Aspects for Pre-emption resp. Time Aware Blocking Shaper (TABS)

September 2012

IEEE 802.1 AVB TG MeetingSanta Cruz

Franz-Josef Götz, Siemens AG

Albert Tretter, Siemens AG

September 2012Page 2 IEEE 802.1 AVB TG Meeting – Santa Cruz

TABS a Solution for Simple Networks

TABS is a solution for networks with the following characteristic:

Simple topologies (e.g. line)

Simple communication relations

point-to-point

Same link speed on all links of the network

Requires an engineered network

Drops Control Data frames received outside the time window

Optimized latency for Control-Data-Traffic

D

D

D

D

D

D

CTx

Rx

Controllere.g. PLC

devicese.g. actorsor sensors

t

t

Tra

nsm

issi

on In

puts

Control Cycle / Transmission Period

ExchangeControl Data

compute

Tx Rx Tx

typical traffic pattern for industrial closed-loop-

applicationsTxFrame

Consideringtransmission order

Rx

Transm

ission Outputs

Goal:minimize

makespan

September 2012Page 3 IEEE 802.1 AVB TG Meeting – Santa Cruz

PLC

PLC

PLC

Motion Control Cell

DEV

PLC

VideoServer

Machinery Cell

DEV

DEV DEV

DEV

PLC overlapping functional cellsfor PLC <-> DEV

control data traffic communication

. . .

. . .

PLC: Programmable Logic Controller for input/output dataDEV: Device for input / output data (sensor / actuator)

Ethernet

PLC – DEV communication relationPLC – PLC communication relation

Pre-emption a Solution for Meshed Networks

Pre-emption in combination with BLSA (Burst Limiting Shaper Algorithm) is a solution for more complex networks with the following characteristics:(for BLSA see Franz Götz's presentation:

http://www.ieee802.org/1/files/public/docs2012/new-goetz-CtrDataScheduler-0712-v1.pdf)

Meshed networks Multiple Transmission Periods within one network Complex communication relations

point-to-point

point-to-multipoint

multipoint-to-multipoint Different link speeds within one network

(e.g. 100MBit/s, 1GBit/s, … ) Robust behavior in case of failure

suitable for safety critical applications Supports periodic and event-based CDT

Low latency for Control-Data-Traffic

September 2012Page 4 IEEE 802.1 AVB TG Meeting – Santa Cruz

Pre-emption versus Time Aware Blocking Shaper

Pro Pre-emption: Easy to add to existing networks

Adding devices or bridges that support pre-emption to a network containing devices or bridges that do not support pre-emption is possible and vica versa

Works well in networks with different link speeds No need to configure the bridges for pre-emption Flexibility, it is easy to change the network (e.g.

add new stations, change the topology, …) No synchronization of the bridges (scheduler)

required Robust against miss-configuration Usage of 100% of the available bandwidth is

possible (see Norman Finn's presentation http://www.ieee802.org/1/files/public/docs2012/new-avb-nfinn-preempt-advantage-0112-v02.pdf)

Transmission periods in the range from 31.25µs to 250µs (@100MBit/s) are not possible without pre-emption

Pro TABS:

Provides the best latency performance (less jitter) for simple topologies

September 2012Page 5 IEEE 802.1 AVB TG Meeting – Santa Cruz

Some relevant Issues for the Bandwidth and Window Size Calculation for TABS

Synchronization inaccuracy (1µs?)

Sync tree is not identical with the data path

Cable length (e.g. assumption 100m real 5m)

Bridge forwarding delay for Control Data Frames must be known

Forwarding mechanism (CT / S&F)

Scheduling has to avoid overtake situation

e.g. huge number of fan in and low number of fan out

Bridges with different link speeds on ingress and egress ports

e.g. 100MBit/s 1GBit/s and one window more switching resources are required

e.g. 100MBit/s 1GBit/s and multiple window

September 2012Page 6 IEEE 802.1 AVB TG Meeting – Santa Cruz

Thank You