1. Distributed Embedded Systems

2. Class Goals
At the conclusion of this class, the student should be able to:
Understand differences between co-processors and accelerators
Understand how to interface an accelerator to a CPU
Compare accelerated single-threaded and multi-threaded applications
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

3. Topic Outline Networking Embedded Systems Networking Fundamentals
The I2C Bus
Ethernet
Internet Protocol (IP) Networks
Analyses of Networks
Summary
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

4. Networking Embedded Systems

5. Network Elements ES
Each embedded system (ES) may communicate with any other ES in the network.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

6. Networks in embedded systems
initial processing
more processing PE
sensor PE PE
actuator
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

7. Why distributed?
Resource constrained platforms cannot perform the entire application
Automated application updates
Functionality is widely varied among a set of embedded platforms
Timing constraints
Centralized control of resources may increase network traffic loading
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

8. Embedded System in a Bakery
App
Server
PE1
PE3
PE4
PE2
PE5
Network
Measure
Temp
Adjust
Mix
Dough
Muffin
Throughput
Cook Time
Recipes
Orders …
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

9. Networking Fundamentals

10. The OSI Seven Layer Model
International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model to describe networks:
Provides a standard way to classify network components and operations.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

11. OSI model application end-use interface presentation data format
session
application dialog control
transport
connections
network
end-to-end service
data link
reliable data transport
physical
mechanical, electrical
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

12. Links Sources Network protocols. Files Files HTTP TCP IP packets
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

13. Distributed Network Architectures
Many different types of networks:
topology;
scheduling of communication;
routing.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

14. Point-to-point networks
One source, one or more destinations, no data switching (serial port):
PE 3
PE 1
PE 2
link 1
link 2
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

15. Bus networks Common physical connection: PE 1 PE 2 PE 3 PE 4
packet format
header
address
data
ECC
EOT
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

16. Bus arbitration Fixed: Same order of resolution every time.
Fair: every PE has same access over long periods.
round-robin: rotate top priority among Pes.
fixed A B C A B C
round-robin A B C B C A t
A,B,C
A,B,C
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

17. Message-based programming
Transport layer provides message-based programming interface:
send_msg(adrs,data1);
Data must be broken into packets at source, reassembled at destination.
Data-push programming: make things happen in network based on data transfers.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

18. The I2C Bus

19. I2C Bus Serial communications bus
Kbps operation (100 Kbps typical)
SDA – Serial DAta Line
SCL – Serial CLock Line
Loosely-defined electrical specification
Low implementation cost
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

20. I2C bus Characteristics:
Supports multiple bus masters
fixed-priority arbitration.
Several microcontrollers come with built-in I2C controllers
Each I2C device is given a unique bus address
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

21. I2C physical layer master A master B SDA SCL slave 1 slave 2
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

22. I2C data format u u /V t/s /V t/s MSB LSB Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

23. I2C electrical interface
Open collector interface: +
SDA +
SCL
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

24. I2C signaling Sender pulls down bus for 0.
Sender listens to bus---if it tried to send a 1 and heard a 0, someone else is simultaneously transmitting.
Transmissions occur in 8-bit bytes.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

25. I2C data link layer
Every device has an address (7 bits in standard, 10 bits in extension)
LSB of address signals read or write
General call address allows broadcast
Address is the indicator for a “general call” or broadcast
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

26. I2C bus arbitration Sender listens while sending address.
When sender hears a conflict, if its address is higher, it stops signaling.
Low-priority senders relinquish control early enough in clock cycle to allow bit to be transmitted reliably.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

27. I2C transmissions (multi-) byte write S Adrs Word-adr Data P Start
Write (LSB of address)
Stop
(multi-) byte write S
Adrs
Word-adr
Data P
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

28. I2C transmissions Write addreses, then read S Adrs Word-adr S Adrs 1
Start
Write (LSB of address)
Read (LSB of address)
Stop
Write addreses, then read S
Adrs
Word-adr S
Adrs 1
Data P
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

29. Ethernet

30. Ethernet Dominant LAN technology Typical: 10/100 Mb/s and 1 Gb/s
Future: 10 Gbps and 40 Gbps
Goal: reliable communication over an unreliable medium.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

31. Ethernet topology Bus-based system, several possible physical layers:
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

32. CSMA/CD Carrier sense multiple access with collision detection:
sense collisions;
exponentially back off in time;
retransmit.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

33. Ethernet packet format
preamble
start
frame
source
adrs
dest
adrs
length
data
payload
padding
CRC
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

