Computer Networks and the Internet
description
The Internet is a computer networks that interconnects hundreds of millions of computing devices through the world. Today not only computers and workstation are being connected to the network, therefore the term computer network may sound a bit dated.
All the devices connected to the Internet are called hosts or end systems. End systems are connected together by a network of communication links and packets switches.
Different links can transmit data at different rates, with the transmission rate of a link measured in bits/second.
When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packages of information, called packets, are then sent through the network to the destination and system where they a reassembled into the original data.
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. The two most prominent types of packets switches are routers and link switches. The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as route or path.
End systems access the Internet through Internet Service Providers (ISPs), including residential ISPs (cable or phone company), corporate, university ISPs … Each ISP in itself is a network of packet switches and communication links. Lower tier (which interconnect end-systems) ISPs are interconnected through national and international upper tier ISP. An upper-tier ISP consists of high speed routers interconnected with high-speed fiber-optic links. Each ISP network is managed independently.
End systems, packet switches and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet.
The Internet can be described as an infrastructure that provides services to applications. These applications (Web, social networks, VoIP…) are said to be distributed since they involve multiple end systems that exchange data with each other. Internet applications run on end systems, not in the packet switches or routers, packet switches facilitate the exchange of data, but they are not concerned with the application that is the source or sink of data.
End systems attached to the Internet provide and Application Programming Interface (API) 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.
All the activity in the Internet that involves two or more communicating remote entities is governed by a protocol.
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as weel as the actions taken on the trasmission and/or receipt of a message or other event
Computers and other devices connected to the Internet are often referred to as end systems as they sit at the edge of the Internet. They are also called hosts as they host, run, applications programs such as a Web Browser or an email client.
Hosts are sometimes further divided into two categories: clients and servers. The former being desktop, mobile pcs, smartphones, the latter being powerful machines that store and distribute Web pages, streams… Nowadays most of the servers reside in large data centers