User's Guide

User’s Guide Modem Basics
2110059 Rev 1.0 Proprietary and Confidential Page 27
the receiving modem, the ETC modem will not be able to negotiate a connection and will
disconnect; irrespective of what other protocols the modems may have in common.
ETC includes the following specific settings to improve immunity to the interference and noise
effects mentioned earlier.
• Small frame size
• Do not allow non error-corrected links,
• Wait for carrier 90 seconds
• Lost carrier hang-up set to 10 seconds
Use of small frame size does reduce direct throughput by 10%, but gives a more robust
connection.
The policy of “start slow and train up” guarantees that the initial negotiation phase has more
chance of success. Start-up speeds of 4800 bps and 9600 bps may be used. In addition if a
connection has not been established after 60 seconds, the modems will fall back to 1200 bps.
Even with all this it is best to limit the maximum DCE speed.
4.5.2.2. Modulation
High speed modem protocols such as V.34 and V.32 use a technique known as Quadrature
Amplitude Modulation (QAM). QAM operates by modulating a carrier sine wave signal in both
amplitude and phase. Each unique combination of amplitude and phase is known as a ‘symbol’
and, therefore, each symbol represents a number. If there are 8 symbols (i.e. 8 unique
combinations of amplitude and phase), then each symbol corresponds to a unique 3-bit number
(0b000 to 0b111). On the receive side, the reverse process of demodulation is performed on that
modulated carrier signal and the symbols or bits of information are retrieved.
The key to achieving high speed on a modem is to use as many symbols as possible so that more
bits can be encoded in them. The number of symbols that can be used is determined by the
receiving modem’s ability to resolve or distinguish between these various symbols.
The more symbols there are, the more difficult it becomes for the modem to resolve them. This is
because there is always a certain amount of noise on the link which causes the symbols to appear
“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.