ECE 101 An Introduction to Information Technology Information Transmission

1. ECE 101An Introduction to Information TechnologyInformation Transmission

2. Information Path Source of Information Digital Sensor Information Display Information Receiver and Processor Information Processor & Transmitter Transmission Medium

3. 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

4. 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

5. 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

6. 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

7. 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

8. Decoding M-ary Signals(figure 8.2, Kuc)

9. Decoding M-ary Signalsin the presence of Noise(figure 8.3, Kuc)

10. 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

11. 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

12. 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

13. Asynchronous Data Transmission and Character Format (figures 8.4 and 8.5, Kuc)

14. 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

15. Infrared Remote Control Signal(figure 8.6, Kuc)

16. 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”

17. 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

18. 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

19. 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

20. 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).

21. Frequency Shift Keying (figure 8.8, Kuc)

22. 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.

23. Modem – Two Way Communication (figure 8.9, Kuc)

24. 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

25. Phase-Shift Keying (figure 8.11, Kuc)

26. 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

27. 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

28. AM and Phase-Shift Keying (figure 8.14, Kuc)

29. 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

30. 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

31. Cellular Telephone Frequency Channels (figure 8.16, Kuc) f

32. Communications(IEEE Web site)

33. 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

34. 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)

35. 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

36. 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

37. Star Architecture for LAN(figure 8.18, Kuc)

38. 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

39. Ring Architecture for LAN(figure 8.18, Kuc)

40. 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

41. 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

42. Bus Architecture for LAN(figure 8.18, Kuc)

43. A Wide-AreaNetwork(figure8.19, Kuc)

44. 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

45. 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

46. 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

47. 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)

48. 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)

49. 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

50. 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