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Video-on-Demand (VoD) systems are being implemented using IP over Ethernet switched transport systems. This paper reviews the capabilities of a combined Singulus G1-T, Tektronix MTM400 MPEG Transport Stream Monitor, and the Tektronix AD-950 MPEG Analysis Software in the installation, verification, and troubleshooting of a VoD system. Parameters such as MPEG protocol integrity, network packet jitter, loss, utilization, and other key parameters can be observed and measured. Measuring and observing how these parameters combine to affect video distribution quality is essential to successfully deploying large production VoD systems.
Introduction
Video-on-Demand implementations today combine packet switched Ethernet networks, cable television hybrid fiber and coax (HFC) distribution networks, system resource management servers, video servers, and various system support resources to deliver a growing number of video stream options to a user at his command. The quality of the user end point video is a function of the MPEG stream source quality, transport network characteristics, and server capacity. Impairments can originate in the video sources (servers or feeds) or the network infrastructure. Streaming video with its inherent sensitivity to delivery time distortions and packet loss is especially demanding on traditional packet switched networks that often have little provision for assuring Quality of Service (QOS). This application note will explore how to use Singulus G1-T and the MTM400 equipment to measure critical network and video source parameters. Typically such measurements are performed during installation and initial verification of a VoD system, to troubleshoot problems as they arise, and on a continuing performance verification basis.
Figure 1 – Simplified Block Diagram of a VoD Network
Figure 1
Figure 1 shows a simplified view of a VOD network. A customer can originate a video request via a set top box (STB) which is then forwarded to a resource manager for accounting, authenticating, billing, and ultimately, initiating video playback at the VOD server. This 2 to 20 Mb/s video stream is packetized for transport over the network infrastructure and delivery to the STB that decodes the MPEG stream for display.
Gaining Access to Network Traffic
To observe and measure base transport infrastructure, access to the live network data is required. Several means have been successfully employed including:
1. Optical Splitter. If the network infrastructure components are connected using optical fibers, passive optical splitters can be inserted at strategic network locations such as at the output of servers, the inputs and outputs of key switch/router components, and at high-speed uplink aggregation points. Such passive splitters are available for both multimode and singlemode fibers with various network and tap attenuations. Since they are passive and fairly economical, they may be inserted without much concern that they will adversely affect the performance or reliability of the network and they may be left in place for ready access to paths should the need for troubleshooting arise. The tap ports can then be connected to test equipment for flow analysis.
- Mirror ports. Many LAN switches provide an option to copy or “mirror” the traffic appearing on an input port to a selected output port on command. This feature can be used to make available a suspect traffic flow to test equipment if a spare switch port is available. This approach, like the Optical Splitter, should not adversely affect the performance or reliability of the network and can be made available on demand for troubleshooting should the need arise.
- Active tap. The Singulus G1-T is an active Ethernet network tap device. Its two ports allow the Singulus G1-T to be inserted in an active link to analyze flows with a minimum of disturbance to the flows. Since each port can be configured for copper or either multimode or singlemode fiber, all common interconnects can be accommodated. Interface speeds can be either 10, 100, or 1000 Mb/s, but both ports must be operated at the same speed. Note that the Singulus G1-T must remain powered and in the TAP mode to prevent interruption of the connected link.
Where to start
A systematic approach is recommended beginning with the characterization of the program source and then following the program flow through the distribution system by tapping the flow at strategic locations using one of the approaches mentioned above. This allows for measuring the accumulating incremental degradation imposed by the interconnect on the flows as well as the identification of any faulty or misconfigured components. Finally, the decoder’s tolerance to a flow’s impairments can be determined.
What to Measure and How
The Singulus G1-T provides the capability to measure the following key network parameters:
· Ethernet packet parameters such as size, network load, CRC errors, arrival time
distribution, addresses, upper layer protocol types, etc.
· IP packet parameters including Checksum errors, packet type identification and frequency
for protocols such as ARP, ICMP, etc.
