Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)14

Remote Monitoring of Building Air-Conditioners using
Internet Web Server Link Communication Technology- Improved Performance and Reliability -CHUZO NINAGAWA*1
TOMOTAKA SATO*2

With the growing demand for energy savings, there is the need for a low-cost,
maintenance-free system to monitor packaged air-conditioners in small to medium-sized officebuildings using only a commercial-off-the-shelf (COTS) Web browser without installing
proprietary monitoring software. Mitsubishi Heavy Industries provides Web gateway products for
COTS Web browsers that utilize our Communication Protocol Interchange technology, whichtransforms data into monitoring data on a Web server in real time. In addition, for remotemonitoring, we developed the Linked Web Server Method, which detects a change of status in the
background Web gateways when multiple Web gateways are monitored. Moreover, to ensure theInternet communication quality, we developed a communication error recovery method for COTS
browsers. These technologies now make it possible to provide customers with low-cost,
maintenance-free, high-performance, and reliable monitoring systems for packaged
air-conditioners.

|1. Introduction
Recently, many control systems for monitoring packaged air-conditioners in buildings havestarted to use Web communication methods. The advantage of Web communication methods is that
one can monitor and control packaged air-conditioners from commercial-off-the-shelf (COTS)
browsers, such as Internet Explorer. In addition, remote monitoring is possible by connecting tointernet protocol (IP) networks. The communication reliability of IP networks, however, differs
significantly between private networks and the Internet.
There are problems to be overcome before realizing Web monitoring over the Internet,
specifically, the communication performance and reliability of the networks. The communication
performance over the Internet is called “best effort”, and it does not guarantee the communication
packet error rate or transmission delay. However, monitoring systems for facilities in small or
medium-sized office buildings require communication performance and reliability without using
expensive private networks and proprietary software.
This paper introduces original technologies for our Web gateways that monitor and control
packaged air-conditioners in multiple buildings located remotely using only inexpensive COTS
browsers.

|2. Packaged Air-conditioner Web Gateway
2.1

Building packaged air-conditioners

Figure 1 shows an overview of a building air-conditioner Web monitoring system. Packaged
air-conditioners are the most common type of air-conditioning system used in small or
medium-sized office buildings. A packaged air-conditioning system consists of several tens toseveral hundreds of indoor units embedded in the ceiling, and roughly one tenth as many outdoor
units installed on the rooftop of a single building. A single refrigerant circuit module consists of
one outdoor unit and approximately ten indoor units. Many refrigerant circuit modules are installed
dispersed within a building.
*1 Ph.D., Deputy Chief Engineer, Air-Conditioner Designing & Engineering Department, Air-Conditioning & Refrigeration Systems
Headquarters*2 Engineering Manager, Air-Conditioner Designing & Engineering Department, Air-Conditioning & Refrigeration Systems Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)15
Commercial
Building

Packaged
Air-conditioner
Outdoor Unit

Packaged
Air-conditioner
Indoor Unit

Packaged
Air-conditioner
Control Network

WEB
Gateway

IP Router
FirewallLAN

IP Network

WEB
Browser

Figure 1

Web monitoring system for packaged air-conditioners in a building

The system collects operation data from packaged air-conditioners installed throughout a medium-sized building
via a control network and creates real-time monitoring screen data in a Web server.

Each indoor or outdoor unit has an embedded microcomputer controller, and makes up acontrol network to transfer sensor data and refrigerant circuit control data. In this example of acontrol network, multiple refrigerant circuit modules make up a single communication channel. As
shown in Figure 1, a communication line connects the indoor and outdoor units via a bus
connection, and creates a single control network for the entire building.
A monitoring device for the packaged air-conditioners is connected to both the control
network and a local area network (LAN), and works as a communication gateway. In Web
monitoring methods, since the device functions as a communication gateway to the control network
and as a Web server with an embedded server inside the gateway, we call it a Web gateway.

