Data Transmission PDF

2 Data transmission In this chapter you will learn about: ★ types and methods of data transmission – how data is broken up into data...

2 Data transmission In this chapter you will learn about: ★ types and methods of data transmission – how data is broken up into data packets before transmission – the structure of data packets (header, payload and trailer) – packet switching (including the role of the router in the process) – methods of data transmission (serial, parallel, simplex, half-duplex and full-duplex) – the Universal Serial Bus (USB) ★ methods of error detection – why error checking methods are needed – error checking methods following data transmission: – parity checks – checksum – echo check – use of check digits to detect data entry errors – use of automatic repeat requests (ARQs) to detect errors ★ encryption – the need for and the purpose of encryption – symmetric and asymmetric encryption – use of public and private keys. Data is frequently transferred from one device to another. The two devices could be in the same building or thousands of kilometres away. Irrespective of the distance travelled, the transmission of data needs to be considered with respect to: l how the data is transmitted l how can errors following transmission be detected and can the data be recovered l the role of encryption to make sure data that falls into the wrong hands can’t be used It is also important to consider ways of checking for errors in data once it has been entered into a computer. 2.1 Types and methods of data transmission Data packets are 2.1.1 Data packets usually referred to simply as ‘packets’ Data sent over long distances is usually broken up into data packets (sometimes called datagrams). The packets of data are usually quite small, typically 64 KiB, which are much easier to control than a long continuous stream of data. 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 45 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 45 18/02/21 3:48 PM  2 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. Header Payload Trailer Sequence Some form of The actual data Method of IP address of IP address of number of Size of the error checking in the packets identifying the the source the destination the packet packet (in to ensure (~64 KiB) – the end of the device (sender) device (receiver) (allowing correct bytes) packet arrives payload packet reassembly) error-free ▲ Figure 2.1 Packet structure 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 64 KiB). 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. 46 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 46 18/02/21 3:48 PM  2.1 Types and methods of data transmission 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: Computer ‘B’ Note: R1 to R10 represent the Computer ‘A’ routers (nodes) in the network R1 between ‘A’ and ‘B’ R3 R2 R4 R5 Router ‘B’ Router ‘A’ R7 R6 R10 R8 R9 ▲ Figure 2.2 Typical network showing possible routes between ‘A’ and ‘B’ 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: ▲ Figure 2.3 » 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: ▲ Figure 2.4 47 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 47 25/02/21 11:16 AM  2 Data transmission Computer ‘B’ will now have to reassemble the packets into the original sequence. Computer ‘B’ Computer ‘A’ R1 R3 R2 R4 R5 Router ‘B’ Router ‘A’ R7 R6 R10 R8 R9 ▲ Figure 2.5 Typical network showing possible paths taken by each packet The benefits of packet switching are: » there is no need to tie up a single communication line Find out more » it is possible to overcome failed, busy or faulty lines by simply re-routing packets Another method of sending packets is » it is relatively easy to expand package usage called circuit switching. » a high data transmission rate is possible. Find out how this differs to packet The drawbacks of packet switching include: switching, and then re-draw Figure 2.5 » packets can be lost and need to be re-sent showing the route the » the method is more prone to errors with real-time streaming (for example, a packets take when live sporting event being transmitted over the internet) using circuit switching. » 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). Router 3 Router 5 Router 8 Router 11 Router 15 Router 21 Hop number: 10 9 8 7 6 5 ▲ Figure 2.6 Hop numbers between routers Advice Hopping is not included Each packet has a maximum hop number to start with. Once a hop number on the syllabus but reaches zero, and the packet hasn’t reached its destination, then the packet is is included here for deleted when it reaches the next router. The missing packets will then be flagged completeness (to by the receiving computer and a request to re-send these packets will be made. help understand how packets can get lost). 48 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 48 25/02/21 11:18 AM  2.1 Types and methods of data transmission Activity 2.1 1 Suppose a video conference is taking place between delegates in two different countries. Packet switching is being used to send video and sound data between the delegates: Delegate 1 in USA Delegate 2 in India Video and sound sent between delegates using packet switching ▲ Figure 2.7 Describe: i any potential problems with sound and video quality ii how these problems could be caused. 