Gates vs. Windows and Scheduled Traffic

H o c h s c h u l e D e g g e n d o r f – H o c h s c h u l e f ü r a n g e w a n d t e W i s s e n s c h a f t e n

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Gates vs. Windows and Scheduled Traffic

Christian [email protected] 802.1 InterimJanuary 2013Vancouver, Canada


Terminology “Scheduled Traffic”

April 19, 2023 2IEEE 802.1 TSN TG – January 2013 Vancouver, BC


What is Scheduled Traffic?

To call everything scheduled is on the one hand correct, but people might think they have to engineer / schedule their best effort traffic

In reality the “schedule for best effort” is a consequence of the schedule for the Scheduled Traffic

802.1 Qbv PAR:

“Virtual Local Area Network (VLAN) tag encoded priority values are allocated allowing simultaneous support of scheduled traffic, credit-based shaper traffic and other bridged traffic over Local Area Networks (LANs).”

Terminology



I think we should stick to Best Effort, Reserved and Scheduled Traffic

What does Scheduled Traffic distinguish from other traffic?- It is scheduled in the end station on a per stream basis

- It has the highest priority/priorities

- It is registered in bridges (domain concept, priority regeneration, no flooding, …)

How do we call the TSN traffic in general?

Terminology



Possible Solution


Time Sensitive Traffic

Reserved Traffic Scheduled Traffic

Best Effort Traffic


Gates and Transmission Windows



In Santa Cruz we had a discussion about alternative ways to configure the Time Aware Shaper

Configuration of the Scheduled Traffic windows instead of the configuration of the gates in order to allow a floating guard band

Background



scheduled traffic framescheduled

traffic framescheduled traffic frame

Transmitted Data:

Scheduled Traffic Gate:

Other Gates:

t0 t1 t2 t3 t4

The general question is:

What should be configured? Gates or transmission windows?

Impacts on the Time Aware Shaper





Transmitted Data:


Other Gates:

t0 t1 t2 t3 t4

Configuring gates is complicated in combination with an intelligent guard band usage (i.e. transmitting lower priority frames during the guard band if they end before the start of the Scheduled Traffic transmission)

If the gate events are defined as the points in time when the queues are getting (dis)connected from/to the transmission selection it is not possible to transfer frames during the guard band

Configuring Gates



“… frames are selected from the corresponding queue for transmission if and only if

…

b) The transmission gate (8.6.8.4) associated with that queue is in the open state; and

c) If there are gate events associated with that queue, then there is sufficient time available to transmit the entirety of that frame before the next gate-close event associated with that queue;”

How it is specified in IEEE 802.1Qbv-D0.2



The gate-closing event is not really defining the time when the queue is disconnected from the transmission selection

It is not possible to start the transmission of a scheduled traffic frame when the corresponding gate is in the open state, if it is not possible to transmit the frame until the closing event

The idea to open the gate only a very short time in order to reschedule the traffic is not possible anymore

But now it is possible to reschedule the frames with the gate-closing events

The usage of the term gates in D0.2 is not really compatible with the definition of gates, it seems to be the definition of transmission windows

Should we change the name?

Implications of the Current Definition





Transmitted Data:


Other Gates:

t0 t1 t2 t3

The example (slide 8) with the D0.2 shaper would look like this:

The queues get disconnected before the gate-closing event, the exact time depends on the frame size of the frames in the other queues.

The gates define now the transmission windows rather than the connection or disconnection from the transmission selection

IEEE 802.1Qbv D0.1



Pros:

Not necessary to define a guard band

Flexible configuration

Gate-closing event allows to reschedule the traffic

Cons:

If the scheduled traffic windows are smaller than a max frame (this might be the usual case) and a frame bigger than the transmission window enters the queue (a talker sends a too big frame (fault)) the queue is blocked (“for ever”)

Pros and Cons of the IEEE 802.1Qbv D0.2 Shaper



Time Aware Shaper – Gate Model


Time Aware Gate

Credit Based Shaper

Time Aware Gate

Transmission Selection

Scheduled Traffic Queue

Reserved Traffic Queue

Best Effort Traffic Queue


Time Aware Gate

Time Aware Gate

Gate Driver

T0:01111111T1:10000000T2:01111111T3:10000000T4:01111111T5:10000000T6:01111111

.

.

.T125:10000000

T126:repeat



gate closed

gate open

scheduled traffic frame

other traffic

Transmitted Data:


Other Gates:

Time Aware Shaper

T1 T2 T3 T4 T5 T6

126

0 0

)(i

x

iiiRx TTntt

t0

tR = reference time

t1 t2 t5t3 t4 t6


Time Aware Shaper – Window Model


T0:1T1:0T2:1T3:0T4:1T5:0T6:1

.

.

.T125:0

T126:repeat

T0:1T1:0T2:1T3:0T4:1T5:0T6:1

.

.

.T125:0

T126:repeat

T0:1T1:0T2:1T3:0T4:1T5:0T6:1

.

.

.T125:0

T126:repeat

T0:0T1:1T2:0T3:1T4:0T5:1T6:0

.

.

.T125:1

T126:repeat

Time Aware Shaper

Credit Based Shaper

Time Aware Shaper


Scheduled Traffic Queue




Time Aware Shaper

Time Aware Shaper

T0:1T1:0T2:1T3:0T4:1T5:0T6:1

.

.

.T125:0

T126:repeat



window closed

windowopen

scheduled traffic frame

other traffic

Transmitted Data:

Scheduled Traffic windows:

Windows other queues:

T1 T2 T3 T4 T5 T6

126

0 0

)(i

x

iiiRx TTntt

t0

tR = reference time

t1 t2 t5t3 t4 t6

T0:0T1:1T2:0T3:1T4:0T5:1T6:0

.

.

.T125:1

T126:repeat


One list per queue

Events:

Gate events:

- Window close event = 0

- Window open event = 1

Other events:

- Repeat

Gate event time interval (T0, T1, …, Tx)- Relative to last gate event

- Granularity: 1ns

- 32 bit unsigned integer in units of 1 ns (max ≈ 4.2s)

- In order to have a defined start condition at tR, T0 must be 0

- tR = PTP epoch

Event List


T0:0T1:1T2:0T3:1T4:0T5:1T6:0

.

.

.T125:1

T126:repeat


Device specific latency t_Device

Necessary to calculate the schedule

Device specific Time Aware Shaper granularity

Necessary to define the minimum window size

Maximum event list length

Important Device Specific Parameters



Shaping in the end stations

Per stream shaping in the end stations

Less complicated than CBS

Ensures that a frame from the scheduled stream is transmitted

End Station – Talker



End Station – Talker



Time Aware Shaper

Credit Based Shaper

Time Aware Shaper


Scheduled Traffic Stream

Queue 1



Time Aware Shaper

Time Aware Shaper

Time Aware Shaper


Queue 2

Time Aware Shaper


Queue n

Reserved Traffic Stream Queue 1



Reserved Traffic Stream Queue n

Credit Based Shaper

Credit Based Shaper

Credit Based Shaper

Credit Based Shaper


Traffic Class and PCP



Time Aware (TA) domain

Time Aware (TA) class

time aware (TA) class:

A traffic class whose frames are scheduled in order to achieve minimum latency. A priority value is associated with each TA class. TA classes are denoted by consecutive letters of the alphabet, starting with A and continuing for up to seven classes.

New PCP for Scheduled Traffic: 4 (default for TA class A)

Scheduled Traffic has the highest priority (above AVB Gen1 SR class A)

Time Aware Domain and Class



Thank You


Gates vs. Windows and Scheduled Traffic

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