REFERENCE MODELS FOR COMPUTER NETWORKS
Much of this material is taken from some developed by Ron Wyllys.
Two major approaches:
1. The seven-layer OSI/ISO model – Open Systems Interconnection,
currently maintained by the International Organization for
Standards.
2. The five-layer TCP/IP model – Transmission Control
Protocol/Internet Protocol.
OSI/ISO
1. Physical layer – controls electrical and mechanical aspects of data
transmission, e.g., voltage levels, cable lengths, and so on.
2. Data-link layer – addresses the transmission of data frames (or
packets) over a physical link between network entities, includes
error correction.
3. Network layer – establishes paths for data between computers and
determines switching among routes between computers, determines
how to disaggregate messages into individual packets.
4. Transport layer – deals with data transfer between end systems and
determines flow control.
5. Session layer – creates and manages sessions when one application
process requests access to another applications process, e.g.,
MSWord importing a spread sheet from Excel.
6. Presentation layer – determines syntactic representation of data, e.g.,
agreement on character code like ASCII/Unicode.
7. Application layer – establishes interface between a user and a host
computer, e.g., searching in a database application.
TCP/IP
1. Physical layer – not really part of this model, since TCP and IP deal
with software; usually thought to refer to all hardware beneath the
network layer.
2. Network or data link layer – defined by whatever the Internet
Protocol will run over, e.g., a token-ring network.
3. Internet or network layer – provides network addressing and
routing, providing a common address space and connecting
heterogeneous networks. IP runs here.
4. Transport layer – manages data-consistency by providing a reliable
[two meanings!!] byte stream between nodes on a network. TCP and
User Datagram Protocol (UDP) run here.
5. Process and applications layer – provides application services to
users and programs.