UNIT III – OVERVIEW OF THE INTERNET
OVERVIEW OF THE INTERNET
In the first section, we introduce local area networks (LANs) and wide area networks (WANs) and give brief definitions of these two types of networks. We define an internetwork, internet, as a combination of LANs and WANs. We show how an organization can create a private internet by connecting its LANs using WANs. Finally, we introduce the internet as a global internetwork made up of backbones, network providers, and customer networks.he largest computer network, the Internet, has more than one billion users. Using wired and wireless transmission media, the system connects small and large computers. It allows users to share an enormous amount of information including text, images, audio, and video. It enables users to send messages to each other. Exploring this extensive system is the major goal of this book. In this chapter, however, we have two goals. Our first is to give an overview of the internet as an inter networks (network of networks) and discuss the components that make up the internet. Part of our first goal is also to introduce protocol layering and give a glance to the TCP/IP protocol suit. In other words, the first goal is to prepare the reader for the rest of the book. Our second goal is to provide pertinent information, which is, however, not needed to understand the rest of the book. We have divided this chapter into four sections.
- In the second section, we use the concept of protocol layering to show how the task to be done by the internet is divided into smaller tasks. We discuss the five-layer protocol suite (TCP/IP) and introduce the duty of each layer and the protocol involved in each layer. We also discuss two concepts in this model: encapsulation/decapsulation and multiplexing/demultiplexing.
- In the third section, we give a brief history of the internet for interested reader. This section can be skipped without the loss of continuity.
- In the fourth section, we introduce the administration of the internet and define the standards and their lifetime. This section is only for information and is not needed to understand the rest of the book.
OVERVIEW OF THE INTERNET
Although the goal of this book is to discuss the internet, a system that interconnects billions of computer in the world, we think of the internet not as a single network. But as an internetwork, a combination of networks. Therefore, we start our journey by first defining a network. We then show how we can connect networks to create small internetworks. Finally, we show the structure of the internet and open the gate to study the internet in the next ten chapters.
A network is the interconnection of a set of devices capable of communication. In this definition, a device can be a host (or an end system as it is sometimes called) such as a large computer, desktop, workstation, cellular phone, or security system. A device in this definition can also be a connecting device such as a router which connects the network to other networks, a switch which connects device together, a modem (modulator/demodulator) that changes the form of data, and so on. These devices is in a network are connected using wired or wireless transmission media such as cable or air. When we connect two computers at home using a plug-and-play router, we have created a network, although very small.
Local Area Network
A local area network(LAN) is usually privately owned and connects some hosts in a single office, building, or campus. Depending on the needs of an organization, a LAN can be as simple as two PCs and a printer in someone’s home office, or it can extend throughout a company and include audio and video devices. Each host in a LAN has an identifier, an address, that uniquely defines the host in the LAN. A packet sent by a host to another host carries both the source host’s and the destination host’s addresses.
In the past, all the hosts in a network were connected through a common cable, which meant that a packet sent from one host to another was received by all hosts. The intended recipient kept the packed; the others dropped the packed. Today, most LANs use a smart connecting switch, which is able to recognize the destination address of the packet and guide the packet to its destination without sending it to all other hosts. The switch alleviates the traffic in the LAN and allows more than one pair to communicate with each other at the same time if there is no common source and destination among them. Note that the above definition of a LAN does not define the minimum or maximum number of hosts in a LAN .
When LANs were used in isolation(which is rare today), they were designed to allow resources to be shared between the hosts. As we will see shortly, LANs today are connected to each other and to WANs(discussed next) to create communication at a wider level.
Wide Area Network
A wide area network(WAN) is also an interconnection of devices capable of communication. However, there are some differences between a LAN and a WAN. A LAN is normally limited in size, spanning an office, a building, or a campus: a WAN has a wider geographical span, spanning a town, a state, a country, or even the world. A LAN interconnects connecting devices such as switchers, routers, or modems. A LAN is normally privately owned by the organization
that uses it; a WAN is normally created and run by communication companies and leased by an organization that uses it. We see two distinct examples of WANs today: point-to-point WANs and switched WANs.
Point-to Point WAN
A point-to-point WAN is a network that connects two communication devices through a transmission media (cable or air). We will see examples of these WANs when we discuss how to connect the networks to one another. Figure 1.2 shows an example of point-to-point WAN.
A switched WAN is a network with more than two ends. A switched WAN, as we see shortly, is used in the backbone of global communication today. We can say that switched WAN is a combination of several point-to-point WANs that are connected switches. Figure 1.3 shows an example of a switched WAN.
Today it is very rare to see a LAN or a WAN in isolation; they are connected to one another. When two or more networks are connected, they make an internetwork, or internet. As an example, we assume that an organization has two offices, one on the east coast and the other on the west coast. Each office has a LAN that allows all employees in the office to communicate with each other. To make the communication between employees at different offices possible, the management leases a point-to-point dedicated WAN from a service provider, such as a telephone company, and connects the two LANs. Now the company has an internet (with lowercase i). Communication between offices is now possible. Figure 1.4 shows this internet.
When a host in the west coast office sends a message to another host in the same office, the router blocks the message, but the switch directs the message to the destination. On the other hand, when a host on the west coast sends a message to a host on the east coast, router R1 routes the packet to router R2, and the packet reaches the destination.
Figure 1.5 shows another internet with several LANs and WANs connected. One of the WANs is a switched WAN with four switches.
