Faq what is V. 90 and how does it work? 3


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ThinkPad V.90 Modem
FAQs






March 12, 1999Table of Contents

FAQs (Frequently Asked Questions):

FAQ-1. What is V.90 and how does it work? 3

FAQ-2. What is the maximum speed achievable with my V.90 modem? 4

FAQ-3. Will my ThinkPad V.90 modem connect at the same speed as

another V.90 modem? 6


FAQ-4. Why does my V.90 modem fail to make a connection sometimes?

or... Why does my V.90 modem drop the connection sometimes? 8


FAQ-5. Why does my V.90 modem connect at low speeds over the PBX

at my office? ...at a hotel?

or... Why does my V.90 modem fail to connect over the PBX

at my office? ...at a hotel? 10
FAQ-6. What kind of protection does my ThinkPad have for preventing

damage if I connect to a digital phone line?

or... A message box appears on my ThinkPad telling me that an

over-current/over-voltage condition has been detected.

What does this mean? Have I damaged my ThinkPad? 12

V.90 URL References 13
FAQ-1. What is V.90 and how does it work?
A1. V.90 is an ITU-T (International Telecommunications Union - Telecommunications Standardization Sector) standard for high speed modem technology which was formally ratified in September 1998. This standard, targeted at internet and on-line service applications, specifies the protocols to be used by modems for transmitting/receiving data over the PSTN (Public Switched Telephone Network). V.90 replaces two earlier protocols, K56flex and x2. After upgrade to V.90, these heretofore incompatible modems can inter-operate at V.90 speeds. Given ideal laboratory conditions, V.90 allows for the download of images and data at speeds up to 56Kbps (56 thousand bits per second) and upload speeds (client to server/ISP) at V.34 data rates (such as 33.6, 31.2, 28.8, 26.4Kbps, etc.). It is important to note that FCC (Federal Communications Commission) regulations currently limit the power levels that can be sent over the PSTN phone lines in order to minimize electromagnetic interference with other electrical devices using adjacent phone lines. This effectively limits the maximum transmission speeds (server/ISP to client) available on phone lines to approximately 53Kbps. To achieve V.90 connections and to exploit this high speed technology requires a modem with V.90 capability, an ISP (Internet Service Provider) with V.90 capable equipment and a telephone line that supports V.90 connections. Due to a variety of limitations found in the PSTN (see other FAQs and URLs for more information), actual speeds vary and are often less than the maximum possible.
NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, has updates that may help to increase the connection speeds over some phone lines and to some servers/ISPs.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html
For more information:
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html

“56K Basics: How it works

http://www.56k.com/basics/basics.shtml

“56K Basics: 3 things you need to use 56K

http://www.56k.com/basics/3things.shtml

“56K Disadvantages: 53K limit in the U.S.

http://www.56k.com/cons/53k.shtml

“What is V.90 Technology

http://www.v90.com/whatis.htm

“How V.90 does its Magic

http://www.v90.com/v90magic.htm

“Agreement Reached on 56K Modem Standard” (ITU Press Release)

http://www.itu.int/newsroom/press/releases/1998/98-04.html

“56K Modem Standard Continues to Break New Ground” (ITU Press Release)

http://www.itu.int/newsroom/press/releases/1998/NP-3.html

FAQ-2a. Why can’t I ever connect at the full 56Kbps speed with my V.90 modem?

FAQ-2b. Why does my V.90 modem only connect at speeds in the low 30Kbps or below?

FAQ-2c. Why does my V.90 modem connect at low speeds from one location, but higher

speeds from another location?

FAQ-2d. I upgraded my modem to V.90, but it still only connects at speeds in the low 30Kbps

or below . Why can’t I achieve higher connection speeds?

FAQ-2e. What is the maximum speed achievable with my V.90 modem?
A2. While the V.90 standard as ratified by the ITU-T (International Telecommunications Union - Telecommunications Standardization Sector) allows for download speeds (server/ISP to client) at up to 56Kbps (56 thousand bits per second) given ideal lab conditions, FCC (Federal Communications Commission) regulations currently limit the power levels that can be sent over the PSTN phone lines in order to minimize electromagnetic interference with other electrical devices using adjacent phone lines. This effectively limits the transmission speeds (server/ISP to client) to approximately 53Kbps. This limit is enforced by the ISP (Internet Service Provider) or server by which you are accessing the internet. Countries other than the United States have differing transmission power limits and, therefore, your modem may have a different theoretical maximum speed. In actuality, the connection speed depends on many factors and is often less than the maximum possible. Depending on such factors, connect speeds in the 40s, 30s and high 20s (Kbps) are not unusual.

