I am looking into getting a few of the i355 radio/phones because of their price and they offer digital communications without the need for a licence (or so I hear). I guess that since they are so old there isn't much information on them and I would like to know if anyone has them and if they had any luck with them. Is it correct that they operate on 900mhz FHSS? And if you buy them with a Nextel SIM card they will work with direct talk? I heard they are a nice option but I asked a former Nextel employee and told him that they are similar to the DTR radio's and he told me that they are nothing alike and they use trunking. I thought they only used trunking when connected to cell towers? With direct talk it should work that any unit with the same channel and code should be able to connect to each other correct? Any other information I should know? Thanks!
My wife and I had i355 NEXTEL phones back when they were on the market. They were great iDEN phones and we used the off-network DirectTalk feature too. The former NEXTEL employee you talked to doesn't know what he is talking about.
The off-network DirectTalk feature operates in the 902-928 MHz band using FHSS and an 8-level FSK digital modulation scheme. The vocoder used is VSELP and is also what the iDEN (NEXTEL, BOOST Mobile, etc.) network used. Note that the DirectTalk feature is not to be confused with the NEXTEL DirectConnect feature which uses the iDEN network. They are two different things. DirectTalk uses the same FHSS system that the 900MHz DTR series and new DLR series radios use, but were purposely coded differently so they will not intercommunicate. It would have been wicked cool and wicked useful if they were compatible.
To enable the DT feature, the phone requires a SIM card previously activated on the iDEN network (NEXTEL, BOOST Mobile, etc.). The DT feature like the DTR and DLR radios use an 11-digit electronic serial number (ESN), aka Private ID. In the i355 and other DT-capable phones, the 11-digit ID is 1 plus the phone's area code and phone number. In the DTR and DLR series radios, the 11-digit ID is programmed at the factory and cannot be changed by the end user. The only reason a DT-capable iDEN phone (NEXTEL, BOOST Mobile, etc.) requires a previously activated SIM card is to use 1 plus the area code and phone number as its Private ID. The DTR/DLR radios and the DT feature use an ID-based system so a DT phone has to have an ID to work at all in DT mode.
Transmit power in DT mode is 890 mW (+29.5 dBm). The DTR and DLR radios also transmit at 890 mW (+29.5 dBm). They all operate under Part 15 of FCC rules under FCC 15.247 where a frequency hopping device in the 902-928MHz band is limited to a maximum output power of 1 W (+30 dBm) and must use a minimum hopset of 50 frequencies. The certified +29.5 dBm output level gives Motorola a 0.5 dB margin under the legal limit to account for uncertainty in measurement and slight variation in output from unit to unit.
The 10 "channels" in DT mode in the i355 and other DT-capable phones are unique hopsets that the phone uses. The 15 codes that can be programmed for selective calling on a given hopset modify the hopping algorithm to function similar to CTCSS on analog radios.
In researching the DTR radios, the FHSS system was originally called MOTOTALK. NEXTEL called it DirectTalk on the i355 and other iDEN subscriber units. In the DTR series radios, the early DTR410 was also called MOTOTALK for a brief period. The DTR series name came a little later and I read the correspondence in the FCC certification explaining the model naming change.
The DTR series have a lot more features than the DT feature on DT-capable iDEN subcriber units (i355) have. I suspect it was evolutionary as Motorola added Public and Private Group features to the DTR models. I suspect this is why the DTRs and new DLR series radios are not compatible with the DT feature even though it uses the same FHSS system and same modulation scheme.
Accessories for Motorola's business radios that are used on the DTR series radios can be used on the i355 phones. The i355 uses the same battery pack and charger that the DTR series use. Factory new batteries are available for them because the DTR series is still current product from Motorola. Some accessories have gone EOL and are NLA but headsets, speakermics, and new batteries are readily available.
The i355 DT-feature should perform identical to a DTR or DLR series radio. The DTR550 and DTR650 might perform slightly better due to being able to use a better antenna than the i355 has. The DTR550 and DTR650 use commercial 900MHz antennas used on other Motorola 900MHz radios such as the MTX series. Performance of the i355 in DT mode should be identical to a DTR series radio using the 3.5" 1/4 wave antenna.
Expect usable range to be similar to 4W UHF commercial handhelds on GMRS/FRS on simplex. They will actually perform better in most cases, particularly indoors due to buildings being much more open at 900MHz compared to 462/467MHz. They were designed to be able to penetrate buildings because on-site business radios need to be able work where businesses are located...inside buildings. When on a cruise ship, the DTRs and DLRs perform WAY better than using 4W UHF handhelds on GMRS. When deep inside the ship, you are essentially inside a compartmentalized metal box and there are lots of reflections everywhere. The ship will be much more open at 900MHz and the reflections actually help you with FHSS. For example, in a given location, what might be a dead spot at say 902MHz might be a hot spot at 915 MHz or 928 MHz. The individual hot spots and dead spots hop around too as they are effectively stirred by the FHSS.
From my experience with DTRs and the new DLR series, they work EXCELLENT. They have replaced my use of GMRS/FRS for local on-site simplex type operations with family and friends. I didn't get them to set any range records and got them specifically for high quality DIGITAL communications for local on-site simplex type use. I still have GMRS/FRS and MURS using good commercial gear but they are no longer my goto modes. I used the DT feature in my i355 years ago and that's when I learned of the DTR series back then. Now that I am a DTR and DLR user I think you'll find the DT feature will work amazingly well for local simplex type use.
Good luck.
