 8000 Framing bits sent per second

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Synchronous Optical Network(SONET)

• The SONET Hierarchy is a more modern digital
  infrastructure than T-carriers.
• Developed in 1980s by BellCore
• Also uses Synchronous Time Division Multiplexing
  to deliver channels that are multiples of 64
  Kbps.
• Designed to only be used over high-quality fiber
  optic transmission systems.

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SONET Systems
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ISDN

• The Idea
• Provides a fully-digital signal between customer
  and telephone network over current telephone
  lines.
• Uses Synchronous Time Division Multiplexing to
  provide customer with three separate digital
  channels at the same time.

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ISDN Basic Rate Interface

• 3 data channels are sent in one digital signal
  using Synchronous Time Division Multiplexing
• 2 B-channels (64-Kbps) used for
• digitized voice
• 56/64 Kbps dial-up data channels
• 1 D-channel (16-Kbps) used for
• Intelligent digital signaling messages carrying
  Caller ID, call parameters, etc.
• Packet-switched data transmission

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ISDN

• ISDN Framing
• Transmit 4000 frames per second
• Each frame contains
• 16 bits for B channel 1
• 16 bits for B channel 2
• 4 bits for D channel
• Resulting channel bandwidths
• B-channels are 4000 16 64 Kbps
• D-channel is 4000 4 16 Kbps

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The ISDN multiplexor stream is also a continuous
stream of frames. Each frame contains various
control and sync info.

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BRI Applications

• Work-at-Home Simultaneously use
• 1 B-channel for voice call
• 1 B-channel for dial-up 64 Kbps data to mainframe
• X.25 data transfer over D-channel
• Internet Access
• up to 128 Kbps using both B-channels
• Low-cost Business Videoconferencing
• Low-speed dial-up LAN Interconnection

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Statistical Time Division Multiplexing A
statistical multiplexor transmits only the data
from active workstations. If a workstation is not
active, no space is wasted on the multiplexed
stream. A statistical multiplexor accepts the
incoming data streams and creates a frame
containing only the data to be transmitted.
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To identify each piece of data, an address is
included.

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If the data is of variable size, a length is also
included.

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The transmitted frame contains a collection of
data groups.

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Packet Switching Statistical Time Division
Multiplexing

• A Packet Switch (or router) acts as a statistical
  time division multiplexor, allowing multiple
  devices to share a single physical line by
  carrying data packets for each one in turn when
  it transmits.

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Packet Switching Statistical Time Division
Multiplexing
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Dense Wavelength Division Multiplexing Dense
wavelength division multiplexing multiplexes
multiple data streams onto a single fiber optic
line. Different wavelength lasers (called
lambdas) transmit the multiple signals. Each
signal carried on the fiber can be transmitted at
a different rate from the other signals.
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Data Communications and Computer Networks A
Business Users Approach

