1. Aspects for Pre-emption resp. Time Aware Blocking Shaper (TABS)
September 2012
IEEE AVB TG Meeting Santa Cruz
Franz-Josef Götz, Siemens AG
Albert Tretter, Siemens AG

2. 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
Line or Comb structures

3. 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:
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
Robust behavior in case of failure
- suitable for safety critical applications -
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Fail operational behavior
Emergency operation features
Robust for safety critical applications
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􀂃 Emergency operation features – (fail operational behavior)

4. 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
Transmission periods in the range from 31.25µs to 250µs are not possible without pre-emption
Pro TABS:
Provides the best latency performance (less jitter) for simple topologies
Compatible Erweiterung möglich

5. 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

6. Thank You