Exchanging Data: Databases, Networks and Web Technologies
Exchanging data is the A Level Computer Science topic covering how computers store, compress, encrypt and transmit data reliably, including database normalisation and SQL, network topologies and protocols such as TCP/IP, and web technologies such as client-side and server-side scripting. It is part of the Component 1 Computer Systems paper and builds on binary data representation.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the difference between lossy compression (permanently discards some data) and lossless compression (keeps all data, so the original can be reconstructed exactly), and when each is appropriate.
- Practise applying run-length encoding (RLE) to a row of repeated values, and calculating hash table indices using a modulo hash function with linear probing.
- Distinguish symmetric encryption (one shared key) from asymmetric encryption (a public/private key pair), and know how HTTPS combines both.
- Normalise an unnormalised table step by step: remove repeating groups to reach 1NF, remove partial dependencies to reach 2NF, then remove transitive dependencies to reach 3NF.
- Write SQL SELECT, JOIN, GROUP BY and simple data-definition statements against a given relational schema.
- Learn the layers of the TCP/IP model and the role of routers, MAC addresses and IP addresses in moving data across a network to its destination.
Worked example
A row of pixels from a bitmap image has the following greyscale values, read left to right: 5, 5, 5, 5, 8, 8, 8, 1, 1, 1, 1, 1, 6. Apply run-length encoding (RLE) to this row of data, giving your answer as a list of (count, value) pairs.
- Scan the row left to right and group consecutive identical values into runs: 5,5,5,5 then 8,8,8 then 1,1,1,1,1 then 6.
- Count how many values are in each run: 4 fives, 3 eights, 5 ones, 1 six.
- Write each run as a (count, value) pair, in the order the runs occur: (4,5), (3,8), (5,1), (1,6).
- Final answer: (4,5), (3,8), (5,1), (1,6).
Practice questions
Try each question, then tap to reveal the answer.
Exam-style questions
Written in the style of a A Level Computer Science exam paper, with a full mark scheme.
Distinguish between symmetric encryption and asymmetric encryption, and state one reason a website might use both types together.
A hash table of size 10 (indices 0 to 9) uses the hash function h(k) = k MOD 10, with collisions resolved by linear probing (checking the next index upward, wrapping to 0 if necessary). The keys 12, 22, 5 and 32 are inserted, in that order, into the empty table. Show the index at which each key is placed.
A college database has an unnormalised table storing each student's repeating set of module entries. Explain the steps needed to convert this table to third normal form (3NF), naming each normal form reached.
Free printable worksheet
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