# QoS Terms

Table 3 defines terms used in this discussion of QoS.

Table 3: QoS Terminology

Term | Description
--- | ---
Assured rate | Bandwidth guaranteed until the node below in the scheduler hierarchy is oversubscribed.
Best effort | Network forwards as many packets as possible in as reasonable a time as possible. This is the default per-hop behavior (PHB) for packet transmission.
Best-effort queue | For a logical interface, the queue associated with the best-effort traffic class for that logical interface,
Best-effort scheduler node | The scheduler node associated with a logical interface and traffic class group pair, and where the traffic class group contains the best-effort traffic class. Also known as best-effort node.
CDV | Cell delay variation. Measures the difference between a cell’s expected and actual transfer delay. Determines the amount of jitter.
CDVT | Cell delay variation tolerance. Specifies the acceptable tolerance of CDV (jitter).
Effective weight | The result of a weight or an assured rate. Users configure the scheduler node by specifying either an assured rate or a weight within a scheduler profile. An assured rate, in bits per second, is translated into a weight. The resultant weight is referred to as an effective weight.
Group node | A scheduler node associated with a {port interface, traffic-class group} pair. Because the logical interface is the port, only one such scheduler node can exist for each traffic-class group above the port. This node aggregates all traffic for traffic classes in the group.
HAR | Hierarchical assured rate. Dynamically adjusts bandwidth for scheduler nodes.
HRR | Hierarchical round-robin. Allocates bandwidth to queues in proportion to their weights.
Latency | Delay in the transmission of a packet through a network from beginning to end.
Proprietary QoS Management Information Base (MIB) | Supported on the E Series router.
Queue | First-in-first-out (FIFO) set of buffers that control packets on the data path.
QoS port-type profile | Supplies the QoS information for forwarding interfaces stacked above ports of the associated interface type.
QoS profile attachment | Applies the rules in the QoS profile to a specific interface.
Rate shaping | Allows you to throttle a queue to a specified rate.
RED | Random early detection congestion avoidance technique.
Scheduler hierarchy | A hierarchical, tree-like arrangement of scheduler nodes and queues. The router supports up to three levels of scheduler nodes stacked above a port. The port scheduler is at level 0, with two levels of scheduler nodes at levels 1 and 2. A final level of queues is stacked above the nodes.
Scheduler node | An element within the hierarchical scheduler that implements bandwidth controls for a group of queues. Queues are stacked above scheduler nodes in a hierarchy. The root node is associated with a channel or physical port.
Shaping rate | Bandwidth in a queue or node can be throttled to a specified rate.
Shared shaper constituent | All nodes and queues that are associated with a logical interface that is being shared shaped are considered potential constituents of the shared shaper.
Weight | Specifies the relative weight for queues in the traffic class.
WRED | Weighted random early detection congestion avoidance technique.