7.1 Anatomy of the Internet

The Internet is a vast collection of computers linked by cable and satellites, not controlled by any one authority, but all operating under common network protocols. The term 'Internet' includes both the hardware (satellites, cable, routing devices and computers) and the software (programs and network protocols) that enable computers to communicate with each other.

When information is sent across the Internet, the Transmission Control Protocol (TCP: the networking-language computers use when communicating over the Internet) first breaks the information up into packets of data. The client computer sends those packets to the local network, Internet service provider (ISP), or online service. From here, the packets travel through many levels of networks, computers, and communications lines until they reach their final destinations. Many types of hardware help the packets on their way. These are:

Hubs, which link groups of computers together and let them intercommunicate through multiple ports.
Bridges, which link local area networks (LANs) with each another.
Gateways, which act like bridges, but also convey data between dissimilar networks.
Repeaters, which amplify the data at intervals so that the signal doesn't weaken.
Routers, which ensure packets of data arrive at their proper destination across different technologies, media, and frame formats.
Servers, which deliver web pages and other services as requested.
Client computers, which make the initial request for Internet services, and run applications to handle those services.
Cables and/or satellite communications, which make the hardware connections.

All hardware units need common operating methods, basic instructions called protocols that specify to all parties how the data will be handled.

Internet Connections

Physical Internet connections are effected with:

1. Twisted wire: two insulated copper wires twisted into pairs for ordinary telephone communications, and 4 pairs of copper cabling for Internet networks. Transmission speeds range from 2 Mbps to 100 Mbps. (Transmission speed or bandwidth is measured in bits per second, where K a thousand, M a million, and G is a thousand million.)

2. Coaxial cables: copper or aluminum wire wrapped with an insulating and flexible material: widely used for cable television systems, office buildings, and for local area networks generally. Transmission speeds range from 200 Gbps to over 500 Gbps.

3. Optical fiber cable: one or more filaments of glass fiber wrapped in protective layers: not affected by electromagnetic radiation. Transmission speeds may exceed 1000 Gbps.

Satellite or Wireless connections are made with:

1. Terrestrial microwave transmitters and receivers placed on 'line of sight' locations on tops of buildings and elevated ground, usually assisted by relay stations spaced approximately 30 miles apart.

2. Communications satellites using microwave radio as their telecommunications medium, which is not deflected by the Earth's atmosphere. Such earth-orbiting systems can receive and relay voice, data, and TV signals.

3. Cellular and PCS systems using radio communications technologies, which are often specific to individual countries. Each area or cell employs a low-power transmitter or radio relay antenna device to relay calls from one cell to the next.

4. Wireless LANs using both high- and low-frequency technologies to enable communication between several devices in a limited area (e.g. Wi-Fi, BlueTooth, WiMax, UWB and ZigBee).

Networks are commonly designated as LAN ( local area network) WAN (wide area network), MAN (metropolitan area network), PAN (personal area network), VPN ( virtual private network), CAN (campus area network) and SAN (storage area network).

Wireless communication spans the electromagnetic spectrum from 9 kHz to 300 GHz. Satellite signals travel at the speed of light, but the distances involved induce a time-delay called 'latency'. A 71,000 km separation of transmitter and receiver, for example, will induce a latency of 473 ms, often noticeable on international calls.

It is often convenient to recognize four levels of Internet connection:

1. The 'backbones' are the main "trunk" connections of the Internet, carrying data at high speeds by fiber-optic cables and satellite links across the countries, continents and oceans of the world. Bandwidth is a measure of data that can be transferred per unit time, and in the US these backbones have bandwidths of 155 Mbps to 2.5 Gbps. Backbones are owned and operated by Network Service Providers, major companies like AT&T, Verizon, and AOL. Built into this network is redundancy, transmission surplus to demand but kept in reserve should there be traffic peaks, or breaks in the network.

2. Backbones 'step down' to regional and local networks at hubs, once called Network Access Points or Metropolitan Area Exchanges but now Internet Exchange Points (IXPs), which are again under the ownership and control of NSPs (Network Service Providers).

3. High-speed switching computers make the connection to the local networks, here leased by NSPs to government departments, campus area networks, large companies and ISPs (Internet Service Providers).

4. ISPs, the most familiar to the public, are retail providers, covering the 'last mile' to offer Internet access to client computers, i.e. those in homes and small businesses. ISPs include telecom giants but are generally national or local companies.

Routers

Routers ensure that all data gets sent to its intended destination by the most efficient route. They open the IP packets of data to read the destination address, calculate the best route, either to its final destination, or to another router closer to that destination, repeating this until the destination is reached. To find the optimal route, routers employ an internal database called a routing table. There are two types. A static table specifies unchanging paths for packets to use. A dynamic table allows a packet to have multiple routes. Sometimes the packets are sent to a router's input port faster than the port can process them, when they pile up in an input queue. If packets overflow that queue, then the TCP protocol has the packets sent again. Routers are a key element of the Internet, and today's models provide great flexibility, security and control over company networks. The one network can link all company employees, even those on out-of-office hotel and conference rooms. Many built-in technologies such as voice, wireless, and advanced security systems can be optimized by the IT management team, and proper measures taken against security lapses and malicious code attacks.

