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Hard and Fast


Doing your head in - numbers

By Antonios Karantze, in , posted: 26-Jul-2009 16:31

Over the last few years, I've had an intense crash course in fixed-line telco and IT technologies, as well as the new world services of VOIP, against a background of projects driving change to enable all of this stuff and transition from one service to the other.

One thing that has done - and still does - my head in, is the discrepancy between speeds and the different technology types. It's easy enough to say 'it don't matter', except when it does, and I find it matters an awful lot, quite often.

What do I mean?

I mean those awful speeds. 24mbps Internet! 7.2mbps HSDPA! 1gbps Fibre, fwoar!

And yet, there is real nuance that affects what you get as a user.

For Internet, since that's the topic of NZ time and again...

The speed that matters I believe, is the IP speed you get for your applications. I use TelstraClear's cable service at home, and I can regularly download a file from a high-capacity server (eg microsoft.com) at 1.02MB/s - that megabytes per second, which measure the size of a file. To get that speed, I have the 10mbps - megabits per seconds, which measure the absolute speed of transfer at that second. As a rough rule of thumb, 10240mb /10 = 1024MB. Take 10 as being how many bits are required to represent a piece of information, with some overhead.

I know that I will be able to get those speeds, assuming all technology is lined up and functioning correctly (I am not getting into a debate over configuration and what TelstraClear does or doesn't do).

But now it get's interesting.

ADSL/ADSL2+ technologies use an older method of transfer called ATM (Asynchronous Transfer Mode)... this doesn't readily map with 100% efficiency into the IP world, and you get something called the ATM-Tax affecting speed. If your ADSL2+ connection can connect at 24mbps/1mbps to the local DSLAM... then the 'tax' will consume between 10-15% of that bandwidth in overhead and syncronisation... or about 2.5mbps... and on it goes. Except of course most people don't get those speeds, and they get something closer to 10mbps, which reflects that it's not a fair comparison between HFC and DSL. With DSL, especially on the Telecom network, sometimes you're up and many time you're not.

802.11a/b/g/n/q/s/x and every other letter also has wonderful overheads. At Radio layer the access point may be able to talk to the wireless network card at speeds from 11mbps to 300mbps... but once you take error correction, delay, latency, and generally poorly implemented software... you're lucky to get IP throughput closer to 5mbps on 802.11b, 20mbps on .11g, and who knows what on the .11n spec. Translation: what you PC gets is not what's on the box. But if you're trying to stream a High-definition video from a media PC to your TV on a wireless connection, you need about 9mbps bandwidth for the IP layer - if you're only getting twice that on average, wow!

Now for something else: Ethernet, which is a way of networking computers together, but also has overhead and loss. A 100mb Ethernet port will not give you 100mb IP - far from it. There will be loss to inefficiency, error correction's, retries and so on. If you purchase a service that is 100mb IP... then the Ethernet access had better be larger than 100mb so you can get the full bandwidth.

Head hurting yet????

Does 1GB fibre connection mean I'm getting 1GB IP bandwidth to my computer? of course not... and most PC's would struggle to process such a torrent of information. 1gb more likely will give about 500mbps, which translates to about 50MB... still huge, but quite different to what's on the box!

I won't go into HSDPA or that lot - that's a whole new level of panadol time...

Now, I'm not a techie, and i'm sure many services could be made to run more efficiently. But I think if you buy a service with a speed promise, then it should be at IP level - which is what your applications will use.

Off to get the panadol....

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Comment by Foo, on 28-Jul-2009 01:51

Nice wee article, all too often the line speed gets confused with the per second speed.

You mention 10 Mbps being equal to 1.02 MB/s by dividing it by 10, but you actually divide by 8 to give 1.25 MB/s. Of course, as mentioned overhead does gobble up some of that.


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antoniosk's profile

Antonios Karantze
Wellington
New Zealand


I'm a born and bred Wellingtonian, and have chosen IT and Telecommunications as my industry, as a Commercial Manager.

Credits include but are not limited to:
- The O2 Xda smartphone range, and O2's range of 3G Mobile Internet services
- Numerous TelstraClear Mobile and IP voice products

In my journey through the industry I have worked at
- Bellsouth NZ
- Telecom NZ
- ICO Global
- T-Mobile International (formerly One2One Communications)
- O2 Plc
- TelstraClear
- Vodafone NZ

I'm a fan of technology, and what it can do for people and business... and I enjoy bringing new things to market and seeing them grow. Enjoy the blog, take the time to think about what I write - it's not technology heavy, and is my reflection on life and the people around me.