2.2

Our Web gateway products

The picture in the title of this paper shows the appearance of our Web gateway product, andits specifications are described in Table 1.
Table 1

Specifications of the Web gateway products

Item
Model
External dimensions
Number of air-conditioners monitored
Embedded OS
Fanless CPU
Alternative to a hard disk
Web browser
Network security
Control monitoring functions
Scheduling
Accounting featureSpecification
SC-WGWNA-B
260 × 200 × 79 mm (W × H × D)
Maximum of 128
ROM Linux
Pentium compatible (500 MHz)
Compact flash ROM and RAM virtual disk
Internet Explorer
IP address filtering
Log-in user classification
On/off, operating mode, setup temperature, room temperature,
fan setting, error indication, etc.
Annual calendar, weekly schedule, execution schedule
Total power consumption of the entire packaged air-conditioners isdivided proportionally among tenants

In general, ordinary Web servers used for corporate homepages are designed to operate in aserver room. However, our Web gateway is embedded in a control board under severe
environments, and must operate stably without maintenance for a long period of time. The use ofmoving components, such as CPU cooling fans and hard disk drives, must be avoided to ensure
reliability and a long lifetime. The required technology consists of a software design that creates abalanced product regardless of hardware limitations, including a fanless CPU and diskless device.
The software embedded in the device must also be able to run continuously, 24 hours a day,for years. With standard server software, administrators regularly perform maintenance asnecessary, such as by restarting the operating system (OS). By contrast, the Web gateway softwaremust run continuously without being restarted for years. For this purpose, the OS has been a type of
embedded software, such as μITRON. However, to enable complicated database management, aMitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)16

large file-processing capability, and IP network connectivity, we developed a technology thatmakes it possible to employ the Linux OS for embedded systems. In this product, the Linux OS,
which is recognized for its stability, is employed as the server OS.
Using this technology, our Web gateway product significantly reduces the cost of building
monitoring systems, and improves the reliability and lifetime of both the hardware and software
compared to existing systems.

|3. Web Monitoring Screen
3.1

Overview monitoring screen

Figure 2 shows the overview monitoring screen of the Web gateway for building packaged
air-conditioners. This screen allows an operator to overview the operating status of all of the
air-conditioners without scrolling. In this example, the screen displays icons that indicate the on/off
status and the operating mode status of each air-conditioner. To view details of a specificair-conditioner, a detailed monitoring window pops up on clicking the location name character
string.
Many large buildings have sophisticated monitoring systems that can display the operating
status of each packaged air-conditioner in a form of floor layout. Such systems are very expensive,
since customized screens must be created for each building. For packaged air-conditioners forsmall or medium-sized buildings, the creation of a customized screen is difficult because of its highcost.
Other than cost, such systems cause a compatibility problem with personal computers (PCs)when customized screens are installed on each PC. Since facility monitoring systems are used forlong periods, i.e., more than a decade, it is essential to maintain compatibility with future PCs, OSs,and other software. To solve these problems, we developed an inexpensive Web monitoring system
for small or medium-sized buildings that requires only a COTS browser, rather than customized
screen software.

Figure 2 Overview monitoring screen of the Web gateway

This screen overviews the operating status of all of the packaged air-conditioners in a building. There is a pop-up
screen for monitoring individual air-conditioners.

3.2

Architecture of Web air-conditioner monitoring

There are various methods for creating the monitoring screens in Web monitoring systems.
The two main methods involve either the client side manipulating the monitoring data andrendering the monitoring screen, or the server side creating and sending the monitoring screen data.
Although the former has an advantage in terms of data transmission quantity, the latter, recently
called “cloud computing”, has been reconsidered because of its manageability, since it maintains allof the monitoring screen data on the server side.
In Web communication, a browser asks the server for Web monitoring screen data. For
ordinary Web sites such as company's homepages, the server returns only static HTML screen data.
In Web monitoring systems, the server needs to construct dynamic HTML data for the monitoring
screen in real time based on the ever-changing operating status of the air-conditioners. In the cloudcomputing method, performance is mainly determined by the mechanism that creates HTML screen
data in real time on the server side.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)17

