AQA A-Level Computer Science: Communication and Networking — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Communication and networking study guide | Communication and networking revision notes

These questions cover section 4.9 of the AQA A-Level Computer Science (7517) specification: communication (4.9.1), networking (4.9.2), the Internet (4.9.3) and TCP/IP (4.9.4). This content is assessed on Paper 2.

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Section A: communication

1. (a) Define baud rate and bit rate. [2] (b) A connection runs at 2,400 baud and encodes 2 bits in each signal change. Calculate its bit rate. [1] (c) State the relationship between bit rate and bandwidth. [1]

2. Explain two advantages of serial transmission over parallel transmission. [4]

3. Describe the purpose of the start bit and stop bit in asynchronous transmission. [2]

4. Define latency and protocol. [2]

Section B: networking

5. Explain how a network that is physically wired as a star can operate as a logical bus. [3]

6. A small design studio with four computers is choosing between peer-to-peer and client-server networking. Recommend one, with two reasons. [3]

7. (a) State three ways a wireless network can be secured. [3] (b) Describe how CSMA/CA with RTS/CTS is used to transmit a frame, and explain the problem that RTS/CTS solves. [5]

Section C: the Internet and security

8. (a) Explain how packet switching is used to send a file across the Internet. [4] (b) State two items in a packet’s header. [2] (c) Explain the difference between a router and a gateway. [2]

9. For the URL https://shop.example.co.uk/basket/view: (a) Identify the protocol, the fully qualified domain name and the domain name. [3] (b) Describe how the browser obtains the IP address of the server. [3] (c) Explain why Internet registries are needed. [2]

10. Describe how a firewall uses stateful inspection, and how this differs from simple packet filtering. [3]

11. Alice sends Bob a signed contract. (a) Describe how Alice creates a digital signature and how Bob checks it. [5] (b) Explain why Bob needs Alice’s digital certificate. [2]

12. Explain the difference between a worm and a trojan, and describe one way improved code quality reduces the threat from malware. [4]

Section D: TCP/IP and addressing

13. Describe what happens to an email message at each of the four layers of the TCP/IP stack as it is sent. [4]

14. (a) Explain what a socket is. [1] (b) Explain the difference between a well-known port and a client port. [2]

15. A host has IP address 10.4.150.23 and subnet mask 255.255.240.0. (a) Calculate the network identifier. [2] (b) Determine whether a host with address 10.4.160.5 is on the same subnet, showing your working. [2]

16. (a) Explain why IPv6 was introduced. [1] (b) Explain the purpose of DHCP. [2] (c) A home has one public IP address and ten devices. Explain how NAT allows them all to use the Internet, and why port forwarding might be needed for a games server on one of the devices. [4]

17. (a) Explain why a live sports score website might use the WebSocket protocol rather than repeated HTTP requests. [2] (b) A web application uses a REST API connected to a database. State the HTTP method and SQL command used to (i) display a list of products (ii) add a new review. [2] (c) Give two reasons why JSON might be chosen instead of XML. [2]

18. A school is choosing between thin-client and thick-client computing for its library computers. Discuss one advantage and one disadvantage of thin-client computing in this case. [4]

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Answers

1. (a) Baud rate: the number of signal changes per second [1]. Bit rate: the number of bits transmitted per second [1]. (b) 2,400 × 2 = 4,800 bit/s [1]. (c) Bit rate is directly proportional to bandwidth [1].

2. Any two, explained [2 each]: no skew, because bits do not travel on several wires that may deliver them at slightly different times, so higher speeds are possible; no crosstalk between neighbouring wires; cheaper, thinner cable with fewer wires; reliable over longer distances.

3. The start bit signals that a character is about to arrive and lets the receiver synchronise its clock to the incoming data [1]. The stop bit marks the end of the character and gives the receiver time to process it before the next [1].

4. Latency: the time delay between data being sent and it starting to be received [1]. Protocol: a set of rules that govern communication between devices [1].

5. Each device is connected by its own cable to a central switch (physical star) [1]. The network uses a bus protocol [1], so every frame is effectively broadcast to all devices and only the device it is addressed to accepts it, as on a shared bus [1].

6. Peer-to-peer [1]: it is cheap because no dedicated server is needed [1], and with only four users it is simple to set up and share files directly [1]. (Accept client-server if justified, e.g. central backups of valuable design files and central security.)

7. (a) Use WPA2 (or WPA) encryption [1]; disable SSID broadcast [1]; use a MAC address allow list [1]. (b) The device listens to check whether the channel is idle; if busy it waits a random back-off period [1]. When idle, it sends a Request to Send to the access point [1]. The access point replies with Clear to Send, which all devices in range hear, so the others delay transmitting [1]. The device sends its frame and the receiver returns an acknowledgement; if none arrives, it assumes a collision and retries after a back-off [1]. RTS/CTS solves the hidden node problem: two devices that cannot hear each other but can both reach the access point would otherwise transmit at the same time [1].

