Transcript Lecture 8 - Mason Gmu

ECE 101
An Introduction to Information
Technology
Information Transmission
Information Path
Source of
Information
Information
Display
Digital
Sensor
Information
Processor
& Transmitter
Transmission
Medium
Information
Receiver and
Processor
Information Transmission
• Procedures for transmitting digital
information over a communication channel
• Data sent over a channel with a limited
channel capacity but > data rate
• Data rate = amount of data that a source
produces in one second
• One and two-way data transmission
• Networks permit data transmission between
remotely located computers
– networks transmit data in data packets
Data Rate
• Source produces data that the transmitter
converts into signal or waveforms to be sent
over communications channel
– Twisted-pair (telephone), coaxial (TV), air
(acoustical) or E&M wave through space
– Binary transmission: two distinguishable
signals (by amplitude, frequency, phase)
– M-ary transmission – more than two signals to
represent data; resulting in faster data
transmission
Data Rate Measurment
• Let R = signal transmission rate (signals
produced every second)
• 1/R is the time duration of each signal
• Data Rate: D = R log2 M
Channel Noise
• Noise – commonly from thermal energy
– Atomic (charged) particles vibrating randomly
– Disturbs the data signal
– Higher temperatures cause greater thermal
motion
–  Sensitive receivers are placed in low-temp
environments
– Noise power level: n2
– Maximum signal power level produced by
transmitter: s2
Channel Transmission
• To transmit more data per second over a
channel, the transmitter could increase M,
the number of distinct signals
• Noise limits the value of M
• Noise level present in the transmission
channel dictates the maximum data rate
Decoding M-ary Signals
(figure 8.2, Kuc)
Decoding
M-ary
Signals
in the
presence
of Noise
(figure
8.3, Kuc)
Channel Capacity
• Measures the amount of data that can be
reliably transmitted over a channel
• Signal passing through a channel is always
contaminated by noise
• Channel capacity C with bandwidth B is
– C = B log2 (1 + s2/ n2) bps
– s2/ n2 is the signal to noise ratio
Channel Capacity
• C = B log2 (1 + s2/ n2) bps
– s2/ n2 is the signal to noise ratio
• Special cases
– n2  0; C  
– ( s2/ n2 ) » 1; C  B log2 (s2/ n2) bps
– n2 » s2  0; C  B log2 (1) = 0
• Long distances: attenuation occurs so s2 is
decreasing, but n2 is increasing
Asynchronous Data Transmission
• Sends data over a transmission one bit at a
time or serially
– channel and receiver are idle much of the time
waiting for data
– data are packaged in a format:
• start bit
• data - one code word at a time (byte sized are
common)
• parity bit - error detection (even or odd)
• stop bit(s) - to terminate data
• all BUT data represent over head to transmit serially
Asynchronous Data Transmission and
Character Format (figures 8.4 and 8.5, Kuc)
One-Way Data Transmission
• Typically used to control remotely a device
such as a TV, projector, VCR, garage door
• Infrared Remote (IR) Control
– Encodes the pressed button into a sequence of
IR light pulses
– The remote control generates a binary signal
that consists of a sequence of light pulses
modulated at 40 kHz for time periods of TB
Infrared Remote Control Signal
(figure 8.6, Kuc)
Infrared Remote Control
• Binary communication, M=2
• Transmits a single bit of information every
TB seconds, or R= 1/TB signals per second
• Data Rate: D =R log2 M =1/TB log2 2 =1/TB
• Number of data bits in a code word depends
upon the number of buttons on the remote
• n bits will take up to 2n buttons
• multiple transmission provides error
correction by repetition; the receiver counts
“votes”
Digital Television
• Standard TV as grid of small squares or
picture elements (pixels) arranged in 700
columns and 400 rows per frame
• assume each pixel is encoded with 8 bits
• TV transmits 30 frames per second
• Data rate D = 67.2  106 bits/second
• or D = 67.2 Mbps
MPEG
• MPEG - Motion Picture Experts Group reduce the number of bits required to
transmit video since many scenes have
static parts. So may only have 2 to 6 Mbps
• Freeze Frame video – if the data rate is
greater than the channel capacity, then each
frame waits till all data received and the
result appears as a series of still pictures
Two-way Data Transmission
With Modems
• Dialog between two systems
• Communication over the same channel
require separation between the signals to
distinguish transmitted and received signals
• Modems - transmit and receive data over
telephone channels - data to audible tones
data rates gone from 300 bps to over 50kbps
Modem Data Transmission Techniques
• Use sinusoidal signals that have features
that can be modified to represent data
– Amplitude-modulation: changes amplitude only
of a single frequency sinusoid,
– Frequency-shift keying: use different
frequencies
– Phase-shift keying methods: change phase of a
single frequency sinusoid
• Baud expresses number of signal intervals
that can be reliably transmitted over a
channel per second (same as R used earlier).
