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applications: serial_lte_modem: Add Linux PPP+CMUX support
Add documentation, scripts and configuration overlay for using Serial LTE Modem as a modem for Linux device. Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
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‎applications/serial_lte_modem/README.rst

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doc/slm_description
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doc/nRF91_as_Zephyr_modem
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doc/PPP_linux
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doc/slm_testing
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doc/slm_extending
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doc/slm_data_mode

‎applications/serial_lte_modem/doc/PPP_linux.rst

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.. _slm_as_linux_modem:
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nRF91 Series SiP as a modem for Linux device
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############################################
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.. contents::
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:local:
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:depth: 2
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Overview
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********
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You can use the Serial LTE Modem (SLM) application to make an nRF91 Series SiP work as a standalone modem that can be used with a Linux device.
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The Linux device can use a standard PPP daemon and ldattach utility to connect to the cellular network through the nRF91 Series SiP.
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The setup differentiates from a typical dial-up modem connection as the GSM 0710 multiplexer protocol (CMUX) is used to multiplex multiple data streams over a single serial port.
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This allows you to use the same serial port for both AT commands and PPP data.
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Prerequisites
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=============
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The Linux device needs to have the following packages installed:
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* pppd from the ppp package
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* ldattach from the util-linux package
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These should be available on all standard Linux distributions.
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Configuration
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=============
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To build the SLM application, use the :file:`overlay-ppp-cmux-linux.conf` configuration overlay.
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You can adjust the serial port baud rate using the devicetree overlay file.
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By default, the baud rate is set to 115200.
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If you change the baud rate, set the same rate in the :file:`scripts/slm_start_ppp.sh` and :file:`scripts/slm_stop_ppp.sh` scripts.
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Building and running
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====================
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To build and program the SLM application to the nRF91 Series device, use the :file:`overlay-ppp-cmux-linux.conf` overlay file.
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Managing the connection
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=======================
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The start and stop scripts are provided in the :file:`scripts` directory of the SLM application.
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The scripts assume that the nRF91 Series SiP is connected to the Linux device using the `/dev/ttyACM0` serial port.
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If needed, adjust the serial port settings in the scripts as follows:
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.. code-block:: none
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MODEM=/dev/ttyACM0
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BAUD=115200
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To start the PPP connection, run the :file:`scripts/slm_start_ppp.sh` script.
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To stop the PPP connection, run the :file:`scripts/slm_stop_ppp.sh` script.
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The scripts need superuser privileges to run, so use `sudo`.
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The PPP link is set as a default route if there is no existing default route.
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The scripts do not manage the DNS settings from the Linux system.
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Read the distribution manuals to learn how to configure the DNS settings.
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The following example shows how to start the connection and verify its operation with various command-line utilities:
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.. code-block:: shell
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$ sudo scripts/slm_start_ppp.sh
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Wait modem to boot
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Attach CMUX channel to modem...
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Connect and wait for PPP link...
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send (AT+CFUN=1^M)
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expect (OK)
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OK
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-- got it
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send ()
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expect (#XPPP: 1,0)
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#XPPP: 1,0
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-- got it
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$ ip addr show ppp0
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7: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1464 qdisc fq_codel state UNKNOWN group default qlen 3
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link/ppp
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inet 10.139.130.66/32 scope global ppp0
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valid_lft forever preferred_lft forever
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inet6 2001:14bb:69b:50a3:ade3:2fce:6cc:ba3c/64 scope global temporary dynamic
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valid_lft 604720sec preferred_lft 85857sec
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inet6 2001:14bb:69b:50a3:40f9:1c4e:7231:638b/64 scope global dynamic mngtmpaddr
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valid_lft forever preferred_lft forever
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inet6 fe80::40f9:1c4e:7231:638b peer fe80::3c29:6401/128 scope link
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valid_lft forever preferred_lft forever
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$ ping -I ppp0 8.8.8.8 -c5
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PING 8.8.8.8 (8.8.8.8) from 10.139.130.66 ppp0: 56(84) bytes of data.
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64 bytes from 8.8.8.8: icmp_seq=1 ttl=60 time=320 ms
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64 bytes from 8.8.8.8: icmp_seq=2 ttl=60 time=97.6 ms
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64 bytes from 8.8.8.8: icmp_seq=3 ttl=60 time=140 ms
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64 bytes from 8.8.8.8: icmp_seq=4 ttl=60 time=132 ms
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64 bytes from 8.8.8.8: icmp_seq=5 ttl=60 time=145 ms
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--- 8.8.8.8 ping statistics ---
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5 packets transmitted, 5 received, 0% packet loss, time 4007ms
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rtt min/avg/max/mdev = 97.610/166.802/319.778/78.251 ms
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$ iperf3 -c ping.online.net%ppp0 -p 5202
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Connecting to host ping.online.net, port 5202
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[ 5] local 10.139.130.66 port 54244 connected to 51.158.1.21 port 5202
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[ ID] Interval Transfer Bitrate Retr Cwnd
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[ 5] 0.00-1.00 sec 0.00 Bytes 0.00 bits/sec 1 17.6 KBytes
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[ 5] 1.00-2.00 sec 0.00 Bytes 0.00 bits/sec 0 25.8 KBytes
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[ 5] 2.00-3.00 sec 0.00 Bytes 0.00 bits/sec 0 32.5 KBytes
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[ 5] 3.00-4.00 sec 128 KBytes 1.05 Mbits/sec 0 35.2 KBytes
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[ 5] 4.00-5.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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[ 5] 5.00-6.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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[ 5] 6.00-7.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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[ 5] 7.00-8.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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[ 5] 8.00-9.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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[ 5] 9.00-10.00 sec 0.00 Bytes 0.00 bits/sec 0 35.2 KBytes
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- - - - - - - - - - - - - - - - - - - - - - - - -
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[ ID] Interval Transfer Bitrate Retr
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[ 5] 0.00-10.00 sec 128 KBytes 105 Kbits/sec 1 sender
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[ 5] 0.00-11.58 sec 89.5 KBytes 63.3 Kbits/sec receiver
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$ sudo scripts/slm_stop_ppp.sh
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send (AT+CFUN=0^M)
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expect (#XPPP: 0,0)
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OK
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#XPPP: 0,0
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-- got it

