basic networking concept

8/14/2019 Basic Networking Concept

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Design By : Abdul Majeed ([email protected])

Data Comm. & Networking: Networking allows one computer to send information to and receive informationfrom another. We may not always be aware of the numerous times we access

information on computer networks. Certainly the Internet i s the most conspicuousexample of computer networking, linking millions of computers around the world,but smaller networks play a roll in information access on a daily basis. Manypublic libraries have replaced their card catalogs with computer terminals thatallow patrons to search for books far more quickly and easily. Airports havenumerous screens displaying information regarding arriving and departing flights.Many retail stores feature specialized computers that handle point-of-saletransactions. In each of these cases, networking allows many different devices inmultiple locations to access a shared repository of data.

Local Area vs. Wide Area

We can classify network technologies as belonging to one of two basic groups.Local area network (LAN) technologies connect many devices that are relativelyclose to each other, usually in the same building. The library terminals thatdisplay book information would connect over a local area network. Wide areanetwork (WAN) technologies connect a smaller number of devices that can bemany kilometers apart. For example, if two libraries at the opposite ends of a citywanted to share their book catalog information, they would most likely make useof a wide area network technology, which could be a dedicated line leased fromthe local telephone company, intended solely to carry their data.

In comparison to WANs, LANs are faster and more reliable, but improvements in

technology continue to blur the line of demarcation. Fiber optic cables haveallowed LAN technologies t o connect devices tens of kilometers apart, while atthe same time greatly improving the speed and reliability of WANs.

The Ethernet

In 1973, at Xerox Corporations Palo Alto Research Center (more commonlyknown as PARC), researcher Bob Metcalfe designed and tested the firstEthernet network. While working on a way to link Xeroxs "Alto" computer t o aprinter, Metcalfe developed the physical method of cabling that connecteddevices on the Ethernet as well as the standards that governed communication

on the cable. Ethernet has since become the most popular and most widelydeployed network technology in the world. Many of the issues involved withEthernet are common to many network technologies, and understanding howEthernet addressed these issues can provide a foundation that will improve your understanding of networking in general.

The Ethernet standard has grown to encompass new technologies as computer networking has matured, but the mechanics of operation for every Ethernetnetwork today stem from Metcalfes original design. The original Ethernet
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described communication over a single cable shared by all devices on thenetwork. Once a device attached to this cable, it had the ability to communicatewith any other attached device. This allows the network to expand toaccommodate new devices without requiring any modification to those devicesalready on the network.

Ethernet is a local area technology, with networks traditionally operating within asingle building, connecting devices in close proximity . At most, Ethernetdevices could have only a few hundred meters of cable between them, making itimpractical to connect geographically dispersed locations. Modern advancementshave increased these distances considerably, allowing Ethernet networks to spantens of kilometers.

Protocols

In networking, the term protocol refers to a set of rules that governcommunications. Protocols are to computers what language is to humans. Sincethis article is in English, to understand it you must be able to read English.Similarly, for two devices on a network to successfully communicate, they mustboth understand the same protocols.

Switches are a fundamental part of most networks. They make it possible for several users to send information over a network at the same time withoutslowing each other down. Just like routers allow different networks tocommunicate with each other, switches allow different nodes (a networkconnection point, typically a computer) of a network to communicate directly withone another in a smooth and efficient manner.

Image courtesy Cisco Systems, Inc. An illustration of a Cisco Catalyst switch.

There are a lot of different types of switches and networks. Switches that provide aseparate connection for each node in a company's internal network are called LAN

switches. Essentially, a LAN switch creates a series of instant networks that contain onlythe two devices communicating with each other at that particular moment.

Networking Basics Here are some of the fundamental parts of a network:
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In the picture above, you see several of the common elements of anetwork.

Network - A group of computers connected together in a way that allowsinformation to be exchanged between the computers.

Node - Anything that is connected to the network. While a node is typically

a computer, it can also be something like a printer or CD-ROM tower.

Segment - Any portion of a network that is separated, by a switch, bridgeor router, from other parts of the network.

Backbone - The main cabling of a network that all of the segmentsconnect to. Typically, the backbone is capable of carrying moreinformation than the individual segments. For example, each segment

may have a transfer rate of 10 Mbps (megabits per second: 1 million bits asecond), while the backbone may operate at 100 Mbps.

Topology - The way that each node is physically connected to thenetwork. Common topologies include:

Bus - Each node is daisy-chained (connected one right after theother) along the same backbone, similar to Christmas lights.Information sent from a node travels along the backbone until itreaches its destination node. Each end of a bus network must be

terminated with a resistor to keep the signal that is sent by a nodeacross the network from bouncing back when it reaches the end of the cable.
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Bus network topology

Ring - Like a bus network, rings have the nodes daisy-chained. Thedifference is that the end of the network comes back around to thefirst node, creating a complete circuit. In a ring network, each nodetakes a turn sending and receiving information through the use of atoken . The token, along with any data, is sent from the first node tothe second node, which extracts the data addressed to it and addsany data it wishes to send. Then, the second node passes thetoken and data to the third node, and so forth until it comes backaround to the first node again. Only the node with the token isallowed to send data. All other nodes must wait for the token tocome to them.

Ring network topology


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Star - In a star network, each node is connected to a central devicecalled a hub . The hub takes a signal that comes from any node andpasses it along to all the other nodes in the network. A hub doesnot perform any type of filtering or routing of the data. It is simply a

junction that joins all the different nodes together.

Star network topology

Star Bus - Probably the most common network topology in usetoday, star bus combines elements of the star and bus topologies tocreate a versatile network environment. Nodes in particular areasare connected to hubs (creating stars), and the hubs are connectedtogether along the network backbone (like a bus network). Quiteoften, stars are nested within stars, as seen in the example below:


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A typical star bus network. Local Area Network (LAN) - A network of computers that are in the same

general physical location, usually within a building or a campus. If thecomputers are far apart (such as across town or in different cities), then aWide Area Network (WAN) is typically used.

