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.