ModemsModems are used to connect two computers over a phone line. Modem isshort for Modulator Demodulator. It’s a device that converts data from digitalcomputer signals to analog signals that can be sent over a phone line. This iscalled modulation. The analog signals are then converted back into digital databy the receiving modem.
This is called demodulation. A modem is fed digitalinformation, in the form of ones and zeros, from the CPU. The modem thenanalyzes this information and converts it to analog signals, that can be sentover a phone line. Another modem then receives these signals, converts them backinto digital data, and sends the data to the receiving CPU. At connection time,modems send tones to each other to negotiate the fastest mutually supportedmodulation method that will work over whatever quality line has been establishedfor that call. There are two main differences in the types of modems for PC,internal and external modems.
Evolution of ModemsIn the last 10 years, modem users have gone from data transfer rates of300bps to 1,200 bps to 2,400 bps to 9,600 bps to 14. 4Kbps to 28. 8Kbps to, and to33. 6Kbps.
Now new modem standards are emerging, reaching speeds of up to56Kbps. Unlike the 33. 6Kbps modems being sold today, 56Kbps is a significantimprovement over 28. 8Kbps modems.
Viewing complex graphics or downloading soundfiles improves significantly with 56Kbps. The modem experts keep telling us thatwe are about maxed out. For instance when the 28. 8 modems where first introducedthey said that we’ve reached our maximum speed, and the same thing was saidabout the 33.
6 and now again for the 56K, but how true is this? The experts saythat the next major improvement will have to come from the telephone companies,when they start laying down fibber-optic cables so we can have integratedservices digital network (ISDN) . The thing that makes digital modems betterthan analog is because with analog modem transmission errors are very frequentwhich results in your modem freezing or just freaking out. These errors arecaused mainly by some kind of noise on the line due to lightning storms,sunspots, and other fascinating electromagnetic phenomena, noise occurs anywhereon the line between your PC and the computer you’re communicating with 2,000miles away. Even if line noise is minimal, most modems will automatically reduceit’s speed to avoid introducing data errors. Baud vs bpsWhile taking about modems, the transmission speed is the source of a lotof confusion.
The root of the problem is the fact that the terms “baud” and”bits per second” are used interchangeably. This is a result of the fact thatit’s easier to say “baud” than “bits per second,” though misinformation has ahand in it, too. A baud is “A change in signal from positive to negative orvice-versa that is used as a measure of transmission speed” and bits per secondis a measure of the number of data bits (digital 0’s and 1’s) transmitted eachsecond in a communications channel. This is sometimes referred to as “bit rate.
“Individual characters (letters, numbers, spaces, etc. ), also referred to asbytes, are composed of 8 bits. Technically, baud is the number of times persecond that the carrier signal shifts value, for example a 1200 bit-per-secondmodem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300 = 1200bits per second). Synchronous vs. Asynchronous Data TransferSynchronous and Asynchronous data transfer are two methods of sendingdata over a phone line.
In synchronous data transmission, data is sent via abit-stream, which sends a group of characters in a single stream. In order to dothis, modems gather groups of characters into a buffer, where they are preparedto be sent as such a stream. In order for the stream to be sent, synchronousmodems must be in perfect synchronization with each other. They accomplish thisby sending special characters, called synchronization, or syn, characters. Whenthe clocks of each modem are in synchronization, the data stream is sent. In asynchronous transmission, data is coded into a series of pulses,including a start bit and a stop bit.
A start bit is sent by the sending modemto inform the receiving modem that a character is to be sent. The character isthen sent, followed by a stop bit designating that the transfer of that bit iscomplete. Modems SpeedsA full page of English text is about 16,000 bits. And in order to viewfull-motion full-screen video it would require roughly 10,000,000 bits-per–second, depending on data compression.
