2012. Eko Didik Widianto

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi

Prokotol Komunikasi Kuliah#9 TSK-612 Sistem Embedded Terdistribusi - TA 2011/2012

Eko Didik Widianto Teknik Sistem Komputer - Universitas Diponegoro

Lisensi

Review Kuliah

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Lisensi

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Sebelumnya telah dibahas tentang metodologi desain sistem embedded terdistribusi I I I I I I

Marketing & Engineering Specs Arsitektur Software Specs Desain Implementasi Pengujian

Tentang Kuliah #9 Protokol Komunikasi

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan

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Jika ingin membuat sistem embedded terdistribusi I

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Pokok Bahasan: I I

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Sistem bus: jaringan embedded, multiplexing, bus Menempatkan bit ke jalur transmisi: antarmuka fisik, enkoding bit Klasifikasi protokol: operasi umum, trade-off (tidak ada protokol yang ’terbaik’), wired vs wireless

Link I

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Maka diperlukan jaringan

Website: http://didik.blog.undip.ac.id/2012/03/06/ kuliah-tsk-612-sistem-embedded-terdistribusi-2011/ Email: [email protected]

Acknowledgement: I

http://www.ece.cmu.edu/~ece649/[ECE649]

Teknik Transmisi Data Protokol Komunikasi Lisensi

Bahasan Topologi Jaringan Topologi Bus Topologi Star Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction Proteksi Terhadap Spike Enkoding (Channel) Message Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks Lisensi

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Lisensi

Topologi Jaringan

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Topologi Bus Topologi Star

Teknik Transmisi Data

I Topologi jaringan sistem

Lisensi

embedded I

Linear: bus I

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satu jalur bus digunakan secara bersama multiplexing akses jalur

Star I

Protokol Komunikasi

satu node menjadi pusat

Bahasan Topologi Jaringan Topologi Bus Topologi Star



Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction Proteksi Terhadap Spike Enkoding (Channel) Message Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks Lisensi

Protokol Komunikasi Lisensi

Topologi Bus


Teknik Transmisi Data Protokol Komunikasi

I Sistem jaringan bus I

Cocok untuk komunikasi berjarak jauh antar-subsistem I

Misalnya: elevator, jalur perakitan

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Digunakan di sebagian besar sistem kontrol embedded

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Beragam opsi protokol dapat diimplementasikan (fleksibel) Putusnya satu segmen kabel akan memisahkan bus Tidak cocok untuk jalur fiber optik karena masalah di splitting dan merging

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Sistem jaringan desktop sebelumnya menggunakan bus

I Contoh: I

serial comm, CAN (Controller Area Network), Inter-Integrated Circuit (I2C), Serial Peripheral Interface (SPI), Firewire (high speed serial bus IEEE 1394)

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Topologi Star


I Sistem jaringan star

Topologi Bus Topologi Star

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Dapat mengemulasikan fungsi bus Mudah untuk mendeteksi dan mengisolasi kegagalan Terputusnya jalur hanya berefek pada satu node Cocok untuk fiber optik

I Memerlukan lebih banyak pengkabelan I

Digunakan secara umum di sistem desktop

I Terputusnya hub (sebagai pusat jaringan) berakibat satu jaringan

terputus I Bisa digunakan untuk mengisolasi node yang membangkitkan terlalu

banyak trafik I Topologi star ini semakin populer I

jaringan kontrol yang safety-critical menggunakan dual redundant star (dua jaringan independen, tiap jaringan mempunyai topologi star)

Teknik Transmisi Data Protokol Komunikasi Lisensi


Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction


Proteksi Terhadap Spike Enkoding (Channel) Message


Transmisi Data (Bit)

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction Proteksi Terhadap Spike

I Rangkaian (digital) harus

memberikan sinyal HI dan LO di jalur bus

Enkoding (Channel) Message


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Contoh: HI = ’1’ = 5 volt dan LO = ’0’ = 0 volt Titik referensi: GND

I Sistem embedded harus tahan

terhadap noise (noise immunity) I

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Isolasi noise di node tunggal sebelum data ditransmisikan Hindari noise di jaringan






Teknik untuk Mengurangi Noise Teknik Differential Driver


I Mengirimkan nilai Data dan Data

Inversenya ke bus 2-jalur I

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Contoh: Data: HI = ’1’ = 5 volt dan LO = ’0’ = 0 volt Inverse Data: HI = ’1’ = 0 volt dan LO = ’0’ = 5 volt Titik referensi: GND

I Receiver mengurangi 2 nilai data

data I

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Menghilangkan tegangan bias common-mode Menyisakan selisih noise yang mempengaruhi jalur



