Idea Transcript
Written and compiled by Mrs Ellis
last updated: 6/10/2013
WJEC A2 Computing CG3 CG3.1 Hardware You need to understand: Data transmission
Describe serial and parallel transmission, their advantages and disadvantages. Describe simplex, half-duplex and full duplex transmission methods. Explain the need for digital transmission of data. Explain the need for multiplexing and switching. (A knowledge of hardware implementation detail is not required.)
Communication networks
Distinguish between the use of circuit switching and packet switching in communication networks. Describe the typical contents of a packet in packet switching. Explain the need for network protocols. Explain network collision detection and how these collisions are dealt with. Describe the function of routers.
Communication standards Internet
Explain why protocols are required.
Explain the concepts of web page design.
Data transmission • Describe serial and parallel transmission, their advantages and disadvantages. • Describe simplex, half-‐duplex and full duplex transmission methods. • Explain the need for digital transmission of data. • Explain the need for multiplexing and switching. (Knowledge of hardware implementation detail is not required.) Serial transmission data is sent in a serial form i.e. bit by bit on a single line. Also, the cost of communication hardware is considerable reduced since only a single wire or channel is require for the serial bit transmission and use simpler interface. Serial data transmission is slow as compared to parallel transmission. Parallel data transmission is less common but faster than serial transmission, data is transferred sometimes a byte or word at a time on multiple wires with each wire carrying individual data bits. Thus transmitting all bits of a given data byte or word at the same time is known as parallel data transmission. Examples of parallel mode transmission include connections between a computer and a printer (parallel printer port and cable). Most printers are within 6 meters or 20 feet of the transmitting computer and the slight cost for extra wires is offset by the added speed gained through parallel transmission of data. Simplex transmission: allows data to travel only in a single, pre specified direction. Data transmission is possible in one direction only.
Written and compiled by Mrs Ellis
last updated: 6/10/2013
Half-‐duplex: both devices can receive and transmit BUT NOT at the same time. Full-‐duplex: simultaneous two way transmission, both devices can receive and transmit at the same time. A digital signal can be transmitted only a limited distance before attenuation endangers the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives the digital signal, recovers the pattern of 1s and Os, and retransmits a new signal, thereby overcoming the attenuation. To achieve longer distances, the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, the amplifier also boosts the noise components. With amplifiers cascaded to achieve long distances, the signal becomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be tolerated and the data remain intelligible. However, for digital data, cascaded amplifiers will introduce errors. Digital data transmission is preferred over analogue, because: • It is less likely to suffer corruption/degradation • Can error checking data • Can compress data and therefore transfer time is quicker Communication networks and communication standards:
Distinguish b e t w e e n the use of circuit switching and packet switching in communication networks. • Describe the typical contents of a packet in packet switching. Explain the need for network protocols. • Explain network collision detection and how these collisions are dealt with. • Describe the function of routers. • Explain why protocols are required. •
Circuit Switching and Packet Switching Switching means: • sending data along the appropriate path. • It has the advantage of sending data to where it is required, thus more bandwidth efficient. Circuit switching works like the old telephone switch network – every pair of sender and receiver (callers) was given a dedicated physical connection line. During circuit switching, a path is set up between the sender and receiver before the start of transmission and is kept open until the end of transmission. All data follows the same path, in order. The path cannot be used by any other data during the transmission. This is expensive and waste of resources when the circuit is not in use, therefore, the need for multiplexing. Multiplexing is where several independent data sources are combined to be sent along a single route to a single destination.
