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Chapter 7

Chapter 7. REAL TIME SYSTEM COMMUNICATION. Contents. Introduction Media communication Network Topology Sending message Network Design Issues Protocol Summary. Introduction.

LeeJohn
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Chapter 7

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  1. Chapter 7 REAL TIME SYSTEM COMMUNICATION

  2. Contents • Introduction • Media communication • Network Topology • Sending message • Network Design Issues • Protocol • Summary

  3. Introduction • Effective communication between various hardware in real time system are crucial to ensure it can function correctly. • Embedded system • Data flow: • From sensor and control panel to central cluster of processor. • Between processors in the central cluster. • From processor to the actuators and output display. • Communication overheads adds to the computer response time.

  4. Real time system structure Sensor and actuator layer Peripheral cluster central cluster

  5. Introduction • Hard Real time system – use communication protocol so that communication overhead can be enclosed . • Soft real time system • Multimedia and video conferencing • Delay in data transmission can effect quality of service but it is not critical. • Protocol communication real time system vs traditional system. • Real Time System – able to transmit message on time (follw the deadline) • Traditional System – able to transmit lots of data from source to destination through network in one time. (throughput).

  6. Introduction • Delay in message can cause by: • Format and message packet • Line of message when waiting to access the communication media. • Sending message form source to destination. • Deformatting the message. • Real time traffic is categorized based on: • Deadline • Arrival pattern • Priority

  7. Introduction • Real time traffic resources have two categories: • Constant rate • The size of fix packet is produced in equal time rate. • easy to manage. • Variable rate • The size of fix packet is produced in unequal time rate or • Various packet size can be produced in fix time rate. • example : voice can have talk spurts ( a lot of packet followed by silent).

  8. Communication media • 3 important media: • Electromagnetic medium • Fiber optic • Wireless a. Electromagnetic Medium • Twisted pair and coaxial cable • Twisted pair – connected using RJ45/RJ11 depends on cable category. • Coaxial cable – hardware is connected using T-junction or vampire tap.

  9. Communication media b. Fiber optic • Transform electric signal to light impulse. • Operates up to 10Gbps ( NOW ???) • Receiver will change the signal back to electric signal. • Point-to-point network and passive star – suitable to use fiber optic. • Advantages • High Bandwidth • No electromagnetic interference. • Disadvantages • Difficult to add node without losing any signal. • High maintenance cost.

  10. … Node Network interface Optical to electrical Electrical to optical electronics interface node Communication media • Point-to-point network • No tap • Interface have optical -> electrical and electrical -> optical. • Interface change optical signal to electrical and vice versa. • Node will check whether the receiving message is for itself or not. Point-to-point network

  11. Communication media • Passive star • Interface will send optical signal to glass cylinder (passive star). • Output will be divided between output fiber frin sylinder. • Interface receive input and choose intended message for itself. • Need sensitive and powerful transceiver to detect signal. Interface 3 Interface 1 Interface 2 Passive star Star architecture

  12. Communication media • WDM (wavelength division multiplexing) • Divide fiber channel into virtual channel. • Every channel has interface and optical detector to change wavelength in virtual channel. • Every channel has enough bandwidth to suite with electronic interface bandwidth. Interface 1 Interface 2 Interface 3 Interface 4 Interface 5

  13. Communication Media c. Wireless • Distance between radio and transmitter, while sustain the receiver depends on: • Transmitter power • Receiver sensitivity • Noise level • Type of error control. • Attenuation barrier (walls, partitions, hardware etc) • Advantages • No need wires to connect node. • Develop Ad hoc network. • Disadvantages • Interference.

  14. Network Topology • To develop and design network, important criterias that should been taken are: • Diameter – maximum length between two nodes in the system. • Node degree – quantity of peripheral that located adjacent to node, shows total of I/O port for each node and total of channel in the system. • Many hardware – higher cost • Fault-tolerance – measure network ability to handle individual channel failure and node failure while operating.

