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[aprssig] Project-25 for amateur radio

Stephen H. Smith wa8lmf2 at aol.com
Sun Aug 17 17:44:50 UTC 2008


Steve Jones wrote:
>
> On Aug 17, 2008, at 12:56 AM, Scott Miller wrote:
>
>> But is there any user-accessible interface for this data, or is it just
>> some reserved bandwidth in the protocol that'd require hacking the radio
>> to get at?
>
> The latest P25 radios from Motorola include text messaging and I 
> believe they have the ability to set an IP address for an external 
> device connected to them.
>
> Other then playing with the text messaging, I have not looked any 
> further as to whether computer to computer communications is possible 
> using the radios as modems.
>

P25 DOES NOT use IP over-the air (unlike Ma/Com's proprietary "OpenSky" 
digital voice radios).  

The "virtual IP address" in a mobile is an artifact of prorietary base 
station software infrastructure (i.e. racks of computer controllers 
associated with the base station radio) that extracts the text payload 
from the interleaved over-the-air data stream carrying digitized voice, 
trunking and control data, and the incidental "low speed data channel" 
simultaneously. 

The base infrastructure software then re-packages the extracted 
auxilliary data stream into more-or-less standard ethernet TCP/IP telnet 
streams.  The application at the other of the ethernet link is then 
"fooled" into thinking it is interacting with an IP address in the 
actual mobile.  

[This is somewhat analogous to the APRS network structure where you 
transmit data over the RF link in an oddball quasi-proprietary format 
(Mic-E encapsulated in AX25 packets), unpack it to raw text at the base 
station's TNC, pass it to the Internet server system, and then finally 
decode and "make sense" of the data at the other end of the IP link in 
the  user's  APRS program. ]

"Real" end-to-end IP (i.e. ethernet jack on the back of the mobile 
radio) is unlikely to ever be practical in the P25 environment since the 
overall transmission rate of P25 Phase I (12.5 KHz digital channels) is 
only 9600 BPS and HALF of that is consumed with handshaking overhead and 
massive forward error correction of  the voice data.  The net throughput 
is only 4800 bps.     (Old time land-line modems anyone???).   

P25 Gen II  is supposed to cram digitized voice into 6.25 KHz channels. 
One proposed approach would halve the raw data rate to 4800 BPS (and the 
net to 2400 bps).   In some ways, this is similar to the Iridium  
satellite phone fiasco where the focus on very-low-bit-rate voice 
transmission has precluded ever using the system effectively for data 
applications.


BTW,  the first gen 9600 BPS P25 uses 4-frequency FSK with the four 
states being +800 Hz, +1600 Hz, -800 and -1600 Hz from the channel 
center.  This is a constant-power transmit mode that can pass through 
existing class-C RF power amps. 

The current plan for the Phase II 6.25 KHz-wide channels would be to 
maintain the current voice coding and base-band bit rate but use QPSK 
(quadrature phase-shift keying) to transmit the same amount of data at 
half the symbol rate.    However, like PSK31, this will require ALL RF 
transmit chains to replaced with LINEAR amplifiers.   

The vender of the AMBE voice coder used in P25 now claims their miracle 
secret-sauce software can now shoehorn the voice information into only 
2400 bps of data, yielding a gross over-the-air rate of 4800 BPS.  As a 
result, the current 4FSK scheme with constant power carriers could be 
used in half the bandwidth to meet the 6.25 Khz channel-splitting 
mandate, along with existing (efficient) class-C RF amplifiers).  

However, this blows up the much-vaunted "interoperability" of P25 since 
there will now be TWO different voice codecs in use. Early adopter P25 
jurisdictions, having spent hundreds of millions on new digital radio 
infrastructure and mobiles will now have to replace or upgrade all this 
stuff very soon if they want to talk to their late-adopting neighbors.  

The P25 standards-setting process, never renowned for timely decision 
making, is now hopelessly gridlocked on whether to adopt the new vocoder 
to save the existing RF hardware, or keep the existing vocoder and 
current baseband coding to facilitate inter-system interconnect.






--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com  --OR--   http://wa8lmf.net

NEW!   World Digipeater Map
  http://wa8lmf.net/APRSmaps

JavAPRS Filter Port 14580 Guide
  http://wa8lmf.net/aprs/JAVaprsFilters.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:






--

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com  --OR--   http://wa8lmf.net

NEW!   World Digipeater Map
  http://wa8lmf.net/APRSmaps

JavAPRS Filter Port 14580 Guide
  http://wa8lmf.net/aprs/JAVaprsFilters.htm

"APRS 101"  Explanation of APRS Path Selection & Digipeating
  http://wa8lmf.net/DigiPaths

Updated "Rev H" APRS            http://wa8lmf.net/aprs
Symbols Set for UI-View,
UIpoint and APRSplus:






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