Why do FreeSpeak ITPs require higher PTP tolerance for DECT RF systems

In the various Clear-Com documents it ask for a PTP value of ± 100 ns. Can you explain why such a tight tolerance is required

 

All AES67 / ST2110-30 networks require high bandwidth, guaranteed bandwidth to support audio streams, low network latency and very low network jitter. ST2110 IP networks use PTPv2 (IEEE-2008) for timing to ensure very low latency and minimum jitter.

Essentially the need for very small PTP jitter is because the PTP packets  – are used to derive RF DECT sync pulses.   

A DECT beltpack will sync itself to a DECT transceiver (TCVR).  Whilst it is locked to that transceiver it has a narrow window for which it can detect other neighbouring (within range) TCVRs.   If you do not tightly sync the TCVRS your beltpack will not be able to see neighbouring  TCVRs to roam to. The sync received at the TCVRs radio is called DECT sync.

 In order to generate a stable DECT sync signal at each TCVR, PTP is used.  If PTP is unstable then DECT sync is unstable and then the Beltpacks will not roam. To reduce the amount of PTP jitter, which effects DECT Sync generated at a TCVR, smaller audio packets should be used to allow the PTP and audio packets to be interleaved closer to their original time of transmission + transit time in an IP network. The varying transit time is jitter. This is why we recommend using PTP packets with a 125μs packet size when using FreeSpeak IPTs (TCVRs)

 

If PTP jitter is low then DECT sync stays within the tolerance window for a successful handover.   

If PTP jitter is high the DECT sync may sway outside tolerance window and beltpacks will not roam.*

*A beltpack can roam between two IPTs (TCVR) as well as within a single IPT .

A Dect IPT (TCVR) can access 10x DECT RF chnls in the EU (5x RF chnls in US) and each RF chnl has a number of timeslots.

A BP can roam to any of the 10 RF chnl and timeslots within a IPT or on another IPT.

The act of roaming requires the tight and stable PTP packet which is converted to a DECT Sync pulse

 

In practice the greater the OFM (Network and Network Equipment dependent)  the more likely jitter is seen at the TCVR, so it is best to keep this value low and constant with small packets flowing through a network.

 

To ensure minimum delay of the PTP clock and audio packets across the network we recommend the use of QoS and VLANS within corporate IT networks, along with the use of dedicated inter-switch trunk ports to carry only AES67 traffic. Alternately the establishment of a dedicated switch network should be considered for FreeSpeak IPTs