8. (a) The file is split into packets [1], each given a header containing addressing information [1]. Each packet is sent independently and routers forward it towards the destination, so packets may take different routes [1]. At the destination they are reassembled in the correct order using their sequence numbers, and any missing packets are requested again [1]. (b) Any two: source IP address, destination IP address, packet sequence number, time to live, protocol [2]. (c) A router forwards packets between networks that use the same protocols [1]; a gateway connects networks using different protocols and translates between them [1].

9. (a) Protocol: HTTPS [1]. FQDN: shop.example.co.uk [1]. Domain name: example.co.uk [1]. (b) The browser sends the domain name to a DNS server [1]. If the server holds the record it returns the IP address; otherwise the request is passed up the hierarchy of name servers until one that knows the address answers [1]. The IP address is returned to the browser, which then connects to the web server [1]. (c) Every IP address and domain name must be globally unique [1], so registries allocate them centrally and keep records of who holds each one [1].

10. Stateful inspection keeps a record of the connections that are open [1] and checks each packet against it, allowing only packets that belong to a legitimate, established connection [1]. Packet filtering examines each packet on its own, using only rules about IP addresses, ports and protocols [1].

11. (a) Alice creates a hash (digest) of the contract [1] and encrypts the hash with her private key to form the signature, which she sends with the contract [1]. Bob decrypts the signature using Alice’s public key to recover the hash [1]. He hashes the contract he received himself [1]. If the two hashes match, the contract came from Alice and has not been altered [1]. (b) The certificate is issued and signed by a trusted Certificate Authority [1] and confirms that the public key really belongs to Alice, not an impostor [1].

12. A worm is self-replicating and spreads across a network on its own [1]; a trojan disguises itself as legitimate software and relies on the user installing it [1]. Code quality: code reviews and thorough testing remove vulnerabilities, such as buffer overflows, that malware exploits [1], so there are fewer weaknesses to attack [1].

13. Application: the email client formats the message using SMTP [1]. Transport: TCP splits it into segments, numbers them and adds the source and destination port numbers [1]. Network: IP adds the source and destination IP addresses to form packets [1]. Link: the network interface adds MAC addresses for the next hop and transmits the frames on the physical medium [1].

14. (a) An IP address combined with a port number, identifying one end of a connection to a specific process [1]. (b) Well-known ports (0-1023) are fixed numbers identifying standard services on servers, e.g. 80 for HTTP [1]; client ports are temporary numbers allocated by the client’s operating system for the duration of one connection [1].

15. (a) Third byte: 150 = 10010110; AND 240 = 11110000 gives 10010000 = 144 [1]. Network identifier 10.4.144.0 [1]. (b) 160 = 10100000; AND 11110000 = 10100000 = 160, so its network identifier is 10.4.160.0 [1]. This differs from 10.4.144.0, so it is not on the same subnet [1].

16. (a) IPv4’s 32-bit addresses were running out; IPv6 uses 128 bits to give far more addresses [1]. (b) DHCP automatically allocates an IP address (and settings such as subnet mask and default gateway) to a device when it joins the network [1], from a pool, so addresses do not have to be configured manually and are not duplicated [1]. (c) Devices have private, non-routable addresses [1]. The router replaces the private source address of outgoing packets with its public address and records the mapping in a translation table, so replies can be sent back to the right device [1]. Incoming connections that were not requested have no entry, so they are not delivered [1]. Port forwarding sets a rule sending all traffic arriving on a specific public port to the games server’s private address, so other players can connect to it [1].

17. (a) WebSocket keeps a persistent, full-duplex connection open [1], so the server can push new scores to the browser immediately, without the overhead of the browser repeatedly sending new requests [1]. (b) (i) GET -> SELECT [1] (ii) POST -> INSERT [1]. (c) Any two: easier for humans to read; more compact; easier to create; easier and quicker for computers to parse [2].

18. Advantage (any one, explained): terminals are cheaper; software is installed and updated once on the server; data is stored and backed up centrally [2]. Disadvantage (any one, explained): if the network or server fails, no computer can work; performance falls when many users are active; dependence on network bandwidth [2].

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Where marks are usually lost

• Calculating subnets in denary instead of ANDing the binary values.
• In the digital signature process, using the receiver’s keys or forgetting to compare the two hashes.
• Describing CSMA/CA as detecting collisions – it avoids them; detection is CSMA/CD.
• Mixing up POP3 (retrieves email) and SMTP (sends email).

Frequently asked questions

Do I need to remember port numbers? The 7517 specification asks you to explain what well-known and client ports are for, not to list numbers. Knowing common ones such as 80 and 443 helps you give clear examples.

Is CSMA/CD examined? No. The specification states that knowledge of CSMA/CD is not required. Learn CSMA/CA with and without RTS/CTS.