Quality of Service (QoS)

Voice traffic and data traffic used to use entirely separate networks.

• Voice traffic useed the PSTN.

• Data traffic used the IP network (enterprise WAN, Internet, etc).

Modern networks are typically converged networks in which IP phones, video traffic, regular data traffic, etc all share the same IP network.

This enable cost savings as well as more advanced features for voice and video traffic, for examples, integrations with collaboration software (Cisoc WebEx, Teams, etc).

However, the different kinds of traffic now have to compete for bandwidth.

QoS is a set of tools used by network devices to apply different treatment to different packets.

QoS is used to manage the following characteristics of network traffic:

1. Bandwidth

   1. The overall capacity of the link, measured in bits per second (kbps, Mbps, Gbps, etc).

   2. QoS tools allow you to reserve a certain amount of a link's bandwidth for specific kinds of traffic. For example: 20% voice traffic, 30% for specific kinds of data traffic, leaving 50% for all other traffic.

2. Delay

   1. The amount of time it takes traffic to go from source to destination = one-way delay.

   2. The amount of time it takes traffic to go from source to destination and return = two-way delay.

3. Jitter

   1. The variation is one-way delay between packets sent by the same application.

   2. IP phones have a 'jitter buffer' to provide a fixed delay to audio packets.

4. Loss

   1. The % of packets sent that do not reach their destination.

   2. Can be caused by faulty cables.

   3. Can also be caused when a device's packet queues get full and the device starts discarding packets.

The following standards are recommended for acceptable interactive audio (ie. phone calls) quality:

• One-way delay: 150ms or less.

• Jitter: 30ms or less.

• Loss: 1% or less.

If these standards are not met, there could be a noticeable reduction in the quality of the phone call.