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2017/11
MULTICAST PACKET SUPPORT
Figure 6 - Multicast Network
On rare occasions, a customer will experience a problem
where a camera sending unicast packets performs
flawlessly, but the NV-EC1701Us fail to deliver multicast
streams.
To understand this, we need to delve a little deeper into
how multicast networking works. To do this, we will first
explore unicast packet transmission.
With unicast packets, there is a source (the camera)
and a destination (the NVR). Packets are generated
by the source and directed to a single destination. The
first time a packet is sent, the switch has never heard
of the destination address. So it sends a broadcast
transmission to ALL destinations. The correct recipient
acknowledges the packet. The switch snoops in on
that response and memorizes which port is associated
with that transmission. Subsequent transmissions are
directed ONLY to the recipient’s port, thereby reducing
bandwidth on other parts of the network.
With multicast packets, the same bandwidth
considerations apply. Switches and routers do not
routinely deliver multicast traffic to all destinations.
Instead they rely on a special control protocol to identify
and report which multicast traffic should go where.
That protocol is called Internet Group Management
Protocol (IGMP). IGMP is a shared “querier” control
resource that is implemented on a host, such as a switch
or router, within the network. Virtually all routers and
most switches are equipped with an IGMP querier.
That host is responsible for sending IGMP queries to
multicast devices, requesting the generation of an IGMP
Report. That report is monitored by switches and routers
within the network. That monitoring is called IGMP
Snooping. These switches and routers keep a state
table and use it to determine to which ports to deliver
each multicast stream.
In practice, a multicast source (camera) sends its data not
to the destination, but to a fictitious “Group IP Address.”
Destination devices (the NVR), in response to an IGMP
querier request, send a request to the IGMP host asking
to be included as a destination for that group.
Switches and routers allow these multicast packets
through so they can be received by those interested
devices (NVR) and block them from being delivered
elsewhere.
It would be inappropriate for the switch
to send a multicast stream everywhere, as it would
clog the network.
The NV-EC1701Us are not point-to-point devices. They
join together to function as a switch. Like a switch, they
listen for IGMP reports and block unknown multicast
packets. This is particularly important in a multi-camera
environment, as we do not want the stream from one
camera being delivered to all other cameras. That could
generate too much traffic.
On rare occasions an installation will be created without
the required IGMP querier. If the switches are not
equipped with IGMP snooping then they just deliver all
multicast traffic everywhere. If the NV-EC1701Us do not
receive IGMP reports, then they will BLOCK UNKNOWN
MULTICAST PACKETS, and the video will not pass
through.
An investigation can be performed as necessary using a
Packet Sniffer, such as WireShark.
The bottom line is that an IGMP querier is required for a
properly configured multicast network. Contact NVT for
further information.
IP Camera
Ethernet over Coax EoC Transceiver
Cat5
Ethernet over Coax EoC Transceiver
55VDC Power Supply
Cat5
Ethernet
Switch
NV-EC1701U
NV-EC1701U
Network
Video
Recorder
LAN/WAN
Firewall
2-wire Cable
Embedded IGMP
Network Querier
Ethernet Switch
Complete Installation Guide
Model NV-EC1701U
EoC Ethernet over Coax Transceiver
with PoE, PoE+, or High Power PoE
