Introduction to Ethernet

Posted On 2005-11-1 by FortyPoundHead
Keywords: Ethernet Network Networking WEP Wireless Gigabit fiber
Views: 1450

Many computers have Ethernet support built into the mainboard. If not, a PCI Ethernet adapter card costs $15 and can be added to any machine. While the USB and Firewire ports connect the computer to secondary devices (printers, scanners, external disks) that it completely controls, an Ethernet connects to peers, such as other computers. In a company, the Ethernet port connects the computer to the corporate network and, through it, to the Internet. At home, the Ethernet connects two or more computers, allowing them to share files and printers. A single PC may be connected to a DSL or Cable modem through a dedicated Ethernet port, or the modem can be connected to a router/firewall so it can be shared by many computers.

Ethernet was invented by Xerox. It is capitalized, because "Ethernet" has been a registered trademark of Xerox, although like the Macintosh/Windows interface that was invented in the same lab at the same time, Xerox never really learned how to make money off the invention.

The first Ethernet was just a thick copper wire encased in a protective sheath. Each computer would connect to the wire with a special tap. Since there was just the one piece of copper, any data transmitted by any computer would be received by all the other computers connected to the wire.

Wire is cheap and dumb. However, the equipment needed to connect to the wire was expensive. Twenty years ago the card that connected a minicomputer to the Ethernet cost $2000, while a bridge (a two port switch to connect two separate Ethernet wires to each other) cost $7000. Then chips got cheaper. Today an Ethernet adapter is $15 and an eight port switch costs $28.

The change in technology and economics transformed the Ethernet physically. It no longer made sense to have a single big dumb piece of copper. With cheap smart circuits, the network could be made simpler and cheaper by connecting computers to switches over ordinary (although high grade) phone wires.

A home or business phone line uses one pair of copper wires. The phone both receives and sends a signal at the same time over the one pair of wires. The phone company generally wires a house with two pairs to provide a backup and to allow two phone lines to be installed in the same residence.

Ethernet uses both pair of wires. Conventional Ethernet operating at speeds of 10 or 100 megabits per second uses one pair of wires to send data in one direction, while the other pair is used for data moving in the opposite direction. It can operate over short distances using old phone wire, but to get the best range and quality you should use modern wires.

The two pair of wires connect two devices at each end. You can connect two computers to each other, for high speed file transfer, or you can connect a computer to a DSL or cable modem. To build a real network, you need to use a switch. While a computer's Ethernet adapter connects to one wire, a switch has 4, 5, 8, or more ports each of which can be connected to a different PC. The switch listens to all of the lines simultaneously. Any data transmitted by any computer is then retransmitted by the switch to its destination device.

Cables and Jacks

An ordinary telephone uses the small size standard phone company jack called an "RJ11". It supports four wires. The phone company also has a larger standard jack called an "RJ45" with room for eight wires. Normally the larger jack is used for corporate systems with many lines. Ethernet standardized on the larger jack even when it only uses four wires. If nothing else, it is useful for distinguishing the network jack from the smaller modem phone line jack on most laptops.

The extra room proved useful in the most recent upgrade to gigabit speeds. Two pair of phone wires cannot handle the speed. It was necessary to add two more pair of wires to fill up the entire RJ45 plug.

At speeds of 10 or 100 megabits, the Ethernet devices at each end of the wire (the computer and the switch) each expect to transmit its data on one pair of wires and receive its data on another pair. They have to choose pairs that match. This is achieved in several ways:

Computers and printers are all wired to transmit on one designated pair. Switches, routers, and modems, on the other hand, expect to receive data from that pair and transmit through the pair computers receive on. So an ordinary cable can connect a computer to a port on a switch.

Sometimes you want to connect similar devices directly to each other. For example, you can create an "Ethernet" simply by connecting two computers to each other. However, since the two Ethernet ports are wired identically, you need a "Crossover" cable. This cable connects each pair of wires to one position on the plug at one end, and the opposite position on the plug at the other end. What one computer regards as transmit, the other regards as receive.

