Overview of IEEE 802.1Qbv Time Aware Shaping

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Overview of IEEE 802.1QbvTime Aware Shaping
Joint IEEE-SA and ITU Workshop on Ethernet

• Don Pannell,
• Principal Systems Architect
• Marvell Semiconductor

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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
  cant be guaranteed in their Networks
• The Network Structure and Usage will have to be
  Engineered, Managed and Controlled

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 http//www.ieee802.o
  rg/1/files/public/docs2011/new-pannell-latency-opt
  ions-0311-v1.pdf
• The max size interfering frame is the determining
  factor
• How does this improve if the interfering frame
  wasnt there?

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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

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 13.898
  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 4.122 uSec
• Can we get rid of the Interfering Frames to get
  the better latency?

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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

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Inside an Qbv Bridge (example)

• Qbv Time Progression Fig 1
• At Bridge t0-16.000 uSec before the start of the
  Burst Window the Green AVB Class B frames are
  being Shaped (gated) by Qav and cant Transmit
• So the Red Max size non-AVB High Priority frame
  n can start

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Inside an Qbv Bridge (example)

• Qbv Time Progression Fig 2
• At Bridge t0-3.664 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

16.000-12.336
30020 bytes 2.560
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Inside an Qbv Bridge (example)

• Qbv Time Progression Fig 3
• At Bridge t0-1.104 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 cant 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

3.664-2.560
6420 bytes 672
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Inside an Qbv Bridge (example)

• Qbv Time Progression Fig 4
• At Bridge t0-0.432 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 cant 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 cant go
  either not enough time

1.104-0.672
6420 bytes 672
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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

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Qbv With Cut Through

• Cut-through bridges generally dont 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

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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 4.122 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 2.074 uSec!
• Assuming a 64 byte Cut Through Point
• This number does NOT change due to frame size!
• And this performance can be Guaranteed!

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Latency By The Numbers

• Qbv supports the lowest latency possible
• But only if the network is Engineered
• It wont 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

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Questions?