What is Ethernet

Ethernet is the most widely-installedlocal area network (LAN) technology. Computers, printers and other devices usethe Ethernet protocol to communicate with each other. Originally devices usingEthernet were connected together by coax cable in a serial, daisy-chainfashion. 

At first a data rate of 10Mbit/sec (million bits per second) was carriedbetween PCs connected together by one or two coax cabling systems:

• 10base2 (thin coax) carried data about 185 meters before a repeater was needed.
• 10base5 (thick coax) could carry data a maximum of 500 meters without a repeater.

10baseT (twisted pair cable) appeared later and carried data 100 meters withouta repeater. Today it is the most widely deployed cabling system used in LANs.

Data Collisions

Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD)to maintain the orderly flow of messages on the network. In CSMA/CD, each PClistens to the line. If the line is quiet any PC can begin transmitting.

Since any PC can transmit when the line is quiet, two will inevitably starttransmitting at or near the same time. The messages collide and the result isthat neither message gets through.

When a collision is detected, all PCs must back off and wait a random amount oftime before transmitting any new messages. If one device keeps transmitting forwhatever reason, the network gets tied up and no one else can use it!

Ethernet Hub

Eventually the hub was developed. It can connect a small number of PCs togetherin a star formation. Think of spokes of a wheel all connected to the hub at thecenter. The hub monitors each PC connected to it and disconnects any PC thatviolates the CSMA/CD rule.

It was a great improvement over the serial, daisy-chain, coax cable system,since it could prevent one over talkative PC from taking down the network.
It also eliminated the problems that often occurred when one PC failedelectronically, or was improperly disconnected from the coax cable.
If one PC failed in a certain way, it could disable the entire network.
If the coax cable at one PC was improperly connected, it split the network intotwo, isolated networks on either side of it. PCs in one network could no longertalk to PCs in the other network

Ethernet Switch

Switches interconnect LANs and transfer messages between them. The developmentof switches allowed the size of networks using Ethernet to become larger.

Without switches the size of an Ethernet system is limited. The reason for thisis that computers at opposite ends of a large network are too far apart tofollow the CSMA/CD protocol required for orderly traffic flow.

Collisions let each transmitting PC know that its message didn't get through.When PCs are too far apart they don't know about collisions that may behappening to their message at the far end of the network due to the very longback and forth time delays. Since they don't know their message collided andfailed, they never retransmit it!

Switches allow a manageable collision domain (area) to be set up for each LAN.When a message crosses from one LAN to another through a switch, it enters anew collision domain and in effect becomes a 'new' message in that domain. Theexplanation of how this works is beyond the scope of this article. With thecollision detection problem solved, very large networks were able to be built.

Ethernet in the Internet

The Internet is made up of millions of PCs and other equipment connectedtogether in thousands of dispersed LANs of varying sizes. The LANs pass data toeach other through banks of switches and routers (very specialized switches)that are operated by communications companies and organizations around theglobe.

If you work in an office, 10baseT Ethernet is probably the way all your localPCs, printers and other equipment is connected together. The 8-pin,telephone-style plug that connects to the Network Interface Card (NIC) in theback of your PC is most likely one end of a 10baseT cable.

The speed of Ethernet keeps getting faster

• 100baseT or Fast Ethernet (100 million bits per second) is widely deployed in LAN backbones. Backbones are high speed data paths that are fed by many slower speed data streams, like 10baseT.
  It is much like smaller roads feeding a few, slow moving cars onto a major highway (backbone), that carries more cars at higher speed.
• Eventually as more and more data comes together, 1000baseX or GB Ethernet (1 billion bits per second) is used to carry a tremendous amount of extremely fast moving data. In the traffic analogy, it is like an interstate highway.
  This data keeps getting merged together into higher speed streams, called Optical Carriers, which use fiber optic cable to carry data. OC-192 (10 billion bits per second) is now being deployed. At this point we've left the highway and are using supersonic jets!

What is a Modem?