When one switch is full, you get additional ports by connecting it to another switch. You could connect the two switches with a special Crossover cable. However, this is such a common requirement that one port on each 10/100 megabit switch is specially wired as the downlink port. That port is wired like a computer instead of the normal switch port. So an ordinary cable can be connected from the downlink port of the switch to any standard port on another switch.

When you move to Gigabit Ethernet, however, there are no dedicated wires. Each wire pair has to carry 250 megabits per second of the aggregate 1 Gigabit load. That means that every pair has to be able to both transmit and receive data. When a Gigabit Ethernet device (computer or switch) is connected to an older 100 megabit device, they not only sense the slower speed but also sense which pair of wires to use as transmit and which as receive.

The original Ethernet standard operated at 10 megabits per second. When run over twisted pair wire, this standard is called "10BaseT". The speed is "10" (megabits/sec), the "T" is for "telephone twisted pair". "Base" standards for a "baseband" signal. In the popular press, "broadband" has been used as a synonym for "high speed". In technical standards, however, "broadband" was used for Cable TV signals (transmitted over long distance as a radio frequency signal) and "baseband" means a short distance signal transmitted as a voltage difference on wires.

The current standard supports 100 megabits over the same type of cable, so it is called "100BaseT". Actually the quality of the cable is slightly higher for 100BaseT than for 10BaseT. Cable quality is designated as Category 3, 4, 5, or 6. Normally this is shorted to "Cat" and you will sound more impressive if you ask for "Cat 5" cable. The cable gets better with every higher number. Higher quality cable may cost a few cents more, but as everyone with a closet full of power cords can testify, wire lasts for decades while technology changes.

The highest current standard is Cat 5E or Cat 6 cable. This is physically different from all the previous generations of Ethernet because it contains four twisted pair of wire that connect to all eight pins on the RJ45 plug. It supports 10 and 100 megabit transmission, but it also support the emerging standard for Gigabit Ethernet or 1000BaseT.

Packets and Hardware Addressing

Today Internet protocols are used for everything. Ethernet, however, predates the Internet and has its own conventions for device addressing and packet formation. Ethernet conventions extend only as far as the wire. An Ethernet may connect devices in your home, but to communicate outside your house you need Internet support.

When an Ethernet was formed from one shielded copper wire, the maximum size for each packet of data was set to be 1500 bytes. Anything bigger has to be broken down into multiple packets. After a device sends one packet it must pause before sending the next packet. All this made sense when devices shared the same wire, but with modern equipment these conventions just slow down large file transfer.

Every Ethernet adapter is assigned a unique six byte number called its "MAC" address. Every packet of data has a source MAC address, of the adapter that sent it, and a destination MAC address. Normal data is sent to one machine, but a packet can be given a "broadcast" address and it will be duplicated by the switches and sent to every computer in the local network. The adapter card in every computer checks the destination MAC address in every packet it receives. It accepts packets addressed to it or containing a broadcast address. It discards all data addressed to another machine.

Modern switches watch the packets that pass through them and learn the port to which each MAC address is connected. However, a residue of the old days when the Ethernet was just a dumb piece of copper is the convention that all packets could be broadcast to all computers and the adapters would ignore packets not addressed to them. The ability of switches to filter out and direct traffic aids performance, but it is not required for the system to work.

Internet protocols were added on top of this system of Ethernet packets. Each Internet device has an IP address. Internet packets are directed to the IP address. Each computer or router maintains a table that maps IP addresses to Ethernet MAC addresses. Traffic to other computers on the local network is sent directly. Traffic to other computers goes out through the gateway router connected to the modem.

Switches, Routers, Gateways, and Firewalls

A DSL or Cable modem frequently comes with an Ethernet adapter for a PC and a cable. Put the adapter in the PC, connect it through the cable to a jack in the modem, install the software, and the computer is connected to the Internet. This creates a simple Ethernet with just two devices.

To share the Internet connection or other devices between two or more PCs, you need a switch or router.

A "switch" is a device typically costing $30 to $50 with a row of jacks. Connect each computer to the switch through phone wire cable. Any data sent by any computer goes through the switch and arrives at the computer or device to which it was directed. A switch knows nothing about Internet protocols. Data move through the switch, but the switch itself neither generates nor receives messages.

