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Samples: iso_connected_benchmark: Fix invalid latency

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The default transport latency was not valid.
Set it to a value that is large enough.
The minimum possible transport latency is 16580 us,
but this assumes the controller ignores the RTN=2.
If the controller uses the RTN=2, the transport latency will
be slightly larger than 30 ms depending on
how the controller chooses to setup the ISO link.

Therefore, we set it to 40 ms to ensure the default
configuration is valid.

How these numbers were found:

For unframed PDUs this is defined in Core_v5.4, Vol 6,
Part G, Section 3.2.2:
  Transport_Latency =
    CIG_Sync_Delay + FT * ISO_Interval - SDU_Interval

Because unframed PDUs are being used, the ISO interval needs
to be a multiple of SDU interval.
If MaxPDU = MaxSDU and the PHY is 2M, this results in that
the maximum subevent size is:
2120 * 2 + 2 * 150 = 4540.
For 2 CISes this is 9080 us > 7500
Therefore, the controller has to set the ISO interval to a
minimum of 15000 us in order to fit the packets.

Based upon this info, the controller has the freedom to ignore
or use the requested RTN=2.
1. Ignore -> NSE is 2: Minimum ISO interval is 15000 us
2. RTN = 2, Increase NSE: 4 * 4540 = 18160 -> ISO_Interval = 22500 us
3. RTN = 2, Increase the FT to 2, NSE = 2 -> ISO_Interval = 15000 us

For (1):
  Transport_Latency = CIG_Sync_Delay + 15000 - 7500
                    = CIG_Sync_Delay + 7500
For (2):
  Transport_Latency = CIG_Sync_Delay + 22500 - 7500
                    = CIG_Sync_Delay + 15000
For (3):
  Transport_Latency = CIG_Sync_Delay + 2 * 15000 - 7500
                    = CIG_Sync_Delay + 22500

As shown in Core_v5.4, Vol 6, Part B, Section 4.5.14,
CIG_Sync_Delay needs to be larger than or equal to the length of
all ISO events. That is, the sum of the subevents:

For (1): CIG_Sync_Delay = 2 * 4540 = 9080
For (2): CIG_Sync_Delay = 4 * 4540 = 18160
For (3): CIG_Sync_Delay = 2 * 4540 = 9080

This results in the following transport latencies:

For (1): Transport_Latency = 9080 + 7500 = 16580
For (2): Transport_Latency = 18160 + 15000 = 33160
For (3): Transport_Latency = 9080 + 22500 = 31580

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>
This commit is contained in:
Rubin Gerritsen 2023-06-09 10:01:40 +02:00 • committed by Anas Nashif
commit 9cb339166e
1 changed files with 1 additions and 1 deletions
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2
samples/bluetooth/iso_connected_benchmark/src/main.c
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@ -30,7 +30,7 @@ enum benchmark_role {
#define DEFAULT_CIS_RTN 2
#define DEFAULT_CIS_INTERVAL_US 7500
#define DEFAULT_CIS_LATENCY_MS 10
#define DEFAULT_CIS_LATENCY_MS 40
#define DEFAULT_CIS_PHY BT_GAP_LE_PHY_2M
#define DEFAULT_CIS_SDU_SIZE CONFIG_BT_ISO_TX_MTU
#define DEFAULT_CIS_PACKING 0