User guide

9
Problem: The video image of the ALIF
receiver shows horizontal lines across
the screen.
This issue is known as Blinding because
the resulting video image looks as though
you’re viewing it through a venetian blind.
When video is transmitted by ALIF units,
the various lines of each screen are divided
up and transmitted as separate data
packets. If the reception of those packets is
disturbed, then blinding is caused. The lines
are displayed in place of the missing video
data packets.
There are several possible causes for the
loss of data packets:
• Incorrectswitchconguration.The
problem could be caused by multicast
flooding, which causes unnecessary
networktrafc.ThisiswhatIGMP
snooping is designed to combat,
however, there can be numerous causes
of the flooding.
• Speed/memorybandwidthissueswithin
one or more switches. The speed and
capabilities of different switch models
varies greatly. If a switch cannot maintain
pace with the quantity of data being sent
through it, then it will inevitably start
dropping packets.
• OneormoreALIFunitsmaybe
outputting Jumbo frames due to the
video resolution (2048 horizontal
pixels) being used. If jumbo frames are
output by an ALIF unit, but the network
switcheshavenotbeenconguredto
use jumbo frames, the switches will
attempt to break the large packets down
into standard packets. This process
introduces a certain latency and could be
a cause for dropped packets.
• OneormoreALIFunitsmaybeusingan
oldrmwareversion.Firmwareversions
prior to v2.1 exhibited an issue with the
timing of IGMP join and leave commands
that caused multicast flooding in certain
congurations.
Remedies:
• EnsurethatIGMP snooping is enabled on
all switches within the subnet.
• WhereeachALIFunitisconnectedas
the sole device on a port connection to
a switch, enable IGMP Fast-Leave (aka
Immediate Leave) to reduce unnecessary
processing on each switch.
• Checkthevideoresolution(s)being
fed into the ALIF transmitters. If video
resolutions with 2048 horizontal pixels
are unavoidable then ensure that Jumbo
frames are enabled on all switches.
• Checktheforwardingmodeonthe
switches.IfStoreandforwardisbeing
used, try selecting Cut-through as this
mode causes less latency on lesser switch
designs.
• Ensurethatonedevicewithinthesubnet
iscorrectlyconguredasanIGMP
Querier, usually a multicast router.
• EnsurethatthermwareineveryALIF
unit is version 2.1 or greater.
• Tryadjustingthetransmittersettings
on each ALIF to make the output data
streamasefcientaspossible.See
Alter ALIF transmitter video settings if
necessary for details.
Problem: The audio output of the ALIF
receiver sounds like a scratched record.
This issue is called Audio crackle and is
a symptom of the same problem that
produces blinding (see left). The issue is
related to missing data packets.
Remedies:
As per blinding discussed above.
Problem: AIM cannot locate working
ALIF units.
There are a few possible causes:
• TheALIFunitsmustberesetbackto
theirzerocongIPaddressesforAIM
discovery. If you have a working network
of ALIF’s without AIM and then add AIM
to the network, AIM will not discover
the ALIFs until they are reset to the zero
congIPaddresses.
• ThiscouldbecausedbyLayer2Cisco
switches that have SpanningTree
Protocol(STP) enabled but do not also
have portfast enabled on the ports to
which ALIF units are connected. Without
portfast enabled, ALIF units will all be
assignedthesamezerocongIPaddress
at reboot and AIM will only acquire them
one at a time on a random basis.
You can easily tell whether portfast is
enabledonaswitchthatisrunningSTP:
When you plug the link cable from a
working ALIF unit into the switch port,
check how long it takes for the port
indicator to change from orange to
green. If it takes roughly one second,
portfast is on; if it takes roughly thirty
seconds then portfast is disabled.
Remedies:
• EnsurethattheALIFunitsandtheAIM
box are located within the same subnet.
AIM cannot cross subnet boundaries.
• ManuallyresettheALIFunitstotheir
zerocongIPaddresses.Pleasereferto
the ALIF user guide for details.
• Enableportfast on all switch ports that
have ALIF units attached to them or try
temporarilydisablingSTPontheswitches
while AIM is attempting to locate ALIF
units.
Troubleshoong
continued