Figure 3 shows the architecture used for rendering the real-time monitoring screens of our
Web monitoring system for building packaged air-conditioners. In the so-called synchronization
mechanism, the server collects the operating status of each air-conditioner upon receiving a requestfrom a browser to refresh the monitoring screen data. Although this method is simple, it takes timeto create the monitoring screen data. The screen response may be a problem when the transmission
time is on the scale of seconds, such as when connecting to the Internet, as described below. To
solve this problem, we developed a mechanism in which the Web server and air-conditioner control
network process operate asynchronously and independently in a Web gateway to create monitoring
screen data in real time. This technology has been registered for a patent called “Communication
Protocol Interchange”.1 This makes it possible for users to display the variance in the operating
data of the air-conditioners in the air-conditioner control network on the client screen without
delay.
In some cases, a client sends control commands to the packaged air-conditioner via a Web
gateway. After the Communication Protocol Interchange converts the commands corresponding to
the air-conditioner control network, the control commands are sent to indoor units through theTransmission Reserve Table (TRT) buffer arranged in parallel, as shown in Figure 3. The TRT
buffer re-sorts the control commands in parallel regardless of the order received, creates
communication packets by grouping the commands by the destination indoor unit according to thepriority of air-conditioning, and sends them to the air-conditioner control network. This is ouroriginal technology and has been registered for a patent called “Parallel Processing Air-conditioner
Control Communications Method”.2
While the speed of Web communication has recently been improved to several tens Mbps,
the speed of the air-conditioner control network is several tens kbps. This makes the transmission
processing from a gateway to the air-conditioner control network a communication bottleneck. In
addition, since the transmission buffer using the conventional First In First Out (FIFO) method
simply sends each control command in order of arrival, the number of transmission packets sent tothe air-conditioner control network is large. However, our TRT buffer method has significantly
improved the transmission speed when sending control commands from a Web gateway to the
air-conditioner control network.3
Web Gateway
Client PC
Monitoring
ScreenCOTS
Web Browser

Web Server
Screen
Data

HTTP

HTTP

Serverside
ScriptProtocolInterchange

TRT buffer
Control Network Com. Process

Air-con. Control Network
Aircon.

Aircon.

Data collection

Aircon.

Figure 3 Architecture for rendering the real-time monitoring screens in the Web monitoring
system
Our original Communication Protocol Interchange method converts the communication data in a control network
for air-conditioners into monitoring screen data on a Web server in real time.

|4. Linked Web Gateways
4.1

Problems of multi-Web gateways monitoring

As described above, our Web monitoring system requires only a COTS browser on the clientside, and does not need special software. Instead, the browser needs to ask the server for data to
refresh the monitoring screen on a certain cycle. By shortening the interval of the refresh request, it
is possible to detect a change of status (CoS) of the air-conditioners that occurred on the serverside.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
18

When the system uses a single Web gateway, there are no problems, since the browser needs
to accesses a single server only. By contrast, in remote monitoring using multiple Web gateways,
as shown in Figure 4, it takes time to scan the status changes because the browser needs to accessall of the Web gateways individually. In addition, when a user selects and focuses a specificmonitoring screen of a Web gateway (foreground), it is impossible to detect a CoS in the other(background) Web gateways. For example, even if an air-conditioner in a background Web
gateway stops abnormally, the abnormal stop alarm cannot notify the error unless the browser
accesses that Web gateway. It is insufficient to simply utilize a general Web browsing method formonitoring.
Needless to say, proprietary software can be installed on the client side to scan the CoS ofmultiple servers. This, however, reduces the advantage of the Web monitoring method, which isthat users need only a COTS Web browser already installed on a PC. Consequently, we developed
the Linked Web Server Method,4 in which a Web gateway can detect a CoSs not only for its own
embedded server, but also for all of the registered linked servers in the Web gateways.
Building N

Building 1
Building Facilities

FieldbusLAN

Web Gateway 1

Web Gateway N

FirewallThe Internet
Web Facility
Monitoring
SystemClient PC

Figure 4

Remote monitoring using multiple Web gateways

Our original technology notifies the user of background changes of status (CoS) of multiple Web gateways using
only a COTS browser.

