1. Local Asynchronous Communications

2. Bit-wise data transmission Data transmission requires: Encoding bits as energy Transmitting energy through medium Decoding energy back into bits Energy can be electric current, radio, infrared, light Transmitter and receiver must agree on encoding scheme and transmission timing

3. Data Transfer Methods Synchronous communication Synchronous communication takes place when people are connected at the same time (real time communication) – real time Examples: phone calls, skype, text chat, face-to-face talk

4. Data Transfer Methods Isochronous communication Time dependent Refers to process where data must be delivered within certain time constraint

5. Data Transfer Methods Isochronous communication fixed amount of time to transmitslotAn isochronous message protocol assigns each data source a fixed amount of time to transmit (its "slot") within each cycle through the sources. That guarantees that each source will have regular opportunities to transmit the latest information. Examples: Real-time voice, video and telemetry

6. Data Transfer Methods Asynchronous communication Used to describe communication in which data can be transmitted at irregular intervals rather than in steady stream

7. Data Transfer Methods Asynchronous communication Asynchronous communication utilizes a transmitter, a receiver and a wire without coordination about the timing of individual bits. Asynchronous communication does not require people to be online at the same time - time delayed

8. Data Transfer Methods Asynchronous communication Examples: RS-232 based serial devices such as on an IBM- compatible computer's COM 1, 2, 3, 4 ports, Asynchronous Transfer Mode (ATM), PS2 ports and computer's keyboard and mouse. snail post, e-mails, springdoo messages, blogs, forums, podcasts, videoblogs or vlogs, discussion boards, and text messaging over cell phones.

9. Asynchronous Communication One definition of asynchronous: transmitter and receiver do not explicitly coordinate each data transmission Transmitter can wait arbitrarily long between transmissions Used, for example, when transmitter such as a keyboard may not always have data ready to send

10. Asynchronous Communication the receiver does not know when the sender will transmit transmit when data is ready variable delays between transmissions no sender-receiver coordination beforehand Technically, the electrical signal does not contain information about where individual bits begin and end

11. Using electric current to send bits Simple idea - use varying voltages to represent 1s and 0s One common encoding use: negative voltage for 1 and positive voltage for 0

12. Communication standards Standards specify operation of communication systems Devices from different vendors that adhere to the standard can interoperate Example organizations: International Telecommunications Union (ITU) Electronic Industries Association (EIA) Institute for Electrical and Electronics Engineers (IEEE)

13. The RS-232 standard Standard for transfer of characters across copper wire Produced by EIA Full name is RS-232-C RS-232 defines serial, asynchronous communication Serial - bits are encoded and transmitted one at a time (as opposed to parallel transmission) Asynchronous - characters can be sent at any time and bits are not individually synchronized

14. RS-232 wiring and connectors

15. Components of standard: Connection must be less than 50 feet Data represented by voltages between +15v and -15v 25-pin connector, with specific signals such as data, ground and control assigned to designated pins Specifies transmission of characters between, e.g., a terminal and a modem Transmitter never leaves wire at 0v; when idle, transmitter puts negative voltage (a 1) on the wire

16. RS-232 connectors and pins RS-232 uses a 25 pin connector (extra pins for control functions) Computer transmits on pin 2 and receives on 3. Opposite on a modem

17. Identifying asynchronous characters Transmitter indicates start of next character by transmitting a zero Receiver can detect transition as start of character Extra zero called the start bit

18. Identifying asynchronous characters Transmitter must leave wire idle so receiver can detect transition marking beginning of next character Transmitter sends a one after each character Extra one call the stop bit Thus, character represented by 7 data bits requires transmission of 9 bits across the wire

19. Identifying asynchronous characters

20. RS-232 terminology: MARK is a negative voltage (== 1) SPACE is a positive voltage (== 0)

21. Timing Transmitter and receiver must agree on timing of each bit Agreement accomplished by choosing transmission rate Measured in bits per second Detection of start bit indicates to receiver when subsequent bits will arrive

22. Timing Hardware can usually be configured to select matching bit rates Switch settings Software Auto detection

23. Measures of transmission rates Baud rate measures number of signal changes per second Bits per second measures number of bits transmitted per second The baud rate need not to be the same as bits rate, it depends on how many levels of signal are use In RS-232, they are the same

24. Framing Start and stop bits represent framing of each character If transmitter and receiver are using different speeds, stop bit will not be received at the expected time Problem is called a framing error RS-232 devices may send an intentional framing error called a BREAK

25. Full-duplex communication Two endpoints may send data simultaneously - full-duplex communication Requires an electrical path in each direction

26. Universal Serial Bus (USB) Faster serial data communication standard Speeds up to 480 Mbps (USB 2.0) 4-wire cable interface 2 for data, 1 for power, 1 for ground signal

27. Limitations of real hardware Hardware cannot instantly change voltage and so imperfect signals must be detectable RS-232 specifies how much tolerance there should be

28. Limitations of real hardware Longer wire, external interference may make signal look even worse RS-232 standard specifies how precise a waveform the transmitter must generate, and how tolerant the receiver must be of imprecise waveform

29. Summary Asynchronous communication - data can start at any time; individual bits not delineated RS-232 - EIA standard for asynchronous character transmission Characters per second and baud rate Bandwidth limits maximum data transmission rate

30. END OF PRESENTATION Thank you..