Maintaining QoS
Nov 1, 2008 12:00 PM, By David Glidden
Providing high-quality video over IP is challenging.
Network impairments
Table 2. The eight priority levels for service, as
defi ned in the IEEE 802.1p standard
Select image to enlarge.
Many of the attributes guaranteed in the service level agreement can be measured to demonstrate a specific QoS. These attributes include the network availability percentage and network impairment measures, such as packet loss, packet reordering, network delay, switching delay and jitter.
Packet loss occurs when IP routers receive packets faster than they can forward them onto the network. Packet reordering measures significant impairments in received video quality when packets take different paths and arrive at the receiver in a different order. Network delay measures or specifies the typical amount of time that it takes for packets to move across the network (in milliseconds). In a service level agreement, this average can be specified over a week or month.
Switching delay causes problems with real-time video when a large number of routers in the transmission path create a large traffic queue, while jitter is the variation in the timing of when packets arrive at the destination. While some jitter is inevitable, it must be minimized in a network design if video is to be successfully decoded and played out in real time. Receivers accommodate jitter by buffering the incoming traffic.
Media delivery index
To improve video-over-IP services, the IETF issued RFC 4445, which specifies a media delivery index (MDI) to determine the quality of the delivered video service. The MDI is calculated through two measurements: delay factor and media loss rate. These two measurements capture many of the network impairments encountered in video-over-IP services.
Delay factor measures the time difference between the arrival of media data and the drain (or playout) of media data, measured for each packet. The delay factor, measured in bytes per second, indicates the amount of jitter that must be accommodated in the receiver buffer. When designing a network, the delay factor will be used to indicate how much latency the network must accommodate to ensure that receive buffers don't underflow from running out of received packets.
Media loss rate counts the number of lost or out-of-order packets over a time interval. Typically, seven 188-byte MPEG transport stream (TS) packets are contained within a single IP packet, so the loss of one IP packet could result in seven lost MPEG TS packets. Because receivers may not be able to reorder packets in the correct order, it is also important to count the number of out-of-order packets as a measure of network performance.
To calculate media loss rate, subtract the actual number of packets received during the measurement interval from the number of packets that were expected, and then scale that calculation to one second. The MDI is the ratio of delay factor (DF) to media loss rate (MLR), displayed as: MDI = DF:MLR. If an MDI equals 90:10, the delay factor is 90ms, and the link lost an average of 10 packets per second.
Testing to ensure QoS
Broadcasters using video-over-IP services can ensure that their services deliver a consistent and effective QoS by capturing and analyzing QoS metrics. An increasing array of commercial test equipment can calculate and display measures like the media loss rate. In addition, broadcasters should ensure that their services are meeting desired QoS levels by using such metrics in the management of their services.
David Glidden is an industry consultant and former industry executive.
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