tailieunhanh - Scalable voip mobility intedration and deployment- P11

Scalable voip mobility intedration and deployment- P11: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | Voice Over Ethernet 99 too full but the fact that it stops dropping when the queue empties gives pressure when the queues are filling and permissiveness when there is plenty of room. On top of RED is a concept called weighted random early detection WRED . WRED uses weights based on the classifications we have seen already to alter the drop probabilities. Using the classifications for voice allows administrators to avoid having WRED kick in for voice which is inelastic and will not respond to being dropped if the administrator has no ability to place voice in a separate queue or route. For data more critical data connections such as TCP-based SIP needed in calls can be given a higher probability by avoiding a higher drop probability while allowing normal data to be slowed down. The problem with RED is the problem with policing. Packets that may have been needed to prevent the queue from going idle even though there are resources for them causing lost work and wasted resources. Explicit Congestion Notification Instead of using RED routers have the option of marking the packets rather than dropping them. TCP endpoints that know to read for the congestion-marked packets will consider it as if the packet had somehow been lost and will back off or slow down but without causing the packet s data to disappear. This increases the performance of the network and improves efficiency though needless to say it does nothing if the endpoints are not aware of the congestion notification scheme. On TCP explicit congestion notification ECN works by the TCP endpoints negotiating that they support this protocol. Both sides need to support it because the only way the sender can know if an intervening router has marked a packet is for the receiver to echo that fact back to the sender over TCP itself. Once a flow is established the sender sets the ECN bit bit 6 in the DSCP usage of the TOS field in IP Section Table . This lets routers know that the packet supports ECN.