· MPEG packet payload analysis including packet sizes, stream presence identification by PID,
IP address and UDP port number, stream rates, MDI including DF and MLR, arrival time
stamps, network utilization by stream.
It also provides the following analysis features:
· Event definition, configuration, and logging
· Stream recording with timestamps for later analysis or playback.
· Stream play back using a software decoder for program identification and verification.
· Built in ASI port is always available for connection to MPEG analyzer or external decoder.
· MPEG test stream source for up to 99 streams
· Customizable, full media bandwidth network traffic loading source
· Multiple standard and user-customizable protocol decodes
· Hardware counters for line rate Gigabit Ethernet statistics such as traffic rates, good and
corrupted packets.
· Hardware filters for capture grooming and custom counters.
· Low power operation.
· Compact portable design for easy field use.
IneoQuest Technologies Singulus G1-T
The Tektronix MTM400 provides real-time MPEG-2 monitoring in applications requiring the continuous monitoring of the transport stream layer. The MTM400 provides the capability to measure the following key MPEG, DVB, and ATSC parameters that are based upon ETSI TR 101 290 Priority 1, 2, and 3.
Priority 1
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Priority 2
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Priority 3
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1.1 Ts_sync_loss
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2.1 Transport error
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3.1a NIT_actual_error
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1.2 Sync_byte_error
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2.2 CRC_error
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3.1b NIT_other_error
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1.3a PAT_error_2
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2.3a PCR_repetition_error
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3.2 SI repetition error
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1.4 Continuity_
count_error
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2.3b PCR_discontinuity_
indicator_error
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3.4a Unreferenced PID
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1.5a PMT_error_2
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2.4 PCR_accuracy_error
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3.5a SDT_actual_error
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1.6 PID_error
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2.5 PTS_error
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3.5b SDT_other_error
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2.6 CAT_error
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3.6a EIT_actual_error
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3.6b EIT_other_error
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3.6c EIT_PF_error
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3.7 RST_error
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3.8 TDT_error
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Measurements with priorities one and two are related to the MPEG-2 definitions, while level three is specific to DVB EN 300 468 or ATSC A.65.
One of the most important tests performed by the MTM400 is the PCR_OJ measurement. This measurement is defined by ETSI TR 101 290 (Annex I), and is beyond the requirements of Priority 1, 2, and 3 testing. The reason for the importance is because of the packetizing characteristics of MPEG over IP. An IP packet is normally created by concatenating seven MPEG-2 transport packets. Each IP packet will be sent over the network at its full rate. On the receive side, each IP packets needs to be disassembled and rate reduced using an algorithm to smooth the bursty nature of IP traffic. Introducing jitter into the MPEG transport may cause timing problems within the set top box.
Transport Stream recordings provided by the MTM400 include a precision time stamp for each transport packet allowing for accurate jitter and arrival time measurements within the AD-950 offline analysis software.
- Additional options for diagnostics and troubleshooting include Triggered recording, Service Information Analysis, Template Testing, Bit Rate Testing, In-depth PCR analysis, and Service bandwidth logging.
Tektronix MTM400
Following the connection of the Singulus G1-T to the network observation point of interest, the following sequence of measurements is suggested:
- General Traffic Analysis
Enter Monitor mode. The Ethernet statistics displayed will indicate if there is Physical or Link layer data corruption such as FCS errors, Alignment errors, Jabbers, Fragments, Symbol errors, etc. in the Rx Total Errors group. These types of errors can be the result of network equipment failure, interconnect plant failures such as connector or cable faults or electrical environment interference. Independent sets of counters are available for data flows in each direction on the link (port 1 to port 2 and port 2 to port 1). Instantaneous frame rates show network load levels. At high loads there is a greater likelihood of packet arrival time distortions which can be investigated further using the MDI measurement discussed below. Packet counters classified by frame size give an immediate indication of the type of traffic present. 1362 Byte MPEG over UDP frames might be expected to be the majority of traffic present, for example. By reviewing the Etherstat counters, significant counts of unknown or unexpected traffic such as high counts for ARP, ICMP, or broadcast traffic in general can indicate network configuration issues.
