Agilent Measurement Journal - Issue 4 - 2008 - (Page 31)

TV service is particularly demanding on the control-plane side because specialized routers must terminate and interpret control-plane packets in order to select channels. Reaction time between receiving a channel join request and commencing service-packet delivery to a user is a vital measurement, especially at high scale (Figure 1). By its very nature, TV can cause coordinated subscriber behavior (e.g., commercial breaks stimulate channel surﬁng) that can cause large peaks in the signaling load. Looking at data content, TV also is the most difﬁcult service to accommodate in the converged IP network. To conserve network bandwidth, TV service needs to use a network mechanism called multicast in which the trafﬁc streams associated with each available channel are sent to a manageable number of replication points distributed throughout the network. From a test point of view, the intermediate routing devices need to be tested to ensure they can properly forward multicast packets. To facilitate that, the test equipment must be capable of emulating the control protocols used to relate video sources to multicast replication points. Even with multicast, it’s necessary to minimize bandwidth per TV channel because subscriber connections are usually limited to a few megabits per second. This requires extreme compression techniques (e.g., MPEG) that are particularly sensitive to packet loss and delay variations. Test trafﬁc needs to simulate the structure of such compressed video. Simulating TCP trafﬁc in particular poses a huge challenge. For routers to recognize and hence prioritize a TCP connection associated with a particular customer, the connection needs to look “real.” To achieve this, test trafﬁc must simulate the handshaking and retransmission mechanisms that deﬁne TCP over long sequences of packets. It also must do so at a realistic scale of tens of thousands of active TCP connections on a single router port. The requirements for realism don’t stop at the TCP level because the purpose of TCP connections is to carry web pages (HTTP), ﬁles (FTP) or peer-to-peer trafﬁc. What’s most demanding is simulating large numbers of fully realistic transactions in a predictable and consistent manner. As with any test stimulus, a lack of precision or determinism put into the system-under-test adds uncertainty to every measurement of what comes out. Therefore, accurate simulations are exceptionally difﬁcult to achieve at high scale without resorting to processor-based schemes. Unfortunately, such approaches lack determinism and also struggle to provide real-time variation in trafﬁc load, which makes it difﬁcult to quickly Sustained performance test: average join latency versus time Test duration = 0:01:30 (hh:mm:ss) Measure interval = 5 seconds Maximum Average Minimum 660 627 594 561 528 495 462 429 396 363 330 297 264 231 198 165 132 99 66 33 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Join latency (ms) Time (s) Figure 1. Measurements of channel-join latency versus time help gauge QoS in IPTV systems. 10010010001110101110100100011010111010110011100010101001110101011010100101101001110100101101101001 10100100100011101011101001000110101110101100111000101010011101010110101001011010011101001011011010 Agilent Measurement Journal 31

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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