Network Interface Card (NIC) - Every computer (and most other devices)is connected to a network through an NIC. In most desktop computers,this is an Ethernet card (normally 10 or 100 Mbps) that is plugged into aslot on the computer's motherboard.

Media Access Control (MAC) Address - This is the physical address of any device, such as the NIC in a computer, on the network. The MACaddress has two parts, each 3 bytes l ong. The first 3 bytes identify thecompany that made the NIC. The second 3 bytes are the serial number of the NIC itself.

Unicast - A transmission from one node addressed specifically to another

node.

Multicast - When a node sends a packet addressed to a special groupaddress. Devices that are interested in this group register to receive
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packets addressed to the group. An example might be a Cisco r outer sending out an update to all of the other Cisco routers.

Broadcast - When a node sends out a packet that is intended for transmission to all other nodes on the network.

CSMA/CD The acronym CSMA/CD signifies Carrier Sense Multiple Access with CollisionDetection and describes how the Ethernet protocol regulates communicationamong nodes. While the term may seem intimidating, if we break it apart into itscomponent concepts we will see that it describes rules very similar to those thatpeople use in polite conversation. To help illustrate the operation of Ethernet, wewill use an analogy of a dinner table conversation.

Lets represent our Ethernet segment as a dinner table, and let several peopleengaged in polite conversation at the table represent the nodes. The termMultiple Access covers what we already discussed above: When one Ethernetstation transmits, all the stations on the medium hear the transmission, just aswhen one person at the table talks, everyone present is able to hear him or her.

Now let's imagine that you are at the table and you have something you wouldlike to say. At the moment, however, I am talking. Since this is a politeconversation, rather than immediately speak up and interrupt, you would waituntil I finished talking before making your statement. This is the same conceptdescribed in the Ethernet protocol as Carrier Sense . Before a station transmits,

it "listens" to the medium to determine if another station is transmitting. If themedium is quiet, the station recognizes that this is an appropriate time totransmit.

Carrier Sense Multiple Access gives us a good start in regulating our conversation, but there is one scenario we still need to address. Lets go back toour dinner table analogy and imagine that there is a momentary lull in theconversation. You and I both have something we would like to add, and we both"sense the carrier" based on the silence, so we begin speaking at approximatelythe same time. In Ethernet terminology, a collision occurs when we both spokeat once.

In our conversation, we can handle this situation gracefully. We both hear theother speak at the same time we are speaking, so we can stop to give the other person a chance to go on. Ethernet nodes also listen to the medium while theytransmit to ensure that they are the only station transmitting at that time. If thestations hear their own transmission returning in a garbled form, as wouldhappen if some other station had begun to transmit its own message at the sametime, then they know that a collision occurred. A single Ethernet segment issometimes called a collision domain because no two stations on the segment
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can transmit at the same time without causing a collision. When stations detect acollision, they cease transmission, wait a random amount of time, and attempt totransmit when they again detect silence on the medium.

The random pause and retry is an important part of the protocol. If two stationscollide when transmitting once, then both will need to transmit again. At the nextappropriate chance to transmit, both stations involved with the previous collisionwill have data ready to transmit. If they transmitted again at the first opportunity,they would most likely collide again and again indefinitely. Instead, the randomdelay makes it unlikely that any two stations will collide more than a few times ina row.

VDSL

How VDSL Works

The use of fast Internet connections has grown rapidly over the last few years. Asmore people buy home computers and create home networks, the demand for broadband (high-speed) connections steadily increases. Two technologies,cable modems and Asymmetric Digital Subscriber Line ( ADSL), currentlydominate the industry.

While both of these technologies provide Internet connections that are manytimes faster than a 56K modem, they still are not fast enough to support theintegration of home services such as digital television and Video-on-Demand.

However, another DSL technology known as very high bit-rate DSL (VDSL) isseen by many as the next step in providing a complete home-communications/entertainment package. There are already some companies,such as U.S. West (part of Qwest now), that offer VDSL service in selectedareas. VDSL provides an incredible amount of bandwidth, with speeds up toabout 52 megabits per second (Mbps). Compare that with a maximum speed of 8to 10 Mbps for ADSL or cable modem and it's clear that the move from currentbroadband technology to VDSL could be as significant as the migration from a56K modem to broadband. As VDSL becomes more common, you can expectthat integrated packages will be cheaper than the total amount for currentseparate services.

DSL Basics A standard telephone i nstallation in the United States consists of a pair of copper wires that the phone company installs in your home. A pair of copper wires hasplenty of bandwidth for carrying data in addition to voice conversations. Voicesignals use only a fraction of the available capacity on the wires. DSL exploits
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though for speed and quality of service reasons, many ADSL providers place aneven lower limit on the distance. At the upper extreme of the distance limit, ADSLcustomers may experience speeds far below the promised maximums, whereascustomers close the central office or DSL termination point may experiencespeeds approaching the maximum, and even beyond the current limit in the

future.You might wonder why, if distance is a limitation for DSL, it's not a limitation for voice telephone calls, too. The answer lies in small amplifiers, called loadingcoils, that the telephone company uses to boost voice signals. These loadingcoils are incompatible with DSL signals because the amplifier disrupts theintegrity of the data. This means that if there is a voice coil in the loop betweenyour telephone and the telephone company's central office, you cannot receiveDSL service. Several other factors might disqualify you from receiving ADSL:

Bridge taps - These are extensions, between you and the central office,that service other customers.

Fiber-optic cables - ADSL signals can't pass through the conversion fromanalog to digital to analog that occurs if a portion of your telephone circuitcomes through fiber-optic cables.

