jkroon

How does one define broadband?  Is it all defined around throuput?  Latency?  Between which endpoints?  Using ADSL currently one can expect a local latency of around 30ms to anywhere within SA, and depending on whether you’ve got a SAIX (Fibre) or IS (both Fibre and Satellite) you can expect anything between 300ms and 800ms to international destinations (obviously depending on the exact geographical location and the type/distance of the peerings being used).

What kinds of speeds is “fast enough”?  What is the trade-off between throughput and latency?

Personally, I could do with >4Mbps links, but this isn’t essential for me.  Heck, I still use a 384 DSL line at home.  However, I obviously won’t be running a company off of it.  Nor am I a big leecher that tries to download a gazillion gigabytes every month.

Half the time, I find that people that complain about “slow” internet isn’t even saturating their link at all.  So why do they consider their links to be slow?  Simple, latency.  Let’s say you’re browsing to google.co.za, (hope page, excluding images is 2909 bytes, that’s _two_ TCP segments).  At 4Mbps that’s a fraction of a second, then why does it feel like google.co.za takes a second to load?  Because it actually does:

1. SYN packet gets sent.
2. ~300ms later the SYN/ACK arrives.
3. ACK gets sent + request gets sent off.
4. ~300ms later the response starts coming in.

So yes, that’s around 600ms (on a low-latency round trip) before we even START receiving the data from google back.  Please note that local caches and other factors obviously influences this.

Then, there is probably the single factor that I consider to be the most crucial piece of the puzzle at the moment:  packet loss!  I’ve recently started seeing round-trip packet loss of around 5 %!  Sounds low?  Well, it’s actually less than 5 % as the 5 % is round-trip, so it’s probably like 2.6 % per direction.  Either way, for VoIP we are trying to send 50 packets per direction per second, at 2 % that means we’re losing a fragment per second (0.02ms worth of audio).

As previously stated we can get away with relatively little bandwidth for VoIP, but there are a few things we do want:

1. Lowish (<100ms) latency.
2. Low jitter (The lower ping’s mdev value, the better, <5ms works well).
3. NO PACKET LOSS.  We do NOT want to lose packets at all.

It is mostly for the latter two reasons that we HIGHLY recommend dedicated internet links for VoIP purposes in SA (No QoS on the network).  However, it seems recently the major providers started competing to see who can provide the highest latency (iBurst is mostly winning here at around 150ms+ latency for local traffic) and packet loss (it’s hard to say who’s leading here … I saw iBurst at 40% the other weekend, I’m getting more DSL lines at 5%+), and let’s not even go into jitter …

No, internet in South Africa at this point in time is a mixture of disappointment and utter frustration.  I’ve been with my hands in my hair the last while, and I’ve no idea where to turn next.  Diginet?  If only it was as good as they claim, and if only it was affordable.

This entry was posted on Tuesday, March 10th, 2009 at 9:10 pm and is filed under Networking, VoIP. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.