QoS Requirements for Voice, Data, Video, and Other Traffic

Later in this chapter (in the "Classification and Marking" section), the Cisco QoS Baseline is presented, representing best-practice recommendations for how QoS should be implemented for various types of traffic. These recommendations are based on the requirements for that traffic, as described in this section.

Voice traffic (discussed in more detail in Chapter 7, "Voice Transport Design") is sensitive to delays, variation in delays (jitter), and packet loss. The guidelines for ensuring acceptable voice quality are as follows:

• The one-way delay should be no more than 150 milliseconds (ms).

• The jitter should be no more than 30 ms.

• No more than 1 percent of packets should be lost.

Note

While 150 ms is the standard for acceptable voice delay, tests have shown that a negligible quality difference is found with a 200-ms delay.

The bandwidth required for voice traffic varies with the algorithm that compresses the traffic and the specific Layer 2 frame type it is encapsulated in, as described further in Chapter 7. Call-signaling traffic requires at least 150 bps (not including Layer 2 overhead), depending on the protocols used.

Interactive video, or video conferencing, has the same delay, jitter, and packet loss requirements as voice traffic. The difference is the bandwidth requirementsvoice packets are small while video conferencing packet sizes can vary, as can the data rate. A general guideline for overhead is to provide an additional 20 percent of bandwidth over that required by the data.

Streaming video has different requirements than interactive video. An example of the use of streaming video is when an employee views an online video during an e-learning session. As such, this video stream is not nearly as sensitive to delay or loss as interactive video isrequirements for streaming video include a loss of no more than 5 percent and a delay of no more than 4 to 5 seconds. Depending on the importance to the organization, this traffic can be given precedence over other traffic.

Note

When you start watching a video stream (a recording) on the Internet, you might see messages such as "Buffering x%" before the video starts in the application that you are running. This buffering is to compensate for any transmission delays that might occur.

Many other types of application data exist within an organization. For example, some are relatively noninteractive and therefore not delay sensitive (such as e-mail), while others involve users entering data and waiting for responses (such as database applications) and are therefore very delay sensitive. Data can also be classified by its importance to the overall corporate business objectives. For example, a company that provides interactive, live e-learning sessions to its customers would consider that traffic to be mission-critical, while a manufacturing company that is one of the e-learning company's customers might consider that same traffic important, but not critical to its operations.

Traffic related to the operation of the network itself must also be considered. One example of this type of traffic is routing protocol messagesthe size and frequency of these messages vary, depending on the specific protocol used and the stability of the network. Network management data is another example, including Simple Network Management Protocol (SNMP) traffic between network devices and the network management station.