AQA A-Level Computer Science: Networking & Communication

A user on a home computer requests a web page from a server on the other side of the country. The request travels across the Internet using the TCP/IP protocol stack and packet switching.

(a) The TCP/IP model is divided into four layers. Name the four layers of the TCP/IP stack, in order, and give one protocol that operates at each. [4 marks]

(b) Describe what is meant by encapsulation, explaining what is added to the user's data as it passes down the stack at the sending computer, and naming the data unit produced at each layer. [4 marks]

(c) The request is too large for a single packet and is split into several packets. Explain how packet switching delivers these packets across the network and how the original message is reassembled at the destination. [4 marks]

The following scenario was written for this exercise.

Harborview Dental is a group of dental practices whose receptionists, dentists and managers use a networked patient-records system from desktops across several sites, with a public-facing website for online appointment booking. The records hold sensitive personal and medical data. The practice manager is concerned about the security of the network and is deciding which protective measures to invest in.

Discuss the main security threats to Harborview Dental's network and evaluate the measures it should use to protect against them. Conclude with a justified recommendation about where its protection effort should be concentrated. [9 marks]

Two transport-layer protocols are available to an application developer: TCP and UDP.

A team is building two products: a file-download service (where the downloaded file must be a perfect, complete copy) and a live voice-chat feature (where keeping the conversation in real time matters more than recovering every lost fragment).

Compare TCP and UDP, and for each of the two products state with justification which protocol is the more appropriate choice. [6 marks]

The following scenario was written for this exercise.

A secondary school is setting up a network for 400 staff and students. All users must log in with central accounts, shared files and the management information system must be stored centrally and backed up nightly, and access to resources must be controlled by the IT team.

Compare the client-server and peer-to-peer network models, and recommend, with justification, which is the more appropriate model for the school. [5 marks]

Two forms of encryption are used to protect data: symmetric encryption and asymmetric (public-key) encryption.

Explain the difference between symmetric and asymmetric encryption in terms of the keys used, and describe how a sender would use asymmetric encryption to send a confidential message to a recipient so that only that recipient can read it. [4 marks]

When a message is sent across a packet-switched network it is divided into packets.

Define what is meant by a packet, and state two items of data, other than the payload, that a packet header must contain and why each is needed. [3 marks]