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V.92/V.90 (56K) Modem FAQ's

Floyd Kling 

(3/6/08)

 

In this FAQ we discuss the two types of modem connections:

1.  Analogue modems: They connect to a conventional phone line like you have in your home.  It may be and internal (ISA or PCI) or external (Stand-Alone or USB).  If you dialed into your ISP to read this FAQ, you are using an analogue modem at your side.

 

2.  Digital modems: The type of modem used by ISP's or larger companies for high-density ‘dial-in’ applications. Digital modems typically 'answer' and are connected directly into the Telephone Company's digital network via high-speed digital connections such as a T1, DS-3, or even higher Fiber connections.  For example, AOL and MSN use Digital Modems at their side that you call into.  Digital modems, while primarily answer calls, are usually capable of originating calls also.

Questions:

 

Under what conditions will 56K analogue modems CONNECT over 33.6Kbps?

ONLY when calling into a digital modem (usually your ISP) and the phone line connection is of sufficient quality to support the higher speeds.

Under what conditions will 56K analogue modems NOT CONNECT at speeds over 33.6Kbps?

 

When analogue modems are answering a call from any (analogue or digital) modem, i.e. when calling into another 56K analogue modem.

 

Will 56Kbps modems really operate at 56Kbps?

No. Under the best conditions, 56K modems top speed in the US is 53.3Kbps.

 

Why?  Obtaining 56Kbps is only possible if the transmit levels (amplitude) are greater than the levels allowed on the phone line by the FCC.  Both Rockwell and US Robotics (the first producers of 56k modems) requested the FCC allow their modems to transmit at a higher (louder) level allowing the 56Kbps speed.  They were both turned down. 

 

Actual connection speed and throughput of any modem depends on the modulation of the signals, and the quality of that particular connection.  The speed of your connection may vary from call to call.

 

Two major factors effecting connection speed for a given modulation are:

  • Noise on the phone line (you may not be able to hear)

  • Distance you are from your central office.  The distance may effect the receive levels of both modems putting the needed signals close to the noise floor. 

56Kbps modems are asymmetric at speeds above 33.6Kbps.  Asymmetric means the upstream speed (the data you send) is different than the downstream speed (the data you receive). 

  • For V.90’s, your downstream (receive) maximum speed is 53.3Kbps, and your upstream (send) maximum speed is 33.6Kbps. 

  • For V.92’s, your downstream max is 53.3Kbps, and upstream is 48Kbps.

My modem is telling me I am connecting at 57.6Kbps and even higher. - Why?

Modems can report two speed indicators: 

1.      DTE speed

2.      DCE speed. 

These speeds are usually, almost always, different.

 

DTE Speed is the Data Terminal Equipment speed: This is the speed setting of the computer COM (communications) port.  This is the port speed between the local computer port and the local modem.  This speed is set during the installation process when you install your modem and modem driver software.  You have the ability to change this setting.  Most modems default to reporting the DTE speed upon connection  i.e.  "CONNECT 57600" (or whatever you port speed is set to)

The DTE (COM PORT) speed setting should ALWAYS be set higher than the maximum capable speed of the modem.  That is, if you have a 33.6K modem, the DTE setting should be 38.4Kbps or 57.6Kbps or higher.  If you have a 56K modem you should make your setting at 57.6Kbps or 115.2Kbps.  (I recommend 57.6Kbps, but nothing greater than 115.2Kbps)

NOTE:  If your DTE setting is lower than the highest possible modem speed, you are basically telling your modem to slow down (that is not a good thing).

DCE speed is the Data Communications Equipment speed.  It is the speed between the two modems.  This is the TRUE speed between the modems.  If you are a long distance from the central office, you will probably have a lower speed connection, even lower than 33.6K in some cases.

How can I tell if my modem is reporting the DTE or DCE speed?

If your modem is reporting ‘CONNECT 57.6’ or ‘CONNECT 115.2’, it is reporting the DTE (terminal) speed, not the true connection speed between the modems (i.e. DCE speed).

 

Here are the standard DTE speeds.  They are defined by the UART settings and are very specific:

  • 300bps

  • 1200bps

  • 2400bps

  • 4800bps

  • 9600bps

  • 19.2K

  • 38.4K

  • 57.6K

  • 115.2K

  • 230.4K

If your modem is reporting ANY other speed, it is reporting the DCE speed (speed between the modems).  The DCE speeds are unique to the modulation.  That is, the DCE speed increments for V.92, V.90, K56Flex, and X2 all vary between themselves because each design implementation is different.

(Hint) If you want your modem to report the real connection speed, the DCE speed, set the W2 option in your modems’ setup string. (my personal recommendation)... atw2<cr>

What are the recognized standards for 56Kbps modulation?

