1. In computer networks, communication occurs between entities in a different system.
2. An entity is anything capable of sending or receiving information.
3.Example include application programs, browsers, database management systems etc.
4. For communication to occur, the entities must agree on a protocol.
5. A protocol is a set of rules or agreements between the communicating parties, to indicate what is communicated, how it is communicated and when it is communicated.
6.Syntax: IT refers to the structure or format of the data, the order in which they are presented.
7. For example, a simple protocol may consider the first 8 bits of data to be source address, next 8 bits to be destination address and remaining its ate data.
8. Some other protocols may assume in a different order.
9.Semantics: It refers to the meaning of each section of bits.
10.Timing: It refers to two characteristics.
(1).When data should be sent.
(2).How fast it can be sent.
11. To reduce the design complexity, most networks are originated as a series of layers or levels, each one built upon the one below it.
12. The number of layers, the name of each layer, the content and the function of each layer differs from network to network.
13.The common purpose of each layer in almost all networks to offer certain services to the higher layers, shielding those layers from the details of how the offered services are actually implemented.
14. Layer n on one machine carries on a conversation with layer n on another machine.
15. The rules and conventions used in this conversation are collectively known as the layer n protocol.
16. The entities comprising the corresponding layers on the different machines are called peers.
17. In reality, no data directly transferred from layer n on one machine to layer n on another machine, instead each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.
18. Between each pair of adjacent layers, there is an interface.
19. The interface defines which primitive operations and services the lower layer offers to the upper layer.
20. A set of layers and protocols is called network architecture.