1. Reflective Memory vs. Ethernet Evaluating Data Network Hardware Solutions for LCLS Fast Feedback Controls Marya Pearson

2. Introduction
The LCLS Fast Feedback project aims to provide a method for controlling the beam parameters and deliver a stable XFEL.
Feedback a technique for controlling these parameters
collect measurements
evaluate algorithms
produce instructions for action
Physics Requirement Document (PRD) outlines all specifications for a data network that will facilitate feedback.

3. The PRD Describes the parameters we wish to control.
Transverse
Longitudinal
Launch
Here, the time budget is introduced.
Data Network requirements
Deterministic
Scalable
Low-Latency
Affordable

4. Time Budget
According to the Physics Requirement Document (PRD) the machine operates at 120 Hz and thereby the 8.3 ms time budget is derived.
1+1+6 = 8ms
1ms BPMs send data
1ms for fast feedback
~6ms for magnets to react
In some cases time budget is adjustable more or less.

5. Time Budget Based on the 120 Hz constraint
The idea is that at least 7ms of the 8.3 ms are fixed. That leaves the feedback 1ms to regulate.

6. Injector Launch IOC:IN20:BP02 BPM9-15 IOC:IN20:MG01
XC04, XC07, YC04, YC07
Loop Rate: 10 Hz
This means that the Inj.L feedback can afford 100ms, however to support 120 Hz beam operation the system should operate within 8.3 ms.

7. Data Network Hardware Solutions
Reflective Memory (RM)
Star topology
Ring topology
Ethernet
Point-to-Point
Multicast

12. Pros Cons Cost (InjL) Latency Scalability Deterministic Other
Reflective Memory-Ring
Built-in redundancy
Eliminates use of hub
No processor involvement in initialization
Error detection and bypass
Data must pass through each node
May lead to difficulty in scaling
$13 k
40 ns each node
difficult
yes
Dynamic packet size 4 to 64 bytes
Transfer rate Mbytes/s
Reflective Memory-Star
Processor identifies it as RAM
Commercially available
Increased scalability
Costly option
More hardware involved
$17 k
Error detection
Redundant transfer mode
Point-to-Point Ethernet
Least expensive
Direct communication between nodes
Not ideal if many devices involved
Latency increases exponentially
price of additional port
Equation:
15n2+85n+3
n= #of nodes
Most readily compatible option
Multicast Ethernet
Selective data delivery
Conserves bandwidth
One-to-many or Many-to-many
Requires significant programming
Worst case scenario and recovery
$10.5 k +price of additional port
6 µs for 64-byte packets
~4.5 µs for 48 byte packets
limited
Throughput: 72 mpps

13. Decision and Future Work
Ethernet with Multicast
chosen for first prototype
simulations must be done to measure performance.
Use Cisco products and collaborate with Cisco engineers to set up a test stand.

14. Acknowledgements
This research was conducted at the U.S. Department of Energy Stanford Linear Accelerator Center. I would like to express my gratitude to my mentors, Ernest Williams Jr. and Sheng Peng. I am also thankful for the assistance of Farah Rahbar, Susan Schultz, and Stephen Rock.

15. Questions?