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Field Report: Harris Intraplex HD Link
All about the data
The system supports two separate LANs, the idea primarily being that the LAN known as HD carries E2X (exporter to exciter) UDP traffic. (You would make use of this if you had both the importer and exporter back at the studio.) The other LAN will also carry traffic in one direction (i.e., UDP). It's important to note that the HD traffic is given priority over any traffic you try to send to the far end via the (lower priority) LAN port. In this way you can ensure that the E2X traffic makes it.
So you may wonder then how you would access the receiver via IP if the traffic only flows in one direction? Of course the answer is that you can't, unless you integrate a full-duplex IP connection into the system. The HD Link is built to do that, and it's a wonderful feature.
To test this feature on my bench, I had to build up the system as best I could, simulating the E2X traffic. The goal of the test then was to pass the UDP traffic along the HD LAN port, and to pass TCP traffic on the lower priority LAN at the same time. It's important to note however that the system can be configured to totally re-route all the LAN traffic, plus all of the audio links, to the duplex connection, should the RF link fail.
See Figure 1. To generate traffic for the E2X link, I set up a Barix pair: the Instreamer was connected to the HD LAN port on the transmit side. Conversely, the Exstreamer was connected to the HD LAN port on the receiver end. The other LAN configuration simulates a typical LAN connection in the rack room of the radio station. The LAN port of the HD Link transmitter connects to a switch-port on a Layer 2 switch. Another port on that same switch (same VLAN) connects to a gateway that gives you access to some type of duplex IP connection to the transmitter site. For my test I used a couple of Cisco routers connected via a T1. On the far end, the other router becomes the gateway for the receiver side. It connects directly to the LAN port of the HD Link receiver.
Now there is some simple configuration necessary to make this arrangement work. You need to configure proxy ARP (Address Resolution Protocol) on the transmitter side with the address of the target on the far end. In my test configuration I considered the HD LAN to have only the ability to pass traffic in on direction; so for this reason, proxy ARP needs to be used at the transmit end on the HD LAN. When the traffic generator (in my case the Barix) sends an ARP request, the HD Link transmitter actually responds with its own MAC address. That way, the Barix can actually build all the frames correctly. The transmitter then sends that data to the far end. In the path redundancy table on the transmit side, tell the system to use both the forward and return IP path, and configure the peer address (which is the address of the LAN link port on the far end receiver). On the transmit side, configure the Ethernet service to fail over from RF to IP. In the path redundancy table on the receive side, tell the system to use forward and return IP path, and configure the peer address (which is the address of the HD Link LAN port on the transmit side).
- continued on page 3
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