We have said that an internet is a combination of links and switches such as the link layer switches and routers we used in the previous sections. In fact, an internet is a switched network in which a switch connects at least two links together. A switch needs to forward data from a link when required. The two most common types of switched networks are circuit-switched and packet-switched networks. We discuss both next.
In a circuit-switched network, a dedicated connection, called a circuit, is always available between the two end systems; the switch can only make it active or inactive. Figure 1.6 shows a very simple switched network that connects four telephones to each end. We have used telephone sets instead of computers as an end system because circuit switching was very common in telephone network today is a packet-switched network.
In the above figure, the four telephones at each side are connected to a switch. The switch connects a telephone set at one side to a telephone set at the other side. The thick line connecting two switches is a high-capacity communication line that can handle four voice communication at the same time; the capacity can be shared between all pairs of telephone sets. The switches used in this example have forwarding tasks but no storing capability.
Let us look at two cases. In the first case, all telephone sets are busy; four people at one site are talking with four people at the other site; the capacity of the thick line is fully used. In the second case, only one telephone set at one side is connected to a telephone set at the other side; only one-fourth of the capacity of the thick line is used. This means that a circuit-switched network is efficient only when it is working at its full capacity; most of the time, it is inefficient because it is working at partial capacity. The reason that we need to make the capacity of the thick line four times the capacity of each voice line is that we do not want communication to fail when all telephone sets at one side want to be connected with all telephone sets at the other side.
In a computer network, the communication between the two ends is done in blocks of data called packets. In other words, instead of the continuous communication we see between two telephone sets when they are being used, we see exchange of individual data packets between the two computers. This allows us to make the switches function for both storing and forwarding because a packed is an independent entity that can be stored and sent later. Figure 1.7 shows a small packet-switched network that connects four computers at the other site.
A router in a packet-switched network has a queue that can store and forward the packet. Now assume that the capacity of the thick line is only twice the capacity of the data line connecting the computers to the routers. If only two computers (one at each site) need to communicate with each other, there is no waiting for the packets. However if packets arrive at one router when the thick line is already working at its full capacity, the packets should be stored and forwarded in the order they arrived. The two simple examples show that a packet-switched network is more efficient than a circuit-switched network, but the packets may encounter some delays.
1.1.3 The Internet
As we discussed before, an internet (note the lowercase i) is two or more networks that can communicate with each other. The most notable internet is called the Internet (uppercase/). And is composed of thousands of interconnected networks. Figures sows a conceptual (not geographical) view of the internet.
The figure shows the internet as several backbones. Provider networks and tomer networks. At the top level, the backbones are large owned by some communication companies such as Sprint, Verizon (MCI), AT&T and NTT. The backbone networks are connected through some complex switching systems called peering points. At the second level, there are smaller networks, called providers networks. It use the services of the backbones for a fee. The providers networks are connected backbones and sometimes to other provider networks. The customer networks are works at the edge of the Internet that actually use the services provided by the Internet. They pay fees to provider networks for receiving services.
Backbones and provider networks are also called Internet Service Provider(ISPs). The backbones are often referred to as international ISPs; the provider networks are often referred to as national or regional ISPs.
1.1.4 Accessing the Internet
The internet today is an internetwork that allows any user to become part of it. The user, however, needs to be physically connected to an ISP. The physical connection normally done through a point – to – point WAN. In this section, we briefly describe how this can happen, but we postpone the technical details of the connection.
Using Telephone Networks
Today most residences and small businesses have telephone services, which means they are connected to the internet. Since most telephone networks already connected themselves to the Internet. One option for the residences and small businesses to connect to the Internet is to change the voice line between the residences or business and the telephone center to the point – to – point WAN. This can be done in two ways
Dial – up service. The first solution is to add to the telephone line a modem that converts data to voice. The software installed on the computers dials the ISP and imitates making a telephone connection. Unfortunately, the dial – up – service is very slow, and when the line is used for internet connection, it cannot be used for telephone (voice) connection. It is only useful for small residences and businesses with occasional connection to the internet. We discuss dial – up – services in Chapter 5.
DSL Service. Since the internet, some telephone companies have upgraded their telephones is to provide higher speed internet services to the residences or small businesses. The DSL service also allows the line to be used simultaneously for voice and data communication. We discuss DSL in chapter 5.
Using Cable Network
More and More residents over the last two decades have begun using cable TV services instead of antennas to receive TV broadcasting. The cable companies have been upgrading their cable networks and connecting to the internet. A residence or a small business can be connected to the internet b using this service. It provides a higher speed connection, but the speed varies depending on the number of neighbors that use the same cable. We discuss the cable networks in Chapter 5.
Wireless connectivity has recently become increasingly popular. A household or a small business can use a combination of wireless and wired connections to access the Internet. With the growing wireless WAN access, household or a small business can be connected to the Internet through a wireless WAN. We discuss wireless access in Chapter 6.
Direct Connection to the Internet
A large organization or a large corporation can itself become a local ISP and be connected to the Internet. This can be done if the organization or the corporation leases a high – speed WAN from a carrier provider and connects itself to a regional ISP. For example, a large university with several campuses can create an internetwork and then connect the internetwork to the Internet.
1.1.5 Hardware and software
We have given the overview of the Internet structure, which is made of small and large networks glued together with connecting devices. It should be clear, however, that if we only connect these pieces nothing will happen. For communication to happen, we need both hardware and software. This is similar to a complex computation in which we need both a computer and a program. In the next section, we show how these combinations of hardware and software are coordinated with each other using protocol layering.