COS (Class of Service)

ATM is famous for it's ability to offer "Class of Service" to customers, so that their PVC's can be prioritized.  Few realize that Frame Relay also offer COS, albeit in a somewhat limited fashion.  

The COS values are loaded into each switch, on a per PVC basis.  For example, Nortel switches have a table with a "Transfer Priority" number assigned to each and every PVC.  

A "fairness" algorithm is applied so that the higher priority packets have "first dibs" on passing through the switch.  They transfer the frames by loading them into buffers and then loading them into queues for transmittal.  The higher the transfer priority number, the sooner the frame is sent on to the next switch in the path.

LAN traffic is bursty in nature and can accept a fair amount of delay. 

SNA requires a more stringent approach because the server sends keep-alive signals to the client, and if an ack is not sent back in time, the session is dropped.

Voice is the least tolerant of delay, which results is static and even periods of silence.  Voice is real-time and requires very tight delay constraints.  Video PVC's are similar, and many customer's request the Voice COS for Video over Frame PVC's.