!BTS-capable SDRs
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agk's phlog
16 Apr 2021 @ 1028
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written on x61 on a sunny morning
while Eddie works on my brakes
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I want a household 2G to IP/VOIP gateway for one
to three obsolete featurephones.
Goal: PSTN calls, XMPP/SMSC messaging, internet
radio listening, IRC chat, telnet/SSH to shell
email, text editor, and gopher/gemini browser.
Stretch goal: Integration with LoRa gateway.
GSM-850 and GSM-1900 carried 2G and 3G traffic
in the United States and most countries in the
Americas. T-Mobile and some other carriers in
the Americas also used GSM-1700.
Data transmission rates
-----------------------
- LoRa (for reference) -- 0.3Kbps to 27 Kbps
- GSM/2G -- 9.6Kbps
- GPRS/2.5G -- 35Kbps to 171kbps
- EDGE/2.75G -- 120Kbps to 384Kbps
- UMTS/3G -- 384Kbps to 2Mbps
Internet radio probably needs 128Kbps to 320Kbps.
Other services should work as expected on GPRS.
Band plan
---------
GSM-850 uses 824-849 MHz to send from the mobile
station to the base station (uplink) and 869-894
MHz for the downlink. Channel numbers are 128 to
251. GSM-1900 uses 1850-1910 MHz uplink and 1930-
1990 MHz downlink. Channel numbers are 512 to 810.
European GSM-900 is unlicensed ISM band in the US,
with a 1 watt/4 watt limit.
Authentication
--------------
If a GSM mobile can't find a BTS for its carrier it
asks other BTS if it may connect. It doesn't auth-
enticate the BTS. UMTS requires authentication of
BTS--the mobile's SIM must be configured with the
correct preshared secret and rolling keys.[1]
Hardware
--------
The cheapest full-duplex BTS-capable SDR trans-
ciever on aliexpress is a $525 Ettus USRP B210
knockoff. A $300 LimeSDR Mini on crowdsupply
runs Osmocom or YateBTS. Pairs of used USRP 1
knockoffs for $150-$250 on ebay run OpenBTS.
The SDR can talk with a $30 GL-MT300N-V2 mini
router (I have one) or any router that runs
OpenWRT. Is a more substantial computer needed?
Hardware questions
------------------
[ ] Are there less expensive full-duplex SDRs?
[ ] Is full-duplex necessary?
[ ] Can a GPRS BTS be built with scavenged or
repurposed tech?
[ ] What tradeoffs go with low price?
[ ] Is other hardware needed?
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[^1]: wikipedia.org/wiki/Network_switching_
subsystem#Authentication_center_(AuC)
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Responses:
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[1]: My friend guofu's response, "My thoughts
toward agk's 2G renaissance" (2021-04-23):
gopher://sdf.org:70/0/users/guofu/phlog/2021/20210423045857.txt
