User guide

Network Engineering Video Services provided by ADSL
3-7
Telesyn Service Guide (Optimizing Video Services)
Some Video encoders produce inherently bursty data streams, particularly those which attempt to rate-limit digi-
tal satellite channels. This must be accounted for in the calculations by assuming that all STBs on the ADSL16
could be watching a different “bursty” channel. Again, the user must assume a 100% non-blocking engineering
rule for bandwidth in order to avoid video disruption for the customer.
Maximum Number of STBs per ADSL16 Card Based on Minimum Downstream Rate Supported - Exam-
ple
For example, if it is found through network inspection at the head-end that the maximum burst rate on the com-
bined audio/video channels is 4Mb/sec., and .5Mb is requested for minimum internet connection for the subscrib-
ers grouped on the SM. The calculation is:
Int(100Mb/(4.0Mb + .5Mb)) = 24 STBs per ADSL16
3.2.4.2.3 Minimum and Maximum Downstream Rate Calculations per System
When an ADSL16 loop trains to a particular data rate the environmental factors at the moment may cause it to
train lower or higher then normal. Examples would include lightning storms.
• Training too low may mean that the loop won't provide enough bandwidth for the number of STBs attached,
causing video disruption.
• Some modems have a tendency to train higher than what can be sustained over time, which may lead to a
higher number of errored seconds than normal, which can also cause video disruption.
There is also an error correction advantage in setting a maximum train rate which is lower then the actual maxi-
mum a loop can support. In such a case the ADSL16 loop can take advantage of excess bandwidth to correct for
noise on the loop. (Note: This requires bitswapping to be enabled in the ADSL16 modem).
It is recommended that a system-wide minimum and maximum train rate be set for all loops. The numbers
used here would be for most subscribers, typically 0-10000 ft., which can expect to reach train rates to support
two STBs.
For longer loops, or loops with interferers on them which prevent training to the Minimum Downstream Rate, a
second training profile can be applied based on a single STB. In other words, if a loop does not train up to the
recommended minimum downstream rate, then it can only support one STB.
The formula for the Minimum Downstream Rate is the same as in Table 3-2. As part of the installation process
the operator should verify that a loop can normally reach the minimum downstream rate without issue. In event
of a poor retrain in the future (example: ADSL16 retrain during a lighting burst) the system will automatically
attempt to recover by retraining the loop until the minimum is reached.