Modem data rates achievable over the PSTN (Public Switched Telephone Network) vary widely based on the location of the caller, the location of the server/ISP being called, the path through the network for the specific call, the modem being used by the individual PC owner, and the modem deployed at the service provider location. This results from the fact that the public network was designed to mainly transmit voice calls and to provide lines which are “clean” enough to support about 5Kbps (5 thousand bits per second) data rates. When data calls are established and data is transmitted across the network, the signal is very sensitive to the electrical characteristics of the telephone line, the length of this line to the central office and the telephone company equipment interference.

Any noise in the phone line can result in diminished connection speeds and data throughputs. This noise may be introduced by such common sources as power lines (inside or outside your home/office) and other equipment connected to the same phone line (cordless phones, answering machines, phone line surge protectors, etc.). These analog impairments adversely affect analog modem signals by reducing the signal to noise ratio. You may be able to make some improvement in connection speed by using another phone extension in your home/office or by disconnecting some of the equipment you may have attached to the phone line.

Some connections are subject to digital discontinuities (i.e., additional signal conversion from analog to digital along the path between client modem and server/ISP modem) which by default will force all V.90 capable modems to fall back to V.34 connections (data rates of 33.6, 31.2, 28.8, 26.4Kbps, etc.). Most other connections are subject to digital impairments of two types: “PADs” and “RBS.” PADs are power attenuations introduced into the phone system by the telephone company in order to protect telephone company equipment from signals that may be strong enough to damage that equipment. RBS (Robbed Bit Signaling) describes the process by which some bits of data are periodically overwritten by the telephone company in order to indicate call status information. These digital impairments may adversely affect the modem signal efficiency and the achievable data throughput and connection speed.




NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, has updates that may help to increase the connection speeds over some phone lines and to some servers/ISPs.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html
For more information:
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html

“56K Disadvantages: 53K limit in the U.S.

http://www.56k.com/cons/53k.shtml

“What is V.90 Technology

http://www.v90.com/whatis.htm

“How V.90 does its Magic

http://www.v90.com/v90magic.htm

“Disadvantages of 56K: Only one A/D conversion allowed

http://www.56k.com/cons/onead.shtml

“Disadvantages of 56K: Won’t work with some PBXs

http://www.56k.com/cons/pbx.shtml

“Fastconnection: No Room Service After 33.6Kbps

http://www.zdnet.com/pcmag/pclabs/nettools/1714/sb1.html

“Why you won’t get 56K

http://www.computers.com/reviews/comparative/substory/0,29,0-14-257672-667659-1,00.html?sr.co.cri.inav.crsu667659

FAQ-3a. Why does my PCMCIA modem or desktop PC modem connect at higher speeds than

my ThinkPad modem?

FAQ-3b. I upgraded my modem to V.90. Why does it connect at lower speeds compared

to my PCMCIA modem or desktop PC modem?

FAQ-3c. Will my ThinkPad V.90 modem connect at the same speed as another V.90 modem?
A3. There exists a wide range of differing line conditions and modem/ISP (Internet Service Provider) parameters including line length, digital impairments, power levels, particular types of server/ISP equipment, etc. One manufacturer’s modem may perform better under certain telephone network scenarios or to certain servers/ISPs, while another manufacturer’s modem (or another model of modem from the same manufacturer) may perform better under an entirely different scenario. No two models of modem will perform identically over all network conditions. For this reason, one user may achieve higher connection speeds with a PCMCIA modem over their particular phone line, while another user on a another phone line (or the same user in another location) may experience higher connection speeds when using the ThinkPad modem. IBM has designed the modem to the ITU-T (International Telecommunications Union - Telecommunications Standardization Sector) V.90 specification with focus on incorporating such critical elements as the TIA (Telecommunications Industry Association) network model of the most common line conditions and the regulations/specifications for telephone networks in the United States and in countries around the world.

