Java Network Programming, Second Edition

The Layers of a Network

Sending data across a network is a complex operation that must be carefully tuned to the physical characteristics of the network as well as the logical character of the data being sent. Software that sends data across a network must understand how to avoid collisions between packets, how to convert digital data to analog signals, how to detect and correct errors, how to route packets from one host to another, and more. The process becomes even more complicated when the requirement to support multiple operating systems and heterogeneous network cabling is added.

To make this complexity manageable and to hide most of it from the application developer and end user, the different aspects of network communication are separated into multiple layers. Each layer represents a different level of abstraction between the physical hardware (e.g., wires and electricity) and the information being transmitted. Each layer has a strictly limited function. For instance, one layer may be responsible for routing packets, while the layer above it is responsible for detecting and requesting retransmission of corrupted packets. In theory, each layer talks only to the layers immediately above and immediately below it. Separating the network into layers lets you modify or even replace one layer without affecting the others as long as the interfaces between the layers stay the same.

There are several different layer models, each organized to fit the needs of a particular kind of network. ...