A-level Computing/AQA/Problem Solving, Programming, Operating Systems, Databases and Networking/Communication and Networking

Communication Methods

Define both serial and parallel transmission methods and describe where they are used.

Serial data transmission

Serial data transmission is when single bits are sent one after another along a single wire. Serial data transmission can happen in many different components such as serial circuits or communication networks that requires serial connection e.g. undersea cables and telephone wires. Binary digits are used to represent data and voltages to their equivalent signal. In terms of communication, it is use for connecting two computers to form a two-way communication. There are other ways of connecting to other machines with a serial data communication like a USB connecting to the computer or a game console connecting to a phone line.

Serial data transmission is used for long-distance communication because the signal travelling on a single pathway is not susceptible to skew. This makes it useful for overseas communications like telecommunication between America and Britain.

Parallel data transmission

Consider the effect of distance on the transmission of data.

Parallel data transmission is the transmitting of single bits along several wires simultaneously. The cable that connects these transmissions consists of many wires.

Parallel data transmission is generally used over short distances. The reason for this is that it is difficult to keep the voltages on the eight wires in line with each other after a certain distance. This problem is known as skew(ed).

Baud

The Baud is the rate of change of signals on a wire or a line. For example, if your Baud Rate was set to 1, the signals would only be able to change at the end of each second. If it were set to 2, there would be 2 changes per second.

To find the time difference between the signal changes, 1 is divided by the baud.

The formula: 1/baud = seconds between signals.

Bit rate

Bit Rate = Bits per signal * Baud Rate
1 = 1 * 1
2000 = 1 * 2000
4000 = 2 * 2000

A Bit rate is the number of transmitted bits within each second, this is very similar to the Baud rate however they are different as the Baud rate is the rate of signal change per second.

Bit rate = Bits per signal * Baud rate

The Bit rate and Bandwidth are closely linked together. The higher the bandwidth of the system, the higher the bit rate allowing more bits to be transmitted.

Bandwidth

Bandwidth is the measurement (Hz) of the speed at which data is transmitted over a transmission medium. The greater the bandwidth, the faster the data transmission.

Low-frequency signals transmitted along a wire from A to B arrives with undiminished strength, while high-frequency signals sent along the same wire from A to B loses a significant amount of strength.

Latency

Latency is the time delay between the moment a message/command is sent and the moment a response is returned.

A very good representation of data transmission can be achieved if the bandwidth is doubled the bit rate. Therefore, the greater the bandwidth, the greater is the bit rate, which the system can transmit.

Asynchronous data transmission

Define asynchronous data transmission.

Asynchronous data transmission is when the arrival of data cannot be predicted by the receiver; so a start bit is used to signal the arrival of data and to synchronise the transmitter and receiver temporarily.

Start and stop bits

Describe the purpose of start and stop bits in asynchronous data transmission. AO / AZ / AL

Stop bit

A stop bit is a character in asynchronous data transmission that lets a receiver know that the byte being sent has ended.Stop bits are very important because this is the way most of our information is sent across the internet. Without a stop bit is possible that a receiving computer will likely prompt an error.

Odd and even parity

7 bits of data
(number of 1s)
8 bits including parity
even odd
0000000 (0) 00000000 10000000
1010001 (3) 11010001 01010001
1101001 (4) 01101001 11101001
1111111 (7) 11111111 01111111

A parity bit is a bit that is added to ensure that the number of bits with the value 1 in a set of bits is odd or even. Parity bits are used in the simplest form of error detecting. For example, if a signal starts of with 3 occurrences of 1 it is in odd parity, once it arrives at its destination and only has 2 occurrences of 1 then the receiver knows there is a problem and will ask for the data to be resent

There are two variants of the parity bit, odd and even.

When using even parity, the parity bit is set to 1 if the number of 1s in a set of bits (not including the parity bit) is odd, making the entire set of bits (including the parity bit) even. For example 1001 0110 (4 bits = even)

When using odd parity, the parity bit is set to 1 if the number of 1s in a set of bits (not including the parity bit) is even, making the entire set of bits (including the parity bit) odd. For example 1000 0110 (3 bits = odd)

In other words, an even parity bit will be set to "1" if the number of 1s + 1 is even, and an odd parity bit will be set to "1" if the number of 1s +1 is odd.

Error detection

If an odd number of bits (including the parity bit) are transmitted incorrectly, the parity bit will be incorrect and therefore shows and error occurred during transmission. Parity bits should only be used to detect errors, they cannot correct any errors as there is no way to determine which specific bit has been corrupted. The data must be scrapped and re-transmitted.

Handshaking

Handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer.