2 Explain how packet switching could be used to download a large web page from a website. 3 a The trailer in a packet will use one form of error checking. Explain what is meant by a cyclic redundancy check. b The payload contains the following data: 11110000 10000011 00110011 00111111 11111110 11100011 Use this data to show how the receiving computer can verify that the received payload was error-free. 4 a Explain how it is possible for packets to be lost during their transmission across a network. b Describe how it is possible for a system to deal with lost packets and prevent them from slowing down the transmission process. c Explain why you think packet switching might improve data security. 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. 49 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 49 18/02/21 3:48 PM  2 Data transmission Simplex, half-duplex and full-duplex Transmission mode Simplex mode Half-duplex mode Full-duplex mode ▲ Figure 2.8 Transmission modes 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 and parallel data transmission Types of data transmission Serial data Parallel data transmission transmission ▲ Figure 2.9 Types of data transmission Serial data transmission occurs when data is sent ONE BIT AT A TIME over a SINGLE WIRE/CHANNEL. Bits are sent one after the other as a single stream. 10011011 Sender Receiver ▲ Figure 2.10 Serial data transmission (Note: Serial data transmission can be simplex, half-duplex or full-duplex.) 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). 50 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 50 18/02/21 3:48 PM  2.1 Types and methods of data transmission Parallel data transmission occurs when SEVERAL BITS OF DATA (usually one byte) are sent down SEVERAL CHANNELS/WIRES all at the same time. Each channel/wire transmits one bit: 1 0 0 1 Sender 1 Receiver 0 1 1 ▲ Figure 2.11 Parallel data transmission (Note: Parallel data transmission can be simplex, half-duplex or full-duplex.) Parallel data transmission works well over short distances. Over longer distances Link (for example, over 20 metres), data can become skewed (that is, the data can arrive unsynchronised) and bits can arrive out of order. The longer the wire, the For more on data transmission within worse this can become. It is, however, a faster method of data transmission than the CPU refer to serial. The internal circuits in a computer use parallel data transmission since the Chapter 3. distance travelled between components is very short and high-speed transmission is essential. Activity 2.2 1 Explain what is meant by: i serial, half-duplex data transmission ii parallel, full-duplex data transmission iii serial, simplex data transmission. 2 Which types of data transmission are being described: i data is sent one bit at a time in one direction only ii data is being sent 8 bits at a time in one direction only iii data is being sent 16 bits at a time in both directions simultaneously iv data is sent one bit at a time in both directions simultaneously v data is sent 16 bits at a time in one direction only? 51 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 51 18/02/21 3:48 PM  2 Data transmission Table 2.1 shows the comparison between serial and parallel data transmission. ▼ Table 2.1 Comparison of serial and parallel data transmission methods Serial Parallel less risk of external interference than with parallel (due to faster rate of data transmission than serial fewer wires) more reliable transmission over longer distances works well over shorter distances (for example, used in internal pathways on computer circuit boards) transmitted bits won’t have the risk of being skewed (that is, since several channels/wires used to transmit data, the bits out of synchronisation) can arrive out of synchronisation (skewed) used if the amount of data being sent is relatively small since preferred method when speed is important transmission rate is slower than parallel (for example, USB uses this method of data transmission) used to send data over long distances (for example, if data is time-sensitive, parallel is the most appropriate telephone lines) transmission method less expensive than parallel due to fewer hardware parallel ports require more hardware, making them more requirements expensive to implement than serial ports easier to program input/output operations when parallel used 2.1.3 Universal serial bus (USB) Link As the name suggests, the universal serial bus (USB) is a form of serial data transmission. USB is now the most common type of input/output port found on Also refer to Section 4.1 on computers and has led to a standardisation method for the transfer of data software drivers between devices and a computer. It is important to note that USB allows both regarding devices half-duplex and full-duplex data transmission. plugged into USB Red (+ 5 V) ports. White (Data +) Green (Data −) Shield – no connection at USB device Black (Ground) ▲ Figure 2.12 Typical USB cable 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). 