Furthermore, some modem designers seek as their primary goal achieving the highest line speed physically possible and in the process may compromise connection reliability and stability. While this has the advantage of reporting higher initial speeds to the application (what the user sees), it has the disadvantage of higher error rates. Transmission errors cause data to be continuously retransmitted resulting in an lower overall data throughput. In other words, downloading an image from the internet involves moving a large quantity of data across the phone line. If a significant amount of that data must be retransmitted due to errors, then the efficiency of the connection is lower and the time to download the image may be longer despite a higher connect speed.

Also, some modem designers focus on achieving the highest, yet most reliable line speed. This approach may result in slightly lower initial connect speeds, but significantly lower error rates and a higher probability of connecting and staying connected to the server/ISP. Data throughput is often higher despite slightly lower connection speed. Using the image download example mentioned above, less data must be retransmitted due to errors and the overall connection efficiency is often higher resulting in downloading the image from the internet in a shorter time.
NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, has updates that may help to increase the connection speeds over some phone lines and to some servers/ISPs.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html

For more information:
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html

“Disadvantages of 56K: Only one A/D conversion allowed

http://www.56k.com/cons/onead.shtml

“Disadvantages of 56K: Won’t work with some PBXs

http://www.56k.com/cons/pbx.shtml

“Fastconnection: No Room Service After 33.6Kbps

http://www.zdnet.com/pcmag/pclabs/nettools/1714/sb1.html

“Why you won’t get 56K

http://www.computers.com/reviews/comparative/substory/0,29,0-14-257672-667659-1,00.html?sr.co.cri.inav.crsu667659

FAQ-4a. Why does my V.90 modem fail to make a connection sometimes?

FAQ-4b. Why does my V.90 modem drop the connection sometimes?

FAQ-4c. My V.90 modem frequently fails to connect to one of my frequently dialed

servers/ISPs. Is there anything that can improve this?

FAQ-4d. My V.90 modem frequently drops the connection to one of my frequently dialed

servers/ISPs. Is there anything that can improve this?

FAQ-4e. My V.90 modem connects at very low speeds to one of my frequently dialed

servers/ISPs. Is there anything that can improve this?

FAQ-4f. Is there anything that I can change in my modem setup or initialization string to

improve my performance? ...improve my ability to make or hold a connection?
A4. Failed connections or dropped connections may occur for a variety of reasons. Some of the more common causes could be a server/ISP (Internet Service Provider) equipment failure, a busy server/ISP line, modem inactivity, the telephone cord not fully engaged in the telephone jack or an incomplete/unsuccessful software installation/upgrade.

In addition, there are several more complicated causes for failed or dropped connections. Error detection and recovery protocols are an integral part of the modem connection process. During the initial phase of modem connection, these protocols are used to establish a viable connection (including negotiation of pulse code modulation (PCM) code points, speed, error correction, data compression, etc.) between your modem and the server/ISP modem. Furthermore, throughout the modem session, changing line or server conditions can trigger a “retrain” (i.e., renegotiation) to reset that connection to another set of parameters that are mutually acceptable to your modem and to the server/ISP. During the exchange of information for these complex error detection and recovery protocols, there is significant opportunity for failure.

As a result of differing interpretations of the V.90 specification, the actual hardware/software/firmware of these modems may have slightly differing implementations of V.90. In addition, modem manufacturers have tuned and optimized their modems to perform best at somewhat differing line conditions and modem parameters. Finally, server/ISP software versions routinely change which can alter the path of the protocol negotiation process and, depending on the manufacturer/model of the modem, may positively or negatively affect the ultimate connection that can be achieved. If a negotiated connection cannot be established or maintained over the course of the session, then a failed or dropped connection will be the outcome.

Telephone company network, client modem and server/ISP modem enhancements in the future are important elements of modem technology evolution.