RS-485 Multi-Master Bus


I Digunakan di jaringan kontrol industrial (seperti Modbus, Profibus) I

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RS-422 differential drivers: high-speed + jangkauan jauh (10Mbps @12 meter) Ditambahkan terminator untuk mengurangi noise Harus hanya satu sistem yang outputnya diaktifkan dalam satu waktu (multiplexing access) I

Sering disebut “master/slave” - satu sistem memerintahkan sistem lain saat gilirannya tiba

Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction Proteksi Terhadap Spike Enkoding (Channel) Message







Isolator Optik untuk Proteksi Tegangan Spike I Tegangan spike dari noise yang besar dapat merusak node-node yang

terkoneksi I Diperlukan isolasi untuk mengatasi tegangan spike yang curam dan besar I Isolator optik menyediakan gap udara secara fisik I LED akan berpendar saat disediakan arus I Photo-transistor menghantar saat LED menyinarinya dengan IR I Dua set di tiap-tiap node I Satu set untuk transmit, dan satu set untuk terima I Menyediakan isolasi yang sangat baik I Tanpa ada koneksi fisik, hanya photon yang melintasi gap I LED akan saturasi, jika diberikan arus yang berlebih (over-drive) I Masih mempunyai noise I Jaringan harus mempunyai sumber daya sendiri I Menyediakan bit dominance I Namun, jika LED terpaku ke “on”, jaringan akan terganggu

Sumber: http://en.wikipedia.org/wiki/Opto-isolator

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Transmisi Data (Bit) Noise Reduction Proteksi Terhadap Spike Enkoding (Channel) Message


Transformer Coupling dan Stuck Node I Stuck node merupakan masalah di sistem terdistribusi

Jika sebuah node terpaku dengan mentransmisikan ’low’ atau ’high’, seluruh jaringan akan mati

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I Solusi: menggunakan current-mode transformer

coupling I

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Komponen AC dari sisi bit akan menyeberang transformer Komponen DC dari node yang stuck diabaikan Induktansi transformer memproteksi tegangan spike Operasi current-mode akan meningkatkan noise rejection Umumnya digunakan di kontrol penerbangan

misalnya: MIL-STD-1553 (defines mechanical, electrical, and functional characteristics of a serial data bus), sistem avionics I Tapi mempunyai keterbatasan I

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TIdak dapat melakukan bit dominance Deteksi collision sangat sulit Sinyal harus DC balanced (energi sinyal high dan low sama)






Protokol Komunikasi

Jenis Enkoding Kanal I

RZ – Return to Zero encoding I

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Topologi Jaringan

Encoding memastikan sinyal kembali ke “zero” sesering mungkin Memaksa transisi setiap satu atau dua bit dengan aturan enkoding sederhana I I

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@2012,Eko Didik Widianto

Memudahkan mensinkronisasi bit stream di sisi penerima Memudahkan penggunaan coupling transformer

NRZ – Non-Return to Zero encoding I

Ditujukan untuk meningkatkan efisiensi hanya dengan mengirimkan nilai bit walaupun tanpa transisi I

Tapi kurangnya transisi menyulitkan untuk mensinkronisasi bit stream

Kode RZ

Kode NRZ



Manchester Coding I

Manchester Encoding I

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Data dienkode dengan transisi dari high-to-low (transisi turun) atau low-to-high (transisi naik) Pasti terdapat transisi di tiap bit I

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Bandwidth terbesar adalah 2 transisi perbit

Kalau terjadi error, dikoreksi dengan menginversikan pasangan bit fisik terbesar

Contoh: 11010001

Enkoding NRZ I Send a Zero as LO; send One as HI I

Kasus terburuk: message mempunyai semua ZERO atau semua ONE

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Solusiya: membatasi panjang data menjadi 8 bit

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Tidak ada transisi di data

SYNC dan END mempunyai nilai yang berlawanan, memastikan ada 2 sisi per message Teknik ini digunakan di port serial UART

Bandwidthnya satu transisi perbit

Contoh: 11010001






Message


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Sync I

Designates start of a message and lets receiver sync to incoming bits



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Header I

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Data I

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Application- or high-level-standard defined data fields (often only 1-8 bytes)

Error detection I

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Global priority information (which message gets on bus first?) Routing information (source, destination)

Detects corrupted data (e.g., using a CRC)

End I

Designates end of message

Prioritas Message


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Local priority I

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Each node transmits its highest priority message when it gets a turn on the bus Or, it can implement some form of round-robin message transmission, etc.

