This column originally appeared in the August 2019 issue of Professional Sound magazine.
By Anthony P. Kuzub
With IP audio, copper and glass are not carrying baseband audio or AES3; packets are moving bits, not a voltage representing sound. Audio is converted to a PCM serialized data stream then encapsulated into synchronized packets. Protocols are rules about packet structures and “conversation” that you are moving around the network. Protocols build on and depend on other protocols. In other words: IEEE 802 + IEEE 1588 + IETF RFC (many) + SMPTE Standard + AES Standard + idea = New Protocol. AES67 audio transport is augmented with AES70 control information.
In the past, an oscilloscope was used to analyze, troubleshoot, measure quality, confirm signal integrity, and identify anomalies. On a network, you
need a “protocol analyzer,” and an open-source initiative called Wireshark is now the industry-standard tool. All professional sound engineers should familiarize themselves with it. Anyone can try this on any network and you can watch protocols talk to one another.
Network problems can be difficult to trace down. It’s in the ether. You would need 12 scopes to probe a 24-channel snake. In IP, you just need a simple laptop ethernet port and Wireshark. (What follows is not a tutorial and is a huge oversimplification of Wireshark’s power. This information is intended to help audio engineers identify AES67 RTP packets.)
Wireshark has three main parts: the filter toolbar, the packet lists, and the packet detail.
Filtering the information gives you the ability to identify select packets of interest. On a scope, you would have the signal gen DUT Probe Filter -> Scope input. Wireshark needs to present you with the signal you care to analyze. You can filter by IP address or by protocol or by a number of other permutations. In the case of an AES67 ST2110-30, you would search for the RTP packets in the Administratively Scoped address space of 239.x.y.z
AES67 is augmented with AES70, Ravenna, SIP, SAP, and NMOS. You can search specific filters for networking protocols used related to those suites. Filter for the connection protocols: sip or rtsp or sap or mdns or dns to see device announcements.
Analyzing the measurement quality is done by looking at the inside of the packets once filtered. On a scope, there is time information on the X and voltage on the Y. Wireshark timestamps each packet and preserves its contents. There are certain formulas (size and structure) that a packet will conform to. There are also certain timings and packet spacings that packets will conform to. In order to know their function, regardless of what type of protocol it is, Wireshark has various toolsets and protocol-specific analysis tools to decode/visualize and probe.
Troubleshooting is to find out why or why not, how come, and always know what the front port is going on! A multicast can only be captured if the switch hardware allows it (read IGMP). Wireshark allows you to see the network traffic of the selected ethernet port, device requests, or responses in order to identify these network-level packet problems.
Recording network activity to analyze “packet captures” and quality of
network is key. The icon of a blue fin is a network/USB record button. Clicking it captures and records this network traffic. The traffic is formed in packets. These packets are routed (out of this article’s scope) with Addresses (IP/COM BUSS ID).
Don’t forget to File -> Save As in order to store for future analysis. Identifying anomalies can be surprising in networking, so let me know what you find.
Run Wireshark. It’s the most amazing network oscilloscope and it’s free: www.wireshark.org.
Anthony P. Kuzub is the senior systems designer at CBC - Radio-Canada and also the Vice Chair for the Toronto Section of the Audio Engineering Society. www.Like.audio.