Types and methods of data transmission

Data sent over long distances is usually broken up into data packets.

The idea of splitting up data in this way means each packet can be sent along a different route to its destination.

This would clearly be of great benefit if a particular transmission route was out of action or very busy.

The only obvious drawback of Data transmission splitting data into packets is the need to reassemble the data when it reaches its destination.

Packet structure A typical packet is split up into:

» a packet header

» the payload

» a trailer.

For each packet, the packet header consists of:

» the IP address of the sending device

» the IP address of the receiving device

» the sequence number of the packet (this is to ensure that all the packets can be reassembled into the correct order once they reach the destination)

» packet size (this is to ensure the receiving station can check if all of the packets have arrived intact).

(Note: the header often also contains another value indicating how many packets there are in total for this transmission.) For each packet, the payload consists of the actual data being sent in the packet (this is usually about 64KiB).

For each packet, the packet trailer consists of:

» some way of identifying the end of the packet; this is essential to allow each packet to be separated from each other as they travel from sending to receiving station

» an error checking method; cyclic redundancy checks (CRCs) are used to check data packets: – this involves the sending computer adding up all the 1-bits in the payload and storing this as a hex value in the trailer before it is sent – once the packet arrives, the receiving computer recalculates the number of 1-bits in the payload – the computer then checks this value against the one sent in the trailer – if the two values match, then no transmission errors have occurred; otherwise the packet needs to be re-sent.

Packet switching Let us now consider what happens when a photograph, for example, is sent from computer ‘A’ to computer ‘B’. The photograph will be split up into a number of packets before it is sent. There will be several possible routes for the packets, between computer ‘A’ (sender) and computer ‘B’ (receiver). Each stage in the route contains a router. A router receives a data packet and, based on the information in the header, decides where to send it next. For example:

Packet switching is a method of data transmission in which a message is broken up into a number of packets. Each packet can then be sent independently from start point to end point. At the destination, the packets will need to be reassembled into their correct order (using the information sent in the header). At each stage in the transmission, there are nodes that contain a router. Each router will determine which route the packet needs to take, in order to reach its destination (the destination IP address is used in this part of the process). Suppose our photograph (Figure 2.3) has been split up into five packets that have been sent in the following order:

» each packet will follow its own path (route)

» routers will determine the route of each packet

» routing selection depends on the number of packets waiting to be processed at each node

» the shortest possible path available is always selected – this may not always be the shortest path that could be taken, since certain parts of the route may be too busy or not suitable

» unfortunately, packets can reach the destination in a different order to that in which they were sent.

Figure 2.5 shows one possible scenario. Notice the different paths taken by each packet from computer ‘A’ to computer ‘B’. Also notice that the packets have arrived in a different order compared to the way they were sent, namely:

Computer ‘B’ will now have to reassemble the packets into the original sequence.

» there is no need to tie up a single communication line

» it is possible to overcome failed, busy or faulty lines by simply re-routing packets

» it is relatively easy to expand package usage » a high data transmission rate is possible.

The drawbacks of packet switching include:

» packets can be lost and need to be re-sent

» the method is more prone to errors with real-time streaming (for example, a live sporting event being transmitted over the internet)

» there is a delay at the destination whilst the packets are being re-ordered. Sometimes it is possible for packets to get lost because they keep ‘bouncing’ around from router to router and never actually reach their destination. Eventually the network would just grind to a halt as the number of lost packets mount up, clogging up the system.

To overcome this, a method called hopping is used. A hop number is added to the header of each packet, and this number is reduced by 1 every time it leaves a router (Figure 2.6).

Each packet has a maximum hop number to start with. Once a hop number reaches zero, and the packet hasn’t reached its destination, then the packet is deleted when it reaches the next router. The missing packets will then be flagged by the receiving computer and a request to re-send these packets will be made.

2.1.2 Data transmission

Data transmission can be either over a short distance (for example, computer to printer) or over longer distances (for example, from one computer to another in a global network).

Essentially, three factors need to be considered when transmitting data:

» The direction of data transmission (for example, can data transmit in one direction only, or in both directions)

» The method of transmission (for example, how many bits can be sent at the same time)

» How will data be synchronised (that is, how to make sure the received data is in the correct order).

These factors are usually considered by a communication protocol.

Simplex data transmission Simplex mode occurs when data can be sent in ONE DIRECTION ONLY (for example, from sender to receiver).

An example of this would be sending data from a computer to a printer.

Half-duplex data transmission Half-duplex mode occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’ along the same transmission line, but they can’t both be done at the same time).

An example of this would be a walkie-talkie where a message can be sent in one direction only at a time; but messages can be both received and sent.

Full-duplex data transmission Full-duplex mode occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’ along the same transmission line simultaneously).

An example of this would be a broadband internet connection.

Serial data transmission works well over long distances. However, the data is transmitted at a slower rate than parallel data transmission. Because only one channel/wire is used, data will arrive at its destination fully synchronised (i.e. in the correct order). An example of its use is when connecting a computer to a printer via a USB connection (see Section 2.1.3).

Parallel data transmission works well over short distances. Over longer distances (for example, over 20metres), data can become skewed (that is, the data can arrive unsynchronised) and bits can arrive out of order. The longer the wire, the worse this can become. It is, however, a faster method of data transmission than serial. The internal circuits in a computer use parallel data transmission since the distance travelled between components is very short and high-speed transmission is essential.

As Figure 2.12 shows, the USB cable consists of a four-wired shielded cable, with two wires for power (red and black).

The other two wires (white and green) are for data transmission. When a device is plugged into a computer using one of the USB ports:

» the computer automatically detects that a device is present (this is due to a small change in the voltage on the data signal wires in the USB cable)

» the device is automatically recognised, and the appropriate device driver software is loaded up so that the computer and device can communicate effectively

» if a new device is detected, the computer will look for the device driver that matches the device; if this is not available, the user is prompted to download the appropriate driver software (some systems do this automatically and the user will see a notice asking for permission to connect to the device website).