Basic Networking Concept

Data Comm. & Networking: Networking allows one computer to send information to and receive information from another. We may not always be aware of the numerous times we access information on computer networks. Certainly the Internet is the most conspicuous example of computer networking, linking millions of computers around the world, but smaller networks play a roll in information access on a daily basis. Many public libraries have replaced their card catalogs with computer terminals that allow patrons to search for books far more quickly and easily. Airports have numerous screens displaying information regarding arriving and departing flights. Many retail stores feature specialized computers that handle point-of-sale transactions. In each of these cases, networking allows many different devices in multiple 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 relatively close to each other, usually in the same building. The library terminals that display book information would connect over a local area network. Wide area network (WAN) technologies connect a smaller number of devices that can be many kilometers apart. For example, if two libraries at the opposite ends of a city wanted to share their book catalog information, they would most likely make use of a wide area network technology, which could be a dedicated line leased from the 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 have allowed LAN technologies to connect devices tens of kilometers apart, while at the same time greatly improving the speed and reliability of WANs.

The Ethernet In 1973, at Xerox Corporation’s Palo Alto Research Center (more commonly known as PARC), researcher Bob Metcalfe designed and tested the first Ethernet network. While working on a way to link Xerox’s "Alto" computer to a printer, Metcalfe developed the physical method of cabling that connected devices on the Ethernet as well as the standards that governed communication on the cable. Ethernet has since become the most popular and most widely deployed network technology in the world. Many of the issues involved with Ethernet are common to many network technologies, and understanding how Ethernet 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 Ethernet network today stem from Metcalfe’s original design. The original Ethernet Design By : Abdul Majeed ([email protected])

described communication over a single cable shared by all devices on the network. Once a device attached to this cable, it had the ability to communicate with any other attached device. This allows the network to expand to accommodate new devices without requiring any modification to those devices already on the network. Ethernet is a local area technology, with networks traditionally operating within a single building, connecting devices in close proximity. At most, Ethernet devices could have only a few hundred meters of cable between them, making it impractical to connect geographically dispersed locations. Modern advancements have increased these distances considerably, allowing Ethernet networks to span tens of kilometers.

Protocols In networking, the term protocol refers to a set of rules that govern communications. Protocols are to computers what language is to humans. Since this 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 must both 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 without slowing each other down. Just like routers allow different networks to communicate with each other, switches allow different nodes (a network connection point, typically a computer) of a network to communicate directly with one 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 a separate 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 only the 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 a network. •

Network - A group of computers connected together in a way that allows information 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, bridge or router, from other parts of the network.

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Backbone - The main cabling of a network that all of the segments connect to. Typically, the backbone is capable of carrying more information than the individual segments. For example, each segment may have a transfer rate of 10 Mbps (megabits per second: 1 million bits a second), while the backbone may operate at 100 Mbps.

•

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


Bus - Each node is daisy-chained (connected one right after the other) along the same backbone, similar to Christmas lights. Information sent from a node travels along the backbone until it reaches its destination node. Each end of a bus network must be terminated with a resistor to keep the signal that is sent by a node across 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. The difference is that the end of the network comes back around to the first node, creating a complete circuit. In a ring network, each node takes a turn sending and receiving information through the use of a token. The token, along with any data, is sent from the first node to the second node, which extracts the data addressed to it and adds any data it wishes to send. Then, the second node passes the token and data to the third node, and so forth until it comes back around to the first node again. Only the node with the token is allowed to send data. All other nodes must wait for the token to come to them.

Ring network topology

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Star - In a star network, each node is connected to a central device called a hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network. A hub does not 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 use today, star bus combines elements of the star and bus topologies to create a versatile network environment. Nodes in particular areas are connected to hubs (creating stars), and the hubs are connected together along the network backbone (like a bus network). Quite often, 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 the computers are far apart (such as across town or in different cities), then a Wide Area Network (WAN) is typically used.

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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 a slot on the computer's motherboard.

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

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Unicast - A transmission from one node addressed specifically to another node.

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Multicast - When a node sends a packet addressed to a special group address. 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 router sending out an update to all of the other Cisco routers.

