Do we still need 50 ms(milliseconds) restoration for running telecom services?

It was always exciting discussing 50ms switching/restoration time perspective for telecom circuits for every engineer who belongs to some part of telecom services including, optical, voice, data, microwave, radio, etc. I was also seeking it since the start of my telecom career, and I believe still somewhere at some point in time, engineers or telecom professionals might be hearing this term and wonder about why (“WHY”) is this? So, I researched over available knowledge pools, and using my experience, I thought of putting it into words to enlighten some of my friends like me.

The 50 ms idea originated from Automatic Protection-based Switching subsystems during early digital transmission systems. It was not actually based on any particular service requirement. The value persists because it is not entirely based on technical considerations which could resolve it, but has roots in historical practices and past capabilities and has been a tool of certain marketing strategies.

Initially, digital transmission systems based on 1:N APS typically required about 20 ms for fault detection, 10 ms for signaling, and 10 ms for the tail-end transfer relay operation, so the specification for APS switching times was reasonably set at 50 ms, allowing a 10 ms margin. 

For information, early generations of DS1 channel banks (1970s era) also had a Carrier Group Alarm (CGA) threshold of about 230 ms. The CGA is a time threshold for the persistence of any alarm state on the transmission line side (such as loss of signal or frame synch loss) after which all trunk channels would be busied out. But the requirement for 50 ms APS switching stayed in place, mainly because this was still technically quite feasible at no extra cost in the design of APS subsystems. 

The apparent sanctity of 50 ms was further entrenched in the 1990s by vendors who promoted only ring-based transport solutions and found it advantageous to insist on 50 ms as the requirement, effectively precluding distributed mesh restoration alternatives under equal consideration start of the SONET era. 

As a marketing strategy, the 50 ms issue served as the “mesh killer” for the 1990s as more traditional telcos were bought into this as reference.

On the other hand, there was also real urgency in the early 1990s to deploy some kind of fast automated restoration method relatively immediately. This lead to the quick adoption of ring-based solutions which had only incremental development requirements over 1+1 APS transmission systems. However, once rings were deployed, the effect was to only further reinforce the cultural assumption of 50 ms as the standard. Thus, as sometimes happens in engineering, what was initially a performance capability in one specific context (APS switching time) evolved into a perceived requirement in all other contexts.

But the “50 ms requirement” is undergoing serious challenges to its validity as a ubiquitous requirement, even being referred to as the “50 ms myth” by data-centric entrants to the field who see little actual need for such fast restoration from an IP services standpoint. Faster restoration is by itself always desirable as a goal, but restoration goals must be carefully set in light of corresponding costs that may be paid in terms of limiting the available choices of network architecture. In practice, insistence on “50 ms” means 1+1 dedicated APS or UPSR rings (to follow) are almost the only choices left for the operator to consider. But if something more like 200 ms is allowed, the entire scope of efficient shared-mesh architectures becomes available. So it is an issue of real importance as to whether there are any services that truly require 50 ms.