There are currently several proposals for setting up and controlling SIP conferences (including back to back client/server
user agents). All of the current proposals have no effective limit on the number of participants involved. The only practical limit would be in the actual mixing servers. These limits are outside the actual protocol and would be enforced by the conferencing application that is controlling the calls. Commercial SIP/H.323 conferencing solutions realistically support hundreds of users per conferencing session and thousands of simultaneous conferencing sessions. The SIP or H.323 signaling used to set up and control the conference would never impose a limit. It is relevant to note that SIP clients can themselves support three-way conference calls without the need for external hardware.
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