nrfconnect · nordicjm · Mar 27, 2025 · Mar 20, 2025 · Mar 25, 2025
@@ -52,21 +52,26 @@ config SLM_UART_RX_BUF_COUNT
 	range 2 4
 	default 3
 	help
-	  Amount of buffers for receiving (RX) UART traffic. If the buffers are full, UART RX will be disabled until the buffers are processed.
+	  Amount of buffers for receiving (RX) UART traffic.
+	  If the buffers are full, UART RX will be disabled until the buffers are processed.
+	  These buffers are not used when CMUX is in use.
 
 config SLM_UART_RX_BUF_SIZE
 	int "Receive buffer size for UART"
 	range 128 4096
 	default 256
 	help
 	  Amount of received (RX), unprocessed, UART traffic that can be held by single buffer.
+	  These buffers are not used when CMUX is in use.
 
 config SLM_UART_TX_BUF_SIZE
 	int "Send buffer size for UART"
 	range 128 4096
 	default 256
 	help
-	  Amount of UART traffic waiting to be sent (TX), that can be held. If the buffers are full, will send synchronously.
+	  Amount of UART traffic waiting to be sent (TX), that can be held.
+	  If the buffers are full, will send synchronously.
+	  These buffers are not used when CMUX is in use.
 
 #
 # GPIO functionality
@@ -181,6 +186,14 @@ endif
 config SLM_CMUX
 	bool "CMUX support in SLM"
 
+config SLM_CMUX_UART_BUFFER_SIZE
+	int "UART buffer size for CMUX"
+	depends on SLM_CMUX
+	default 6000
+	help
+	  Size of the buffer for data received in CMUX mode.
+	  Same buffer size is used for both RX and TX.
+
 if SLM_CMUX && SLM_PPP
 
 config SLM_CMUX_AUTOMATIC_FALLBACK_ON_PPP_STOPPAGE

@@ -18,6 +18,7 @@ See the subpages for how to use the application, how to extend it, and informati
 
    doc/slm_description
    doc/nRF91_as_Zephyr_modem
+   doc/PPP_linux
    doc/slm_testing
    doc/slm_extending
    doc/slm_data_mode

