Wireless Design Considerations for Industrial Applications

Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved.

PUBLIC INFORMATION

Wireless Design Considerations for Industrial Applications


Introduction

This presentation is an overview of the joint Rockwell Automation/Cisco®

application guide

ENET-TD004 “Wireless Design Considerations for Industrial Applications”

Please refer to the complete guide for:

Full list of recommendations

Details of Wireless Local Area Network (WLAN) implementation

Test details and full results

Links to Cisco documentation on wireless technology

Copyright © 2014 Rockwell Automation, Inc. All Rights Reserved. 3

Agenda

Wireless Application Considerations

Technology Overview

WLAN Design Considerations

Wireless Equipment Use Cases

Wireless Application Evaluation


Technology Overview

4

Advantages of a wireless network include:

Lower installation costs due to cabling and hardware reduction

Lower operational costs by eliminating cable failures

Ability to connect hard-to-reach, restricted and remote areas

Gains in productivity and efficiency due to equipment mobility

Higher productivity and less downtime due to personnel mobility


Technology Overview

5

Wireless is different from wired media

Half-duplex shared medium: only one device can transmit at a time

Wireless signal may vary with time and direction

Wireless coverage area cannot be precisely defined

Signal may reach beyond the intended area

Some packet loss compared to wired Ethernet

Less protected from interference

Dynamic network topologies due to wireless roaming

Advantages > Challenges (Protocol + Device + Environment )


Technology Overview

6

Wireless

sensors

Process Control

WirelessHART ISA-100.11a

3G / 4G LTE / WiMAX FHSS 900 MHz / 2.4 GHz

Wi-Fi: 802.11a/g/n

Remote site

connectivity

Long haul SCADA

Monitoring and supervisory control

I/O control

Peer-to-peer control

Safety control

Mobile HMI

Performance

Industrial Wireless Technologies

Different characteristics and use areas


Technology Overview

8

Wireless Client Types

WGB

WGB Access Point (AP)

Bridge

Embedded Adapter: • Higher cost • Antenna limitations • Placement limitations

Universal Bridge: • Single wired client

(MAC address)

Workgroup Bridge (WGB): • Multiple wired

clients (MAC addresses)

• Viewed as a single wireless client on the network

Workgroup Bridge is the main method of connecting to WLAN


Technology Overview

10

Autonomous WLAN Architecture

WGB

Autonomous AP

…

SSID1

5 GHz

Access Switches

Distribution Switch

1

2

3

1

EtherNet/IP (Wired)

2

EtherNet/IP (Wireless)

3

AP management

1

Autonomous AP

…

SSID2

2.4/5 GHz

3

• Each autonomous Access Point (AP)

is managed individually

• Limited coordination between APs

• Stand-alone IACS applications

• Fixed clients and predetermined traffic


Technology Overview

11

Unified WLAN Architecture

• Functionality is split between

lightweight APs and Wireless LAN

Controller (WLC)

• Centralized management of WLAN

parameters

• Zero-touch AP deployment

• Large scale plant-wide coverage

WGB

Lightweight AP

…

SSID1 5 GHz

…

SSID2 2.4/5 GHz

2

3

Lightweight AP

WLC


MSE ISE NCS

1

1

EtherNet/IP (Wired)

2

3

WLC / AP management

CAPWAP tunnel (Data / Control)

1

1


Agenda


Technology Overview





Wireless use cases include…

Equipment Wire Replacement / Mobility

Hard to reach (costly)

Valuable data without system connection requirement

Moving parts on static machines

Portable or moving equipment

Personnel Mobility

Mobile HMI and operator access

Maintenance and engineering access

Wireless voice communication

Vendor guest access


13

Asset and Personnel Tracking

Radio Frequency Identification

(RFID)

Real Time Location Services

(RTLS)

Remote Device Monitoring

Condition based maintenance

Instrumentation of existing

machinery

Video surveillance

Long Haul SCADA Communication

Process Instrumentation



14

Single Coverage Cell (No Roaming)

WGB

Autonomous AP

…

SSID1

5 GHz

WGB

Fixed Position • Equipment is static while operating • Associated to the same AP • Installed in a permanent location


WGB


15


WGB

Autonomous AP

…

SSID1

5 GHz

Nomadic (Non-operational Relocation) • Equipment is static while operating • Associated to the same AP while

operating • Moves to a new location in the

shutdown state

Autonomous AP



16


Autonomous AP SSID1

5 GHz

Mobile (No Roaming) • Equipment is moving while

operating • Associated to the same AP while

operating • May rapidly change position

and orientation

WGB



17

Multiple Coverage Cells (Roaming)