Distance - Even if you know where your central office is (don't besurprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal musttravel between your house and the office. The wire may follow a veryconvoluted path between the two points.

Fiber-optic cables, one of the major disrupting factors of ADSL, is actually whatenables VDSL technology. In the next section, you'll find out why.

VDSL Speed VDSL operates over the copper wires in your phone line in much the same waythat ADSL does, but there are a couple of distinctions. VDSL can achieveincredible speeds, as high as 52 Mbps downstream (to your home) and 16Mbps upstream (from your home). That is much faster than ADSL, whichprovides up to 8 Mbps downstream and 800 Kbps (kilobits per second) upstream.But VDSL's amazing performance comes at a price: It can only operate over thecopper line for a short distance, about 4,000 feet (1,200 m).
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Photo courtesy Corning A fiber optic wire

The key to VDSL is that the telephone companies are replacing many of their main feeds with fiber-optic cable. In fact, many phone companies are planningFiber to the Curb (FTTC), which means that they will replace all existing copper lines right up to the point where your phone line branches off at your house. Atthe least, most companies expect to implement Fiber to the Neighborhood(FTTN) . Instead of installing fiber-optic cable along each street, FTTN has fiber going to the main junction box for a particular neighborhood.

By placing a VDSL transceiver in your home and a VDSL gateway in the junctionbox, the distance limitation is neatly overcome. The gateway takes care of theanalog-digital-analog conversion problem that disables ADSL over fiber-opticlines. It converts the data received from the transceiver into pulses of light thatcan be transmitted over the fiber-optic system to the central office, where thedata is routed t o the appropriate network t o reach its final destination. When datais sent back to your computer, the VDSL gateway converts the signal from thefiber-optic cable and sends it to the transceiver. All of this happens millions of times each second!

ADSL and VDSL are just two representatives of the DSL spectrum. On the nextpage, you'll find a chart that lists the variations and how they compare to eachother.

Comparing DSL Types There are several variations on DSL technology. In fact, there are so many thatyou will often see the term xDSL , where x is a variable, when the discussion isabout DSL in general.

Asymmetric DSL (ADSL) - It is called "asymmetric" because thedownload speed is greater than the upload speed. ADSL works this waybecause most Internet users look at, or download, much more informationthan they send, or upload.

High bit-rate DSL (HDSL) - Providing transfer rates comparable to a T1line (about 1.5 Mbps), HDSL receives and sends data at the same speed,but it requires two lines that are separate from your normal phone line.

ISDN DSL (ISDL) - Geared primarily toward existing users of IntegratedServices Digital Network (ISDN), ISDL is slower than most other formsof DSL, operating at fixed rate of 144 Kbps in both directions. Theadvantage for ISDN customers is that they can use their existingequipment, but the actual speed gain is typically only 16 Kbps (ISDN runsat 128 Kbps).

Multirate Symmetric DSL (MSDSL) - This is Symmetric DSL that iscapable of more than one transfer rate. The transfer rate is set by theservice provider, typically based on the service (price) level.
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Rate Adaptive DSL (RADSL) - This is a popular variation of ADSL thatallows the modem to adjust the speed of the connection depending on thelength and quality of the line.

Symmetric DSL (SDSL) - Like HDSL, this version receives and sendsdata at the same speed. While SDSL also requires a separate line fromyour phone, it uses only a single line instead of the two used by HDSL.

Very high bit-rate DSL (VDSL) - An extremely fast connection, VDSL isasymmetric, but only works over a short distance using standard copper phone wiring.

Voice-over DSL (VoDSL) - A type of IP telephony, VoDSL allows multiplephone lines to be combined into a single phone line that also includesdata-transmission capabilities.

The chart below provides a comparison of the various DSL technologies:

DSLType

Max.Send

Speed

Max.ReceiveSpeed

Max.Distance

LinesRequired

PhoneSupport

ADSL 800 Kbps 8 Mbps 18,000 ft(5,500 m) 1 Yes

HDSL 1.54Mbps 1.54 Mbps 12,000 ft(3,650 m) 2 No

IDSL 144 Kbps 144 Kbps 35,000 ft(10,700 m) 1 No

MSDSL 2 Mbps 2 Mbps 29,000 ft(8,800 m) 1 No

RADSL

1 Mbps 7 Mbps 18,000 ft(5,500 m)

1 Yes

SDSL 2.3 Mbps 2.3 Mbps 22,000 ft(6,700 m) 1 No

VDSL 16 Mbps 52 Mbps 4,000 ft(1,200 m) 1 Yes

As you can see, VDSL provides a significant performance boost over any other version. But for VDSL to become widely available, it must be standardized. In thenext section, we'll talk about two potential VDSL standards.

Competing VDSL Standards There are two competing consortiums that are pushing to standardize VDSL. Theproblem is that their proposed standards use carrier technologies that areincompatible with one another. The VDSL Alliance, a partnership betweenAlcatel, Texas Instruments and others, supports VDSL using a carrier systemcalled Discrete MultiTone (DMT). According to equipment manufacturers, mostof the ADSL equipment installed today uses DMT.

DMT divides signals into 247 separate channels, each 4 kilohertz (KHz, or 1,000cycles per second) wide. One way to think about it is to imagine that the phone
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company divides your copper line into 247 different 4-KHz lines and attaches amodem to each one. You get the equivalent of 247 modems connected to your computer at once! Each channel is monitored and, if the quality is too impaired,the signal is shifted to another channel. This system constantly shifts signals,searching for the best channels for transmission and reception. In addition, some

of the lower channels (those starting at about 8 KHz) are used as bidirectionalchannels, for both upstream and downstream information. Monitoring and sortingout the information on the bidirectional channels, and keeping up with the qualityof all 247 channels, makes DMT more complex to implement than other carrier technologies, but also gives it more flexibility on lines of differing quality.

