Definition
of a Modem (Analog and Digital)
The modem was initially developed to link two computers together by utilizing an ordinary telephone connection. As with all technology, the first modems were very large, bulky affairs that would seem primitive by today's standards. You might think that it should be a simple task to link two computers together since there is already a telephone network stretching to virtually any location you would be. There is one major hurdle.
Computers normally communicate using digital signals, which involves sending a series of binary data bits (or ones and zeros) back and forth. Digital information can be related to an ordinary light switch (not a dimmer), as it is either on or off. In computer chips, for example, the digital information is transmitted within the chip and outside the chip by either having an open or closed electrical current. A zero is transmitted by zero voltage, and a one is transmitted by the full power voltage of the chip (3.3V, 5V or lesser voltage in the portable CPUs). Compact discs store digital information by using pits and spaces on the surface of the disc. As the CD player reads the disc, it translates pits in the surface into ones and spaces on the surface into zeros. And so forth.
Since the primary intention of the telephone network was to transmit the sounds of the human voice, the network is designed for analog communication. Analog communication involves sound waves with crests and troughs of any selectable interval. Analog information can be related to the dimmer light switch, as you can have the light fully on or fully off, or you can select any of a range of lighting levels in between in a smooth increasing range.
Thus, what people needed was a device which could take the ones and zeros being sent from the computer and "modulate" it into sound waves for transmission (a digital-to-analog conversion, or DAC), then when information was being received this device would take the sound waves and "demodulate" it into ones and zeros again (an analog-to-digital conversion, ADC) so that the computer could process the data. The clumsy name of "modulator/demodulator" evolved into the term modem.
The first mass-production modems were capable of transmitting at a 110 baud rate (the word baud is derived from the name Baudot, a 19th century French inventor, and originally measured the speed proficiency at which someone could send Morse Code). To operate this modem, assuming the computer was already up and ready, you picked up the handset and dialed the number you were going to connect with. Once you heard the other modem answer, you physically placed the telephone handset down on top of the modem, which was designed with rubber cups to hold the mouthpiece and earpiece. You can actually see one of these beasts if you rent the movie Wargames with Matthew Broderick. As you can imagine, all you had to do was slam a door and the data stream could easily be corrupted.
The next modems were high-tech and lightyears ahead of the previous ones. These actually had a phone line jack built-in so that you didn't have to muck about with the handset. These modems were capable of transmitting data at a blistering 300 baud.
The next modems were the 1200 bps modems. These were the first models with a full line of the funky lights on the front that blink to let you know that the computer is effectively mangling your data.
Next came the 2400 bps modems. These arrived at least a full 10-12 years after the first 110 baud modems hit the market.
Beyond 2400 bps modems lay the 9600 bps modems, 14.4 kbps, 19.2 kbps, 28.8 kbps, 33.6 kbps, 56.6 kbps, shotgun 112 kbps modems, and 128 kbps ISDN adapters.
A typical modem connection follows the same routine:
The originate modem is sent an initialization string by the computer. The originate then takes the line off-hook, checks for a dial-tone, and dials the series of phone numbers the computer has instructed it to dial.
The answer modem detects a ring on the line and takes the line off-hook. The answer then begins sending a tone of a set frequency across the line. Once it sends the first guard tone, it immediately sends an unmodulated carrier consisting of varying frequencies determined by the conditions on the line, the speed of the modem hardware, and the modem software settings. This is the beginning of the handshake. In the example below, a 33.6 kbps modem at an ISP modem bank answers with all of the frequencies used for a 33.6 kbps connection.
The originate modem listens specifically for an initial guard tone frequency that it understands. Once it hears a familiar tone, it knows it has dialed in to another modem. It then begins examining the incoming frequencies and responds with a different carrier of its own for each incoming frequency that it sees. In the example below, the 14.4 kbps originating modem will respond only to the 14.4 kbps carriers that it knows.
