S122 SDS v1.0: Advanced Central connection timing

In some advanced use-cases, the Central may require links to be closer together than what is supported on the initiating PHY. This can be achieved by choosing a shorter connection event length and initiating the PHY update procedure to the faster PHY after establishing a connection.

When connection event length checking is disabled, the Central may create connections that have scheduling conflicts on the initiating PHY. This is illustrated in Too short connection event length causes scheduling conflicts. The central connections C0 and C1 alternate on blocking each other's timing-events. This degrades the average link performance.

Figure 1. Too short connection event length causes scheduling conflicts

Three links configured with a connection event length shorter than the required time to transmit a TX/RX pair on the active PHY. The links alternate blocking each other.

After C0 has been updated to a faster PHY, it no longer blocks the timing-events of C1. However, C1 and C2 are still in conflict, and their average link performance is degraded. See No scheduling conflict for first link after PHY update.

Figure 2. No scheduling conflict for first link after PHY update

Now the first central connection has switched to a faster PHY. The two remaining links will still block each other.

When all links have been updated to the faster PHY, the Central has achieved the closest packing of links that is possible for a given connection interval. In Optimal packing of four links and a scanner, all four central links are on the faster PHY, and a scanner is consuming the remainder of the available time.

Figure 3. Optimal packing of four links and a scanner

All central connections (C0-C3) are using the faster PHY. An additional Scanner (S) fills the gap between the last central connection and the start of the next connection interval.