The word Modem is an acronym for Modulate-Demodulate. A modem converts (modulates) digital signals into analog signals thatcan be sent over telephone lines. The Modem at the receiving end converts theanalog signals back into digital signals.

Getting pages from this web site to your PC requires a modem of some type(dial-up, cable or DSL), which converts the digital data into analog signalsthat travel to the phone company and then out over the Internet.

The move to Broadband

Most people still use dial-up modems. If you do, you should consider switchingto a cable or DSL (broadband) modem. Both operate at much higher speed thandial-up modems.

We will provide information on thissite that can help you decide.

PCs, printers and other digital output devices can not connect directly totelephone lines.

Digital devices produce outputs that can be in one of two distinct states, 0 or1. Like a stair step, they jump from one level to the other, never stopping inbetween.

In contrast, analog signals like that which is produced by a human voice, varycontinuously in time in a smoother, more hill shaped manner.

Digital signaling produces very high frequencies, that get attenuated (reduced)as the signal goes down the line. Loss of these frequencies at the receivingend can result in a loss of information.

The way around this is to use the digital 1s and 0s to vary an analog carrier,which is sent down the line.

Modulate

Modulate means to encode. A simple modulation type is Frequency Shift Keying.In this modulation scheme when the modem sees a digital 1 it sets the carrierto one tone (A) and sends it down the phone line. When it sees a 0 it changesthe carrier to a different tone (B) and sends it down the line.

Demodulate

Demodulate means to decode. The receiving modem upon detecting a tone A convertsit to a 1. And when it detects tone B, it converts it to a 0.

Training Tones

Data is not sent until after the modems have connected andtrained. When your computer first connects to a line, the sporadic tones youhear are your modem and the one at the other end going through a training mode.They are searching for a rate they can both support using the current phoneline. If your modem is 56K and the other is 14.4k, the fastest they can go is14.4K. If during training the line won't support 14.4k, due to noise whichcauses errors, they have to drop to a lower rate that the line can support,like 9.6K for instance.

Morse Code

Morse code is a very old code where each character (letter, number or control)is assigned a unique sequence of dots and dashes. A Morse code operator sendingdots and dashes down a telegraph line is modulating a carrier tone. A dot is ashort burst of an analog tone. A dash is long burst of the same analog tone.Morse code was first used in telegraph communications. Many ham radio operatorsstill use it today.

The Morse code operator receiving the stream knows what the relative length ofthe tones mean. He knows that a short tone represents a dot and a long tone isa dash. So he demodulates (decodes) the tones he hears on the line back intodots and dashes. Between each coded character sequence of dots and dashes is ashort rest period which is required so that the the beginning and end of eachcharater is known.

In Morse code the sequence 'dot dot dot' represents the letter 'S' and 'dashdash dash' represents the letter 'O.'

So the sequence:

'dot dot dot - rest - dash dash dash - rest - dot dot dot'
coming down the line represents the letters SOS which happens to be theuniversal distress signal. People who have become trapped will often tap outSOS, hoping that rescuers will hear their call for help.

Summary

In Morse code, dots and dashes are sent down the line at a very slow rate,about 50 characters a minute. 10baseT Ethernet by contrast sends about1,000,000 characters per second!

In modern digital communications the digital stream of 1s and 0s are modulatedonto phone lines by modems using amplitude, frequency and phase modulation orvarious combinations of them. The receiving modem uses a compatibledemodulation scheme to convert the modulated signal back into the originalsequence of 1s and 0s.

Memory - what type, speed and quantity should you use?

Your PCs main memory is called by a number of names: RAM, DRAM,SRDRAM, DDR SDRAM. Don't be confused by these names. They are all types of RAM(Random Access Memory).

PCs first used DRAM (Dynamic Random Access Memory) for main memory. These wereimproved and became SRDAM (Synchrounous DRAM). The latest version is a DDRSDRAM which is a (Double Data Rate SRDAM). It runs twice as fast as SDRAM.

Main memory is used to store programs and data electronically when your PC isrunning. It communicates with the processor over the front side bus. Memorycapacity is measured in megabytes Mb (millions of bytes).