52 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 52 25/02/21 11:27 AM  2.1 Types and methods of data transmission We will now consider the benefits and drawbacks of using the USB system: ▼ Table 2.2 Benefits and drawbacks of USB systems Benefits Drawbacks devices plugged into the computer are automatically detected and device standard USB only supports a maximum drivers are automatically loaded up cable length of 5 m; beyond that, USB hubs connections can only fit one way preventing incorrect connections being made are needed to extend the cable length it has become an industry standard, which means considerable support is even though USB is backward compatible, available very early USB standards (V1) may not always be supported by the latest can support different data transmission rates (from 1.5 Mbps to 5 Gbps) computers no need for external power source since cable supplies +5 V power even the latest version 3 (V3) and version USB protocol notifies the transmitter to re- transmit data if any errors are 4 (V4) USB-C systems have a data transfer detected; this leads to error-free data transmission rate which is slow compared to, for example, Ethernet connections (Note: USB it is relatively easy to add more USB ports if necessary, by using USB hubs V2 has a maximum data transfer rate of USB is backward compatible (that is, older versions are still supported) 480 Mbps.) A new type of USB connector, referred to as USB-C, is now becoming more common in laptops and tablets/phones. This is a 24-pin symmetrical connector which means it will fit into a USB-C port either way round. It is much smaller and thinner than older USB connectors, offers 100 watt (20 volt) power connectivity, which means full-sized devices can now be charged and it can carry data at 10 gigabits per second (10 Gbps); this means it can now support 4K video delivery. USB-C is backward compatible (to USB 2.0 and 3.0) provided a suitable adaptor is used, and is expected to become the new industry standard (universal) format. Activity 2.3 Ten statements are shown on the left in the table. Each of these statements is either True or False. For each statement, tick (✓) the appropriate column to indicate which statements are true and which are false. Statement True False Packets have a header which contains the IP address of the sender and the receiver Packets don’t require any form of error checking USBs use a protocol that allows for error-free data transmission between device and computer Serial data transmission suffers from data skewing The longest cable length supported by USB is 5 metres or less Simplex data transmission occurs when data is transmitted one bit at a time Full-duplex data transmission involves sending 8 bits of data at a time USB uses serial data transfer Packet switching prevents loss of any data packets USB connections can transfer data using half-duplex or full-duplex 53 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 53 18/02/21 3:48 PM  2 Data transmission 2.2 Methods of error detection 2.2.1 The need to check for errors When data is transmitted, there is always a risk that it may be corrupted, lost or even gained. Errors can occur during data transmission due to: » interference (all types of cable can suffer from electrical interference, which can cause data to be corrupted or even lost) » problems during packet switching (this can lead to data loss – or it is even possible to gain data!) » skewing of data (this occurs during parallel data transmission and can cause data corruption if the bits arrive out of synchronisation). Checking for errors is important since computers are unable to understand text, for example, if the words are not recognised by its built-in dictionary. Look at the following example of some corrupted text: Can you raed tihs? “I cnduo’t bvleiee taht I culod aulaclty uesdtannrd waht I was rdnaieg. Unisg the icndeblire pweor of the hmuan mnid, aocdcrnig to rseecrah at Cmabridge Uinervtisy, it dseno’t mttaer in waht oderr the lterets in a wrod are, the olny irpoamtnt tihng is taht the frsit and lsat ltteer be in the rhgit pclae. The rset can be a taotl mses and you can sitll raed it whoutit a pboerlm. Tihs is bucseae the huamn mnid deos not raed ervey ltteer by istlef, but the wrod as a wlohe. Aaznmig, huh? Yeah and I awlyas tghhuot slelinpg was ipmorantt! See if yuor fdreins can raed tihs too” (from an unknown source at Cambridge University) ▲ Figure 2.13 Example of data corruption on a message Whilst you probably had little problem understanding this text, a computer would be unable to make any sense of it. Data corruption is therefore a very real problem to a computer. Figure 2.13 could be the result of some data corruption following transmission which would make the text unintelligible to a computer. This is why error checking is such an important part of computer technology. The following section considers a number of ways that can be used to check for errors, so that you don’t end up with text as shown in Figure 2.13 above! There are a number of ways data can be checked for errors following transmission: » parity checks » checksum » echo check. 