QoS and its relationship to bandwidth management is becoming more critical in order to deliver network performance while not investing too heavily into equipment. 

Quality of Service is a way to mark network data packets in various prioties - or classes - across a network.  This is important, because more and more real time traffic - such as Voice over IP (VoIP) and video are increasingly flowing across data networks.  Also, large SMS flows of traffic - like anti-virus updates - can crowd out mission critical traffic.  QoS provides a tool to protect your most critical applications.

But as QoS has become more of a reality, the price of bandwidth continues to fall.  This leads most enterprises to weigh the extra costs of providing sophisticated infrastructure to provide advanced routing protocols against the less and less expensive costs of bandwidth.  After all, given huge amounts of bandwidth, QoS becomes less and less of an effective tool.

It is only in network bottlenecks where QoS is required.  If large speeds can be configured throughout a network, traffic shaping, weighted fair queing and any other sophisticated routing protocols are not needed.  A network refresh is then not needed to procure the latest routers and switches to run QoS policies.

But a robust and effective QoS deployment does allow maximum use of existing bandwidth.  Since most traffic is not real-time, and some delay is acceptable, a quality of service packet scheme can maximize fixed bandwidth.  Rather than upgrading circuits to larger speeds, QoS allows for the efficient use of smaller circuits. 

There are lower speeds however when QoS is just not effective.  For instance, any circuit below 256k QoS routing is just not going to make much of a difference.  But again, given the declining price of bandwidth, an upgrade to a WAN circuit up to the underlying access of T1 is typically not cost prohitive compared to the cost of a new router.

Increases in router processing, which allows for more sophisticated QoS service, versus the declining price of bandwidth requires careful analysis of how much investment in either will provide for the best network experience.