1. Overview of IEEE 802.1Qbv Time Aware Shaping
Joint IEEE-SA and ITU Workshop on Ethernet
Overview of IEEE 802.1Qbv Time Aware Shaping
Don Pannell,
Principal Systems Architect
Marvell Semiconductor
Geneva, Switzerland, 13 July 2013

2. Need/Desire/Goal of Qbv
Get The Lowest Latency Possible – Any Way Possible
Want Many Long, ~32 hop, Daisy Chains
Small Bursts of frames at known regular intervals (e.g., a 40 uSec long burst of data every 125 uSec)
Willing to Engineer Network Segments to meet this goal
Non-Engineered (i.e., Consumer) Networks will not be able to depend on this very low latency as it can’t be guaranteed in their Networks
The Network Structure and Usage will have to be Engineered, Managed and Controlled
Geneva, Switzerland, 2 13 July 2013

3. How Fast Can A Bridge Go? Total time for the Critical frame is:
Internal delay of the Bridge
Plus the time to transmit the max size interfering frame
Plus the time to get the bits down a 100 meter cable
Tx of max frame overlaps the Rx of the Critical frame
Detailed analysis is shown in
The max size interfering frame is the determining factor
How does this improve if the interfering frame wasn’t there?
Geneva, Switzerland, 2 13 July 2013

4. How Fast If No Interference?
Total time for the Critical frame is:
Internal delay of the Bridge
Plus the time to transmit the largest frame size of the Critical stream
Plus the time to get the bits down a 100 meter cable
Now the max size of the Critical frame is the determining factor
This is due to the Store & Forward nature of most Bridges
Since most Critical streams use small frames this is a great improvement
And the Critical stream frame size can be part of the ‘Engineering of the Network’
Geneva, Switzerland, 2 13 July 2013

5. Interfering Frames are the Problem
Some GE speed examples with numbers:
The Bridge Latency with Interfering Frame is:
Equal to the Size of Interfering frame + Bridge Delay + Cable Delay (max size interfering frame = 1522 bytes)
With Max Size interfering frame this is uSec
The Bridge Latency without an Interfering Frame is:
Equal to the Size of AVB/TSN frame + Bridge Delay + Cable Delay
With a 300 byte AVB/TSN frame this is uSec
Can we get rid of the Interfering Frames to get the better latency?
Geneva, Switzerland, 2 13 July 2013

6. Qbv – Time Aware Shaper
Qbv takes advantage of the target low latency data pattern
i.e., That they are typically Small Bursts of frames at known regular intervals (for example: a 40 uSec long burst of data every 125 uSec)
Then use this information to delay the start of non-Critical frames just before the start of the Burst Window
This insures the egress port is idle so the Critical burst is not interfered – All interference is removed!
Smart designs can allow non-Critical frames that fit to use the available bandwidth
Geneva, Switzerland, 2 13 July 2013

7. Inside an Qbv Bridge (example)
Qbv Time Progression – Fig 1
At Bridge t uSec before the start of the Burst Window the Green AVB Class B frames are being Shaped (gated) by Qav and can’t Transmit
So the Red Max size non-AVB High Priority frame ‘n’ can start

8. Inside an Qbv Bridge (example)
Qbv Time Progression – Fig 2
At Bridge t uSec before the start of the Burst Window the interfering Red Non-AVB frame is done
Now the Green AVB Class B frames are available for transmit with enough credits to burst two frames =
bytes = 2.560

9. Inside an Qbv Bridge (example)
Qbv Time Progression – Fig 3
At Bridge t uSec before the start of the Burst Window the 1st Green AVB Class B frame is done
Now the next Green AVB Class B frame has credit to go, but it can’t because there is not enough time before t0 - the start of the Burst Window nor can the Red ‘m’ frame
But the 64 byte low priority Yellow non-AVB frame can go and does =
64+20 bytes = 672

10. Inside an Qbv Bridge (example)
Qbv Time Progression – Fig 4
At Bridge t uSec before the start of the Burst Window the 64 byte Yellow frame is done
The next Green AVB Class B frame has credit to go, but it still can’t because there is not enough time before t0 (its credits are actually increasing) nor can the Red ‘m’ frame
The next low priority 64 byte frame can’t go either – not enough time =
64+20 bytes = 672

11. Inside an Qbv Bridge (example)
Qbv Time Progression – Fig 5
At Bridge t0 - the start of the Burst Window the port is idle so the newly arrived Blue Critical frames are allowed to egress without any interference!
The burst of Blue frames will continue as long as Qbv leaves it queue open for transmission as they are the highest priority queue

12. Qbv – With Cut Through
Cut-through bridges generally don’t help normal network performance due to the low percentage of improved latency and that this improvement cannot be guaranteed
With Qbv the improved latency can be guaranteed
Cut-through only works when the target ports are idle and Qbv does exactly that – thus the guarantee
With Cut-through the Bridge latency is:
Geneva, Switzerland, 2 13 July 2013

13. Latency By The Numbers Non-Qbv Bridge Latency is 13.898 uSec
Due to the Max Size interfering frame
Store & Forward Qbv Bridge Latency is uSec
Assuming a 300 byte maximum size AVB/TSN frame
This number will go up with increasing Critical frame sizes
Cut Through Qbv Bridge Latency is uSec!
Assuming a 64 byte Cut Through Point
This number does NOT change due to frame size!
And this performance can be Guaranteed!
Geneva, Switzerland, 2 13 July 2013

14. Latency By The Numbers Qbv supports the lowest latency possible
But only if the network is ‘Engineered’
It won’t work for all topologies
And proper configuration of the timing ‘gates’ is not easy
The Qbv Standard will support Control of the timing ‘gates’ – but will not figure the timing out
3rd Party tools will be required to compute the ‘gate’ timing
Multiple timing ‘gate’ windows will be supported per port with each window having nSec configuration resolution
TSN Bridges already have network timing information by their support of IEEE 802.1AS
Geneva, Switzerland, 2 13 July 2013

15. Questions?
Geneva, Switzerland, 2 13 July 2013