What Is the Internet

What Is the Internet

In this blog, we'll use the public Internet, a specific computer network. as our principal vehicle for discussing computer networks and their protocols. But what is the Internet? There are a couple of ways to answer this question. First, we can describe the nut and bolts of the Internet, that is, the basic hardware and software components that make up the Internet. Second, we can explain the Internet in terms of a networking infrastructure that gives services to distributed applications. Let's begin with the nuts-and-bolts description, using the following figure to demonstrate our discussion.

some pieces of the internet

A Nuts-and-Bolts Description

The Internet is a computer network that interconnects hundreds of millions of computing devices throughout the world. Not too long ago, these computing devices were mostly traditional desktop PCs. Linux workstations, and so-called servers that store and broadcast information such as Web pages and e-mail messages. Increasingly, though. nontraditional Internet end systems such as TVs, laptops, gaming consoles, cell phones, Web cams, automobiles, environmental sensing devices, picture frames, and home electrical and security systems are being linked to the Internet. In fact, the term computer network is beginning to sound a bit dated, given the many nontraditional devices that are being hooked up to the Internet. In Internet terminology, all of these devices are called hosts or end systems. As of July 2008, there were nearly 600 million end systems connected to the Internet [ISC 2009], not counting the cell phones, laptops, and other devices that are only occasionally connected to the Internet.

End systems are attached together by a network of communication links and packet switches. Well see in "The Network Edge" that there are many kinds of communication links, which are made up of different types of physical media, including coaxial cable, copper wire, fiber optics, and radio spectrum. 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, known as packets in the terminology of computer networks, are then sent through the network to the destination end system, where they are 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. Packet switches come in various shapes and flavors, but the two most famous types in today's Internet are routers and link-layer switches. Both types of switches forward packets toward their final destinations. Link-layer switches are normally used in access networks, while routers are normally used in the network core. 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 a route or path through the network. The exact amount of traffic being carried in the Internet is not easy to estimate [Odylsko 2003]. PriMetrica [PriMetrica 2009] estimates that 10 terabits per second of international capacity was used by public Internet providers in 2008, and that capacity doubles about every two years.

Packet-switched networks (which transport packets) are in many ways similar to transportation networks of highways, roads, and intersections (which transport vehicles). Think about, for instance, a factory that needs to move a large amount of cargo to some destination warehouse located thousands of kilometers away. At the factory, the cargo is segmented and loaded into a fleet of trucks. Each of the trucks then alone travels through the network of highways, roads, and intersections to the destination warehouse. At the destination warehouse, the cargo is unloaded and grouped with the rest of the cargo arriving from the same shipment. Thus, in many ways, packets are similar to trucks, communication links are similar to highways and roads, packet switches are similar to intersections, and end systems are similar to buildings. Just as a truck takes a path through the transportation network, a packet takes a path through a computer network.

End systems access the Internet through Internet Service Providers (ISPs), including residential ISPs for example local cable or telephone companies; corporate ISPs; university ISPs; and ISPs that provide WiFi access in airports, hotels, Coffee shops, and other public places. Each ISP is in itself a network of packet switches and communication links. ISPs give a variety of types of network access to the end systems, including 56 kbps dial-up modem access, residential broadband access such as cable modem or DSL, high-speed local area network access, and wireless access. ISPs also give Internet access to content providers, connecting Web sites directly to the Internet. The Internet is all about connecting end systems to each other, so the ISPs that give access to end systems must also be interconnected. These lower-tier ISPs are interconnected through national and international upper-tier ISPs such as AT&T and Sprint. An upper-tier ISP comprises high-speed routers interconnected with high-speed fiber-optic links. Each ISP network, whether upper-tier or lower-tier, is managed independently, runs the IP protocol (see below), and conforms to certain naming and address conventions. We'll examine ISPs and their interconnection more closely in "The Network Core".

End systems, packet switches, and other pieces of the Internet run protocols that manage the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol identifies the format of the packets that are sent and received among routers and end systems. The Internet's principal protocols are collectively known as TCP/IP. We'll begin looking into protocols in this introductory section. But that's just a start - much of this blog is concerned with computer network protocols!

Given the importance of protocols to the Internet, it's important that everyone agree on what each and every protocol does. This is where standards come into play. Internet standards are made by the Internet Engineering Task Force (IETF)[IETF 2009]. The IETF standards documents are called requests for comments (RFCs). RFCs started out as common requests for comments (hence the name) to resolve network and protocol design problems that faced the forerunner to the Internet. RFCs tend to be quite technical and detailed. They describe protocols such as TCP, IP, HTTP (for the Web), and SMTP (for e-mail). There are currently more than 5,000 RFCs. Other bodies also specify standards for network components, most particularly for network links. The IEEE 802 LAN/MAN Standards Committee [IEEE 802 2009], for instance, specifies the Ethernet and wireless WiFi standards.


distributed applications, packet switches, transmission rate

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