22-Elec-B4 Information Technology Networks · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Engineers Canada / Professional Engineers of Ontario, National Examinations — December 2017, 16-Elec-B4 Information Technology Networks. Three hours, closed book, a PEO-approved non-programmable calculator permitted. Five questions of 25 marks each; any four constitute a complete paper worth 100 marks, and the marks are printed in the left margin against every sub-part. All five questions are solved here, because this set is a study resource rather than an exam attempt.
Reference texts.
Canadian context. The addressing and numbering practice assumed throughout is the Canadian one: IPv4 and IPv6 blocks used by Canadian networks are allocated by ARIN, of which Canada is part, and the national research network CANARIE has run production IPv6 since the mid-2000s, which is why the IPv6 answer in Question 1(c) is the operational rather than the theoretical response. Circuit-switched telephony in Question 5 refers to the Canadian PSTN as regulated by the CRTC.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The printed paper writes part (b) as “three layers from your answer in part b”, which is self-referential; part (a) is the list of OSI layers being drawn on, so it is read as “part a” here.
Approach. Part (a) is a definition question and the instruction to be brief is a real marking rule, so each layer gets one sentence naming its service, not its protocols. Part (b) collapses that stack onto the four-layer TCP/IP model, and part (c) is answered by asking, for each item, at what scope the function is performed: one link, one network, end to end between processes, or above that.
Part (a) — the seven layers. The OSI reference model divides communication into seven layers, each offering a service to the layer above and using the service of the layer below.
| No. | Layer | One-sentence description |
|---|---|---|
| 7 | Application | Provides the network service that the user's program actually invokes, such as file transfer, mail or web access. |
| 6 | Presentation | Fixes the syntax of the transferred data — character coding, data representation, compression and encryption — so that dissimilar machines agree on meaning. |
| 5 | Session | Establishes, manages and synchronises a dialogue between two applications, including checkpointing and recovery of long exchanges. |
| 4 | Transport | Delivers data end to end between processes on the two end systems, with whatever multiplexing, sequencing, error recovery and flow control the service requires. |
| 3 | Network | Provides logical addressing and chooses the route by which packets cross one or more intervening networks. |
| 2 | Data link | Turns the raw bit stream of one link into frames and controls access to the medium and errors on that single link. |
| 1 | Physical | Transmits raw bits over the medium, defining voltages, modulation, frequencies, timing and connectors. |
Part (b) — the four TCP/IP layers. The TCP/IP model keeps the application, transport and network layers of the OSI stack — renaming the third the Internet layer — discards the separate session and presentation layers, whose functions are left to the application, and adds one layer that is not an OSI layer at all: the network interface (or network access / host-to-network) layer, which merges the OSI data link and physical layers into a single “whatever the local network technology does” layer.
Part (c) — where each function lives. The scope test settles every item: a function confined to one link is layer 2 (with its signalling in layer 1), a function that spans networks is layer 3, a function between the two end processes is layer 4, and a function about the meaning or representation of the data is layer 6 or 7.
| Item | Layer(s) | Reason |
|---|---|---|
| i. Local-area networking | 2 (data link), with 1 (physical) | A LAN is defined by its medium-access and framing rules — the MAC sublayer of the data link layer — carried on a physical medium; IEEE 802 standards are written across layers 1 and 2. |
| ii. Encryption | 6 (presentation) | It transforms the representation of the data rather than moving it; in the OSI model that is exactly the presentation layer's job (in the TCP/IP world TLS sits between transport and application). |
| iii. Routing | 3 (network) | Choosing a path across interconnected networks is the defining service of the network layer. |
| iv. End-to-end error correction | 4 (transport) | “End to end” means between the two end systems, which is the transport layer's scope; TCP's retransmission is the standard example. |
| v. SMTP, HTTP, or FTP | 7 (application) | These are the protocols invoked directly by user programs. |
| vi. Specifications for LTE bands | 1 (physical) | Carrier frequencies, bandwidths and modulation are properties of the signal on the medium. |
| vii. Character display, such as ASCII | 6 (presentation) | Character coding is a data-representation convention, the presentation layer's classic example. |
| viii. Error detection over a single link | 2 (data link) | The word “single” fixes the scope at one hop, where the frame check sequence lives — the deliberate contrast with item iv. |
Items iv and viii are the pair the examiner is testing: the same function, error control, performed at two different scopes, and the answer is decided by the words “end-to-end” and “single link” rather than by the mechanism.
| Part | Result |
|---|---|
| (a) OSI layers, 7 down to 1 | Application, Presentation, Session, Transport, Network, Data link, Physical |
| (b) TCP/IP layers | Application, Transport, Internet (network), Network interface — the last being the non-OSI “layer” |
| (c) i / ii / iii / iv | Data link (2), with physical (1) / Presentation (6) / Network (3) / Transport (4) |
| (c) v / vi / vii / viii | Application (7) / Physical (1) / Presentation (6) / Data link (2) |
