1.2.1 Data transmission

• show understanding of what is meant by transmission of data • distinguish between serial and parallel data transmission • distinguish between simplex, duplex and half-duplex data transmission • show understanding of the reasons for choosing serial or parallel data transmission • show understanding of the need to check for errors • explain how parity bits are used for error detection • show understanding of the use of serial and parallel data transmission, in Universal Serial Bus (USB) and Integrated Circuit (IC)

Show understanding of what is meant by transmission of data THE PHYSICAL TRANSFER OF DATA OVER A COMMUNICATIONS CHANNEL. Data in digital computers consist of numbers. To transmit numbers across a channel the numbers have to be converted to a physical form such as electrical currents or pulses of light. Numbers in computers are in binary (base 2) so only two physical states are required for transmission e.g. 0v (off) and 5v (On). Communication may be in guided (e.g. along a wire) or unguided (e.g. broadcast to  ir) channels: in both cases the data are encoded as electromagnetic signals. Signals decrease in strength with distance and need repeating and error checking mechanisms to ensure recovery of the data.

Distinguish between serial and parallel data transmission Serial data transmission

Serial transmission This is where data bits move along a single wire, one behind the other. The pattern of 1s and 0s may be represented by 5v (on) and 0v (off) respectively, giving a square wave. Two-way communication requires two lines. Serial communication devices use additional lines for acknowledgement and synchronisation. Long distance communication uses serial transmission because it is easier to regenerate the signal on a single line (by repeaters). It is also easier to route single lines through switches and cheaper than using multiple cables.

Distinguish between serial and parallel data transmission So what do you think the difference is? Kinetic exercise to show difference?

Distinguish between simplex, duplex and half-duplex data transmission. SIMPLEX DATA TRANSMISSION CAN ONLY TRANSFER DATA IN ONE DIRECTION ONLY (BROADCAST RADIO). HALF-DUPLEX DATA TRANSMISSION CAN TRANSFER DATA IN TWO DIRECTIONS BUT NOT AT THE SAME TIME (WALKIE-TALKIE). FULL-DUPLEX DATA TRANSMISSION CAN TRANSFER DATA IN TWO DIRECTIONS AT THE SAME TIME (PHONE). 

Distinguish between simplex, duplex and half-duplex data transmission.

Show understanding of the reasons for choosing serial or parallel data transmission

Show understanding of the use of serial and parallel data transmission, in Universal Serial Bus (USB) and Integrated Circuit (IC) Universal Serial Bus (USB) is an industry standard developed in the mid- 1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices. USB was designed to standardize the connection of computer peripherals to personal computers. They communicate data and supply power and have replaced other interfaces

What is a bus A bus is a subsystem that is used to connect computer components and transfer data between them. For example, an internal bus connects computer internals to the motherboard. A bus may be parallel or serial. Parallel buses transmit data across multiple wires.

Parallel Transmission in Integrated Circuits Communications between devices such as computers and printers and across networks is performed adequately with serial lines. Data transmission over much shorter distances inside integrated circuits (ICs) is best done using parallel channels. ICs usually employ three buses for communication: Data bus: carries data between memory and processor and also between ports and other parts e.g. sound card, graphics card, have own processors and memory and some data are moved to processor and main memory; bi-directional (duplex), data flow to processor and back to memory. Address bus: carries address from processor to location in main memory (some ports may be memory- mapped so have addresses too); unidirectional, no need for an address to flow back to processor. Control bus: carries signals from processor to parts of CPU and system to activate something e.g. to read or write data to a memory location or I/O device.

Show understanding of the need to check for errors Error Checking in Data Communications Errors occur in data as it moves around a computer and across networks. The most extreme distances that data travel is across space e.g. from Voyager. Bits 'flip' are when 1s change to 0 and 0s to 1. In most circumstances this is potentially serious e.g. in financial data. There are various strategies to overcome this, including: repetition, parity and checksums. Repetition Data bits are repeated n times e.g. 1 is sent as 111 (3 times repetition) for bit repetition, or the whole sequence is sent again (block repetition)

Repetition: Block and bit.

Explain how parity bits are used for error detection – We have looked at this previously A parity is reserved and set to 0 or 1 according to the number of 1s in a data word. In even parity an even number of 1s will set the parity bit to 0 (so there is an even number of 1s) while an odd number of 1s will set the parity bit to 1 (so there is an even number of 1s). In odd parity an even number of 1s will set the parity bit to 1 (to maintain the odd number of 1s) while an odd number of 1s will set the parity bit to 0 (to maintain the odd number of 1s). Checksum The numeric value of data words is added before transmission and the checksum sent with the data. The same calculation is performed after transmission to see if the checksums match. If the checksums do not match an error occurred and the data are transmitted again. Check Digit e.g. ISBN 10  ISBN 13

Question (a) State what is meant by the terms: Parallel data Serial data transmission Serial data transmission [2] (b) Give one benefit of each type of data transmission. Parallel data transmission: Benefit Serial data transmission: Benefit (c) Give one application of each type of data transmission. Each application must be different. Parallel data transmission: Application Serial data transmission: Application

Answer 1 (a) parallel – 8 bits/1 byte/multiple bits sent at a time – using many/multiple/8 wires/lines (1 mark) serial – one bit sent at a time – over a single wire (1 mark) [2] (b) parallel – faster rate of data transmission (1 mark) – more accurate/fewer errors over a longer distance – less expensive wiring – less chance of data being skewed/out of synchronisation/order (1 mark) [2] (c) parallel – sending data from a computer to a printer – internal data transfer (buses) (1 mark) – connect computer to a modem

Question 2 (a) State what is meant by the term USB. ................................................................................................................................................... [1] (b) Describe two benefits of using USB connections between a computer and a device. 1 ................................................................................................................................................ 2 ................................................................................................................................................ [2]

Answer 2 (a) – universal serial bus – description of USB [1] (b) Any two from: – devices are automatically detected and configured when initially attached – impossible to connect device incorrectly/connector only fits one way – has become the industry standard – supports multiple data transmission speeds – lots of support base for USB software developers – supported by many operating systems – backward compatible – faster transmission compared to wireless