Communications & Networks – Introduction

Alexander Nelson

February 19th, 2018

University of Arkansas - Department of Computer Science and Computer Engineering

Networking

Number of Connected Devices

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How can 50B devices all be connected?

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Wireless Networking Categories

• LPWAN – Low Power

Wide Area Network

• WWAN - Wireless

Wide Area Network

• WPAN – Wireless

Personal Area

Network

• WLAN – Wireless

Local Area Network

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WWANs

Wireless Networking Categories

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1“Smart Cities Technologies” Ivan Nunes Da Silva and Rogerio Andrade –

ISBN 978-953-51-2808-3

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WWAN – Wireless Wide Area Network

WWAN – Set of technologies that provide wireless access to

geographically distant sources

Includes:

• Cellular Technologies

• 2G

• 3G

• LTE

• LTE-M

• IEEE 802.16 – WiMAX

• LPWAN Technologies

• LoRA

• SigFox

• Ingenu

• Mesh Networking?

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Cellular Communications

Cellular Network – Communication network where last link is

wireless

Distributed over land areas – “cells”

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2By Andrew pmk - SVG version of Frequency reuse.fig by Mozzerati, CC

BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=2155967 5

Cellular Communications

Advantages:

• More capacity than a single large transmitter, same frequency

used for multiple links in different cells

• Cell towers are closer – Mobile devices use less power than

with single transmitter

• Larger coverage area than a single terrestrial transmitter –

additional cell towers can be added indefinitely

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Cellular Communications

Range is inversely proportional to frequency

CDMA2000 Network

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WiMAX

WiMAX – Worldwide Interoperability for Microwave Access

Microwave meaning 300MHz-300GHz frequencies

Meant to provide “last-mile” broadband over wireless

Originally 30-40Mbps – Upgraded to 1Gbps in 2011

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WiMAX

WiMAX – Worldwide Interoperability for Microwave Access

Microwave meaning 300MHz-300GHz frequencies

Meant to provide “last-mile” broadband over wireless

Originally 30-40Mbps – Upgraded to 1Gbps in 2011

Most WiMAX deployments are winding down

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LPWAN Communications

LPWAN – Low Power Wide Area Network

Designed for long range communications at low bit rates

Data Rates – 0.3kbps-50kbps

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LPWAN Communications

LPWAN – Low Power Wide Area Network

Designed for long range communications at low bit rates

Data Rates – 0.3kbps-50kbps

Intended for sensor-type deployments at long range

Power IoT/Smart-City type applications

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LPWAN Communications

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Mesh Networks

WANETs – Wireless Ad hoc Networks

MANETs – Mobile Ad hoc Networks

ad hoc – latin, literally “for this” – Does not rely on static

infrastructure

MANETs allow for nodes to move freely geographically

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Mesh Networks

Mesh Networks as WWAN?

Communications technologies similar to cellular networks

Can provide WAN access to small devices through sink nodes

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Wireless Network Challenges

Wireless Communication

Given a certain amount of RF frequency spectrum, how to

optimize communication?

Metrics:

• Throughput – Total network bits-per-second

• Latency – Average time form packet creation to reception

• Fairness – Do devices receive equal share of resource

(airtime)?

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Multiple Access Techniques

If multiple wireless clients exist, how to divide resources?

Multiple Access Methods – Techniques for dividing resources

• CSMA – Carrier Sense Multiple Access

• TDMA – Time Division Multiple Access

• FDMA – Frequency Division Multiple Access

• CDMA – Code Division Multiple Access

• SDMA – Space Division Multiple Access

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Carrier-Sense Multiple Access

CSMA – Carrier Sense Multiple Access

Nodes check to see if anyone is communicating

If channel is empty, they may start

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Hidden Node Problem

Not detecting communication does not mean channel is clear

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CSMA/CA

CSMA/CA – CSMA with Collision Avoidance

1. Carrier Sense – Listen for other transmission

2. Random Backoff – If channel is busy, backoff for random

amount of time

3. Request to Send/Clear to Send (RTS/CTS) – Access

Control Signals

4. Transmission – Send entire packet to Access Point (AP)

5. Acknowledgment – AP sends acknowledgment if packet is

received

6. Exponential Backoff – If no ACK, assume collision and

backoff

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CSMA

Advantages

• Low latency if channel is not busy

• No need for centralized control

Disadvantages

• Busy channel causes many collisions – low throughput & high

energy waste

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TDMA

TDMA – Time Division Multiple Access

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3By Mozzerati at English Wikipedia - 13:13, 2004 Sep 5 (UTC), CC BY-SA

3.0, https://commons.wikimedia.org/w/index.php?curid=32927708

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TDMA

Advantages

• Nodes only need to listen during frame – save power

• Guaranteed no collision

Disadvantages

• Lower Throughput – A node may not need to use their frame

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FDMA

FDMA – Frequency Division Multiple Access

Divide available frequency into channels

Nodes communicate on their own channel

Wi-Fi FDMA – 5MHz channel width – 22MHz spectrum spread

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CDMA

CDMA – Code Division Multiple Access

Use orthogonal psuedorandom codes to allow additive

communication on same spectrum

Four mutually orthogonal codes24

CDMA Example

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CDMA

Advantages

• Multiple nodes may communicate at the same time

• Flexible length of codes – dynamic allocation

Disadvantages

• Reduced throughput per device per frame

That is, takes n times as long to transmit a single bit, where n

is length of the code

• Subject to Near-Far problem

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Near Far Problem

Problem with SNR for geographically disparate nodes

Near node Noise may be similar to magnitude of far node signal

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Phase Shift Keying

Phase Shift Keying – PSK

Code division multiple access – Codes are multiplexed by phase

rather than time

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