Discrete MultiTone divides the available carrier band into 247distinct 4-KHz channels.

The other VDSL group is called the VDSL Coalition. Led by Lucent andBroadcom, the Coalition proposes a carrier system that uses a pair of technologies called Carrierless Amplitude Phase (CAP) and QuadratureAmplitude Modulation (QAM).

CAP operates by dividing the signals on the telephone line into three distinctbands: Voice conversations are carried in the zero- to 4-KHz band, which is in allstandard phone circuits. The upstream channel (from the user to the server) iscarried in a band between 25 and 160 KHz. The downstream channel (from theserver to the user) begins at 240 KHz and goes up to a point that varies withsuch conditions as line length, line noise and the number of users in the switch,

but it has a maximum of about 1.5 megahertz (MHz). This system, with the threechannels widely separated, minimizes the possibility of interference between thechannels on one line, or between the signals on different lines.

Carrier Amplitude Modulation divides the line into threedistinct bands, with space between each band.

QAM is a modulation technique that effectively triples or quadruples theinformation sent over a line, depending on the version used. It accomplishes thisby modulating (varying the shape of the carrier wave) and phase shifting (varying the angle of the carrier wave). An unmodulated signal provides for onlytwo states, 1 or 0, which means that it can send a single bit of information per cycle. By sending a second wave that is shifted 90 degrees out of phase with thefirst one, and then modulating each wave so that there are two points per wave,you get eight states. This allows you to send 3 bits per cycle instead of just 1.
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Why 3 bits? Remember that you are sending binary information. Two statesequal a single bit (2 1 = 2). Four states are equivalent 2 bits (2 2 = 4). Eight statesequal 3 bits (2 3 = 8).

By adding four more waves, shifted 15 degrees out of phase, you get 16 statesand can send 4 bits per cycle (2 4 = 16). Adding another bit increases the number of phase shifts geometrically. To go beyond 4 bits per cycle becomesincreasingly difficult because the number of necessary states doubles for eachbit: 2 5 = 32, 2 6 = 64 and so on. This site provides a detailed look at QAM, and thisone has a great QAM animation.

There is a possibility that VDSL will encompass both standards, with providersselecting which technology they will implement across their system. No matter what happens, the future of VDSL is very bright. It has the potential to provide uswith that elusive dream of interactive television and Video-on-Demand.

The Origin of Modems The word "modem" is a contraction of the words modulator-demodulator . Amodem is typically used to send digital data over a phone line.

The sending modem modulates the data into a signal that is compatible with thephone line, and the receiving modem demodulates the signal back into digitaldata. Wireless modems convert digital data into radio signals and back.

Modems came into existence in the 1960s as a way to allow terminals to connectto computers over the phone lines. A typical arrangement is shown below:

In a configuration like this, a dumb terminal at an off-site office or store could"dial in" to a large, central computer. The 1960s were the age of time-shared computers, so a business would often buy computer time from a time-sharefacility and connect to it via a 300-bit-per-second (bps) modem.

A dumb terminal is simply a keyboard and a screen. A very common dumbterminal at the time was called the DEC VT-100 , and it became a standard of theday (now memorialized in terminal emulators worldwide). The VT-100 coulddisplay 25 lines of 80 characters each. When the user typed a character on theterminal, the modem sent the ASCII code for the character to the computer. Thecomputer then sent the character back to the computer so it would appear on thescreen.
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When personal computers started appearing in the late 1970s, bulletin boardsystems (BBS) became the rage. A person would set up a computer with amodem or two and some BBS software, and other people would dial in toconnect to the bulletin board. The users would run terminal emulators on their computers to emulate a dumb terminal.

People got along at 300 bps for quite a while. The reason this speed wastolerable was because 300 bps represents about 30 characters per second,which is a lot more characters per second than a person can type or read. Oncepeople started transferring large programs and images to and from bulletin boardsystems, however, 300 bps became intolerable. Modem speeds went through aseries of steps at approximately two-year intervals:

300 bps - 1960s through 1983 or so

1200 bps - Gained popularity in 1984 and 1985

2400 bps

9600 bps - First appeared in late 1990 and early 1991

19.2 kilobits per second (Kbps)

28.8 Kbps

33.6 Kbps

56 Kbps - Became the standard in 1998

ADSL, with theoretical maximum of up to 8 megabits per second (Mbps) -

Gained popularity in 1999(Check out How DSL Works and How Cable Modems Work f or more informationon the progression of modem technology and current speeds.)

300-bps Modems We'll use 300-bps modems as a starting point because they are extremely easyto understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shiftkeying, a different tone (frequency) is used for the different bits (see How Guitars

Work for a discussion of tones and frequencies).When a terminal's modem dials a computer's modem, the terminal's modem iscalled the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and ittransmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because theoriginate and answer modems transmit different tones, they can use the linesimultaneously. This is known as full-duplex operation. Modems that can
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transmit in only one direction at a time are known as half-duplex modems, andthey are rare.

Let's say that two 300-bps modems are connected, and the user at the terminaltypes the letter "a." The ASCII code for this letter is 97 decimal or 01100001binary (see How Bits and Bytes Work for details on binary). A device inside theterminal called a UART (universal asynchronous receiver/transmitter) convertsthe byte into its bits and sends them out one at a time through the terminal's RS-232 port (also known as a serial port ). The terminal's modem is connected tothe RS-232 port, so it receives the bits one at a time and its job is to send themover the phone line.

Faster Modems In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shiftkeying (PSK), and then quadrature amplitude modulation (QAM). Thesetechniques allow an incredible amount of information to be crammed into the3,000 hertz of bandwidth available on a normal voice-grade phone line. 56Kmodems, which actually connect at something like 48 Kbps on anything butabsolutely perfect lines, are about the limit of these techniques (see the links atthe end of this article for more information).

