[VoiceOps] Experiences with VoIP and 100+ seat sites

Tim Bray tim at kooky.org
Wed Feb 1 12:08:52 EST 2012

On 01/02/12 16:25, Darren Schreiber wrote:
> Hi folks,
> I'd love to hear some stories (good or bad) of hosted PBX VoIP installs
> on 100+ seat sites (single site). Specifically if you've done this with
> Broadsoft or another solidified switch. I have mixed opinions on how
> this type of scenario can be successful and now I'm being pressed by a
> client on a formal opinion. I figure having it based on experience from
> others on a similar product is worth hearing about.
> Specifically curious about how you addressed call quality issues and
> ensured bandwidth and uplink were sufficient.

I help look after a site with 140 ish SIP phones on the same site. Works 
very well.  Phones all on www.voipfone.co.uk

The sums for network links are easy.   Bandwidth per call * calls.  Then 
spec the right circuit.

Things people forget:

0) To plan the user experience.  You can just slap a new phone on a desk 
and expect people to use it.  You need to do training for end users. 
Even if you show them how to make a call.

Do not tell the end users it is voip.  Just `A new phone system`.

1) IP header allowance in bandwidth sums.   RTP, UDP, IP, then Ethernet 
or ATM depending on the circuit.

2) Consider Packets per second through the router/firewall.  VoIP is 
lots of small packets.  Many firewalls have a low session count limit. a 
25$ router is not going to cope with all those phones.

3) Just buy enough bandwidth

4) Protect the ethernet infrastructure.  You want to be using managed 
switches which can
- drop rogue DHCP servers
- drop a port if somebody pretends to be the default gateway
- cope when somebody makes a loop in the network or attaches a device 
which floods then lan with broadcasts

5) Put the Router in a HA setup with 2 routers and 2 WAN connections. 
With VRRP or CARP or similar.  Or agree with the customer in writing 
that if the WAN fails, the phones fail.

- or sell divert to mobile as part of the solution.

6) Manage all the phones on a configuration server.   Lock all the 
phones down so people can't mess with them.

7) Don't use wifi to connect phones.

8) Avoid Active SIP ALGs.   You don't want anything modding SIP packets 
on the router.     Passive devices which detect SIP to do traffic 
prioritization are ok.  Anything which modifies packets is bad.

- Sometimes the SIP aware routers get hacked.

9) Don't use low rate codecs.   711 all the way.  Or 722.

10) Primary and failover DHCP and DNS servers onsite.


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