FAQ

                                                           Expecting 56K or 33,600 or 28,800 bps? Your results will vary. If you find that you’re unable to achieve full speed connections and/or your new high speed modem often behaves erratically, the reason is probably a phone line impairment. That’s not to imply you have a bad phone line, merely one which may not be capable of what your modem is demanding of it. Most often the issue is insufficient bandwidth, poor frequency response, low signal to noise ratio, an impedance mismatch that causes echo, an imbalance in the phone lines, or a combination thereof... either yours, the phone lines of the system you are calling, or in the lines and telephone switch equipment -anywhere- along the way.
Here are the minimums:
                                        33,600 bps V.34 requires 3429 Hz bandwidth, from 244 - 3674 Hz.
                                        31,200 bps V.34 requires 3200 Hz bandwidth, from 359 - 3559 Hz.
                                        28,800 bps V.FC/V.34 requires 3200 Hz bandwidth, from 320 - 3520 Hz.
                                        26,400 bps V.FC/V.34 requires 3000 Hz bandwidth, from 375 - 3375 Hz.
                                        24,000 bps V.FC/V.34 requires 2800 Hz bandwidth, from 467 - 3267 Hz.

56k: (see "Expecting 56k?" toward end of article, below)
I should qualify the above by stating that at 28.8 speeds and below, V.34 can shift the whole baseband down a bit further to compensate for a poor top end but I’m sure everyone gets the basic idea.
By contrast... a 14.4k bps (V.32bis) link requires a usable bandwidth of only 2400 Hz, from 600 - 3000 Hz.
Alas, while most U.S. domestic phone lines can easily support the requirements of 14400/V.32 bis, many fall short of meeting the minimum technical specifications necessary for V.FC and V.34 to function at _FULL_ speed. This, in a nutshell, is what you (and several others) are experiencing. The bottom line is that the modem manufacturers have built a product that is capable of substantially exceeding the design intent of the analog phone line.
Consider this: Compared to 14.4k V.32 bis, 33,600 requires 43% more available bandwidth, 28,800 bps requires 33% more bandwidth, 26,400 bps requires 25% more bandwidth and 24,000 bps needs 17% more bandwidth. This additional bandwidth _MUST_ be there from end to end, all the way from one modem to the other. Either you have it or you don’t, and your modem is as much as telling you via its performance.
Now, before you go screaming at your phone company, beware that phone company tariffs on file with the FCC and state Public Utility Commissions guarantee a voice grade bandwidth of only 300 - 3000 Hz. Meaning that if 14.4/v.32bis works ok and you’re now achieving at least 14.4k bps performance =or better= with your 28.8 (or faster) modem, then you can rest fully assured that your line at least -meets- the minimums. The phone company is under no obligation to provide anything beyond 300-3000, though most of us do enjoy considerably better than minimum spec lines.
Here are a few things you can TRY for improving your high speed modeming:
Go throughout the house and disconnect -ALL- telephonic devices attached to the phone line. This includes extension phones, answering machines, fax machines, caller-id boxes, line-in-use indicators, cordless phone base units, demon dialers, and voltage spike protectors or line filters like those commonly found in PC Desktop master-switch power directors and power line conditioning units. Don’t forget remote utility meter reading devices and burglar alarms which may also be attached to the phone line. If you find disconnecting any of this helps, then start plugging things back in one by one, until the culprit is identified. It could even be a combination of things. At the office you may discover that your modem will perform better when provisioned with an outside line, i.e., one which does not go through the office PBX or a multi-line key system.
If your telephone wiring is a rat’s nest, and/or you’ve strung some extension lines yourself and not used genuine twisted-pair telephone wiring, consider having a professional replace your haywired additions. Your telephone wiring should also be well *away* from the A/C power wiring in walls, ceilings & floors. AC power wiring should be crossed only at right angles. The old (non-twisted) 4-conductor "cloverleaf" wire (red/green/yellow/black) should never be used for 2-line service since the parallel conductors will cause crosstalk. If you have to make a vertical drop down a wall that also has power wires in it, a good rule of thumb is to allow at least 24-inches horizontal separation. Don’t waste your money putting in "Category-5" data grade cable - it isn’t required, not even for ISDN lines. However, twisted-pair phone wiring is sometimes hard to find whereas ‘Cat-5’ wire is often plentiful, so use Cat-5 if you can’t find anything else. When using ‘twisted pair’ wire, never split a "pair". Your first phone line should use the white/blue "pair". If you have more than one phone line, connect your 2nd phone line wires to the white/orange pair, and if still more put line 3 on the white/green and line 4 on the white/brown pair.
If your phone service arrives in your area via a small neighborhood hut inside which is a subscriber loop concentrator or "remote terminal" or other "pair gain" device such as the ubiquitous SLC-96 ("SLICK") or DMS-1/Urban, then all bets are off and you may have to settle for 24,000 and below on most calls, with 26,400 being a rare treat, indeed. I’ve not heard of too many folks regularly achieving 28,800 through a non-integrated slick.
If you’re using a USR modem, be SURE you have your serial port speed set to operate at 38,400 bps **MINIMUM**. The new USR’s will not permit the -data LINK- to operate at speeds any faster than the serial port... this has already bitten several unsuspecting users.