54 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 54 18/02/21 3:48 PM  2.2 Methods of error detection 2.2.2 Parity checks, checksum and echo checks Parity checks Parity checking is one method used to check whether data has been changed or corrupted following data transmission. This method is based on the number of 1-bits in a byte of data. The parity can be either called EVEN (that is, an even number of 1-bits in the byte) or ODD (that is, an odd number of 1-bits in the byte). One of the bits in the byte (usually the most significant bit or left-most bit) is reserved for a parity bit. The parity bit is set according to whether the parity being used is even or odd. For example, consider the byte: 1 1 0 1 1 0 0 parity bit Advice In this example, if the byte is using even parity, then the parity bit needs to be Note for the 7 bit set to 0, since there is already an even number of 1-bits in the byte (four 1-bits). number 1110000, the We thus get: even parity bit would be 1 and the odd parity 0 1 1 0 1 1 0 0 bit would be 0. The parity bit can be set parity bit as a 1 or a 0 for either choice of parity – it just In this example, if the byte is using odd parity, then the parity bit needs to be depends on how many set to 1, since we need to have an odd number of 1-bits in the byte. We thus get: 1s are in the byte. 1 1 1 0 1 1 0 0 parity bit Before data is transferred, an agreement is made between sender and receiver regarding which type of parity is being used. Parity checks are therefore being used as a type of transmission protocol. Activity 2.4 Find the parity bits for each of the following bytes: 1 1 0 1 1 0 1 even parity being used 0 0 0 1 1 1 1 even parity being used 0 1 1 1 0 0 0 even parity being used 1 1 1 0 1 0 0 odd parity being used 1 0 1 1 0 1 1 odd parity being used 1 1 1 1 1 1 0 even parity being used 1 1 1 1 1 1 0 odd parity being used 1 1 0 1 0 0 0 odd parity being used 0 0 0 0 1 1 1 even parity being used 1 1 1 1 1 1 1 odd parity being used 55 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 55 18/02/21 3:48 PM  2 Data transmission If a byte has been transmitted from ‘A’ to ‘B’, and if even parity is used, an error would be flagged if the byte now had an odd number of 1-bits at the receiver’s end. For example (assuming even parity is being used): byte being sent: 0 1 0 1 1 1 0 0 parity bit byte being received: 0 1 0 0 1 1 0 0 parity bit In this case, the byte received has three 1-bits, which means it now has odd parity; while the sender’s byte was using even parity (four 1-bits). This means an error has occurred during the transmission of the byte. The error is detected by the recipient’s computer re-calculating the parity of the byte sent. If even parity had been agreed between sender and receiver, then a change in parity in the received byte indicates that a transmission error has occurred. Activity 2.5 1 Which of the following received bytes indicate an error has occurred following data transmission? 1 1 1 0 1 1 0 1 even parity being used 0 1 0 0 1 1 1 1 even parity being used 0 0 1 1 1 0 0 0 even parity being used 1 1 1 1 0 1 0 0 odd parity being used 1 1 0 1 1 0 1 1 odd parity being used 1 1 1 1 1 1 1 1 odd parity being used 0 0 0 0 0 0 0 0 even parity being used 1 1 1 0 0 0 0 0 odd parity being used 0 1 0 1 0 1 0 1 even parity being used 1 1 1 0 0 0 1 1 odd parity being used 2 In each case, in question 1, where an error occurred, can you work out which bit in the byte was changed during data transmission? If two of the bits change value following data transmission, it may be impossible to locate the error using parity checking. Let us imagine we are transmitting the following byte, using even parity: 0 1 0 1 1 1 0 0 Suppose more than one bit has been modified during data transmission. This means the byte could have reached the destination as any of the following: 0 1 1 1 1 1 0 1 six 1-bits 56 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 56 18/02/21 3:48 PM  2.2 Methods of error detection 0 1 0 1 0 0 0 0 two 1-bits 0 1 0 1 0 1 1 0 four 1-bits In all these cases, the byte has clearly been corrupted, but the bytes have retained even parity. Therefore, no error would be flagged in spite of the obvious errors in transmission. Clearly it will be necessary to have other ways to complement parity when it comes to error checking to ensure errors are never missed. One such method is called checksum – see the next section. You should have concluded that any of the bits in question 2 (Activity 2.5) could have been changed where there was a transmission error. Therefore, even though an error has been flagged, it is impossible to know exactly which bit is in error. One of the ways round this problem is to use parity blocks. In this method, a block of data is sent and the number of 1-bits are totalled horizontally and vertically (in other words, a parity check is done in both horizontal and vertical directions). As the following example shows, this method not only identifies that an error has occurred but also