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Data Security
  If data is transmitted through a network there needs to be measures to ensure the data is secure. If users are sending their credit card details over the Internet then it is important that hackers cannot access them.  

Security measures include -

  
Call-back procedures Some companies operate a dial-back system. A user logs on to a computer which immediately disconnects the line and dials the user back. This would stop a user logging on with someone else's password.

 
Firewalls A dedicated gateway machine with special security precautions on it, used to monitor network, especially Internet, connections. The idea is to protect a network segment and its files from hackers.

An iron box is a  special environment set up to trap a hacker logging in over remote connections. May include a modified shell restricting the hacker's movements in unobvious ways, and "bait" files designed to keep him interested and logged on until he can be traced.

 
An advert for a firewall system
Data encryption Data transmitted over a network is coded before transmission. This means that anybody intercepting the transmitted data would not be able to understand it. The data needs to be de-coded by the proper recipient. PGP = "Pretty Good Privacy" is free software which encrypts your emails.
 
  Data Integrity

The Integrity of the data is ensured by -

Parity A Parity Bit is a single bit (0 or 1) added onto the end of a byte of data. If even parity is used then the number of 1 bits in any transmitted data must always be even.

In the following two bytes, the red bit is the parity bit :

0010110 1
1110111 0

The receiving computer will check the number of 1-bits in the data. If data is received with an odd number of bits then the computer will know the data has been corrupted during the transmission - and will ask for the data to be sent again.

Odd parity uses an odd number of 1-bits in every byte.

 
Checksum When data is transmitted the total of all the bytes is added and transmitted along with the data. This total is called a checksum. The receiving data will check the checksum is correct - if not, then data has been corrupted or some is missing so will not accept the data.

 
Cyclic Redundancy
Check		 The CRC is a calculation performed on a set of data and transmitted along with it. The receiving computer will perform the same calculation and will be able to see if the data has been corrupted during transmission.