Network Stream Analysis
Enter MPEG Analysis mode. Right click in the Available Streams window and select “Watch for New Streams.” Any MPEG streams present on the network will be listed in the Available Streams window within a few seconds along with their IP addresses, Port numbers, and Bitrates. A stream may be selected for more detailed analysis by double clicking on the stream line in the Available Streams window which activates the tabs in the lower section of the window. Note that after a stream has been selected for analysis, the stream census will not be updated until the analysis mode has been terminated.
To verify that the expected stream has been selected, select the Record tab, select Start Record, then Stop Record, then Post Process Raw Data, and then Launch MPEG Player as the screen shown below indicates. The program segment just recorded by the Singulus G1-T will be played in the software MPEG player for convenient viewing and program confirmation. Alternatively, after double clicking on the stream line in the Available Streams window, choose the Send Out ASI tab. Selecting the Start ASI Stream option will output the stream to the rear mounted ASI BNC connector for attachment to an external decoder for viewing and confirmation. The ASI output is available continuously while other analysis is performed as described below.
Now that the stream of interest has been selected and the selection has been verified, return to the Media Delivery Index tab for a more in-depth analysis of the stream per the screen already shown above. The measured Average Rate and Deviation (Dev) from the MPEG encoded bit rate in Mb/s is shown. Significant deviations usually indicate server configuration issues or possibly stream encoding problems. The measured Min Pkt Size and Max Pkt Size showing in this window should be checked to confirm that the expected packets and only the expected packets are being received as part of this stream. For example, an MPEG-2 constant rate stream with seven 188 byte Transport Stream Packets per Ethernet packet with UDP/IP encapsulation should result in a stream’s constant packet size of 1362 bytes. Mixed size packets may indicate the presence of a misaddressed MPEG or non-MPEG IP stream.
Note the MDI value on the right side of the window. It is composed of two parts: the Delay Factor (DF) and the Media Loss Rate (MLR). It is displayed as DF:MLR.
If DF is high (Say, over 10 ms for a 2.5 Mb/s stream on a Gigabit Ethernet. See the “Measuring Quality of Video-Over-IP” reference for more details.) and MLR is 0, then significant packet jitter is present on the network but packets have not been lost up to the measurement point. However, this can result in downstream buffer overflows or underflows that will result in packet loss and picture blocking.
If DF is low but MLR indicates packet loss (which can be confirmed by checking the accumulated CC Errors displayed), then the MPEG Transport Stream may be corrupted. If MLR indicates packet loss and DF is high, packets are likely being lost in the network path. Tracking MDI through strategic network locations will indicate the source of the loss.
Other dashboard displays indicate MPEG stream statistics such as:
o Instantaneous Flow Rate (IFR) and Instantaneous Flow Rate Deviation (IFRD). The
measured IFR and IFRD confirm a stream’s nominal rate and, if not constant over time,
gives insight into how a stream is being corrupted.
o Average Rate in Mb/s. This measure indicates whether a stream’s rate being analyzed
conforms to its specified rate over a measurement time. This is the longer term
measurement of IFR.
o Stream Utilization in percent of network bandwidth. This measure indicates how much of
the available network bandwidth is being consumed by the stream being analyzed.
For more intermittent corruptions or long term event tracking, right-click the Port line for the chassis of interest and select the Monitor/Log Options tab. Event severity and log options can be configured here. Selecting the View Log option will display a timestamped log of captured events. This can yield clues as to what event clusters caused certain types of problems. For example, multiple new streams starting up at the same time stressing the network system’s capacity may cause a high DF or MLR resulting in a poor picture display.
Selecting the Arrival Time Histogram tab provides a graphical view of packet arrival times. This view makes it easy to detect burst patterns that may overflow network or decoder equipment buffers resulting in lost data.