indicates where the error is. Example 1 In this example, nine bytes of data have been transmitted. Agreement has been made that even parity will be used. Another byte, known as the parity byte, has also been sent. This byte consists entirely of the parity bits produced by the vertical parity check. The parity byte also indicates the end of the block of data. Table 2.3 shows how the data arrived at the receiving end. It is now necessary to check the parity of each byte horizontally (bytes 1 to 9) and vertically (columns 1 to 8). Each row and column where the parity has changed from even to odd should be flagged: ▼ Table 2.3 Parity block showing nine bytes and parity byte Parity Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 bit Byte 1 1 1 1 1 0 1 1 0 Byte 2 1 0 0 1 0 1 0 1 Byte 3 0 1 1 1 1 1 1 0 Byte 4 1 0 0 0 0 0 1 0 Byte 5 0 1 1 0 1 0 0 1 Byte 6 1 0 0 0 1 0 0 0 Byte 7 1 0 1 0 1 1 1 1 Byte 8 0 0 0 1 1 0 1 0 Byte 9 0 0 0 1 0 0 1 0 Parity 1 1 0 1 0 0 0 1 byte A careful study of Table 2.3 shows the following: » byte 8 (row 8) now has incorrect parity (there are three 1-bits) » bit 5 (column 5) also now has incorrect parity (there are five 1-bits). First of all, the table shows that an error has occurred following data transmission (there has been a change in parity in one of the bytes). 57 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 57 18/02/21 3:48 PM  2 Data transmission Secondly, at the intersection of row 8 and column 5, the position of the incorrect bit value (which caused the error) can be found. The 1-bit at this intersection should be a 0-bit; this means that byte 8 should have been: 0 0 0 1 0 0 1 0 which would also correct column 5 giving an even vertical parity (now has four 1-bits). This byte could therefore be corrected automatically as shown above, or an error message could be relayed back to the sender asking them to re-transmit the block of data. Activity 2.6 1 The following block of data was received after transmission from a remote computer; odd parity was being used by both sender and receiver. One of the bits has been changed during the transmission stage. Locate where this error is and suggest a corrected byte value: Parity bit Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Byte 1 0 1 1 0 0 0 1 0 Byte 2 1 0 1 1 1 1 1 1 Byte 3 1 0 0 1 1 0 0 0 Byte 4 0 1 1 0 1 0 1 0 Byte 5 1 1 1 0 0 1 1 0 Byte 6 1 0 0 0 0 1 0 1 Byte 7 0 1 1 1 0 0 0 0 Byte 8 0 0 0 0 0 0 0 1 Byte 9 0 1 1 1 1 0 1 0 Parity 1 0 1 1 1 1 0 0 byte 2 The following block of data was received after transmission from a remote computer. Even parity was being used by both sender and receiver. One of the bytes has been changed during the transmission stage. Locate where this error is and suggest a corrected byte value. Parity bit Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7 Bit 8 Byte 1 1 1 0 0 0 0 0 0 Byte 2 0 0 1 1 1 1 0 0 Byte 3 0 1 0 0 0 1 1 1 Byte 4 1 0 1 0 1 1 1 1 Byte 5 0 0 0 1 0 0 0 1 Byte 6 0 0 1 1 1 1 1 1 Byte 7 1 0 1 1 0 1 0 0 Byte 8 0 1 0 1 0 0 0 1 Byte 9 1 1 1 0 0 1 0 0 Parity 0 0 1 1 1 0 0 1 byte 58 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 58 18/02/21 3:48 PM  2.2 Methods of error detection Checksum A checksum is a method used to check if data has been changed or corrupted following data transmission. Data is sent in blocks, and an additional value, called the checksum, is sent at the end of the block of data. The checksum process is as follows: » when a block of data is about to be transmitted, the checksum is calculated from the block of data » the calculation is done using an agreed algorithm (this algorithm has been agreed by sender and receiver) » the checksum is then transmitted with the block of data » at the receiving end, the checksum is recalculated by the computer using the block of data (the agreed algorithm is used to find the checksum) » the re-calculated checksum is then compared to the checksum sent with the data block » if the two checksums are the same, then no transmission errors have occurred; otherwise a request is made to re-send the block of data. Echo check With echo check, when data is sent to another device, this data is sent back again to the sender. The sender’s computer compares the two sets of data to check if any errors occurred during the transmission process. As you will have no doubt worked out, this isn’t very reliable. If the two sets of data are different, it isn’t known whether the error occurred when sending the data in the first place, or if the error occurred when sending the data back for checking. However, if no errors occurred, then it is another way to check that the data was transmitted correctly. In summary: » a copy of the data is sent back to the sender » the returned data is compared with the original data by the sender’s computer » if there are no differences, then the data was sent without error » if the two sets of data are different, then an error occurred at some stage during the data transmission. Data is sent Sender Data returned to sender Receiver Data is re-sent if any errors found ▲ Figure 2.14 Echo check diagram 2.2.3 Check digits A check digit is the final digit