Network Protocols and Packet Switching: Computer Science 9618 (Cambridge International AS & A Level)

A city library's email system uses several different application-layer protocols to handle its members' messages.

A librarian finishes writing a message in the library's email app and sends it. The message is passed on from the library's own outgoing mail server to the mail server that looks after the recipient's mailbox, ready for the recipient to collect it later.

Which protocol is responsible for this step: passing an outgoing email message on from one mail server to another?

A telecommunications company is training new engineers on the difference between circuit switching and packet switching, using four candidate descriptions.

Which one of these four statements correctly describes packet switching?

A student's laptop runs a web browser, which sends a request over the internet to the school's web server to fetch a homework page.

(a) State the four layers of the TCP/IP protocol suite, in the order the request passes through them as it leaves the laptop. [2]

(b) Describe the purpose of the Transport layer and the purpose of the Link layer for this request. [4]

(c) State which TCP/IP layer's protocols include the specific protocol used to request and receive the homework page over the web, and name that protocol. [2]

Oakwood Books is a small independent bookshop that relies on several different application-layer protocols to run its online services.

(a) Oakwood's web developer uses an FTP client to upload updated web pages to the bookshop's web hosting server. State the purpose of FTP in this context. [2]

(b) Every month, Oakwood's mail server sends a newsletter on to each subscriber's own mail server, ready for the subscriber to read later. Describe how SMTP is used to deliver this newsletter from Oakwood's mail server to a subscriber's mail server. [2]

(c) The shop's owner checks the same order-enquiries mailbox from a desktop computer in the shop and from a phone while travelling to book fairs, and expects to see the same read/unread status and the same folders on both devices. State which is more appropriate here, IMAP or POP3, and justify your answer by describing how your chosen protocol manages the mailbox. [3]

(d) Oakwood also distributes a large digital course catalogue (several gigabytes) to several regional distributors at once, using a torrent file rather than a single direct download from Oakwood's own server. Identify the protocol being used here, and describe one way it differs from the client-server pattern used in parts (a) to (c). [3]

A remote weather sensor station needs to send a large environmental data file across the internet to a research lab's server, via two intermediate routers, Router X and Router Y.

(a) Explain why the sensor station and the research lab's server must both use the same agreed protocols for this data transfer to succeed. [2]

(b) Before transmission, the data file is divided into 5 numbered packets. Describe how packet switching, including the role of Router X and Router Y, is used to move these packets from the sensor station to the research lab's server, and explain why the packets might not arrive at the server in the order 1, 2, 3, 4, 5. [4]

(c) State one benefit and one drawback of using packet switching, rather than circuit switching, for this data transfer. [2]

(d) At the same time, two of the researchers at the lab are having a live video call with a colleague at another site. State which switching method, circuit switching or packet switching, would traditionally offer a more constant and predictable data rate and delay for the whole duration of this call, and justify your answer. [2]

A group of A Level Computer Science students is making revision flashcards, one for each layer of the TCP/IP protocol suite. On the flashcard for the Internet layer, they want to write a single sentence that correctly describes what that layer is responsible for.

Which sentence correctly describes the purpose of the Internet layer?

A hospital ward uses wireless patient monitors that each send heart-rate and oxygen-level readings over the hospital's network to a central monitoring server. Each reading leaves a monitor, passes through two internal routers, Router P and Router Q, and arrives at the server.

(a) Describe the general purpose of the Application layer for this scenario. [2]

(b) Describe the general purpose of the Internet layer for this scenario, including the role played by Router P and Router Q. [3]

(c) Explain why the readings need logical (IP) addressing at the Internet layer, in addition to the physical (MAC) addressing already used at the Link layer, as they travel from a monitor to the central server. [3]

Two offices, one in London and one in Leeds, are connected by a traditional landline telephone network. An employee in the London office picks up a handset and dials a colleague's extension in the Leeds office, and the two have a single, continuous phone conversation.

(a) Describe how circuit switching would be used to connect and carry this phone call between the two offices. [3]

(b) State one drawback of using circuit switching, rather than packet switching, for this phone call. [2]

A network technician has written four statements about protocols and switching methods on the internet, to test new trainees' understanding. Exactly one of the four statements is correct.

Which statement is correct?

A retired hobbyist checks his personal email using a single, old laptop that is permanently switched on at home, and he never needs to read his email from any other device. His mail provider gives him only a small amount of storage space on its server, so he wants every message downloaded fully onto his laptop and then removed from the server, to free up that limited space.

Which application-layer protocol is most suitable for retrieving his email in this way?