To capture and examine a stream’s trend information, select the Record tab, then Start Record, then Stop Record when sufficient data has been captured. Select Post Process Raw Data. Selecting Analyze Recorded File provides a graphical trend of arrival times. This is useful for analyzing how the network is transferring data. Burst frequency and inter packet gap (IPG) can be analyzed. A recorded clip may be stored for later playback and/or analysis by entering a file name in the Record tab’s Record File Information section.
3. System Margin
It is possible to detect lost packets in a network infrastructure by monitoring the links between network devices, but it is important to note that even if the infrastructure does not drop packets, it may affect delivery times to a video decoder such that the decoder buffer overflows or underflows resulting in packet loss within the decoder. Therefore it is equally important to monitor, log, and trigger alarm conditions based on packet jitter values. By tracking jitter values as network utilization grows, the margin for error due to buffer overflows or underflows will indicate how close the dynamic behavior of the network is to the limit at which visible errors occur. Using the ability to provide time delivery distortion on a transmitted file as described below, the maximum values that the network can handle can be determined. Once determined and logged, subsequent measurements with perhaps a growing traffic load will indicate how much margin is available. Event thresholds may then be set to cause alarms indicating shrinking margin but before a viewed stream shows corruption.
To transmit a test file, right click on the Stimulus line of the chassis display and select Send MPEG File. Select a file to be sent in the Select File section and select Download File To Target. The MPEG file can then be transmitted by selecting Start/Resume. Use Pause to temporarily cease transmitting or STOP to select a new MPEG file or to leave the Send MPEG File function. Use the Edit option to configure link, IP, or UDP address information to be used for the transmit stream. The transmit bit rate can be adjusted to stress the network links and/or decoder. The file may be sent in bursts by specifying the Inter-Burst Interval or the transmitted file may be corrupted with packet loss by selecting the Drop rate to determine how the network behaves in the presence of corruption.
4. Detailed MPEG Analysis
The Tektronix AD950 software provides conformance testing against the MPEG-2 13818-1 Transport Stream standard as well as the –2 Video and –3 Audio. Dolby-Digital (AC-3) is also supported as an elementary stream with MPEG-2. There are hundreds of syntax and semantic protocol measurements required ranging from the 0x47 sync byte presence, all of the way down to the video slice and macroblock verification.
The recorded file can be analyzed at the MPEG transport and elementary stream layer using the AD-950 MPEG Analysis software. When the IP measurements fall within specification, but the decoded video continues to generate a poor picture, then verification of the transport and elementary stream layer is required. The two most important tests within the four-byte transport header include the MPEG-2 synchronization byte and continuity counter. The presence of a non-0x47 sync-byte indicates a major problem in the multiplexer, or possibly a transmission error. The continuity counter is a simple 4-bit counter that increments by one (modulo 15) for each successive packet Id (PID) of the same value. A PID with a continuity counter jumping forward or backward by more than one indicates a loss of one or more packets.
Conclusion:
Streaming Video transported by packet switched networks is an emerging technology and requires a systematic approach to network performance measurement to assure quality. Assuring high quality at the end-points of such networks requires measuring and monitoring of the payload quality, packet loss,
packet jitter and its relation to buffer limits in the system components, and other network parameters such as instantaneous flow rate and network utilization. As higher numbers of streams are served, it is critical is that system parameter bounds are maintained. Measurement and monitoring at several points in the system may be required.
Further Reading:
1. “Measuring Quality of Video-Over-IP”,
http://www.ineoquest.com/downloads/index.shtml 2. “A Guide to MPEG Fundamentals and Protocol Analysis (including DVB and ATSC)”,
http://www.tek.com/3. “PCR Measurements”,
http://www.tek.com/ 4. “A Layman’s Guide to PCR Measurements”,
http://www.tek.com/ 5. “Media Delivery Index”,
http://www.ineoquest.com/downloads/index.shtml James Welch - Ineoquest Technologies, Inc.
jim.welch@ineoquest.comDennis Kucera - Tektronix, Inc.
Dennis.w.kucera@tek.com
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