  15. Network Topology • Network topology clasification: • Point-to-point topology • Shared topology (broadcast) 1. Point to point • Node is connected using dedicated link • If node send messages not to an adjacent node, that message will pass through intermediate node (router, bridge). 2. Shared topology • All node can use the communication channel but only one node can send message at one time.

  16. Network Topology Network path interface interface Node 1 Node n Shared network Node 1 interface interface Node 2 Node 3 interface interface Node 4 Point-to-point network

  17. Network Topology • Types of network topology: 1. Bus • Both end will be terminated to prevent electromagnetic interference. • Interface consists of tap or forwarding points 2. Ring • Receive bit will be copy into buffer, process and then send to output. Topologi Bas Topologi Cincin

  18. Network Topology 3. Mesh 4. Hypercube • 2n node in n-dimensional hypercube • Node is label binari 0 to 2n – 1 and is connected using line between node with differentiation one bit. • N-dimensional = two (n-1)-dimensional hypercubes Rectangular mesh 10 11 00 01 Hypercube network : two-dimensional

  19. Network Topology • 5. Multistage network • Built out of switchboxes • 4 confguration. 4 configuration 2 X 2 switch Physical Topology vs Virtual Topology???

  20. Sending Message • 3 methods to send message: 1. Packet switching • Message will be break apart into packet. • Packet has header that contain source address, destination and other important information. • Sending to destination using routing and flow control algoritma. 2. Circuit switching • Circuit will establish between source and destination before sending message. • Other message must wait until the sending process complete. 3. Wormhole routing • Pipelining sending packet in multihop network. • Can cause deadlock

  21. Sending Message S D Circuit switching

  22. Network Design Issues • High level design/architecture • High level distributed system contains set of node that communicate through network. • Every node can be a multiprocessor that have application, system and network processor. • Network architecture can support scalability, easy to run, reliable and support communication 1 to 1 and 1 to many. • Low level design/architecture • Provide packet processing , routing and error control. • In real time distributed system, there are deadline, time management and housekeeping issues. • Network processor (NP) – function to handle those issues.

  23. Network Design Issues • Network Processor (NP) • Execute sending message from source to destination operation. • Task that send message will give information to NP about the receiver and message destination • NP tasks • Establish connection between source and destination • Handle error detection and resend message. • Choose alternative route. • Give bandwidth. • Packet the information to data block and segment. • Assemble packet when reach its destinantion.

  24. Network Design Issues • Continue… • In point-to-point: NP choose switching technique. • In token ring: NP choose suitable protocol to ensure it meets its deadline. • NP perform framing, synchronization and packet sequencing.

  25. Network Design Issues • I/O Architecture • To enhance I/O, the I/O hardware will be divided to multiple access path (multi accessibility) and handle by a controller. • Multi accessibility – combination of I/O hardware and then assign a controller to manage access to hardware in each cluster. • Controller is connected with three node in the system. I/O controller 3 4 node 0 1 0 1 2

  26. Protocol • Protocol category • Contention-based protokol • VTCSMA • Window protocol • Token-based protocol • Timed token protocol • IEEE 802.5 Token ring protocol • Stop-and-go Multihop Protocol • Polled bus protocol • Hierarchical round-robin protocol • Deadline based protokol • Fault tolerance routing

  27. Protocol : VTCSMA • VTCSMA (virtual time carrier sensed multiple access) • for single-channel broadcast network, ring and bus topology • Node monitor channel and sending message if the channel is idle. • If many nodes sending packet at one time, this can cause collision. • If collision happen, the sending node will abort and try to send again after the channel is idle. • CSMA-distributed algorithm, every node can determine time to send message. How to determine??

  28. Protocol : VTCSMA • Information in each node: • Channel state ( idle or busy ) • Priority packet wait in the buffer before sending through network. • Time follows syncronized clock • VTCSMA algorithm used two clock for each node: • Real clock (RC) – tells real time that synchronized with node’s clock. • Virtual clock (VC) • If channel is busy, VC freeze. • If channel free, VC reset at rate n>1. • VC is more accurate than RC when it is free.