Frequency Shift Keying
(figure 8.8, Kuc)
Frequency Shift Keying
Frequency-shift keying uses different
frequencies
– 300 to 3300 Hz bandwidth of the telephone
network
– example, two different frequencies might
represent 1s & 0s
– Or, more practically, four frequencies, each one
assigned to a two-bit value – Baud rate the
same, but the data rate doubles with the two bits
per sample period.
Modem – Two Way Communication
(figure 8.9, Kuc)
Phase-Shift Keying
• Changes the phase at a constant frequency
and amplitude
• Can make M-ary transmission by having
each value have a different phase shift
relative to the immediately preceding
sinusoidal signal
o
– M=4: dibits with dibit varying by 360/4 = 90
o
– M=8: tribits, with tribits varying by 360/8=45
• Phase shift occurs every Tbaud seconds
Phase-Shift Keying (figure 8.11, Kuc)
Phase-Shift Keying
• Phase shift occurs every Tbaud seconds and if
M=4, every shift encodes 2 bits, so the data
rate is twice the baud rate.
• Modem factor: 1 bit/cycle = 1 bps/Hz
• If M=8, we transmit 3 bits every 2 cycles of
the waveform for a modem factor of 1.5
bps/Hz
Phase-Shift Keying with
Amplitude Modulation
• Can go to quadbits, shifting the amplitude
to two different levels and using phase shift
of 45o
• Now transmit 4 bits per 2 cycles of the
waveform for a modem factor equal to 2
bps/Hz
AM and Phase-Shift Keying
(figure 8.14, Kuc)
Establishing Modem Communication
• No energy for 48 Tbaud
– after answering the ring, both modems listen to
channel to determine the noise level and if little
noise use higher data rate
• Alternation between 2 known signals for
128 Tbaud to synchronize the two modems
• Pseudo-random alternations between known
signals for 384 Tbaud
– compensate for distortions in the telephone line
• Transmission of known data sequence for
48 Tbaud to verify all circuits are ok
Digital Cellular Telephone
• Uses wide frequency band width radio
channel to transmit electromagnetic signals
• Frequency band divided into channels with
each having a transmit & receive frequency
• Each user uses the first sub-baud pair as a
control channel to communicate to all users
(a code determines who can actually receive
the message)
• Voice channel is assigned to a user when a
call is made or received
Cellular Telephone Frequency
Channels (figure 8.16, Kuc)
f
Communications
(IEEE Web site)
Satellites
• Must always be visible to the antenna with
which it communicates
• Uses a geosynchronous orbit as the satellite
remains stationary at 36,000 km (22,300
miles) above a point on the earth
• Signal delay Tt = (dt + dr)/c, c=3108 m/s
• Delays can be large fraction of a second;
hence one-way communications better than
two
Data Packets
• Transmission of multiple-byte units over
networks of interconnected computers
• Five parts or fields:
– address with routing information about the
desired destination and address of the source
– data length indicating the number of bytes in
the data field (46 to 1500 bytes)
– tag - a number that indexes the data packet
(often single byte with numbers 0 to 255)
Data Packets
– data field contains the information to be transmitted for internet applications the data segment is
approximately 500 bytes - compromise, smaller needs
more packets, larger would cause delays for access to
communication links
– cyclic redundancy clock (CRC) - error detection often a one byte number simply adding up all the 1s
that are in the data and retaining the smallest 8 bits of
the sum. This is modulo-256 of the sum. Alternative
is parity bit
Data Networks
• Local Area Network (LAN)
– connects computers and peripheral devices
– can use various means or protocols to transfer
data
• Wide Area Networks (WAN)
– Connects devices wherever long-distance
communications exist
– Most common is international network known
as the Internet
Star Architecture for LAN
(figure 8.18, Kuc)
Star Architecture
• All nodes connect to hub computer called a
server
– fast since message only goes to server then its
destination
– server can store message if it is not delivered
– all communication stops if the server is “down”
– limited number of connections to server
Ring Architecture for LAN
(figure 8.18, Kuc)
Ring Architecture
• Each node connects to two neighboring
nodes and the data packets flow around the
loop in one direction.
