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Problem in SCPC link

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Ex Member
Ex Member

Oct 18th, 2011 at 6:42am  
Dear All,

We have a SCPC link which is operating in C Band. I have tested my link several times on spectrum and I have done successful loop backs on my link. My link is very much stable but when I ping from site A to Site B am getting ping drops. Some times I get excellent response but most of the time I am getting very poor response. I have check CRC errors on the serial port and there were no CRC on both sites.
I have changed frequencies even but problem is still there. Now I need your expert opinion to resolve this problem.

Thank you.
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Reply #1 - Oct 18th, 2011 at 9:11am  
If there are no CRC errors then the satellite link is error free and the problem must be elsewhere.

Is the link very congested with traffic due to intensive use, VoIP/skype calls, large file transfers, viruses etc ?  That would cause a proportion of pings to be lost, particularly at peak traffic times.

Leave a ping running for a long time ( -t option ) and see if you can observe what else occurs when the lost pings appear.

Do the pings to the IP address of the local modem and also to the IP address of the remote modem. Starting with your your PC you need to go step by step, testing further each time, till the problem is localised.

If there is a ping generator inside the local modem, use it outwards in both directions, as well as testing from your PC.

Turn off all the pingers when you have finished !

Best regards, Eric.
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Reply #2 - Oct 18th, 2011 at 11:22am  
Dear Eric,

Thank you for your reply. No application is running at the time when I ping from site A to site B.
I am using Comtech Data 570 Modem with the combination on Juniper FW/Router.
I am doing ping from Router to router and still I am finding ping drops... with all my efforts I am getting 80% response from both sites.
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Reply #3 - Oct 18th, 2011 at 11:38am  
If you're using Microsoft's PING utility, the default wait time I believe is about 500ms. In many circumstances, that's not long enough for satellite hops. Bottlenecks in the path can delay the PING return beyond the default timeout. In those occasions the packet is counted as dropped, even if it actually did make it back.

When I experience suspected dropped packets, first thing I do is extend the PING wait time with the -w suffix. Another way to PING the IP address rather than the PLA. Example: rather than PING satsig.net, try PING -w2000. That will eliminated the DNS conversion lag and wait 2 seconds for the PING to return to be counted. If the number of dropped packets decreases, then I run TRACERT to see if I can isolate the bottleneck.

If you are interested in the other PING (and TRACERT) variants, type PING /? (or TRACERT /?) at a command prompt.

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Reply #4 - Oct 19th, 2011 at 10:44am  

YOU Change the frequncy but still the prob is presist.

your link is getting high BER from unknown source which is operating periodically please check around area if it is cause for any interferance.

2 it is also possible to get this problem from bad UPS and power sources whcih casuse harmonic noise for the MODEM SIG especially if you are using comtech.

3 this is some time come from modem internally as check please use two modem one for tx and one for rx then see the ping .

Thanks Amiree VSAT Engineer Afghanistan
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Eric Johnston
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Reply #5 - Oct 19th, 2011 at 4:32pm  
Try testing each part of the link to localise the problem.

Local router to local modem.
Local modem to distant modem (over satellite).
Distant modem to distant router.

The CDM570-IP modem contains its own ping generator, so try testing with that.

I said earlier "If there are no CRC errors the satellite link must be error free."  I am now not quite sure about this. If the system is idle and no traffic then there are no packets being sent or received, so no CRC count. There could be errors occurring which are not detected because no traffic is flowing.

Do you have any congestion - too much traffic at times ?

Added thoughts....

Are the clocks slipping?.  This might cause some regular brief outage.   Check how the clocks are configured at each end. Do you have exactly the same bit rate set at each end ?

If the modem is set to revert to home state it may loose connection very briefly, every so often.

The FEC and scrambling etc must match exactly between both ends.

On your routers, set the ethernet port speeds fixed, rather than leaving them to autonegioate from time to time, which causes brief outages.

Familiarise yourself with the CDM570 "IP Statistics Page", "Ethernet Statistics page" and the "WAN statistics".  If you have some problem that only occurs at time during the day, save the data and reset the counters to zero, every hour, for example.

Do you have any device on the network with wrong IP address or subnet mask?.  

Maybe ping packets are set to low priority in QoS, so they are delayed or dropped in favour of normal customer traffic during high traffic periods.

Check you don't have default route ( Point-to-Point ToSat) pointed at satellite at both ends. If so, a packet with invalid address will go round and round till the TTL expires, causing a brief traffic spike.

Best regards, Eric.
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« Last Edit: Oct 19th, 2011 at 5:52pm by Eric Johnston »  
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