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IPTV Technology Overview


The real-time nature of IPTV prevents (in most cases) the network from performing retransmissions to correct errors; the end user’s perceived quality of experience (QoE) may therefore be affected on a number of different levels. Independent studies have shown that contrary to voice service customers, IPTV subscribers are not expected to compromise on the quality of their service, thus the signal quality across the IPTV network must be routinely tested or monitored to minimize and quickly resolve potential threats to service revenue.

IPTV is all about consistent image quality

Figure 1. Eliminating video disruption and pixelization is the key to retaining subscribers.

IPTV Network Topology

IPTV technology is part of a new breed of services designed to facilitate access to video entertainment. It provides access to digital TV over the IP transport medium from a head-end device to the end user’s TV set-top box (STB). Most service providers use a dedicated transport network to support IPTV.

A typical IPTV network is comprised of the following functional blocks (see figure below):

  • National headend: Where most of the IPTV channels enter the network from national broadcasters
  • Core network: Usually an IP/MPLS network transporting traffic to the access network
  • Access network: Distributes the IPTV streams to the DSLAMs
  • Regional headend: Where local content is added to the network
  • Customer premises: Where the IPTV stream is terminated and viewed

General IPTV network architecture

Figure 2. General IPTV network architecture

Broadcast information coming from an antenna or a satellite dish at the national headend is mainly distributed using MPEG-2 multiprogram transport stream (or MPTS) to the video service node. Note that other more efficient, less bandwidth-hungry compression algorithms such as H.264 (MPEG-4 Part 10) or the Society of Motion Picture and Television Engineers (SMPTE) 421M (also known as VC-1) are making their way to the marketplace to complement this first offering.

The distribution of the actual SDTV or HDTV channel content is performed using various devices on the access network. Among these devices, digital subscriber line access multiplexers (DSLAM) as well as other technologies like fiber-to-the-home (FTTH) can be used to interface with the user’s STB. For IPTV, each channel is distributed using a multicast IP address.

Factors Affecting Service

Encoding and Compression

The quality of the video being distributed across the network can be affected right at the source; i.e., at the video headend. The encoding and compression process usually creates a trade-off between the quality of the video and the desired compression level.

Jitter

Defined as a short-term variation in the packet arrival time, typically caused by network or server congestion. If the Ethernet frames arrive at the STB at a rate that is slower or faster, as determined by the network conditions, buffering is required to help smooth out the variations. Based on the size of the buffer, there are delivery conditions that can make the buffer overflow or underflow, which results in a degradation of the perceived video.

Limited Bandwidth

As core IP infrastructure is usually based on optical networks with a low level of congestion, bandwidth limitations (and the total amount of video-stream data that can be sent) is limited mostly by the access network or the customer’s home network supported rate. When traffic levels hit the maximum bandwidth available, packets are discarded, leading to video quality degradation.

Packet Loss

Loss of IP packets may occur for multiple reasons—bandwidth limitations, network congestion, failed links and transmission errors. Packet loss usually presents a bursty behavior, commonly related to periods of network congestion.

 

Quality of Experience (QoE)

The basic structure of Ethernet and IP networks means that the quality of the video/audio traffic is primarily influenced by network jitter and packet loss. Considering the type of video encoding that is used in MPEG or other similar compression algorithms, the actual impact on user perception will depend on the packet type that is lost in the network. In MPEG-2, the transported packets that are used to form an image are divided into I-frames, P-frames and B-frames. Simply put, I-frames contain a complete image, whereas P-frames and B-frames contain predicted information from the other frames.

Typical group of picture (GOP) relationship in MPEG

Figure 3. Typical group of picture (GOP) relationship in MPEG.

Figure 3 provides a sample of the relationships between the various types of frames included in a group of pictures (GOP). As shown, I-frames are independent and provide data to support the other frames; this means that an error in the I-frames will have more repercussions on the image being viewed than losing P-frames or B-frames.

Key QoE Parameters

Several metrics exist to quantify the impact of the network on the quality of the channel that is received by the end user. The most popular parameters are the media delivery index (MDI) as well as PCR jitter for MPEG-2 TS. Other parameters are also used in the IPTV network, but they typically require further packet inspection to collect the information necessary for deeper analysis.

IPTV is an evolving technology and is not governed by specific testing and monitoring standards. However, the aforementioned parameters serve as a basis on which to qualify a user’s quality of experience (QoE) for the IPTV services being transported.

Media Delivery Index (MDI) as a Testing Metric

The nature of an IPTV service has inherent characteristics that are the primary drivers affecting the quality of the image being viewed; namely, bandwidth availability, packet loss and jitter. The use of MDI as a testing metric provides users with the tools to measure and diagnose network-induced impairments for IPTV streaming media. MDI is the only standards-based (RFC 4445) video-quality metric available today and it is endorsed by the IP Video Quality Alliance.

MDI is comprised of two distinct measurements: delay factor (DF) and media loss rate (MLR), which together provide a QoS of the delivered media stream that can be directly correlated to the end users’ quality of experience.

Some of the key benefits of using MDI:

  • MDI does not perform any type of stream decoding to achieve its metrics and therefore does not require significant real-time processing power.
  • MDI can be used with encrypted media payloads.
  • MDI is not dependent on any one type of video-encoding technique, so it can easily be scaled to monitor video quality on hundreds of simultaneous channels.
  • MDI is typically sampled at multiple points throughout the stream path with the measurements serving as indicators of problems in the network that can be proactively addressed before they become service-affecting issues.
  • Since MDI relies on transport-layer metrics (DF and MLR), it can be used to set network margins and it directly correlates to impending network problems with respect to video quality.
  • Since MDI uses packet-level metrics, it helps validate the performance of network equipment such as switches and routers that play a key role in determining whether a packet is delayed or dropped.


Typical core-to-access IPTV testing application with media delivery index (MDI) measurement across the network.

Figure 4. Typical core-to-access IPTV testing application with media delivery index (MDI) measurement across the network.

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