[VOIPSEC] OT: What would be a good Service Level Agreement - VoIP trunking - for SMEs

[Dan Wing]

Marco Mouta wrote:
> Hi guys,
> 
> I know that this could be considered a bit off the topic, but I couldn't
> find out other good Mailing list to post my question and I do believe this
> topic could be interesting for both of those who work with VoIP.
> 
> So the point is for traditional telephony we expect service availability of
> 99,999%

According to [1], the class 5 switch, itself, has 99.995% availability,
the access circuit (between your house or your business and the class 5 
switch) has 99.99% availability.  The remote class 5 switch has 99.995%
availability and the remote access link has 99.99% availability.  If you
consider a class 4 (long distance) switch between the two class 5 
switches, it has its own availability (99.98%) and its access links
have 99.995% availability.  When you're done, a single phone call
across two class 5 switches and a single class 4 switch has availability
of 99.94%.  On modern PSTN systems, class 4 switches are no longer
common, so availability is slightly improved from this model.

So -- 99.999% looks pretty on paper, and ~5 minutes per year of outage
sound pretty impressive.  But, please consider, that the PSTN's own
availability model does not meet that value, at least according to
Bellcore.

(Also part of the PSTN's availability model is that 'small outages', 
affecting only a dozen or so users (I forget the number) are not counted 
at all against system availability.  The number was chosen to be the 
size of the interface cards available at the time.)

[1] "Notes on the Networks - Section 4.6: Reliability of
     Equipment and Systems," Bellcore Special Report SR-2275,
     December 1997.

 > and what about VoIP providers around the world what is currently
> available?
> 
> I know that some operators are not owners of the infrastructure so this
> becomes even harder to warranty the high level requirements that telephony
> demands.

Yeah, so you have to get one SLA from the access provider, and a 
different SLA from the telephony (SIP or whatever) provider.

> The best info and explanation I've found until now is: from bandwidth at
> http://www.bandwidth.com/pdf/voip/bandwidth_voip_sla_062105.pdf
> 
> It would be interesting to discuss the recommend values needed and available
> at SLA agreements:
> 
> Max Latency
> Max Jitter
> Max Packet Lost

Some service providers have provided SLAs on latency/jitter/loss for the
last 5-6 years.  For example, here is an article from 2003 about Sprint
including jitter in their SLA:
http://www.networkworld.com/news/2003/1020carrsprint.html

> Mean Time Between Failures
> Mean Time To Repair
> 
> Mean Opinion Score at least 4 ?

MOS depends on your codec's perceived quality considering the latency,
jitter, and loss it experiences.  Said another way, different codecs
will sound 'better' over a network with 0.1% packet loss, but those
same 'good' codecs would sound worse with 0.0% packet loss but high
jitter.

> Service Availability, this point as you probably are aware is very
> important, what I just notice a few time a go is that the difference between
> 99,9% and 99,999% is big!
> 
> 99,9%    ->  Max time for Outage during one month is 43,2 minutes ,
> considering 30 days per month
> 99,999% ->  Max time for Outage during one month is 0,432 minutes
> 
> If any of you around the world is aware of  this values for VoIP SLAs I
> would be thankful to exchange and discuss this info.

Planned outages are also important to consider.  Planning a one-hour
outage at 3am on Sunday morning, for example, has absolutely no impact
on a traditional business.  Planning a 5-minute outage at 9am on Monday,
however, has a significant impact.  Not all outages have the same
cost or impact.


If you're serious about availability on the PSTN, with Internet access,
or with VoIP, you need multiple links to the Internet, need to be
multi-homed, duplicate datacenters, and so on.  It is a considerable
amount of work, and seldom done by the typical business with their PRI
lines.  Even ignoring VoIP, as more and more business-critical data
(point of sales terminals, inventory management, etc.) travels over IP,
it is vital that IP networks have backup links and the capability to
run in a degraded state (because backup links are more cost effective
if they have less bandwidth than the primary links).

-d


> Thanks in advance.
> 
> Best regards,
> Marco Mouta
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