ELE00040M DATA COMMUNICATION

TECHNIQUES

Introduction

Dr David Chesmore

Objectives (Learning Outcomes) 1

n  Understand the differences between synchronous and asynchronous communications.

n  Understand the necessity for flow control and the techniques that can be employed to provide it.

n  Understand the sources of errors and error control techniques to ensure data integrity within digital communication systems.

Objectives (Learning Outcomes) 1

n  Understand practical digital communications techniques such as clock embedding and recovery, line coding, DC balancing, serialisation and de-serialisation, buffering and buffer control.

n  Understand the operation of standard communication hardware for embedded systems (e.g. RS232, I2C, SPI, CAN, USB).

Syllabus 1 n  Introduction: communications issues; network

toplogies; ISO 7 layer protocol.

n  Communications media: radio (propagation mechanisms); cable; fibre optic; sources of errors (random, burst).

n  Data transmission: asynchronous, synchronous; multiplexing (FDM, WDM, TDM); line codes (NRZ, AMI, 2B1Q, etc); clock embedding; modulation (ASK, FSK, m-ary).

Syllabus 2

n  Error checking and control: Stop-and-wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ; hardware & software flow control, XON/XOFF; Hamming codes; cyclic codes and CRCs; interleaving; probability of error.

n  Practical serial communication systems: point to point and multidrop; RS232, RS422, RS423, RS485; I2C, SPI, USB, CAN; high speed issues.

Recommended Text

n  Forouzan, B.A., “Data Communications and Networking”, McGraw-Hill International, fourth edition 2007.

n  There are many other data communication books available

Communication Issues 1

n  Analogue or digital? – DIGITAL n  Does the data need to be sent/received in real

time? n  Are delays acceptable? n  Does the data need to be protected? n  What communications medium is available/

required? n  What is the transmission distance? n  What data rate is required/available?

Communication Issues 2

n  Asynchronous, synchronous? n  Data in blocks? n  Data protection (CRC, ECC, encryption)? n  Overheads

−  e.g. address, synchronisation, ECC add overhead in terms of transmission time and bandwidth

n  1 or 2 channels available (half-duplex or full-duplex)?

n  What is the error rate on the channel? n  Point to point or network?

Summary of Issues

n  What is the available bandwidth? −  determines bit rate

n  Does the data require protection? −  increases bit rate & BW

n  Does the signal need to be transmitted in real time? −  impacts on bit rate, storage requirements

n  What type of communication channel is available? −  full duplex needs 2 channels

Drax Power Station

http://www.nsf.gov/od/lpa/news/02/pr02100_images.htm

Meteorological Station

Network Topologies 1

POINT TO POINT

MESH

need n(n-1) links

Network Topologies 2

HUB STAR

BUS

BUS

Network Topologies 3

RING

Networks – Advantages/Disadvantages 1

n  Point to point −  not a network, single connection, dedicated

n  Mesh −  dedicated link guarantees data load means no

reduction in data rate due to sharing −  fault detection (e.g. cable) easy −  expensive due to cost of n(n-1) links and equipment

n  Star −  lower cost than mesh −  system depends on hub operating correctly

Networks – Advantages/Disadvantages 2

n  Bus −  ease of installation – only 1 bus cable −  reconnection? fault detecton difficult −  fault in bus stops communication even if on same side

as fault due to reflections −  need to cope with potential collisions if more than 1 unit

communicates at the same time

n  Ring −  data circulates in 1 direction – easy to install −  fault in 1 unit causes ring to fail if it can’t pass signal on

Categories of Networks

n  Local Area Network (LAN) −  often privately owned and covers building/site/campus −  Usually limited to sveral km

n  Wide Area Network (WAN) −  long distance over large geographic area

n  Metropolitan Area Network (MAN) −  size >LAN and <WAN, e.g. city or town −  w.g. telephone company providing DSL

Network Model – N layer Service

Layer N-1

Layer N

Layer N+1

Layer N-1

Layer N

Layer N+1 service

provided service request

transmission medium

N protocol service access point (SAP)

OSI Model

n  International Stanards Orgnisation (ISO) developed an open system model of communications to facilitate communication between different systems −  OSI (Open Systems Interconnect) −  7 layer model

OSI 7 Layer Protocol

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL

high level application support tools

conversion between different machine representations

applications sync & connection management

process to process delivery of messages

network addressing and routing

link control and data transmission

physical medium control, transmission and timing

networking protocols

application protocols

MEDIUM

OSI 7 Layer Model

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL INTERMEDIATE

NETWORK

transmission medium transmission medium

Protocol Data Units (PDU)

n  When application sends data, it appends a header and passes the unit (PDU) downwards

n  Next layer down appends header – new PDU n  Continues downwards until layer 2 – appends a

trailer = frame for transmission over medium

n  Headers removed as the data goes back up the layers at the receiver

PDUs

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

LINK

PHYSICAL

H

T

H

H

H

H

H

data

Layer 7 PDU

Layer 3 PDU

Layer 6 PDU

Layer 5 PDU

Layer 4 PDU

ENCAPSULATION

DECAPSULATION