Service Class	Quality of Service Parameter
constant bit rate (CBR)	This class is used for emulating circuit switching. The cell rate is constant with time. CBR applications are quite sensitive to cell-delay variation. Examples of applications that can use CBR are telephone traffic (i.e., nx64 kbps), videoconferencing, and television.
variable bit rate–non-real time (VBR–NRT)	This class allows users to send traffic at a rate that varies with time depending on the availability of user information. Statistical multiplexing is provided to make optimum use of network resources. Multimedia e-mail is an example of VBR–NRT.
variable bit rate–real time (VBR–RT)	This class is similar to VBR–NRT but is designed for applications that are sensitive to cell-delay variation. Examples for real-time VBR are voice with speech activity detection (SAD) and interactive compressed video.
available bit rate (ABR)	This class of ATM services provides rate-based flow control and is aimed at data traffic such as file transfer and e-mail. Although the standard does not require the cell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable for switches to minimize delay and loss as much as possible. Depending upon the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate, which is guaranteed to the connection by the network.
unspecified bit rate (UBR)	This class is the catch-all, other class and is widely used today for TCP/IP.

Table 1. ATM Service Classes

Technical Parameter	Definition
cell loss ratio (CLR)	CLR is the percentage of cells not delivered at their destination because they were lost in the network due to congestion and buffer overflow.
cell transfer delay (CTD)	The delay experienced by a cell between network entry and exit points is called the CTD. It includes propagation delays, queuing delays at various intermediate switches, and service times at queuing points.
cell delay variation (CDV)	CDV is a measure of the variance of the cell transfer delay. High variation implies larger buffering for delay-sensitive traffic such as voice and video.
peak cell rate (PCR)	The maximum cell rate at which the user will transmit. PCR is the inverse of the minimum cell inter-arrival time.
sustained cell rate (SCR)	This is the average rate, as measured over a long interval, in the order of the connection lifetime.
burst tolerance (BT)	This parameter determines the maximum burst that can be sent at the peak rate. This is the bucket-size parameter for the enforcement algorithm that is used to control the traffic entering the network.

Table 2. ATM Technical Parameters

Class of Service	CBR	VBR–NRT	VBR–RT	ABR	UBR
CLR	yes	yes	yes	yes	no
CTD	yes	no	yes	no	no
CDV	yes	yes	yes	no	no
PCR	yes	yes	yes	no	yes
SCR	no	yes	yes	no	no
BT @ PCR	no	yes	yes	no	no
flow control	no	no	no	yes	no

Table 3. ATM Classes of Services