OpenBTS: Konfigurasi Asterisk untuk OpenBTS

Setting Up Asterisk to Work with OpenBTS¶

OpenBTS integration with Asterisk is simple in principle: each SIM is configured in Asterisk as a SIP user, using the IMSI as the SIP user name. So provisioning an OpenBTS subscriber has two steps: # Get the IMSI from the SIM. # Make the corresponding entries in sip.conf and extensions.conf to support the new SIP user. Getting the IMSIs¶

There are several ways to get the IMSI for a SIM so that it can be programmed into Asterisk's configuration files. Which one you use will depend on the volume of SIMs you need to provision. Getting the IMSI with OpenBTS Itself¶

This is the easiest approach for small numbers of SIMs. Whenever a handset tries to register with OpenBTS, it will send a mobile identity. You can force the handset to use the IMSI as its identity and then capture the IMSI from the OpenBTS console output when the handset tries to register. Here are the steps:

  1. Be in the correct GSM band.
        1. Be sure the handset is configured to use the same band as you OpenBTS binary, and no other.
  2. Reset the handset's TMSI.
     This procedure is derived from GSM 04.08.
        1. Turn off the handset.
        2. Remove the battery and SIM.
        3. Replace them and turn the phone back on. This will clear any TMSI stored in the phone.
  3. Let the phone try to register.
     What happens next depends on the version.

For releases prior to "Jean Lafitte": Start running the full OpenBTS (OpenBTS + transceiver + Asterisk). Pipe out output of the OpenBTS binary through "| grep Control" and watch for something like this:

FACCHDispatch.cpp:53 ControlLayer FACCHDispatcher waiting for ESTABLISH RadioResource.cpp:87 ControlLayer AccessGrantResponder RA=27 when=0:7734 RadioResource.cpp:127 ControlLayer AccessGrantResponder sending PageMode=(0) DedicatedModeOrTBF=(TMA=0 Downlink=0 DMOrTBF=0) ChannelDescription=(typeAndOffset=SDCCH/4-3 TN=0 TSC=0 ARFCN=29) RequestReference=(RA=27 T1'=5 T2=12 T3=33) TimingAdvance=0 SDCCHDispatch.cpp:101 ControlLayer SDCCHDispatcher got MM Location Updating Request MobileIdentity=(IMSI=310410186585294) MobilityManagement.cpp:111 ControlLayer LocationUpdatingController MM Location Updating Request MobileIdentity=(IMSI=310410186585294) MobilityManagement.cpp:121 ControlLayer LocationUpdatingController waiting for registration MobilityManagement.cpp:137 ControlLayer LocationUpdatingController : SIPRegistration -> FAIL SDCCHDispatch.cpp:136 ControlLayer SDCCHDisptacher waiting for RELEASE SDCCHDispatch.cpp:97 ControlLayer SDCCHDisptacher waiting for ESTABLISH

For "Jean Lafitte" and later: Start OpenBTS and Asterisk with the OpenBTS logging level set to "INFO" and watch for something like this in the log:

RadioResource.cpp:152: AccessGrantResponder RA=0x15 when=0:1192710 age=25 TOA=0.0000 RadioResource.cpp:196: AccessGrantResponder sending PageMode=(0) DedicatedModeOrTBF=(TMA=0 Downlink=0 DMOrTBF=0) ChannelDescription=(typeAndOffset=SDCCH/4-0 TN=0 TSC=0 ARFCN=975) RequestReference=(RA=21 T1'=3 T2=12 T3=24) TimingAdvance=0 MobilityManagement.cpp:119: LocationUpdatingController MM Location Updating Request LAI=(MCC=901 MNC=55 LAC=0x29b) MobileIdentity=(TMSI=0x49ffcddd) MobilityManagement.cpp:172: LocationUpdatingController registration FAIL: IMSI=234100223456161

In either version, the SIP registration attempt will return "FAIL" because this IMSI is not yet provisioned in Asterisk.

If you're really clever, you'll write some kind of Python script to catch the log output and automatically update the Asterisk configuration files. If you do that, please share your scripts with the rest of us. That would make this technique applicable to very large numbers of SIMs in situations where acceptance rules can be applied based on the upper digits of the IMSI. Use the GSM "AT" Interface¶

Many GSM devices support a Hayes-style "AT" command interface, defined in GSM 07.07. This interface supports the "AT+CIMI" command, described in GSM 07.07 Section 5.6, which will return the IMSI of the device's installed SIM. This is a good approach if you have a device that affords easy SIM changes, like a MultiTech GSM modem and need to provision a few dozen subscribers.

Again, if you're really clever you'll write a script to automate that. And if you're a good person you'll share it with the rest of us. Order Lots of SIMs¶

If you need even larger numbers of SIMs provisioned you might consider ordering a batch of them with sequential IMSIs that can be easily programmed into Asterisk with a script.

Programming IMSIs Into Asterisk¶

To provision an IMSI into Asterisk you will need to modify two files: sip.conf and extensions.conf. These files are normally located in /etc/asterisk. Creating a SIP User in sip.conf¶

The minimal entry for the IMSI in sip.conf looks like this:

r310410186585294
canreinvite=no
type=friend
allow=gsm
context=sip-external
host=dynamic

This defines a SIP user called "310410186585294". Assigning an Address in extensions.conf¶ We recommend this macro for dialing OpenBTS users:

[macro-dialGSM]
exten => s,1,Dial(SIP/${ARG1})
exten => s,2,Goto(s-${DIALSTATUS},1)
exten => s-CANCEL,1,Hangup
exten => s-NOANSWER,1,Hangup
exten => s-BUSY,1,Busy(30)
exten => s-CONGESTION,1,Congestion(30)
exten => s-CHANUNAVAIL,1,playback(ss-noservice)
exten => s-CANCEL,1,Hangup

(And if any Asterisk expert has improvements to that, please feel free to submit them.) With that macro in place, the GSM handset can be assigned an extension just like any other Asterisk terminal device. For example, this macro assigns extension "2103" to the handset with ISMI "310410186585291":

the name in the brackets is the context, it should be the same with the context line in sip.conf

[sip-external]
exten => 2103,1,Macro(dialGSM,310410186585291)

And this macro allows a specific block of 1,000 AT&T SIMs (310410186585000-310410186585999) to be dialed by the last 4 digits of the IMSI:

exten => _5XXX,1,Macro(dialGSM,310410186585${EXTEN:-3})