Quality-of-Service



	Synchronizing high and low bandwidth users:
	Consider the case where participants in one area are connected through a low-speed link to the majority of the conference participants who enjoy high-speed network access. Instead of forcing everyone to use a lower-bandwidth, reduced-quality audio encoding, an RTP-level relay called a mixer may be placed near the low-bandwidth area. This mixer resynchronizes incoming audio packets to reconstruct the constant 20 ms spacing generated by the sender, mixes these reconstructed audio streams into a single stream, translates the audio encoding to a lower-bandwidth one and forwards the lower- bandwidth packet stream across the low-speed link.

	BANDWIDTH ISSUES
	RTP is designed to allow an application to scale automatically over session sizes ranging from a few participants to thousands. In an audio conference the data traffic is inherently self- limiting because only one or two people will speak at a time, so with multicast distribution the data rate on any given link remains relatively constant independent of the number of participants. However, the control traffic is not self-limiting. If the reception reports from each participant were sent at a constant rate, the control traffic would grow linearly with the number of participants.

	To maintain scalability, the average interval between packets from a session participant should scale with the group size. The control traffic should be limited to a small and known fraction of the session bandwidth:

	a) Small so that the primary function of the transport protocol to carry data is not impaired;
	b) Known so that each participant can independently calculate its share.

	It is suggested that the fraction of the session bandwidth allocated to RTCP be fixed at 5%.

	MIXER BASICS
	1) Receives streams of RTP data packets from one or more sources, possibly changes the data format, combines the streams in some manner and then forwards the combined stream.

	2) All data packets forwarded by a mixer will be marked with the mixer's own SSRC identifier. In order to preserve the identity of the original sources contributing to the mixed packet.