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Digital Television Modern Systems Monitoring

20 February 2012
Provision of full “triple-play” service has nearly become a standard practice for broadband operators. Traditional broadcast networks of digital cable TV allow the arrangement of data transfer and telephony with use of DOCSIS technology. However, due to limited frequency resource, there exists a problem of ensuring the high exchange rate during extension of subscriber base. Further upgrade of such utilities – in particular, transition to new DOCSIS 3.0 standard – requires the substantial financial investments.

At the same time, it has been a while since Ethernet technology was used in local data transmission networks only. Due to substantial improvement in performance of active equipment and reduction of its cost, Ethernet standard is actively utilized to build the modern broadband networks. Use of fiber-optic technology at backbone and distribution sectors enabled the significant increase of bandwidth for such networks. Transmission rate ensured by service providers for end users is essentially sufficient for such resource-intensive traffic as IPTV (Internet Protocol Television) streams of standard and high quality. IPTV service provision enables the competition between small Internet providers and traditional cable operators, while providing the user with the same packages of TV programs that are also added with extra range of services that are not available to clients of cable TV operators.
Except for enabling the optimization of users’ expenses for received content, TV services of the next generation also ensure the access to new potential video services ("video on demand", "time-shift ordering of program", "network VCR", etc.).

IPTV Network Building

For IPTV technology implementation it is necessary to build up network (Fig. 1), comprising of the head-end IPTV station, data communication network itself, and subscriber terminals. Also in order to provide traditional TV services special IP/DVB-gates may be located on the network (Arris Edge-QAM D5, Appear TV etc.). These gates convert IPTV streams into signal of either digital video standard (DVB).

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  Fig. 1  IPTV network structure

However, not all Ethernet networks are feasible to provide optional IPTV services. During implementation of video-services network should meet following requirements:
• all compounds must support Internet Group Management Protocol (IGMP) v. 2 or higher.
• IGMP query function support by the router or switch, which receives all multicast streams from all sources.
• If multicast stream routing is foreseen, it is necessary to provide Protocol Independent Multicast (PIM) support.

Though, even with modern Ethernet broadcast networks it may be difficult to provide high-quality video signal due to the specifics of packet data communication network performance. Wrong network calculation, configuration of network equipment, time packet delays, noise interference of copper lines and other troubles lead to visible deterioration of the picture, hanging, picture “dispersion” and etc.

IPTV system like any other data communication system works properly only when:
• Data packages reach the end user;
• They arrive timely and in correct order.

Therefore, it is essential for IPTV operator to objectively and promptly determine the reason for de-rating the video signal, and watch the quality parameters change with time. Practice of IPTV networks operation shows that search for a solution to some problems can be time consuming, and sometimes even impossible without the use of specialized diagnostic tools.

Organization of monitoring within IPTV networks
Let’s consider the generalized scheme of the signal passage from the source to the consumer and the stages of quality control (Fig. 2). At the first stage, the "ideal" non-compressed signal (270 Mbit/s in case of standard quality) goes to the encoder to convert it into a format suitable for transmission over the network. Naturally, reduction of the flow rate, for example, 100 times, causes an irreversible deterioration of the quality of video signal. Then this encoded channel enters the data transmission network, from which it reaches out to the end user, where the signal is decoded by the subscriber terminal - set-top box (STB). 

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Fig. 2 Quality control scheme for IPTV network

Image quality can be affected only by operation of the coder on the transmitting side (it all depends on the degree of compression and quality of the used codes), and set-top box. These elements are checked only once at the initial stage before they are installed into the network. In order to recovery the signal correctly at the receiving side, it is necessary that all UDP (User Datagram Protocol) packets, formed by the source, received by the recipient on time and in proper sequence. Therefore, the network must be constantly monitored.

Control jitter parameters in IPTV networks

All UDP packets arrive at the receiver one by one, after a certain period of time, Inter-arrival time (IAT, time interval between arrival of the packets). This time interval should ideally be constant. But in reality it is constantly changing positively or negatively (Fig. 3), and can reach unacceptable values.

