LTE Modem connected to two char devices

tramsch · 05-09-2014, 01:28 AM

Hi,

I have a custom build server-box here that has two LTE Modems (Sierra Wireless, Inc. MC8700 Modem) installed.
After playing around with wvdial and the test SIM cards that I have, I found out that the first modem is reachable via

/dev/ttyUSB3 and
/dev/ttyUSB4

and the second modem uses

/dev/ttyUSB8 and
/dev/ttyUSB9

In both cases both char devices successfully make a connection, but it is not possible to connect both (e.g USB8+9) at the same time, which of course is plausible.
I am now wondering why there are two char devices per modem.

Is this normal behaviour? And does anybody know why?
Furthermore it is important which of the devices I use and how do I know which one to use?

Some system details:
# cat /etc/*-release

Code:

CentOS release 6.5 (Final)
LSB_VERSION=base-4.0-amd64:base-4.0-noarch:core-4.0-amd64:core-4.0-noarch:graphics-4.0-amd64:graphics-4.0-noarch:printing-4.0-amd64:printing-4.0-noarch

# uname -a

Code:

Linux test 2.6.32-431.17.1.el6.x86_64 #1 SMP Wed May 7 23:32:49 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

# lsusb

Code:

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub
Bus 001 Device 003: ID 03f0:0024 Hewlett-Packard KU-0316 Keyboard
Bus 001 Device 004: ID 0424:2240 Standard Microsystems Corp. 
Bus 003 Device 002: ID 04b4:6560 Cypress Semiconductor Corp. CY7C65640 USB-2.0 "TetraHub"
Bus 003 Device 003: ID 04b4:6560 Cypress Semiconductor Corp. CY7C65640 USB-2.0 "TetraHub"
Bus 003 Device 004: ID 1199:68a3 Sierra Wireless, Inc. MC8700 Modem
Bus 003 Device 005: ID 1199:68a3 Sierra Wireless, Inc. MC8700 Modem

dmesg was not very helpful:

Code:

# dmesg|grep Sierra
# dmesg|grep 8700
# dmesg|grep 68a3
# dmesg|grep ttyUSB

thank you for your time and effort,
Florian

ondoho · 05-09-2014, 02:18 PM

last time i tried on a brand new ubuntu install, it was possible to get an lte modem working, but only as a 3g modem.
centos uses a rather old kernel and i see a long and painful road ahead of you if you insist on using an LTE (aka 4G) mobile broadband modem with it.

(i guess there's also other reasons why it would not be advisable to run a server on a mobile broadband connection)

jefro · 05-09-2014, 02:42 PM

I know the current debian works like a charm on my 4g hotspot attached usb. Basically it is a Samsumg phone tethered.

So the question remains as to if two connections can be made at a time. I'd ask if any other situations where two usb modems could connect? I suspect it is a limit of the dialer.

ondoho · 05-09-2014, 02:53 PM

Quote:

Originally Posted by jefro

I know the current debian works like a charm on my 4g hotspot attached usb.

from the computers point of view, this is a wlan connection.

tramsch · 05-09-2014, 03:05 PM

Than you very much for your replies.

The connections are working very well, one per modem. And also the multi route balancing (iproute2, http://lartc.org/) that I set up works fine.
It is just the detail that there is another char/modem device available per modem and I have no clue why.

The system itself is going to be installed in vehicles and will have no other choice than to use broadband to connect to the internet. ;-)
And yes the CentOS kernel is pretty old, that always seems to be part of the deal if you're choosing enterprise linux...

jefro · 05-09-2014, 07:47 PM

Well, at some point it becomes a wan connection only by fooling the OS.

ondoho · 05-10-2014, 02:00 AM

@tramsch:
i sort of assumed you're talking about an external (dongle) modem; never even knew they exist internally. so i guess i'm less of an expert than i thought.

however (shot in the dark) the word "dual carrier" comes to mind - it's something 4g/lts modems do when there's no "real" 4g connection- they just use 2 3g connections to get higher speeds.