• If the packet address corresponds to the
node address the message is read if not it is
just passed on
• Does not require a separate server but it
performs properly only when all the nodes
are operational
Bus Architecture
• Most common LAN
• all nodes (users) connect to the same bus
• Each node can transmit and each much
recognize its address to receive
• Doesn’t require a separate server
• Additional nodes easily added
• Highly reliable since it remains operational
when a node fails or is turned off
Bus Architecture for LAN
(figure 8.18, Kuc)
A WideArea
Network
(figure
8.19,
Kuc)
Data Packets
– Recall earlier we looked at the transmission of data in
“data packets”
– tag - a number that indexes the data packet (often
single byte with numbers 0 to 255)
– data field contains the information to be transmitted for internet applications the data segment is
approximately 500 bytes - compromise, smaller needs
more packets, larger would cause delays for access to
communication links
Wide-Area Network
• Consists of many switching computers or
routers between the source and destination
• Moving packets around the wide-area
network is packet switching
• The exact path of a particular packet is
random – otherwise bottlenecks
• More sophisticated networks offer the
fastest paths
• Recall that each packet has the destination
and a tag to help it arrange the packets in
order
Ethernet
• Most common communication channel for
transmitting data packets
• Standard has capacity of 10 million bps
• Fast ethernet = 100 Mbps, Gigabit ethernet
= 1 billion bps
• Special data signal using two wires to
transmit data and two wires to receive data
Ethernet
• Hence etherner uses dedicated cables to
interconnect computers directly
• Computer connects to network through a
special network interface card (NIC)
– packages the data bytes from the computer into
data packets
– at the receiving end another NIC receives the
data packets, checks for errors, and delivers the
data bytes (typically 46 to 1500 bytes)
Data Packets on Ethernet
• Preamble – 7 repetitions of 10101010 to
synchronize the receiver (7 bytes)
• Start byte with a value of 10101011 to
indicate the start of the information fields (1
byte)
• Destination Address (6 bytes)
• Source address (6 bytes)
Data Packets on Ethernet
• Tag/Length field that indicates the packet
number and length of data (2 bytes)
• Data – varies in length (46 to 1,500 byte)
• A cyclic redundancy check (CRC) for error
detection (4 bytes)
• Total overhead of 26 additional bytes
Asynchronous Transfer Mode
(ATM)
• Ethernet packets have variable length fields.
• To simplify server design, ATM is used
• ATM packets are always 53 bytes long (5
for routing and 48 for data
• All ATM packets use the same path to the
destination, so path designate by just 5
bytes to reduce the routing information
• Error checking done only at the destination
Transmission Protocols on the
Internet
• Data on the internet are transmitted as data
packets
• Methods of data transfer are protocols such
as:
• TCP/IP guarantees that the received data is
correct hence reliable
• UDP/IP transmits data quickly but does not
retransmit erroneous packets hence speed
TCP/IP
• Transmission control protocol/Internet
protocol (TCP/IP)
• Uses parity bits and check character to
ensure the integrity of the data.
• When the data packet is received correctly it
sends an acknowledgement (ACK) to the
transmitter
• If ACK is not received it sends the message
again hence the transmission rate is reduced
UDP/IP
• Universal datagram protocol/Internet
protocol or UDP/IP
• Transmits data with minimum delay
– it finds the quickest available route to send the
data and does not acknowledge receipt or
retransmit erroneous packets
• Music uses this protocol
Internet (“Introduction to Internet” - S. James)
• Who runs it?
– Backbone funded by NSF
– Internal Advisory Board - helps to set standards
• Growing exponentially
–
–
–
–
1980’s - 213 hosts on internet
1986 - 2,300 hosts
now millions
1991 - business use > academic use
Internet (“Introduction to Internet” - S. James)
• Computers available in late 1950’s
• Immediate need to communicate with one
another
• ARPA Net formed (Advanced Research
Projects Agency) in 1969
– developed Transmission Control
Protocol/Internet Protocol (TCP/IP)
Internet (“Introduction to Internet” - S. James)
• Etiquette - prescribed forms and practices of
correct behavior
• Netiquette - rules for the internet
–
–
–
–
avoid “flame” wars
update address
don’t use all caps
reply to questions
Internet (“Introduction to Internet” - S. James)
• Advantages
–
–
–
–
Access information anytime
Blind to race, religion, sex, creed
Direct cost minimal, generally your time
Communicating by writing - tends to be more
organized
– Send many messages of nearly any length
relatively quickly to many people
Internet (“Introduction to Internet” - S. James)
• Disadvantages
–
–
–
–
Credibility of information
Internet gets “crowded” - connection time slow
Addictive
People may write what they wouldn’t say faceto-face
– Mistakes get amplified
– Junk mail
Internet (“Introduction to Internet” - S. James)
• Hosts
– computers on internet that provide some service
(such as e-mail, file transfer, web site, etc.)
• Hostname
– all computers that are registered on the internet
have a unique host name and domain name:
•
•
•
•
teal.gmu.edu
teal - computer name
gmu.edu - domain name
edu - extension
Internet (“Introduction to Internet” - S. James)
• IP Address
– all computers on internet
must have an Internet
Protocol IP address
– handed out by Internet
Network Information
Center
• Unix
– popular operating system
for computers
– runs on PC’s and
mainframes
– original TCP/IP computers
ran Unix
Internet (“Introduction to Internet” - S. James)
• Internet 2
– universities & research organizations joining
together to create another internet exclusivley
for their use
• Internet Service Providers (ISP)
– computer companies that have the necessary
hardware/software to allow your computer to
dial into the ISP and in turn connect you to the
internet
• some use cable for higher speeds rather than phone
lines or use satellites
Web (“Introduction to Internet” - S. James)
• Origin goes back to need to communicate
• Hypertext Markup Language (HTML)
– text stored in electronic form with crossreference links between pages (example - our
syllabus)
– In 1993 almost 100 computers were equipped
to serve up HTML pages - those linked pages
were called the World Wide Web (WWW).
– Means for referencing text on the Internet
• Web Browsers
– view graphic images were developed like
Netscape Navigator
Data Networking Laboratory
(Room 228, S&T 2)