Unspecified Bit Rate (UBR) has no bandwidth guarantee. The other forms of QoS all require the adapter to know that if it doesn't exceed a certain limit, the switch will not drop packets. UBR provides no contract whatsoever. An adapter just gets whatever bandwidth is available at the time. Cells being sent out onto the net may all be dropped, or they could all be sent. UBR is of course the cheapest, because the carrier makes absolutely no guarantees at all. It's similar to being on standby. UBR is useful, because it's like UDP (User Datagram Protocol) today on TCP/IP. UDP provides no guarantee. There may be no bandwidth available, and no way to determine if a packet reaches its destination. A good example of UBR is a Dow-Jones quote feed coming to a window on a group of PCs. If someone misses a stock quote when it scrolls by, they'll see it on the next pass, and it's not important to see it immediately.