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A-Level Computer Science OCR > Exchanging Data > Flashcards

Flashcards in Exchanging Data Deck (102)
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1

Lossy Compression

Actual data is removed from the file in order to reduce its size. An algorithm is used to strip out the least important data. The original file cannot be restored as data is physically removed.

2

Lossless Compression

Reduces the file size but allows the original data be perfectly reconstructed from the compressed data. Actual data is still removed however the data is encoded in such a way that the original can be recreated.
Less effect at reducing file size than lossy.

3

Run Length Encoding

Run length encoding is used when there is repeated redundant data. It stores the item once and then an index with how often the item is repeated.

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Dictionary Coding

Dictionary coding is when an index is built where every data item in the file is recorded along with an indexed reference. The final compressed file will just consist of the dictionary index along with the sequence of occurrences needed to recreate the original file.

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Purpose of Compression

1. Reduce download times
2. Reduce requirements on file storage
3. Make best use of bandwidth

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uses of lossy

multimedia files e.g. MP3, JPEG, MPEG

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uses of lossless

text files and computer programs

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Encryption

Encryption is the process of encoding a message so that it can be read only by the sender and the intended recipient.

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Symmetric Encryption

Symmetric Encryption is when the same key is used to encrypt and decrypt the message. Both party’s must know the key and also keep it secret.

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Negatives of Symmetric encryption

There is a security risk as the key may be intercepted or the process of creating the key may be duplicated meaning the data can be decrypted by a third party.

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Asymmetric Encryption

Asymmetric Encryption (Public Key Cryptography) is when a public key and a private key is used. The public key is used to encrypt the data and the private key is used to decrypt the data. If you use person X’s public key to encrypt the data only person X’s private key will be able to decrypt it

12

Hashing

Hashing is the process used to transform a data item into something different. A hashing function provides a mapping between an arbitrary length input and a fixed length output. It is a one-way transformation meaning you cannot get back to the original form.

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Uses of Hashing

- generating disk address
- storing and checking passwords

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digital signature

hash total is encrypted using the users private key. If the receiver can decryptit using the senders public key they know that the message is authentic

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Uses of assymetric encryption

used for transfering data e.g. online shopping

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Uses of symmetric encryption

Used when the same person is accessing and saving data e.g. for backing up

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benefits of symmetric

- encrypted very quickly
- simple and easy

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Benefits of asymmetric

- no movement of keys, more secure
- can be used for digital signatures

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drawbacks of assymmetric

- not as fast

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Database

Databases are structured, persistent collections of data.

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Flat File

single table database. It is inefficient as it is difficult to query and leads to data redundancy which can cause errors.

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Relational Database

A relational database has more than one related table. It is more efficient that a flat file database as no data is duplicated meaning querying is easier.

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Entity Relationship Modelling

when the relationships between tables are shown in abstracted view.

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relationships

one-to-one
one-to-many
many-to-many

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primary key

A field that uniquely identifies each record

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Foreign key

a foreign key is a field in one table that uniquely identifies a row of another table

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composite key

A composite key is a combination of two or more columns in a table that can be used to uniquely identify a row.

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Secondary key

any field in a database which is not a primary, candidate or foreign key. They are used to order queries.

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Method of capturing data (Automated)

barcode readers, scanners, sensors, magnetic ink character recognition, smart card readers

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Method of capturing data (Manual)

Paper data capture form (read by OCR & OMR or typed in manually)