  • K56Flex by Conexant - (formerly Rockwell)

  • V.90 standardized by ITU-T (formerly CCITT)

  • V.92 standardized by ITU-T (ditto)

  • K56Flex by Conexant<Rockwell> K56Flex is practically obsolete now

  • X2 by 3Com - (formerly USR: US Robotics) X2 is practiaclly obsolete now

Modems made from Conexant (Rockwell) chips, support V.92, V.90 and K56Flex.  Modems from 3Com (USR) support V.92, V.90 and X2 modulations.  If two non-compatible standards attempt to connect to each other, they will begin connection attempts at the highest compatible standard.  If one end is not a 56K, they will begin attempting at the lower speed V.34, which begins at 33.6Kbps because that was the last unified standard.

 

For the most part, K56Flex and X2 are no longer supported, so don't use these standards in your modem selection consideration.  If you feel you must have a 56k modem, look for a V.90 or V.92 (I personally would look for V.92). 

 

 

What is V.92?
V.92 is the current ITU-T dial-up modem modulation specification.  V.92 defines modulation and speeds up to 56Kbps.  It seamlessly supports the older V.90 and is fully compatible with all lower modem standards. (56Kbps is the highest data speed speed available on dial line phone system because of the telco network sampling rate).

 V.92 benefits over V.90     V.92 adds 4 main features to the older V.90 protocol. 

  • Faster Upstream – up to 48Kbps (PCM Modulation)

  • Faster Connection Negotiation Time

  • Call waiting w/o disconnect also called MOH (Modem On Hold)

  • V.44   ... better compression for Internet related files

There is no downside to use V.92 over V.90   

IMPORTANT NOTE:  V.92 and V.90(56K) speeds are available only when the ANSWER Modem is a DIGITAL V.92/V.90(56K) Modem.  If the ANSWER modem is an ANALOG modem, regardless if it is a V.92 or V.90, the maximum connection standard is V.34, (33.6Kbps) in both directions.    Read this explanation : 56K Speed Limits

 Faster Upstream

  • V.92 Upstream (PCM) – up to 48Kbps … vs older V.90 QAM upstream – 33.6Kbps

  •  V.92 (and V.90) modems are asymmetrical.  Meaning their upstream (sending direction) is a different speed than the downstream (receiving direction). 

  •  V.90 under the best conditions, has a maximum downstream of 53.3Kbps (PCM) and an maximum upstream of 33.6Kbps (QAM).

  •  V.92 added PCM upstream allowing upstream speeds as high as 48Kbps under best conditions.  Downstream of 53.3Kbps max was not changed.

 Faster Connection Negotiation

  • Quick connect – appx 10-20 seconds – vs V.90 at appx. 30-45 seconds.

  •  V.90 (and V.92) modulations are very complex.  The calculations of the anomalies (noise) on the phone connection are laborious for the modems' processor.  ITU-T surmised that once your modem makes the calculations for the first connection, these calculations should not change unless you change locations. 

  •  The V.92 protocol makes these calculations only on the first connection after a power cycle or reset.  Those settings are saved in volatile memory.  Since the second and subsequent calls would not need to make those calculations, subsequent connection times will usually be shorter.

  •  As long as you do not turn off the modem, the subsequent time to connect should be noticeably faster.  If you cycle power or inject a 'hard reset' after every connection there is no advantage.

Call Waiting/Modem On Hold

  • 'Modem on hold' (MOH) allows you to put your modem session on hold while you take another call (Call Waiting), or you can place a voice call, without disconnecting the modem. 

  •  During the MOH function, data transmission and reception is stopped. You can then resume the data connection without re-dialing. 

 For MOH to function you must have cooperation with:

  • Your ISP

  • Your phone carrier (Call waiting and caller ID)

  • Your modem (It must support the MOH and have the software to manage it.)

The actual time you are allowed to put the modem on hold is controlled by the ISP.  The ISP can set the time from 0 to 16 seconds or wait indefinitely.  ISP’s will probably discourage the MOH function because while you have their modem on hold it cannot be used by another caller.

 V.44 Better compression for Internet related files

  • Existing V.90 and earlier standards incorporate V.42 compression.  V.42 claims to compress as much as 4:1.  V.44 claims compression as much as 6:1.  These claims are NOT real world.

  • The amount of compression is directly related to the file type.  Compression operates by removing redundancy in a file.  Text files are usually most compressible.  Most graphic files already have some compression already built in.  Executable and already compressed files have almost 0 redundancy and therefore any compression algorithm will offer little <if any> benefits.  (“You cannot compress a compressed file”)

  •  V.44 in itself, in a practical sense, is really NOT worth any expense in time or money to upgrade over V.42.

 Here is a site that ran actual throughput tests:

 http://www.digit-life.com/articles/compressv44vsv42bis/

 Here are some selected measurements from their tests:

------------- Begin Quote -------------

Comparison of the efficiency of data compression with the V.42bis, V.44 and usual pkzip.

Table 3:

text

graphics

exe-file

archives

compound

LT Win V.42bis

5.53

2.39

1.43

1.00

3.56

LT Win V.44

7.16

3.29

1.57

1.00

3.97

Pkzip

13.92

7.20

1.88

1.00

6.62

 You can see that the pkzip is much more effective than any modem compression, and the difference in the results between the pkzip and V.44 is much more than that between the V.44 and V.42bis.

 ----------------- End of Quote --------------

 END

 

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