Queueing in data networks

Modern data networks route or switch relatively small ‘packets’ of data across shared links that along with the switching nodes, form the wider data network.

One of the roles of the switches is to receive packets on one link, and send them onwards on the most appropriate link. Since links may be at different speeds, and many links may source packets to be sent on any link, there exists a mechanism in the switch to store packets pending transmission, in the simplest case it is a first come first served link queue.

The function of the queue then is to hold packets until they can be sent on the link, and to offer them in first come first served order. That raises two important questions:

  • how long will packets be delayed;
  • how many slots does the queue need.

Queuing theory gives us a method of estimating these quantities.

Lets make some assumptions about the traffic:

  • service requests arrive randomly in time; and
  • service time is exponentially distributed with an average time of 1.

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Above is a plot of normalised average response time (service time + queue wait) vs resource (link) utilisation (pu means per unit). It can be seen that when the link utilisation is 0.5pu (50% busy), that response time is 2pu (ie twice the average service time), twice that needed to send an average packet at very low utilisation. Response time rapidly degrades:

  • at 70% link busy, response time is 3.3 times packet transmission time; and
  • at 90% link busy, response time is 10 times packet transmission time.

To ensure packets are not discarded, the queue need sufficient slots to hold packets even in most peak bursts. Whilst at 70% link utilisation, the average queue size is 2.3 slots (3.3-1), a larger queue size accommodates bursts better. Discarded packets can severely affect performance, not only are they likely to be resent after some delay and network overhead, they can break a higher level protocol unit in simple systems and waste the link capacity and other links used to send the rest of that protocol unit. Continue reading Queueing in data networks

iinet – pre-connect experience

The NBN is coming….

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Our NBN street cabinet, or Node in FTTN. The pic is not crooked, the cabinet is out of level.

We received advice that NBN access is now available in our street… about 10 months after the street cabinet was installed.

This is the best hope we have had of escaping Telstra’s appalling service that we have endured for 6 years.

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Above is a plot of the median download speed of the Telstra 8Mb/s ADSL service.

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Above, the distribution for last week show that although HTTP speeds would appear to be capped at 2Mb/s on an 8Mb/s access pipe, the inevitable slide to poorer performance is shown where around 10% of the measurements are below 0.25Mb/s. Continue reading iinet – pre-connect experience

Another chapter in the NBN debacle – Jul 2016

I mentioned that NBN is coming….

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This NBN cabinet has been in place for about eight months, no electronics in it… but an important showpiece in the government’s desire to show that Malcolm Turnbull’s copper based NBN was on track. Empty cabinets probably persuaded some naive voters at the federal election earlier this month.

The deception worked, Malcolm was returned, albeit with barely the majority to allow majority coalition government.

NBN is a wholly government owned ‘enterprise’ operating on a ‘commercial’ basis. Who else could afford to spend capital on infrastructure that delivers no service.
Continue reading Another chapter in the NBN debacle – Jul 2016

NBN is coming… it seems

The NBN was the Australian government’s response to a dominant carrier that would not engage in government’s attempts to guide industry to develop an equitable access network beyond that delivered off the telephony copper that existed in the street (the 2007 RFP).

That ‘initiative’ sees us return the good old bad old days when government owned most of the street infrastructure used to deliver telecommunications, and of course it has been in their interest as a provider to weaken consumer controls that otherwise gave some measure of protection of service levels.

The above graph shows the performance of my ADSL Internet access which reliably achieved  greater than 7Mb/s when I moved here in 2009 but fell to around 0.5Mb/s. A recent upgrade has seen around 2Mb/s fairly reliably, though not enough for reliable IPTV, a stunning improvement on the previous 3+ years. Continue reading NBN is coming… it seems

An example of Eb/N0 design with the Field strength / receive power converter

I have been asked whether the Field strength / receive power converter can be used to solve a Eb/N0 (Eb/N0) design problem.

Eb/N0 is a method often used for specifying the relationship of signal and noise that will give adequate bit error rate in a data demodulator.

Whilst the calculator was not specifically designed for that purpose, and you cannot directly enter the desired Eb/N0, with the help of a hand calculator for simple calculations, a solution can be found. Continue reading An example of Eb/N0 design with the Field strength / receive power converter

When governments tinker in telecommunications carriage…

I had cause to document Internet access performance recently, an interesting exercise.

Background

When I moved into this house about five years ago, we enjoyed an excellent broadband service delivered on an ADSL1 link of about 800m to the RIM/DSLAM. The downstream rate was capped at 8Mb/s and most of the time, most of that speed was available to the end user.

For years I have run a test file transfer every half hour to document access performance, and when asked about recent performance that was an ideal source. I needed to go back to the previous week as last week started with an outage, the RIM batteries seem to have gone flat and Telstra had not been proactive in responding to the condition that left it running on batteries. Continue reading When governments tinker in telecommunications carriage…

Australia’s NBN debacle limps on under a new government and new CEO

In an attempt to fix up a government orchestrated flawed regulatory and market environment that saw the dominant carrier digging its heels in to regain the right to install infrastructure that it was not required to share with competitors (ie, to allow it to reestablish a monopoly), the Labour government called for tenders from organisations to provide a National Broadband Network on its terms.

No contracts were awarded, government decided the way forward was for it to own the access network, at least for the interim, and so return to the situation that existed prior to the partial privatisation of Telstra in 1997.

Continue reading Australia’s NBN debacle limps on under a new government and new CEO