ut at opposite ends of the spectrum and have moved towards each other. For example, SIP was initially deployed exclusively with UDP, but TCP support has grown and become more impor- tant over the years. H.323 on the other hand initially could not be used over UDP exclusively, but now has been extended to be used over UDP. Another example relates to the number of messages used to set up sessions. H.323 started with a large number of messages, then reduced that number as it evolved using FastStart. Some SIP applications that use extensions such as UPDATE and PRACK to perform precondition negotiation prior to session establishment now use many more messages to set up a session than the three in the original specifi- cation. (However, this complexity in these SIP preconditions applications sug- gests that they, like H.323, will never achieve wide deployment, and will instead be displaced by the simpler SIP model.) Finally, H.323 started with a larger specification than SIP [3], but as SIP has grown and added functionality, it now wins the "paperweight" war in terms of more pages of specification text. As a result, most SIP versus H.323 comparisons are badly out of date due to the revi- sions in each protocol. 8.4.2 Strengths of Each Protocol H.323 has carved out two niche areas in current deployed systems-it is widely deployed in small PSTN replacement networks for handling simple phone calls, and it dominates the IP videoconferencing market. Simple PSTN replacement networks that only originate and terminate phone calls without even basic fea- tures do not use most of the key advantages of SIP. As a result, they have little incentive to upgrade to SIP until the widespread adoption of SIP eventually makes SIP gateway ports much cheaper than H.323 ports. However, since SIP is earlier in its development cycle than H.323, this is not likely to happen for a number of years. New implementations of these types of systems will likely deploy SIP from the start, seeking to "future proof" the investment, but there is little inc