In the past, the fastest modem (V.34) used the telephone lines to send data at a maximum rate of 33600 bps (bits per second). 33.6k modems functioned under the assumption that the connection between the user and the ISP (Internet Service Provider) was totally analog. Today, we have 56k modems that treat the telephone network as a partially digital connection. In fact, the connection between the STN (Switched Telephone Network) and the ISP must be digital to support a data transfer rate greater than 33.6k bps. Below is an illustration of the connection between the user and ISP.

The codec located at the Switched Telephone Network converts analog signals into digital pulses and vice versa. These digital pulses are called Pulse Code Modulation and are transmitted at a rate of 64k bps. 56k modems transmit and receive data asymmetrically. The upstream (user to ISP) is limited to 33600k bps. The downstream (ISP to user) is limited to 53333 bps (by the FCC). Downstream data flow is the advantage of 56k and Pulse Code Modulation. The conversion from digital to analog causes less complication for a PCM modem (56k modem) than the conversion from analog to digital. 56k modems cannot establish >33600 bps downstream if more than one conversion exists on the telephone network between the ISP and user.

Modem stands for MOdulation/DEModulation. A modem converts digital signals generated by the computer into analog signals (modulation) for transmission over telephone lines. Incoming signals from a remote modem are then converted back(demodulated) to digital data.

The specific techniques used to encode the digital data into analog signals are called modulation protocols. The modulation protocol defines the actual method of encoding data. The modulation protocol also determines the data transfer speed. Most modems currently in use support data transmission rates greater than 9600 bits per second. These are considered high-speed modems.

The International Telecommunications Union (ITU) is responsible for the standardization of modulation protocols. Standardized protocols include: V.90, V.34, V.32bis, V. 32, V.22bis, V.22, and V.21. Below is a brief explanation of each protocol.

Standardization of K56Flex and X2 protocols. Supports 53333 bps

receive and 33600 bps transmit rates. This standard is soon to be supported by most modem manufacturers and Internet Service Providers.

Relies on digital connection between the user�s local exchange and the remote host modem.

Standard for 33600 bps connections. Originally standard for 28800 bps,

but was update to include 33600 bps in 1996.

Standard for 14400 bps modem. Downward compatible with V.32

allowing fall back to 12000, 9600, 7200, 4800 bps

Standard for 9600 bps, supports fall back speed of 4800 bps.

Standard for 1200/75 bps.

Standard for 2400 bps communications.

Standard for 1200 bps.

Standard for 300 bps.

Note: not all modulation protocols are standardized. Many other protocols exist or have existed. Understanding of those protocols is beyond the scope of this document.

There are two current non-standard 56k modulation protocols in use. These are K56Flex and X2. Rockwell and Lucent manufacture chips that use Flex modulation. US Robotics (3COM) produces all X2 modulation chips. To achieve 56k connectivity, the user must share the same proprietary protocol as the ISP. When making a connection, the user�s modem will send out a tone based on the type of modulation in use. If the receiving modem supports that modulation, a connection will be made. If the receiving modem does not support the tone modulation, the modems attempt a fall back to a lower type of modulation that is the highest rate they have in common. A K56Flex modem connected to an X2 modem will communicate with the shared V.34 modulation. In other words, the Flex modem will try to negotiate a Flex connection but will fail. The modem will then fall back to the next highest modulation in common to both modems, which is commonly V.34 (33600 bps). Fall back allows two modems to speak in a modulation they both share, and that sustainable under various conditions (line noise, etc.).

The ITU developed the standardization of Flex and X2 called V.90. V.90 permits a 56k connection without regard for the ISP�s non-standard protocol provided the ISP supports V.90. Most 56k modems are currently shipping with this protocol installed. Many ISPs have already upgraded to V.90 with others soon to follow. Once again, a modem using Flex or X2 will not establish a 56k connection to a v.90 modem. It will fall back to v.34.

Many people currently own a 56k modem and want to know how they will use V.90. There are a variety of scenarios for upgrading to V.90. The following explanation should help in deciding which route is appropriate.

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The modern modem uses a Digital Signal Processor to perform the modulation and demodulation functions. The program that controls the DSP is stored in the memory. Memory can be implemented two ways: Fixed and Open. Fixed memory is permanent and cannot be changed without a new memory chip. Open memory (Flash ROM) is electrically changeable, but is not lost when the power is off. Flash ROM can be altered (upgraded) through software. A flasher is a program that rewrites the memory and then performs a check to verify the memory�s integrity.

A modem with Flash ROM can upgrade to V.90 from X2 or Flex. Some models (mostly Flex) have a restricted amount of available memory. These modems can not simultaneously hold the code for Flex and V.90. In these cases, the user must decide which protocol is best for their purposes. This issue has been addressed and more recently manufactured modems can support both protocols. So, to summarize upgrade procedures:

1. If modem is not Flash ROM capable, must purchase new V.90 modem.

2. If modem is Flash ROM capable, can flash to desired protocol. If memory size is sufficient, can flash to both protocols

simultaneously.

3. Flash ROM flashers are available usually from the manufacturer's web site or Bulletin Boards.