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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 l.jpg

Chapter 7


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  • Introduction

  • Media communication

  • Network Topology

  • Sending message

  • Network Design Issues

  • Protocol

  • Summary

Introduction l.jpg

  • 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.

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Real time system structure

Sensor and actuator layer

Peripheral cluster

central cluster

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  • 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).

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  • 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

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  • 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).

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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.

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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.

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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

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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

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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

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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 l.jpg
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.

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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.

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Network Topology

Network path



Node 1

Node n

Shared network

Node 1



Node 2

Node 3



Node 4

Point-to-point network

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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

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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 l.jpg
Network Topology

  • 5. Multistage network

    • Built out of switchboxes

    • 4 confguration.

4 configuration 2 X 2 switch

Physical Topology vs Virtual Topology???

Sending message l.jpg
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

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Sending Message



Circuit switching

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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.

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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.

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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.

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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 l.jpg

  • 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 l.jpg
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 l.jpg
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 l.jpg
Protokol : VTCSMA

Operation of virtual clock



Channel busy



Channel idle







Protokol window protocol l.jpg
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 l.jpg
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 l.jpg
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.

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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 l.jpg
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

Protokol l.jpg

  • 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.

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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.

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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.

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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

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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


Protocol40 l.jpg

Protocol for real time system

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Communication media??

sending message??

Network topology??