Question 1 of 7: The OSI Reference Model and ISO

Question 1: The OSI Reference Model and ISO (20 marks)

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.

(a) How OSI and ISO are related

ISO (the International Organization for Standardization) is the international standards body — a federation of national standards organizations — that develops and publishes voluntary technical standards across many fields, including information technology and telecommunications. OSI (Open Systems Interconnection) is not itself a body; it is a specific reference model that ISO standardized (jointly with the ITU-T, as ISO/IEC 7498-1 and ITU-T Recommendation X.200) in the early 1980s. OSI describes, in seven layers, how two heterogeneous computer systems can interoperate over a network without either side depending on the other's proprietary hardware or software. In short, ISO is the organization; OSI is one particular standard that organization produced. The OSI model is used today mainly as a reference and teaching framework for describing and comparing the functions of real protocol stacks (including the de facto TCP/IP suite, whose layers are routinely explained by mapping them onto the OSI layers).

(b) The seven OSI layers, in sequence

From the physical medium up to the end-user application, the seven OSI layers are:

1. Physical (Layer 1). Transmission of raw bits over the physical medium — voltages, radio waves, or light — including connectors, line coding, and bit timing.
2. Data Link (Layer 2). Reliable transfer of frames across a single physical link, including framing, MAC addressing, and link-level error detection (e.g. Ethernet).
3. Network (Layer 3). Logical addressing and routing of packets across multiple links between the source and destination hosts (e.g. IP).
4. Transport (Layer 4). End-to-end delivery between processes on the source and destination hosts, including segmentation and, where required, reliability and flow/congestion control (e.g. TCP).
5. Session (Layer 5). Establishes, manages, and terminates dialogues (sessions) between two communicating hosts, including synchronization checkpoints within a long exchange.
6. Presentation (Layer 6). Data representation — encryption/decryption, compression, and translation between application-level data formats (e.g. character encoding, serialization) — so the two end applications agree on how bits map to meaning.
7. Application (Layer 7). User-facing network services (e.g. HTTP, FTP, SMTP, DNS) that interface directly with application software.