  29. Protokol : VTCSMA Operation of virtual clock t4 t3 Channel busy VC t2 Channel idle t1 t1 t2 t3 t4 RC

  30. Protokol : Window Protocol • Window protocol • Based on collision sensing. • Cannot guarantee message will be send on time. • Suitable for soft real time system. • System contain set of node which is connected through bus. • Every node will monitor bus line to receive message dedicated for them. • Window = sela masa (setiap node mempunyai sela masa yang sama). • Node can send packet if latest-time-to-transmit (LTTT) packet is located in the window and the channel is idle. • If so many sending packet, refer to LTTT value.

  31. Protokol : Token Based Protocol • Token based protocol • Token – Grant for node to send packet in the network. • When done, node will send the token to other node. • Suitable for bus and ring topology/

  32. Protokol : Token Based Protocol • Token algorithm melibatkan beberapa overhead: • Medium propagation delay – mengambil masa untuk mesej bergerak dari satu node ke node yang lain • Token transmission time – penghantaran token keluar mengambil masa. • Token capture delay – sela masa (time lag) antara node mengambil token dan node menghantar mesej. • Network interface latency – masa antara bit diterima oleh antaramuka rangkaian dan apabila ia dihantar.

  33. Protokol : Time Token Protocol • Timed token protocol 1. Synchronous traffic – real time traffic. - protokol ensure each node can send synchronous trafic for each T unit time. 2. Asynchronous traffic - non-real time traffic. - take unused bandwidth from synchronous traffic. - Key control – TTRT (target token-rotation time) - when token arrive, node will check the value • Cycle time >TTRT = LATE , transmit synchronous traffic • Cycle time < TTRT = EARLY, transmit synchronous and asynchronous traffic.

  34. Protokol : Token Ring Error control code SD AC ED DA ED FS message SA IEEE 802.5 token ring SD –start delimiter AC – access control ED – ending delimiter FS – Frame status SA – Source address DA – Destination address Disemak oleh sender bila data frame kembali semula. After that, remove the FS FS = 00 -> destination node not available FS =10 -> frame tidak boleh disalin ke destinasi FS =11 -> frame diterima oleh destinasi

  35. Protokol • Packet Scheduling • Cannot preempted packet transmission. If occur, the sender must resend the packet. • Overhead occur while sending the message again. • Decide priority for each packet earlier.

  36. Protocol : Stop and Go Multihop Protocol • Stop and go multihop protocol • For hard deadline packet delivery • Distributed algorithm • Every node works without any control. • All nodes can send and served in nonpreemptive priority order. • Node idle when there is no packet to be send.

  37. Protocol : Polled Bus Protocol • Polled bus protocol • Assume bus network with a bus busy line. • When processor broadcast on the bus, it maintains this high line. • When finish, this line is reset. • Line execute wired-OR operation. • When 2 signal, A and B put on a line simultaneously, the signal is A.OR.B.

  38. Example:Round Robin • Execute using FIFO • Assume : 5 processes P1, P2, P3, P4 and P5 with execution time are 10, 29, 3, 7 and 12 (unit ms) • Assume start time, t = 0 ms and quantum time = 10ms 0 10 20 23 30 40 50 52 61 • Waiting time; • P1 = 0ms • P2 = (10 + (40 - 20) + (52-50)) = 32ms • P3 = 20ms • P4 = 23ms • P5 = (30 + (50-40)) = 40ms • Average waiting time = (0 + 32 + 20 + 23 + 40) / 5 = 23ms

  39. Protocol : Deadline Based Protocol • Deadline based protocol • Preemptive vs nonpreemptive • 3 types of traffic • Guaranteed traffic System ensure that every packet must meet its deadline. • Statistical real time traffic Packet cannot miss deadline exceeding certain percentage that be in agreement • Non real-time traffic Packet did not guarantee/meet deadline (deadline is not important)

  40. Protocol Protocol for real time system

  41. Summary Communication media?? sending message?? Network topology?? Protocol???

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