@@ -0,0 +1,141 @@
+.. _slm_as_linux_modem:
+
+nRF91 Series SiP as a modem for Linux device
+############################################
+
+.. contents::
+   :local:
+   :depth: 2
+
+Overview
+********
+
+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.
+The Linux device can use a standard PPP daemon and ldattach utility to connect to the cellular network through the nRF91 Series SiP.
+
+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.
+This allows you to use the same serial port for both AT commands and PPP data.
+
+Prerequisites
+=============
+
+The Linux device needs to have the following packages installed:
+
+* pppd from the ppp package
+* ldattach from the util-linux package
+
+These should be available on all standard Linux distributions.
+
+Configuration
+=============
+
+To build the SLM application, use the :file:`overlay-ppp-cmux-linux.conf` configuration overlay.
+
+You can adjust the serial port baud rate using the devicetree overlay file.
+By default, the baud rate is set to 115200.
+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.
+
+Building and running
+====================
+
+To build and program the SLM application to the nRF91 Series device, use the :file:`overlay-ppp-cmux-linux.conf` overlay file.
+
+Managing the connection
+=======================
+
+The start and stop scripts are provided in the :file:`scripts` directory of the SLM application.
+The scripts assume that the nRF91 Series SiP is connected to the Linux device using the `/dev/ttyACM0` serial port.
+
+If needed, adjust the serial port settings in the scripts as follows:
+
+.. code-block:: none
+
+   MODEM=/dev/ttyACM0
+   BAUD=115200
+
+To start the PPP connection, run the :file:`scripts/slm_start_ppp.sh` script.
+To stop the PPP connection, run the :file:`scripts/slm_stop_ppp.sh` script.
+
+The scripts need superuser privileges to run, so use `sudo`.
+The PPP link is set as a default route if there is no existing default route.
+The scripts do not manage the DNS settings from the Linux system.
+Read the distribution manuals to learn how to configure the DNS settings.
+
+The following example shows how to start the connection and verify its operation with various command-line utilities:
+
+.. code-block:: shell
+
+   $ sudo scripts/slm_start_ppp.sh
+   Wait modem to boot
+   Attach CMUX channel to modem...
+   Connect and wait for PPP link...
+   send (AT+CFUN=1^M)
+   expect (OK)
+
+
+   OK
+   -- got it
+
+   send ()
+   expect (#XPPP: 1,0)
+
+
+
+
+   #XPPP: 1,0
+   -- got it
+
+   $ ip addr show ppp0
+   7: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1464 qdisc fq_codel state UNKNOWN group default qlen 3
+      link/ppp
+      inet 10.139.130.66/32 scope global ppp0
+         valid_lft forever preferred_lft forever
+      inet6 2001:14bb:69b:50a3:ade3:2fce:6cc:ba3c/64 scope global temporary dynamic
+         valid_lft 604720sec preferred_lft 85857sec
+      inet6 2001:14bb:69b:50a3:40f9:1c4e:7231:638b/64 scope global dynamic mngtmpaddr
+         valid_lft forever preferred_lft forever
+      inet6 fe80::40f9:1c4e:7231:638b peer fe80::3c29:6401/128 scope link
+         valid_lft forever preferred_lft forever
+
+   $ ping -I ppp0 8.8.8.8 -c5
+   PING 8.8.8.8 (8.8.8.8) from 10.139.130.66 ppp0: 56(84) bytes of data.
+   64 bytes from 8.8.8.8: icmp_seq=1 ttl=60 time=320 ms
+   64 bytes from 8.8.8.8: icmp_seq=2 ttl=60 time=97.6 ms
+   64 bytes from 8.8.8.8: icmp_seq=3 ttl=60 time=140 ms
+   64 bytes from 8.8.8.8: icmp_seq=4 ttl=60 time=132 ms
+   64 bytes from 8.8.8.8: icmp_seq=5 ttl=60 time=145 ms
+
+   --- 8.8.8.8 ping statistics ---
+   5 packets transmitted, 5 received, 0% packet loss, time 4007ms
+   rtt min/avg/max/mdev = 97.610/166.802/319.778/78.251 ms
+
+   $ iperf3 -c ping.online.net%ppp0 -p 5202
+   Connecting to host ping.online.net, port 5202
+   [  5] local 10.139.130.66 port 54244 connected to 51.158.1.21 port 5202
+   [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
+   [  5]   0.00-1.00   sec  0.00 Bytes  0.00 bits/sec    1   17.6 KBytes
+   [  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec    0   25.8 KBytes
+   [  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec    0   32.5 KBytes
+   [  5]   3.00-4.00   sec   128 KBytes  1.05 Mbits/sec    0   35.2 KBytes
+   [  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   [  5]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   [  5]   6.00-7.00   sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   [  5]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   [  5]   8.00-9.00   sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   [  5]   9.00-10.00  sec  0.00 Bytes  0.00 bits/sec    0   35.2 KBytes
