In March 2011, the IEEE 1733 standard was approved for publication. It specified how to transport Layer 3-encapsulated AV data. The awesome Layer 2 Quality of Service (QoS) provided by the IEEE 802.1 AVB standards is one of the best things to come with this standard.
Also big: the standard is reliable, well known, what people like to call robust. So, one would not be unreasonable in assuming that AVB is a Layer 3 protocol.
Not so fast.
AVB networking is actually a set of protocols. Layer 2 standards (IEEE standards 802.1AS, 802.1Q(at/av), and 1722.1) are absolutely part of that, like the cheese in ham and cheese. Sandwich isn’t a sandwich without both.
Let’s talk layers.
Layer 2:
Used to design local area networks (LANs). Advantages of Layer 2 are lower costs ( switching, no routing) and very low latency. BUT Layer 2 networks also forward all traffic, especially ARP and DHCP broadcasts. Anything transmitted by one device is forwarded to all devices. When the network gets too large, the broadcast traffic begins to create congestion and network efficiency drops. So, there is a limit to the size of a layer 2 network
Layer 3:
Allows for communication between different networks. For example, might be transition between a LAN and a wide-area network (WAN). They handle all kinds of things, including broadcast traffic such as ARP and DHCP broadcasts to the local network, reducing overall traffic levels by allowing administrators to divide networks into smaller parts and restricting broadcasts to only that sub-network. AND properly configured layer 3 network with the correct knowledge and hardware can have infinite growth.
AVB has a bit of a caveat: AVB functions brilliantly over layer 2, but we also live in a layer 3 world. And just because the IEEE has ratified a part of the AVB suite for transporting data across Layer 3 networks, that alone is not enough to assure full AVB-over-Layer-3 functionality in every situation, with every application.
What that means for us: unless my Layer 3 WAN is really carefully designed, takes bandwidth, latency QoS guarantees, and other considerations into account – which may not even be possible in some WANs – it may not be able to guarantee complete functionality in all applications. There, I said it.
All is not lost.
Here is how Jason Damori of Biamp puts it:
“You can build very large Layer 2 networks such that the layer of the underlying protocol is inconsequential. For example, let’s say you build a network in a building and you have a Layer 2 network on each floor. You might connect all the Layer 2 networks via a Layer 3 network. If you wanted to send AVB audio from floor 1 to floor 3 though, such a network would be a problem. You would need a transmission method which relied only on Layer 3 protocols to make that scenario work. However, because of the intelligence of modern AVB-enabled switches/bridges, a single Layer 2 network can be very large and encompass a multi-floor scenario.
Even campuses (corporate, educational, hospital, etc.) with multiple buildings are possible. Only a properly engineered network can perform according to its promise, regardless of which layer it’s operating on. The features provided by AVB help to reduce the amount of engineering required, and in the simplest cases nearly eliminate the human interaction required with infrastructure equipment.”
Conclusions for the video designer:
AVB + layer 2 is well trodden ground, with all the video kinks worked out. There is no doubt that a proper layer 2 network will transport your time-sensitive data, all nicely sync’d, very low-latency, reserved bandwidth.
Layer 3 is incredibly powerful, and almost limitless in it’s capacity. That being said, it may be some time before we see people designing AVB into layer 3 scenarios with anything near the ease of a layer 2 version. It’s up the IEEE and the manufacturers to make that happen sooner rather than later.
Note that Pakedge has just released a reasonably priced, true Layer 3, AVB switch. Description here; in short, dynamic routing protocols, multicast, 24 PoE+ capable ports and four independent 10 Gigabit SFP+ fiber ports.