QoS Overview
Quality of service (QoS) configurations give special treatment to certain
traffic at the expense of others. This helps make your network performance
more deterministic and predictable for this traffic. Using QoS in the network
addresses the following problems:
■ Lack of bandwidth for important applications
■ Delay of sensitive data, such as voice and video
■ Jitter (variable delay)
■ Packet loss due to data being dropped at a congested interface
Bandwidth
In a network with several hops, the available bandwidth is only as much as
the smallest link. When multiple applications and multiple flows use the
same links, the available bandwidth per application is even smaller—it
equals the smallest link bandwidth divided by the number of flows.
Insufficient bandwidth especially affects time-sensitive and interactive
traffic, and traffic with large flows.
You can increase link speeds to get more bandwidth—that can be expensive,
time-consuming, and introduce technological difficulties. Alternatively, QoS
mechanisms can guarantee bandwidth to specific applications.
Compressing the traffic on slower links creates more useable bandwidth;
because each frame is smaller, there are fewer bits to transmit. However,
compressing data uses processor and memory resources and introduces some
latency while the compression is being done. Because of this, use compression
only on T1 links or less. You can compress the whole payload or just
compress the protocol headers with TCP or Real-time Protocol (RTP) header
compression (cRTP). Cisco supports three payload compression algorithms:
■ Stacker
■ Predictor
■ Microsoft Point-to-Point Compression (MPPC)
For voice, use Low Latency Queuing (LLQ) and cRTP compression, and for
data, use Class-Based Weighted Fair Queuing (CBWFQ) and TCP compression.
LLQ and CBWFQ are discussed later in this chapter.