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Fig. 3 Jitter of UDP packets

Change of the packets arrival time interval relative to the average value is called a jitter. According to standard ETSI TS 102 034, difference between the maximum deviations towards positive and negative side should not exceed 40 ms. Otherwise, buffer over- or overflow of the subscriber set-top box is possible, that can lead to visible image defects.

Increased jitter may be caused by different reasons, but the main of them are as follows:
• Misconfigured routers or switches (no priority by QoS, multicast flows programming, equipment overloading).
• Noise pollution of DSL lines, it results in formation of queues and buffer overflow on the station DSLAM multiplexers.
• Different time of passing between the routers, which may result from different routes of the packets of the same flow.

Let’s consider the jitter mapping by measuring equipment by the example of analyzer flow interface BridgeTech VB120 via network monitoring server VideoBridge Controller (Fig. 4). The most demonstrative method for jitter mapping is a histogram showing the number of packets. The normal pattern for jitter is a histogram is in the form of a normal Gaussian distribution of variance not exceeding 20 ms, i.e., half of the maximum jitter. As we can see in the illustrations for the first flow, the maximum jitter is less than 2 milliseconds, which is negligible compared with the threshold value.
  

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Fig. 4. Jitter mapping by flow analyzer BridgeTech VB120


Fig. 5 illustrates a different situation for the same network. The difference is that the first diagram is constructed for the flow of constant rate (bit rate) and the second one – for the variable rate. Change of the flow rate at the encoderoutput results in a significant change in the UDP packets sending interval, and often exceeding of the 20 ms threshold. Such scheme cannot reflect the real situation. Since the provider does not always has the opportunity to equalize the flow rates, which are mostly broadcasted with a variable bit rate, a reference flows generator BridgeTech VB230 with a constant rate is used for control of variations introduced by the network .The generator is installed at the head end, and the jitter values are read at the control points by means of the analyzers of series VB12, VB120, VB20 or VB220.

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Fig. 5 Jitter mapping in the network with a variable bit rate

Definition of error parameters in IPTV networks

Since on of the monitoring system objectives is control the packet delivery to the end subscriber, the second important parameter is the rate of packet loss or MLR (Media Loss Rate). When the transmitting equipment forms a UDP packet, it is usually encapsulated with up to seven 188-byte transport packets TS (Transport Stream), thus loss of at least one UDP packet will result in visible image defects. Standard ETSI TS 102 034 allows packet loss rate not more than one an hour.
The most common causes for MLR are as follows:
  • • Overflow of the network equipment buffers.
    • Interference in communication lines resulting in CRC errors.
    • Change of the packets sequencing.
    • Inconsistency between the routers, bad checksums (often found in the networks built with equipment of different generations and different manufacturers).
    • DoS attacks.
    • Going beyond the threshold number of subscribers using resource-intensive services (e.g., 90% of users watch video on demand).

An important quality parameter, derived from IAT, is a factor propagation within the transmission medium MDI-DF (Media-delivery index delivery factor). It is calculated as the maximum IAT value for measurement period 1 s.
If, during the second measurement interval there was an IAT surge beyond the threshold, or there was a packet loss (MLR), then the second is considered to be false (Errored Second, ES).
Since the network parameters may fluctuate over time, it is important for the operator to estimate the IPTV quality parameters distribution over time. An example of such a successful solution can be the quick display technology for flow status BridgeTech MediaWindow (Fig. 6), implemented in all the equipment of line VB IP-PROBE and microVB. The essence of this technology is that the horizontal axis is the time axis, and vertical axis is bi-directional: DF values (or size of the occupied bandwidth) are laid off along positive direction, and MLR – along negative direction. If the set thresholds are exceeded, the columns are colored yellow or red, depending on the extent of the threshold crossing.

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Fig. 6 Flow status display interface in time (BridgeTech MediaWindow)

Information display in monitoring system BridgeTech

The display method for statistical information about flows is very important. It should be demonstrative, easy to read, allow to asses the flow transmission quality, frequency and extent of errors at a glance. It is convenient when information is given in a graphical form for several channels at a time (Fig. 7), as it reduces the need for multiple switches in the user interface.
With the help of technology BridgeTech MediaWindow, the input jitter can be measured even for flows with a variable bit rate, by comparing their DF values immediately after the source and at the control point on the network.