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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 Collision Detection and describes how the Ethernet protocol regulates communication among nodes. While the term may seem intimidating, if we break it apart into its component concepts we will see that it describes rules very similar to those that people use in polite conversation. To help illustrate the operation of Ethernet, we will use an analogy of a dinner table conversation. Let’s represent our Ethernet segment as a dinner table, and let several people engaged in polite conversation at the table represent the nodes. The term Multiple Access covers what we already discussed above: When one Ethernet station transmits, all the stations on the medium hear the transmission, just as when 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 would like to say. At the moment, however, I am talking. Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I finished talking before making your statement. This is the same concept described in the Ethernet protocol as Carrier Sense. Before a station transmits, it "listens" to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit. Carrier Sense Multiple Access gives us a good start in regulating our conversation, but there is one scenario we still need to address. Let’s go back to our dinner table analogy and imagine that there is a momentary lull in the conversation. 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 approximately the same time. In Ethernet terminology, a collision occurs when we both spoke at once. In our conversation, we can handle this situation gracefully. We both hear the other 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 they transmit to ensure that they are the only station transmitting at that time. If the stations hear their own transmission returning in a garbled form, as would happen if some other station had begun to transmit its own message at the same time, then they know that a collision occurred. A single Ethernet segment is sometimes called a collision domain because no two stations on the segment Design By : Abdul Majeed ([email protected])

can transmit at the same time without causing a collision. When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to transmit when they again detect silence on the medium. The random pause and retry is an important part of the protocol. If two stations collide when transmitting once, then both will need to transmit again. At the next appropriate chance to transmit, both stations involved with the previous collision will have data ready to transmit. If they transmitted again at the first opportunity, they would most likely collide again and again indefinitely. Instead, the random delay makes it unlikely that any two stations will collide more than a few times in a row.

VDSL How VDSL Works The use of fast Internet connections has grown rapidly over the last few years. As more 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), currently dominate the industry. While both of these technologies provide Internet connections that are many times faster than a 56K modem, they still are not fast enough to support the integration of home services such as digital television and Video-on-Demand. However, another DSL technology known as very high bit-rate DSL (VDSL) is seen by many as the next step in providing a complete homecommunications/entertainment package. There are already some companies, such as U.S. West (part of Qwest now), that offer VDSL service in selected areas. VDSL provides an incredible amount of bandwidth, with speeds up to about 52 megabits per second (Mbps). Compare that with a maximum speed of 8 to 10 Mbps for ADSL or cable modem and it's clear that the move from current broadband technology to VDSL could be as significant as the migration from a 56K modem to broadband. As VDSL becomes more common, you can expect that integrated packages will be cheaper than the total amount for current separate services.

DSL Basics A standard telephone installation in the United States consists of a pair of copper wires that the phone company installs in your home. A pair of copper wires has plenty of bandwidth for carrying data in addition to voice conversations. Voice signals use only a fraction of the available capacity on the wires. DSL exploits

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this remaining capacity to carry information on the wire without disturbing the line's ability to carry conversations. Standard phone service limits the frequencies that the switches, telephones and other equipment can carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 400 to 3,400 Hertz (cycles per second). In most cases, the wires themselves have the potential to handle frequencies of up to several-million Hertz. Modern equipment that sends digital (rather than analog) data can safely use much more of the telephone line's capacity, and DSL does just that.

How an ADSL network is laid out

ADSL uses two pieces of equipment: one on the customer end and one at the provider end: •

Transceiver - At the customer's location, there is a DSL transceiver, which may also provide other services.

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DSL access multiplexer (DSLAM) - The DSL service provider has a DSLAM to receive customer connections.

Most residential customers call their DSL transceiver a "DSL modem." The engineers at the telephone company or ISP call it an ATU-R, which stands for ADSL Transceiver Unit - Remote. Regardless of what it's called, the transceiver is the point where data from the user's computer or network is connected to the DSL line. The transceiver can connect to a customer's equipment in several ways, though most residential installation uses universal serial bus (USB) or 10BaseT Ethernet connections. Most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, but the devices used by businesses may combine network routers, network switches or other networking equipment in the same box. The DSLAM at the access provider is the equipment that really makes DSL happen. A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. DSLAMs are generally flexible and able to support multiple types of DSL, as well as provide additional functions such as routing and dynamic IP address assignment for customers. For more information about ADSL, check out How DSL Works. DSL is a distance-sensitive technology: As the connection's length increases, the signal quality and connection speed decrease. ADSL service has a maximum distance of 18,000 feet (5,460 m) between the DSL modem and the DSLAM, Design By : Abdul Majeed ([email protected])