Written and compiled by Mrs Ellis
last updated: 6/10/2013
This is normally done by time division – each stream gets a share of the time for transmitting. In the simplest case, think you have two 100Mb/s streams to share a 200Mb/s line. One stream can send the first 100Mb and the second can send another 100Mb one second later using the same line. Packet switching is where the data is split into packets before transmission. Each packet may be transmitted by different routes through network. They may arrive out of order and are reassembled on arrival. Packet switching is usually preferred because: • better security as it is very difficult to intercept and reconstruct the packets. • more efficient use of data lines as there is no waiting during gaps. • less likely to be affected by network failure because multiple paths are used. A typical packet is made of the following (data and header): • the actual data (payload) • the sequence number of the packet • control signals / bit(s) (sender address, destination address, and packet size) • error control signals / bit / check sum etc Network Protocols Many different devices and computers are in need of communication with others. In networking, the term protocol refers to a set of rules that govern communications. Protocols are to computers what language is to humans. For two devices on a network to successfully communicate, they must both understand the same protocols. Protocols are like common languages to enable those computing devices to identify, connect, send and receive messages among them. A node is a computer or device, such as a printer or a smartphone in a network with its own unique network address (IP address). A unique network address allows a node to be located in networks. You have learned some network protocols during AS study, such as STMP, POP, FTP, 802.11x etc. The purpose of network protocols is to establish communication rules and standards to enable computing devices to connect and transfer information over networks. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets. Network collision, collision detection and collision Imaging you and your friend both trying to talk to each other face to face at the same time, the result is no one can understand the other. The same situation can occur when two nodes trying to send messages over the same channel at the same time. This
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last updated: 6/10/2013
phenomenon is called collision. Data collision occurs when two sets of data are detected on the network simultaneously. Collision detection is a process which a node determines when a collision has happened. Collisions are dealt with commonly in following ways: If no transmission is taking place at the time, the particular node can transmit. If two nodes attempt to transmit simultaneously, this causes a collision, which is detected by all participating nodes. After a random time interval, the nodes that collided attempt to transmit again. If another collision occurs, the time intervals from which the random waiting time is selected are increased step by step. This is known as exponential back off. A router is a device in the network which holds information about the address of computers in its network (or address of other networks) can therefore send data to the correct computer. (exam definition) Typically, routers perform the following tasks: • direct data to the correct computer • direct data around network in (most) efficient way • act as a gateway connecting to a larger network. More detailed info: • Directing packets to their destinations as specified in the packet headers • Managing the paths along which information is forwarded within a network. • Standing as gatekeepers between networks to control the flow of data over the Internet. Determining when certain segments of the Internet are busier than others. When one portion experiences some congestion, nearby routers simply send packets of information along another route. • Blocking unwanted traffic to networks Some routers can handle tens of millions of packets per second! Internet: • Explain the concepts of web page design
All about the web: Web site is a collection of linked web pages. A web page typically will have text, images, hyper links to other sites or pages. Web pages should be designed with the following considerations: 1. Should comply with established web standards 2. Should be easily navigable with links to allow easy navigation 3. All links should be correct
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last updated: 6/10/2013
4. The page should be as accessible as possible for user with visual impairment 5. It should make sensible/imaginative used of colour, fonts, multi-‐media such as graphics, & video/audio. Cloud computing:
A modern term you should be familiar with. Cloud computing means files and applications over the Internet. Those files and applications are served by a virtual cloud of hardware and software. VLE: Virtual learning environment (VLE) is a software system that allows teachers and pupils to manage and use of learning resources. Typical contents in a VLE include: • homework or coursework assignments • feedback from teachers to students • additional or background teaching materials Extranet and Intranet: An intranet can only be accessed by authorised users within an organisation and is isolated from the global Internet. What a school intranet may typically be used for? An extranet is an intranet that is accessible to some people from outside the organisation. What a school extranet may typically be used for? 2013W Q1-Q3, Q6-Q9 Describe what is meant by the term virtual learning environment and give one example of information which could sensibly be held on the school VLE. [2] Describe what is meant by the term intranet and give one example of information which could sensibly be held on the school intranet. [2] Give one example of information which would not normally be held on the school VLE or intranet, but which could sensibly be available on the school web site. [1]
Give one advantage of transmitting data in digital form rather than analogue form. [1] Data may be transmitted through a computer network using simplex, half duplex or full duplex. Describe each of these transmission methods. [3]
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Explain what is meant by the term switching in a computer network. Why is switching useful in a computer network? [2] Explain what is meant by a router. Describe the main function of a router.
Written and compiled by Mrs Ellis
last updated: 6/10/2013