Once all carriers have been fired and responded to by both ends, any carriers not receiving a response are dropped from the connection, leaving the carriers that were successfully negotiated. The modem speakers usually mute themselves and the computers will begin sending data to the modem to be converted into sound and transmitted. The diagram below shows this final carrier setup.
If both modems in these examples had been 33.6 kbps modems, the originate modem would have responded to more carriers (as it would have known how to see them) and more carriers would normally be established as a result. Similarly, if the answer modem had been a 14.4 kbps modem, it would have only sent carriers that the 14.4 kbps would understand, as they would be speaking on the same level.
Now that the link between the two modems has been established, each modem will continue to monitor the status of the line conditions and the condition of the data. If errors occur, the modem lets the other modem know that it needs to re-send the block of data. If errors continue to occur and the conditions do not improve, more drastic measures are necessary.
Retraining occurs when the modems realize that there may be serious problems with the line conditions. Both modems agree to halt the flow of data and interrupt the session. The modems send a low burst tone across the line. Then the modems re-establish the carriers in the same manner as when they initially connect (the answer sending all known carriers, the originate responding to all heard carriers, and the unresponsive carriers dropped from the stream).
If conditions are bad enough, retraining may result in a 2400 bps connection. This way the minimum number of carriers are being used (two upstream, two downstream), and is almost guaranteed to function properly. This is known as fall back.
If the conditions on the line improve and the data does not appear with the errors seen before, the modems retrain again and attempt to use more carriers than before. This is known as fall forward.
Sometimes a connection will simply terminate. If a modem manufacturer has cut costs by disabling the ability to fall back and fall forward, without the ability to retrain the modem has no other choice but to drop the connection so that you can connect again. This can also occur if the line conditions are so damaging to the data stream that it is impossible to re-establish the carriers.
There is a set time that the computer will go without a carrier being detected, and this can be changed through software settings. The retrain period can also be changed. Raising these values will often allow a modem connection to remain connected if the line noise conditions make stable connections difficult. The specific modem command to change these strings is located within the Initialization Strings section below.
When we speak about a modem transmitting at a rate of 110 baud, we are referring to a series of pulses or tone changes on the phone line at a particular frequency. Each pulse or change in tone represents a specific bit of information (either a one [1] or a zero [0]). A 110 baud modem is capable of transmitting and receiving 110 bits per second (bps) using 110 pulses or tone changes per second. A 300 baud modem is capable of changing the tones up to 300 times per second by shortening the length of the tones, and thus being able to send 300 bps (still sending one bit with each baud). Here's where the technology changes.
Due to the physics of telephone lines, 600 pulses per second is close to the maximum speed limit. Transmitting information much faster than this baud rate results in tones which do not physically last long enough to be transmitted across the line and received accurately on the other end. The tones start to blend together.
We all know what happens to speed limits, though. In order to surpass the 600 baud limit, a 1200 bps modem must use two separate 600 baud carriers. Different carriers are achieved by using different forms of modulation on the transmission waves. In this way, it is possible to transmit 1200 bps using 600 tone changes per second but this time by sending 2 bits with each baud instead of one. Note that we now only refer to the modems using a bps moniker; all of them still transmit at 600 baud, they just stack numerous types of modulation on top of it. 2400 bps modems still only transmit 600 tone changes per second but use 4 different levels of modulation on the data. By keeping a careful eye on where the modulations intersect in the data stream, you can provide multiple places to encode data. The following diagrams may explain this more adequately:
The diagram above is close to how the technology actually operates. Each of the red points in the second diagram is a location within the carriers which is an exact intersection of different modulations. This allows data to be encoded in multiple locations as in this diagram, whereas with the original modem technology, you had two carrier frequencies that could only carry one bit in each pulse. The diagram above only contains 16 intersections of different modulations. Modern modems apparently bump the total number of data points up to 256! Different forms of modulation used to today include amplitude modulation, frequency modulation, quadrature modulation, and phase modulation, etc.