A big memory allows your PC to run multiple programs with out slowing down andwith much less of a chance of crashing. Your memory also needs to be fast andmanufacturers offer it in a number of speed grades.

Multiple memory chips are packaged together on a small circuit board. Thisassembly is called a DIMM (Dual Inline Memory Module). DIMMs plug into themotherboard. SDRAM and DDR SDRAM memory chips are still found in DIMMS, but theSDRAM chip have pretty much been replaced by the much faster DDR SDRAM chips.

Recommendation:

Get at least 128Mb of DDR SDRAM at a speed grade of PC2100. Make sure your PCmotherboard has sockets that accept DDR DIMMS and a memory controller capableof running PC2100 memory. Any new PC motherboard should be able to supportthese requirements.

SDRAM DIMMs are still available in the after market. They have 168 pins andrequire a socket different than that used by the 184 pin DDR DIMM.

Some motherboards have both types of sockets that allow you to use either typeof DIMM. You can't use both types of DIMM together. It's all one or all theother.

Protect your PC with a Surge Protector

This is an area that is critical to thecontinued operation of your PC. Power surges and lightning strikes cansometimes get coupled into your PC through the power line or phone line. Thesesurges can and often do damage PCs.

Surge protected power strips have snubber circuits that filter out thesepotentially dangerous power line transients.

The AC line protection feature is easy to use.

1. Plug the surge strip into any working AC receptacle.
2. Then plug your PC into one of the receptacles on the strip that is protected. Usually they all are, but read the markings on the face of the strip to be sure.

In addition to AC line protection many surge protectors alsohave protected phone line connectors. Use these to protect your PC'smodem:

1. Plug a standard telephone cable between the phone wall connector and the 'In' connector on the surge protector.
2. Use a second telephone cable to connect your PC's modem to the 'Out' connector.

Your modem and PC are now protected from dangerous phone linetransients.

Surge protectors with these features cost around $20 dollars and are well worthevery penny!

Internet Addresses - IPv4 and IPv6

IPv4 stands for Internet Protocol version 4. It is the originalstandard set up for handling IP addresses when the Internet was initialdeveloped by DARPA (Defense Advanced Research Projects Agency) in the early1970s.

IPv4 uses a 32 bit address field which provides for 4,294,967,296 uniqueInternet addresses. This is the number of computers/devices that can beconnected to and use the Internet. In the early 1970's the population of theearth was less than 4 billion people, personal computers did not exist, and atmost there were hundreds, perhaps thousands of mainframe and mini computersthat had been assigned Internet addresses. So the 4 billion plus address spacewas deemed to be more than enough to last beyond any foreseeable requirements.

IPv4 addresses are all but consumed

By 1992, the rapid explosion of the Internet fueled by the vast number ofpersonal computers attaching to it, made it clear that the IPv4 address spacewas already consumed to the point that a replacement had to be found.

IPv6 was developed in response to this situation. IPv6 allocates 128 bits tomap the Internet address space. The number of bits were not just doubled, butinstead quadrupled from IPv4's 32 bits to insure that this address space wouldnot run out any time soon!

IPv6 addresses will probably neverrun out

128 address bits provide IPv6 with 340,282,366,920,938,463,463,374, 607,431,768,211,456unique addresses. It may seem like overkill to have this many addressesavailable, However, many visionary individuals believe that eventually everywired and wireless computer, cell phone, PDA, household appliance, securitycamera, devices that haven't yet been invented, will each have their own uniqueInternet address.

Besides the huge number of IP addresses, IPv6 provides for better handling ofvoice than IPv4 which was not initially set up to handle it. This means thatphone conversations over the Internet will be smooth and clear instead ofchoppy and broken up like they often are now.

The time is almost upon us when any device with an Internet address and aconnection to the Internet can be monitored and controlled from anywhere in theworld. While you're away on vacation you could turn on lights, change yourthermostat, check security cameras around your home, etc. The possibilities areonly limited by our imagination!