Network Protocols and Packet Switching: Question 7

Worked solution

Part (a): Purpose of the Application layer

The Application layer is where the actual reading data originates and is ultimately used. The monitor’s own monitoring software creates each heart-rate and oxygen-level reading and formats it ready to be passed down through the lower layers to be sent across the network. At the receiving end, the central server’s monitoring application is what receives, interprets and displays the reading once it arrives. This layer is not responsible for moving the data across the network itself, only for producing it at the source and making sense of it at the destination.

[2 marks]: [1] for identifying the Application layer as the layer at which the reading data is created/used by the monitor’s and server’s own software, [1] for a correct, complete description linking this to both ends (sending monitor and receiving server).

Part (b): Purpose of the Internet layer, and the role of Router P and Router Q

The Internet layer adds logical (IP) addressing to each packet of reading data, recording the source and destination host, and is responsible for getting each packet across from the monitor’s own network to the central server’s network, however many networks lie in between.

Router P and Router Q both operate at this layer. When a packet reaches Router P, Router P examines the packet’s destination IP address and independently decides the most appropriate next hop for that packet, for example, forwarding it on towards Router Q. Router Q then repeats this process, examining the same packet’s destination IP address and deciding the next hop needed to reach the central server’s network.

[3 marks]: [1] for identifying logical (IP) addressing as the Internet layer’s job, [1] for stating Router P and Router Q operate at this layer, [1] for describing that each router independently examines the destination address and decides the next hop.

Part (c): Why logical (IP) addressing is needed alongside MAC addressing

A MAC address only identifies a device on the one local link it is directly attached to. As a packet crosses from the monitor’s local network, through Router P and Router Q, to the server’s network, the MAC addressing used is actually replaced at every hop, since each router re-addresses the packet for the next local link along the way. This means MAC addressing alone cannot describe the packet’s overall journey from monitor to server.

A logical (IP) address, by contrast, stays the same for the entire journey. The packet’s source and destination IP addresses do not change as it passes through Router P and Router Q. This constant, end-to-end addressing is what allows each router to work out which network the packet still needs to reach, and so which direction to forward it in, something the constantly-changing, purely local MAC addressing cannot provide on its own.

[3 marks]: [1] for stating a MAC address only identifies a device on a single local link and changes at each hop, [1] for stating an IP address stays constant for the whole source-to-destination journey, [1] for linking this to why routers need it to decide each packet’s onward direction between networks.

Final answers

• (a) The Application layer is where the readings are created (by the monitor’s software) and used (by the server’s monitoring application), not where they are moved across the network.
• (b) The Internet layer adds logical (IP) addressing and gets each packet to the correct destination network; Router P and Router Q each independently read a packet’s destination IP address and decide its next hop.
• (c) MAC addressing only identifies a device on one local link and changes at each router; IP addressing stays constant end-to-end, which is what lets routers direct the packet between networks.