It is usually a process that takes place when a computer is about to communicate with a foreign device to establish rules for communication. When a computer communicates with another device like a modem or a printer it needs to handshake with it to establish a connection. Much like humans greet each other by a handshake to establish a connection.

Handshaking may be used to negotiate parameters that are acceptable to equipment and systems at both ends of the communication channel, including, but not limited to, information transfer rate, coding alphabet, parity, interrupt procedure and other protocol or hardware features.

Handshaking makes it possible to connect relatively heterogeneous systems or equipment over a communication channel without the need for human intervention to set parameters. One classic example of handshaking is that of modems, which typically negotiate communication parameters for a brief period when connection is first established, and thereafter use those parameters to provide optimal information transfer over the channel as a function of its quality and capacity.

Protocol

A protocol is a set of agreed signals, codes and rules which is used for communication across a network consisting of computers and peripherals.

Baseband

Baseband is a transmission medium used for a network over short distances. Usually, a network is used between several computers so data is sent simultaneously. However, a baseband system only allows one station to be sent at a time.

Definition - Single/data signal sent at a time over a full bandwidth of the cable.

Broadband is also a transmission medium used for a network but it is a multichannel system which combines several data channels into one so that the bandwidth of the transmission can be shared between several channels. It is mainly used for long distance communication.

Definition - Several data signals sent simultaneously each at a different frequency

Networks

Wide Area Networks

A set of links that connect geographically remote computers and LANs.

Network adapter or Network Interface Card

A computer communicates on the network through a network adapter. A network adapter plugs into the motherboard of a computer and into a network cable. They convert data from the form stored in the computer to the form transmitted or received on the cable. Data is passed through electronics that calculate a checksum value and its own and destination address. It is now known as a frame. The frame is transmitted a bit at a time onto the network cable, with the address sent first, then data and lastly, the checksum. Each network adapter card has a unique 48 bit hexadecimal MAC address.

Topology

The structure of the connections that connect devices to a network

• Bus
Bus Network
If a computer goes down it doesn't take out the entire system
Cannot have two computers transmitting at the same time. Messages 'collide'
As several computers are using the same wire you can eavesdrop on messages for other machines
A break in the cable breaks the entire network

• Star
Star Network
If a node fails the entire system stays up
Message collisions do not occur, the switch in the centre stores the frames of information until the backbone is ready to receive
Hard for people to eaves drop on your messages. Each computer has a dedicated cable used to connect it to the switch
If the central server / switch goes down the entire network fails

Bus Networks

Bus layout
• A bus network relies on each computer being connected to one wire, the bus.
• When a computer on the network wishes to transmit data, it is possible to do so immediately if no other data is detected on the bus.
• If there is already data being transmitted on the wire then the computer has to retry later.
• If two or more computers transmit at exactly the same time a collision occurs.
• If a collision occurs the computer that was trying to send the data detects that the sending has failed and proceeds to try again at a different time.
• At times when the network is in heavy use, lots of computers will be transmitting data therefore leading to a number of collisions. This causes the overall performance of the network to deteriorate.

Star networks

Star layout
• A star network will have a switch at its centre.
• All data transmitted from the individual computers on the network will go through the switch then sent on to their destination, therefore removing any congestion.

Candidates should be able to compare local area networking with standalone operation. AG

Network Segment

A segment refers to a string of computers. Segmentation refers to non switched Ethernet bus networks that are split to improve performance.

Peer-to-peer networking

This a network system where each workstation has equivalent capabilities.

Peer-to-Peer LANs Good for less than 10 users where users are located in a close proximity and security isn't an issue. However provides a limited growth for future network extensions.

Peer-to-Peer WANs used to share files among a large number of users connected temporarily. E.g.: BitTorrent

Server-based networking

Where resources, security and administration on a network is carried out by dedicated servers.

Thin client computing

Thin-client computing when all networks procession comes from a central server. The Thin-clients is used to connect are computers to one central server to be able to form a network. Some star formatted systems use thin-client networks. Thin-client networks do not require any hard disk storage and it has little processing. Thin-client network would be use in small office because thin-client network has very little bandwidth. They are often called dumb terminals because of this. Central server is the application server. Central server can run applications like Word, multimedia and send to different users terminals. Also, Central server controlled users login to see if the terminals are connected to the server.

Web services

Explain these terms and describe situations where these might be used. Compare and contrast thin client computing (software as a service, AJAX, Web 2.0, etc.) vs rich client computing (client-server, peer-to-peer), web services as examples of ‘systems architectures’.

Wireless networking

• Wi-Fi
• Bluetooth

Radio-based LAN protocols for connecting mobile/portable devices.