Lightweight AP SSID1

5 GHz

Mobile (Fast Roaming) • Equipment is moving while

operating • Changes association (roams)

between APs while operating • Roaming delays do not cause

application timeouts (target <50 ms)

WLC

… WGB WGB

Lightweight AP



18


WGB

AP

…

SSID1 5 GHz

Access Switches

Distribution Switch Stack

1

2

1

EtherNet/IP (Wired)

2 1

Coverage Cell/Area

WGB

Fixed PAC

Mobile I/O

I/O, Safety I/O

Mobile I/O

Wired PAC to Wireless I/O



19


1

EtherNet/IP (Wired)

2

Wired PAC to Wireless PAC

WGB

AP

…

SSID1 5 GHz

Access Switches


2

1

Coverage Cell/Area

WGB

Fixed PAC

Mobile PAC

P/C, Safety P/C, MSG

1

Mobile PAC



20


1

EtherNet/IP (Wired)

2

Wireless PAC to Wireless PAC or I/O

WGB

AP

…

SSID1 5 GHz

Access Switches


1

Coverage Cell/Area

WGB

Fixed PAC

Mobile I/O

1

Mobile PAC

WGB

1

Mobile PAC

2 2

Not recommended: x2 bandwidth, higher latency


Agenda


Technology Overview






24

Preparing for WLAN Implementation

1. Identify Site Requirements:

Number of wireless channels available and in use

IT policy regulating wireless spectrum in the facility

Existing and potential sources of wireless interference in the area

Locations, dimensions, material compositions of required coverage areas

Environmental characteristics of the site

Any obstructions that may enter and leave the coverage areas

Installation limitations for the antennas, APs, and cabling

Has a site survey been done? What was the survey equipment

and parameters?

Proper RF design and comprehensive site survey are crucial



25


2. Identify Network Requirements:

WLAN architecture (Autonomous or Unified)

Existing WLAN and switch infrastructure

Who is responsible for managing WLAN?

Required WLAN security

Required network redundancy

IP addressing, DHCP, VLAN requirements

Early collaboration with IT personnel is critical



26


3. Identify Application Requirements:

Number and type of wireless and wired devices

Type of CIP and non-CIP protocols required by the application

Packet intervals, size, and packet per second (PPS) rate for each type

of traffic

Explicit Messaging

Standard P/C and I/O

Safety P/C and I/O

Motion P/C (Virtual Axis), CIP Sync

Directional flow of the traffic per protocol

Total PPS per wireless channel

Is wireless technology appropriate for your application?



27


3. Identify Application Requirements:

Application timeout requirements per protocol

Maximum tolerable latency and jitter per protocol

Handling of lost or late data packets by the application

Time synchronization requirements

Equipment mobility requirements

Does it require fast roaming?

If multiple identical applications need to operate throughout the plant

Number of installations and distance between each operation area

Is wireless technology appropriate for your application?


Agenda


Technology Overview






29

Packet Rate Limitations

Total packet rate in a wireless channel is the main factor that determines application

performance

Do not exceed 2,200 PPS in a wireless channel

Reduce packet rate in environments with RF issues and interference

Reserve 20% of bandwidth for HMI and maintenance traffic

All communication should be accounted for, including non-CIP packets and traffic from

neighboring WLAN sharing the same channel

Use rack-optimized I/O (vs. direct) connections when possible. Minimize the number of

individual connections by using produce/consume tags, large arrays, data aggregation and

other techniques

Direct communication between wireless clients should be limited, since each packet is

transmitted twice (upstream to the AP and downstream from the AP)

Use techniques to reduce packet rate as much as possible



30

Wireless Node Limitations

Number of wireless clients in the cell affects the performance, especially in

the heavily loaded channel

Do not exceed 20 wireless nodes (WGBs or embedded adapters) per AP

Number of nodes may need to be smaller with high packet rate

Do not exceed 19 wired clients per WGB

Total number of Ethernet devices on a single VLAN (wired or wireless)

should be below 200 to restrict the amount of broadcast traffic



31

Latency and Jitter

Average wireless latency and jitter should meet the requirements of many

applications with these conditions:

A channel is loaded below the limit

Proper Quality of Service (QoS) policy is applied

Very small percentage of packets are delayed significantly. An application should

be able to handle delayed packets.

