110 Rate Adaptation

If you set the I-modem to V.110, it will negotiate only for V.110 connections. If a V.110 connection cannot be made, the I-modem does not negotiate for other types of connec-tions.

Analog Modem or Fax Emulation

The I-modem does not try to make a digital connection and works only in Analog mode. If you know youÕll make and receive only analog calls, this setting shortens the connect time.

Analog Mode Handshaking

When making analog connections, the I-modem defaults to V.34 and tries for the highest possible speed (33.6 Kbps) when it attempts to connect with a modem. The V.34 range spans 33.6, 31.2, 28.8, 26.4, 24, 21.6, 19.2, 16.8, and 14.4 Kbps, and 9600, 7200, 4800, and 2400 bps. If the remote analog device is not V.34 capable, a connection is made using

Handshaking, Error Control, Data Compression, and Throughput • Page 11-6

the highest compatible modulation scheme (V.FC, V.32terbo, V.32bis, and so on, down to as low as Bell 103, or 300 bps).

If the remote analog device has V.34 capability, the I-modem uses a line-probing technique to determine the highest speed possible under current line conditions, then completes the connection. If the remote analog device does not have V.34 capability, the I-modem listens to the deviceÕs answer tones to identify the standard rate at which the remote analog device is operating, and then adjusts to that rate.

When the I-modem answers a call from an analog device, the I-modem sends out a series of answer tone signals until both devices negotiate the best connection rate.

Fast Class (V.FC) Handshaking

After trying V.34, the I-modem tries for the fastest possible V.Fast Class (28.8 Kbps) con-nection. The V.FC range spans 28.8, 26.4, 24, 21.6, 19.2, 16.8, and 14.4 Kbps. If the remote device is not V.FC- capable, a connection is made using the highest compatible modulation scheme (V.32terbo, V.32bis, and so on, down to as low as Bell 103, or 300 bps).

If the remote device has V.FC capability, the I-modem uses a line-probing technique to determine the highest speed possible under current line conditions, then completes the

Handshaking, Error Control, Data Compression, and Throughput • Page 11-7

connection. If the remote analog device does not have V.FC capability, the I-modem lis-tens to the deviceÕs answer tones to identify what standard rate the remote analog device is operating at, and then adjusts to that rate.

When the I-modem answers a call from an analog device, the I-modem sends out a series of answer tone signals until both devices negotiate the best connection rate.

USR V.32terbo to USR V.32terbo

On these analog connections, I-modems have two features that result in outstanding per-formance: Quick Connect and Adaptive Speed Leveling (ASL).

Quick Connect allows the devices to connect in approximately 7 seconds, a far shorter time than with most devices.

ASL (described below in Other V.Protocols) is used by I-modems operating in V.32terbo and V.32bis modes.

Other V. Protocols

Earlier, lower-speed ÒV.Ó protocols do not employ line probing. Instead they use pre-deÞned answer tones to specify or identify speed capabilities. These protocols deÞne the following maximum speeds:

Handshaking, Error Control, Data Compression, and Throughput • Page 11-8

¥ V.32terbo: 19.2 Kbps, with an additional USR device-to-USR device speed of 21.6 Kbps.

¥ V.32bis: 14.4 Kbps.

¥ V.32, V.22bis, etc.: 9600 bps and lower.

ASL (used in V.32terbo and V.32bis modes) is a technique that allows the I-modemÕs receivers and transmitters to act independently of each other. One transceiver may slow down and then speed up without affecting the data ßow on the other. The result is more efÞcient line operation.

Note When answering using V.32terbo, I-modems shift their serial port rate up to 38.4 Kbps (for 21.6 Kbps connections) if the calling V.32terbo device dials in at 21.6 Kbps. The answering V.32terbo device then sends data to its computer at 38.4 Kbps. V.32bis I-modems also shift their serial port rate to 19.2 Kbps to make 14.4 Kbps connections. If your computer does not sup-port these higher serial sup-port rates, disable V.32terbo and/or V.32bis. (See Register S34, in Appendix A, Alphabetic Command Summary.)

Note While most modems on the market now use higher speeds, there may be a problem in answering older, V.32 modems at 9600 bps. Use Register S28 to modify the duration of the extra tones used in V.32 negotiations, in the rare

Handshaking, Error Control, Data Compression, and Throughput • Page 11-9

instance that this may be necessary. (See Appendix A, Alphabetic Com-mand Summary.)

Dual Standard Handshaking

We recommend that I-modems retain the default B0 and &N0 settings. This allows them to make analog connections with

ÒV.Ó protocol and HST modems in both Originate and Answer modes at a variety of speeds.

When originating an analog call and the I-modem is set to B1, it sends out a Bell answer tone, which is the prevalent standard in the United States and Canada for connections at 2400 bps and lower. At higher speeds, the I-modem also recognizes the ITU answer tones necessary for connecting with V. protocol modems, and adjusts to the answering device.

However, when answering a call, an I-modem sending out the Bell answer tone (B1) won't be recognized by V. protocol modems. The calling modem, instead, will wait until it detects a tone it recognizes. The V.22bis tone used at 2400 bps.

Handshaking, Error Control, Data Compression, and Throughput • Page 11-10

If you want the I-modem connect with V. protocol modems at high speeds, make sure it is set to B0 for the ITU answer tones. It will also connect with HST modems at speeds up to 16.8 Kbps.