Quality of service is the user's view of the service. In connection-oriented networks, users establish an end-to-end connection in the network prior to transferring information. As part of this process, the users and the ATM network layer must negotiate a contract defining the service to set up a virtual connection between two or more nodes and specify connection parameters. The first part of the contract is the traffic descriptor, which characterizes the load to be offered. The second part of the contract specifies the quality of service desired by the user and accepted by the network operator (or carrier).
To provide traffic contracts, the ATM standard defines a number of QoS parameters. For each QoS parameter, a worst-case performance is specified, and the network operator is required to meet or exceed it.
Some of the most important QoS parameters are listed below.
Peak cell rate (PCR)
The maximum rate (in cells per second) at which the sender wants to send the cells.
Sustained cell rate (SCR)
The expected or required cell rate averaged over a long interval.
Minimum cell rate (MCR)
The minimum number of cells per second that the user considers acceptable. If the carrier cannot guarantee this much bandwidth, it must reject the connection.
Cell Delay Variation (CDV or Jitter)
How uniformly the ATM cells are delivered. ATM layer functions may alter the traffic characteristics by introducing cell delay variation. When cells from two or more ATM connections are multiplexed (MUX), cells of a given connection may be delayed when cells of another connection are being inserted at the output of the multiplexer. (Multiplexing at the transport layer refers to placing several transport connections onto one virtual circuit).
Cell Delay Variation Tolerance (CDVT)
The amount of variation present in cell transmission times. CDVT is specified independently for peak cell rate and sustained cell rate. For a perfect source operating at PCR, every cell will appear exactly 1/PCR after the previous cell. However, for a real source operating at PCR, some variation in cell transmission time will occur. CDVT controls the amount of variation acceptable using a leaky bucket algorithm, described later in this section.
Cell Loss Ratio (CLR)
The fraction of the transmitted cells that are not delivered or are delivered so late as to be useless (for example, for real-time traffic).
Cell Transfer Delay (CTD)
The average transit time from source to destination.
Cell Error Ratio (CER)
The fraction of cells that are delivered with one or more bits wrong.
Severely Errored Cell Block Ratio (SECBR)
The fraction of N-cell blocks of which M or more cells contain an error.
Cell Misinsertion Rate (CMR)
The number of cells/second that are delivered to the wrong destination because of an undetected error in the cell header.
The QoS of an ATM connection refers to the cell loss, the delay, and the delay variation incurred by the cells belonging to that connection. QoS of an ATM connection is closely linked to the bandwidth it uses. For example, using more bandwidth increases the cell loss, the delay, and the delay variation incurred, therefore decreasing the QoS for cells of all connections that share those resources.