Calculating Calls Per Second (VoIP Deployment)

When you have data that identifies your maximum concurrent calls, you need to figure out how fast you’ll send those calls to your carrier — another concern that’s rather unique to VoIP. Long-distance carriers use simple metrics to determine the statistical probability of when a call would arrive in non-VoIP, traditional telephony. The carriers use one device to provide a dial tone to every phone line, and they know (by mathematical probability) how often they need to provide call setup for an incoming call.
VoIP has changed this dynamic — with one IP circuit, you can send hundreds or thousands of calls to your carrier in a single second. But even if you can send 200 calls per second to your carrier, you shouldn’t. If you’re a small or medium-sized business, you may need to send a maximum of only one or two calls per second to your carrier. It’ll love you for it, and you shouldn’t have to worry about ever overloading its VoIP proxy server with incoming calls.

Your carrier won’t appreciate being inundated with 100 — or 1,000 — calls in the same second. The first 100 calls overload your carrier’s VoIP proxy server. This condition, whereby your VoIP carrier’s proxy is incapacitated, both prevents the remainder of your calls from completing and also prevents any other customer that the long-distance carrier has assigned to that specific VoIP proxy server from completing any calls. This type of situation usually earns you a quick call from the network security folks at your carrier, telling you to stop sending so many calls. If you have a habit of flooding their network with calls, they may simply block your IP and stop providing service to you.
If you’re a larger telecom customer and have dedicated circuits running the equivalent of hundreds or thousands of phone lines, you need to do some quick analysis. Three factors impact the maximum number of Calls Per Second you require:

Your average call duration

Your average duration of time your outbound calls ring at the destination (ring-back time) before the call connects
The maximum number of concurrent calls you need
Traditional dedicated telecom services have two main benchmarks for maximum concurrent calls. Many companies that are replacing existing service with VoIP or deploying a new office and phone system frequently use these traditional telephony units of measure to place their required call volume into a known grouping:
‘ DS-1: Represents 24 phone lines ‘ DS-3: Represents 672 phone lines
If your carrier can manage even one call per second, you can make a DS-1′s full 24 calls in less than half a minute.
Table 6-1 shows two calling profiles’ maximum concurrent calls at an increasing number of allowed calls per second.

The left column in Table 6-1 represents the calls per second available. The center and right columns represent the maximum number of consecutive calls possible at two call-per-second rates:
55-second total duration: Represents the values for calls whose average duration is 55 seconds from the moment the last digit of the phone number is sent to the time the call is disconnected.
This duration breaks down, on average, to 25 seconds of ring-back time before the call is answered and a call connect duration of 30 seconds.
Your maximum concurrent call level must be attained within the time of your average total call duration. After you cross that threshold, you’re gaining calls only as quickly as your active calls are disconnecting.
The right column (85-second total duration): Shows that you can get a larger number of total consecutive calls when you have a longer total call duration (ring-back time and the total time the call is connected from end to end). An 85-second call identifies a call with a total off-hook time of 85 seconds (25 seconds of ring-back and a 60-second connect duration). The longer call duration provides 30 more seconds to send new calls before you begin disconnecting your previous calls. This enables you to reach a higher total consecutive call number.

You can use Table 6-1 to try to calculate the total calls per second that you need for the volume of calls you have on existing non-VoIP circuits. Either call-duration profile can easily fill seven to ten DS-1s with only three calls per second. The values that each call-duration profile needs to match the volume of a DS-3 (672 phone lines) appear in bold.

If your business is telemarketing, you must factor in an initial failure rate on your calls of up to 20 percent because of bad phone numbers in your dialing list. A call that receives a busy signal or recorded out-of-service message ends quickly, freeing up a line. If you’re pushing to fill the equivalent of a DS-3 in calls, you have to negotiate a higher call per second rate.
Your carrier bases your number-of-consecutive-calls and calls per second restrictions on the capacity of its Session Boarder Controller (SBC) assigned to receive all your calls. You may need more calls per second or sessions than your carrier will provide. If you find yourself in this situation, ask your carrier to split up your traffic over two or more SBCs. It shouldn’t have a problem meeting your 20 call per second requirement as long as two SBCs, which each have you allocated for 10 calls per second, handle your calls.