A "router" is a slightly more expensive and more intelligent device. Home users typically purchase a router that controls the DSL or Cable modem connecting to the Internet. A router knows Internet protocols. It has an address just like the computers. Modern routers frequently have a built in Web Server and can be controlled from a PC Web Browser.

To clarify obsolete terminology, a "hub" is an older device that does a subset of the functions of a modern switch. Given current prices, it makes no sense today to use hubs.

A switch has memory to hold some amount of data from each device. This allows different computers to connect to the same switch at different speeds. For example, an old printer could connect to the switch at 10 megabits per second while a more recent computer connects at 100 megabits per second. The computer sends data 10 times faster than the printer can read it. The switch buffers a block of the data and sends it on to the printer at whatever speed the printer can manage. Fortunately, the higher level communication protocols (like the Internet's TCP) require every computer to after sending a certain amount of data and wait for it to be received and acknowledged before

The switch has memory to receive each packet of data, and a processor to examine the message. Switches learn the MAC address of every machine connected to them. When an Ethernet packet has a destination MAC address that they know is connected to a particular port, then they only transmit the message out that one port. They can receive data simultaneously from two or more devices, and transmit data simultaneously to multiple devices. Buffering also allows a switch to receive data at 100 Megabits per second and then transmit it at the slower rate of 10 Megabits per second to any old devices still connected to the network. Switches are, therefore, more flexible, more efficient, and more secure than hubs.

Switches operate transparently as part of the Ethernet hardware. They do not have any understanding of the Internet, or Windows, or any of the applications. However, if you have a DSL or Cable modem and want to share Internet access among several different computers, then you may want to spend $80 to get an even more intelligent device called a "router" or "gateway".

At this point the terminology gets a little fuzzy. There are technical definitions for routers and gateways in terms of communications theory, but they are not particularly useful building a home network. Many companies sell versions of what is essentially the same device that combines features of a "switch", "router", "gateway", and "firewall" all combined together.

A switch is part of the Ethernet fabric, but the type of switch used in a home network doesn't appear itself as a device on the network. Message pass through it, but it has no MAC address and neither sends nor receives messages itself. Routers not only have an address, but they can be configured by utilities on the PC.

A popular low cost vendor is Linksys, although Netgear and D-Link also have products. These devices can be used to share a single DSL or Cable modem among several computers connected by Ethernet cable, and some also contain a Wireless Access Point for 802.11 devices. They are configured to logon to the DSL or Cable service supplying any userid, password, or station ID assigned to you. Other machines are configured to send all Internet traffic to the Router.

Messages transmitted across the Ethernet indicate that they are being transmitted from one machine to another. The "gateway" function in the Router box changes the form of these messages so that all traffic going out to the Internet appears to be coming from different programs on a single computer. Responses coming back from the Internet are converted back and transmitted over the Ethernet to the proper designation machine. This trick, called Network Address Translation or "NAT", is necessary because the DSL or Cable systems are only designed to talk to a single computer, and the NAT Gateway function makes them believe that there is only one computer even when the traffic is coming from a whole home network.

The Router allows local computers to connect to any service on the Internet, but generally machines on the Internet are not allowed to connect through the Router to any machines on your Ethernet. This function is called a "Firewall" and protects the casually configured home computer user from Code Red, Slammer, and all the other worms and hackers that may try to infect a machine.

Internet Addressing

Ethernet adapters, switches, and MAC Addresses provide basic low level communication between machines. Microsoft used to support simple communication protocols that allowed Windows machines to share files and printers directly on top of the basic Ethernet protocol. However, connectivity to the Internet has become essential for almost every computer. It no longer makes sense to support networking without Internet support. Today every network has to be configured for internet protocols just to let the machines talk to each other.

Internet protocols require a second address number called the "IP Address". The IP address is a four byte number, and by convention it is represented as the decimal numeric value of each byte (0 to 255) separated by periods. Yale University, for example, has IP addresses beginning with 130.132.*.* and the machine on which PCLT is hosted at the time this is being written has address Every source or destination of messages on the Internet has to be assigned one of these numbers. There are enough consumers who cannot set the clock on their microwave oven, so expecting them to correctly enter a number like this into the system is unreasonable. Most of the time the number is provided automatically over the network.