For some connections to servers/ISPs that frequently suffer from failed or dropped connections or very poor connection speeds, it may be possible that forcing the modem to default to the V.34 protocol may actually improve the connectivity or performance. This may be accomplished through ThinkPad Modem - Options - Settings by selecting a maximum speed of “33600 (V.34).” Alternatively, there are commands that may either be included in the modem initialization string (i.e., a list of active settings executed during the negotiation process with the remote modem) or entered in terminal mode as “AT” commands. It is important to remember that the maximum speed as set through the ThinkPad Modem application or as invoked through editing the modem initialization string will remain in effect until manually returned to their original settings. In other words, you will only be able to achieve V.34 data rates (33.6, 31.2, 28.8, 26.4Kbps, etc.) as a maximum for all dialed servers/ISPs until you change the default maximum speed back to V.90.

NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, has updates that may overcome some instances of failed or dropped connections.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html
For more information:
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html

FAQ-5a. Why does my V.90 modem connect at low speeds over the PBX at my office?

...at a hotel?

FAQ-5b. I upgraded my modem to V.90, but it still only connects at V.34 speeds over the PBX

at my office. Why can’t I achieve higher connection speeds?

FAQ-5c. I upgraded my modem to V.90, but it still only connects at 32Kbps over the PBX

at my office. Why can’t I achieve higher connection speeds?

FAQ-5d. Why does my V.90 modem fail to connect over the PBX at my office? ...at a hotel?

FAQ-5e. Are there any compatibility issues that might affect the ability of my V.90 modem

to dial out or to connect over a PBX?
A5. Private branch exchange (PBX) telephone systems operate as a separate network from the public switched telephone network (PSTN). PBXs are deployed in thousands of corporations, offices and hotels around the world to carry voice traffic. Companies with PBXs can route telephone calls within their site without ever accessing the public telephone network. In doing so, these companies gain additional control of their telephone system, save money by avoiding “per employee/per room” telephone line fees and can offer employees/guests advanced features such as extension dialing, call forwarding and conference calling. These PBXs connect many internal lines to a “trunk” (i.e., groups of external lines) to allow employees or hotel guests to access the PSTN. There are two general PBX types -- digital and analog. Furthermore, digital PBXs may have both digital extensions and analog extensions.

Analog PBX Lines or Digital PBX / Analog Extensions: Although your ThinkPad modem will be able to achieve high speed V.90 connections over many digital PBXs with analog extensions and over many analog PBX lines, it is not unusual to obtain lower connection rates over some PBX networks. PBX networks are typically designed for “voice” traffic in a specific corporate, office or hotel environment. PBXs vary widely in both line quality and type of termination to the PSTN. These variations can create problems for analog modems in general and may manifest themselves as low V.90 connection speeds. Analog modems are designed to comply with the ITU-T (International Telecommunications Union - Telecommunications Standardization Sector) standards for modems and to comply with the specific government specifications/regulations defining power levels, etc. PBXs are not subject to these standards and regulations for internal traffic (over the PBX).

In addition, small digital PBXs with analog extensions often convert those analog extensions to digital in the PBX and then back to analog to connect to the PSTN. These conversions are called digital discontinuities and result in the introduction of noise and distortion into the data. Digital discontinuities by default will force all V.90 capable modems to fall back to V.34 connections (data rates of 33.6, 31.2, 28.8, 26.4Kbps, etc.).




Furthermore, some PBXs use a nonstandard dial tone which, if not recognized by the modem, prevents dialing out. The typical range of frequencies for dial tones are derived from recommendations contained in the Bellcore Technical Reference (TR-NWT-000506) and the EIA/TIA Requirements for PBX Switching Equipment standards document (EIA/TIA-464B). IBM has designed the modem software to accommodate these recommended frequencies and to accommodate many other nonstandard dial tone frequencies. However, the sheer magnitude of varied PBX implementations preclude any modem manufacturer from guaranteeing compatibility across all PBXs. Fortunately, for many of those cases of nonstandard dial tone, there may be a simple workaround called “blind dialing.” This involves disabling dial tone detection in the modem by the issuance of the command “ATX1” as part of the modem initialization string. Blind dialing can also be selected from within most dialing applications through modem properties or within Windows 95/98 modem properties by deselecting the check-box for “wait for dial tone before dialing”. While this does not guarantee connect capability, it can help is most such cases.

