Packet Switching (Cambridge (CIE) O Level Computer Science)

Worked Example

A local market shop wishes to arrange a delivery of goods from a supplier. Anna, the shop owner, decides to send an email to request the delivery of the goods at a certain date and time.

Describe how packet switching is used to send this email and how it can be protected from corruption. 

[8]

Answer

• The business email is first broken down into packets which are given a source address (where its come from) and a destination address (where its going to) [1]

• Each packet receives a packet number so that the email can be reassembled when it reaches its destination  [1]

• Each packet also receives an error check such as a parity bit. A parity bit checks whether any bits have been flipped due to corruption  [1]

• Each packet is sent over the internet via routers. Routers contain routing tables that determine the next closest router to the destination  [1]

• Packets may take different routes depending on internet traffic and arrive at their destination in any order  [1]

• Packets are checked for errors using the error checks and missing packets can be requested to be resent [1]

• Once all packets have been received then they can be put together in order using the packet numbers [1]

• Once assembled the original email can be read by the other business [1]

Exam Tip

For high marks make sure your answer is coherent, that is it follows logically from one point to the next.

Some marks depend on previous points you have made.

Explaining parity bits without mentioning error checking first may not gain you additional marks