Protocols
Jyrki T. J. Penttinen · 2015
This chapter describes theories of protocols, including the Open Systems Interconnection (OSI) model of International Organization for Standardization (ISO), and typical protocols adapted to modern communications systems. It also describes the realities of live networks, and provides reasons why the theoretical models are not necessarily deployed as such everywhere. The aim of OSI is to present standardized abstraction layers for communications systems. Each of the OSI layer communicates with underlying and overlying protocol layers such that underlying protocol adds a certain amount of information to the information flow, like headers and bits required by the protection mechanisms. Transfer control protocol/Internet protocol (TCP/IP) is the facto standards of the IP networks for long time. User Datagram Protocol (UDP) is a transport layer protocol, and is thus working in the OSI model. Link access protocol in D channel (LAPD) is capable of multiplexing several connections into a single physical channel between users.