FIFO queueing performs no prioritization of data packets on user data traffic. It entails no concept of priority or classes of traffic. When FIFO is used,ill-behaved sources can consume available bandwidth,bursty sources can cause delays in time-sensitive or important traffic, and important traffic may be dropped because less important traffic fills the queue.
Consider these differences in deciding whether to use CQ or PQ:
• CQ guarantees some level of service to all traffic because you can allocate bandwidth to all classesof traffic. You can define the size of the queue by determining its configured packet-count capacity,thereby controlling bandwidth access.
• PQ guarantees strict priority in that it ensures that one type of traffic will be sent, possibly at the expense of all others. For PQ, a low priority queue can be detrimentally affected, and, in the worst case, never allowed to send its packets if a limited amount of bandwidth is available or if the transmission rate of critical traffic is high.

In deciding whether to use WFQ or one of the other two queueing types, consider these differences among WFQ and PQ and CQ:
• WFQ does not require configuration of access lists to determine the preferred traffic on a serial interface. Rather, the fair queue algorithm dynamically sorts traffic into messages that are part of a conversation.
• Low-volume, interactive traffic gets fair allocation of bandwidth with WFQ, as does high-volume traffic such as file transfers.
• Strict priority queueing can be accomplished with WFQ by using the IP RTP Priority, Frame Relay IP RTP Priority, low latency queueing (LLQ), distributed low latency queueing, lw latency queueing for Frame Relay, or Frame Relay PVC Interface Priority Queueing features. Strict PQ allows delay-sensitive data such as voice to be dequeued and sent before packets in other queues are dequeued.