Digital fibre and distributed CMTS provides a weaker video viewing experience…

…but this can be solved if the components are designed carefully.

First of all, I find it funny to call fibre either analog or digital. Like fibre would know what kind of light it carries. In the cable TV industry, the wording ’digital fibre’ is often used together with the next generation DOCSIS architectures that split traditional CMTS functionality in HE functionality and in remote functionality. In these new architectures, aka distributed data access, the ’digital fibre’ carries data traffic deeper into the network. What is funny is that current ’analog fibres’ also carry, in digital format, data that is modulated and upconverted at the headend. In the following text, I use the terms ‘RF data’ (instead of analog fibre) and ‘baseband data’ (digital fibre) to point out the difference, although they are not 100% correct terms either.

When digital video, in a baseband format, is carried from headend to the deeper levels of the network, it faces new challenges.

In IP networks, multicast video is transmitted over the UDP (User Datagram Protocol) layer, which is unreliable by default. While TCP (Transmission Control Protocol) is able to request retransmission, the UDP doesn’t. Traditional video transmission over UDP does not use forward error correction and fixing bit errors becomes impossible. Some faulty bits mean that several video packets can be lost, which again means that end customers will see pixels. The UDP layer is not able to understand if data packets arrive in the wrong order. To solve it, a receiver must support other protocols or methods, e.g., RTP (real time protocol). In old “RF data” solutions, the FEC encoding is already at the headend and set top boxes are able to fix faulty packets. The whole access network’s transmission is covered by the FEC. In the distributed DOCSIS architectures, this same error correction only covers the last mile of coaxial cabling.

While distributed CMTS architectures are still under construction, the centralized architectures currently demonstrate higher speeds every day. Optical components improve and around 10 Gbit/s over single wavelength is already on the horizon. Although distributed architecture technologies will solve many current issues, they are not yet mature technologies like traditional RF data. Once distributed architectures reach a point where they are solving all current issues that centralized architectures may have, the next thing on the list is to solve all new issues that centralized architectures do not have.

Teleste is continuously developing and studying distributed CMTS solutions to solve video transmission and other issues that new architectures will introduce. Apart from this, we are also developing our RF fibre offering further; this technology is still rapidly evolving. The RF fibre has been a part of the cable data triumph, something that distributed architectures cannot put on their CV, at least not yet.


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Arttu Purmonen

I joined Teleste in 1997 and engineered video processing and data transmission products. I have worked as engineer, project manager, product manager and business director but understanding customer perspective has always motivated me. It brought me to be responsible for system and technical marketing where my internal driving force and former experience can party together. See my LinkedIn.

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