Inter-networking

• Internetworking is formed when computers from one network communicates with computers from other networks.
• These networks communicate via gateways.
• An example of internetworking is the internet, which has now become important and widely used in everyday life.

Routers

Routers and Bridges allow communication between computers on different segments. A Router is a device that receives packets from a router or a host and uses the destination IP address the packets contain to pass them to another router or host. Routers are commonly connected to a network of other routers. An example of how routers are used: If a host of a network (network A) in one country(country A) wants to communicate with a host on a network(network B) of another country (country B), it will have to send packets to the gateway router on network A; this will pass the packets to the

```local router -> regional router -> national router -> international router
```

until the network of country B is reached.

Bridges

A bridge holds a table of addresses for each connected machine. Data packets received from one segment are only forwarded if they have a valid destination address on the other segment. This helps to reduce overall traffic.

Gateways

A device used to connect networks using different protocols so that information can be passed from one system to another. Gateways are used to connect LANs through the internet; in order to do this, they must translate the LANs frame into equivalent WAN frame, vice versa; this is because the LANs uses a protocol that is different from the protocols used on the internet which is WAN.

Define these and consider where and why they are used. In particular, consider how routing is achieved across the Internet and how local area networks are connected to the Internet via gateways.

Subnetting

Subnetting is used to divide host networks in to further networks, e.g a diffrent ip address within a host i.p address. By adding another octect (.000 > to > .255) to the ip adress you can add another smaller network within a larger one. A subnet mask is used to identify which bits in an IP adress specify its subnet, and hence which adresses it can send packets directly to, not via a router or switch.

Server-side scripting

Common Gateway Interface (CGI) Server-side scripting – the basis of dynamic web page content. Practical experience of writing simple server-side scripts. Accessing data from a DBMS using server-side scripts.

Internet Security

Firewalls

A firewall is a device or program that monitors and controls data traffic between the internet and a private network (such as your network at home). Every firewall can be customised and assigned rules which determine which data packets are allowed through from the internet and which are not.

Firewalls can also be used to block data from certain IP addresses, domain names or port numbers. Many firewalls also have the capability of being able to search individual packets for specific matches of text.

Packet filtering -

• When using the packet filtering method, the firewall analyses the packets that are sent from the internet against a set of filters (firewall rules) which determine whether or not the packet is allowed to go through.

Proxy server -

• Proxy servers prevent the user of a private network coming into direct contact with the computer that hosts a web page on the internet. This works by the proxy requesting the data from the internet and then passing it on to the private network user.

Packet filtering.

Proxy Server. JE / AG

• Encryption is used to hide sensitive messages from illegitimate recipients by using encryption algorithms and an encryption key to convert plain text to cipher text, illegible to those without the encryption and decryption key. Private/Public key encryption is when both parties have a pair of keys, one private and one public. The Public Key is kept in the open freely usable by anyone as is the encryption algorithm, however the Private Key is kept hidden.
• A message encrypted with A's public key can only be decrypted with A's private key.
• A message encrypted with A's private key can only be decrypted with A's public key.
• A message encrypted with B's public key can only be decrypted with B's private key.
• A message encrypted with B's private key can only be decrypted with B's public key.

Digital signatures

are a way for the sender to prove to the receiver that the message did in fact originate from them. A digital signature is obtained through the following process:

Processes required before A sends the message to B Processes required to ensure the message is from A
Message is hashed to get a message digest. B decrypts the message with B's private key.
The message digest is encrypted with A's private key, this then becomes the signature. B decrypts the signature with A's public key to get the original message digest.
The signature is appended to the message. The decrypted message is hashed again, reproducing the message digest.
The message is encrypted using B's public key. The message has not been tampered with if the decrypted message digest is the same as the reproduced digest.
The encrypted message is sent to B.

A Digital Certificate is a way of proving that the public key of the sender is authentic. Digital Certificate are only issued by the Certification Authorities (CAs). The certificates are encrypted into the message via the CA's private key, and can only be decrypted with the CA's public key. SY / MT / KT

• Virus detection - Discuss worms, spam, phishing, pharming as well as viruses, also vulnerabilities that these exploit and how to address them through improved code quality, monitoring, protection. OA / ZA/ LA / MD

Computer Security Procedures

Authentication, Authorisation, Accounting. TH / LL / AO / NR

- Authentication: verification of user of the computer system. It use for passwords, biometric data, security token and digital signatures.

Server Side Scripting

 Specification: Server-side scripting Common Gateway Interface (CGI) Server-side scripting – the basis of dynamic web page content. Practical experience of writing simple server-side scripts. Accessing data from a DBMS using server-side scripts.