!Initial thoughts on GPRS/EDGE small cells
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agk's phlog 
9 Apr 2021 @ 0212
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written on X61 in bed
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In retrocomputing it's common to use an 
intermediate computer to interface old hardware 
with current networks. The link might be serial 
(for a teletype), WLAN, or bluetooth dial-up 
network (for a palm treo). 

The host--an OpenWRT router or general-purpose 
computer--provides LAN and internet access. It 
also often handles encryption: the old machine 
telnets into the host, which can ssh to remote 
systems. The insecure telnet protocol is only 
used on the LAN for the first hop.

2G networks are at end of life in my country. I 
want to extend the life of durable, low-power 
feature phones with 2G-only connectivity. These 
phones don't contain bluetooth low-energy or 
802.11 wlan radios. 

One approach is to add connectivity. Desolder 2G 
radios and baseband processors from poorly-
documented boards. Add BLE, WLAN, and/or LORA. 
This seems too hard.

The other approach is to build "small cells" that
like hosts described above let obsolete 2G feature 
phones transmit and recieve IP packets on local 
networks and the internet. This is possible.

The hardware in one deployed 2G voice and SMS small 
cell was antenna --> duplexer --> amplifier --> 
USRP2 software radio peripheral --> general-purpose
computer. It communicated with 650 phones 
containing custom SIMs.

OpenBTS (Open Base Transceiver Station) software 
and the USRP2 radio peripheral provided a 2G GSM 
access point. GSM phones became SIP endpoints in 
a voice over IP network. Call routing was handled 
by asterisk or yate.

2G data can also be handled by small cells. 
Bandwidth depends on how many channels per base 
transciever station are dedicated to data, if 
devices are capable of GPRS or EDGE, proximity, 
and how many devices are simultaneously 
transmitting/receiving data.

GPRS DATA examples:
-   up to 85.6kbps (DL) 21.4Kbps (UL)
-   up to 64.2kbps (DL) 42.8Kbps (UL)

EDGE DATA examples:
-   up to 8kbps (DL) 59.2Kbps (UL)
-   up to 6kbps (DL) 118.4Kbps (UL)

What is the minimal hardware required to build a 
small cell for 2G feature phones to: 
-   call (SIP)
-   chat (jabber/xmpp), preferably with OMEMO 
    encryption, 
-   listen to internet radio streams (mpg123), 
-   telnet or ssh (remotely access email/usenet, 
    edit text, browse gopher/gemini)?

If you have any ideas or knowledge about building a 
small cell with cheap/scavenged tech, please email: 
agk@sdf.org

 ------------------------------------------------
 Responses:
 ------------------------------------------------
[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