though for speed and quality of service reasons, many ADSL providers place an even lower limit on the distance. At the upper extreme of the distance limit, ADSL customers may experience speeds far below the promised maximums, whereas customers close the central office or DSL termination point may experience speeds 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 loading coils, that the telephone company uses to boost voice signals. These loading coils are incompatible with DSL signals because the amplifier disrupts the integrity of the data. This means that if there is a voice coil in the loop between your telephone and the telephone company's central office, you cannot receive DSL 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.

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

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

Fiber-optic cables, one of the major disrupting factors of ADSL, is actually what enables 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 way that ADSL does, but there are a couple of distinctions. VDSL can achieve incredible speeds, as high as 52 Mbps downstream (to your home) and 16 Mbps upstream (from your home). That is much faster than ADSL, which provides 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 the copper 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 planning Fiber 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. At the 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 junction box, the distance limitation is neatly overcome. The gateway takes care of the analog-digital-analog conversion problem that disables ADSL over fiber-optic lines. It converts the data received from the transceiver into pulses of light that can be transmitted over the fiber-optic system to the central office, where the data is routed to the appropriate network to reach its final destination. When data is sent back to your computer, the VDSL gateway converts the signal from the fiber-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 next page, you'll find a chart that lists the variations and how they compare to each other.

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

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

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High bit-rate DSL (HDSL) - Providing transfer rates comparable to a T1 line (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.

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ISDN DSL (ISDL) - Geared primarily toward existing users of Integrated Services Digital Network (ISDN), ISDL is slower than most other forms of DSL, operating at fixed rate of 144 Kbps in both directions. The advantage for ISDN customers is that they can use their existing equipment, but the actual speed gain is typically only 16 Kbps (ISDN runs at 128 Kbps).

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Multirate Symmetric DSL (MSDSL) - This is Symmetric DSL that is capable of more than one transfer rate. The transfer rate is set by the service provider, typically based on the service (price) level.

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Rate Adaptive DSL (RADSL) - This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line.

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

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

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

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

Max. Send Speed

Max. Receive Speed

ADSL

800 Kbps

8 Mbps

HDSL

1.54 Mbps

1.54 Mbps

IDSL

144 Kbps

144 Kbps

MSDSL

2 Mbps

2 Mbps

RADSL

1 Mbps

7 Mbps

SDSL

2.3 Mbps

2.3 Mbps

VDSL

16 Mbps

52 Mbps

Max. Distance 18,000 ft (5,500 m) 12,000 ft (3,650 m) 35,000 ft (10,700 m) 29,000 ft (8,800 m) 18,000 ft (5,500 m) 22,000 ft (6,700 m) 4,000 ft (1,200 m)

Lines Required

Phone Support

1

Yes

2

No

1

No

1

No

1

Yes

1

No

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 the next section, we'll talk about two potential VDSL standards.

Competing VDSL Standards There are two competing consortiums that are pushing to standardize VDSL. The problem is that their proposed standards use carrier technologies that are incompatible with one another. The VDSL Alliance, a partnership between Alcatel, Texas Instruments and others, supports VDSL using a carrier system called Discrete MultiTone (DMT). According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. DMT divides signals into 247 separate channels, each 4 kilohertz (KHz, or 1,000 cycles 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 a modem 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 bidirectional channels, for both upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of 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 247 distinct 4-KHz channels.

The other VDSL group is called the VDSL Coalition. Led by Lucent and Broadcom, the Coalition proposes a carrier system that uses a pair of technologies called Carrierless Amplitude Phase (CAP) and Quadrature Amplitude Modulation (QAM). CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the zero- to 4-KHz band, which is in all standard phone circuits. The upstream channel (from the user to the server) is carried in a band between 25 and 160 KHz. The downstream channel (from the server to the user) begins at 240 KHz and goes up to a point that varies with such 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 three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.