• Chapter 6
• Errors, Error Detection, and Error Control

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Introduction Noise is always present. If a
communications line experiences too much noise,
the signal will be lost or corrupted. Communicatio
n systems should check for transmission
errors. Once an error is detected, a system may
perform some action. Some systems perform no
error control, but simply let the data in error
be discarded.
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White Noise Also known as thermal or Gaussian
noise Relatively constant and can be reduced. If
white noise gets too strong, it can completely
disrupt the signal.
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Impulse noise One of the most disruptive forms of
noise. Random spikes of power that can destroy
one or more bits of information. Difficult to
remove from an analog signal because it may be
hard to distinguish from the original
signal. Impulse noise can damage more bits if the
bits are closer together (transmitted at a faster
rate).
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Crosstalk Unwanted coupling between two different
signal paths. For example, hearing another
conversation while talking on the
telephone. Relatively constant and can be reduced
with proper measures.
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Echo The reflective feedback of a transmitted
signal as the signal moves through a medium. Most
often occurs on coaxial cable. If echo bad
enough, it could interfere with original
signal. Relatively constant, and can be
significantly reduced.
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Jitter The result of small timing irregularities
during the transmission of digital
signals. Occurs when a digital signal is repeater
over and over. If serious enough, jitter forces
systems to slow down their transmission. Steps
can be taken to reduce jitter.
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Attenuation The continuous loss of a signals
strength as it travels through a medium.
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Error Prevention To prevent errors from
happening, several techniques may be applied -
Proper shielding of cables to reduce
interference - Telephone line conditioning or
equalization - Replacing older media and
equipment with new, possibly digital components -
Proper use of digital repeaters and analog
amplifiers - Observe the stated capacities of the
media
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Error Detection Despite the best prevention
techniques, errors may still happen. To detect an
error, something extra has to be added to the
data/signal. This extra is an error detection
code. Lets examine two basic techniques for
detecting errors parity checking, and cyclic
redundancy checksum.
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Parity Checks Simple parity - If performing even
parity, add a parity bit such that an even number
of 1s are maintained. If performing odd parity,
add a parity bit such that an odd number of 1s
are maintained. For example, if the character
1001010 is to be sent, using even parity, a
parity bit 1 would be added to the
character. If the character 1001011 is to be
sent, using even parity, a parity bit 0 would
be added to the character.
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Parity Checks what about 2 errors? What happens
if the character 10010101 (parity bit is the last
bit) experiences two bit errors and the first two
0s accidentally become two 1s? Thus, the
following character is received 11110101. Will
there be a parity error? NO. Problem Simple
parity only detects even numbers of bits in error.
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Parity Checks Longitudinal parity detects more
errors by adding a row of parity bits after a
block of characters. The row of parity bits is
actually a parity bit for each column of
characters. The row parity bits plus the column
parity bits add a greater redundancy to a block
of characters.
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Parity Checks Both simple parity and longitudinal
parity do not catch all errors. Simple parity
only catches odd numbers of bit
errors. Longitudinal parity catches errors if an
odd number of bit errors occur in any row OR any
column. Longitudinal parity also requires many
check bits to be added to a block of data. We
need a better error detection method. Cyclic
Redundancy Checksum
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Cyclic Redundancy Checksum The CRC error
detection method treats the packet of data to be
transmitted as a large polynomial. Both
transmitter and receiver must know the same
generating polynomial. The transmitter takes the
message polynomial and divides it by the
generating polynomial. The quotient is discarded
but the remainder is placed in a CRC data field
in the message trailer.
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Cyclic Redundancy Checksum The message is
transmitted to the receiver. The receiver divides
the message (including CRC field at the end) by
the same generating polynomial. If the remainder
equals zero then there was no error during
transmission! If the remainder does not equal
zero, there was an error during transmission!
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CRC Advantages CRC calculation can easily be
implemented in hardware, so this operation
requires no processor time at sender or
receiver. CRC does not generate much overhead.
It adds 2 bytes or 4 bytes to a message (which
may contain thousands of bytes of data). CRC
catches the vast majority of all possible errors
that can occur on a communications line!!
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Error Control Once an error is detected, what is
the receiver going to do? 1. Do nothing 2. Return
an error message to the transmitter 3. Fix the
error with no further help from the transmitter
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Error Control Do nothing (discard the data) Seems
like a strange way to control errors but some
Data Link (layer 2) protocols, such as Frame
Relay, and some Network Layer (layer 3)
protocols, such as the Internet Protocol (IP),
perform this type of error control. The idea is
that we do not want to slow down our network by
correcting errors in the network equipment. The
error will eventually be resolved through a
timeout and retransmission request that will be
sent by higher-level software (such as Transport
Layer).
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Error Control Return a message to the
transmitter This method has three basic
formats 1. Stop-and-wait ARQ 2. Go-back-N ARQ 3.
Selective-reject ARQ ARQ stands for Automatic
Repeat Request and indicates that errors are
resolved by retransmitting the data again.
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Error Control Stop-and-wait ARQ is the simplest
of the error control protocols. A transmitter
sends a frame then stops and waits for an
acknowledgment. If a positive acknowledgment
(ACK) is received, the next frame is sent. If a
negative acknowledgment (NAK) is received, the
same frame is transmitted again.
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Error Control Under Stop-and-wait ARQ, the
transmitter also starts a timer each time a data
frame is transmitted. If the transmitter sees NO
response (ACK or NAK) after a certain timeout
period, it will assume that something went wrong
and it will retransmit the data frame.
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Error Control Go-back-N ARQ and selective reject
are more efficient protocols. They assume that
multiple frames are in transmission at one time
(sliding window protocols). A sliding window
protocol allows the transmitter to send up to the
window size frames before receiving any
acknowledgments. When a receiver does acknowledge
receipt, the returned ack contains the number of
the frame expected next.
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Error Control Using the go-back-N ARQ protocol,
if a frame arrives in error, the receiver can ask
the transmitter to go back to the Nth frame and
retransmit it. After the Nth frame is
retransmitted, the sender resends all subsequent
frames.
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Error Control Selective-reject ARQ is the most
efficient error control protocol. If a frame is
received in error, the receiver asks the
transmitter to resend ONLY the frame that was in
error. Subsequent frames following the Nth frame
are not retransmitted. Figure 6-10 shows a normal
transmission of frames with no errors, while
Figures 6-11 and 6-12 show examples of errors.
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Error Control Forward Error Correction For a
receiver to correct the error with no further
help from the transmitter requires a large amount
of redundant information accompany the original
data. This redundant information allows the
receiver to determine the error and make
corrections. This type of error control is often
called forward error correction.