The first modem engineers used the optimum range of the telephone lines to transmit information, usually remaining in the 300-1500 Hertz (Hz), or cycles-per-second range. This helped to minimize the risk of damaging the data with the usual line interference that is an "added feature" of the telephone system. Pops and hums are usually found in lower frequencies below 300 Hz, while clicks and static are found in frequencies higher than 1500 Hz. As you add the additional frequencies to increase the bits per second throughput of a modem, you start to use the frequencies that are susceptible to being interrupted by noises on the line.
300 baud connections will lose 1
bit of data to a pop on the line.
1200 bps connections will lose 2 bits of data to
the same pop.
2400 bps connections will lose 4 bits of data to
the pop.
9600 bps connections will lose 16 bits with this
same pop.
14.4 kbps connections will lose 24 bits of your
data.
28.8 kbps connections will lose 48 bits of data.
33.6 kbps connections will lose 56 bits if a pop
came through the line.
This is the reason why the phone companies only guarantee a 9600 bps connection. All 9600 bps modems still operate using frequencies between 300 Hz and 1500 Hz, the cleanest frequencies on the telephone line. Once you jump into the 14.4 kbps range, you are in the land of line noise above 1500 Hz, and nothing can be guaranteed.
If it weren't for some very specific error-checking technologies, we'd be reloading pages and files quite a bit. And this is just a simple "pop" on the phone line. Have you ever heard where it seems like your phone wires are twisted together with the neighbor's? How much damage do you think that would do to a connection? The house in Winter Park that I used to live in had a phone system [Hail Sprint!] that you could press the hook button and you could actually hear a Spanish radio station for a brief 3 seconds as it faded to zero. Press the hook again, and you get another fading 3 seconds of Spanish radio. Keep pressing the hook every 3 seconds and you have very bad Spanish Radio reception through the telephone.
As modem speeds have increased by leaps and bounds, the necessity of having "clean" telephone lines has also increased. Any time that there is enough interference on the line, it can cause noticeable delays in uploading or downloading, and it can cause the initial handshake of the modems to continue without resolving into stable carriers. It can also cause the two modems to decide the connection is too disrupted to continue and drop the connection. This interference can be caused by the phone lines themselves, the telephone pole equipment, the lines going into the house, the actual phone lines in the walls of a house (especially houses without twisted-pair telephone wires), specifically damaged telephone cables, the wrong telephone cables, and any extra machinery on the telephone system within the house, such as: answering machines, portable phones, caller ID boxes, two-way (or more) cable splitters, burglar alarms connected to the phone lines, surge protectors, and wireless telephone jacks, and anything else that connects to both a telephone line and the power system at the same. If one of these devices is suspected of causing interference, it should be disconnected from the phone system and the modem connection should be attempted again. In a worst case scenario, disconnect anything connected to telephone jacks except for the modem, then try the connection again. If the connection improves, add devices one by one, trying the connection each time, to zero in on the culprit.
Without some sort of gauge of the amount of data being transferred, you quickly wind up with piles of ones and zeros that mean practically nothing. A need arose for a way to let the computers signal each other that they were either ready to send or ready to receive information.
Most electrical equipment transmits information in a synchronous manner. When speaking about modem communication, this means that the data stream is continuous with only the data being sent through the stream.
When a computer uses a modem to communicate, the stream is asynchronous in nature. This means that extra bits are added into the stream first to let the other computer know that it is about to start sending a data byte (start bits) and bits to let the other computer know that it has completed sending a data byte (stop bits).
A hardware chip was developed to fulfill this need. A Universal Asynchronous Receiver Transmitter (UART) chip uses hardware to add these start and stop bits into the data stream automatically. UART chips on the receiving side strip the start bits and stop bits off the stream and then output the original information.
The UART also performs another function for modem communication. Computer processors send information in sets of 8, 16, or 32 bits at a time. Modems do not send multiple bits at a time in parallel, instead they send the bits sequentially. The UART is capable of handling the multiple-bit packets and converting them into a steady stream of binary digits. The diagrams below show this process as the computer sends information to the modem to transmit, and as the modem receives information and provides it to the computer.
