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P300H P300 Series Modem Installation and Operating Handbook Page 147
Rates: BB:2048000 Frm:2056031
(O/h=0.39%) RS:2056031 Sym:1370687
Info, View Config, Tx, (Down, Down)
Tx ESC:All Intfc:RS485
(Async set:4800,8,N Avail:6413)
Info, View Config, Tx, (Down, Down, Down, Down, Down etc)
8.7 CLOSED NETWORK PLUS ESC
8.7.1 Overhead Rates
The following table shows example overhead rates for different async ESC Baud rates at a variety of main
channel data rates. The table was generated (using a modem) with the async character format set for 8
bits no parity, the overhead percentages will vary slightly for async character formats different from this.
Figures: Overhead rate with backward alarm [/ Overhead rate without backward alarm, if different]
Main data channel rates
ESC rates 9.6kbps 16kbps 64kbps 256kbps 512kbps 1024kbps 2048kbps
2400 Baud
33.3% 20% 5% / 4.8% 1.25% / 1.19% 0.62% / 0.59% 0.39% * 0.39% *
4800 Baud
100% 50% 10% 2.5% / 2.38% 1.25% / 1.19% 0.62% / 0.59% 0.39% *
9600 Baud
n/a 100% 20% 5% / 4/76% 2.5% / 2.38% 1.25% / 1.19% 0.62% / 0.59%
19200 Baud
n/a n/a 50% 10% 5% / 4.76% 2.5% / 2.38% 1.25% / 1.19%
38400 Baud
n/a n/a 100% 20% 10% 5% / 4.76% 2.5% / 2.38%
* see notes on absolute minimum overhead in para 8.7.3 on page 148
The formula for deriving the overhead rate is moderately complex, and relies on many different parameters.
For other mixes of data rate and async settings the simplest way to determine the overhead is to enter the
parameters into a modem and then select Info, View Config, Tx. A screen such as the one shown below
is displayed (as the second or third screen accessed by Down Arrow). This screen shows the exact
overhead percentage as well as the Framed data rate ("Frm"), the data rate including any Reed-Solomon
encoding ("RS"), and the final symbol rate including the effects of the `inner ` FEC and modulation ("Sym").
There is of course a similar screen for the Rx path.
In addition a later screen summarises the ESC settings as show below
Note that the displayed available ESC rate ("Avail") may well exceed the set ESC rate. This is because the
hardware can only provide overhead in 1/Nth increments (eg 1/1=100%, ½=50%, 1/3=33%, 1/4=25% etc
up to 1/256=0.39%) and so the modem must pick the appropriate value of N which provides at least the
required async ESC Baud rate.