One of the many wonderful things I get to do at $employer is work on voice systems and convincing my customers to move from old clusters of analog trunks to new, shiny Primary Rate Interface (PRI) trunks to carry their calls. PRIs are wonderful things, capable of taking up to 23 calls at a time, providing calling party and called party information, and dispensing with the need to have kludgy “rollover” analog trunks. However, in my experience with turning these circuits on, the worksheet the telco provider sends out tends to look like speaking Greek to most network enginee…rock stars. It took a while for me to figure out what all the obscure acronyms meant, since the telco just assumed that I knew what they all stood for. In an effort to provide help to my readers that may not be telco people, or might be getting forced into working on a PRI worksheet, I thought it might be helpful to provide some translations.
PIC/LPIC - Probably the most confusing acronym out of the bunch. PIC stands for Primary Interexchange Carrier. This is your long distance carrier. This is a code that is kept in a database and when you need to make a long distance call, the telco consults this database to know whose network to send the call along. A great explanation of long distance calls can be found HERE. Conversely, the LPIC is the Local Primary Interexchange Carrier. In other words, they are the company that handles your local calls that aren’t long distance. These two providers can be different, and in many cases they are. In rural areas, the LPIC is the local telco, and the PIC is a larger carrier like AT&T or Verizon. I’ve found that many companies will give you a deal if you specify them for both PIC and LPIC. Most of the time, the PIC/LPIC choice will be whomever is installing the PRI for you, such as AT&T or Cox Communications.
DID – Another one that confuses people. In this case, DID stands for Direct Inward Dial. This is a huge change from the way an analog circuit works. With an analog circuit (like my house), when you call my number it sends an electrical signal along the wire telling the device at the other end to ring. When we hook this circuit up to a CUCM/CCME system, we usually have to configure Private Line Automatic Ringdown (PLAR) in order to be sure something gets trigger when the electrical signal arrives. A PRI doesn’t use electric signals to trigger ringing. Instead, they are configured with two different fields, the Calling Party and the Called Party. In this example, the Calling Party is what is most often referred to as “Caller ID”. The Called Party on a PRI is the DID. This is a number that is delivered to the PRI and sent to the PBX equipment on the other end. The name comes from the fact that these numbers are most often used to directly reach internal extensions without the need to reach a PBX operator or automated attendant. The DID can be configured to ring a phone, a group of phones, or even a recording. The numbers that used to belong to your analog circuits will usually be moved over to a group of DIDs and pointed at the PRI.
Outpulsed Digits - This one sounds straight forward. Digits are being sent somewhere, right? Remember that this worksheet is from the perspective of the service provider, so the outpulsed digits are what the provider is sending to your equipment. You have tons of options, but most providers will usually limit your options to 4, 7, or 10 digits. This is the number of digits that you get from the PRI to determine where your calls get sent. Since I’m a big fan of using translation patterns on my systems to send the digits around, I tend to pick 7 or 10 digits. In areas like Dallas, you may be forced to take 10 digits, as most metro areas are now mandatory 10-digit dialing. This also helps me avoid dial plan collisions when a phone number for a site is the same as a 4 digit extension internally. If I get 7 digits coming from the PRI, I can be pretty sure that none of my extensions will have the same number. If you don’t want to configure translation patterns and have a lot of DID numbers that correspond to phone extensions, you may want to consider a 4-digit outpulse setup from the telco.
NFAS – This one I don’t use very often, but it might come up. NFAS stands for Non-Facility Associated Signaling. This is used when you have more than one PRI configured in your environment. With a 24-channel PRI, 23 of those channels are used to provided data/calls. These are bearer channels or B-channels. The 24th channel is used to send control and signalling data. This is the Data Channel or the D-channel. When you configure your environment with multiple PRIs, you have multiple D-channels to provide signalling. However, you can pay a premium for each of those D-channels. In an effort to save some money, the idea of NFAS allows one D-channel to provide the signalling for up to 20 PRI lines. The catch is that if the D-channel goes down for any reason, so does the signalling for all the PRIs participating in the NFAS setup. Usually, if you designate NFAS on your worksheet, the telco will make you choose whether or not to have a backup D-channel. This is a good idea just in case, because you can never go wrong with a backup.
Station Caller ID – I include this one because of more than one issue I’ve gotten into with a telco over it. Like, a full-on yelling match. If you are given the option of using the station ID as the outbound caller ID, use it. You have much more control over how the caller ID is represented inside of CUCM than you do if you the telco takes over for you. If you don’t use the station ID as the caller ID, they will usually use the first DID number in your list, or set it to the billing number of the main telephone line. As most PRIs I setup are usually for multi-site deployments, this creates issues. People see the caller ID of the headquarters or the administration building instead of the individual unit number. They call that number back expecting to get their child’s school (for instance), but instead get the board of education building. Some telcos will go to war with you about the inherent danger in letting the user specify their station ID for use with emergency services like 911 or 999. I usually tell the telco rep to get stuffed, since my route lists will get the Caller ID more correct that their ham-handed attempts to just slap a useless billing ID number on the PRI and call it good. If they pick a DID number that doesn’t appear in the phone book or in the PS/ALI database for the local emergency service provider, then you can get into a liability issue. Better to just check the “station ID” box and build your system right.
These were the most confusing parts of the PRI worksheets that I’ve filled out from multiple providers. I hope that my explanations help if and when you need to fill out your own sheet. If it saves time having to Google what LPIC and NFAS mean, then I’ll sleep happy knowing that you were able to conserve some of your Google-fu.