included in a code; it is calculated from all the other digits in the code. Check digits are used for barcodes on products, such as International Standard Book Numbers (ISBN) and Vehicle Identification Numbers 59 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 59 18/02/21 3:48 PM  2 Data transmission (VIN). Check digits are used to identify errors in data entry caused by mis-typing or mis-scanning a barcode. They can usually detect the following types of error: » an incorrect digit entered, for example 5327 entered instead of 5307 » transposition errors where two numbers have changed order, for example 5037 instead of 5307 » omitted or extra digits, for example 537 instead of 5307 or 53107 instead of 5307 » phonetic errors, for example 13 (thirteen), instead of 30 (thirty). I S B N 97 8 -0 -34 0- 98 3 8 2 -9 9 780340 983829 ▲ Figure 2.15 Sample barcode (ISBN 13 code with check digit) There are a number of different methods used to generate a check digit. Two common methods will be considered here: » ISBN 13 » Modulo-11 Example 1: ISBN 13 The check digit in ISBN 13 is the thirteenth digit in the number. We will now consider two different calculations. The first calculation is the generation of the check digit. The second calculation is a verification of the check digit (that is, a recalculation). Calculation 1 – Generation of the check digit from the other 12 digits in a number The following algorithm generates the check digit from the 12 other digits: 1 add all the odd numbered digits together 2 add all the even numbered digits together and multiply the result by 3 3 add the results from 1 and 2 together and divide by 10 4 take the remainder, if it is zero then use this value, otherwise subtract the remainder from 10 to find the check digit. Using the ISBN 9 7 8 0 3 4 0 9 8 3 8 2 (note this is the same ISBN as in Figure 2.15): Odd digits 9 7 8 0 3 4 0 9 8 3 8 2 Even digits ▲ Figure 2.16 ISBN (no check digit) 1 9 + 8 + 3 + 0 + 8 + 8 = 36 2 3 × (7 + 0 + 4 + 9 + 3 + 2) = 75 3 (36 + 75)/10 = 111/10 = 11 remainder 1 4 10 – 1 = 9 the check digit 60 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 60 18/02/21 3:48 PM  2.2 Methods of error detection So we end up with the following thirteen-digit number (which matches the number shown in Figure 2.15): Odd digits 9 7 8 0 3 4 0 9 8 3 8 2 9 Check digit Even digits ▲ Figure 2.17 ISBN (including the check digit) Calculation 2 – Re-calculation of the check digit from the thirteen-digit number Advice (which now includes the check digit) You will not need To check that an ISBN 13-digit code is correct, including its check digit, a similar to remember the process is followed: steps shown in these 1 add all the odd numbered digits together, including the check digit algorithms; the steps 2 add all the even number of digits together and multiply the result by 3 will be given to you, 3 add the results from 1 and 2 together and divide by 10 but it is important that 4 the number is correct if the remainder is zero. you understand how Using the ISBN 9 7 8 0 3 4 0 9 8 3 8 2 9 (including its check digit) from Figure 2.17: to use an algorithm 1 9 + 8 + 3 + 0 + 8 + 8 + 9 = 45 to calculate or verify 2 3 × (7 + 0 + 4 + 9 + 3 + 2) = 75 check digits. 3 (45 + 75)/10 = 120/10 = 12 remainder 0 4 remainder is 0, therefore number is correct. Example 2: Modulo-11 The modulo-11 method can have varying lengths of number which makes it suitable for many applications, such as product codes or VINs. The first calculation is the generation of the check digit. The second calculation is a verification of the check digit (that is, a recalculation). Calculation 1 – Generation of the check digit from the other digits in a number (In this example, we will assume the original number contained only 7 digits.) The following algorithm generates the check digit from the other 7 digits: 1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3 or 2 starting from the left (weightings start from 8 since the number will become eight-digit when the check digit is added) 2 the digit is multiplied by its weighting and then each value is added to make a total 3 the total is divided by 11 4 the remainder is then subtracted from 11 to find the check digit (note if the remainder is 10 then the check digit ‘X’ is used). The example to be used has the following seven-digit number: 1 7-digit number: 4 1 5 6 7 1 0 weighting values: 8 7 6 5 4 3 2 2 sum: (8 × 4) + (7 × 1) + (6 × 5) + (5 × 6) + (4 × 7) + (3 × 1) + (2 × 0) = 32 + 7 + 30 + 30 + 28 + 3 + 0 total = 130 3 divide total by 11: 130/11 = 11 remainder 9 4 subtract remainder from 11: 11 – 9 = 2 (check digit) So we end up with the following eight-digit: 4 1 5 6 7 1 0 2 61 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 61 18/02/21 3:48 PM  2 Data transmission Calculation 2 – Re-calculation of the check digit from the eight-digit number (which now includes the check digit) To check that the eight-digit number is correct, including its check digit, a similar process is followed: 1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3, 2 or 1 starting from the left 2 the digit is multiplied by its