P300H
P300 Series Modem Installation and Operating Handbook
Page 149
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 150
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").
Rates: BB:2048000 Frm:2056031
(O/h=0.39%) RS:2056031 Sym:1370687
Info, View Config, Tx, (Down, Down)
There is of course a similar screen for the Rx path.
In addition a later screen summarises the ESC settings as show below
Tx ESC:All Intfc:RS485
(Async set:4800,8,N Avail:6413)
Info, View Config, Tx, (Down, Down, Down, Down, Down etc)
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%, 1/2=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.
