SLA for voice and video over IP/MPLS

William Herrin bill at herrin.us
Mon Feb 28 21:44:19 CST 2011


On Mon, Feb 28, 2011 at 9:08 PM, Anton Kapela <tkapela at gmail.com> wrote:
> On Mon, Feb 28, 2011 at 5:23 PM, William Herrin <bill at herrin.us> wrote:
>
>> Who uses BER to measure packet switched networks?
>
> I do, some 'packet' test gear can, bitstream oriented software often will, etc.

Hi Anton,

So... Not really, no.

You get a bit error on an Ethernet in the middle, the next router
flunks the Ethernet CRC and you never see the packet.

You get congestion in the middle, the router drops the packet and you
never see it.

You get a bit error on a 802.11 link in the middle, it retransmits and
you get a clean packet with a little jitter and maybe out of order.


Point is, you don't get a measurement that looks like Bit Error Rate
because you don't have access to layer 1 and you see a very incomplete
layer 2. Evaluating an MPLS virtual circuit, you want metrics that
make sense for layer 3 in a packet switched network: loss at various
sizes, delay, jitter, packet order.


Don't take this the wrong way, but someone starts asking me about BER
rates in the SLA on a packet switched network and the message I hear
is that they're asking to be lied to. Like when I describe DSL quality
in terms of the birds perched pooping on the lines. His mental model
for datacom is stuck in the '80s and I'll have to accommodate that if
I want to do business. And when he calls to complain that we owe him a
day's credit because of a high BER, he'll be the nice gentleman who we
humor because he pays his bills on time and the occasional service
credits are built in to our price.

Loss. Delay. Jitter. Not BER. BER is the wrong tool for even
attempting to evaluate the end to end performance of an MPLS virtual
circuit.



> For example, if we have 1 bit lost in 1000000, that'd be ~1 packet
> lost every 82 packets we receive,

If you're losing 1 packet in 82, you're fired. Seriously, that's an
order of magnitude off even for tasks less demanding than VoIP and
streaming video. Doesn't matter if you flipped 1 bit or 20, 1.2%
packet loss one way (2.4% round trip) is way excessive. That's at the
level where you start to notice sluggish web browsing because of TCP's
congestion control algorithms.

-Bill


-- 
William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004




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