22-Elec-B4 Information Technology Networks · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Engineers Canada / Professional Engineers of Ontario, National Examinations — May 2017, 16-Elec-B4 Information Technology Networks. Three hours, closed book, one approved Casio or Sharp 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 spectrum, licensing and equipment-certification framework assumed throughout is the Canadian one: Innovation, Science and Economic Development Canada (ISED) licenses the cellular bands under the Radiocommunication Act and publishes the Standard Radio System Plans (SRSP) that fix the duplex spacing referred to in Question 1(e), and Canadian carriers deploy the same 3GPP LTE numerology used in Question 1(b).
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.
Part (a) — the advantage of a layered architecture. Layering is the application of modular decomposition to a distributed system. Each layer is defined by the service it offers to the layer above and the protocol it runs with its peer at the far end, and it treats everything below it as an opaque pipe. Because the service interface is stated independently of how the layer is built, any implementation that honours the interface may be substituted, and a change confined to one layer cannot propagate upward or downward.
Four practical benefits follow, and they are what a network designer is buying. Manageable complexity: the problem of moving bits between arbitrary applications on arbitrary machines is decomposed into seven tractable problems, each solved once. Independent evolution: Ethernet has been replaced by Wi-Fi and by fibre beneath an unchanged IP layer, and HTTP has gone through three major revisions above an unchanged TCP, precisely because neither side of an interface needs to know the other's internals. Interoperability and open competition: a published interface lets equipment from different vendors, and standards written by different bodies, work together — which is the reason ISO published the reference model at all. Reuse: one transport layer serves every application, so reliability, flow control and congestion control are implemented once rather than in each program.
The cost is real and worth stating in one sentence: layering adds per-layer headers and per-layer processing, and it hides information a lower layer holds from an upper layer that could use it — which is exactly the pathology of Question 2(c), where TCP cannot see that a loss came from radio fading rather than a queue.
Part (b) — the seven layers of the OSI reference model. Named from the top down, with the one-sentence description the question asks for:
The peer-to-peer principle ties the list together: layer n at the sender communicates logically with layer n at the receiver using the header it adds, while physically the data passes down to layer 1, across, and back up. Note also that an intermediate router implements only layers 1 to 3; layers 4 to 7 exist only in the end systems, which is the structural statement of the end-to-end argument that Question 2(b) relies on.
Part (c) — where each item lives in the model. The eight items are placed as follows, with the one-line justification that earns the mark.
| Item | OSI layer | Why |
|---|---|---|
| i. The HTTP protocol | Layer 7 — Application | It is the request/response protocol a browser and a web server speak to each other; it defines a user-facing service, not a transfer mechanism. |
| ii. The TCP/IP protocol | Layers 4 and 3 — TCP is Transport, IP is Network | The name is a protocol suite, not one protocol: TCP provides the reliable end-to-end byte stream (layer 4) and IP provides addressing and forwarding (layer 3). |
| iii. Ethernet | Layers 2 and 1 — Data link (and Physical) | IEEE 802.3 defines both the MAC sub-layer — framing, 48-bit addresses, CSMA/CD — and the physical signalling of 10BASE-T, 100BASE-TX and their successors. |
| iv. Routing | Layer 3 — Network | Choosing a path across interconnected networks, and forwarding on a longest-prefix match of the logical address, is the defining function of the network layer. |
| v. End-to-end packet delivery | Layer 4 — Transport | Only the transport layer joins the two end systems process to process; layer 3 delivers host to host, hop by hop, with no end-to-end guarantee. |
| vi. Encryption and decryption | Layer 6 — Presentation | In the OSI model, transforming the syntax of the data — including ciphering — is a presentation-layer function. (In the practical TCP/IP stack, TLS sits between layers 4 and 7, and link ciphers such as WPA3 encrypt at layer 2.) |
| vii. Character display, such as ASCII | Layer 6 — Presentation | Agreeing how characters are represented as bit patterns is exactly the common-syntax role of the presentation layer. |
| viii. Signal voltages | Layer 1 — Physical | Voltage levels, timing, line codes and connector pinouts are the mechanical and electrical specification of the physical layer. |
Two of the eight deserve the extra sentence a marker looks for. Item (ii) is worth two marks precisely because it spans two layers, and an answer that names only one has missed the point of the item. Item (vi) is the place where the OSI model and real deployed practice diverge, and the strongest answer says so: the model assigns encryption to the presentation layer, while the protocols actually in service encrypt at layer 2 (WPA3, MACsec), at layer 3 (IPsec) or just above layer 4 (TLS).