If you use a dial up phone line to connect to the Internet, the Internet Service Provider gives you a phone number to dial and an id and password to logon to their system. During the logon the ISP passes back to your machine an IP address it should use during the connection.

The same approach is used when a high speed DSL or Cable modem is connected to a home network through an Ethernet Router box. The router is provided with a node name, userid, or password to logon to the ISP network. The ISP passes back an IP Address value that the Router box then uses to communicate with the outside world.

In either case, the IP address provided by the ISP, even temporarily, allows one computer or the one Router box to communicate with any mail, Web, or other server anywhere in the world. This still leaves the question of how computers inside your home talk to each other or to the Router box. The NAT function in the Router translates all messages from other computers so that they look, to the outside world, like programs running inside the Router itself. Therefore, other computers in the home network don't have to be assigned addresses that are meaningful outside the home.

The Internet reserves sets of IP Addresses for non-public use. These numbers can be assigned to machines that are isolated from the public network and either do not communicate at all or else only communicate through gateways. A popular range reserved for non-public use are the addresses beginning 192.168.1.*.

The simplest way to assign IP Addresses to all the computers of a home network is to let the Router box that provides connectivity to the Internet assign numbers on request to any machine that asks for one. By default, the Linksys Router assigns itself the address in the home network. It then skips numbers 2-99 and assigns numbers as requested by computers starting at The protocol for serving up IP Address values on request is called DHCP. All of these values can be configured in the advanced control panels of the Router, but there typically is no reason to change them.

So having explained how this all works, the equipment and services are generally configured so you don't need to know the details.

The ISP will provide you with a DSL or Cable modem, some software for a computer, and the names and passwords needed to access the system. Since some ISP agreements don't allow multiple machines in a home network to share the same line, it may be a good idea while the installer is in the house to hide any Router in a closet and install and test everything on one computer.

After the ISP equipment has been tested, replace the single computer with the Router box and connect at least one computer Ethernet adapter to the Router. The computer should be set to pick up its IP Address automatically from the network, and if it is the same computer used to test the modem it should probably be rebooted so it picks up a new address from the Router. Now follow the instructions in the Router manual to configure the Router with the same ID and password that the ISP provided to make the previous connection. It may be helpful to know the buzzword that identifies the particular type of logon protocol used by the ISP (for example, "PPoE" is a popular choice) since this has to be selected from a menu of options in the Router.

Once the Router logs on successfully to the ISP, computers connected to it through Ethernet should be able to access Web sites. The IP addresses vended by the Router also allow the computers to talk to each other to share files and printers.


A new generation of Gigabit Ethernet is beginning to appear. It comes standard on some high end motherboards. A Gigabit Ethernet adapter costs $45 (instead of $15 for a 100 Megabit device). The real pricing problem, however, comes in the switches where an eight port device costs $180, four times the cost of a 100 Megabit switch.

In theory a Gigabit Ethernet can run at 10 times the speed of commodity 100BaseT. In practice, you will be lucky to do better than twice the speed. Other things start to become the limiting factor.

* All desktop and home computers have the standard PCI bus with 32 bit slots running at 33 MHz. This produces a theoretical limit of 133 Megabytes per second, but overhead reduces this to a practical limit of 80 Megabytes per second. So a Gigabit Eithernet adapter cannot reach it full speed unless it is connected at both ends to a system with 64 bit PCI slots, and today they are only found on servers and expensive workstations.

* A typical desktop hard disk running at 7200 RPM can read or write data at only about 30 to 40 megabytes a second. So if the data is coming from or going to a disk, its performance will be the limiting factor.

* The real problem is that the Microsoft Windows network and Internet support is too slow. No matter how fast the CPU, bus, and other hardware, the current operating system simply cannot run Gigabit Ethernet at full speed.

