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Did you know 'modem' is actually an acryonym? The term was coined from the fact that a device accomplishing digital transmission over analog
lines must 'modulate' the original digital signal to analog for transmission, and the receiving device must 'demodulate' the received analog
signal to be interpreted by a digital device (in our case, a PC). MOdulate-DEModulate. MO-DEM. Modem.
As stated, a modem is any device that is capable of both sending digital signals in analog form, and converting received analog signals back
into digital form.
Today's modem generally is hosted on a backplane of some sort, connected to a host bus in some fashion. This can be either a PC mainboard,
or a rack-mount server. Other types of modems in lesser use nowadays are in external enclosures connected to a serial port, or have an
attachment called an acoustic coupler that attaches to a phone handset when the actual phone jack is unable to be used for some reason.
The very nature of acoustic couplers generally forces very low throughput, depending on the engineering of the telephone itself, since the
signal will not be very clear and background noise will be high.
In the old days, modems did not have built-in dialer tones, nor intelligence enough to dial. When a user called another user, first a
telephone connection was made. When each user acknowledged that the party on the other end wanted to connect via modem, they would then
enter their terminal software and send their modems the command to connect. However, the problem here is that each modem has no idea whether
or not it should be the one to talk or to listen. If both modems try to talk, neither of them can hear anything over their own screeching.
If both modems try to listen, nothing happens because each is waiting for the other to move first. This problem was solved by specifying
a handshake mode for each modem to be in. There are two possibilities for the handshake mode: Originate (ORG) or Answer (ANS). If the
modem is in ANS mode, it will send the first carrier tones assuming the other modem is in ORG mode and is listening for it. If the modem is
in ORG mode, it will wait indefinitely for the carrier tones of a modem in ANS mode.
With the 'old days' now officially over, modems have gained the capability to dial the line without handset (or user) intervention. Handshake
modes have been all but forgotten as another 'automatic' nuance of modern modem technology. The dialing modem is always ORG, and the
answering modem is always ANS by default. The details of handshaking modes can now basically be all but forgotten.
In the beginning, there was Bell. Naturally, Bell was the first to jump into the wild fray and create a modulation standard for the emerging
PC modem. Bell, in the beginning, defined the standards for the American modem marketplace, while another group known as the CCITT began
to form standards for use all over the world. CCITT today is known as the ITU-T and is responsible for all recent and emerging modem
standards. Any protocol denoted with a V-dot (such as V.90, V.23, V.42, etc) is an ITU-T international standard.
Modulation standards are needed, because without standards, modems can attempt to send data to each other, but without first agreeing HOW the
data is going to be sent, the receiving modem is clueless as to how to decode the transmitted data. (Should this spline sequence be a 010?
Should this relatively straight sine wave of amplitude 0.5 be decoded as all 1's or all 0's?)
Obviously there needed to be standard modulation protocols so that not every modem on the market was incompatible with each other one. Bell
jumped up to bat with its original Bell 103 protocol, which later became CCITT V.21. This allowed for connections of up to 300 bps (bits per
second).
Bell had conquered, and everything was good. Until, the demand for more speed became very clear. Bell whipped out a new standard for 1200 bps
transmissions, Bell 212A, which would later become CCITT V.22. Modems were mass-produced and thrown into the market, when people started to
realize, "Hey wait, 1200 bps is much faster than 300 bps, so it can slow down to connect to the slower modem, right?"
Well, the problem is that the original Bell 103 modems expected to connect to none other than another Bell 103 on the other end. If anything
came out even slightly different than intended, the connection was aborted immediately. This really didn't leave much room for future
protocol development.
Good thing Bell had thought ahead. No, the new 212A protocol could not inherently connect to an old 103 modem. It would be like an Italian of
today trying to talk to an ancient Latin figure. Yes, Italian is a derivative of Latin, as 212A was a derivative of 102, but the modems would
make faces at each other just as the Latin man would make faces at the Italian. DERIVATION does not include compatibility as a property.
As well, if Bell were to continue to make improvements in the modulation protocol, they really had no choice but to leave their past mistakes
(cough 103 cough) behind.
Bell Labs came up with a brilliant hack, the results of which can be heard every day today as our modems struggle and screech at our ISP's modems.
They defined the process of 'modem handshaking', where modems can be given the code for a range of protocols in their low-level software.
So a modem can support, say, all of V.32, V.22, and V.21. Then, a handshaking priority is specified; by default, a modem will generally attempt
to connect first at its highest supported speed, but this can be changed in case a protocols has trouble with certain remote modems. The ORG
modem listens for the ANS modem's handshake tones. It internally determines the highest-speed modulation protocol common to both modems, from
the tones it hears. In our example, if it heard the tones for an answering V.32 modem, then it would decide on V.32. If it heard V.22 tones,
however, it would have to fall-back to V.22 since the remote modem would have no idea how to connect at V.32. The ORG modem then generates its
own tones indicating to the ANS modem just how high it can go, based on the protocols that the remote modem supports.
At this point, the modems have established a modulation protocol which they will use to communicate. However, a connection has not yet been
established. The next thing the ORG modem attempts is to send a carrier signal. If received by the remote modem, the modems continue to
pass this 'token' back and forth until the connection is manually aborted. If the carrier signal is ever lost, the modems will retrain (covered
later, but basically means 'reconnect at lower speed without redialing') if they are modern, or else just continue to attempt sending and receiving
carrier. If, after a carrier loss, the modems once again fail to keep the carrier signal for a set length of time, or if one of the modems gives
up too soon on the retrain/resend, a disconnect will result, and the modems will report the familiar NO CARRIER to the user, indicating
that not only has the carrier signal been lost, but as far as the modem can tell, there IS no carrier signal anymore!
The carrier signal is mainly just something that lets each modem know that, yes in fact, another modem IS on the other end listening to it!
Loss of the carrier signal is generally attributed to line noise, and can be triggered by bugs running on a poorly grounded wire outside, sister
picking up handset, poor solder in the house, etc. Any of these events can trigger a retrain, and frequent retrains will probably end up in a
disconnect because of all the variables involved in each one.
Before modems had error correction, line noise would frequently result in loss of carrier and disconnects. Line noise would have such
a severe impact on the transmission that frequently a modem would give up on the connection even before the receiving modem had gotten the
results of the noise! The impact of line noise also worsened as transmission speeds rose from 1200 to 2400 to 9600 bps and above, and clearly something had to
be done about it.
Contrary to popular belief, loss of carrier does not equal a disconnect!
Carrier loss is simply a signal to the modems that something needs to be
done about the connection, and if they can't decide together what to do,
the connection is then dropped. Most modems have a register that defines
how long the modem will stay connected waiting for retrain (if the remote
modem supports retrin) or a carrier resend.
Modems being sold for PC's today exist in either the industry-standard PCI or ISA form factor. The ITU-T
Most PCI modems sold today are controllerless. The nature of PCI is more conducive to many short bursts of information and needs less time
spent debugging hardware and thus allows more time and resources to be spent improving the driver software, which is the 'heart' of a
controllerless modem.