ti- mate this total signal traffic in order to plan STP capacity. To meet SS7 redundancy requirements, there must be two STPs, and the total ca- pacity at peak demand must equal no more than 80 percent of the total capacity of one device. This allows for the peak demand to be met with each STP running at only percent capacity. If one should fail, then the other can take over the whole load while still operating at only 80 percent of capacity so that it has room to spare. The SS7 requirements apply whether the STP is a separate physical device, as is the case with Telekelic Eagle STPs, or are integrated with the MSC switch itself, as is the case with switches from Nortel. 34.6.4 Short message service center (SMSC) planning The size of the database for the SMSC is the size of the home subscriber community. Like the HLR, there should be no surprises in the number of short message entities (SMEs) served by an SMSC. We have to plan the SMSC to handle the short message activity of those subscribers, and this can vary considerably. The estimate of SMS activity per subscriber depends on the service mix of the subscriber base. We might expect data service subscribers to send fewer short messages than voice subscribers. The good news for SMSC planning is that the SMSC is associated with the home MSC rather than with the visitor MSC of a user terminal. This means that a late flight at the airport generates a lot of short messages, but those short messages are distrib- uted over the home SMSCs of the passengers. Unless the airline sold its tickets to a lot of people who live in the same place, this surge of SMS messages will be distributed over many SMSCs. The traffic engineering job for an SMSC is less critical than the VLR because the wireless system will still operate when the SMSC is slow. Short messages are a fea- ture, a service, but wireless telephone life can go on during a few minutes of slow SMSC performance. This is in contrast to the VLR, where a slowdown affects all roam- ing traffic. Downloaded from Digital Eng