Global priority I

Which node gets the next turn on the bus? I I I

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Could be a function of round-robin selection of nodes Could be a function of the node’s inherent priority Could be a function of the priority of the highest message on the node

Fundamental tension: latency vs fairness I

Reducing latency for high-priority nodes/messages vs Ensuring fairness/no starvation for low-priority nodes/messages



Protokol Komunikasi Embedded

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

Lisensi

Message Coordination

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA)

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Message coordination: I I I I I

Bus Master Appoarch Transmit and Hope (CSMA, Carrier Sense Multiple Access) Explicit token Implicit tokens Binary countdown (bit dominance)

Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

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Coordination: Bus Master Approach I

Bus Master can poll for messages & wait for response I

Problem: missing/slow slave I I

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Master uses worst-case timeout waiting for response If slave gets confused/is late, protocol fails

Problem: broken master


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Master can send a time tick – TDMA I I

Other nodes select response time from that time tick Then becomes a form of time slice/time slot protocols

Protokol Komunikasi

Polling

@2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

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Operation I I

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Centrally assigned Master polls the other nodes (slaves) Non-master nodes transmit messages when they are polled Inter-slave communication through the master

Examples I

MIL-STD-1553B, 1773, Profibus, Bacnet, AN192

Lisensi

Polling Tradeoff

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data

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Advantages

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• Simple protocol to implement I I

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historically very popular Bounded latency for real-time applications

Disadvantages I I I

Single point of failure from centralized master Polling consumes bandwidth Network size fixed during installation (not robust) I

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Or, master must discover nodes during reconfiguration

Prioritization is local to each node I I

But, can use centralized load balancing Polling need not be in strict order; it could be, for example: 1, 2, 1, 3, 4, 1, 5, 1, 3, 1, 6, ...(repeats)

Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

Lisensi

TDMA - Time Division Multiplexed Access


Lisensi

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Operation I I

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Master node sends out a frame sync to synchronize clocks Each node transmits during its unique time slot

Examples I

Satellite Networks, static portion of FlexRay

TDMA Tradeoff


I Advantages

Teknik Transmisi Data Protokol Komunikasi

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Simple protocol to implement Deterministic response time No wasted time for Master polling messages

I Disadvantages I I I I I

Single point of failure from the bus master Wasted bandwidth when some nodes are idle Requires stable clocks Network size fixed during installation (not robust) Prioritization is local to each node I

can use centralized load balancing

I Variation: Variable Length TDMA I I I

Unused time slices are truncated to save time More efficient use of bandwidth Used in FlexRay Dynamic Segment (http://en.wikipedia.org/wiki/FlexRay)

Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

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Coordination: Transmit and Hope (CSMA) Carrier Sense Multiple Access I

Send a message and hope it made it I I I

Useful for satellites & systems with no collision detection Vulnerable for entire time a message is transmitting No direct way to know if message was delivered successfully


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IMPLICIT collision detection I I

Response might not make it even if message makes it Iterate until some node pair gets lucky twice in a row

Transmit And Collide (CSMA/CD) I

Transmit message I I I

If you get lucky network transitions to active But If you get unlucky, you get a collision event Vulnerability window is about 2 tpd I I

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Two propagation delays along length of network The time during which the receiver hears nothing, but a frame from another transmitter is already in-flight

After collision, back off a certain time I I

Amount of time to back off should vary with network load Repeated collisions result in increasing backoff times


Lisensi

Transmit And Collide (CSMA/CD) Carrier Sense Multiple Access / Collision Detection

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance)

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Operation

Emulation Wireless Networks

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A node waits for an idle channel before transmitting Collisions can occur if two or more nodes transmit simultaneously If a collision is detected, the nodes stop transmitting I

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Resolve contention using random backoff algorithm (2x longer interval each retry)

Examples I

Ethernet (IEEE 802.3 LAN), Bacnet, CAB, CEBus

Lisensi

CSMA/CD Tradeoffs


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Advantages I I I

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Small latency for low traffic load Network initialization/configuration is not required Node can enter or leave the network without any interruption Supports many nodes Probabilistic global prioritization is possible

Disadvantages I

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Designed for aperiodic traffic - not ideal for synchronized control loops Collision detection is an analog process which is not always practical Unbounded individual message latency Poor efficiency under heavy loads


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Coordination: Explicit Tokens I Token value says which node is transmitting and/or should transmit next I I

Token holder = OWNER; only the owner may transmit Master/slave polling is a special form where token is passed by master and returned to master by slave

I Problems: Lost token / Duplicated token(s) / Who starts?