Intel's 8250 UART chip was one of the first produced, and can be found in many older PCs. One of the problems with the 8250 UART chip was that it has a buffer capable of storing only one byte (1 byte = 8 bits). Once it had filled up the 1 byte buffer, it had to get rid of the stored information before it could receive more. If the software or the computer could not handle this much information, or was busy while the modem continued to receive new data, the information was simply lost.
The 16550 UART is a 16 bit chip, and has a much larger buffer, capable of storing 16 bytes in a First-In First-Out (FIFO) buffer. FIFO buffers mean that even though the buffer is written dynamically and in differing locations depending on which registers are open, the information heading from the modem to the computer is in the same exact order that it was received from the foreign modem.
The larger buffer size means that the modem does not have to continually remind Windows to accept the data from the modem. Anything less than a 16550 UART chip is just begging for transmission problems. This is precisely why a 16550 UART is a minimum system requirement.
Now that we have an effective method to measure and monitor the data stream as it is being transmitted and received, there are some additional tricks we can use to increase the accuracy of the modem communication.
One method of checking for errors is called a checksum. Every 128 bytes (characters) or less, a checksum can be performed on the data received and sent across the line to the opposite modem. This is performed by taking the most recent block of data that was transferred and adding it together. It then keeps these numbers and transfers them along with later signals. By diligently monitoring these checksums, it becomes readily apparent when errors occur. The checksums stop matching up and begin to differ. When this occurs, the modems agree to re-send the block to see if the checksums agree again. If they do agree, the modems continue sending. If the checksums still do not agree, the block is re-sent until the opposite is true. Cyclic redundancy checks (CRC) can also be used to actively search the data stream for errors. Various overrun, alignment, and timing errors are also helpful in detecting problematic conditions..
Parity is another type of error correction. With parity, the last block of data that was sent is analyzed and added together to determine whether it was an even or an odd number. For example, the string 011010 becomes the odd number 3 when added together. The string 101101 becomes the even number 4 when added together. Whether the computers and modems will be looking for even or odd parity must be decided before the data transmission begins. This is usually set in a software setting. Once both sides have determined to use even parity, for example, both sides will actively watch each block of data to determine whether the sum of the block is an even or odd number. If the answer is odd, a parity bit (1) is added to the data stream to make it an even number. If the sum is already even, a different parity bit (0) is added to the stream to keep it even.
Error correction allows the modems to keep track of the data that is being transmitted and when discrepancies are discovered, allows them to request that a block be re-sent to maintain the integrity of the data.
Another sneaky technology that was invented was that of compression. Compression uses techniques to scan data for patterns that can be reduced mathematically. Let us say that the compression routine receives the following stream of data:
1100 1100 1100 1100 1100 1100 1100 1100 0010 0010 0010 0010 0010 0010 0010 0010
Using compression techniques, this stream can be significantly reduced while accurately retaining the information:
1100*8 0010*8
Since the computers only output binary code to the modems, you would need to use the binary number equivalent of the decimal number 8 (1000), and you would need to use a binary number to tell it to repeat the number 1100 by 8, but the idea is the same. On the receiving end, mechanisms use the same compression techniques to decompress the data so that it may be processed by the computer. This is a greatly simplified example, but is the main operating factor in compression. This allows even greater amounts of data to be squeezed through the connection.
In order to compress data being sent from the computer, the computer must send information to the modem faster than it would normally send the information. When the data is compressed, the timing synchronizes and the data flow is smooth. This is why you always set the maximum modem speed to 19200 for a 14.4 kbps modem, 38400 for a 28.8 kbps modem, etc. This is also accomplished utilizing a 16550 UART chip in the modem itself as well as the computer. While the modem is actively sending information across the line, it can receive bursts of new data which it then has a brief time to compress before being required to send it along the line.
Computers need to take a moment every now and then to attend to other business. This might have the capacity to seriously disrupt a communication program from functioning properly. Now that the computer is sending information to the modem faster than it is actually transmitting the data, it becomes important to have a mechanism to let the data stream start and stop; this is called flow control.
