12 Appendix F: Quality of service – Setting Packet priority
Normal IP routing is by best effort. This does not work well for broadcast television as the video and audio components needs to be transport as a continuous flow of packets without inference caused by other traffic over the internet. There are different techniques to secure a high quality of service. The main ones are:
oMPLS (Multi Protocol Label Switching) o Layer 3 routing priority
o Layer 2 routing priority
12.1 MPLS
In networks running MPLS, the packets are forwarded along a predefined path from an ingress router to an egress router. Packet switching is then done according to the label and packets will be switched expediently. The MPLS label is added to the IP packet by the ingress router and removed on the egress router. The labelling is done on the basis of packet classification.
12.2 Layer 3 Routing
An alternative technique to get QoS is to use layer 3 routing and give video content packets higher priority than other data. IP packets are put into queues according to their priority. Packets with high priority are forwarded expediently and have a lower probability to being discarded due to buffer overflow.
There are two ways prioritize IP packets called Differentiated services
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
Figure 43 Differentiated services (Diff-serve) or by the precedence bits (TOS )
Layer 3 prioritisation may also be combined with MPLS where layer 3 routing is used in the aggregation network and MPLS in the core network. The DSCP priority setting may be used for MPLS tagging.
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