When you need parts-per-billion frequency accuracy, “Let's synchronise our watches” doesn't cut it. Take LTE and 5G for example: they need tight synchronisation, both in frequency and phase, and that makes time-signalling an important part of the network.
CableLabs, the US cable industry's research arm, is proposing a technology to help with synch, because it's a key part of mobile backhaul.
The R&D team's distinguished technologist for wireless Jennifer Andreoli-Fang said the rise of microcells to the use of GPS as a synchronisation standard: adding high-resolution GPS sync to microcells is relatively costly, and the devices are often deployed indoors where they can't see the satellites.
Just how tight those timing requirements can be is shown in this table from CableLabs:
|Frequency Sync||Phase Sync||Latency|
|LTE FDD||± 50 ppb (parts per billion)||-||User plane, varies|
|LTE TDD||±50 ppb (wide area)
±100 ppb (local area)
±250 ppb (home)
|10 µs (wide: cell radius >3km)
3 µs (local: cell radius <3km)
1.33 µs + Tprop (home eNB radius >500m)
3 µs (home eNB radius <500m)
|User plane, varies|
|CoMP||n/a||± 1.5 µs||< 5 ms|
|eICIC||n/a||± 1.5 - 5 µs||< 5 ms|
|5G TDD||± 50 ppb (wide area)
±100 ppb (local area)
|≤ ± 1.5µs||User plane, varies|
Andreoli-Fang wrote that CableLabs is proposing its 2012-authored DOCSIS Time Protocol to solve the problem, and yesterday, it published (PDF) “Synchronization Techniques for DOCSIS Technology Specification” – which also happens to mark the group's first entry into applying DOCSIS specs to mobile applications.
The spec addresses a problem specific to using cable networks to provide backhaul: their asymmetry hamstrings PTP, the Precision Time Protocol (IEEE 1588-2008).
In the specification, CableLabs explains that, courtesy of the ITU, there are standards for carrying IEEE 1588 timing from a master clock to a mobile base station – but only if the kit between the two points is Ethernet.
Rather than create a wholly new time protocol, what CableLabs has specified is a way to translate between PTP and its existing DOCSIS timing, so "the DOCSIS-based timing distribution chain [can] fit into the standard ITU-T synchronisation framework".
The framework has:
- Full timing support for phase synchronisation distribution, if all network elements are IEEE 1588-aware;
- Partial support for phase synchronisation if some network elements don't support IEEE-1588; and
- Physical layer timing support, if the network supports Synchronous Ethernet and the end application only needs a Layer 1 clock.
The DOCSIS Timing Protocol provides two services needed to stop asymmetry from breaking PTP: there's a hardware path to carry timestamps, and a signalling path which “determines the downstream timing offset, which is used as a correction factor for PTP”.
Where Ethernet is in the timing distribution chain, some of the Ethernet network's error budget is reallocated to the DOCSIS end of the network, the specification explained, to maintain an end-to-end ±1.5µs error budget. ®