Requested Packet Intervals (RPIs) faster than 5ms may not be useful

Overloading the channel will quickly lead to excessive latency and timeouts

Larger number of wireless nodes increases maximum latency

Certain events can also cause significant delays and packet drops:

Wireless roaming

Periodic RF monitoring of channels, if enabled

Persistent interference in the channel



32

Packet Loss and Reliability

If a wireless frame is not received, it will be retransmitted until retry limit

is reached

An application should be able to tolerate occasional packet loss

With normal channel load, RF conditions, and recommended QoS

configuration, the expected application-level packet loss is very small

Excessive packet rate causes high packet loss and application timeouts

Large number of wireless nodes may increase chance of timeouts

Multicast and broadcast traffic is much less reliable than unicast traffic

Changes in RF environment, interference, or unauthorized channel

transmissions may decrease reliability or even completely disrupt wireless

communication. This risk should always be considered for the application.

Wireless is not lossless but can be sufficiently reliable



33

Unicast vs. Multicast

Multicast wireless frames are not acknowledged and not repeated if lost.

Use only unicast EtherNet/IP connections with I/O or Produced / Consumed

Do not use ControlLogix® Redundancy System with wireless communication

CIP Sync uses multicast at a low rate

Configure IGMP snooping and querier in the network infrastructure

Traffic Type Unicast Support / RSLogix™ version

Standard I/O v18 (ControlLogix Redundancy - multicast only)

Standard Produced /

Consumed

v16 (ControlLogix Redundancy – multicast only for consumed tags)

Safety I/O v20

Safety Produced /

Consumed

v19

CIP Sync Multicast only (as of v21)



34

Application Protocols with 802.11

IACS Protocol Type CIP Standard Use with

Wireless

Constraints

Information and

diagnostics;

Peer to peer messaging

CIP Class 3

(HMI, MSG)

Yes

Peer to Peer Control;

I/O Control

CIP Class 1

(P/C, I/O)

Yes Higher latency and jitter may be an issue if an

application depend on exact timing of updates

Safety Control CIP Safety Yes Very fast safety reaction times may not be

supported

Time Synchronization

(IEEE 1588 PTP)

CIP Sync Limited Accuracy of ~150 µs can be achieved; suitable

for Sequence of Events (SoE) and event

logging applications;

Motion Control CIP Motion;

Produce/Consume

Virtual Axis

Caution:

Experimental

Position accuracy depends on CIP Sync

performance;

Direct CIP Motion control is not feasible;

Virtual Produce/Consume axis may be possible

for low performance applications

Additional detail on protocol characteristics see Application Guide



36

CIP Safety & Standard over wireless

Based on Reference Architecture lab testing:

Safety RPI as low as 15ms can be supported (Produced / Consumed)

Standard RPI of 20ms can be supported

As low as 10ms depending on the application sensitivity to jitter and delay

Connection Reaction Time Limit (CRTL) for a CIP Safety connection over wireless should

be at least:

60 ms for Safety Produced / Consumed

72 ms for Safety I/O

CRTL should be x4 greater than the RPI value (can handle 2 lost packets in a row).

CRTL may need to be increased further to prevent safety connection timeouts by changing

Safety multipliers.

Assuming packet rate below the limit and proper QoS policy



37

8 WGBs

Min. 6,000,000 samples

Avg. latency <2ms vs. 0.3ms wired

99.99% samples <10ms latency

No connection timeouts

Test results example: Wireless Latency

I/O or P/C

Packet rate,

pps

Total

Packet

rate, pps

Std I/O or

P/C RPI,

ms

Safety

Input RPI,

ms

Safety Task

Period, ms

Measured Network Latency

AP to WGB / WGB to AP, ms

Avg. Max.

99.99% samples

Max.

100% samples

2,222 (I/O) 2,375 20 18 30 0.7 / 1.2 2.5 / 6.8 12.5 / 20.8

2,133 (P/C) 2,723 15 15 15 0.7 / 1.5 2.2 / 7.5 27.1 / 16.8

For complete test results see Application Guide

WGB to AP, ms AP to WGB, ms



38

8 WGBs

Min. 1,000,000 samples

Measured screw-to-screw time

Up to 14 days continuos run with no

connection faults

99.99% samples < No-fault

theoretical worst case SRT

Test results example: Safety Reaction Time

I/O or P/C

Packet

Rate, pps

Total

Rate, pps

Safety

Input RPI,

ms

Safety

Task

Period, ms

Input I/O

or P/C

CRTL, ms

Theoretical SRT, ms

No fault / Single fault

Observed SRT, ms

Avg.