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

QAM is a modulation technique that effectively triples or quadruples the information sent over a line, depending on the version used. It accomplishes this by modulating (varying the shape of the carrier wave) and phase shifting (varying the angle of the carrier wave). An unmodulated signal provides for only two 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 the first 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. Design By : Abdul Majeed ([email protected])

Why 3 bits? Remember that you are sending binary information. Two states equal a single bit (21 = 2). Four states are equivalent 2 bits (22 = 4). Eight states equal 3 bits (23 = 8). By adding four more waves, shifted 15 degrees out of phase, you get 16 states and can send 4 bits per cycle (24 = 16). Adding another bit increases the number of phase shifts geometrically. To go beyond 4 bits per cycle becomes increasingly difficult because the number of necessary states doubles for each bit: 25 = 32, 26 = 64 and so on. This site provides a detailed look at QAM, and this one has a great QAM animation. There is a possibility that VDSL will encompass both standards, with providers selecting 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 us with 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. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back. Modems came into existence in the 1960s as a way to allow terminals to connect to 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-share facility 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 dumb terminal at the time was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user typed a character on the terminal, the modem sent the ASCII code for the character to the computer. The computer then sent the character back to the computer so it would appear on the screen.

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When personal computers started appearing in the late 1970s, bulletin board systems (BBS) became the rage. A person would set up a computer with a modem or two and some BBS software, and other people would dial in to connect 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 was tolerable 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. Once people started transferring large programs and images to and from bulletin board systems, however, 300 bps became intolerable. Modem speeds went through a series of steps at approximately two-year intervals: •

300 bps - 1960s through 1983 or so

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1200 bps - Gained popularity in 1984 and 1985

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2400 bps

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9600 bps - First appeared in late 1990 and early 1991

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19.2 kilobits per second (Kbps)

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28.8 Kbps

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33.6 Kbps

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56 Kbps - Became the standard in 1998

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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 for more information on 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 easy to understand. A 300-bps modem is a device that uses frequency shift keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, 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 is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. 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, and they are rare. Let's say that two 300-bps modems are connected, and the user at the terminal types the letter "a." The ASCII code for this letter is 97 decimal or 01100001 binary (see How Bits and Bytes Work for details on binary). A device inside the terminal called a UART (universal asynchronous receiver/transmitter) converts the byte into its bits and sends them out one at a time through the terminal's RS232 port (also known as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over 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-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines, are about the limit of these techniques (see the links at the 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 line cannot 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 modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated Design By : Abdul Majeed ([email protected])

copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central 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 is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data. The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. 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 an individual computer. Instead, we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet 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 these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same process occurs to get data from the ISP to your computer. See this page for additional information on PPP. If you want to know more about modems, protocols, and especially if you wish to delve into things like PSK and QAM in more detail, check out the links on the next page!

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Photo courtesy Corning

A fiber-optic wire

You hear about fiber-optic cables whenever people talk about the telephone system, 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 mechanical engineering 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 the diameter 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 following parts: •

Core - Thin glass center of the fiber where the light travels

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•

Cladding - Outer optical material surrounding the core that reflects the light back into the core

•

Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. 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 in telephones and cable TV)

•

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

Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared laser light (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 1,300 nm) 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 = 650 nm) 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 is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber.

Diagram of total internal reflection in an optical fiber

Design By : Abdul Majeed ([email protected])

The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called total internal reflection. Because the cladding does not absorb any light from the core, the light wave can travel great distances. However, some of the light signal degrades 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 the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibers 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 look at an example from a World War II movie or documentary where two naval ships in a fleet need to communicate with each other while maintaining radio silence or on stormy seas. One ship pulls up alongside the other. The captain of one ship sends a message to a sailor on deck. The sailor translates the message into Morse code (dots and dashes) and uses a signal light (floodlight with a venetian blind type shutter on it) to send the message to the other ship. A sailor on the deck of the other ship sees the Morse code message, decodes it into English and sends the message up to the captain. Now, imagine doing this when the ships are on either side of the ocean separated by thousands of miles and you have a fiber-optic communication system in place between the two ships. Fiber-optic relay systems consist of the following: •

Transmitter - Produces and encodes the light signals

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Optical fiber - Conducts the light signals over a distance

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

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Optical receiver - Receives and decodes the light signals

Transmitter The transmitter is like the sailor on the deck of the sending ship. It receives and directs 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 lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, nonvisible portions of the spectrum).

