V.90 Modem Compatibility
Marcio Saito, Revision v.1.10 June 2000

About this document
    Internet Service Providers (ISP) providing dialup access to home users have the difficult task of supporting their customer base and dealing with a very heterogeneous set of hardware and software. Some of the most frequent support calls at an ISP are the ones related to difficulties in getting reliable V.90 modem connections.
   The objective of this document is to help ISPs using the Cyclades-PR4000 to better understand the problem and suggest actions to minimize and handle the compatibility issues. It is not in the scope of this document to explain the V.90 technology.

The Cyclades-PR4000 and the V.90 Digital Modems
   The Cyclades-PR4000 is a Digital Remote Access Server that can digitally terminate both analog (V.90, V.34 analog client modems) and digital (ISDN-BRI clients) calls on a T1 or E1 trunk. To be able to terminate analog calls the PR4000 chassis must be populated with digital modems.
   The PR4000 digital modems are implemented in software running on Digital Signal Processors (DSP). We work in close partnership with Analog Devices Inc (ADI) for both the DSPs and the modem technology. Other Remote Access vendors including some of the major equipment vendors for the telecommunication industry share this same technology.
   Our modems run on general purpose DSPs with downloadable software. This means that they can evolve to accommodate improvements or changes in the modem protocol standards without requiring hardware upgrades.
   The digital modems in the PR4000 support the following modem protocols:

• V.90 (the official ITU/T standard for 56kbps operation)
• K56flex (the provisory 56k standard supported by Lucent and Rockwell)
• V.34, V.34bis (the 28.8/33.6kbps modem standard)
• All other important modem standards (V.FC, V.32bis, V.22bis, V.22, etc)
• V42 LAPM and MNP2-4 error correction
• V.42bis and MNP5 data compression

V.90 and Modem Compatibility Issues
    V.90 (a.k.a. "56k"), in contrast to previous standards, assumes digital modems in the server side and analog modems on the client side.
   The V.90 standard evolved from two competing standards (X2 and K56flex) and was defined late in 1998. It takes advantage of sophisticated modulation techniques to push the maximum theoretical throughput up to 56kbps over regular analog phone lines.
   Because it is relatively new and because it is sophisticated, V.90 is still evolving and there are many less-than-perfect implementations in the field.
   Several major modem suppliers are implementing the V.90 standards on the server side (ADI, Lucent, Rockwell and few proprietary implementations by the largest vendors). Together, they encompass more than 90% of the access server market.
   That means the situation is stable on the server-side of the equation. All the V.90 implementations by the major vendors have been compliant with the defined standards for some time and there should be no compatibility issues.
   Things are different on the client side. There are hundreds of client-modem vendors distributed around the world. Most manufacturers of external modems use chipsets sold by a few major vendors (Rockwell, Lucent). Most of the modems built into new PC motherboards are "soft modems", implemented using a general-purpose hardware interface with the modem protocols implemented in software.
    While these client-side modem V.90 implementations are converging to the standard, consumers purchased less-than-perfect or flawed implementations of V.90 in client modems (or PC motherboards) and rarely keep the firmware updated. There are many early and experimental implementations of V.90 in client modems in the field.
    The result is that some amount of modem compatibility problems is unavoidable. A client modem that is implemented within the V.90 standards will connect well with any server modem. A client modem with a slightly non-standard or outdated V.90 implementation may connect to server A but not connect to server B. Another client modem may connect to server B but not to server A.
    The next sections provide some suggestions on what to do if you face modem incompatibility problems and explain why the Cyclades-PR4000 helps you to minimize them.

Identifying and Solving V.90 Modem Compatibility Issues
    Here is a list of common problems related to modem connections and interoperability.

• No connection. After the phone connection is established, the modems are not able to synchronize and the line is dropped. This can be caused by line quality problems and modem incompatibility.
• Unstable connections. After the connection, the line performance deteriorates and the line ends up being dropped (when the error counters go above a threshold). Poor line quality is the most common cause, but modem compatibility issues can also cause this problem.
• Low connection speed. V.90 users expect connections speed higher then the 28.8/33.6 of V.34 connections. While 56kbps is not practical today (it is the theoretical maximum connection speed), connections speed in the range of 40kbps are typical. Several factors that can impact connection speed, including quality of the line, analog phone switches, V.90 compatibility issues, quality of wiring, etc.

