OCR A-Level Computer Science: Networks & Communication

When an email client uploads a message to a mail server across the Internet, the data is handled by the TCP/IP protocol stack and carried by packet switching.

(a) The TCP/IP model has four layers. Name the four layers in order from top to bottom, state the role of each, and describe encapsulation — what happens to the user's data as it passes down the stack at the sending host. [6 marks]

(b) The message is too large for one packet and is split into several. State two items of data, other than the payload, that a packet header must contain, and describe how the packets are routed independently across the network and reassembled at the destination. [4 marks]

(c) Give one advantage of packet switching over circuit switching. [2 marks]

The following scenario was written for this exercise.

Ledgerwise is a small accountancy firm of twelve staff that is moving to remote working. Employees will now connect from home over the public Internet to a central server that holds clients' financial records and tax data, and the firm has launched a customer portal where clients can log in to upload documents. The staff are accountants, not IT specialists, and several reuse the same simple password across systems. The partners want to understand the security risks of this change before committing to it.

Evaluate the threats to Ledgerwise's network security and the measures it could take to protect against them, reaching a justified conclusion about where its protection effort should be concentrated. [9 marks]

Different protocols operate at different layers of the TCP/IP stack.

(a) For each of the following four protocols, state the layer of the TCP/IP stack at which it operates and state, in one line, its purpose: HTTPS, SMTP, TCP, IP. [4 marks]

(b) HTTPS is preferred over HTTP for the login page of a website. State the one difference between them that makes HTTPS suitable for a login page. [1 mark]

(c) Both TCP and IP are needed to deliver a web page. State, in one line, why both are needed — i.e. the distinct job each does. [1 mark]

The following scenario was written for this exercise.

A video-streaming start-up is deciding how to deliver its library of films. The same popular films are watched by millions of users, and the company wants to avoid the cost of a single huge server farm and to keep the service running even if some machines go offline. An engineer suggests distributing the content using a peer-to-peer (P2P) model in which each viewer's device also shares the parts of a film it has already downloaded, rather than the conventional client-server model where every user streams from the company's own servers.

Compare the client-server and peer-to-peer models for this use, giving one advantage of each, and recommend, with justification, which model is more appropriate here. [5 marks]

A network can be wired in different physical topologies. Two options being considered are a star topology and a (full) mesh topology.

Compare the star and mesh topologies by giving one advantage and one disadvantage of each. [4 marks]

Two common pieces of network hardware are a router and a switch, and two kinds of address are a MAC address and an IP address.

(a) Distinguish between a router and a switch by stating the job of each. [2 marks]

(b) State one difference between a MAC address and an IP address. [1 mark]