When configuring a DSLAM, each user port is configured like this: "qos ubr" .
What this ubr stands for?ubr stands for Unspecified Bit Rate.
Constant Bit rate (CBR),Variable Bit Rate(VBR), Available Bit Rate (ABR), Unspecified Bit Rate(UBR) are some of the services available in ATM.
UBR is not for Real time applications. Real time applications need constant bit rate and tranfer delay should be exceed certain limit for real time applications. But, broadband subscribers are normally Internet users. Hence UBR is the configuration generally used.
UBR has its own advatages: You need not reserve bandwidth for the user for each Virtual Circuit. Hence more users can be multiplexed. UBR is best for applications which are tolerent to delay, such as email,ftp,browsing,etc.