Here's a look inside a typical 56K modem:

All of these high-speed modems incorporate a concept of gradual degradation ,meaning they can test the phone line and fall back to slower speeds if the linecannot handle the modem's fastest speed.

The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modemssend data faster in one direction than they do in another. An ADSL modem takesadvantage of the fact that any normal home, apartment or office has a dedicated
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copper wire running between it and phone company's nearest mux or centraloffice. This dedicated copper wire can carry far more data than the 3,000-hertzsignal needed for your phone's voice channel. If both the phone company'scentral office and your house are equipped with an ADSL modem on your line,then the section of copper wire between your house and the phone company can

act as a purely digital high-speed transmission channel. The capacity issomething like 1 million bits per second (Mbps) between the home and the phonecompany ( upstream ) and 8 Mbps between the phone company and the home(downstream ) under ideal conditions. The same line can transmit both a phoneconversation and the digital data.

The approach an ADSL modem takes is very simple in principle. The phone line'sbandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertzbands, and a virtual modem is assigned to each band. Each of these 249 virtualmodems tests its band and does the best it can with the slice of bandwidth it isallocated. The aggregate of the 249 virtual modems is the total speed of the pipe.

Point-to-Point Protocol Today, no one uses dumb terminals or terminal emulators to connect to anindividual computer. Instead, we use our modems to connect to an Internetservice provider (ISP), and the ISP connects us into the Internet. The Internetlets us connect to any machine in the world (see How Web Servers and theInternet Work for details). Because of the relationship between your computer,the ISP and the Internet, it is no longer appropriate to send individual characters.Instead, your modem is routing TCP/IP packets between you and your ISP.

The standard technique for routing t hese packets through your modem is calledthe Point-to-Point Protocol (PPP ). The basic idea is simple -- your computer'sTCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams arehanded to the modem for transmission. The ISP receives each datagram androutes it appropriately onto the Internet. The same process occurs to get datafrom the ISP to your computer. See this page f or additional information on PPP.

If you want to know more about modems, protocols, and especially if you wish todelve into things like PSK and QAM in more detail, check out the links on thenext page!
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Photo courtesy Corning A fiber-optic wire

You hear about fiber-optic cables whenever people talk about the telephonesystem, the cable TV system or the Internet. Fiber-optic lines are strands of optically pure glass as thin as a human hair that carry digital information over long distances. They are also used in medical imaging and mechanicalengineering inspection.

In this edition of HowStuffWorks , we will show you how these tiny strands of glass transmit light and the fascinating way that these strands are made.

What are Fiber Optics? Fiber optics (optical fibers) are long, thin strands of very pure glass about thediameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

Parts of a single optical fiber

If you look closely at a single optical fiber, you will see that it has the followingparts:

Core - Thin glass center of the fiber where the light travels
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Cladding - Outer optical material surrounding the core that reflects t helight back into the core

Buffer coating - Plastic coating that protects the fiber from damage andmoisture

Hundreds or thousands of these optical fibers are arranged in bundles in opticalcables. The bundles are protected by the cable's outer covering, called a jacket .

Optical fibers come in two types:

Single-mode fibers - Used to transmit one signal per fiber (used intelephones and cable TV)

Multi-mode fibers - Used to transmit many signals per fiber (used incomputer networks, local area networks)

Single-mode fibers have small cores (about 3.5 x 10 -4 inches or 9 microns indiameter) and transmit infrared laser l ight (wavelength = 1,300 to 1,550

nanometers). Multi-mode fibers have larger cores (about 2.5 x 10-3

inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300nm) from light-emitting diodes ( LEDs).

Some optical fibers can be made from plastic . These fibers have a large core(0.04 inches or 1 mm diameter) and transmit visible red light (wavelength = 650nm) from LEDs.

Let's look at how an optical fiber works.

How Does an Optical Fiber Transmit Light? Suppose you want to shine a flashlight beam down a long, straight hallway. Just

point the beam straight down the hallway -- light travels in straight lines, so it isno problem. What if the hallway has a bend in it? You could place a mirror at thebend to reflect the light beam around the corner. What if the hallway is verywinding with multiple bends? You might line the walls with mirrors and angle thebeam so that it bounces from side-to-side all along the hallway. This is exactlywhat happens in an optical fiber.

Diagram of total internal reflection in an optical fiber
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The light in a fiber-optic cable travels through the core (hallway) by constantlybouncing from the cladding (mirror-lined walls), a principle called total internalreflection . Because the cladding does not absorb any light from the core, thelight wave can travel great distances. However, some of the light signaldegrades within the fiber, mostly due to impurities in the glass. The extent that

the signal degrades depends on the purity of the glass and the wavelength of thetransmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium opticalfibers show much less signal degradation -- less than 10 percent/km at 1,550 nm.

A Fiber-Optic Relay System To understand how optical fibers are used in communications systems, let's lookat an example from a World War II movie or documentary where two naval shipsin a fleet need to communicate with each other while maintaining radio s ilence or on stormy seas. One ship pulls up alongside the other. The captain of one shipsends a message to a sailor on deck. The sailor translates the message intoMorse code (dots and dashes) and uses a signal light (floodlight with a venetianblind type shutter on it) to send the message to the other ship. A sailor on thedeck of the other ship sees the Morse code message, decodes it into English andsends the message up to the captain.

Now, imagine doing this when the ships are on either side of the oceanseparated by thousands of miles and you have a fiber-optic communicationsystem in place between the two ships. Fiber-optic relay systems consist of thefollowing:

Transmitter - Produces and encodes the light signals

Optical fiber - Conducts the light signals over a distance

Optical regenerator - May be necessary to boost the light signal (for longdistances)

Optical receiver - Receives and decodes the light signals

Transmitter The transmitter is like the sailor on the deck of the sending ship. It receives anddirects the optical device to turn the light "on" and "off" in the correct sequence,thereby generating a light signal.