weighting and then each value is added to make a total 3 the total is divided by 11 4 the number is correct if the remainder is zero Using the 8-digit number: 4 1 5 6 7 1 0 2 1 weighting values: 8 7 6 5 4 3 2 1 2 sum: (8 × 4) + (7 × 1) + (6 × 5) + (5 × 6) + (4 × 7) + (3 × 1) + (2 × 0) + (1 × 2) = 32 + 7 + 30 + 30 + 28   + 3 + 0 + 2 total = 132 3 divide total by 11: 132/11 = 12 remainder 0 4 remainder is 0, therefore number is correct Activity 2.7 1 Using the algorithm for ISBN-13 calculate the check digit for: 978151045759 2 Find the check digits for the following numbers using both modulo-11 and ISBN 13 methods: i 213111000428 ii 9 0 9 8 1 2 1 2 3 5 4 4 2.2.4 Automatic Repeat Requests (ARQs) We have already considered parity checks and echo checks as methods to verify that data has arrived at its destination unchanged. An Automatic Repeat Request (ARQ) is a third way used to check data following data transmission. This method can best be summarised as follows: » ARQ uses positive and negative acknowledgements (messages sent to the receiver indicating that data has/has not been received correctly) and timeout (this is the time interval allowed to elapse before an acknowledgement is received) » the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check – refer to Section 2.1.1); this is used to detect whether the received data contains any transmission errors » if no error is detected, a positive acknowledgement is sent back to the sending device » however, if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the data » a time-out is used by the sending device by waiting a pre-determined amount of time …. »... and if no acknowledgement of any type has been received by the sending device within this time limit, it automatically re-sends the data until a positive acknowledgement is received …. »... or until a pre-determined number of re-transmissions has taken place » ARQ is often used by mobile phone networks to guarantee data integrity. 62 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 62 18/02/21 3:48 PM  2.3 Symmetric and asymmetric encryption 2.3 Symmetric and asymmetric encryption 2.3.1 The purpose of encryption When data is transmitted over any public network (wired or wireless), there is always a risk of it being intercepted by, for example, a hacker. Under these circumstances, a hacker is often referred to as an eavesdropper. Using encryption helps to minimise this risk. Encryption alters data into a form that is unreadable by anybody for whom the Link data is not intended. It cannot prevent the data being intercepted, but it stops it For more on cyber from making any sense to the eavesdropper. This is particularly important if the security see data is sensitive or confidential (for example, credit card/bank details, medical Chapter 5. history or legal documents). Plaintext and ciphertext The original data being sent is known as plaintext. Once it has gone through an encryption algorithm, it produces ciphertext: Plaintext Ciphertext Ciphertext Encryption Decryption Plaintext algorithm algorithm Encryption Internet Decryption key key ▲ Figure 2.18 Plaintext and ciphertext 2.3.2 Symmetric and asymmetric encryption Symmetric encryption Symmetric encryption uses an encryption key; the same key is used to encrypt and decrypt the encoded message. First of all, consider a simple system that uses a 10-digit denary encryption key (this gives 1 × 1010 possible codes); and a decryption key. Suppose our encryption key is: 4291362856 which means every letter in a word is shifted across the alphabet +4, +2, +9, +1, and so on, places. For example, here is the message COMPUTER SCIENCE IS EXCITING (plaintext on the top line of Figure 2.19) before and after applying the encryption key (forming the ciphertext shown on the bottom line of Figure 2.19): C O M P U T E R S C I E N C E I S E X C I T I N G 4 2 9 1 3 6 2 8 5 6 4 2 9 1 3 6 2 8 5 6 4 2 9 1 3 G Q V Q X Z G Z X I M G W D H O U M C I M V R O J ▲ Figure 2.19 Plaintext into ciphertext using 10-digit encryption key 63 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 63 25/02/21 11:21 AM  2 Data transmission To get back to the original message, it will be necessary to apply the same decryption key; that is, 4 2 9 1 3 6 2 8 5 6. But in this case, the decryption process would be the reverse of encryption and each letter would be shifted –4, –2, –9, –1, and so on. For example, ‘G’ ‘C’, ‘Q’ ‘O’, ‘V’ ‘M’, ‘Q’ ‘P’, and so on. However, modern computers could ‘crack’ this encryption key in a matter of seconds. To try to combat this, we now use 256-bit binary encryption keys that give 2256 (approximately, 1.2 × 1077) possible combinations. (Even this may not be enough as we head towards quantum computers.) The real difficulty is keeping the encryption key a secret (for example, it needs Find out more to be sent in an email or a text message which can be intercepted). Therefore, One of the ways of the issue of security is always the main drawback of symmetrical encryption, mitigating the risk of since a single encryption key is required for both sender and recipient. symmetric keys falling into the wrong