ECE 101 An Introduction to Information

Technology

Information Transmission

Information Path

InformationDisplay

Information Processor

& Transmitter

InformationReceiver and

Processor

Source ofInformation

DigitalSensor

TransmissionMedium

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: n

2

– Maximum signal power level produced by transmitter: s

2

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 MNoise level present in the transmission

channel dictates the maximum data rate

Decoding M-ary Signals(figure 8.2, Kuc)

Decoding M-ary Signalsin 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 + s

2/ n2) bps

s2/ n

2 is the signal to noise ratio

Channel Capacity

• C = B log2 (1 + s2/ n

2) bps s

2/ n2 is the signal to noise ratio

• Special cases n

2 0; C – ( s

2/ n2 ) » 1; C B log2 (s

2/ n2) bps

n2 » s

2 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– M=4: dibits with dibit varying by 360/4 = 90o

– M=8: tribits, with tribits varying by 360/8=45o

• 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=3108 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 Wide-Area

Network(figure8.19, 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 face-

to-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 cross-

reference 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)