***** EXPECTING 56K? (X2/K56Flex/V.PCM)
As of this writing (Jan ‘98) the vendors offering these radically new (and somewhat controversial) modems all have one thing in common; the new modems aren’t living up to all the advertising hype or meeting all customer’s performance expectations. "X2" is more like X1.3 to X1.5 and K56Flex is no better. On phone lines good enough for these new modems to work at all, the speeds I’ve been seeing are often in the low to mid 40’s, rare exceptions notwithstanding.
Unless you *read closely* or *listened closely*, the advertising hype would have us believing that the new X2/K56Flex modems will operate at twice the normal speed of your 28.8/V34 modem. The fact is these new modems often won’t achieve anything close to those implied speeds. If you did read the ad or press release closely, you may have noticed that those statements were all couched in very vague and ambiguous terms. In close analysis no one "promised" anything. Words like "nearly" and "almost" and a vague reference to "standard modem speeds" are used to lead you into expecting something that the new modems simply did not promise and for many users fall considerably short of delivering.
In actual performance testing, the new "56k" modems can, DEPENDING ON PHONE LINE CONDITIONS, operate at speeds that, on average, yield about 42-48k. True there are some -rare- exceptions where users report 52-53k, but I need to emphasize that those are _highly unusual_ exceptions where the user’s dumb luck affords them access to a "near lab-grade" telephone line. These are often too the instances where both the user and the ISP are served out of the same telco switching office and the user is located very, very near to his local telco office, typically less than a mile or so.
One modem manufacturer, during a nationally televised interview on CNBC, suggested that up to 90% of all US Domestic phone lines could be used with this new technology. If we take into account that well in excess of 90% of all those phone lines are located in high-density commercial business areas (and thus near the telco central office), then his statement is factually correct. However, if we interpret his statement to imply that 90% of everyone will be able to use the new technology, then his statement is blatantly false. In suburban residential areas around the large metropolitan cities and in many rural areas the telephone companies make frequent use of ‘subscriber-loop-concentrators’ or slicks as they are known. By the manufacturer’s own acknowledgement, the new 56k modems won’t work in this situation. The result then is in actuality, only about 50-60% of all residential users can utilize the magical new 56k technology, and only then if they’re reasonably "close" to their telco switching office (usually less than 18,000 cable feet).
The modem manufacturers have established some long distance dial-up ‘test numbers’ for you to call in and check your lines, but these tests are anecdotal at best and absolutely meaningless unless the test equipment is located at *YOUR* local service provider and you’re placing a *LOCAL* call to reach it. The reason here is that toll-free and long distance calls are completed over 100% full digital telephone circuits. By contrast, a local call from one side of town to the other may *not* be carried over full digital facilities or may encounter a technique known as "robbed-bit-signaling" to indicate status of the call or digital pads or zero-code suppression. Therefore, the manufacturer’s long distance or toll-free test numbers may yield false or misleading positive results that usually are not achievable over a local call.
Here are a few phone line bandwidth and frequency response parameters expected by the new 56k modems:
3600 Hz of usable bandwidth between 150 - 3750 Hz with
Less than 10 db rolloff between 150 and 300 Hz
Less than 24 db rolloff between 3450 and 3750 Hz
Better than -50 db receive level at 3750 Hz Any phone company expert will tell you these are pretty extraordinary conditions for the average subscriber’s phone line.