Max.

99.99%

samples

Max.

100%

samples

2,222 (I/O) 2,375 18 30 72 117 / 171 52 79 152

2,133 (P/C) 2,723 15 15 60 125 / 158 58 78 89

Wired

Wireless


Agenda


Technology Overview






40

Radio Spectrum

5 GHz frequency band is recommended for critical applications

Channel availability:

2.4 GHz band: 3 non-overlapping channels (1, 6, 11)

5 GHz band: based on regulatory domain

Rules are subject to change

Use non-DFS (Dynamic Frequency Selection) channels when possible

Weather / military radars cause disruption of service in DFS channels

If DFS channels are used, site survey and monitoring is required

Particular caution when operating near airports, sea ports, or military bases.

Exclusive use of a channel bandwidth is expected in most cases

Early dialogue and collaboration with the IT department is important

Country

examples

Available 20 MHz

Channels in 5 GHz

No DFS DFS

U.S., Canada,

Australia

9 12

Europe 4 15

China 5 0

Wireless spectrum management policy is critical!



41

Site Survey Recommendations

Thorough RF spectrum survey is critical

Prolonged monitoring for interference in various locations

Active survey type is required

Evaluates link performance at the actual data rate

Survey conditions must match production environment

AP and antenna type, RF channels, transmit power

Installation restrictions, moving obstacles

Complete walk-through of the coverage area

Changes in the environment may require a follow-up survey

Adequate signal level and cell overlap should be maintained (see App. Guide)

Survey for performance, not just coverage

Survey experience in industrial environment is important



42

Antenna Recommendations

An accurate site survey is necessary to determine appropriate antenna type and placement

Follow manufacturers‟ recommendations about antenna orientation, mounting hardware, and installation procedures

Set the correct antenna gain in the AP/WGB configuration

Antennas should be mounted clear of any obstructions, particularly metal obstacles

Length of the antenna cable should be minimal

High ceilings may present problems in coverage

Low-gain omnidirectional antennas are typically appropriate for most applications



43

Channel Re-use

Wireless cells using the same channel can interfere at a great distance

Must defer transmission as long as can detect valid Wi-Fi signal

Co-channel Interference (CCI) can be decreased but hard to eliminate

Channel bandwidth may be essentially shared between applications

Do not reuse channels with high utilization and client count, unless

complete signal separation can be reliably achieved

Ch. 36 WGB

AP

Ch. 36

WGB

AP

Ch. 40

Tx Power + Antenna Gain – Attenuation > Rx Sensitivity

Just an example:

free space signal propagation

Receive sensitivity at 6

Mbps (5 GHz radio)

-92 dBm

Transmit power 0 dBm

Transmit antenna gain 4 dBi

Receive antenna gain 4 dBi

Attenuation needed -100 dB

Distance with free

space loss (5180 MHz)

460

meters



47

QoS Recommendations

802.11 wireless uses statistical QoS mechanism to

give preference to certain classes of traffic

Still half-duplex media: cannot transmit while

someone is using the channel

Traffic is placed into one of the queues based on

selected criteria

DSCP (L3 QoS) is recommended

TCP/UDP port numbers can be used

Transmission parameters are adjusted for each

queue (see App. Guide)

Backoff time

Number of retries

Packet timeout

Traffic Type DSCP Queue

PTP event 59 Voice

PTP management 47

Video

CIP class 0 / 1

(I/O, P/C, Safety, Motion)

55

47

43

31

CIP class 3 (MSG, HMI) 27

Best Effort

Unclassified 0

Media contention between stations

BK

Classification

BE VI VO

Internal contention between queues



48

Security Recommendations

WPA2 security with AES encryption is the only mechanism recommended for IACS wireless

applications.

Hardware AES encryption does not significantly affect application performance

WPA2-PSK (pre-shared key) is a common method of authentication in WGB-based

topologies, but it has limitations:

No user-based authentication

Does not provide fastest roaming time

May not satisfy organization requirements

802.1X/EAP-based authentication is most secure

May require additional infrastructure support

EAP-FAST is recommended (reduced complexity, local authentication support)

MAC address authentication is not a secure method by itself

Security is organic to the standard….USE IT!


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Wireless Design Considerations for Industrial Applications

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