ALERT2 Frequently Asked Questions
What is ALERT2?
Like ALERT, ALERT2 is a system for transporting short data messages over terrestrial radio paths. It differs from ALERT in that it is much faster, carries more information, and operates error free. Using a channel sharing technique called TDMA, where each transmitter has its own brief time slot in which to transmit, the message contention and data loss problems of ALERT can be eliminated. It has a large enough ID space to eliminate the ID assignment problems common in some areas of the country.
What is TDMA?
TDMA (Time Division Multiple Access) is simply a scheme where every device has its own assigned time slot in which to transmit. If you are familiar with GOES satellite, it uses TDMA to avoid collisions between transmissions from different sites. ALERT2 TDMA is much faster – Each slot can be as short as half a second, and most cycles (frames) are under a minute. ALERT2 TDMA presently uses time signals from GPS satellites to keep the transmitter’s clocks accurate, which is necessary for TDMA to work correctly.
I have a substantial investment in my ALERT system. Would I have to replace everything in order to use ALERT2?
No. The transition path is designed so that ALERT and ALERT2 gages can work side by side. In most systems, the process begins by installing an ALERT2 receiver at the base station and ALERT2 repeaters at the final hop repeater site(s). The ALERT2 repeater has two input ports; one is configured to receive messagesfrom an ALERT receiver/decoder, and the other is configured to receive the audio output of the ALERT 2 receiver. All messages from both sources are buffered during the frame period and combined into a single ALERT2 message. At the base station decoder, ALERT messages are extracted from the ALERT2 message and fed to an ALERT base station. ALERT2 messages are decoded to a serial data packet that is read by an ALERT2-capable base station. ALERT gages can be replaced with ALERT2 gages on the agency’s timetable.
What is an ALERT Concentrator?
A concentrator is the part of an ALERT2 repeater that receives and forwards ALERT messages bundled into an ALERT2 transmission. Typically, a concentrator receives messages for several seconds and compacts them into a single ALERT2 message that is transmitted in its own TDMA time slot. Thus the hop from gage to repeater is ALERT, and the hop from the repeater to the base is ALERT2. This greatly increases the output channel capacity and eliminates all errors and contention on the output channel.As mentioned above, an ALERT2 repeater can do both concentration and repeat incoming ALERT2 messages. As ALERT2 gages are added to a system, ALERT messages are replaced by error free ALERT2 messages.
Does ALERT2 have to use TDMA?
TDMA is optional, but recommended for larger systems. ALERT2 can operate in ALOHA (random transmission) mode, but collisions between gage reports will cause loss of data, and this problem is greatest when the system is busiest during a rain event. ALERT2 transmissions are shorter than ALERT transmissions, so fewer collisions will occur, and ALERT2 will filter out the errors which often occur with collisions, either by correcting them or by throwing away the contaminated message. The price differential for adding GPS to support TDMA is relatively small because of the high volume production of GPS receiver chips.
Should I be considering ALERT2 for my system?
You should consider ALERT2 carefully if you are starting a new system, adding significantly to an existing one, or undertaking a system rehab. If your system is large enough to have data collision problems during rain events, you should be considering what ALERT concentration can do as part of a phased transition to ALERT2.
Is there base station software that supports ALERT2?
As of Q3 2011, there are two software providers that support ALERT Concentration and ALERT2 messages. Others will likely follow suit; we suggest you contact the various software providers directly for the latest information on ALERT2 software products.
Will ALERT2 provide higher accuracy and/or resolution to my data?
ALERT2 enables higher accuracy by providing a larger “data space” that can express greater precision through the use of larger integers or floating point numbers. However, the accuracy is only as great as the sensor and analog-to-digital converter support, so it is important to understand the capabilities of your equipment.
Can ALERT2 and ALERT operate on the same frequency?
Yes, and this may be desirable if a separate ALERT2 frequency is not readily available. Since ALERT transmissions occur at random intervals, they will occasionally collide with ALERT2 transmissions that are in TDMA slots. As more and more gages are converted to ALERT2 TDMA, the number of interfering ALERT transmissions will fall until finally the full benefit of TDMA is achieved when the system is fully TDMA.
What is the available ID range for sites/sensors in ALERT2?
The present specification calls for an ID range of 0 to 65,535 sites, and each transmitting site can have up to 255 sensors. ALERT IDs were generally assigned in blocks of 10 for each site, so the unique identification capability has increased about eighty-fold.
Can a user choose any IDs desired for their system?
The ALERT2 Technical Working Group (TWG) is presently working on this question. The consensus is that some minimal coordination is needed to prevent re-use of IDs in the same radio coverage area.
Can there still be multiple “receive” stations listening to ALERT2 reports?
Yes. ALERT2 is still a sensor-initiated data broadcast architecture like ALERT. Multiple base stations can independently receive and process ALERT2 reports at separate locations.
Can gauge configuration parameters be changed remotely with ALERT2?
No, not yet. A two way capability has been designed into the ALERT2 protocol, but hardware and software to execute it is not yet available. We expect to see this capability developed and supported in the next few years.
Must the entire ALERT gauge be replaced in order to convert it to ALERT2?
Some manufacturers are planning to offer ALERT2 “add-ons” to their existing ALERT products, and will offer a retrofit option. Others are starting “from the ground up” to maximize the advantages of ALERT2. Check with the vendor of your present equipment.