5.27.4 Hold and Forward Buffering mechanism

DS-TT ports and NW-TT ports support a hold and forward mechanism to schedule traffic as defined in IEEE Std 802.1Q [98] if 5GS is to participate transparently as a bridge in a TSN network. That is, the hold and forward buffering mechanism in this release of the specification provides externally observable behaviour identical to scheduled traffic with up to eight queues (clause 8.6.8.4 in IEEE Std 802.1Q [98]) and with protected windows (Annex Q.2 in IEEE Std 802.1Q [98]). Frames are only transmitted from a given buffer according to the open time interval of the corresponding transmission gate; otherwise, frames are hold back (which corresponds to a closed transmission gate). The protected windows scheme implies that only a single transmission gate is open at any single time. Thus, the Hold and Forward buffering mechanism allows PDB based 5GS QoS to be used for TSC traffic.

For Ethernet frames that contain a VLAN tag, DS-TT and NW-TT determine the priority based on the PCP value contained in the VLAN tag. For Ethernet frames that do not contain a VLAN tag, DS-TT and NW-TT apply a priority value of 0.

To achieve externally observable behaviour according to the protected windows scheme, 5GS provides AdminControlList, AdminBaseTime, AdminCycleTime and TickGranularity as defined in IEEE Std 802.1Q [98] on a per Ethernet port basis to DS-TT and NW-TT for the hold and forward buffering mechanism as described in clause 5.28.3.

NOTE: The details of how Hold and Forward buffering mechanism is provided by the DS-TT and NW-TT is up to implementation.