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Fig.7 Distribution of jitter parameters in time for different flows


Monitoring equipment for various network sections

Ideology for building an effective monitoring system involves placing the monitoring equipment in the following network points.
1. Head-end
 

In this case (Fig. 8) the main tasks of the system are as follows:
• Quality control of the signal source (it includes monitoring of flows from the receiver/descriptor, as well as visual inspection of the image for presence of fading pictures and sound).
• Monitoring of the subscribers connection status (it often happens that the provider learns that the card did was not prolonged, and thus the channel was no longer open, from the people who called the customers call-center, instead of noticing the described problem earlier and taking the problem correction measures before it is noticed by a large number of subscribers).
• Control of the flow processing system (scramblers, transraters, multiplexers, etc.).
• Generation of CBR test flows for monitoring the jitter introduced by the network at different sections.
• Gathering of information from all devices of the network monitoring system. This function, in particular, is performed by server Controller.

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Fig. 8 Information gathering from head-end by monitoring system VideoBridge

2. Backbone and access network

The main objective for installing the analyzers in this case is control of jitter and loss introduced by network equipment in the network hub, as well as switches and routers access level. As previously mentioned, due to noises in communication lines at some sections, packet queues may form in the routers, which can cause their delay and loss. It shall be also noted that equipment installed in the network hub must be able to handle traffic in different virtual local networks (VLAN), and also to perform simultaneous monitoring of the large quantity of IPTV flows. An example of such device can be analyzer BridgeTech VB220, which can analyze up to 260 flows simultaneously.
Analyzer BridgeTech VB20 can be used on the other side of the backbone (Fig. 9), it allows control of flows quality on the access switch or DSLAM (access multiplexer of digital subscriber loop xDSL).

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Fig. 9 Information gathering by system VideoBridge on the backbone and access network

3. Subscriber equipment

Monitoring equipment installed in the customer premise is primarily aimed at addressing the problems which occur either due to mishandling of subscribers themselves, or due to problems associated with the "last mile". Thus, cost cut is achieved by reducing the number of technicians’ visits on subscriber equipment installation site. Installation of monitoring devices of such type should not require a highly skilled installer; there should also be a possibility to install it by the subscriber himself. An example of such a decision is BridgeTech microVB analyzer installed at the subscriber side, into the subscriber line rupture (Fig. 9), which can register in the network monitoring system on its own at turn-on.

Organization of monitoring in digital broadcast networks

Monitoring system BridgeTech is fundamentally suitable not only for use on the IPTV networks. IP network can be also used as a transport network for construction of regional and local networks of digital cable television (on-air, cable). In some segments of these networks it is important to control the signals parameters transmitted in the formats DVB-C, DVB-T, DVB-S (Fig. 10).
It is necessary to control the parameters of radio signal itself at the output of multiplexer and modulator (SNR, BER, MER, signal level), and check the contents of service information transmitted in the flow for compliance with ETSI ETR 101 290. These features are implemented in the line of devices VB120/220 with the use of additional interface modules for operation in broadcast networks of standards DVB-S/S2 (VB270), DVB-T (VB250), DVB-C (VB260, VB262).

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Fig. 10 Signal quality control in networks DVB-C/T/S

Generating a report on the monitoring results

There should be a possibility of generating the informative reports to provide information about the network operation and quality of services provided over a specific period of time. Fig. 11 shows example of the report page generated on the monitoring server VideoBridge Controller. It displays general information about the generated report, shows graphs displaying the percentage of channels on which errors appeared («no signal», MLR and DF). Time history curve for parameters IAT, MLR and «no signal» (with indication of the number of erroneous seconds). Thresholds of key parameters set in accordance with SLA (Service Level Agreement) are provided as well.

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Fig. 11 Example of the report page generated by VideoBridge Controller

Thus, construction of an effective monitoring system is a complex task, and its every factor affects its efficiency to some extent and, consequently, the quality of services provided. The monitoring system should be flexible and scalable able to adapt to any provider’s requirements. Such feature is characteristic for decisions based on equipment of BridgeTech Company, whose official representative in Ukraine is DEPS Company.

Systems Integration Department
DEPS Company.


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