The transmitter is physically close to the optical fiber and may even have a lensto focus the light into the fiber. Lasers have more power than LEDs, but varymore with changes in temperature and are more expensive. The most commonwavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum) .

Optical Regenerator As mentioned above, some signal loss occurs when the light is transmittedthrough the fiber, especially over long distances (more than a half mile, or about
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1 km) such as with undersea cables. Therefore, one or more opticalregenerators is spliced along the cable to boost the degraded light signals.

An optical regenerator consists of optical fibers with a special coating ( doping ).The doped portion is "pumped" with a laser. When the degraded signal comesinto the doped coating, the energy from the laser allows the doped molecules tobecome lasers themselves. The doped molecules then emit a new, stronger lightsignal with the same characteristics as the incoming weak light signal. Basically,the regenerator is a laser amplifier for the incoming signal (see this page on fiber amplifiers for more details).

Optical Receiver The optical receiver is like the sailor on the deck of the receiving ship. It takesthe incoming digital light signals, decodes them and sends the electrical signal tothe other user's computer, TV or telephone ( receiving ship's captain). Thereceiver uses a photocell or photodiode to detect the light.

For a good discussion of lightwave transmission systems, see this page f rom BellLabs.

Advantages of Fiber Optics Why are fiber-optic systems revolutionizing telecommunications? Compared toconventional metal wire (copper wire), optical fibers are:

Less expensive - Several miles of optical cable can be made cheaper than equivalent lengths of copper wire. This saves your provider (cableTV, Internet) and you money.

Thinner - Optical fibers can be drawn to smaller diameters than copper

wire. Higher carrying capacity - Because optical fibers are thinner than copper

wires, more fibers can be bundled into a given-diameter cable than copper wires. This allows more phone lines to go over the same cable or morechannels to come through the cable into your cable TV box.

Less signal degradation - The loss of signal in optical fiber is less than incopper wire.

Light signals - Unlike electrical signals in copper wires, light signals fromone fiber do not interfere with those of other fibers in the same cable. This

means clearer phone conversations or TV reception. Low power - Because signals in optical fibers degrade less, lower-power

transmitters can be used instead of the high-voltage electrical transmittersneeded for copper wires. Again, this saves your provider and you money.

Digital signals - Optical fibers are ideally suited for carrying digitalinformation, which is especially useful in computer networks.
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Non-flammable - Because no electricity is passed through optical fibers,there is no fire hazard.

Lightweight - An optical cable weighs less than a comparable copper wire cable. Fiber-optic cables take up less space in the ground.

Flexible - Because fiber optics are so flexible and can transmit andreceive light, they are used in many flexible digital cameras f or thefollowing purposes:

Medical imaging - in bronchoscopes, endoscopes, laparoscopes

Mechanical imaging - inspecting mechanical welds in pipes andengines (in airplanes, rockets, space shuttles, cars)

Plumbing - to inspect sewer lines

Because of these advantages, you see fiber optics in many industries, most

notably telecommunications and computer networks. For example, if youtelephone Europe from the United States (or vice versa) and the signal isbounced off a communications satellite, you often hear an echo on the line. Butwith transatlantic fiber-optic cables, you have a direct connection with no echoes.

How Are Optical Fibers Made? Now that we know how fiber-optic systems work and why they are useful -- howdo they make them? Optical fibers are made of extremely pure optical glass .We think of a glass window as transparent, but the thicker the glass gets, theless transparent it becomes due to impurities in the glass. However, the glass inan optical fiber has far fewer impurities than window-pane glass. One company's

description of the quality of glass is as follows: If you were on top of an oceanthat is miles of solid core optical fiber glass, you could see the bottom clearly.

Making optical fibers requires the following steps:

1. Making a preform glass cylinder

2. Drawing the fibers from the preform

3. Testing the fibers

Making the Preform Blank The glass for the preform is made by a process called modified chemical vapor deposition (MCVD).
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Photo courtesy Fibercore Ltd. MCVD process for making the preform blank

In MCVD, oxygen is bubbled through solutions of silicon chloride (SiCl 4),germanium chloride (GeCl 4) and/or other chemicals. The precise mixture governsthe various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the insideof a synthetic silica or quartz tube (cladding) in a special lathe . As the latheturns, a torch is moved up and down the outside of the tube. The extreme heatfrom the torch causes two things to happen:

Photo courtesy Fibercore Ltd. Lathe used in preparing

the preform blank The silicon and germanium react with oxygen, forming silicon dioxide

(SiO 2) and germanium dioxide (GeO 2).

The silicon dioxide and germanium dioxide deposit on the inside of the

tube and fuse together to form glass.The lathe turns continuously to make an even coating and consistent blank. Thepurity of the glass is maintained by using corrosion-resistant plastic in the gasdelivery system (valve blocks, pipes, seals) and by precisely controlling the flowand composition of the mixture. The process of making the preform blank ishighly automated and takes several hours. After the preform blank cools, it istested for quality control (index of refraction) .
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Drawing Fibers from the Preform Blank Once the preform blank has been tested, it gets loaded into a fiber drawingtower .

Diagram of a fiber drawing tower used to draw optical glass fibersfrom a preform blank

The blank gets lowered into a graphite furnace (3,452 to 3,992 degreesFahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until amolten glob falls down by gravity. As it drops, it cools and forms a thread.

The operator threads the strand through a series of coating cups (buffer coatings) and ultraviolet light curing ovens onto a tractor-controlled spool. Thetractor mechanism slowly pulls the fiber from the heated preform blank and isprecisely controlled by using a laser micrometer to measure the diameter of thefiber and feed the information back to the tractor mechanism. Fibers are pulled
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from the blank at a rate of 33 to 66 ft/s (10 to 20 m/s) and the finished product iswound onto the spool. It is not uncommon for spools to contain more than 1.4miles (2.2 km) of optical fiber.