4.2

Linked Web server method

Figure 5 shows the Overview Monitoring Screen for multiple Web gateways using ourLinked Web Server Method. This screen notifies the user of a CoS of the background Web
gateways by changing the color of the tabs used for monitoring screen switching and sounding abuzzer, even if the user is viewing the currently connected foreground Web gateway.
Foreground
ScreenBackground Screen Tab
indicating Alarm

Figure 5 Overview monitoring screen for multiple Web gateways
When the status of a packaged air-conditioner changes in a background Web gateway, the color of the tab formonitoring screen switching changes and a buzzer sounds to notify users of the change.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)19

Figure 6 shows the architecture used to realize this Linked Web Server Method. In thisexample, while a client communicates with the foreground server, the server-side script is used tocommunicate with other linked background servers simultaneously to realize communication
between servers. This allows users to obtain a CoS report for the background servers not beingbrowsed currently, without affecting the currently connected foreground screen.
Building Facility
Remote Monitoring
ScreenClient PC
CoS

COTS Web Browser
Jscript
AJAX
Reload Screen

The Internet

Background CoS Push
Server CoS Scan

Embedded Web Server

Jscript

Embedded Web Server
More

PHP fsockPHP fsock

CoS
Jscript

Protocol Interchange

Protocol Interchange
Background Web Gateways

Foreground Web Gateway

Figure 6 Architecture for realizing the Linked Web Server Method
Our original method, in which a foreground Web gateway scans and monitors the changes of status (CoS) of
air-conditioners in the background Web gateways, and notifies the browser of these changes.

Figure 7 shows the communication sequences of the Linked Web Server Method. Since theforeground and background sequences are independent, loading the foreground screen is notaffected. The background Web communication enables background servers to push information asa CoS Push. We have applied for a patent for our Linked Web Server Method called“Air-conditioner monitoring system with a linked Web server”.
Browser

FocusedServerBackground
ServerBackground
ServerReloadAjax XmlHttpRequest

Firewall
Fsock Http Get
Fsock Http Get

Screen Data

Ajax XmlHttpRequest

Fsock Http Get

ReloadCoS

Fsock Http Get

CoS

Background CoS Push
Server CoS Scan
Screen Data

Figure 7 Communications sequences of the Linked Web Server Method
This facilitates monitoring without affecting the users’ browsing operation by scanning the background Web
gateways asynchronously with respect to the foreground screen.

|5. Improvement in Communications Performance and Reliability
5.1

Communications performance

When monitoring a remote monitoring system via the Internet using a Web monitoring
system, as described above, the reloading time of the monitoring screen can be a problem. As an
example of a Web monitoring system via the Internet, Figure 8 shows the time it takes for abrowser in London to reload the monitoring screens from a Web gateway in Nagoya- in Japan.5
Since the status of the Internet varies constantly, the Web monitoring screen was reloaded every
hour for 7 days to show trends. As Figure 8 shows, the average reloading time of the monitoring
screen varied widely from 1.1 to 3.4 s. Therefore, we need to consider this factor fully whenassessing Web data transmission performance via the Internet.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)20

Web Access Time TA [s]5
Web Access Time
Ping RTT

4
3
2
1
0

July 180:00
July 19
July 20
July 21
July 22
July 23
July 24
0:00
0:00
0:00
0:00
0:00
0:00
0:00
Wed
Thu
Fri
Sat
Sun
Mon
Tue
Calendar Day

Figure 8 Field test of the intercontinental Internet communication
The measured transmission performance for overview monitoring screen data between Nagoya and London. Thisgraph shows the transmission status changes at every moment.

Figure 9 compares the reloading time of the monitoring screen for the packaged
air-conditioners Web gateway in Nagoya from browsers in several locations worldwide by taking
an average for 2 days. Although our Web gateway shows high performance and can reload themonitoring screen in 0.35 s when connected to a LAN directly, it may take several seconds when
the Web gateway is connected to a low-speed part of the Internet. This may cause a problem after
actual operation start. In an environment that uses a low-performance wireless LAN, carefulconsideration is needed since the data transaction time may be extremely slow because ofretransmission due to communication packet losses.
When integrating a Web monitoring system with the Internet environment, it is advisable tostudy the Web data transmission performance for the relevant Internet connection in the planningphase. We have been exploring ways to assess the Web communication access time using aninternet communication traffic simulator6 Although it is difficult to assess the Internet datatransmission performance since the network components are unknown in most cases, it is expectedthat this technology will be put to practical use.