Wireless (a, b, and g)

The FCC in the US and its international counterparts license various frequencies to radio, TV, military, and other users. Specific bands of frequency are assigned for "unlicensed" use by household devices. The first devices to use these frequencies were cordless telephones. Computers quickly followed.

The first unlicensed frequency range was 900 MHz. There are still wireless phones in this frequency, but an initial generation of non-standard wireless computer cards has now been phased out. A second band of frequencies was opened at 2.4 GHz. This is the most popular choice for high end wireless phones and the current standard "802.11b" ("WiFi") wireless Ethernet equipment. A new band of frequencies at 5 GHz is now becoming available. It is used for new "802.11a" wireless Ethernet equipment, but there are no wireless phones currently operating in this range.

The 2.4 GHz frequency is preferred by wireless phones because it has good performance, long range, and some ability to pass through the walls of a house. Its disadvantage is that the frequencies are crowded with devices, and they are subject to interference from microwave ovens. The 5 GHz devices are free from interference, but they don't stretch as far and have serious problems passing through walls.

The 2.4 GHz range is divided into 11 "channels". A low speed device like a cordless phone can operate on one channel. Computers will also fall down to lower speeds when there is a lot of interference. However, to run at the maximum 11 Megabits per second allowed by the standard, an 802.11b device has to spread out across adjacent channels. In practice, there are only three independent full speed wireless Ethernet frequencies centered on channels 1, 6, and 11.

If you have to go a short distance through an open area, the new 5 GHz 802.11a devices provide speeds up to 54 Megabits per second, compared to the 11 Megabits for the 2.4 GHz 802.11b. You need different hardware for each standard, although some new adapter cards and wireless base stations have both types of hardware and can operate on both the a and b standards.

Existing wireless networks in schools, offices, cybercafes, and airports are based on the established 802.11b standard. Existing laptop computers with built in wireless support only work on b networks. So while someone searching for the best possible performance might get an adapter that runs at both the a and b standards, getting an a-only adapter is a bad idea.

The a and b standards run at two entirely different frequency ranges. They also use different modulation techniques. Modulation converts information into waves that can be transmitted over a radio frequency. Everyone is familiar with the AM (Amplitude Modulation) radio technology which encodes a sound signal as differences in the intensity of the radio signal. The problem with AM is that there are many other sources of radio frequencies (electric mixers, lightning) that produce interference to an AM signal. The alternative is FM (Frequency Modulation) radio that encodes its signal as the variation in frequency of a carrier signal. If you were to use the same techniques on a signal flag system, then AM would generate a signal by varying the size of the flag (small or large) while FM would vary the color of the flag (red or blue). The problem is that a system designed to detect one form of modulation cannot detect the other form, just as a person who is color blind would see differences in size but not in color.

In the digital world, better forms of modulation allow higher data rates over the same assigned frequency band. Given a fixed number of flags, one could generate more signal by varying the size, shape, and color simultaneously rather than changing only one characteristic. More sophisticated modulation is limited only by the processing power of the chip that generates and decodes it. As chip technology improves, so does data capacity. The a standard runs at 54 megabits per second while the b standard runs at 11 because of differences in the modulation.

A new standard proposes to use the high speed modulation designed for the a standard on the 2.4 GHz frequency range used by the b standard. The result is called 802.11g. All devices that support g will also communicate to b devices, although at lower speed. When both b and g devices coexist in the same area, the slower speed b devices interfere, even when they are inactive, with the ability to get a full 54 megabit data transfer rate.

This produces several classes of devices

Equipment that supports only the 802.11b standard is cheapest option and will run almost everywhere.

Equipment that supports the 802.11g standard also supports b. It is a bit more expensive. It has to support two types of modulartion and enough firmware to be able to switch back and forth between the two standards. If the station is close enough to the access point to run at full speed then it is fast enough to support multimedia and really fast file transfer.

Equipment that supports only the 802.11a standard seems like a generally poor choice. It could not itself talk to the large number of devices that come with b support built in. The 5 GHz frequency range also has trouble going through walls, so this type of equipment may be limited to a single room.

The best all around choice are devices that support a, g, and b simultaneously. They will negotiate the best form of transfer available and can simultaneously support slow speed 11 megabit b traffic and high speed 54 megabit a traffic in the same area over the two different frequencies.

