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Cable internet won't need dose of fibre to stop feeling bloated

Standards group CableLabs tackling TCP 'bufferbloat' in 3.1 standard

CableLabs, the industry group that develops data transmission standards for cable television networks, is experimenting with Active Queue Management (AQM) to deal with the issue of bufferbloat in cable networks.

Bufferbloat is one of the underground problems of Internet performance: the way that buffers in network kit add to latency, because TCP will helpfully fill up buffers, however much memory equipment providers put in their equipment.

That's because TCP doesn't reduce its flow-rate until packets start getting dropped. While packets are getting through – even with long latency – TCP assumes that the network is fine, and it can just keep transmitting.

In fact, more buffer can make the problem worse. As CableLabs puts it here, “if network equipment implements larger and larger buffers to avoid dropping packets, and TCP by its very design won’t stop increasing its data rate until it sees packet loss, the result is that the network has large buffers that are being kept full whenever a TCP flow is moving files, and packets have to sit and wait in a queue to be processed.”

It's only a problem for latency-sensitive traffic – game-clicks, or live video – and as the Bufferbloat project records, there are a variety of proposals on the table.

One of those, Active Queue Management, is what CableLabs is investigating. In particular, the organisation is looking at getting smarter kit at each end of that part of the link that a cable broadband provider controls, the user's modem and the CMTS (cable modem termination system) in the headend.

Under AQM, the modem and CMTS include software that watches over their buffers, and if TCP is keeping buffers so full that it has an impact on latency, the devices will drop “just enough packets to send TCP the signal that it needs to slow down, so that more appropriate buffer levels can be maintained”.

AQM is mandated in DOCSIS 3.1, and in cable modems following that spec, the group is mandating the adoption of a Cisco approach called Proportional Integral Enhanced Active Queue Management (described here).

The group also says it's putting forward a strong recommendation that future DOCSIS 3.0 equipment start implementing AQM.

That AQM is coming to cable internet kit will make a difference to subscribers of such services. In nations like Australia, where the National Broadband Network build is relying on cable as a way to reduce network build costs, it could make a big difference to network performance. And therefore to acceptance of cable as a carriage medium, as one of the objections raised to using DOCSIS in Australia is the poor performance of current cable internet services. ®

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