‎applications/serial_lte_modem/overlay-ppp-cmux-linux.conf

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#
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# Copyright (c) 2025 Nordic Semiconductor ASA
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#
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# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
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#
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CONFIG_SLM_SKIP_READY_MSG=y
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CONFIG_SLM_CMUX=y
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CONFIG_SLM_PPP=y
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CONFIG_SLM_CR_TERMINATION=y
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# Assume at least baudrate 115200 for UART
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# so CMUX frame can be received in 12 ms (134*10/115200)
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CONFIG_MODEM_BACKEND_UART_ASYNC_RECEIVE_IDLE_TIMEOUT_MS=12
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CONFIG_MODEM_CMUX_MTU=127
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CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE=536
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CONFIG_SLM_CMUX_UART_BUFFER_SIZE=6000
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# For sending full 6000 bytes at 115200 baudrate
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# 6000 * 10 / 115200 = 521 ms
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CONFIG_MODEM_BACKEND_UART_ASYNC_TRANSMIT_TIMEOUT_MS=521
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# These buffers are unused after AT#CMUX is enabled
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# so use minimal buffer size
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CONFIG_SLM_UART_RX_BUF_COUNT=3
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CONFIG_SLM_UART_RX_BUF_SIZE=128
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CONFIG_SLM_UART_TX_BUF_SIZE=128
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# When using PPP, disable commands of IP-based protocols to save flash space.
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CONFIG_SLM_FTPC=n
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CONFIG_SLM_TFTPC=n
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CONFIG_SLM_HTTPC=n
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CONFIG_SLM_MQTTC=n
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# nRF Connect SDK modules
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CONFIG_PDN=y
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CONFIG_AT_CMD_CUSTOM=y
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CONFIG_NRF_MODEM_LIB_SHMEM_TX_SIZE=22528
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# Zephyr modules
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CONFIG_NET_NATIVE=y
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CONFIG_NET_L2_PPP=y
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CONFIG_MODEM_MODULES=y
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CONFIG_MODEM_CMUX=y
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CONFIG_MODEM_PPP=y
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CONFIG_MODEM_BACKEND_UART=y
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# L2 protocol
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CONFIG_NET_L2_PPP_MGMT=y
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CONFIG_NET_L2_PPP_OPTION_MRU=y
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CONFIG_NET_L2_PPP_OPTION_SERVE_IP=y
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CONFIG_NET_L2_PPP_OPTION_SERVE_DNS=y
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CONFIG_NET_L2_PPP_TIMEOUT=5000
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# IP stack
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CONFIG_NET_IP_ADDR_CHECK=n
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CONFIG_NET_SOCKETS_PACKET=y
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# network buffering
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CONFIG_NET_BUF=y
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CONFIG_NET_BUF_POOL_USAGE=y
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CONFIG_NET_PKT_RX_COUNT=44
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CONFIG_NET_BUF_RX_COUNT=88
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CONFIG_NET_PKT_TX_COUNT=44
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CONFIG_NET_BUF_TX_COUNT=88
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CONFIG_NET_TC_RX_COUNT=0

