Janne Peisa

Unlike Tomas, I have spent most of my career in telecommunications optimizing the air interface for IP-based applications. While doing so the focus was (almost) always on the applications using TCP. We quickly realized that one of the fundamental problems with the first cellular packet data access systems (especially GPRS) was the round-trip time (which was close to one second), and we became almost obsessed with reducing the air interface round-trip time. This culminated in the work for High Speed Packet Access (which introduced two millisecond transmission time interval) and Long Term Evolution of UTRAN (which will introduce an even shorter TTI).

It never occurred to me that there would be any interest in providing a high capacity voice service over the HSPA channels we had created. The design goal of the HSPA had always been interactive applications, and we explicitly ruled out any conversational services over HSPA. But suddenly this changed. Preliminary analysis showed that it was theoretically possible to reach or exceed the CS capacity for voice service, and I spent a lot of effort trying to understand how this is possible (for curious readers, the reasons are explained in Section 7.3). The outcome was surprising: when designing the HSPA we had accidentally designed an air interface that was capable of supporting voice applications with higher efficiency than the existing CS bearers could.

As soon as I understood that it would be better to provide the voice service with HSPA access, it also become apparent that suddenly we have both the flexibility of the IP-based applications, allowing one to quickly introduce new codecs, add new modes of communications, such as video calls or instant messaging, and the efficient performance the CS service.

After redesigning the air interface, it was time to redesign the basic telephony application - to replace the voice telephony with Multimedia Telephony.

I hope the reader can appreciate both the flexibility and the efficiency of the IMS Multimedia Telephony.

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