GLOBALCOM ACS to ACS IP Multicast Layer 3 RTP Network Requirements

GLOBALCOM ACS to ACS IP Multicast Layer 3 Network Requirements 

For ACS → ACS audio transport in a GLOBALCOM system between VLANs, IED employs IP Multicasting. This enables one ACS to source audio that may be received by N receivers. To support this capability, there are prerequisite network equipment and configuration settings which must be in place. 


Switches must support IGMP (Internet Group Management Protocol). The current standard is IGMPv3, but versions 1 and 2 are compatible. There are other settings related to IGMP that may be configured. IED has found that on some installations, enabling IGMP Snooping has a detrimental effect on the local CobraNet audio. So even though it may help IP Multicasting, it may hurt local isochronous Ethernet audio traffic, due to some impacts internal to the switches. 

Multicast Router 

To route multicast traffic between subnets and maintain multicast group membership lists, a multicast-capable router is required. If more than one router is necessary, the routers must communicate multicast information to each other. This is typically done using Protocol Independent Multicast (PIM). There are several types of PIM. One common type is Sparse Mode, typically referred to as PIM-SM. There is also a Dense Mode version of the protocol. Either mode may be employed with a GLOBALCOM system, as recommended by the site’s network engineer or switch manufacturer for the type of installation. 

Multicast Groups 

Multicast groups are generated using Class D network addresses ( – in combination with a port number. In a VLAN situation, 224.0.0.X addresses cannot be used. In the GLOBALCOM configuration software (System Management Console or SMC), there are defaults assigned. These may be manually over-ridden as dictated by the site’s network engineer. By default, the ACS multicast group is 239.192.0.x where x is the system number. So System 1 uses, System 2 transmits on, etc. The default port numbers used IP multicasts are 5001 – 5008 for system 1, 6001 – 6008 for system 2, 7001 – 7008 for system 3, etc. Again, these can be manually over-ridden if different ranges of ports are opened up between VLANs. 

Testing the Network Settings

IED has written a simple multicast network tester application (see attached ZIP file).  To use this tool, you copy it to two or more computers on the network, either the GLOBALCOM/GCK controller or laptop/client computers on the same network segment as the controllers.  To use this tool, one enters the Multicast Address into  the Address box of the application (e.g., the default one assigned in SMC when you add RTP transmitters, such as and the Port from one of these transmitters in the Port box (e.g., 5002).  Then hit the “Register” button.  Do the same at the “other end” (the remote system/controller, such as, the client computer in another building).  Then type in some text in the Message edit box, followed by Send button.  The message should appear in both the sender’s Received Messages list box and the other end’s Received Messages box.  One can expand this experiment to add a second or third remote end on the same multicast address and port and see messages arriving in all locations when entered from any location.

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