You may want to check if there are any special regulations in your area. If there are not, then I would suggest 10 DID's would work well. Given that most Telcos sell DID numbers in groups of 10 at a minimum, you could use 2 per floor (ELIN/ERL) for 2 floors and 3 per floor (ELIN/ERL) for the 2 other floors that perhaps have more users/phones. Here is some info on the relationship between DID's and ERL's;
Emergency Responder converts a caller's extension to an emergency location identification number (ELIN), and this number is used to route the call and to enable PSAP callbacks.
To set up the ELIN numbers, you must first obtain direct inward dial (DID) numbers from your service provider. Because you must pay for each number, you might want to limit the number of DIDs you obtain to two or three per ERL. The DIDs must be unique for each ERL.
Emergency Responder will reuse the ELIN numbers assigned to an ERL if necessary. For example, if you configure two numbers for an ERL, and three emergency calls are made within a three hour window, the first emergency caller's ELIN mapping is replaced by the third caller's extension. Thus, if the PSAP tries to call the first caller, the PSAP will reach the third caller. Keep this in mind as you determine the number of DIDs you will obtain for each ERL.
An emergency response location (ERL) is a building, area within a building, or outside area (if you extend phone service outdoors) that is to be treated as a single location for emergency response purposes. All telephones within the ERL are treated as coming from the same location.
Thus, when someone makes an emergency call, the public safety answering point (PSAP) and your onsite alert (security) team are notified of the ERL. If the emergency requires locating the individual who placed the emergency call, the response teams will have to find the person within the ERL. You can include more specific information using the Phone Location field for individual switch ports. This level of detail is only available for automatically tracked phones, and only appears on the Web Alert screen for onsite alert personnel.
This is similar to the way emergency calls are handled for individual home users: emergency response teams know the house from which the call was placed, but have to search from room to room until they find the caller. The bigger the house, the longer the potential search. Likewise, the larger you make your ERLs, the longer it might take a response team to find an emergency caller.
The laws relating to size of ERLs can vary for different cities, states, and countries. You are responsible for learning your local statutes and developing ERLs that satisfy those statutes. Work with your telephone service provider; they can help you understand the laws. Ultimately, you will have to submit the automatic location information (ALI) for your ERLs to your service provider so that calls from your ERLs are routed to the appropriate PSAPs.
â¢You have a 25-story building, each floor has 10,000 square feet of office space. You might create 25 ERLs, one per floor. Better, you could divide each floor in half and create 50 ERLs, two per floor.
â¢You have 5 buildings. Each building was a former home, and they are approximately 3000 square feet. You might create 5 ERLs, one per building, even though some of the buildings are multi-story.
â¢You have a 5 story building, but the building is very large, so that each floor has 100,000 square feet of office space. You might create 20 ERLs per floor for a total of 100 ERLs, each ERL covering approximately 5,000 square feet.
â¢You have a high concentration of telephones, and local standards require that an ERL have no more than 48 telephones. In this case, you will have to define zones based on telephone coverage, rather than on physical space. Try to create zones that are recognizable as a physical location, for example, BldJFloor5Row3.
To add to great information from Rob, you also need to accomandate the default ERL, which will be invoked if for some reason switchport was not allocated to any ERL. Here is my rule of thumb, configure 2-3 DID (ELINs) per ERL, plus 1 ELIN for the default ERL.
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