Xon/Xoff flow control is performed by actually using two different ASCII characters (Ctrl-Q) (Ctrl-S) to tell the other modem to either stop sending and hold on a moment or to start sending again when the modem is ready. As modem technology evolved, these control characters eventually began being used by the modems to actually transmit data. This type of flow control is also called inband signaling because the control characters are inserted into the data as it travels.
Since modem varieties became faster and faster, many times the inband signaling does not function properly. In this case, out-of-band signaling is used to communicate information about the data stream between the modems. Out-of-band signaling utilizes electrical signals that do not disrupt the data as it streams from one modem to the other. There are several different out-of-band flow control signals which comprise the RS-232 standard. The last two flow indicators, Send Data and Receive Data, are the two lights that blink in the Systray while connected under Windows 98/95.
RS-232 Standard Flow Control Signals |
|||
Control |
Code | Controlled By | Description |
| Data Set Ready | DSR | Modem | Tells the computer that the modem is ready to receive incoming data from the computer. |
| Request To Send | RTS | Modem | Tells the computer that the modem is ready to transmit data across the line. |
| Carrier Detect | CD | Modem | Tells the computer that a carrier exists, meaning it is connected to a remote modem. |
| Ring Indicator | RI | Modem | Alerts the computer to an incoming call that the modem has detected. |
| Data Term Ready | DTR | Computer | Tells the modem that the computer is ready to receive incoming data from the modem. |
| Clear To Send | CTS | Computer | Tells the modem that the computer is ready to accept data for reception from the modem. |
| Signal Ground | GND | This is the reference for a modem and computer to obtain a mutual zero. | |
| Send Data | TD | Computer | This signal is only a flow indicator, saying that data is flowing out. |
| Receive Data | RD | Modem | This signal is only a flow indicator, saying that data is flowing in. |
Hayes designed a series of in-band controls that can be used to directly control many modem parameters. These commands are usually grouped together in the form of an initialization string, which is placed either in a dialer program, such as ShivaPPP or in a DUN connectoid configuration, or in the master modem software settings, such as in the Modem Control Panel (Windows 98/95).
Because Hayes did not formally patent the Hayes Command Set that it designed, most modem manufacturers have begun customizing and modifying the command set. The following listing does not contain every command possible, but it does contain many commands that should work with most, if not all modems.
Hayes Command Set - All commands must begin with AT
and then the command code. (attention) |
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| Command | Description | Function | ||||||||||||
| A | Answer | When a call rings into a modem, a RING indicator will appear on the screen. Giving an ATA command will force the modem to take the line off-hook and start sending a guard tone. | ||||||||||||
| &Bx (&B1) |
DSR Behavior | This setting controls how DSR
(data send ready) hardware flow control reacts with your modem. If hardware flow
control is being used, this setting should usually be set to &B1
so that the computer can always speak to the modem.
|
||||||||||||
| &Cx (&C1) |
Carrier Detect | This setting determines how the
modem reacts to a dropped carrier. Then set to &C1, the modem will
let the computer know when the carrier has dropped so that it can redial and re-establish
the connection.
|
||||||||||||
| D | Dial | This is followed by the number that you wish to
call. Some special suffixes can be used to enhance this command:
|
||||||||||||
| &Dx (&D2) |
DTR Response | This setting determines the modem
reaction when DTR is toggled one way or another. This should usually be set to &D2,
as it will cause the modem to disconnect as soon as DTR drops. Most software drops DTR to
intentionally sever a connection. &D0 may cause the connection to
remain after sending a disconnect signal. &D3 will send an ATZ
to the modem each time it hangs up.
