As more broadcasters are looking to use the internet and the cloud to contribute and distribute live video, they are challenged to maintain the quality of transport.

The internet, of course, was not architected with live video in mind, which leaves an unmanaged network prone to loss, jitter, and latency. The most valuable Tier 1 applications are at the most risk. Even the slightest issue of loss, jitter or delay leads to massive visual or revenue impact.

The established approach to overcome this challenge is using retransmission (ARQ) technology enabling broadcasters to deliver high quality video even when bandwidth is limited.

ARQ (Automatic Repeat Request) | Net Insight

Managing video over IP

Bandwidth is an important consideration when broadcasting over the public internet. The quality and stability of internet bandwidth vary greatly, and it is vital that it doesn’t have a negative impact on the video being consumed by viewers. Retransmission technology is often described as a groundbreaking solution to this problem.

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ARQ is a fundamental part of TCP, the most widely used protocol for IP delivery, and a part of the TCP/IP stack. Ultimately, this provides much-needed properties such as guaranteed delivery and fairness for users. Its sibling, UDP, is instead a ‘fire and forget type of protocol which is well suited for streaming applications with high throughput and low latency.

What is generally regarded as retransmission technology within media is a way to marry the two. The aim is to create a solution that is ideal for live streaming broadcast-quality video contribution over lossy networks.

Specifically, the UDP-based protocol leverages an error-correction mechanism for packet recovery called Automatic Repeat ReQuest.

Here’s how it works: If the receiver (server) recognizes a gap in the stream, it re-requests those packets from the sender (encoder) via a negative acknowledgment packet. Even though the sender and receiver remain in ‘conversation’ throughout the transmission to make sure all packets make their way to the finish line, only the missing packets get retransmitted. In this way, it doesn’t add unnecessary overhead or extensive latency. Since video is delay-sensitive and the receiver needs data to consume, the ARQ mechanism has a deadline. If lost packets have not been retransmitted and received before the deadline, the video stream is displayed with the potentially lost packets meaning a disturbance.

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ARQ and IP Media Transport

Video transport protocols using ARQ were developed to enable video providers to contribute and distribute video over standard internet connections. This makes it much more accessible, especially for live contributions from the field, as well as driving costs down as it negates the need for a private IP connection.

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Streaming protocols may prevent the following needing to be deployed:

Dedicated internet connections (hardwired or cellular):

  • Point-to-point network connections (fiber, MPLS, T1)
  • Cellular bonding services
  • Satellite uplinks

Looking at the demands that live broadcast video has on the underlying IP transport network, there are two fundamental properties that are specifically applicable to retransmission technology.

Loss sensitivity: Mostly related to the properties of the embedded codec, loss can be challenging for streaming media. In MPEG, as an example, losing an I-Frame means worst -case several seconds of outage depending on the GOP length. Other methods such as Forward Error Correction (FEC) can also be combined with retransmission to improve quality over lossy links.

Latency sensitivity: While this depends on the application itself, latency could end up being a deal breaker for using retransmission technology. In most cases the codec delay is the large overshadowing factor for the end-to-end chain. From a retransmission technology perspective, these are the two parameters that are constantly balanced against each other.

For retransmission technology itself there are also a number of other factors at play such as packet pacing and bandwidth variation – challenges that have been solved within the specifications of ARQ protocols.

The rule of thumb is: If avoiding loss is the primary concern, increase latency. If low latency is key, reduce latency at the cost of increased risk of impact to the video quality.

Another key concept of retransmission are control plane establishments. Due to the nature of the media environment, firewalls are often the rule rather than the exception. To overcome this challenge, most transport over IP solutions introduce what is often called a push/pull or caller/listener concept.

This is based on the idea that firewalls typically pass traffic outbound but block inbound. It means , that the device, or function, initiating the video channel should be behind the firewall, whereas the listener device is not. If both sides are behind a firewall traffic can still be passed, however port-forwarding is required. The caller/listener concept is a fundamental part of ARQ and provides a practical solution for most use cases.

ARQ (retransmission) technology uses additional bandwidth to resend lost information. On low-bandwidth links, adding protection using ARQ based streaming protocols has an impact on the amount of content that can be carried. Bandwidth savings typically range from 5%- 15%. This bandwidth can be re-used for error recovery, or, in a multi-stream scenario, it allows for additional content to be carried.

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Open Source Streaming Protocols

Because retransmission technology can easily be implemented in software, a number of vendors have developed their own technologies for internet contribution. Fundamentally these are based on the concept of retransmission in one form or another, but for professional broadcast networking, there are really only three implementations that warrant a closer look. These have been chosen because of either their reach and large ecosystems, or their open-source nature.

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These are Zixi (a proprietary solution), Secure Reliable Transport (SRT), and Reliable Internet Streaming Transport (RIST).

Having been early into the market, Zixi has a strong field-proven implementation that in addition to standard ARQ also supports video encryption, ARQ with FEC, link bonding, primary and secondary backups, and hitless mode.

However, both RIST and SRT have their implementation specifications published in the open, making them relatively simple to implement and offering interoperability with third-party solutions and networks.

As broadcasters need to interwork with multiple implementations to maximize their content reach, building solutions that can work across technology domains while still providing the same fundamental properties will be key

Net Insight believes we need solutions, products, and vendors that are flexible enough to work with more than one of these ARQ implementations. And we need products and solutions to bridge different technology domains in cases where two or more organizations wish to interconnect but are already invested in different primary retransmission technologies.

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Automatic Repeat ReQuest is a group of error control protocols for transmission of data over noisy or unreliable communication network. ARQ is also called with Retransmission because it provides automatic retransmission of frames that are corrupted or lost during transmission.

ARQ protocols are used to provide reliable transmissions of video and data transmission over unreliable (unmanaged) internet services.