Page 36 - ITUJournal Future and evolving technologies Volume 2 (2021), Issue 1

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 1

Wait for Stream
Removal, Addition,
Stream Stream
Addition and Pending Event Removal
Stream
Pending
Resources Locate Stream in Registration Table and
No Available? release resources consumed by stream
Accepted?
No
TAS Traffic Class Yes
Slot Full? Yes
No Destination Stream completed
Switch? No Successfully? Yes
Yes Report
Yes Report Failed Admitted
No Stream Stream
Terminate Registration Allocate Resources or TAS
Procedure and Report Traffic Class Slot (per stream
Stream Rejection properties)
Forward CDT packet to next
hop in path and wait for Recompute the TAS gating
Pending Event schedule for remaining Streams in
Register Stream Record and
Report Stream Acceptance to Registration Table
Source (in reverse direction)
De-registration
completion
Registration
completion
Fig. 4 – The main logical steps performed by each switch along the stream’s path are shown to apply stream registration and reservation. Each switch
generally waits for an event (addition, removal, or pending) for each stream. For instance, a stream removal is usually based on the resource utilization
time (stream lifetime) that was speci ied at stream establishment. The bridges that allocated resources for the stream can remove the stream after the
stream lifetime has expired. For the cases of stream addition or pending, the event is the CDT message received (whether in the forward or reverse
direction). Towards completing ( inalizing, con irming) a stream reservation (registration), the pending event is the event when a CDT message is
received in the reverse direction where each switch (not the last switch) waits for the approval (con irmation of reservation) of the next‑hop switch.

tion switch). generally by allocating more resources from BE Traf ic.
The stream lifetime is reported by the source to the net‑
4.1.4 Traf ic shaper — Time‑Aware Shaper work as User/Network Information (UNI). Each UNI is
(TAS) propagated by each switch along the path which allows
the switch to register the stream and store the stream’s
The TAS is the main shaping and scheduling mechanism resource utilization time (stream lifetime) among other
that controls the gating schedules for all the traf ic classes critical information. Any information pertaining to the
within the TSN domain. All bridges are synchronized UNI of a stream is recorded in the stream registration ta‑
to the same gating schedule GCL CT that is initially pre‑ ble. IntermsofGCEsforTASwiththesupportofSTandBE
de ined by the network administrator. Ideally, we want traf ic classes, only two GCEs within a GCL (1/0 (ST/BE)
the CT to be large enough for all streams from all traf‑ for the irst entry and 0/1 (BE/ST) for the second entry)
ic classes to be accommodated and short enough that all are necessary with a total of three outbound queues for
streams meet their delay requirements. In our current each egress channel port in a TSN switch; two queues for
evaluations, the CT is prede ined at 50 microseconds. each traf ic class (ST and BE), and another queue for CDT
traf ic (signaling traf ic).
4.1.5 Recon iguration calculus Upon initialization, each switch allocates 20% of the CT to
ST traf ic, and BE traf ic is initially allocated the remain‑
The recon iguration (dynamic con iguration) of the TAS ing 80% of the CT. These initial settings are chosen ar‑
schedules (switch GCL/GCE) for each egress port is dy‑ bitrarily to start up the network system. As streams get
namically invoked according to two principle events, ) registered, the ST slot time is recomputed (according to
adding a new stream, and ) removing an existing stream. the stream packet size, ST injection rate, and current slot
The switch’s gating ratio (for a particular stream belong‑ time). If the stream is the irst stream to the switch, then
ing to a de ined traf ic class) reports certain parame‑ the ST slot size is set at a minimum to 11% (a minimum of
ters (e.g., packet injection rate, maximum packet size, la‑ 1% step size for the added ST low plus the minimum ST
tency requirement, deadline, and application response partition of 10%) of the CT. Thus, as ST streams are ad‑
time) which are then used to check if enough slot time mitted and exit the system, the ST vs. BE allocation is dy‑
is available (which corresponds to attempting bandwidth namically adapted in the recon iguration scenarios. The
reservation). In the event that no slots are available, minimum step size of 1% of the CT is considered so as to
the GCE slot size is recomputed (according to the reg‑ limit the adaption granularity to a reasonable level. Note
istered stream properties within the registration table), that the ST to BE slot size (or gating ratio) is limited to

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