User's Guide

The Operational View SB300 Series

Page 28 Proprietary and Confidential 2110059 Rev 1.1

“fuzzy” on the receive side. The receiver can mistake one symbol for another. This is where the

protocol speed training algorithm comes into play. It is constantly measuring the amount of noise

on the line and based on that measurement will select a speed where it knows it will be able to

resolve symbols reliably. In other words, the modem speed is a function of noise level.

Note that “noise” here is not just noise in the traditional sense of the word, such as white noise,

crackle, etc. It also includes distortions of the carrier signal caused by the various filtering stages

present between the two modems, starting at the transmitting modem itself, all the way through the

cellular network, the PSTN network, and finally the receiving modem.

Because cellular networks optimize their system for voice, additional distortions are created which

result in a higher noise figure and hence lower speed. ETC attempts to counteract these cellular

network distortions at both the transmit and receive ends.

The smarter protocols such as V.34 implement adaptive filtering techniques where the filter

characteristics are constantly adjusted to produce the lowest noise figure on both the transmit and

receive channels. V.32bis and lower speed protocols are not as sophisticated in that they will not

adapt to any situation. This is why a V.34 modem will achieve higher speeds than a V.32 modem

in a given environment.

The SB320 modem is capable of establishing both ETC and non-ETC connections while in CSC

data mode. The fastest protocol supported by the SB320 is V.34. This means that the highest

speed theoretically achievable with this modem is 33600 bps. However, this occurs under ideal

conditions as found typically in a PSTN wireline environment. In a cellular network, experience

shows that it is quite rare for such speeds to be achieved. Instead, speeds of up to 19200 bps are

more typical, depending on the characteristics of both the cellular and the PSTN sections of the

link between two modems. Common connection rates are 9600 and 12800 bps.

The table below shows the fall back speeds from V.34 to V.22 as supported by the SB320 modem.

When ETC is enabled on the SB320 using the “super” AT command AT&F5 (described in the

CSC Operations section), the maximum speed of the modem is limited to the DTE speed (set at

19200 bps).

Protocols

Speed (bps)

V.34 4800 – 33600

V.32bis 12000 – 14400

V.32 4800 – 9600

V.22bis 2400

V22 600 – 1200

4.5.2.3. Configuration Issues

Using an SB320 in Wireline or CSC modes requires careful use of the AT commands that tune the

various elements controlling the modem handshaking process. Optimizing for Wireline and CSC

modes requires different settings. Where one mode is improved with a particular profile or

settings, the other can be impaired by the same profile.

The specific sections on CSC and Wireline Operations (Chapters 6 and 7 respectively) give a

description of some of the key AT commands and S-registers associated with performance so that

the user can modify the modem configuration from its default settings if necessary.

We highly recommend reading the Primer on CSC Data Communications (Document 2110128)

for more detailed information. This is available from our web site at