09 — Practice solutions
These are independently written explanations, not an official marking scheme.
09A
Standardised protocols/interfaces let products from different vendors communicate. Modularity allows a layer’s implementation or medium to change while preserving its interface, reducing changes elsewhere. Separating functions helps isolate faults to a layer, making diagnosis more manageable. Simply writing “standardisation, flexibility, troubleshooting” omits the explanations.
Reasoning
Three benefits need three distinct mechanisms. Interoperability concerns agreement between implementations; modular change concerns replacing a component; diagnosis concerns locating a fault. Rewording “easier to maintain” three times would not establish three distinct points.
09B
R receives and removes the incoming link frame, processes the IP destination and selects an outgoing route, then encapsulates the packet in a new link frame for the next hop. The destination IP normally remains B’s address, while destination MAC identifies the receiver on the current Ethernet link. TCP is transport; HTTP is application.
Reasoning
Separate the two links A–R and R–B. Each needs its own frame. The routing task uses the packet’s IP destination; the outgoing link needs an address for the next receiver. Then answer the two layer classifications explicitly rather than leaving them implied.