|
||||||||||||
| Ex (E1) |
Echo | Determines whether you see the letters that you type. This should always be set to E1 to allow local echo. | ||||||||||||
| &Fx | Reset to Factory Defaults | This command resets the modem to a
factory default setting. Factory settings usually specify:
Most modern modems contain more than one factory default setting. If AT&F does not work, try AT&F1 up through AT&F8 and see if the condition improves. This may be easier than guessing and configuring the other settings. |
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| Hx | Hook Switch | Command to instruct the modem to activate or
deactivate the hook switch.
|
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| Kx (K1) |
Break During Error Control |
This setting permits or denies a modem to retrain during error control detection. This should always be set to K1, to allow retrains. | ||||||||||||
| Mx (M1) |
Speaker Mode | This setting determines how the modem speaker
reacts during a connection.
|
||||||||||||
| Qx (Q0) |
Quiet Mode | This setting determines whether or not the modem sends responses to commands issued to it. This should be left at Q0 to enable the responses. | ||||||||||||
| S7=x (S7=60) |
Wait for Carrier | This setting determines how long the modem will wait to detect a carrier before hanging up the line. This setting also determines how long the modem will wait during a retrain attempt. This setting should usually be set to 90 or higher (S7=90) | ||||||||||||
| S9=x (S9=5) |
Carrier Detect Time for Recognition | This setting determines how long a modem will listen to a carrier before it decides it is a valid and functional carrier. Increasing this value may assist connections on noisy or dirty lines. This value should not be set above 10 (S9=10). | ||||||||||||
| S10=x | Wait on Carrier Loss | This setting will determine how long a modem will wait after the remote carrier is lost before halting attempts to retrain. This should be set to a higher number - S10=60 is adequate. | ||||||||||||
| S11=x | Dial Delay | This setting determines how long a
modem will wait between dialing another digit. It really does not affect performance, but
setting it to S11=50 will cause the modem to dial quickly. If dialing errors occur after setting this to 50, remove the setting. |
||||||||||||
| Vx (V1) |
Verbose Mode | This setting determines whether the modem sends responses as text or as number codes. This should remain at V1, to enable textual messages. | ||||||||||||
| Xx (X5) |
Response and Dialing Mode |
This command determines what
responses and checks will be performed during a connection. If you constantly receive NO
DIALTONE or BUSY errors while trying to dial, disabling the
checks for these two features may allow you to connect normally.
|
||||||||||||
| Z | Reset to NVRAM Defaults | This command resets the modem parameters to the default settings stored in the NVRAM. AT&F is usually a better choice, but ATZ will work in certain situations. | ||||||||||||
Other initialization string settings are available through various resources on the internet. Customized command sets should also be available from each respective modem manufacturer.
Now that we know exactly how the regular modems have operated to this point, it's possible to understand how a 56K modem works and how it is able to transmit so much data where it did not seem possible. The primary operating factor is another form of modulation known as pulse code modulation. This technology requires certain features that separate it from normal modem functionality.
Pulse code modulation (PCM) involves extra processing levels over ordinary modems, and will only operate when the data can travel out through a completely digital pathway, without needing even one analog-to-digital conversion (ADC). This usually means the information is flowing from an ISP, and that the ISP has digital trunks leading out of their location, such as a T1 or T3 line. Phone companies that run their own Internet Services almost always support this. If the ISP has regular phone lines out of their location, 33.6 kbps is the maximum output speed possible.
So there are 3 requirements for obtaining a 56.6 kbps connection.
| A 56.6 kbps modem | |
| An ISP who supports the 56.6 kbps modem | |
| No analog-to-digital conversions (ADC) in the path from the ISP to the user |
The third requirement is what angers most people upon returning home with their shiny new 56K modem. This is what happens.
On the ISP side of the equation, the ISP 56.6 kbps modems use pulse code modulation to examine the data and send it out in digital form into the Public Switched Telephone Network (PSTN). The data may only be transmitted using the 256 discrete digital signal levels available on the PSTN for this to work. Assuming there are no analog-to-digital conversions (ADC) in the pathway from the ISP to the internet user, the signal is converted right outside the consumer's house in a digital-to-analog conversion (DAC) (as it leaves the PSTN and travels the analog lines into the house). DACs do not disrupt the data at all, as there are many more signal levels available on the analog lines than the 256 digital signal levels on the PSTN. ADCs, however, involve information travelling from the multitude of analog signals and being converted into a maximum of 256 signal levels for the digital network. This means that information is lost going through an ADC, whereas a DAC maintains the integrity of the data.
