The Internet is a computer network that interconnects billions of computing devices throughout the world. In Internet jargon, the devices that exploits Internet connection are called end systems or hosts.
End systems are connected together by a network of communication links and packet switches. Different links can transmit data at different rates, with the transmission rate of a link measured in bits/seconds. When one system wishes to send some data to another system, it splits the data and put resulting segments into the packages which are called packets.
A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links.
End systems access the Internet through Internet Service Providers (ISPs). Each ISP is in itself a network of packet switches and communication links. An upper-tier ISP consists of high-speed routers interconnected with high-speed fiber-optic links.
End systems, packet switches, and other pieces of the Internet run protocols that control sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and Internet Protocol (IP) are two of the most important protocols in the Internet.
From another side of view, Internet can be defined as an infrastructure that provides services to applications. End systems attached to the Internet provide a socket interface that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system. Socket Interface is a set of rules that the sending program must follow.
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.
End systems are also referred to as hosts because they host application programs. Hosts are sometimes further divided into two categories: clients and servers.
Access networks — the network that physically connects and end system to the first router on a path from the end system to any other distant end system.
Today, the most prevalent types of broadband residential access are digital subscriber line (DSL) and cable.
Physical media fall into two categories: guided media and unguided media
| Media | Brief | Rates | Prices |
|---|---|---|---|
| Twisted-Pair Copper Wire | The least expensive and most commonly used guided transmission medium. It consists of two insulated copper wires, each about 1 mm. | 10Mbps — 10 Gbps (6a cable) | Low, the most spreaded |
| Coaxial Cable | It consists of two copper conductors, but they are concentric rather than parallel. | Hundreds of Mbps | Still quite a common |
| Fiber Optics | Thin, flexible medium that conducts pulses of light. | from 51.8 Mbps up to 39.8 Gbps. | Moderately higher |
| Terrestrial Radio Channels | Radio channels carry signals in the electromagnetic spectrum. Highly dependent on the propagation environment. | Unspecified | Unspecified |
| Satellite Radio Channels | Links two or more Earth-based microwave transmitter/receivers known as ground stations. The satellite receives transmissions on one frequency band, regenerates the signal using a repeated and transmits the signal on another frequency. Subdivided into geostationary satellites and low-earth orbiting (LEO) satellites | Unspecified | Unspecified |
In a network application, end systems exchange messages with each other. Messages can contain anything the application designer wants. Between source and destination, each packet travels through communication links and packet switches.