22-Elec-B4 Information Technology Networks · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Professional Engineers of Ontario annual examination, 07-Elec-B4 Information Technology Networks, May 2014. Three hours, closed book, one PEO-approved non-programmable calculator permitted. Marks are printed in the left margin; the cover page states that there are five questions and that any four constitute a complete paper worth 100 marks. All five questions and every sub-part are answered below, because this set is intended as a study resource rather than as a sat examination.
Reference texts. A. Leon-Garcia and I. Widjaja, Communication Networks: Fundamental Concepts and Key Architectures, 2nd ed. — the text listed by the Engineers Canada syllabus for this examination code; J. F. Kurose and K. W. Ross, Computer Networking: A Top-Down Approach, 8th ed.; A. S. Tanenbaum and D. J. Wetherall, Computer Networks, 5th ed.; W. Stallings, Wireless Communications and Networking, 2nd ed.; T. S. Rappaport, Wireless Communications: Principles and Practice, 2nd ed. Normative documents cited: ISO/IEC 7498-1 (the OSI reference model), IEEE 802.3 (CSMA/CD), IEEE 802.11 (wireless LAN), 3GPP TS 23.401 (the LTE Evolved Packet Core), RFC 768 (UDP) and RFC 959 (FTP).
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.
Layering decomposes an intractably large design problem into a stack of small ones. Each layer offers a defined service to the layer above through an interface, obtains a service from the layer below, and holds a protocol conversation with its peer layer at the far end. Crucially, the interface says what the layer does and says nothing about how, so an implementation can be replaced wholesale without the neighbours noticing.
That single property yields the benefits that matter in engineering practice. The same IP layer runs unchanged over Ethernet, Wi-Fi, LTE and optical fibre, because each of those is merely a data-link service that delivers a frame across one hop; conversely TCP and UDP both use IP without duplicating any of its work. Standards bodies can specify one layer at a time and different vendors can implement different layers, which is what makes multi-vendor interoperability possible at all. Development proceeds in parallel, testing can exercise one layer against a stub of its neighbours, and a fault can be localised by asking which layer's service is failing — the everyday discipline of checking the link light, then the IP address, then the routing table, then the application.
The costs deserve a sentence, since a complete answer weighs both. Every layer adds a header, so a small packet may be more overhead than payload. Functions get duplicated: error detection appears at layer 2 and again at layer 4. And information hiding can prevent an optimisation that a layer-crossing view would make obvious — the classic case being TCP interpreting a wireless bit error as congestion and slowing down, because the link layer was not permitted to tell it otherwise.
| Layer | Name | What it does |
|---|---|---|
| 7 | Application | Provides the network service the user's program actually wants, and defines the protocol it speaks with its peer. |
| 6 | Presentation | Fixes the syntax and meaning of the transmitted data, so that differing internal representations, character sets, compression and encryption are reconciled. |
| 5 | Session | Manages a dialogue between two end systems: who may transmit, checkpointing, and resynchronisation after a failure. |
| 4 | Transport | Delivers data end to end between processes on the two end systems, adding whatever multiplexing, reliability, ordering and flow control the application needs. |
| 3 | Network | Addresses and routes packets across an interconnection of networks, choosing a path and forwarding hop by hop. |
| 2 | Data link | Turns the raw bit stream into framed, error-checked delivery across a single link, and arbitrates access when the link is shared. |
| 1 | Physical | Transmits raw bits over the medium, defining signal levels, timing, modulation, connectors and pinouts. |
| Item | Layer | Reason |
|---|---|---|
| i. The FTP protocol | 7 — Application | FTP (RFC 959) is the protocol a file-transfer program speaks with its peer; it uses transport services rather than providing them. |
| ii. The UDP protocol | 4 — Transport | UDP multiplexes datagrams to processes by port number end to end, deliberately adding nothing else. |
| iii. Ethernet | 2 — Data link (MAC), and 1 — Physical | IEEE 802.3 specifies both the MAC sublayer, which frames and addresses, and the physical layer variants (10BASE-T, 1000BASE-SX) that put bits on the medium. |
| iv. Routing | 3 — Network | Choosing a path across an interconnection of networks, and forwarding on it, is the defining function of the network layer. |
| v. End-to-end packet delivery | 4 — Transport (resting on 3 — Network) | The transport layer is the lowest layer that is end to end, i.e. present only in the end systems; the network layer carries packets host to host, hop by hop, and is what makes it possible. |
| vi. Encryption and decryption | 6 — Presentation | Transforming the representation of data without changing its meaning is a presentation function in the OSI model. |
| vii. Point-to-point protocol (PPP) | 2 — Data link | PPP frames, error-checks and negotiates a single point-to-point link, exactly the data-link service. |
| viii. Modulation and demodulation | 1 — Physical | Impressing bits on a carrier and recovering them is the definition of the physical layer's job. |
Two of these deserve a note, because the honest answer is not a single number. Item (v) turns on what "end to end" is taken to mean: the network layer delivers packets between host machines, and if the phrase is read that way the answer is layer 3, but the phrase "end to end" is the standard description of the transport layer, the lowest layer implemented only in the end systems and not in the routers between them. Naming layer 4 and explaining the relationship to layer 3 is the complete answer. Item (vi) is layer 6 in the OSI model as ISO defined it, but the real deployed stack does not use a presentation layer: TLS sits between the application and the transport layer, IPsec sits at layer 3, and WPA2 encrypts at layer 2. The OSI answer is Presentation; the observation that encryption is in practice applied at whichever layer defines the boundary of trust is worth a sentence.
