Page 13 of 14
MULTICAST PACKET SUPPORT
LAN / WAN
Ethernet Switch
Enbedded
IGMP
Network
Host
IP Camera
Cat5
NV-EC1701U
NV-EC1701U
Model NV
-EC1701
www
.nv
t.com
● (+1
) 650.
462.
8100 ● (
+44) (
0) 208 9
77 6614
Eth
ernet over
C
oa
x T
ra
nsc
eiv
er
Model NV-EC1701
www.nvt.com ● (+1) 650.462.8100 ● (+44) (0) 208 977 6614
Ethernet over Coax Transceiver
Power Supply
Figure 6 - Multicast Network
Network
Video
Recorder
Firewall
Cat5
2- wire Cable
On rare occasions, a customer will experience a problem where
a camera sending unicast packets performs flawlessly, but the
NV-EC1701s 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 desti-
nation (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 imple-
mented 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, and for generating an IGMP Report. That report is moni-
tored 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) 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 inappropri-
ate for the switch to send a multicast stream everywhere, as
it would clog the network.
The NV-EC1701s 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-EC1701s 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.
