There is no all-inclusive rule regarding the implementation of QoS because all networks and their traffic characteristics are unique. A good practice is to baseline the IP telephony response on a network without QoS and then apply QoS as necessary. Avaya Professional Services (APS) can help with baselining services. Conversely, enabling multiple QoS features simultaneously without knowing the effects of respective features is a bad practice.
For newer network equipment, best practices involve enabling Layer 3 (DiffServ) QoS on WAN links traversed by voice. Tag voice and data with DiffServ Code Point 46 (Expedited Forwarding), and set up a strict priority queue for voice. If voice quality is still not acceptable, or if QoS is desired for contingencies such as unexpected traffic storms, QoS can then be implemented on the LAN segments as necessary.
Caution:
There is one caution to keep in mind about QoS with regard to the processor load on network devices.
Simple routing and switching technologies have been around for many years and have advanced significantly. Packet forwarding at Layer 2 and Layer 3 is commonly done in hardware. Cisco calls this fast switching, with switching being used as a generic term here, without heavy processor intervention. When selection criteria such as QoS and other policies are added to the routing and switching process, it inherently requires more processing resources from the network device. Many new devices can handle this additional processing in hardware and maintain speed without a significant processor burden. However, to implement QoS, some devices must move a hardware process to software. Cisco calls this process switching. Process switching not only reduces the speed of packet forwarding, but it also adds a processor penalty that can be significant. Processor penalty can result in an overall performance degradation from the network device and even device failure. You must examine each network device individually to determine if enabling QoS will reduce its overall effectiveness by moving a hardware function to software or for any other reason. Since most QoS policies are implemented on WAN links, the following points increase the effectiveness of QoS remains:
Hardware platforms such as the 2600, 3600, 7200, 7500 series, or later are required. Newer platforms such as the 1800, 2800 and 3800 series can handle QoS well because of powerful processors.
Newer interface modules such as WIC, and VIP are required.
Note:
If you are using Cisco devices with the interfaces such as WIC, and VIP, you must consult Cisco to determine which hardware revision is required for any given module.
Sufficient memory is required: device dependent.
Recommended IOS 12.0 or later.
Examine the following when you enable QoS on a network device.
First, the network administrator should examine the processor load on the device and compare the load to the levels before QoS was enabled. The levels are likely to have gone up but the increase might not be significant. If it is, then it is likely that the QoS process is being done by software.
Also, the processor load must remain at a manageable level (50% average, 80% peak). If the processor load is manageable, then the IP telephony response, for example, voice quality should be checked to verify that it has improved under stressed conditions, for example, high congestion. If the IP telephony response has improved, the other applications should be checked to verify that their performances have not degraded to unacceptable levels.
