OCS won’t be an acceptable telephony solution for all users for some time.  My last post concluded with an introduction to an OCS architecture that would make it easier to migrate all users to OCS over time.  Here’s more detail on how to deploy OCS for the users who need it, while leaving the existing PBX phones in place for users who need it.

The approach involves an architectural concept called ‘front-ending.’  In a nutshell, this means putting one system in front of another, allowing the first (front-end) system to process the call first, make a decision about what to do with it.  The main decision is; whether to send to an OCS user directly, or pass it along to the second (back-end) system if needed.  See architecture below:

Enabling has done this for clients who already know they want to use OCS as a PBX.   Our first experience with front-ending came about in a roundabout way.  Working with a small/mid-sized business key system, we found out that it was not capable of a trunk-to-trunk transfer.  So when the key system took an inbound call on the phone company’s T1, it couldn’t pass the call over another T1 trunk to the OCS VoIP Gateway.  Most fully functional PBXs do have this capability, but key systems typically don’t get used as ‘tandem’ switches. 

In this case, we flipped the boxes – the PSTN T1 went into the OCS VoIP Gateway, and connected the gateway to the PBX via another T1 trunk.  The VoIP Gateway handles the call and sends everything to OCS.  OCS now handles the call first, and sends to the PBX only calls that are destined for the users still on PBX phones. 
Now that we’ve seen the pros and cons of this solution, we are suggesting it to IT Pros that know they want to eventually migrate all users off the PBX to OCS.  We are helping them evaluate some of the benefits and tradeoffs below.

Benefits:
- Everyone can get the benefits of IM/Presence, video, and web conferencing, even users staying with their existing handset.
- This approach allows key groups to stay with the existing handset and system.
o One of the key groups is usually a contact center group, with prompting and queuing, announcements, possibly integration to a back-end database, and/or call analysis software for workforce management and staffing analysis.  Such groups have lots of business logic built into them that cannot be transitioned overnight. 
- OCS call control enables new features for the users that want them (i.e.  dual ringing, self-administered call forwarding settings)
- As a user transitions from the PBX to OCS as their call controller, the PBX doesn’t need to be changed.  The IT pros can thus minimize Moves/Adds/Changes on the PBX over time.  The IT pro can simply enable the user for ‘Enterprise Voice’ within OCS and remove their PBX phone from the desktop. 
o If the architecture were flipped, and the PBX stayed in front of OCS, then every time a user moved off of their PBX phone to OCS, the PBX would need to be changed.  Specifically, the user’s coverage path would have to be altered, so calls would be immediately forwarded to OCS.  This can get unwieldy as you move one extension at a time, or even chunks of users from discontinuous extension ranges.

Toss-ups:
- Another box (and point of failure) has been added between the outside world and the PBX.  If the PBX users take more critical calls than the OCS users, then this may keep you up at night.  Redundant boxes can keep your calls coming/going if one fails, but if the Gateway or the cable or card on the PBX fails during a live call, you’ll get dead air.  Gateways are PSTN-Grade equipment running embedded operating systems in case you were wondering.
- Troubleshooting issues on the PBX phones may result in looking in the PBX and the VoIP Gateway, instead of only the PBX. Then again, troubleshooting voice issues for OCS users becomes easier, as you needn’t look inside the PBX at all.
- Some gateway vendors offer 911-compliant solutions, so that if you dial 911 and all outgoing trunks are busy, the 911 call bumps another call to take priority. 

Negatives
- PBX vendors won’t buy you lunch anymore, so if it’s important to maintain that relationship for other business reasons, keep your plan on the down-low.