Data transfer in mpt-1327 network

[acehigh]

In the AVL application, we use voice channel to transfer GPS data. The question is, why the baud rate should be set 2400 or 1200?

[Mike_P]

Hi,

Your question is not very specific, and you don't give details about where the baud rate is set to 1200/2400 - the GPS? the modem (if any)? the radio?.

If it is the radio, the most common modem ICs used in MPT1327 radios, made by CML, support 1200 and 2400 bps, so this is why the radios natively support these speeds. Some other radios have custom made modem chips, so they may be able to support higher baud rates, although I have not seen or heard of any such unit.

Besides, MPT1327 uses a 12.5kHz channel width (usually), which means that a higher operating speed would have to be achieved with an external modem using something else other than FFSK, which raises the cost of the application and hardware.

If you give us a few more details, we may be able to answer your question better.

Regards,

Mike
www.trunksniffer.com

[acehigh]

In our application, GPS data is transferred in voice channel. We use CML’s FX429A modem. The baud rate is set 1200bps, I think the error rate will be higher if the baud rate is higher than 2400. Am I right?

MPT-1327 system is common in China. Our solution for police is very well (dozens of vehicles). But as for monitoring and dispatching taxis, we don’t satisfy with the results. One channel can only control 20~30 cars, far from enough for cities.

[Mike_P]

OK, that's made it far clearer then. In any communications system, merely increasing the baud rate is likely to also increase error rate, as a spurious noise in the channel will affect more bits at higher speed than at lower speed (this is one reason why in HF such low rates, such as 50 bps are used).

The big question here is: why do you use traffic channels to send the GPS data? It is FAR more efficient and fast to send them on the control channel as Short Data Messages every so often. Each GPS message will only take a few slots of the control channel, so you would be able to monitor many more vehicles. Here we have fleets with hundreds of vehicles being monitored using GPS over the control channel.

A traffic call will be much slower to setup, will tie the receiving radio for a much longer time than compared with SDMs.

Best regards,

Mike
www.trunksniffer.com

[jsharp2316]

Originally Posted by acehigh

In the AVL application, we use voice channel to transfer GPS data. The question is, why the baud rate should be set 2400 or 1200?

trunked radio manufacturers. This mixed-mode feature may or may not be used in conjunction with the CBS features in the system.

The key feature that sets the MPT-1327 standard above other trunking standards is its “designed-in” ability to transport data. The system supports 1200 KBPS call protocol to send a single byte message (SDM1, useful for status messaging), a 22-byte message (a SDM2, useful for AVL and bar-code and other low volume data uses)(some manufacturers support multiple SDM-2 for larger messages [with resulting increased control channel congestion]) and a free-form data message format that can run to limit of the system call timers known as a LDM. Although the SDM messages use the inherent 1200 BPS control channel signaling, the throughput is quite quick as the mobile is already synchronized with the control channel and no call setup time or modem training or framing time is required. The LDM can use the FFSK modem at 1200 BPS already built into the radios or an external non-defined modem. The LDM can be used for any data requirement. These messages may be broadcast (SDM1 Only), sent unit to unit, unit to dispatch, dispatch to unit, unit to PSTN and unit to PABX or unit to a data gateway (thus a specific port could be a IP or X.25 or generic data gateway). Careful implementation of application software can provide reasonable throughput speeds.

Nearly all MPT mobiles (and some portables) have a data port (this may be a "add-on" option). This data port capability is part of the MPT standard and is normally implemented in units complying with MAP-27, the protocol standard for that interface. It also provides access to a non-defined (non-prescribed) data transmission ability. Thus, the call setup and maintenance is provided within the MPT standard for a data transmission technique to be determined later. The non-prescribed data rate may be any rate that is physically possible within the constraints of radio channel signal to noise ratio and bandwidth. A 25 KHz bandwidth channel should be theoretically capable of up to 19.2 KBPS; a 12.5 KHz bandwidth channel should be theoretically capable of up to 9.6 KBPS; a [DX implemented] <10 KHz channel over 4.8 KBPS, and the infamous Securicor LM 5 KHz channel is claimed to have supported up to 16.8 KBPS; although actual, achievable data rates are probably 9.6 KBPS; 4.8 KBPS; 2.4 KBPS and 9.6 KBPS respectively. Good application software for MPT transport must minimize over the air transport, that is, forms and formats should be stored at each end in intelligent devices and only transport the dynamic or changing parts of the data. Since the mobiles are typically simplex, the data calls are simplex and care must be taken to minimize training and framing times and call setup times to provide a reasonable throughput.

If a port is truly MAP-27 compliant it is accessible to any data terminal device designed for that protocol. These devices include automatic vehicle location units, mobile data terminals, card swipe units, bar code readers, printers, video frame grabbers and many other mobile data devices. MAP-27 provides a common standard for manufacturers to build data accessibility. You may run the MAP 27 port at any speed you like but tehbove sped constraints apply.