To pick-up where we left off in our post about debunking AVB myths, let’s take a deeper look at the Layer 2 vs. Layer 3 discussion. There are some important considerations to keep in mind, as it’s not as simple as saying one layer is better than the other.

Most of the protocols in the AVB suite of protocols are currently only specified for Layer 2 operation, but to simply state that AVB can’t run on Layer 3 isn’t the whole story. In March 2011, the IEEE 1733 standard was approved for publication. It specifies how to transport Layer 3-encapsulated AV data while taking advantage of the robust Layer 2 Quality of Service (QoS) provided by the IEEE 802.1 AVB standards. That means AVB is now a Layer 3 protocol, right? Not quite. AVB networking requires an entire suite of protocols working together in order to deliver its promised benefits. Some of those protocols remain Layer 2 standards (IEEE standards 802.1AS, 802.1Q(at/av), and 1722.1).

Layer 3 isn’t as easy as 1-2-3
Layer 2 protocols work on local area networks (LANs), while Layer 3 protocols allow for communication between different networks. A common example of this is the transition between a LAN and a wide-area network (WAN). It takes more than a Layer 3-capable protocol to ensure that the application functions as intended across such boundaries. Assuming you’ll have this whole new world of networking open up is only true in the most generic sense. Unless the Layer 3 WAN is very specifically engineered – where someone has taken into account all the bandwidth and latency QoS guarantees that need to be in place (often not possible on many types of WANs) – then the applications using Layer 3 protocols won’t be able to provide the expected functionality.

Layer 2 networking at its best
While Layer 3 provides the means for communicating between Layer 2 networks of different types, with proper consideration 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.

Better is Best
While the IEEE has ratified a part of the AVB suite for transporting data across Layer 3 networks, that ingredient alone is not enough to bake the AVB-over-Layer-3 cake.

On one hand, in general, Layer 3 networking makes things more flexible and gives you more options in the networking world. Simply being able to transport data over Layer 3 doesn’t make a system function, however. You may have a Layer 3 network, but your system could be nonfunctional or under-functional if you’re not getting the kinds of QoS guarantees that you need to make your system work properly.

On the other hand, Layer 2 via AVB promises transportation of time-sensitive data over time-synchronized, low-latency streams. It’s crucial to have network bandwidth and latency guarantees, and that is currently less likely (and in many cases not possible) to happen at Layer 3 with the same precision and ease as on Layer 2 with AVB.

Hopefully that helps clarify some of the confusion surrounding Layer 2 and Layer 3 protocols. If not, email us.

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