‎applications/serial_lte_modem/scripts/slm_start_ppp.sh

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#!/bin/bash -eu
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#
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# Copyright (c) 2025 Nordic Semiconductor ASA
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#
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# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
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#
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# Script to start PPP link inside CMUX channel
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# using Serial LTE Modem
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#
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MODEM=/dev/ttyACM0
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BAUD=115200
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PPP_CMUX=/dev/gsmtty2
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AT_CMUX=/dev/gsmtty1
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CHATOPT="-vs"
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cleanup() {
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set +eu
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pkill pppd
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pkill ldattach
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echo "Failed to start..."
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exit 1
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}
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trap cleanup ERR
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if [[ ! -c $MODEM ]]; then
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echo "Serial port not found: $MODEM"
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exit 1
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fi
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stty -F $MODEM $BAUD pass8 raw crtscts clocal
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echo "Wait modem to boot"
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chat -t5 "" "AT" "OK" <$MODEM >$MODEM || true
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echo "Attach CMUX channel to modem..."
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ldattach -c $'AT#XCMUX=1\r' GSM0710 $MODEM
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sleep 1
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stty -F $AT_CMUX clocal
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echo "Connect and wait for PPP link..."
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test -c $AT_CMUX
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chat $CHATOPT -t60 "" "AT+CFUN=1" "OK" "\c" "#XPPP: 1,0" >$AT_CMUX <$AT_CMUX
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pppd $PPP_CMUX noauth novj nodeflate nobsdcomp debug noipdefault passive +ipv6 \
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noremoteip local linkname nrf91 defaultroute defaultroute-metric -1

‎applications/serial_lte_modem/scripts/slm_stop_ppp.sh

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#!/bin/bash -u
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#
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# Copyright (c) 2025 Nordic Semiconductor ASA
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#
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# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
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#
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# Script to stop PPP link inside CMUX channel
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# using Serial LTE Modem
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#
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MODEM=/dev/ttyACM0
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BAUD=115200
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PPP_CMUX=/dev/gsmtty2
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AT_CMUX=/dev/gsmtty1
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CHATOPT="-vs"
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if [[ ! -c $AT_CMUX ]]; then
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echo "AT CMUX channel not found: $AT_CMUX"
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pkill pppd
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pkill ldattach
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exit 1
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fi
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chat $CHATOPT -t30 "" "AT+CFUN=0" "#XPPP: 0,0" >$AT_CMUX <$AT_CMUX
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sleep 1
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test -f /var/run/ppp-nrf91.pid && kill $(head -1 </var/run/ppp-nrf91.pid)
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pkill ldattach

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