+   - - - - - - - - - - - - - - - - - - - - - - - - -
+   [ ID] Interval           Transfer     Bitrate         Retr
+   [  5]   0.00-10.00  sec   128 KBytes   105 Kbits/sec    1             sender
+   [  5]   0.00-11.58  sec  89.5 KBytes  63.3 Kbits/sec                  receiver
+
+   $ sudo scripts/slm_stop_ppp.sh
+   send (AT+CFUN=0^M)
+   expect (#XPPP: 0,0)
+
+
+   OK
+
+
+
+   #XPPP: 0,0
+   -- got it
@@ -0,0 +1,65 @@
+#
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+
+CONFIG_SLM_SKIP_READY_MSG=y
+CONFIG_SLM_CMUX=y
+CONFIG_SLM_PPP=y
+CONFIG_SLM_CR_TERMINATION=y
+
+# Assume at least baudrate 115200 for UART
+# so CMUX frame can be received in 12 ms (134*10/115200)
+CONFIG_MODEM_BACKEND_UART_ASYNC_RECEIVE_IDLE_TIMEOUT_MS=12
+CONFIG_MODEM_CMUX_MTU=127
+CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE=536
+CONFIG_SLM_CMUX_UART_BUFFER_SIZE=6000
+# For sending full 6000 bytes at 115200 baudrate
+# 6000 * 10 / 115200 = 521 ms
+CONFIG_MODEM_BACKEND_UART_ASYNC_TRANSMIT_TIMEOUT_MS=521
+
+# These buffers are unused after AT#CMUX is enabled
+# so use minimal buffer size
+CONFIG_SLM_UART_RX_BUF_COUNT=3
+CONFIG_SLM_UART_RX_BUF_SIZE=128
+CONFIG_SLM_UART_TX_BUF_SIZE=128
+
+# When using PPP, disable commands of IP-based protocols to save flash space.
+CONFIG_SLM_FTPC=n
+CONFIG_SLM_TFTPC=n
+CONFIG_SLM_HTTPC=n
+CONFIG_SLM_MQTTC=n
+
+# nRF Connect SDK modules
+CONFIG_PDN=y
+CONFIG_AT_CMD_CUSTOM=y
+CONFIG_NRF_MODEM_LIB_SHMEM_TX_SIZE=22528
+
+# Zephyr modules
+CONFIG_NET_NATIVE=y
+CONFIG_NET_L2_PPP=y
+CONFIG_MODEM_MODULES=y
+CONFIG_MODEM_CMUX=y
+CONFIG_MODEM_PPP=y
+CONFIG_MODEM_BACKEND_UART=y
+
+# L2 protocol
+CONFIG_NET_L2_PPP_MGMT=y
+CONFIG_NET_L2_PPP_OPTION_MRU=y
+CONFIG_NET_L2_PPP_OPTION_SERVE_IP=y
+CONFIG_NET_L2_PPP_OPTION_SERVE_DNS=y
+CONFIG_NET_L2_PPP_TIMEOUT=5000
+
+# IP stack
+CONFIG_NET_IP_ADDR_CHECK=n
+CONFIG_NET_SOCKETS_PACKET=y
+
+# network buffering
+CONFIG_NET_BUF=y
+CONFIG_NET_BUF_POOL_USAGE=y
+CONFIG_NET_PKT_RX_COUNT=44
+CONFIG_NET_BUF_RX_COUNT=88
+CONFIG_NET_PKT_TX_COUNT=44
+CONFIG_NET_BUF_TX_COUNT=88
+CONFIG_NET_TC_RX_COUNT=0
@@ -0,0 +1,48 @@
+#!/bin/bash -eu
+#
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+#
+# Script to start PPP link inside CMUX channel
+# using Serial LTE Modem
+#
+
+MODEM=/dev/ttyACM0
+BAUD=115200
+PPP_CMUX=/dev/gsmtty2
+AT_CMUX=/dev/gsmtty1
+CHATOPT="-vs"
+
+cleanup() {
+	set +eu
+	pkill pppd
+	pkill ldattach
+	echo "Failed to start..."
+	exit 1
+}
+trap cleanup ERR
+
+if [[ ! -c $MODEM ]]; then
+	echo "Serial port not found: $MODEM"
+	exit 1
+fi
+
+stty -F $MODEM $BAUD pass8 raw crtscts clocal
+
+echo "Wait modem to boot"
+chat -t5 "" "AT" "OK" <$MODEM >$MODEM || true
+
+echo "Attach CMUX channel to modem..."
+ldattach -c $'AT#XCMUX=1\r' GSM0710 $MODEM
+
+sleep 1
+stty -F $AT_CMUX clocal
+
+echo "Connect and wait for PPP link..."
+test -c $AT_CMUX
+chat $CHATOPT -t60 "" "AT+CFUN=1" "OK" "\c" "#XPPP: 1,0" >$AT_CMUX <$AT_CMUX
+
+pppd $PPP_CMUX noauth novj nodeflate nobsdcomp debug noipdefault passive +ipv6 \
+		noremoteip local linkname nrf91 defaultroute defaultroute-metric -1
@@ -0,0 +1,29 @@
+#!/bin/bash -u
+#
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+#
+# Script to stop PPP link inside CMUX channel
+# using Serial LTE Modem
+#
+
+MODEM=/dev/ttyACM0
+BAUD=115200
+PPP_CMUX=/dev/gsmtty2
+AT_CMUX=/dev/gsmtty1
+CHATOPT="-vs"
+
+if [[ ! -c $AT_CMUX ]]; then
+	echo "AT CMUX channel not found: $AT_CMUX"
+	pkill pppd
+	pkill ldattach
+	exit 1
+fi
+
+chat $CHATOPT -t30 "" "AT+CFUN=0" "#XPPP: 0,0" >$AT_CMUX <$AT_CMUX
+
+sleep 1
+test -f /var/run/ppp-nrf91.pid && kill $(head -1 </var/run/ppp-nrf91.pid)
+pkill ldattach
@@ -32,13 +32,13 @@ static struct {
 	struct modem_pipe *uart_pipe;
 	bool uart_pipe_open;
 	struct modem_backend_uart uart_backend;
-	uint8_t uart_backend_receive_buf[RECV_BUF_LEN];
-	uint8_t uart_backend_transmit_buf[TRANSMIT_BUF_LEN];
+	uint8_t uart_backend_receive_buf[CONFIG_SLM_CMUX_UART_BUFFER_SIZE];
+	uint8_t uart_backend_transmit_buf[CONFIG_SLM_CMUX_UART_BUFFER_SIZE];
 
 	/* CMUX */
 	struct modem_cmux instance;
-	uint8_t cmux_receive_buf[RECV_BUF_LEN];
-	uint8_t cmux_transmit_buf[TRANSMIT_BUF_LEN];
+	uint8_t cmux_receive_buf[CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE];
+	uint8_t cmux_transmit_buf[CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE];
 
 	/* CMUX channels (Data Link Connection Identifier); index = address - 1 */
 	struct cmux_dlci {