Servers

Equally important is the server, a powerful computer (or often groups of computers) that handle requests for web pages, email data, and an increasing variety of services. The computers will use the Unix, Windows, Linux, or Macintosh operating systems, which have the TCP/IP protocols built in, but run different types of software, depending on the service offered: http servers, network servers, ftp servers or database servers. Simplest are the http (Hypertext Transfer Protocol) servers, which comply with requests from website visitors, sending the data back to the client computer for the browser software to assemble as familiar web pages. Generally, however, servers will also employ scripts (Perl, Common Gateway Interface scripts, .NET and others) to engage with external mini-programs like database lookup or interactive forms processing. A Yahoo search for information on telecommunications, for example, will appear like http://search.yahoo.com/bin/search?p=telecommunications, where the 'bin' indicates where the scripts are located and the 'search?p=telecommunication' instructs the script to search the associated databases for the term 'telecommunications'.

Repeaters

Repeaters maintain the signal strength and use technologies appropriate to the transmission medium. Even backbone fiber-optical cables may carry optical amplifier repeaters in the form of erbium-doped amplifiers spaced several tens of kilometers apart.

Hubs

Transmission step-downs at Internet Exchange Points are achieved by the use of hubs, an electronic device with multiple ports. Transmission rates vary considerably across these hubs: as this Wikipedia listing indicates.

Gateways

Technically, a gateway is a network node designed to interface with another network that uses a different protocol. Not only must the gateway contain protocols translators, but also impedance matching devices, rate converters, fault isolators, and/or signal translators. Mutually acceptable administrative procedures have also to be agreed between the two networks.

Bridges

A bridge connects numerous local area networks for the purpose of collaboration and/or exchange of information. All networks have to be using the same network protocols.

Client Computers

Client computers are those used by the general public, on which they run applications, or make requests for Internet services.

How Everything Fits Together

A simplified tabulation:


Level

Term

Devices

Activity

Bandwidths

Owner

1

Backbone

Cables


Satellite

Data

transmission

155 Mbps

to 2.5 Gbps

Network

Service Providers

1-2

Network

Access Points

Hubs

Bandwidth

step-down

-

Network

Service Providers

2

T1-T3

Regional

Networks

Cables


Satellites


Routers


Repeaters

Data

transmission

1.5 Mbps (T1)


45 Mbps (T3)


768Kbps-

5 Mbps

(satellites)

Local Bell

Operating Companies

(RBOCs)


Local

Telecommunication

Companies

2-3

Internet

Exchange

Points

Hubs

Bandwidth

step-down

-

Local Bell

Operating Companies

(RBOCs)


Local

Telecommunication

Companies

3

Local Area

Networks

Cables


Routers


Repeaters


Mobile

Transmission

Towers

Data

transmission

up to 45 Mbps

but generally

less

Government

Institutions


Larger Companies


Internet Service

Providers


Third Party

Hosting Companies

Local Area

Networks

Bridges

Link LANs

-

ditto

Local Area

Networks

Gateways

Convey data

between LANs

-

ditto

Campus Area

Networks

Cable


Routers


Repeaters


Mobile

Transmission

Towers

Data

Transmission

10-100 Mbps

Third Party

Providers to Universities

3-4

Internet

Exchange Points

Hubs

Bandwidth

step-down

-

Internet Service

Providers

4

'Last Mile'

Retail

Cable


Telephone

system


Routers


Repeaters


Mobile

Transmission

Towers

Data

Transmission

30 Kbps

to 2 Mbps

Leased by smaller

companies


and individual

subscribers


Internet Service Providers provide a range of services, from simple telephone dial-up to T3 high speed transmission connections that media companies require. The terminology is much more complex than shown here, and varies somewhat with country concerned. Narrowband generally refers to the connection through the traditional (landline) telephone system, however, and broadband refers to cable connection at speeds suitable for audio and video streaming. Transmission speeds downstream (to the customer) may be markedly different from those upstream (from the customer), depending on the technology. Speeds given refer to the maximum. A Broadband Index by country intention is provided by the gBBi.

Dial-up Modems: a modem device (modulator-demodulator) converts Internet data to telephone signals. A transmission speed of 56 Kbps is theoretically possible, but noise generally imposes packet resending and reduces speeds to 30 Kbps or less.

Digital Subscriber Line (DSL) uses a telephone technology with transmission speeds of 256 Kbps to 20 Mbps. Subscribers must live within 2 miles of the local telephone switching exchange.

Cable modem is a technology that uses an existing cable TV service to provide transmission speeds of 1 Mbps to 15 Mbsp.

T1 and T3 are international standards for digital communication, which guarantee transmission rates of 1.54 Mbps and 45 Mbps respectively.

Satellite access is offered in rural areas where other ISP services are unavailable. Transmission speeds can reach broadband levels, but a small satellite dish has to be installed and there are often penalty payments for excessive bandwidth use.

ISP charges have been declining, and a rough comparison is as follows:


Access

Typical Charge

/Month

Transmission

Speed to Desktop

Dial-up Modem

$10 - $25

30 - 56 Kbps

Digital

Subscriber Line

$15 - $50

256 Kbps - 20 Mbps

Cable Modem

$20 - $50

1 Mbps - 20 Mbps

Satellite

$20 - $50

768 Kbps - 5 Mbps

T1

$300 - $1,200

1.5 Mbps

T3

$2,500 - $10,000

45 Mbps

Questions

1. What is the Internet exactly? Explain how it differs from the World Wide Web.
2. List and briefly explain the functions of the hardware involved.
3. What is the server-client model, and why is it important?
4. Draw a rough map of the Internet, from backbone to last mile facilities.
5. Where may technological advances be expected?

Sources and Further Reading

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