The ASTi system is fully capable of interoperating with Multicast Routers using the Internet Group Management Protocol (IGMP). This scheme allows you to route specific voice packets onto specific segments, thereby segregating unwanted traffic from narrow segments.

The current version of Model Builder has the ability to independently assign DIS radios or intercoms to three IP modes: Broadcast, IGMP Multicast and Non-IGMP Multicast. IP mode assignment is based on DIS exercise number. This is done by assigning ranges of DIS Exercise Number values to each IP mode. Here's an example:

• DIS Exercise ID: 1-100 = Broadcast IP (transmission to all hosts)
• DIS Exercise ID: 101-200 = IGMP Multicast (transmission to specific hosts through IGMP routers)
• DIS Exercise ID: 201-255 = Non-IGMP Multicast (transmission to specific hosts on the LAN - not through IGMP routers.)

IGMP Multicast requires that you connect a IGMP-capable router (most, if not all, off-the-shelf Ciscos fall into this category) to the LAN at each site. Typically, a number of ASTi DACSs are connected to the LAN, "behind" each router. In other words, only one IGMP router is required for each LAN. The routers connect the LANs together.

Then (for this example), Model Builder automatically assigns multicast addresses for each DIS exercise assigned to a radios or intercoms. Every time an exercise ID in the range (101-200 for the example above) is created in the model, the DACS sends an IGMP subscription request to the router. Then the router adds the multicast address to its subscription list. The router will only pass voice traffic assigned to group addresses in its subscription list, and block traffic with addresses that are not in the list.

The IGMP-capable Multicast routing software included on every Telestra system will achieve this type of bandwidth conservation when used with properly-configured DACS systems. It will not, however, act as a true "network router" in that it cannot route all traffic between two LANs.