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

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chapter 7

Chapter 7


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

Real time system structure

Sensor and actuator layer

Peripheral cluster

central cluster

  • 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).
  • 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
  • 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).
communication media
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.
communication media9
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.
communication media10


Network interface

Optical to electrical

Electrical to optical




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

communication media11
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

communication media12
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

communication media13
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.
network topology
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.
network topology15
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.
network topology16
Network Topology

Network path



Node 1

Node n

Shared network

Node 1



Node 2

Node 3



Node 4

Point-to-point network

network topology17
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

network topology18
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





Hypercube network :


network topology19
Network Topology
  • 5. Multistage network
    • Built out of switchboxes
    • 4 confguration.

4 configuration 2 X 2 switch

Physical Topology vs Virtual Topology???

sending message
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
sending message21
Sending Message



Circuit switching

network design issues
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.
network design issues23
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.
network design issues24
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.
network design issues25
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.











  • 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
protocol vtcsma
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??
protocol vtcsma28
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.
protokol vtcsma
Protokol : VTCSMA

Operation of virtual clock



Channel busy



Channel idle







protokol window protocol
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.
protokol token based protocol
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/
protokol token based protocol32
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.
protokol time token protocol
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.
protokol token ring
Protokol : Token Ring

Error control










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

  • 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.
protocol stop and go multihop protocol
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.
protocol polled bus protocol
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.
example round robin
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










  • 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
protocol deadline based protocol
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



Protocol for real time system


Communication media??

sending message??

Network topology??