This module is a continuation of Data Communication and Networking module from S3.
Why a Communications Architecture?
Communications systems are inherently complex — both to reason about and to build. Technologies in this space change rapidly, and systems often involve heterogeneous components from different vendors. To manage this complexity, three core strategies are used.
Modularization
The process of breaking a large, complex system into smaller, self-contained pieces (aka. modules). Each piece has a clearly defined job. Each module can be designed, implemented, and tested independently of the others, as long as it adheres to the agreed-upon interfaces for communicating with other modules.
Layering
Communication system is divided into layers.
Each layer performs a specific function and provides services to the layer above it. Each layer is designed to be independent of the others. The number of layers should be large enough to avoid mixing unrelated functions, but small enough to remain manageable.
An interface defines how adjacent layers interact. Each layer has 2 interfaces: one to the above and one to the below.
Matching layers on 2 different systems can communicate with each other using lower layers.
Standardization of Interfaces
To avoid non-compatible implementation of layers, the interfaces must be standardized. Different implementors, following different implementation strategies, can still interoperate as long as they conform to the same standardized interfaces.
Why Standards? Standards ensure large markets for equipment/software, enabling mass production and lower costs. They allow multi-vendor interoperability and give users flexibility in selecting products. Standards can be voluntary (e.g. ISO, CCITT) or regulatory (government-mandated).
Goals of Layered Protocols To decompose complex networks into manageable layers, provide standard interfaces between network functions, predict/control network changes, ensure symmetry across nodes, and give designers/vendors a common language.
The Seven Layers Physical (L1) — handles bit transmission; covers mechanical, electrical, functional, and procedural characteristics (e.g. RS232-C). Data Link (L2) — makes the physical link reliable; handles error detection/control, link activation and deactivation (e.g. HDLC, LAP-B). Network (L3) — provides transparent data transfer between transport entities; manages connection establishment, maintenance, and termination. Transport (L4) — ensures reliable, ordered, error-free data exchange between processes; complexity scales inversely with network layer quality (e.g. TCP). Session (L5) — controls application dialogue; manages session establishment and supports dialogue types (simultaneous, alternate, one-way) and recovery checkpoints. Presentation (L6) — handles data syntax; resolves format differences between systems (e.g. encryption, virtual terminal protocol). Application (L7) — provides application processes access to the OSI environment; supports distributed applications and management functions.
Key Terminology SAP — Service Access Point, where layers interface. The four primitives are request, indication, response, and confirm. SDU is the user data passed between layers. PCI is control information exchanged by peer entities. PDU = PCI + SDU. ICI is a temporary inter-layer parameter. IDU (= PCI + SDU + ICI) is the full unit crossing a layer boundary via the SAP.
Protocol Functions Segmentation/reassembly, encapsulation, connection control, ordered delivery, flow control, error control, multiplexing.
Connection Oriented vs Connectionless CO requires three-party agreement, has a defined lifetime, supports negotiation and connection identifiers, and maintains data unit relationships. CL is a two-party, single-access service with no negotiation; data units are self-contained and independent.
Conclusion OSI is a reference model — it defines how systems should communicate without mandating identical implementations, only requiring the same layered functions and shared peer protocols.
Terminology
Node
A router or a switch in a network.
Communication Link
A connection between two nodes for communication purposes.