CSCI1412 Lecture 14

1. phones off(please) CSCI1412Lecture 14 Hardware 8 Basic Communications Dr John Cowell

2. Overview • Communication issues • speed (baud, bps, cps) • direction (simplex, half-duplex and full-duplex) • data bits and parity • modes (asynchronous and synchronous) • packets and frames • bandwidth and capacity • Communication protocols • serial and parallel communication • other communication methods • Data compression techniques

3. Communication Issues

4. Speed • Maximum speed of data transmission is limited by the hardware. • the type of cable / line / media • the capabilities of the device controllers • Speed is measured in several (related) ways • bits per second (bps) • The preferred way. The number of bits of information transmitted every second. A 2400 bps second modem may have a baud rate of 1200 sending symbols of 2 bits. • baud rate • the rate at which ‘symbols’ can be sent. Each symbol may be 1 or more bits. • characters per second (cps) • the number of characters transmitted every second

5. Direction • Communication takes place in certain directions • Simplex transmission • allows data to flow in a single direction only, e.g. • sending data to a basic printer • sending display information to screens in airports / stations • Half-duplex transmission • data can flow in both directions, but not at same time • e.g. CB radio or ‘walkie-talkies’ • Full-duplex transmission • data can flow in both directions at the same time, e.g. • a (voice) telephone line • most network communications

6. Data-bits • Original ASCII only uses 7 bits to code characters • most computers store a character in one byte (8 bits) • 8th bit is always zero, so it does not have to be transmitted! • only 7 bits need to be transmitted • Original ASCII encoded only limited characters and so it has been extended, e.g. • 8th bit is used to code an extra 128 characters • quotes (‘ = 145), bullets (• = 149), pound-sign (£ = 163), etc. • in this case 8 bits need to be transmitted • International character sets (UNICODE) now use 16 bits to encode each single character

7. 1 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 ASCII ‘A’ 1 0 0 0 0 0 1 1 1 0 0 0 0 0 1 ASCII ‘C’ 1 0 0 0 0 1 1 0 1 0 0 0 0 1 1 Parity • Parity is a technique where an extra bit is included in each transmitted character to detect errors • e.g. for 7 bit ASCII an eighth parity bit is added • In even-parity, the eighth bit is set to ‘0’ or ‘1’ so that there are an even number of ‘1’s in the byte ASCII ‘A’ ASCII ‘C’ • In odd-parity, the eighth bit is set to ‘0’ or ‘1’ so that there are an odd number of ‘1’s in the byte

8. Modes • Transmitting data only works if the receiving device is ready for it • There are two aspects to being ready • available • in step • There are two communication modes used to ensure that the transmitting and receiving devices keep in step once transmission has begun • asynchronous and synchronous modes

9. Asynchronous Mode • A special start signal is transmitted at the start of each group of message bits • a group is usually just a single character • Another special stop signal is transmitted at the end of each group • When the receiving device gets the start signal, it sets up the timing mechanism to accept the group of message bits • Usually, start / stop signals are additional (fixed value) bits, e.g. start is a ‘0’ bit, stop is a ‘1’ bit’

10. Synchronous Mode • A larger group of message bits • usually many characters are transmitted together in a continuous stream • There is a single start and stop signal at the beginning and end of each message group • there are no start / stop signals for each character • the transmitting and receiving devices must synchronise their clocks at the start of transmission • these must be accurate enough to keep time with each other • Usually, error-checking bits are transmitted at the end of each message to ensure accuracy

11. Comparison asynchronous start character stop start character stop start character stop • Synchronous mode is • faster (less start / stop signals) • more complex • more expensive synchronous start message (many characters) error checking stop

12. start control information packet 1 error checking stop start control information packet 2 error checking stop Frames - 1 • Frames or packets are a further extension used in synchronous transmission • a transmitted message is divided into a series of message groups called frames (carrying packets) • frames may arrive at any time, in any order • the receiving device has to rebuild the message by • testing the received frames • requesting the re-transmission of damaged/missing frames

13. Frames - 2 • Control information includes • source address • destination address • important for routing • actual number of data bytes • sequence number • important for when frames arrive out of order • frame type • start of message • continuation of message • end of message • Not all bits transmitted are useful data • data transfer rate is less than byte transfer rate

14. Bandwidth • For analoguemedia, bandwidth is the difference between the highest and lowest frequencies at which a medium can transmit • measured in Hertz (cycles per second) • for example, telephone bandwidth is from 300 Hz to 3300 Hz = 3000 Hz • but available bandwidth on copper wire  2 MHz • ADSL technology exploits this gap

