IGCSE Data Transmission

What is serial transmission of data?

What is serial transmission of data?

When a single wire is used to link the two devices, data is sent down the wire in the form of bits

How is a byte that represents a single character transmitted in serial data transmission?

As eight bits in sequence, one signal for each bit

What are the advantages of serial transmission of data?

Simple and reliable because the next bit is not transmitted until the current one has been received

What is the disadvantage of serial transmission?

Slow transmission because only one bit can be transmitted at a time

What type of data transmission is used by all peripheral devices that connect through a Universal Serial Bus (USB)?

Serial data transmission

What is parallel data transmission?

When more than one wire is used to connect two devices so that more bits can be sent simultaneously

What would be a sensible number of wires in parallel transmission of data and why?

Eight because then a whole byte can be sent at the same time

Why is parallel data transmission faster than serial data transmission?

More bits can travel at the same time

Why is parallel transmission less reliable than serial transmission?

The fine tolerances(公差) in the transmission means that bits can get muddled(糊涂) up

What is skew(斜)?

When the bits come out of sequence if a bit is delayed long enough to arrive in time to be counted as a bit in the next byte due to the resistance(电阻) on its wire

What are the advantages of parallel data transmission?

Faster than serial, more bits can be sent simultaneously

What are the disadvantages of parallel data transmission?

Only suitable for short distances because the bits may come out of sequence (skew) due to resistance of wires, less reliable than serial data transmission

What is simplex transmission?

Data can only travel in one direction

Why was teletext(图文) information a popular use of simplex transmission?

Information passed to TV - television receiver (no way to send data back in other direction)

What is duplex transmission?

Data can travel in both directions at the same time

Explain how a telephone conversation is an example of duplex transmission

Users can speak and be heard from both sides at the same time

What is half duplex transmission?

When data can pass in both directions along the wire but only in one direction at a time

Give an example of half duplex transmission

A radio(无线电) system in which each handset can be set to either receive mode or send mode

What is the bit rate/baud rate of data transmission?

The number of bits that can be sent in one second - i.e. the rate at which data is transferred

What is the unit of bit rate?

Baud (bits per second)

If a higher amount of data needs to be transferred, is it more desirable to have a higher or lower bit rate and why?

Higher bit rate - more bits can be sent in one second so larger data can be sent faster

What is the bandwidth of a communication medium?

The amount of data that can be sent in a given time period along this medium

How does the bandwidth effect the amount of data that can be sent along a medium in a given time?

The higher the bandwidth, the more data can be sent along the medium in a given time

Give an example of when the size of a file does not matter in data transmission

When a word document is being transmitted - the user has time to wait for it to be fully downloaded

Why would the file size matter when the data being transmitted is a video to be streamed?

The video is being watched as it is being transmitted, so the data needs to arrive quickly enough to compose and display the media to the user

What are the four main methods of error checking used in data transmission?

Echoing back, parity check, parity blocks, checksum

Describe the echoing back method of error checking

Data sent back from receiving device to the sender. If data sent back matches original data, the original data must have been transmitted correctly. If not, an error occurred and the data must be sent again.

What transmission mode does there need to be when echoing back is used as the error checking method in data transmission?

Duplex or half duplex to allow bi-directional transfer of data

What is an odd parity?

Number of 1's in the byte must be odd

What is an even parity?

Number of 1's in the byte must be even

Describe the parity check method of error checking

Before communicating, two devices decide on an odd or even parity. An extra bit is added to the byte being transmitted to ensure the byte always has an odd/even parity in accordance with the parity decided pre-transmission. If byte received has opposite parity to that decided, an error occurred and data has to be resent.

What is added to the byte pre-transmission to make sure that the parity has the decided parity before being sent in the parity check method?

An extra bit

Why may the parity block error checking method be used instead of parity check?

Used to prevent error being overlooked if two mistakes made in the same byte cancel each other out

What is added to each group of bytes being transmitted when the parity block method of error checking is used

An additional byte (parity byte) is calculated and transmitted for each group of bytes. Data byte and parity bit together are called a parity byte.

Describe the parity blocks method of error checking

Parity checked for parity in the byte and in the place value - if there is an error in both (i.e. wrong parity in both), there is an error. The intersection is the faulty bit, and is changed to a 1 if it is a 0 & vice versa. If >1 error in one block, the entire data is retransmitted.

Describe the checksum method of error checking

The data bytes are added together in a block (data transmitted in blocks of bytes) and stored in a checksum. The bytes are added again after transmission to produce a 2nd checksum. If the two checksums don't match, there was an error in data transmission and the whole block of data is resent.

Describe the process of packet switching

Data split into packets, packets sent along communication lines towards destination, each time a packet reaches a node (point of intersection) on the network, the node decides which direction to send it, packets are reordered at destination and assembled to create original data

What are the advantages of packet switching?

Optimal use of network, more secure - difficult to track down data and intercept data, loss of part of message will not be fatal as only one packet needs to be resent

What are the disadvantages of packet switching?

Data has to be reassembled at the destination - slower, only as fast as its slowest packet

What is a protocol?

A set of rules that governs the transmission of data

What is a communications protocol and why is it needed?

A protocol that governs how two devices will communicate - needed to standardise communication otherwise no messages will be able to be transferred or understood due to different frequencies, etc.

Name four parts of a protocol that would be necessary to allow communications in a network

Bit rate/baud rate, error checking method, character set used, type of data transmission used

Why would the error checking method need to be established as part of a protocol to allow communications in a network?

Two devices need to agree on method otherwise messages will never be accepted or incorrect messages may be overlooked

Why would the type of data transmission need to be established as part of a protocol to allow communications in a network?

To be able to transmit data in an agreed method e.g. serial or parallel/simplex or duplex