Since 802.11g runs at the same 2.4 GHz frequency as 802.11b, both can be supported on the same hardware. The difference between them is mostly firmware programming, and an adapter card or base station can use g when possible and fall back to b when necessary. A device that supports both a and b has to have two separate pieces of hardware and therefore must cost more. The problem is that as soon as any 802.11b device appears in the area, all 802.11g devices (base stations and adapter cards) have to fall back to b mode in order to support the newcomer. In contrast, device with both a and b hardware can support both standards simultaneously (since they operate on different frequencies and do not interfere).

The simplest way to add Wireless capability to a home system is to use a Wireless Router instead of a conventional Router to connect to the DSL or Cable modem. Wireless routers have all the functions and Ethernet ports of the standard router, and a set of antenna and wireless capability added in. Prices may vary, and you may prefer another vendor. However, for reference the following four Linksys routers were priced in Feb, 2003:


4 RJ45 ports (no wireless)

802.11b Router + 4 RJ45 ports

802.11g Router + 4 RJ45 ports

802.11 and b Router + 4 RJ45 ports

802.11a, b, and g Router + 4 RJ45 ports

The most common Wireless Ethernet adapter is a Cardbus device that plugs into laptop computers. To make a wireless connection from a desktop computer, the most convenient option is probably to use an external network adapter device with a USB connector to the PC.


Wireless Ethernet broadcasts data for at least a hundred feet. The signal may go much farther if the recipient uses more sensitive professional equipment. To provide even the most basic elements of privacy, the data should be encrypted.

Wireless standards provide for data encryption called 'WEP". WEP comes in 64, 128, and 152 bit versions. The larger number is better, but it must be supported by all of the devices in the network. It is generally agreed that 64 bit WEP is not particularly good, but it is still better than nothing. Use at least 128 bit if possible.

WEP is driven by an encryption key. You can generate the key manually, but there is typically an algorithm that will generate a key from a password. The key is initially generated on the Access Point. It must then be entered into the configuration panels of every computer that you want to connect to the Access Point. Since it is very easy to get this wrong the first time you try to do it, make sure that there is at least one Wired Ethernet computer that can connect to the Access Point and run the configuration panels. Otherwise, if something goes wrong you may not be able to get back to the Access Point with any Wireless device to check or change the WEP configuration.


Wireless Ethernet adapter cards can be configured to run in "ad hoc" or "infrastructure" modes. The "ad hoc" mode allows any two computers that come within range of each other to begin communicating. This is not, however, an easy configuration to debug. Furthermore, low level Ethernet connectivity has already been shown to be useless without also getting an IP Address. Since "ad hoc" operation requires manual configuration of IP, it is difficult to set up.

Normally the adapter is configured for "infrastructure" mode. It then searches not for another computer, but for Wireless Router or "Access Point" such as the devices listed in the previous table.

A 2.4 GHz device (b or g) has a range of 100 to 150 feet indoors, less through thick walls. A 5 GHz device has a range of 25 to 75 feet and generally cannot penetrate a real wall. To provide full coverage, a company may scatter Access Points around a building. By luck, somebody is going to be located midway between two Access Points with the opportunity to connect to either.

Access Points are configured with a network identifier (SSID) and a channel number (recommended to be 1, 6, or 11). Access points that cover adjacent territory should be assigned to different channels so their signals do not interfere with each other. Generally, Access Points shared by workers in the same company, or Access Points at opposite ends of a really big home, will have the same SSID. You can configure the Access Point to either broadcast the SSID or to be quite. Broadcast SSID makes it easy to select particular Access Points when there are several networks close to each other, but keeping the SSID a secret improves security.

If you live in an apartment building, it is possible that the signal from a neighbor's Access Point will leak into your apartment. It would then be strongly recommended that you choose a different SSID and a different channel.

When you install a Wireless Ethernet adapter in a computer and set it up for "infrastructure" mode, the Windows support will display the SSID of all the Access Points close enough to read their broadcast. The user must select one Access Point, and if it is secured must provide a WEP Key.

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