15. Capacity • For digital media, bandwidth is the rate at which data can be transmitted • usually measured in bits-per-second • it is sometimes referred to as the capacity of the link • limited by parameters of transmission medium • With all data transmission, not all data sent is useful e.g. • start/stop characters, control info., error-checking, etc. all slow down the effective data rate

16. Communication Protocols

17. 01000100 01000001 01100011 01000001 D A T A Serial Communication • In serial communication, each bit is transmitted one at a time over a single wire • Only a single wire (plus an earth) is needed for each direction of transmission, i.e. • simplex requires just one wire plus earth • full-duplex requires two wires plus earth • this makes it very cheap

18. Serial Uses • Because serial communication is so cheap, it is widely used for slow speed peripherals • modems • slow printers • mice • other input devices • barcode readers, magnetic card readers, etc. • Unfortunately, there are a wide variety of cable standards and specifications e.g. • RS232 , RS432

19. 0 0 0 0 1 1 1 1 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 1 1 D A T A Parallel Communications • In parallel communication, all eight bits in one byte (character) are send over eight different wires all at once • faster than serial, but more expensive

20. Parallel Uses • Printers • original PC parallel ports were simplex ports capable of transmitting information only • suitable for printers • modern printers are often half- or full-duplex so that they can send status information such as ‘out-of-paper’ • Add-on peripheral devices • external hard disk drives • external CD-ROM drives • tape backup devices

21. Other Protocols • Parallel and serial connectors are point to point • need one port (connector) per device • USB (Universal Serial Bus) combines serial communications with bus technology • multiple devices (up to 128) from one port • is now the ‘standard’ • Infra red technology is similar in concept • multiple ‘serial’ devices from one port • no physical connection (wires) required

22. Data Compression

23. Data Compression Techniques • The effective data rate of a communications link can usually be increased through the use of data compression (and decompression) techniques • as the communications link is the slowest link in the chain, a processor at either end of the link can afford to spend time converting data into a compressed form • Data is examined prior to transmission and then recoded so that unnecessary redundant and duplicate bits are eliminated • text can often be compressed by 75% or more • graphics (bitmaps) can often be compressed over 90%

24. Text Compression • Some compression methods are used for all types of information (but especially text) • ‘lossless’ compression / decompression • all information is preserved (perfectly) on decompression • Huffman coding • determines how often each character occurs • The more common the character, the shorter the code that replaces it. • Lempel-Ziv-Welch (LZW) • searches for repeated strings in a document • e.g. ‘communications’, ‘compression’ • replaces these with special short codes

25. Graphics Compression • Other techniques can be used for graphics files • including ‘lossy’ compression / decompression • decompressed image is an approximation of original • Run length encoding (RLE) • many graphics files have long sections of identical value bytes, e.g. 0’s for an area of black image • repeated sequences are replaced by a code for • ‘the next 2000 bytes are all zero’ • JPEG (Joint Photographics Experts Group) • a ‘lossy’ technique designed specially for photographic images, taking into account capabilities of the human eye in distinguishing adjacent colours and pixels

26. MPEG • A set of standards for video and audio compression developed by the Moving Picture Experts Group. Started in 1988. • Achieves video compression between 25:1 and 50:1 • MPEG-1 - coding video at about 1.5MBits/s. The audio layer 3 is MP3. • MPEG-2 – coding for transmission rates above 4Mbits/s. Used for DVD and digital TV and HDTV. • MPEG-3 – never released – incorporated into MPEG-2. • MPEG-4 – used for Blu-ray disk encoding.

27. MPEG Compression • Uses 5 different compression techniques: • A frequency based transform – Discrete Cosine Transform (DCT). • Quantization, lossy compression, removes detail which may not be too noticeable to the viewer. • Huffman coding (lossless compression) using code table based on encoded data. • Motion compensated predictive coding – comparing the differences between successive images. • Bi-directional prediction – images are predicted from images before and after the image.

28. MPEG encoding • There are many references on the web giving great detail.

29. Summary • Communication issues • speed (baud, bps, cps) • direction (simplex, half-duplex and full-duplex) • data bits and parity • modes (asynchronous and synchronous) • packets and frames • bandwidth and capacity • Communication protocols • serial communication • parallel communication • other communication methods • Data compression techniques