The Past 300 bps (both ways)1 200 bps (both ways)2 400 bps (both ways)9 600 bps (both ways)14 400 bps (both ways)Current Speeds 28 000 bps (both ways)33 600 bps (both ways)X2 or K56Plu56 000 bps (downloading)33 600 bps (uploading)ISDN single channel64 000 bps (both ways)ISDN two channel128 000 bps (both ways)SDSL384 000 bps (both ways)Satellite integrated modem 400 000 bps (downloading)ADSL (T-1)1 544 000 bps (downloading)128 000 bps (uploading)Cable modem (T-1)1 600 000 bps (both ways)(Videotron)Ethernet (T-2)10 000 000 bps (both ways)Cable modem (T-2) 10 to 27 000 000 bps (both ways)(in general)FDDI(T-3)100 000 000 bps (both ways)In some cases, the modem-equipped PC with a 28. 8Kbps modem would befaster than a 33. 6Kbps or even 56K modem, especially with sites that don’t havea great deal of graphics. That’s because there are several factors thatdetermine how long it takes to reach and display a Web site. These include thespeed of your PC, your connection to your Internet service provider, your ISP’sconnection to the Internet itself, traffic on the Internet and the speed andcurrent traffic conditions on the site you’re visiting. A good example would be,say you drive a fancy sports car and I drove along in my family minivan, you’llcertainly beat me on an open stretch of road.
But if we’re both stuck in atraffic jam, you’ll move just as slowly as me. In short, any modem willsometimes operate below its rated speed. According to the vice president of amajor 33. 6Kbps modem company, you can expect a full 33.
6Kbps connection aboutone out of 10 tries. X2 56K ModemU. S. Robotics, Cardinal, Rockwell, and other manufacturers havedeveloped modems capable of 56K speeds over standard phone lines. U. S.
Roboticsline of modems called X2, uses an “asymmetric” scheme. Basically, it lets youdownload data at up to 56Kbps from any on-line service or Internet serviceprovider using matching U. S. Robotics modems.
The company says AOL, Prodigy,Netcom, and others are committed to deploying the X2 technology. The only catchis the data you upload to the provider is still limited to 33. 6Kbps or 28. 8Kbps.
The main reason why everyone has not yet leap to 56Kbps is because there are noset standards yet. Not all modem vendors are supporting the same 56Kbpsspecification. That means your Rockwell-based modem won’t work with a U. S. Robotics or Logicode model.
ISDNISDN (Integrated Services Digital Network) is a way to move more dataover existing regular phone lines. ISDN cards are like modems, but approximately5 times faster then regular 28. 8 modems. They require special telephone lines,which cost a little or a lot, depending on your phone company. It can providespeeds of roughly 128,000 bits-per-second over regular phone lines. ISDN has acouple of advantages.
It uses the same pair wire found in regular phone lines,so the phone company won’t necessarily have to run new wires into your house orbusiness. A single physical ISDN line offers two 64Kbps phone lines calledchannels that can be used for voice and data. Unfortunately, ISDN isn’t cheap. Installation fees can run a couple hundred dollars and setup can be confusing. ISDN also requires a special digital adapter for your PC that costs around $200. And though you could replace your old phone line with ISDN, I wouldn’t recommendit.
An ISDN line goes through a converter powered by AC current and if yourpower fails, so does your phone line. Satellite ModemsThe access service to Internet by satellite is called DirecPC. It wascreated by an American company of telecommunications called Hughes NetworkSystems Inc. DirecPC offers speeds of up to 400 Kbps. That’s nearly 14 timesfaster than a standard 28. 8Kbps modem and four times faster than ISDN(integrated system digital network).
The draw back to this system is that it’stoo expensive, requires a relatively elaborate installation and configurationand, in the end, doesn’t necessarily speed up your access to the World Wide Web. The price for the 21″ dish, PC card and software is about $499 U. S. retail.
Then there is a $49. 95 U. S. one-time activation fee.
The monthly chargesstart at $9. 95 U. S. , but that is for a limited account that also requires you topay to download data.
The “Moon Surfer” account, which costs $39. 95 U. S. , givesyou unlimited access nights and weekends. If you want unlimited access duringthe day, you’ll have to pay $129 U. S.
a month for the “Sun Surfer” plan. Customers pay between $149 and $199 U. S. for professional help, or $89 U. S. perhour plus materials if custom installation is required.