Testing the Finished Optical Fiber

Photo courtesy Corning Finished spool of optical fiber

The finished optical fiber is tested for the following: Tensile strength - Must withstand 100,000 lb/in 2 or more

Refractive index profile - Determine numerical aperture as well asscreen for optical defects

Fiber geometry - Core diameter, cladding dimensions and coatingdiameter are uniform

Attenuation - Determine the extent that light signals of variouswavelengths degrade over distance

Information carrying capacity (bandwidth) - Number of signals that canbe carried at one time (multi-mode fibers)

Chromatic dispersion - Spread of various wavelengths of light throughthe core (important for bandwidth)

Operating temperature/humidity range

Temperature dependence of attenuation

Ability to conduct light underwater - Important for undersea cables

Once the fibers have passed the quality control, they are sold to telephonecompanies, cable companies and network providers. Many companies arecurrently replacing their old copper-wire-based systems with new fiber-optic-based systems to improve speed, capacity and clarity.

How Internet Infrastructure Works


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One of the greatest things about the Internet is that nobody really owns it. It is aglobal collection of networks, both big and small. These networks connecttogether in many different ways to form the single entity that we know as theInternet . In fact, the very name comes from this idea of interconnected networks.

Since its beginning in 1969, the Internet has grown from four host computer systems to tens of millions. However, just because nobody owns the Internet, itdoesn't mean it is not monitored and maintained in different ways. The InternetSociety, a non-profit group established in 1992, oversees the formation of thepolicies and protocols that define how we use and interact with the Internet.
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VLANs

As networks have grown in size and complexity, many companies have turned toVirtual Local Area Networks (VLANs) to provide some way of structuring thisgrowth logically. Basically, a VLAN is a collection of nodes that are groupedtogether in a single broadcast domain that is based on something other thanphysical location. You learned about broadcasts earlier, and how a router doesnot pass along broadcasts. A broadcast domain is a network (or portion of anetwork) that will receive a broadcast packet from any node located within thatnetwork. In a typical network, everything on the same side of the router i s all partof the same broadcast domain. A switch that you have implemented VLANs onhas multiple broadcast domains, similar to a router. But you still need a router toroute from one VLAN to another; the switch can't do this by itself.

Here are some common reasons why a company might have VLANs:

Security - Separating systems with sensitive data from the rest of thenetwork decreases the chance that someone will gain access toinformation they are not authorized to see.

Projects/Special applications - Managing a project or working with aspecialized application can be simplified by the use of a VLAN that bringsall of the required nodes together.

Performance/Bandwidth - Careful monitoring of network use allows thenetwork administrator to create VLANs that reduce the number of router hops and increase the apparent bandwidth for network users.

Broadcasts/Traffic flow - Since a principle element of a VLAN is the factthat it does not pass broadcast traffic to nodes that are not part of theVLAN, it automatically reduces broadcasts. Access lists provide thenetwork administrator with a way to control who sees what network traffic.An access list is a table the network administrator creates that lists whataddresses have access to that network.

Departments/Specific job types - Companies may want VLANs set up for departments that are heavy network users (such as Multimedia or Engineering), or a VLAN across departments that is dedicated to specifictypes of employees (such as managers or sales people).

You can create a VLAN using most switches simply by logging into the switch viaTelnet and entering the parameters for the VLAN (name, domain and portassignments). After you have created the VLAN, any network segmentsconnected to the assigned ports will become part of that VLAN.

While you can have more than one VLAN on a switch, they cannot communicatedirectly with one another on that switch. If they could, it would defeat the purposeof having a VLAN, which is to isolate a part of the network. Communicationbetween VLANs requires the use of a router.
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VLANs can span across multiple switches and you can have more than oneVLAN on each switch. For multiple VLANs on multiple switches to be able tocommunicate via a single link between the switches, you must use a processcalled trunking ; trunking is the technology that allows information from multipleVLANs to be carried over just one link between switches.

The VLAN Trunking Protocol (VTP) is the protocol that switches use tocommunicate among themselves about VLAN configuration.

In the image above, each switch has two VLANs. On the first switch, VLAN A andVLAN B are sent through a single port (trunked) to the router and throughanother port to the second switch. VLAN C and VLAN D are trunked from thesecond switch to the first switch, and through the first switch to the router. Thistrunk can carry traffic from all four VLANs. The trunk link from the first switch tothe router can also carry all four VLANs. In fact, this one connection to the router allows the router to appear on all four VLANs, as if it had four, different, physicalports connected to the switch.

The VLANs can communicate with each other via the trunking connectionbetween the two switches using the router. For example, data from a computer on VLAN A that needs to get to a computer on VLAN B (or VLAN C or VLAN D)must travel from the switch to the router and back again to the switch. Becauseof the transparent bridging algorithm and trunking, both PCs and the router thinkthat they are on the same physical segment!

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How Network AddressTranslation Works

The Internet has grown larger than anyone ever imagined it could be. Althoughthe exact size is unknown, the current estimate is that there are about 100 millionhosts and more than 350 million users actively on the Internet. That is more thanthe entire population of the United States! In fact, the rate of growth has beensuch that the Internet is effectively doubling in size each year.

So what does the size of the Internet have to do with NAT? Everything! For acomputer to communicate with other computers and Web servers on the Internet,it must have an IP address . An IP address (IP stands for Internet Protocol) is aunique 32-bit number that identifies the location of your computer on a network.Basically, it works like your street address -- as a way to find out exactly whereyou are and deliver information to you.

When IP addressing first came out, everyone thought that there were plenty of addresses to cover any need. Theoretically, you could have 4,294,967,296unique addresses (2 32). The actual number of available addresses is smaller (somewhere between 3.2 and 3.3 billion) because of the way that the addressesare separated into classes, and because some addresses are set aside for multicasting, testing or other special uses.