When a 56.6 kbps modem initially establishes carriers with another 56.6 kbps modem, the first thing they do is watch the connection to observe if any ADCs are present. If they detect even one, they will revert to the 33.6 kbps speed and that is the end of the story. Most times this involves the physical distance that the lines must travel from the consumer's home to the entry switch into the PSTN. If this distance is greater than 3 miles, the signal must usually be pumped through an ADC to boost the signal so that it will make it to the entry point in tact. Consumers should usually be able to contact their local phone operator and find the physical location of the PSTN switch. Then it is a simple matter to drive there in their car, watching the odometer to roughly gauge the distance in miles.
So assuming there are no ADCs in the pathway, the data coming from the ISP arrives at the home of the internet user with all of the digital information intact. Due to overhead constraints, it is not possible to use all 255 digital signal levels available on the PSTN, and this is why it is a 56K connection maximum instead of a 64K connection (which 255 signal levels would allow). Line conditions, of course, will knock this level down below 56K on most occasions, but if the connection is above 33.6 kbps, the pulse code modulation is working to some extent.
As the data leaves the house of the internet user and heads toward the ISP, it leaves the house through an analog line, and must be almost immediately sent through an ADC to convert it into a digital signal for the remainder of the trip through the PSTN. This means all upstream communication from a 56K modem is limited to the usual 33.6 kbps speeds.
One hybrid version of the 56K modem that has appeared is the shotgun modem. This is a modem which is actually two separate 56K modems integrated together, offering up to 112Kbps speeds. This utilizes the newer features of Windows 98 and 95's Dial-up Networking that will let you multilink two modems together to increased bandwidth.
Integrated
Services Digital Network (ISDN)
Integrated Services Digital Network (ISDN) involves paying extra money every month to have a digital line installed all the way to your home or business. This establishes a direct connection from a location into the PSTN. Once this direct digital pathway is available, there is no need to mess with the analog conditions of the rest of the telephone system.
Now all that is needed is an ISDN adapter to connect the computer and allow it to send signals out through the digital line into the PSTN. The ISDN adapter takes the place of the modem, as modulation and demodulation is no longer necessary.
It is important to understand that ISDN consists of 3 separate data channels. ISDN has two B-channels, each one capable of transmitting and receiving at 64 kbps. ISDN also includes a smaller bandwidth D-channel, enabling the ISDN adapters to send error-checking and other connection information.
ISDN technology is capable of sending and receiving all 256 discrete digital signal levels available to the PSTN. This means that the connections are a standard 64 kbps. ISDN contains two separate B-channels, however, which make it possible to bond two 64 kbps B-channels together to form a 128 kbps connection. One B-channel can also be used for browsing the internet while the other B-channel allows voice communication like a regular phone. Note that a two B-channel ISDN subscription will cost more money per month than a single B-channel.
That's really all there is to ISDN. It is just a digital transmission medium (as opposed to the analog telephone medium), and therefore requires a different interface between the computer and the medium. As long as the user has Windows 98, or if Dial-Up Networking 1.3 Update (or 1.2) has been installed for Microsoft Windows 95, or Service Pack 3 has been installed for Windows NT 4.0, and the ISDN adapter itself is functioning and installed properly, ISDN is not a problem.
The Microsoft Windows 95 Dial-Up Networking 1.2 Update can be downloaded directly from Microsoft. Windows NT 4.0 Service Pack 3 may also be downloaded from Microsoft. This update installs the software necessary for using ISDN adapters with a Windows 95 system and upgrades the system to the ability to bond the two B-channels together, if they are both available.
| daniel mcdonald | revised: 8 october 1998 |