    For the large majority of cases, compatibility problems are caused by old, non-compliant client modem software.
   Unfortunately, there is not much we can do about it. While the modems implemented in the PR4000 (or any other current RAS for that matter) are standard V.90, while also somewhat flexible to accommodate slightly non-standard client modems, it cannot perform miracles. When dealing with a large number of client modems, there will always be a few that are so non-standard that they wonÕt be able to connect.
   The only solution to solve V.90 compatibility problems caused by outdated modem firmware is to upgrade it with the latest revision released by the client-modem manufacturer.
   We understand that it is not easy for an ISP to get home users to update their modems, but sometimes there is no other alternative. The WEB site http://www.56k.com can help you not only with support information, but also in identifying problematic modems in the field and getting the latest firmware for the popular client modems.
   If the client modem cannot be updated (either because it is an obsolete product or because the modem implementation is hardware-based), an alternative would be to disable V.90 in the client modem by issuing the appropriate AT commands. This way, the modem would connect using the more stable V.34 protocols.
   Here is a list of additional sources of connection problems.

• Sometimes, modem connection problems are not caused by V.90 incompatibility, but rather are due to the quality of the phone connection. The PR4000 allows you to monitor and collect statistics related to the T1/E1 trunk. Check the error counters and contact the phone carrier if they keep increasing.
• Even when the line is perfect, connection speeds above 33.6 are not possible if there is even one analog phone switch between the client and the server. Most installations in North America are digital, but there are still some analog central offices.
• V.90 is an evolving standard and, while in most cases the problem is on the client side, you should make sure to maintain the RAS firmware up to date. That may be difficult or expensive, depending on the RAS being used. For the PR4000, it is a simple flash software update, free from the Cyclades FTP site. Read the firmware release documents or contact our technical support if you are in doubt about the need for a firmware upgrade.
• Modem configuration. The PR4000 allows you to control some of the modem parameters. For example: our modems support K56flex, which is an early 56k standard. Some very old modems confuse the K56flex tone with a V.32 tone, which causes an incomplete connection. If your customer base has a lot of very old modems, you may consider disabling K56flex support. Contact technical support for help with this.
• T1/E1 Line configuration. The PR4000 allows you to set some line configurations, such as T1/E1 line build out and receiver sensitivity. The default values should work, but in some cases you may want to vary these parameters if you have problems with errors in the T1/E1 trunk and the T1/E1 provider cannot detect problems with the line. Consult technical support for assistance.

The Best RAS Product, The Best Partner.
    The design of the Cyclades-PR4000 is newer than most of the popular remote access servers on the market. Because of this, we were able to take advantage of the latest developments in hardware technology.
    High power consumption, difficult or impossible modem firmware upgrades, incompatibility, and high component count are just some of the problems that are common in older RAS designs. All those problems are eliminated by the PR4000 architecture.
    The modems in the PR4000 can be fully upgraded by software (both modem data pump and controller code) and can evolve to accommodate improvements and changes in modem protocols. With the PR4000, you can rest assured that you will always have access to the latest in modem technology without having to upgrade the DSP hardware.
    The DSP and modem technology provided by ADI is one of the most current in the market, being used by Cyclades and some of the major telecommunication companies. While it is impossible to assure interoperability with non-compliant V.90 client modems, ADI, Cyclades, and other ADI partners work very hard to provide a V.90-compliant implementation that connects to the largest possible number of client modems on the market.
    Cyclades offers free and unlimited technical support and software updates for the life of the product to all customers. The best available modem firmware is always available to you and can be downloaded from our FTP area. There is no need for expensive service contracts. Having current modem firmware means that you will always be able to provide the largest possible client-modem coverage.
    In contrast to the large vendors providing remote access solutions focused on the carrier market, our focus is the small and medium Internet Service Provider. Because of this, your problem is our problem and we will work with you to help you identify and solve your problems.

Comments or suggestions to this document can be sent to Marcio Saito marcio@cyclades.com