Testing an AG's Network

Methods and guidance on testing the network of your Access Grid.


A critical aspect and performance of an Access Grid is its networking capability. For an Access Grid to function to its optimal performance, it needs multicast capability.

This guide will demonstrate how to test your multicast capability, as well as providing methods on how to check whether there are any particular locations that are unable to participate due to multicast connectivity problems.

What is Multicast?

Multicast traffic refers to packets that are sent from one source host address to a special group-based destination address. The destination represents only the hosts that are interested in receiving the packets, and no others. Two extremes are covered here—a unicast, which travels from host to host, and a broadcast, which travels from one host to everyone on a segment. Multicast falls somewhere in the middle, where the intention is to send packets from one host to only the users that want to receive them, namely those in the designated multicast group. In the case of the Access Grid, this would be the different venues which people can join.

Ideally, the recipients of multicast packets could be located anywhere, not just on the local segment, with the advantage that only people that wish to receive the video presentation would be affected by the traffic without flooding all the rest of the network with unwanted traffic. In the case of video conferencing, the amount of traffic is very high, which could create a lot of bottle necks and delays in the network if multicast didn’t exist, and it was necessary to send broadcast or multiple unicast streams instead.

Using tools to conduct multicast tests

Using a multicast beacon


The computer program “beacon” communicates with a beaconserver which allows it to test your multicast connectivity.


A guide for “Installing Beacon v1.3” on Linux can be found at

Multicast Issues

  • No Multicast

    Generally speaking, if your Access Grid node is unable to send or receive multicast traffic to the Beacon server it will be classified as a Blind Beacon. Therefore it can be assumed that you have no multicast connectivity.

    As you can see from the examples listed below, the beacon server is reporting that two “Beacons” are classified as Blind Beacons and therefore those two beacons currently have no multicast connectivity.

  • Unidirectional Multicast

    In some cases, an Access Grid Node might have Unidirectional Multicast problems. This means that the AccessGrid node that the ability to send or receive multicast traffic, but not both (ie you might be able to see and hear other Access Grid nodes, but they are unable to hear or see you through the use of the media tools whilst using Multicast).

    “Unidirectional Multicast” problems can be easily diagnosed using the multicast beacon. As can be seen from the image below, the beacon “jastestmachine” is able to receive multicast traffic from some of the other beacons, but is unable to transmit multicast to any of the other beacons.


    As can be seen from the image above, the “jastestmachine” is assigned a beacon number of 4. Therefore, if you look horizontally along R4 (Receive for Beacon number 4), you can see that there are few “green” boxes, which indicates that beacon 4 is able to receive multicast traffic from a number of senders. Whereas, if we look vertically along S4 (Sending for Beacon number 4), we can see that other than it’s self, it is unable to send multicast traffic to any of the other beacons.

  • Partial Accessibility Multicast

    “Partial Accessibility Multicast” is another common problem that you can use the Beacon Server to try and diagnose. In teams of the Access Grid, this might be demonstrated by ability to connect to some Access Grids, but not others. As can be seen from the image below, beacon *“”* has the ability to send and received multicast traffic to some of the beacons, but not all of them.


Using the AudioReflector to test multicast


AudioReflector is a simple program which listens on a multicast address and port and reflects all valid RTP packets back with a certain delay. As described in Testing an AG's Sound, the AudioReflector can be used to help test an AccessGrids audio. The interesting part of the AudioReflector is that you can use the AudioReflector as a quick test to determine if you have multicast connectivity to the AudioReflector Service (Which is located at ANU).

Using the Audio Reflector as a simple test

  1. Start the VenueClient

  2. Go to the Audio Reflector Venue. This is achieved by:
    • Going to the APAG Lobby (
    • Going to the Services Lobby
    • Going to the Audio Reflector Venue

  3. Once in the Audio Reflector Venue, RAT should show (after a small delay) three audio sources. Your own name or node name should appear twice, and there should be an AudioReflector source as well as in screen shot of rat below.


  4. Select the Options button

  5. From the Category pull-down list select Interface

  6. Click on the Reception quality matrix item, this should pop up the window shown in the image below


  7. If the window shows all 0% and green as in the sample image, then your multicast connectivity to the AudioReflector is working well.

Using unicast to connect to Access Grid Sessions

Another way to confirm that you have multicast issues, is to try connecting to an Access Grid session using unicast. Provided that you have no firewall issues with using unicast, it should hopefully work. If you are able to connect to an Access Grid session, then either you or the other AG’s are having some form of multicast issues which can be diagnosed using the tools recommended above.

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