Transcript Chapter 1. Introduction to Data Communications

Digital Transmission: Advantages
• Produces fewer errors
– Easier to detect and correct errors, since transmitted data
is binary (1s and 0s, only two distinct values))
• Permits higher maximum transmission rates
– e.g., Optical fiber designed for digital transmission
• More efficient
– Possible to send more digital data through a given circuit
• More secure
– Easier to encrypt
• Simpler to integrate voice, video and data
– Easier to combine them on the same circuit, since signals
made up of digital data
Data Flow (Transmission)
data flows move in one direction only,
(radio or cable television broadcasts)
data flows both ways, but only one
direction at a time (e.g., CB radio)
(requires control info)
data flows in both directions
at the same time
Amplitude Modulation (AM)
• Changing the height of the wave to encode data
• One bit is encoded for
each carrier wave
change
– A high amplitude
means a bit value
of 1
– Low amplitude
means a bit value
of 0
• More susceptible noise than the other modulation methods
Frequency Modulation (FM)
• Changing the frequency of carrier wave to encode data
• One bit is encoded for each carrier wave change
– Changing carrier
wave to a higher
frequency
encodes a bit
value of 1
– No change in
carrier wave
frequency means
a bit value of 0
Phase Modulation (PM)
• Changing the phase of the carrier wave to encode data
• One bit is encoded for each carrier wave change
– Changing
carrier wave’s
phase by 180o
corresponds to
a bit value of 1
– No change in
carrier wave’s
phase means
a bit value of 0
Bit Rate vs. Baud Rate
• bit: a unit of information
• baud: a unit of signaling speed
• Bit rate (or data rate): b
– Number of bits transmitted per second
• Baud rate (or symbol rate): s
– number of symbols transmitted per second
• General formula:
b=sxn
Example: AM
n=1
b=s
b = Data Rate (bits/second)
s = Symbol Rate (symbols/sec.)
n = Number of bits per symbol
Example: 16-QAM
n=4
b=4xs
where
Multiplexing
• Breaking up a higher speed circuit into several
slower (logical) circuits
– Several devices can use it at the same time
– Requires two multiplexer: one to combine; one to
separate
• Main advantage: cost
– Fewer network circuits needed
• Categories of multiplexing:
– Frequency division multiplexing (FDM)
– Time division multiplexing (TDM)
– Statistical time division multiplexing (STDM)
Frequency Division Multiplexing
Makes a number of smaller channels from a larger frequency band
3000 Hz available bandwidth
Used mostly
by CATV
FDM
FDM
Host computer
• Guardbands needed
to separate channels
– To prevent interference
between channels
– Unused frequency bands
,wasted capacity
circuit
Four
terminals
Dividing the circuit “horizontally
Time Division Multiplexing
Dividing the circuit “vertically”
• Allows multiple
channels to be used by
allowing the channels
to send data by taking
turns
4 terminals sharing a circuit,
with each terminal sending
one character at a time
Statistical TDM (STDM)
• Designed to make use of the idle time slots
– (In TDM, when terminals are not using the multiplexed
circuit, timeslots for those terminals are idle.)
• Uses non-dedicated time slots
– Time slots used as needed by the different terminals
• Complexities of STDM
– Additional addressing information needed
• Since source of a data sample is not identified by the
time slot it occupies
– Potential response time delays (when all terminals try to
use the multiplexed circuit intensively)
• Requires memory to store data (in case more data come
in than its outgoing circuit capacity can handle)
Sources of Errors and Prevention
Source of Error
What causes it
How to prevent it
More important
mostly on analog
Line Outages
Faulty equipment, Storms,
Accidents (circuit fails)
White Noise (hiss)
(Gaussian Noise)
Movement of electrons (thermal
energy)
Increase signal strength
(increase SNR)
Impulse Noise
Sudden increases in electricity
(e.g., lightning, power surges)
Shield or move the wires
Cross-talk
Multiplexer guard bands are too
small or wires too close together
Increase the guard bands, or
move or shield the wires
Echo
Poor connections (causing signal to
be reflected back to the source)
Fix the connections, or
tune equipment
Attenuation
Gradual decrease in signal over
distance (weakening of a signal)
Intermodulation
Noise
Signals from several circuits
combine
Use repeaters or
amplifiers
Move or shield the wires
Jitter
Analog signals change (small
changes in amp., freq., and phase)
Tune equipment
Harmonic
Distortion
Amplifier changes phase (does not
correctly amplify its input signal)
Tune equipment
(Spikes)
Error Detection Techniques
• Parity checks
• Longitudinal Redundancy Checking (LRC)
• Polynomial checking
– Checksum
– Cyclic Redundancy Check (CRC)
Examples of Using Parity
To be sent: Letter V in 7-bit ASCII: 0110101
EVEN parity
sender
01101010
number of all
transmitted 1’s
remains EVEN
ODD parity
receiver
parity
sender
number of all transmitted
1’s remains ODD
receiver
01101011
parity
Using LRC for Error Detection
Example:
Send the message “DATA” using ODD parity and LRC
Letter
ASCII
Parity bit
D 10001001
A 10000011
T 10101000
A 10000011
BCC 1 1 0 1 1 1 1 1
Note that the BCC’s parity bit
is also determined by parity
Polynomial Checking
• Adds 1 or more characters to the end of message
(based on a mathematical algorithm)
• Two types: Checksum and CRC
• Checksum
– Calculated by adding decimal values of each character
in the message,
– Dividing the total by 255. and
– Saving the remainder (1 byte value) and using it as the
checksum
– 95% effective
• Cyclic Redundancy Check (CRC)
– Computed by calculating the remainder to a division
problem:
Discrete ARQ
Sender
Sends the packet,
then waits to hear
from receiver.
Receiver
Sends
acknowledgement
Sends the
next packet
Sends negative
acknowledgement
Resends the
packet again
Continuous ARQ
Sender sends packets
continuously without
waiting for receiver to
acknowledge
Notice that
acknowledgments now
identify the packet
being acknowledged.
Receiver sends back
a NAK for a specific
packet to be resent.
Asynchronous Transmission
Sometimes called start-stop transmission
Used by the
receiver for
separating
characters
and for
synch.
Each character is
sent independently
Sent
between
transmissi
ons (a
series of
stop bits)
Used on point-to-point full duplex circuits
(used by Telnet when you connect to Unix/Linux computers)
SDLC –
Synchronous Data Link Control
• Bit-oriented protocol developed by IBM
• Uses a controlled media access protocol
Beginning
(01111110)
Ending
(01111110)
data
Destination
Address (8
or 16 bits)
CRC-32
Identifies frame type;
• Information (for transferring of user data)
• Supervisory (for error and flow control)
Transmission Control Protocol
• Links the application layer to the network layer
• Performs packetization and reassembly
• Breaking up a large message into smaller packets
• Numbering the packets and
• Reassembling them at the destination end
• Ensures reliable delivery of packets
used in message
reassembly
TCP Header: 192 bits (24 bytes)
Internet Protocol (IP)
• Responsible for addressing and routing of
packets
• Two versions in current in use
– IPv4: a 192 bit (24 byte) header, uses 32 bit addresses.
– IPv6: Mainly developed to increase IP address space
due to the huge growth in Internet usage (128 bit
addresses)
• Both versions have a variable length data field
– Max size depends on the data link layer protocol.
– e.g., Ethernet’s max message size is 1,492 bytes, so max
size of TCP message field:
1492 – 24 – 24 = 1444 bytes
TCP header
IPv4 header
IP Packet Formats
IPv4 Header: 192 bits (24 bytes)
IPv6 Header: 320 bits (40 bytes)