ProSoft Technology ILX34-AENWG - Manual (Page 58)

PSW-PWD ♦ ProSoft Software Planning and Implementing Wireless Networks
ProSoft Wireless Designer User Manual
Page 58 of 91
ProSoft Technology, Inc.
July 8, 2009
What are the bandwidth and latency requirements?
Bandwidth (data rate) and latency (delay time) are important considerations
when designing a wireless network. There are many different factors that impact
bandwidth and latency requirements including the application process, amount of
I/O at each site, number of devices/sites sharing the network, interface/protocol,
and if multimedia data types are used.
For SCADA applications where RTUs are polled from a central PLC, bandwidth
requirements are minimal and latency time can be hundreds of milliseconds (or
longer) without impacting the process or usability of the system.
This is because update times for SCADA systems are usually acceptable once
every minute two. For this type of system, a relatively slow wireless network may
suffice and be the best choice. However, if video data is also included across the
wireless network, then data rate is very important as the video data requires
much greater bandwidth.
If wireless is used to transmit I/O, then how quickly does the program need to
read/write to each point? Will the process suffer if the I/O is delayed? Will delays
cause process errors in closed loop systems? Will safety or machine integrity be
compromised if data is lost? If so, it may be better to use remote controllers
(PLCs) for local I/O control and wireless network for exchanging data and
commands.
The bandwidth (data rate) and latency (packet delay) of the wireless network is a
key consideration when choosing wireless technologies and wireless network
design.
Who will need access to the wireless network?
Wireless networks are frequently used to connect workers to plant networks
throughout a facility. These wireless local area networks (WLANs) create
wireless "hotspots" for connecting laptops, PDAs, and other portable devices to
the network.
WLANs are commonly used by maintenance engineers to access automation
programs and diagnostic tools when at the machine that failed. Other popular
uses of WLANs include warehouse automation, process control, and production
monitoring.
When designing a wireless LAN for portable access, it is important to know
where in the plant the works will need network access. It is also important to
ensure that the WLAN and all portable devices support the same wireless RF
and security standards.
Does the Equipment Move?
Another popular usage of industrial wireless is to communicate with controllers,
HMIs, or I/O devices on moving equipment. Integrating wireless into systems that
use overhead cranes, transfer cars, rotating platforms and automatic guided
vehicles (AGVs) can reduce mechanical failures and improve production.
When designing a wireless network for this type of application, it is important to
consider where the devices will move for proper RF network coverage.