hands Asymmetric encryption (known as the key Asymmetric encryption was developed to overcome the security problems distribution problem) associated with symmetric encryption. It makes use of two keys called the public is to use a system based on modulo-11, key and the private key: where both sender and » public key (made available to everybody) receiver can calculate » private key (only known to the computer user). the encryption key without it actually being exchanged in any Both types of key are needed to encrypt and decrypt messages. way. Find out how this We will use an example to explain how this works; suppose Tom and Jane work for system works. the same company and Tom wishes to send a confidential document to Jane: 1 Jane uses an algorithm to generate a matching pair of keys (private and public) that they must keep stored on their computers; the matching pairs of keys are mathematically linked but can’t be derived from each other. 2 Jane now sends her public key to Tom. Jane sends Tom Tom Jane her public key Public key Public key Private key ▲ Figure 2.20 Jane sends Tom her public key 3 Tom now uses Jane’s public key ( ) to encrypt the document he wishes to send to her. He then sends his encrypted document (ciphertext) back to Jane. Tom sends Jane Tom Jane an encrypted Public key Public key document Private key ▲ Figure 2.21 Encrypted document sent from Tom to Jane 4 Jane uses her matching private key ( ) to unlock Tom’s document and decrypt it; this works because the public key used to encrypt the document and the private key used to decrypt it are a matching pair generated on Jane’s computer. (Jane can’t use the public key to decrypt the message.) 64 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 64 18/02/21 3:48 PM  2.3 Symmetric and asymmetric encryption Activity 2.8 1 At the moment Jane can only receive encrypted documents from Tom. Describe what would need to happen for Jane to be able to send encrypted documents back to Tom. 2 Explain why this method is much more secure than symmetric encryption. Jane can also exchange her public key with any number of people working in the company, so she is able to receive encrypted messages (which have been encrypted using her public key ) and she can then decrypt them using her matching private key: Tom Susan Mike Megan Public key Public key Public key Public key Jane Public key Private key ▲ Figure 2.22 The sharing of Jane’s public key However, if a two-way communication is required between all five workers, then they all need to generate their own matching public and private keys. Once this is done, all users then need to swap public keys so that they can send encrypted documents/files/messages between each other. Each worker will then use their own private key to decrypt information being sent to them. Find out more 1 Using Figure 2.22 as your template, draw a new diagram showing the public keys and private keys that need to be swapped if Jane wishes to have a two-way exchange of encrypted documents between Tom, Susan, Mike and Megan. 2 Consider the complexity, if all five people want to have secure two-way communication between each other (and not just with Jane). This would mean each of the five workers sharing their own public keys with each of the other workers. Activity 2.9 For each of the following ten questions, choose which of the five options corresponds to the correct response. a What is meant by the term ciphertext when used in encryption? A an encryption or decryption algorithm B an encrypted message C a type of session key D another name for plaintext E text following an encryption algorithm 65 318281_C02_CAM_IGCSE CO_SCI_045_074.indd 65 25/02/21 11:22 AM  2 Data transmission b When carrying out asymmetric encryption, which of the following users would keep the private key? A the sender D all recipients of the message B the receiver E none of the above C both sender and receiver c In encryption, which of the following is the term used to describe the message before it is encrypted? A simpletext D ciphertext B plaintext E firsttext C notext d Which of the following is the biggest disadvantage of using symmetric encryption? A it is very complex and time consuming B it is rarely used anymore C the value of the key reads the same in both directions D it only works on computers with older operating systems E there is a security problem when transmitting the encryption key e Which of the following is the correct name for a form of encryption in which both the sender and the recipient use the same key to encrypt and decrypt? A symmetric key encryption D same key encryption B asymmetric key encryption E block cipher encryption C public key encryption f What of the following is the final number in a code, which is calculated from all the numbers in the code; its purpose is to find errors in data entry? A parity check D parity bit B checksum E check digit C cyclic redundancy check g Which of the following is a form of error detection that makes use of a system of acknowledgements and timeouts? A automatic repeat request D parity bit B echo check E cyclic redundancy check C check digit