Modem Guide
Alternatives to modems are still quite expensive in the UK. Even though modems should be old news they're still with us and with quite a bit of verity. To get the best from your modem (or choose the best modem) it pays to know a little bit about the workings so you can make an informed decision.
HARDWARE MODEMS
The name Modem is a shortening of the name Modulator/Demodulator; that is the computer sends information to the port and it translates it into audio frequencies (or a horrible noise) to be sent down the phone line. Incoming signals are translated back into useable code and sent to the computer. In the case of hardware modems they are completely invisible to the computer (aside from initialisation codes). As far as it is concerned it is directly linked to the other computer via an RS232 serial port.
The port is represented to the CPU by means of a UART chip. You can see some of the UARTS settings in the modem-setting box under windows. For instance the ‘maximum connect’ speed is not the Modems Max speed but the maximum data rate between the Computer and the UART. The UART acts as a controller for the Comm-port, modern systems having UARTS that support data rates up to 155Kb/S. In my experience it is best to leave this set to 115K so the CPU can dump its data to the modem quickly.
Serial communication rates are measures in baud; that is the amount of bits that are transmitted down the line in a second. The term baud is not used for faster modems because they actually transmit more than 1 bit at a time, therefore not begin serial.
SOFTWARE MODEMS
Software modems are undeniably the cheaper alternative, you can see why when you look at the fact they have like one chip and 2 resistors!
The key difference is that the System is VERY aware of the presence of a software modem. As the name suggests all the modulation and demodulation is done via software, all the board is actually doing is acting as a DAC/ADC. Everything such as handshaking has to be processed by the CPU rather than the Modem. I would go as far as to say ‘Modem’ is the wrong term for them as they are not modulating or demodulating at all! I would assume that the effects of this would be shared between pings and system performance.
In a world where all peripherals are trying to reduce the workload of the CPU this does seem a little strange but I don’t think it’s actually that bad. I can’t see that all the new systems that software modems would be found on should have the small amount of power kicking around that the modem requires and, again considering the peripherals are doing all the work nowadays, I should hope a 700MHz CPU was used for something.
Having said that I’m still on the side of Hardware modems, for the following reasons.
1) Software modems seem to be able to renegotiate the connection as well as hardware modems. i.e. If someone picks the phone up or you get noise on the line it's more likely to loose your concoction.
2) They use up another IRQ (hardware modems share IRQs with your existing serial port or are external.
3) Some get much lower connection speeds.
However, manufactures can increase the performance of software modems by writing better drivers. Hardware modems may require a 'firmware' (a term for something between hardware and software) flash upgrade. This is a more risky procedure. I've had one flash fail before (I've performed about 10) rendering the modem useless.
33.6, 56K and ISDN
The British telephone system is based on digital channels for each standard line. Analogue from sampled at 8-bit 8KHz. This gives a maximum data rate of 8000 x 8 = 64000 or 64K/sec.
But why can’t we get 64K analogue? Well it all lies with ‘Nyquist’s theorem’, which states that the reliable bandwidth of a signal is half its sampling rate. In other words when the signal is converted between analogue and digital it loses half its data carrying ability, explaining the previous 32K limit.
With ISDN the analogue to digital conversion does not take place in the exchange. Instead there is a digital 8-bit 8KHz link giving the full 64K data rate. If you want to use your phone the ISDN BOX installed in your house handles the analogue to digital conversion.
56K technologies are a half-breed between Analogue and ISDN connections. The incoming Signals are viewed as digital signals by the modem avoiding Nyquist’s theorem. In practice data rates of about 56K are achievable.
56K STANDARDS - WHATS THE DIFFERENCE?
Well I can’t say I know the technical details, but I have had practical experience with both 56K flex and V.90. X2 was only really found on expensive externals and was not really very common. I have read somewhere that "V.90 is just 56K flex with a bit of X2 thrown in". Since the two standards are an agreement between Rockwell and Robotics, I’d say this is pretty accurate. However the performance in practical terms is quite strange.
I started off browsing and playing games with a 33.6 modem I found it adequate for both. Along came 56K Flex. My download speeds went through the roof. But there was a grey lining in this otherwise silver cloud. My gaming died. I found although downloading seamed extremely fast uploading was pig slow. I don’t know why. The result was that I ended up watching everyone else run around smoothly while I had absolutely no control. I was not a happy bunny. A v.90 flash later and it was all sorted.
CONNECTION RATES
Connection rates are governed between the firmware (or software) driving your modem and the quality of your phone line. I've already mentioned that some modems seem to get better rates than others do so I'll concentrate on what effects the line.
1) Low gain:
If you're suffering from disconnects then one possible fix is having the gain on your line increased, to do this you have to contact a BT engineer. However they have got quite upset in the past with all the modem users pestering them.
2) Where you live:
I live in a rural area with very little source of electrical noise and very few phone lines in the bundle. I can get connection rates of up to 52K. The very same modem in the very same computer in London gets 36K. It also suffers from regular disconnection along with the TV picture distorting and any speakers cracking, obviously there's a bad source of interference near by.
3) What you have connected:
If you have too many items connected to your line it starts drawing too much power and decreases the quality of your signal (this is why BT go through what you've got connected before even thinking of increasing the gain, they're not in the business of power supply). Bare in mind that if something connected to your line has got a main supply, it probably isn't drawing much power (your modem included), look for things like too many basic phones and ring amplifiers.
