[Dave Klee]

Hey Marc, I’m not really a networking guy, but here’s what I know about Ethernet flow control:

In a typical network, there are both data senders and data receivers — and, in a TCP network using full-duplex mode with flow control enabled, they listen to each other (kinda). The sender just keeps sending data as fast as it can — until it hears a “PAUSE” command. A PAUSE gets issued by a receiver that can’t keep up. Receivers can’t keep up for a variety of reasons from time to time; it could be that the receiver’s processor is overwhelmed or the receiver’s network connection had too much stuff coming in at one time and/or a buffer got full. Regardless, the receiver issues a PAUSE — telling the sender to wait for just a fraction of a second before sending additional data. That’s flow control. It all centers around those PAUSE commands.

And, while it’s built-in to the TCP protocol, it’s not always turned on or supported by everything on your network. The sender’s network adapter (Ethernet connection) needs to know how to listen for these PAUSE commands and briefly stop sending data. Receivers need to know how to issue these PAUSE commands, and the switch has to know how important these PAUSE commands are to pass along to the data senders.

Flow control does add just a touch of latency. Data senders and switches are, in a way, constantly listening for a PAUSE command rather than just blindly sending data as fast as they can. Before sending a file, there is, in a way, a brief check performed to make sure the receiver is ready to accept the file. And, if a PAUSE command is issued by a receiver, that’s where things can slow down — because the sender actually stops sending to that receiver for a very brief amount of time (we’re talking nanoseconds, literally).

I’ve heard rumors of networks mixed with gigabit (1Gbps) and fast-ethernet (100Mbps) devices getting bogged down by flow control because the slow receivers are constantly telling the data senders (often these are servers) to PAUSE. Similarly, let’s say ten computers with gigabit connections all start sending a big file to a computer with a fast-ethernet connection at the same time. The slow receiver is repeatedly going to tell all those gigabit computers to PAUSE and wait for it to be ready to receive more information.

Anyway, for video-heavy Ethernet networks, flow control is generally pretty helpful. While it might add just a tiny, tiny, tiny fraction of latency, it makes sure that your receivers (clients, desktops and edit suites) aren’t overwhelmed by a sender (server) that’s trying to send stuff too fast. If the receiver wasn’t ready for the data, you might get dropped frames during video playback or worse — bad file copies or corrupted data. The reverse situation is also helpful when your clients or desktops are sending new data to the server (when capturing new video, for example). You want to make sure the data gets written correctly and the server isn’t already busy receiving data from lots of other computers.

However, there are times when flow control is not helpful on an Ethernet network — the most specific one I know of is an Xsan metadata network. In Xsan, a client (edit suite) requests a file over a separate, private Ethernet network that goes directly to the server, and the server returns the requested file to the client over Fibre-Channel. This private, metadata network only handles very small files — only these simple requests. Here, there’s almost no chance any device is overwhelmed by these tiny request files (even if a ton of them come in at the same time), and minimal latency is really your utmost concern. If there is ANY lag in the server receiving your file request, the file won’t be sent over Fibre-Channel by the time you need it. This all happens in tiny fractions of a second — and flow control, as helpful as it is, can get in the way by slowing these requests down just a tiny, tiny bit.

Aside from these specific cases, though, flow control makes your network smarter. It makes sure senders don’t push out data faster than it can be received and minimizes the chance that things get lost in the transfer. The price, in terms of latency, is so small that most people in most situations would never notice.

I hope at least some of that is new / accurate, and I hope everything’s working well on your end!

[Dave Klee]

Hey Baz, I was hoping someone would respond using one of the new Blackmagic boxes!

I have a couple older cards (Decklink HD Pro), and when using Final Cut Pro, they give me a spinning color wheel (of death) on a fairly regular basis: before capturing a clip, after capturing a clip, when changing clip capturing settings, when opening log and capture, when closing log and capture, when changing open sequences — and sometimes for no reason I can tell at all. The spinning wheel lasts from 1-10 seconds, and it almost never causes the computer to crash. It just spins and makes you wait. Then it’s done and works fine until the next time.

Does this happen when using your newer Multibridge? Do you ever see the spinning color wheel? I’ve been hesitant to go back to Blackmagic after these initial experiences.

Thanks for the help!

[Dave Klee]

Hey Bob, thanks much! That’s a really good summary, and the analog in is something I would like to have the option for without a converter (if possible). And, we’re currently a DVCPROHD house, so I appreciate your feedback on the Io HD.

I’m using a MOTU V4HD right now in one suite, and it does do up/down/cross conversion really well — I think that’s one of its strong points. However, it is a somewhat complicated interface, and instead of using Final Cut Pro to make all your video settings for input and output, you pretty much have to set everything twice — once with Final Cut Pro settings, then go to the MOTU software utility to tell it exactly what you want to do.

Also, the way the MOTU handles audio annoys me. It really only works well with all analog or all digital audio. In fact, it can only output one of those at a time — not both. For input, both can be active, but channel assignments are very strange.

I thought the MOTU V4HD would be the perfect box, but these quirks have me a little weary. I guess it’s too much to ask something to be powerful and easy at the same time.