Studyledge

Data Transmission, Error Checking and Encryption: Question 3

Syllabus 2.1

Structured 9 marks

A university research group in London emails a 400 MB dataset file to a collaborator's laptop in Singapore over the internet.

(a) Describe how the dataset file is prepared before it can be sent across the network, referring to packets, headers, payload and trailer. [3]

(b) The packets that make up the file are sent using packet switching rather than as one continuous stream of data. Describe what happens to each packet during packet switching, referring to the role played by routers. [3]

(c) State why packets belonging to the same file might arrive at the collaborator's laptop in a different order from the order in which they were sent, and explain how the laptop is still able to reconstruct the original file correctly. [3]

Show worked solution Hide worked solution

Worked solution

Part (a): Preparing the file for transmission

Before the 400 MB file can be sent, it is split into a large number of smaller packets. Each packet is built from three parts:

  • a header, holding the destination address (the collaborator’s laptop), the originator’s address (the London server) and a packet number showing where that packet belongs in the file;
  • a payload, the actual small chunk of the file’s data carried by that particular packet;
  • a trailer, which marks where the packet ends.

Part (b): Packet switching and the role of routers

Rather than travelling as one continuous stream, each packet is transmitted independently across the network. Whenever a packet reaches a junction in the network, a router examines the destination address in its header and forwards it along whichever available path looks best at that moment, for example, the least congested connection. Because this decision is made separately for every packet, packets from the same file can end up travelling along different routes between London and Singapore, rather than all following one identical path.

Part (c): Why packets can arrive out of order, and how the file is rebuilt

Since routers choose a path for each packet independently, one packet might cross a busy or physically longer route while another takes a quicker path, so packets do not necessarily arrive in the order they were sent. This is not a problem, however, because every packet’s header includes a packet number. Once all the packets belonging to the dataset file have reached the collaborator’s laptop, it uses these packet numbers to put the payloads back into the correct sequence, reconstructing the original 400 MB file exactly.

Final answers

  • (a) The file is split into packets, each with a header (destination address, originator’s address, packet number), a payload (a chunk of file data) and a trailer (marks the packet’s end).
  • (b) Each packet is routed independently; routers read the destination address in the header and forward each packet along the best available path at that moment, so packets can take different routes.
  • (c) Independent routing means packets can arrive out of order; the packet number in each header lets the laptop reassemble the file correctly regardless of arrival order.