With the explosion of the Internet and the increase in home networks andbusiness networks, the number of available IP addresses is simply not enough.The obvious solution is to redesign the address format to allow for more possibleaddresses. This is being developed (called IPv6 ), but will take several years toimplement because it requires modification of the entire infrastructure of theInternet.

The NAT router translates traffic coming into and leaving the private network.

This is where NAT (RFC 1631) comes to the rescue. Network AddressTranslation allows a single device, such as a router, to act as an agent betweenthe Internet (or "public network") and a local (or "private") network. This means
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that only a single, unique IP address is required to represent an entire group of computers.

Behind The Mask NAT is like the receptionist in a large office. Let's say you have left instructionswith the receptionist not to forward any calls to you unless you request it. Later on, you call a potential client and leave a message for that client to call you back.You tell the receptionist that you are expecting a call from this client and to putthem through.

The client calls the main number to your office, which is the only number theclient knows. When the client tells the receptionist that he or she is looking for you, the receptionist checks a lookup table that matches your name name withyour extension. The receptionist knows that you requested this call, and thereforeforwards the caller to your extension.

Developed by Cisco, Network Address Translation is used by a device (firewall,router or computer) that sits between an internal network and the rest of theworld. NAT has many forms and can work in several ways:

Static NAT - Mapping an unregistered IP address to a registered IPaddress on a one-to-one basis. Particularly useful when a device needs tobe accessible from outside the network.

In static NAT, the computer with the IP address of 192.168.32.10 willalways translate to 213.18.123.110.

Dynamic NAT - Maps an unregistered IP address to a registered IPaddress from a group of registered IP addresses.

In dynamic NAT, the computer with the IP address 192.168.32.10 willtranslate to the first available address in the range from

213.18.123.100 to 213.18.123.150.
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Overloading - A form of dynamic NAT that maps multiple unregistered IPaddresses to a single registered IP address by using different ports. Thisis known also as PAT (Port Address Translation), single address NAT or port-level multiplexed NAT.

In overloading, each computer on the private network is translated tothe same IP address (213.18.123.100), but with a different port

number assignment. Overlapping - When the IP addresses used on your internal network are

registered IP addresses in use on another network, the router must

maintain a lookup table of these addresses so that it can intercept themand replace them with registered unique IP addresses. It is important tonote that the NAT router must translate the "internal" addresses toregistered unique addresses as well as translate the "external" registeredaddresses to addresses that are unique to the private network. This canbe done either through static NAT or by using DNS and implementingdynamic NAT.

The internal IP range (237.16.32.xx) is also a registered range used byanother network. Therefore, the router is translating the addresses toavoid a potential conflict with another network. It will also translatethe registered global IP addresses back to the unregistered local IP

addresses when information is sent to the internal network.

The internal network is usually a LAN (Local Area Network) , commonly referredto as the stub domain . A stub domain is a LAN that uses IP addressesinternally. Most of the network traffic in a stub domain is local, so it doesn't traveloutside the internal network. A stub domain can include both registered andunregistered IP addresses. Of course, any computers that use unregistered IPaddresses must use Network Address Translation to communicate with the restof the world.

NAT can be configured in various ways. In the example below, the NAT router isconfigured to translate unregistered (inside, local) IP addresses, that reside onthe private (inside) network, to registered IP addresses. This happens whenever
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a device on the inside with an unregistered address needs to communicate withthe public (outside) network.

An ISP assigns a range of IP addresses to your company. The assignedblock of addresses are registered, unique IP addresses and are calledinside global addresses . Unregistered, private IP addresses are split intotwo groups. One is a small group ( outside local addresses ) that will beused by the NAT routers. The other, much larger group, known as insidelocal addresses , will be used on the stub domain. The outside localaddresses are used to translate the unique IP addresses, known asoutside global addresses , of devices on the public network.

IP addresses have different designations based on whether they are onthe private network (stub domain) or on the public network (Internet),

and whether the traffic is incoming or outgoing. Most computers on the stub domain communicate with each other using

the inside local addresses.

Some computers on the stub domain communicate a lot outside thenetwork. These computers have inside global addresses, which meansthat they do not require translation.

When a computer on the stub domain that has an inside local addresswants to communicate outside the network, the packet goes to one of theNAT routers.

The NAT router checks the routing table to see if it has an entry for thedestination address. If it does, the NAT router then translates the packetand creates an entry for it in the address translation table. If thedestination address is not in the routing table, the packet is dropped.

Using an inside global address, the router sends the packet on to it'sdestination.

A computer on the public network sends a packet to the private network.The source address on the packet is an outside global address. Thedestination address is an inside global address.

The NAT router looks at the address translation table and determines thatthe destination address is in there, mapped to a computer on the stubdomain.


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The NAT router translates the inside global address of the packet to theinside local address, and sends it to the destination computer.

NAT overloading utilizes a feature of the TCP/IP protocol stack, multiplexing ,that allows a computer to maintain several concurrent connections with a remotecomputer (or computers) using different TCP or UDP ports . An IP packet has aheader that contains the following information:

Source Address - The IP address of the originating computer, such as201.3.83.132.

Source Port - The TCP or UDP port number assigned by the originatingcomputer for this packet, such as Port 1080.

Destination Address - The IP address of the receiving computer, such as145.51.18.223.

Destination Port - The TCP or UDP port number that the originatingcomputer is asking the receiving computer to open, such as Port 3021.

The addresses specify the two machines at each end, while the port numbersensure that the connection between the two computers has a unique identifier.The combination of these four numbers defines a single TCP/IP connection.Each port number uses 16 bits, which means that there are a possible 65.536(2 16) values. Realistically, since different manufacturers map the ports in slightlydifferent ways, you can expect to have about 4,000 ports available.
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