Optical Regenerator As mentioned above, some signal loss occurs when the light is transmitted through 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 optical regenerators 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 comes into the doped coating, the energy from the laser allows the doped molecules to become lasers themselves. The doped molecules then emit a new, stronger light signal 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 takes the incoming digital light signals, decodes them and sends the electrical signal to the other user's computer, TV or telephone (receiving ship's captain). The receiver uses a photocell or photodiode to detect the light. For a good discussion of lightwave transmission systems, see this page from Bell Labs.

Advantages of Fiber Optics Why are fiber-optic systems revolutionizing telecommunications? Compared to conventional 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 (cable TV, Internet) and you money.

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

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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 more channels to come through the cable into your cable TV box.

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

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Light signals - Unlike electrical signals in copper wires, light signals from one fiber do not interfere with those of other fibers in the same cable. This means clearer phone conversations or TV reception.

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Low power - Because signals in optical fibers degrade less, lower-power transmitters can be used instead of the high-voltage electrical transmitters needed for copper wires. Again, this saves your provider and you money.

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Digital signals - Optical fibers are ideally suited for carrying digital information, 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.

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

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Flexible - Because fiber optics are so flexible and can transmit and receive light, they are used in many flexible digital cameras for the following purposes:


Medical imaging - in bronchoscopes, endoscopes, laparoscopes




Mechanical imaging - inspecting mechanical welds in pipes and engines (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 you telephone Europe from the United States (or vice versa) and the signal is bounced off a communications satellite, you often hear an echo on the line. But with 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 -- how do 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, the less transparent it becomes due to impurities in the glass. However, the glass in an 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 ocean that 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 (SiCl4), germanium chloride (GeCl4) and/or other chemicals. The precise mixture governs the various physical and optical properties (index of refraction, coefficient of expansion, melting point, etc.). The gas vapors are then conducted to the inside of a synthetic silica or quartz tube (cladding) in a special lathe. As the lathe turns, a torch is moved up and down the outside of the tube. The extreme heat from 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 (SiO2) and germanium dioxide (GeO2).

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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. The purity of the glass is maintained by using corrosion-resistant plastic in the gas delivery system (valve blocks, pipes, seals) and by precisely controlling the flow and composition of the mixture. The process of making the preform blank is highly automated and takes several hours. After the preform blank cools, it is tested for quality control (index of refraction).

Design By : Abdul Majeed ([email protected])

Drawing Fibers from the Preform Blank Once the preform blank has been tested, it gets loaded into a fiber drawing tower.

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

The blank gets lowered into a graphite furnace (3,452 to 3,992 degrees Fahrenheit or 1,900 to 2,200 degrees Celsius) and the tip gets melted until a molten 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. The tractor mechanism slowly pulls the fiber from the heated preform blank and is precisely controlled by using a laser micrometer to measure the diameter of the fiber 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 is wound onto the spool. It is not uncommon for spools to contain more than 1.4 miles (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/in2 or more

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

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

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

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

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

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Operating temperature/humidity range

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Temperature dependence of attenuation

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Ability to conduct light underwater - Important for undersea cables

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

How Internet Infrastructure Works

Design By : Abdul Majeed ([email protected])

One of the greatest things about the Internet is that nobody really owns it. It is a global collection of networks, both big and small. These networks connect together in many different ways to form the single entity that we know as the Internet. 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, it doesn't mean it is not monitored and maintained in different ways. The Internet Society, a non-profit group established in 1992, oversees the formation of the policies 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 to Virtual Local Area Networks (VLANs) to provide some way of structuring this growth logically. Basically, a VLAN is a collection of nodes that are grouped together in a single broadcast domain that is based on something other than physical location. You learned about broadcasts earlier, and how a router does not pass along broadcasts. A broadcast domain is a network (or portion of a network) that will receive a broadcast packet from any node located within that network. In a typical network, everything on the same side of the router is all part of the same broadcast domain. A switch that you have implemented VLANs on has multiple broadcast domains, similar to a router. But you still need a router to route 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 the network decreases the chance that someone will gain access to information they are not authorized to see.

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

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

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

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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 specific types of employees (such as managers or sales people).

You can create a VLAN using most switches simply by logging into the switch via Telnet and entering the parameters for the VLAN (name, domain and port assignments). After you have created the VLAN, any network segments connected to the assigned ports will become part of that VLAN. While you can have more than one VLAN on a switch, they cannot communicate directly with one another on that switch. If they could, it would defeat the purpose of having a VLAN, which is to isolate a part of the network. Communication between VLANs requires the use of a router.

