Broadcast Engineering - April 2008 - (Page 39)

FEATURE QOE FOR IPTV END USERS between the streaming server and the residential gateway located at an IPTV end user’s home are illustrated in Figure 1. The ﬁrst scenario illustrates the situation when network conditions are ideal and the packets are separated by a time difference of 1.5ms. In this environment, the intrapacket gap stays the same as it traverses each of the network hops on route to the residential gateway. This scenario is unlikely to occur across a live network. In the second scenario, two of the IPTV packets have been injected onto the distribution network without time gap, and the third packet is 3.5ms behind the other pair of packets. Although no queues are occurring, the injection of another pair of packets back-to-back results in the formation of a mini-queue. The third scenario is more representative of what occurs across a network that is used to transport triple-play IP services. In this ﬁnal scenario, the VoIP, IPTV and Web servers have simultaneously injected IP packets into the network. Once on the network, the intrapacket delays of all three streams are more or less the same as they arrive on three different router interfaces at the same time. The router is unable to process three streams simultaneously so a queue is formed, and packets need to wait a ﬁnite period of time in memory before being forwarded to the correct output port. In this example, the queue overﬂows, and a packet storing Internet access content is discarded. Meanwhile, the VoIP and IPTV trafﬁc receive preferential treatment in the queue, and the intrapacket time gap remains the same at the output of the router. This is a simplistic model of the types of network patterns that could develop across an IPTV network. Video jitter IPTV services are particularly sensitive to delays caused by overloaded servers, routing, network congestion and queuing as the IP video packets traverse the network. The quality of a video signal depends on the delivery of a lossless IP stream at a constant bit rate. The decoder in the IPTV consumer device requires a steady, dependable incoming IP stream. This is achieved through a sophisticated synchronization and clocking process that occurs between the decoder in the IPTV consumer device and the encoder at the IPTV data center. Variations associated with packets arriving too early or too late result in a behavior called jitter. Figure 2 on page 40 depicts the effect that jitter can have on an IPTV stream. As illustrated, the ﬁrst stream pat- April 2008 | broadcastengineeringworld.com 39 http://broadcastengineeringworld.com

Table of Contents Feed for the Digital Edition of Broadcast Engineering - April 2008

Broadcast Engineering - April 2008 Contents Editorial HD Newsrooms Wavelet Compression Mobile TV Audio Processing for HDTV, Part 1 QoE for IPTV End Users NAB Update Asset Management Advertisers Index

Broadcast Engineering - April 2008

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