Web Access Time TA [s]

8
6
4
2

Figure 9Thai(Wireless)
NagoyaLondonNagoyaBangkok
NagoyaNagoyawithin city

LAN

0

Assessing the transmission performance of Web monitoring screen data

This compares the transmission performance for the overview monitoring screen data by taking the averages for
two weekdays. The graph shows that the data transmission time varies markedly depending on the Internet
environment.

5.2

Recovery from a Web communication error

In remote monitoring with the Internet, a browser may remain stopped at the communication
error screen due to communication anomalies. If an air-conditioner error occurs at this time, a fatal
problem may occur, such as failure to display an alarm.
As described above, our product uses a pre-installed COTS browser because of its low costand better operability. Nevertheless, COTS browsers assume that a user is in front of the screenbrowsing various Web sites. When a Web communication error occurs, the browser only prompts
the user to perform manual recovery. With the Web monitoring system, since a user is not in frontof the screen most of the time, manual recovery cannot be expected. Consequently, we decided toMitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
21

implement a mechanism that automatically recovers a COTS browser from the communication
error screen.
Since the specifications of the communication error screen differ depending on the browser,
we decided to implement an auto-recovery system mechanism in our Web gateway side.
Specifically, we developed an original mechanism that embeds our special password character
strings and a program that scans data and searches for the password in each of the transmitting Web
monitoring screen data streams. The program searching for the password is coded in Java script andis embedded in the monitoring screen data.
With this mechanism, a browser on the client side finds the above Java script code by parsing
the monitoring screen data, and automatically executes the script code to search for the password
character strings in the current screen data. At this time, if the communication error screen is being
displayed because the complete monitoring screen data cannot be obtained, the password character
strings cannot be detected, even if they are embedded at the end of the monitoring screen data. In
such a case, HTTP GET is sent again automatically to the Web gateway in order to reload the same
monitoring screen. We have applied for a patent for this original method.
Figure 10 shows the laboratory test results for assessing the Internet communication
reliability and the communication error recovery effect. Laboratory testing was employed rather
than field testing because it is difficult to create extremely undesirable conditions at will. The
horizontal axis indicates each communication packet loss rate. The vertical axis indicates thecommunication error occurrence rate while reloading the monitoring screen (solid line) andauto-recovery failure rate from the error (dashed line). The higher the communication packet loss
rate is and the more averse the network conditions are, the higher the communication error rate isduring the time the screen is reloading. Even with a communication packet loss rate of several tensof percent, which reflects very adverse network conditions, the auto-recovery failure rate was 0%,
which means that it is possible to recover from all errors automatically, even if a communication
error occurs frequently.
Using our original “Password Embedding Error Recovery Method”, even in a wireless LAN
or a poor Internet environment, it is possible to recover from communication errors automatically
24 hours a day. This makes it possible for us to provide users with reliable monitoring systems.
Successful Com Error Recovery Frequency
Poisson Packet Loss generated by Dummynet

Recovery / Failure Rate

200
High
Successful Error Recovry

150

Error Recovery Failure

100

50
Low

0
0

Low

10

20
Packet Loss Rate

30

High 40

Figure 10 Web communication error auto-recovery
We assessed the communication error occurrence rate and auto-recovery failure rate from errors resulting fromthe network communication packet loss rate in laboratory tests.

|6. Conclusion
This paper has described the technologies that support our Web system for monitoringbuilding packaged air-conditioners via the Internet.
When monitoring packaged air-conditioners in small or medium-sized buildings, there is ademand to use only COTS browsers, instead of installing proprietary monitoring software, becauseof their low-cost and better maintainability. By using our Communication Protocol Interchange andTransmission Reserve Table technologies that transfer the operating data for the packaged
air-conditioners into Web server screen data and vice versa in real time, we have developed Web
gateway products for COTS browsers to respond to market needs. In addition, we developed a
Linked Web Server Method that can detect a CoS, even in the background Web gateways when
multiple Web gateways are monitored for remote monitoring. In addition, to compensate for theMitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
22

Internet communication quality, we developed a communication error recovery method for COTS
browsers. Utilizing these technologies, we can now provide customers with a low-cost,high-maintainability, high-performance, reliable packaged air-conditioner monitoring system.
Demand for integrated monitoring via the Internet is expected to grow. We have beendeveloping technologies for building packaged air-conditioners to meet such customer needs.

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