No TCP tricks to thwart VoIP – Telecom

Wholesale DSL isn't meant to support VoIP anyway, telco says

Telecom denies it has plans to alter TCP packet interleaving to in order to discourage Voice over IP calls by its broadband customers, as reported by US Public Broadcasting Service commentator Robert X Cringely in his online Pulpit column last month.

According to Cringely, the alteration of TCP packet interleaving would entail "bunching all packets in the first half of each second", causing "half a second of dead air" to be added to each VoIP conversation. This increase in latency would only hurt VoIP as it requires real-time support, whereas streaming video and audio applications use buffering of incoming data to overcome gaps in the transmission.

Telecom's spokesman for internet affairs, Nick Brown, says the telco has no plans to alter the TCP interleaving in order to put an end to customers using VoIP. He says interleaving is a normal parameter of the DSL standard, adding that Telecom has used it on all DSL services from the outset. Interleaving is necessary, according to Brown, to minimise noise and interference from affecting internet grade services provided over the copper network, but he refutes the idea that it is a specific approach to destroy voice quality.

Brown says that when any service on a network has to compete with others — Telecom's DSL network is shared among customers, with an unknown contention ratio — only two outcomes are possible: delay variation or packet loss.

Delay varation is friendler to TCP/IP traffic, Brown says, and shaping is used in an attempt to avoid packet loss.

Telecom uses terms such as "internet grade" and "best effort" for both its wholesale and retail DSL plans. In the case of the wholesale Unbundled Bitstream Service (UBS), this means unspecified packet loss and delays of up to one second in each direction, according to Telecom's specification documents.

The Commerce Commission specified that its hitherto un-implemented UBS should not support real-time services — which could be interactive applications such as VoIP, online games and VPNs. Telecom has decided to apply this limitation to its commercial proxy UBS as well, along with the slow 128kbit/s upload speed.

However, the Commission's recommendation to the Government says the regulated — but not yet finalised or implemented — UBS should have a minimum throughput of 32kbit/s.

Telecom's commercial proxy service only offers 10kbit/s committed download speed and 5kbit/s uploads during congestion, according to Telecom's UBS service specification.

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