CCEA GCE A2 ICT

A client server network consists of a controlling network server which handles processing requests from the other computers on the network. A peer-to-peer network has no controlling server. All nodes are of equal status, with each node being a supplier and consumer of resources. Nodes make their resources available to other nodes, however each node is in charge of its won security and decides which other nodes get access to its resources. 

Centralised databse - Data consistency 

• Data consistency is ensured as there is only a single copy of the database

Distributed database - Data consistency

• There may be multiple values of the same data in different locations e.g. a fild may be updated at only location only or a filed may be updated to different values at two different locations. The data has to be reconciled at regular intervals

Centralised databse - User access to data 

• All users access the single copy of the data so access times are greater due to the distances involved. The volumes of access at the central location will be high and there may be deterioration in erformance. I fhte central location fails, the entire database fails

Distributed database - User access to data 

• Users have faster access to their local data as the data is stored locally. Volumes of access at each location will be reduced. If one location fails, only that part of the database is affected

Centralised database - Data security

• It is simpler to enforce centralised security measures on the single copy of the database. The increased volume of data traffic may be more vulnerable to interception.

Distributed database - Data security

• It is more complicated to neorce uniform security measures across a number of locations. It is simpler to enforce security measures at a single location. The decreased volume of data traffic will be less vulnerable to interception.

Advantages of a distributed database include more efficient processing, more efficient use of resources; it is more secure and is less reliant on telecommunications.

Disadvantages of a distributed database are that there is more difficulty when updating the master file and more difficulty controlling security.

A mobile phone network uses radio frequency waves to communicate information. When a mobile phone connects to a network it communicates with the nearest base station. The area covered by a base station is known as a cell. Each cell is usually split into three sectors which overlap with the sectors of neighbouring cells to create an uninterrupted network. The signal is passed from one base station to the next as people travel. Cells are connected to cellular telephone exchange switches which are connected to the public telephone network or other exchange switches. 

A wireless LAN connects computers together within a small geographical area. All network computers are wireless enabled equipped with wireless network interface cards. Access points act as hubs for the wireless network which transmit and receive radio signals for stations to communicate with. 

Communication standards have an important role to play in networked systems. Communication standards are required: to enable different hardware, software, and information systems to pass data to one another coherently, securely and compatibly using agreed formats and rules. 

An OSI model was developed as part of the Open Systems Interconnection initiative by the International Organization of Standardization (ISO)/ANSI. It consists of an abstract or basic model of networking and a set of specific protocols. It consists of seven layers with each layer dealing with specific functionality. Each layer interacts directly with the layer immediately beneath it and provides facilities for use by the layer above. 

Application

Presentation

Session

Transport

Network

Data

Physical

A parity bit is an additional bit calculated from the other bits in a byte of data set to make the total number of bits odd (or even). The parity bit is checked after transmission. If the parity bit is not correct, a transmission error has occurred. With simple parity checking, it cannot determine which bits have flipped and the data will have to be re-transmitted. With latitudinal and longitudinal parity checks, if one bit is transmitted incorrectly the error can be located and corrected.

A checksum is calculated by adding together all the bytes in a block of data. The checksum is recalculated after data transmission. If the checksum is incorrect, the data is very likely to be an error. Some checksums will automatically correct the checksum.

An echo checking is when the receiving data sends the received data back to the transmitting device. The transmitting device can compare this data with the original data and make corrections as appropriate, retransmitting the data. 

• Authentication – identifying an individual through information unique to them such as a username and password, or biometrics

• Authorisation – granting or denying access to network resources once a user is authenticated. The data and services which a user can access will depend on their authorisation level

• Accounting – tracking a user’s activity while accessing network resources. Accounting software records each event within a system and can be used for auditing purposes

Auditing is the process of monitoring usage statistics and user habits. It may be done for performance reasons or it may be done to monitor undesirable practices. An audit trail is a record of events carried out by a computer system and is generated by accounting software. It allows an action to be traced through all stages of processing beginning when data is input to information is output.

Information stored on an audit trail includes the software used, log in/out times, files opened, number of read-write operations and how often a server is accesssed

An audit trail is needed for formal/legal requirements, billing purposes, maintenance reasons and security reasons