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VLANs can span across multiple switches and you can have more than one VLAN on each switch. For multiple VLANs on multiple switches to be able to communicate via a single link between the switches, you must use a process called trunking; trunking is the technology that allows information from multiple VLANs to be carried over just one link between switches. The VLAN Trunking Protocol (VTP) is the protocol that switches use to communicate among themselves about VLAN configuration.

In the image above, each switch has two VLANs. On the first switch, VLAN A and VLAN B are sent through a single port (trunked) to the router and through another port to the second switch. VLAN C and VLAN D are trunked from the second switch to the first switch, and through the first switch to the router. This trunk can carry traffic from all four VLANs. The trunk link from the first switch to the 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, physical ports connected to the switch. The VLANs can communicate with each other via the trunking connection between 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. Because of the transparent bridging algorithm and trunking, both PCs and the router think that they are on the same physical segment! .

Design By : Abdul Majeed ([email protected])

How Network Address Translation Works

The Internet has grown larger than anyone ever imagined it could be. Although the exact size is unknown, the current estimate is that there are about 100 million hosts and more than 350 million users actively on the Internet. That is more than the entire population of the United States! In fact, the rate of growth has been such 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 a computer 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 a unique 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 where you 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 4,294,967,296 unique addresses (232). The actual number of available addresses is smaller (somewhere between 3.2 and 3.3 billion) because of the way that the addresses are 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 and business networks, the number of available IP addresses is simply not enough. The obvious solution is to redesign the address format to allow for more possible addresses. This is being developed (called IPv6), but will take several years to implement because it requires modification of the entire infrastructure of the Internet.

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

This is where NAT (RFC 1631) comes to the rescue. Network Address Translation allows a single device, such as a router, to act as an agent between the 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 instructions with 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 put them through. The client calls the main number to your office, which is the only number the client 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 with your extension. The receptionist knows that you requested this call, and therefore forwards 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 the world. NAT has many forms and can work in several ways: •

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

In static NAT, the computer with the IP address of 192.168.32.10 will always translate to 213.18.123.110. •

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

In dynamic NAT, the computer with the IP address 192.168.32.10 will translate 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 IP addresses to a single registered IP address by using different ports. This is 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 to the 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 them and replace them with registered unique IP addresses. It is important to note that the NAT router must translate the "internal" addresses to registered unique addresses as well as translate the "external" registered addresses to addresses that are unique to the private network. This can be done either through static NAT or by using DNS and implementing dynamic NAT.

The internal IP range (237.16.32.xx) is also a registered range used by another network. Therefore, the router is translating the addresses to avoid a potential conflict with another network. It will also translate the 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 referred to as the stub domain. A stub domain is a LAN that uses IP addresses internally. Most of the network traffic in a stub domain is local, so it doesn't travel outside the internal network. A stub domain can include both registered and unregistered IP addresses. Of course, any computers that use unregistered IP addresses must use Network Address Translation to communicate with the rest of the world. NAT can be configured in various ways. In the example below, the NAT router is configured to translate unregistered (inside, local) IP addresses, that reside on the 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 with the public (outside) network. •

An ISP assigns a range of IP addresses to your company. The assigned block of addresses are registered, unique IP addresses and are called inside global addresses. Unregistered, private IP addresses are split into two groups. One is a small group (outside local addresses) that will be used by the NAT routers. The other, much larger group, known as inside local addresses, will be used on the stub domain. The outside local addresses are used to translate the unique IP addresses, known as outside global addresses, of devices on the public network.

IP addresses have different designations based on whether they are on the 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.

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

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

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

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

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

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

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The NAT router translates the inside global address of the packet to the inside 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 remote computer (or computers) using different TCP or UDP ports. An IP packet has a header that contains the following information: •

Source Address - The IP address of the originating computer, such as 201.3.83.132.

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

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Destination Address - The IP address of the receiving computer, such as 145.51.18.223.

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

The addresses specify the two machines at each end, while the port numbers ensure 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 (216) values. Realistically, since different manufacturers map the ports in slightly different ways, you can expect to have about 4,000 ports available.

Design By : Abdul Majeed ([email protected])