User's Guide

User’s Guide Modem Basics
2110059 Rev 1.1 Proprietary and Confidential Page 27
4.5.2. CSC vs. Wireline
Older modem protocols, such as V.32 and earlier, were developed and optimized for PSTN
wireline networks. However, with the advent of cellular phone networks, and the need for
wireless modems to operate within these networks, it became apparent that some adjustments had
to be made to the existing protocols to enable them to achieve performance similar to their
wireline counterparts.
4.5.2.1. Enhanced Throughput Cellular (ETC
®
)
ETC was developed by Paradyne Corporation to bring cellular link performance up to wireline
levels. Although it does not exist as a layer of its own, it interacts with various layers by adjusting
some of their critical parameters to compensate for network differences. It uses a number of
techniques to improve the robustness of the data link.
At the lowest layer, ETC adjusts the filter characteristics that modulate and demodulate the carrier
signal to compensate for differences between PSTN and cellular networks. At the Error
Correction layer, it enforces the use of small data packets so that retransmissions are short in the
event of data corruption, which will be more frequent in a wireless environment.
Because ETC does not exist as a separate layer in the stack shown above, but instead makes use of
existing protocols, it is possible for an ETC-enabled modem to communicate with a non-ETC
modem. Although it is preferred that both sides of the modem link support ETC, one of the
attributes is that even if only one side supports ETC, significant benefits will be gained.
Also, due to the nature of ETC making use of existing protocols, ETC imposes certain constraints
on which protocols must be present at both ends for a connection to be established. The required
protocols are LAPM for Error Correction and V.42bis for Data Compression. Without these on
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