CAP7001 · Unit 1: Network basics
Protocols, layered architecture and the OSI model
12 practice questions · in the MTE and ETE syllabus
Key points
- Protocol elements: syntax, semantics, timing
- OSI has 7 layers; PDUs: bit, frame, packet, segment
- Presentation = translation/encryption; session = dialog control
- Router works up to the network layer
A protocol defines how devices communicate; its three key elements are syntax (format), semantics (meaning) and timing (when and how fast). The OSI model splits communication into seven layers, and the same layers on two hosts talk by peer-to-peer processes.
| # | Layer | PDU / job |
|---|---|---|
| 7 | Application | User services |
| 6 | Presentation | Translation, encryption, compression |
| 5 | Session | Dialog control, synchronization |
| 4 | Transport | Segments, process-to-process |
| 3 | Network | Packets, logical addressing, routing |
| 2 | Data link | Frames, MAC addressing, framing |
| 1 | Physical | Bits, signals, media |
On the way down each layer adds a header (the data link layer also adds a trailer); this is encapsulation.
Worked example: 1000 bytes of data plus three 20-byte headers and a 4-byte trailer gives 1064 bytes on the wire.
Exam pitfall: a router looks only as high as the network layer; it does not read transport data when forwarding.
Try two questions
Two hosts exchange integers over a TCP connection. All segments arrive in order and error-free, but the receiver reads each integer wrongly because the sender stores bytes in a different order. Which OSI layer's function is meant to resolve this mismatch?
- Session layer
- Presentation layer
- Transport layer
- Data link layer
Show the answer
B. Translating data formats such as byte order between systems is a presentation-layer job. The transport layer already delivered the bytes correctly, so it is not at fault.
Why not A: The session layer handles dialog control and checkpoints, not data formats.
Why not C: The transport layer already provided correct, ordered delivery here.
Why not D: The data link layer works on frames between neighbours and does not interpret the data.
A 4000-byte message is sent in segments of 1000 data bytes. Each segment gets a 20-byte transport header and a 20-byte network header, and each packet gets an 18-byte data link header and trailer. How many bytes cross the link in total?
- 4058 bytes
- 4160 bytes
- 4232 bytes
- 4072 bytes
Show the answer
C. There are 4 segments and each is 1000 + 20 + 20 + 18 = 1058 bytes, so the total is 4232. Counting the headers once, or leaving out the frame overhead, misses the per-segment cost.
Why not A: That adds the three headers just once to the whole message.
Why not B: That leaves out the 18-byte data link overhead on each frame.
Why not D: That counts only the 18-byte frame overhead for each of the four frames.