Digital PBX / Digital Extensions: Analog modems, such as the type in your ThinkPad, as well as most integrated notebook computer modems and PCMCIA modems, are not designed to work with digital extensions on digital PBXs. In fact, connecting to some digital PBX phone lines will damage many analog modems. Fortunately, your ThinkPad modem has over-voltage/over-current protection which will disconnect from the digital line before any damage can be done. A message box will then appear informing you that you are connected to an incompatible phone system.
NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, has updates that may help overcome some limitations of connecting through PBXs.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html
For more information:
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html

“Disadvantages of 56K: Only one A/D conversion allowed

http://www.56k.com/cons/onead.shtml

“Disadvantages of 56K: Won’t work with some PBXs

http://www.56k.com/cons/pbx.shtml

“Fastconnection: No Room Service After 33.6Kbps

http://www.zdnet.com/pcmag/pclabs/nettools/1714/sb1.html

“Why you won’t get 56K

http://www.computers.com/reviews/comparative/substory/0,29,0-14-257672-667659-1,00.html?sr.co.cri.inav.crsu667659

FAQ-6a. Why does my modem display a message telling me that I can’t dial out over the PBX

at my office? ...at a hotel?

FAQ-6b. Why does my modem display a message telling me that I can’t dial out when I am

in another country?

FAQ-6c. A message box appears on my ThinkPad telling me that an over-current/over-voltage

condition has been detected. What does this mean? Have I damaged my ThinkPad?

FAQ-6d. What kind of protection does my ThinkPad have for preventing damage if I connect

to a digital phone line?
A6. Analog modems, such as the type in your ThinkPad, as well as most integrated notebook computer modems and PCMCIA modems, are not designed to work with digital extensions on digital PBXs. In fact, connecting to some digital PBX extensions will damage many analog modems. Your ThinkPad modem hardware has been designed with a feature to detect over-voltage and/or over-current conditions that often exist on digital phone lines and to instantaneously go off-hook (disconnect), thus protecting your ThinkPad investment. Your modem immediately resets and is fully operational for you to try another phone line.

This feature is also used to protect your ThinkPad in the event that you have not yet configured your modem for use while in another country. This configuration is automatic once you select the country in which you are using your modem. Country selection is performed using either the Windows - Dialing Properties application or the ThinkPad Modem - Country Selection application.
NOTE: ThinkPad ACP Modem V.90 Upgrade Version 1.50WW (for ThinkPad 770/ED/X/Z, 600/E models with integrated ACP modem) available on the IBM ThinkPad web site, is the latest version of the V.90 modem upgrade.
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html

V.90 URL References
ThinkPad ACP Modem V.90 Upgrade Version 1.50WW

For ThinkPad 770/ED and 600 users running Windows 95/98:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVPYF.html

For ThinkPad 770/ED and 600 users running Windows NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS2J.html

For ThinkPad 770X/Z and 600E users running Windows 95/98/NT 4.0:

http://www.pc.ibm.com/qtechinfo/LWIK-3ZVS8H.html
“V.90 Analog Modems”

http://www.pc.ibm.com/qtechinfo/DSHY-45VSWQ.html
“56K Basics: How it works

http://www.56k.com/basics/basics.shtml
“56K Basics: 3 things you need to use 56K

http://www.56k.com/basics/3things.shtml
“56K Disadvantages: 53K limit in the U.S.

http://www.56k.com/cons/53k.shtml
“What is V.90 Technology

http://www.v90.com/whatis.htm
“How V.90 does its Magic

http://www.v90.com/v90magic.htm
“Agreement Reached on 56K Modem Standard” (ITU Press Release)

http://www.itu.int/newsroom/press/releases/1998/98-04.html
“56K Modem Standard Continues to Break New Ground” (ITU Press Release)

http://www.itu.int/newsroom/press/releases/1998/NP-3.html
“Why you won’t get 56K

http://www.computers.com/reviews/comparative/substory/0,29,0-14-257672-667659-1,00.html?sr.co.cri.inav.crsu667659
“Disadvantages of 56K: Only one A/D conversion allowed

Http://www.56k.com/cons/onead.shtml
“Disadvantages of 56K: Won’t work with some PBXs

http://www.56k.com/cons/pbx.shtml
“Fastconnection: No Room Service After 33.6Kbps

http://www.zdnet.com/pcmag/pclabs/nettools/1714/sb1.htm

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