QoS for voice and video

Quality of service (QoS) tells network equipment which traffic to send first when links get busy. Voice and video are sensitive to delay, jitter and loss, while file downloads and backups are not, so QoS puts real-time traffic at the front of the queue. It only works if traffic is marked consistently and every device along the path, wired and wireless, honours those marks.

Why voice and video need priority

A network link can only send one packet at a time. When traffic arrives faster than a link can send it, packets wait in a queue. A large file transfer can fill that queue and delay the small, steady packets of a phone call. The call does not need much bandwidth, but it needs its packets on time.

Common planning targets for good voice quality are:

• One-way delay under about 150 milliseconds, the figure in ITU-T recommendation G.114
• Jitter low enough for the phone's jitter buffer to smooth out, often quoted as under about 30 milliseconds
• Packet loss well under 1 percent

Video conferencing has similar needs and uses much more bandwidth, so it needs its own planning. Treat these figures as guidance and check your phone system and conferencing platform's own requirements.

How QoS works

1. Classify: identify traffic as voice, video, signalling or general data.
2. Mark: label each packet so other devices know its class. On IP networks this is the DSCP value in the packet header.
3. Queue: on each link, put marked traffic in the right queue, with a strict-priority queue for voice and a guaranteed share for video.
4. Police and shape: limit each class so priority traffic cannot starve everything else, and shape traffic to match the real speed of slower links.

Typical markings

These are widely used conventions. Your phone system or conferencing vendor may recommend different values; what matters is that the whole network uses the same scheme.

Keep the number of classes small. Four to eight classes is typical for a business network; a scheme with a dozen classes is hard to maintain and rarely performs better. Document the scheme so that whoever configures a new switch, access point or firewall applies the same rules.

The trust boundary

Any device can mark its own traffic as high priority, including a laptop downloading a large file. Switches therefore decide whose markings to trust. A common approach trusts markings from IP phones and conferencing systems, re-marks or ignores markings from general user ports, and classifies softphone traffic by application or port. Setting the trust boundary carefully stops QoS from being undone by misbehaving or misconfigured devices.

QoS on WiFi

WiFi uses WMM (WiFi Multimedia), which maps traffic into voice, video, best effort and background access categories. Voice gets shorter waiting times to transmit. For this to work, DSCP markings must map correctly to WMM categories on access points and controllers, and devices must mark their own traffic. Good WiFi QoS also depends on good design: enough signal, low channel utilization and fast roaming. See WiFi roaming and handoff and high-density WiFi design.

Checking that QoS works

• Capture packets at several points and confirm markings survive end to end
• Check switch and router queue statistics for drops in the priority queue
• Run test calls while the network is under heavy load
• Use your phone system's call quality reports to find problem sites or users
• Review QoS after network changes, firmware updates or new applications

QoS cannot fix a link that is simply too small, or a network with cabling faults. Bandwidth, design and QoS work together. For the bigger picture, see VoIP explained.

How ehZee can help

Our VoIP design and build service includes checking the network that calls will travel over, designing QoS across switches, WiFi and the internet edge, and testing call quality before cutover.

Common questions