Local Asynchronous Communications Bitwise data transmission Data transmission

Local Asynchronous Communications

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

Data Transfer Methods • 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

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

Data Transfer Methods • Isochronous communication • An isochronous message protocol assigns each data source a fixed amount of time to transmit (its "slot") 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

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

Data Transfer Methods • 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 delayed

Data Transfer Methods • Asynchronous communication • Examples: • RS-232 based serial devices such as on an IBMcompatible computer's COM 1, 2, 3, 4 ports, Asynchronous Transfer Mode (ATM), PS 2 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.

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

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

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

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)

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

RS-232 wiring and connectors

RS-232 wiring and connectors • Components of standard: • Connection must be less than 50 feet • Data represented by voltages between +15 v and -15 v • 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 0 v; when idle, transmitter puts negative voltage (a 1) on the wire

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

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

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

Identifying asynchronous characters

Identifying asynchronous characters • RS-232 terminology: • MARK is a negative voltage (== 1) • SPACE is a positive voltage (== 0)

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

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

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

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

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

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

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

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

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

Thank you. . END OF PRESENTATION