34. Ethernet performance
Quality-of-service tends to non-linearly decrease at high load levels.
Can’t guarantee real-time deadlines. However, may provide very good service at proper load levels.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

35. Internet Protocol (IP) Networks

36. Internet Protocol Internet Protocol (IP) is basis for Internet.
Provides an internetworking standard: between two Ethernets, Ethernet and token ring, etc.
Higher-level services are built on top of IP.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

37. IP in communication application application presentation presentation
session
session IP
transport
transport
network
network
network
data link
data link
data link
physical
physical
physical
node A
router
node B
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

38. IP packet Includes: Maximum data payload is 65,535 bytes.
version, service type, length
time to live, protocol
source and destination address
data payload
Maximum data payload is 65,535 bytes.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

39. IP addresses 32 bits in early IP, 128 bits in IPv6.
Typically written in form xxx.xx.xx.xx.
Names (foo.baz.com) translated to IP address by domain name server (DNS).
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

40. Internet routing Best effort routing: Routing can vary:
doesn’t guarantee data delivery at IP layer.
Routing can vary:
session to session;
packet to packet.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

41. Higher-level Internet services
Transmission Control Protocol (TCP) provides connection-oriented service.
Quality-of-service (QoS) guaranteed services are under development.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

42. The Internet service stack
FTP
HTTP
SMTP
telnet
SNMP
TCP
UDP
User
Datagram
Protocol IP
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

43. Analyses of Networks

44. Networks Network-based design. Internet-enabled systems.
Communication analysis.
System performance analysis.
Internet-enabled systems.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

45. Communication analysis
First, understand delay for single message.
Delay for multiple messages depends on:
network protocol;
devices on network.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

46. Message delay Assume: Delay: single message; no contention.
tm = tx + tn + tr
= xmtr overhead + network xmit time + rcvr overhead
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

47. Example: I2C message delay
Network transmission time dominates.
Assume 100 kbits/sec, one 8-bit byte.
Number of bits in packet:
npacket = start + address + data + stop
= = 18 bits
Time required to transmit: 1.8 x 10-4 sec.
20 instructions on 8 MHz controller adds 2.5 x 10-6 delay on xmtr, rcvr.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

48. Multiple messages
If messages can interfere with each other, analysis is more complex.
Model total message delay:
ty = td + tm
= wait time for network + message delay
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

49. Arbitration and delay
Fixed-priority arbitration introduces unbounded delay for all but highest-priority device.
Unless higher-priority devices are known to have limited rates that allow lower devices to transmit.
Round-robin arbitration introduces bounded delay proportional to N.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

50. Example: adjusting messages to reduce delay
Task graph:
Network: 3 4
execution time
Transmission time = 4
allocation P1 P2 M1 M2 M3 d1 d2 P3
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

51. Initial schedule M1 P1 M2 P2 M3 P3 network d1 d2 Time = 15 time 5 10
time 5 10 15 20
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

52. New design Modify P3: reads one packet of d1, one packet of d2
computes partial result
continues to next packet
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

53. New schedule M1 P1 M2 P2 M3 P3 P3 P3 P3 network d1 d2 d1 d2 d1 d2 d1
Time = 12
time 5 10 15 20
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

54. Hardware platform design
Need to choose:
number and types of PEs;
number and types of networks.
Evaluate a platform by allocating processes, scheduling processes and communication.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

55. I/O-intensive systems
Start with I/O devices, then consider computation:
inventory required devices;
identify critical deadlines;
chooses devices that can share PEs;
analyze communication times;
choose PEs to go with devices.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

56. Computation-intensive systems
Start with shortest-deadline tasks:
Put shortest-deadline tasks on separate PEs.
Check for interference on critical communications.
Allocate low-priority tasks to common PEs wherever possible.
Balance loads wherever possible.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

57. Internet-enabled embedded system
Internet-enabled embedded system: any embedded system that includes an Internet interface (e.g., refrigerator).
Internet appliance: embedded system designed for a particular Internet task (e.g. ).
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

58. Examples Laser printer. Personal digital assistant (PDA).
Home automation system.
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

59. Summary
Resource constrained embedded systems may require communication with other systems to complete a job
The selected networking strategy determines the overall performance of a set of networked embedded systems
The designer must consider system performance as well as the cost to implement
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

60. Assignment Work on final projects and term papers
Complete remaining class presentations
Schedule to present final projects/papers
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

61. Questions ?
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007

62. Acknowledgements
Some of the presentation materials were provided by Morgan Kaufmann Publishers
Additional material was developed by the GWU Computer Science course instructors
Y. Williams/J. Hoenig
Csci-339/HFU, Spring 2007