Lisensi

I Token passed as node number or other similar value I I

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May be tacked on to end of data-bearing message Can be either node# that has token or node# that gets token next Null messages with tokens must be passed to prevent network from going idle

Token Bus


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Operation I

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A token signal is passed from a node to node on a bus (virtual ring) Only the token holder has permission to access the media

Examples I

IEEE 802.4, Arcnet, AN192, MAP, Profibus

Token Bus Tradeoffs


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Advantages I I I

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Bounded latency for real-time control applications High throughput during heavy traffic On-the-fly reconfiguration

Disadvantages I I I I I

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Token passing latencies under light traffic conditions Prioritization local to each node Lengthy reconfiguration process Token initialization, loss, and duplication recovery overhead Collisions may occur during initialization and reconfiguration Complex protocol (especially at MAC sublayer) Token bus was popular for a while, but is used less often now


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Coordination: Implicit Tokens I

Length of waiting period is used as a time-domain implicit “token” I

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Owner of bus determined by what time it is instead of explicit token message

Time slices – waiting period is a whole message long I

TDMA


Lisensi

I

Time slots – waiting period is as short as possible ~ 2tpd I

CSMA/CA (collision avoidance)

CSMA/CA (Implicit Token)


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Operation I I

IDLE: Active station transmits immediately After each message, reserve S slots for N nodes I

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BUSY: Transmit during your assigned slot I I

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Slots are normally idle – they are time intervals, not signals!

If S=N, no collisions - known as Reservation CSMA If S
Example I

Echelon LONTalk

Lisensi

CSMA/CA Tradeoffs


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Advantages I I I

Small latency for light traffic Good throughput under heavy traffic Global prioritization through fixed slots I

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prioritized implicit token passes

Bounded latency through rotating slots


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non-prioritized implicit token passes

Disadvantages I

Restarting time slots from an idle bus can be difficult I

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Send dummy messages to avoid idle state

Collisions can occur Node complexity in mapping Sth slot to Nth node

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Lisensi

Binary Countdown (Bit Dominance)

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation

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Operation I I

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Each node is assigned a unique identification number All nodes wishing to transmit compete for the channel by transmitting a binary signal based on their identification value A node drops out the competition if it detects a dominant state while transmitting a passive state Thus, the node with the LOWEST identification value wins

Examples I

CAN, SAE J1850

Wireless Networks

Lisensi

Contoh Bit Dominance


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Binary Countdown Tradeoffs

Protokol Komunikasi @2012,Eko Didik Widianto Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach

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Advantages I I I

High throughput under light loads Local and global prioritization possible Arbitration is part of the message - low overhead

Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation

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Disadvantages I I

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Propagation delay limits bus length (2 tpd bit length) Unfair access - node with a high priority can "hog" the network Poor latency for low priority nodes

Wireless Networks

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Before CAN


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After CAN


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CAN is Central of Automotive Networks


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Protokol Komunikasi

Emulation

@2012,Eko Didik Widianto Topologi Jaringan

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You can use one protocol to emulate another

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Use Ethernet (CSMA/CD) to emulate: I

Master/slave polling I

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Token bus – use explicit token messages I

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application only transmits when it has the token

TDMA I

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slaves only respond when polled

slaves measure time from message from master and transmit appropriately

But, there is cost I I

“Slot” time involves round-trip through OS “Slice” time must account for CPU/OS jitter, not just HW oscillator drift

Teknik Transmisi Data Protokol Komunikasi Coordination: Bus Master Approach Coordination: Transmit and Hope (CSMA) Coordination: Explicit Tokens Coordination: Implicit Tokens Binary Countdown (Bit Dominance) Emulation Wireless Networks

Lisensi

Emulation Capability I

Protocols higher in picture can emulate protocols lower in picture I

Example: you can pass a token around on a CAN network in software


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Wireless Network I

Strength is installation flexibility I I I

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Weakness is potential unreliability for critical operations I I

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No wiring harnesses to install (except for power) Can make/break networks without physical connections Can have overlapping/interacting/hierarchical networks (e.g., Bluetooth) Geometry may introduce standing waves/fading Conflicts with other wireless systems (EMC = ElectroMagnetic Compatibility) Interference from RF emitters (EMI = ElectroMagnetic Interference) Limited spectrum space Where does a wireless node get its power – who changes the batteries? In general, unsuitable for use in critical applications that aren’t fail-safe!

Also, cost I

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Bluetooth is getting cheap enough to be in consumer electronics But has to be able to beat a piece of copper and a plastic connector And that cost has to include power supply strategy


Lisensi

Protokol Komunikasi

Lisensi

@2012,Eko Didik Widianto

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Topologi Jaringan Teknik Transmisi Data Protokol Komunikasi Lisensi

2012. Eko Didik Widianto

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