If you chose to installthe dish on ground level, Hughes Network Systems also has designed a hollowfiber glass camouflage that looks like a huge rock which can be put over thedish in order to prevent it from it being stolen. In addition to these charges, you also need to be signed up with anInternet service provider, or ISP, which approximately costs about $20 a month. You can use any ISP other than on-line services such as Prodigy or America On-line. The reason you need an ISP is because DirecPC is a one-way system. Thesatellite sends data to your PC, but you need to use a standard modem and aregular ISP to send data or commands to the DirecPC network. The data you sendflows at the speed of your modem, normally a 28.
8 Kbps modem. The fact that thesatellite is only one-way isn’t as bad as it might seem. Most users send verylittle data compared with what they receive. If you wish to view a Web site, forexample, you would send the Web address to the system via the modem, but thesite’s text and graphics would rush back to you via the satellite. Since theaddress is typically only a few bytes, that takes almost no time at all, even ifyou have a slow modem. The data from the site itself takes up far more time,especially if it has a lot of graphics.
Those who upload a lot of data,including people who need to update their own Web sites, will get no advantagefrom the satellite system while they are uploading. In addition to the dish, you get a 16-bit card that plugs into an ISAport of a desktop PC. The draw back to the system is that it eliminates Macs,notebook PCs and any other machines that don’t have available slots. You will find a noticeable difference when viewing sites with videoand lots of graphics. This could eventually be a big advantage as an increasingnumber of information providers start using the Internet for full-motion videoand other multimedia presentations.
But DirecPC for now doesn’t offerspectacular advantages for normal Web surfing. And if you’re thinking about along-term investment, consider that in the future there will be other optionsfor high-speed Net access. ADSL / SDSLADSL (Asymmetric Digital Subscriber Line) a method for moving dataover regular phone lines. An ADSL circuit is much faster than a regular phoneconnection, and the wires coming into the subscriber’s home are the same copperwires used for regular phone service.
An ADSL circuit must be configured toconnect two specific locations. A commonly used configuration of ADSL is toallow a subscriber to download data at speeds of up to 1. 544 megabits per second,and to upload data at speeds of 128 kilobits per second. ADSL is often used asan alternative to ISDN, allowing higher speeds in cases where the connection isalways to the same place. SDSL (Symmetrical Digital Subscriber Line) is adifferent configuration of ADSL capable of 384 Kilobits per second in bothdirections.
Cable modemsAnother type of modems are cable modems. It uses the same black coaxialcable that connects millions of TVs nationwide and is also capable of carryingcomputer data at the same time. It’s able to uploading and downloadingapproximately 10 to 27 megabits per second. A 500K file that would take 1.
5minutes to download via ISDN but would take about one second over cable. Classification Of ModemsA classification of modems that are capable of carrying data at1,544,000 bits-per-second are called T-1. At maximum capacity, a T-1 line couldmove a megabyte in less than 10 seconds. That is still not fast enough for full-screen, full-motion video, for which you need at least 10,000,000 bits-per-second. T-1 is the fastest speed commonly used to connect networks to theInternet.
Modems that are capable of carrying data at 3,152,000 bits-per-second are refereed to as T-1C. Modems that are capable of carrying data at6,312,000 bits-per-second are refereed to as T-2. And modems that are capableof carrying data at 44,736,000 bits-per-second are refereed to as T-3. This ismore than enough to do full-screen, full-motion video. Modems that are capableof carrying data at 274,176,000 bits-per-second are refereed to as T-4.
EthernetA very common method of networking computers in a LAN (local areanetwork) is called Ethernet. It will handle about 10,000,000 bits-per-second andcan be used with almost any kind of computer. FDDIFDDI, (Fiber Distributed Data Interface) is a standard for transmittingdata on optical fiber cables at a rate of around 100,000,000 bits-per-second. It’s 10 times as fast as Ethernet, and approximately twice as fast as T-3.
Most modems mentioned such as T-1, T-2, T-3, etc. are not intended forhome use. These high speed connections are use mainly for big businesses. Buteven such speeds as T-4 and FDDI are use very little among big companies, butmore of the Army, NASA, the Government, etc. They’re highly priced which makesthem only available to larger corporations and organizations who need to sendhuge amounts data from one place to another in little time or no time at all.Apart the price factor when would you need to transfer data that is on a CD-ROMdisk holding it’s full capacity (650 Mb) across the world in 52 seconds? Technology
