Integrate the multiplexer into the AT chat library

1 files changed, 625 insertions, 0 deletions

diff --git a/gatchat/gsm0710.c b/gatchat/gsm0710.c
new file mode 100644
index 00000000..e9d128d0
--- /dev/null
+++ b/gatchat/gsm0710.c
@@ -0,0 +1,625 @@
+/*
+ *
+ *  AT chat library with GLib integration
+ *
+ *  Copyright (C) 2009  Intel Corporation. All rights reserved.
+ *  Copyright (C) 2009  Trolltech ASA.
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License version 2 as
+ *  published by the Free Software Foundation.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ */
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <alloca.h>
+#include <string.h>
+#include <stdio.h>
+
+#include "gsm0710.h"
+
+/* Frame types and subtypes */
+#define GSM0710_OPEN_CHANNEL		0x3F
+#define GSM0710_CLOSE_CHANNEL		0x53
+#define GSM0710_DATA			0xEF
+#define GSM0710_DATA_ALT		0x03
+#define GSM0710_STATUS_SET		0xE3
+#define GSM0710_STATUS_ACK		0xE1
+#define GSM0710_TERMINATE_BYTE1		0xC3
+#define GSM0710_TERMINATE_BYTE2		0x01
+
+/* Status flags */
+#define GSM0710_FC			0x02
+#define GSM0710_DTR			0x04
+#define GSM0710_DSR			0x04
+#define GSM0710_RTS			0x08
+#define GSM0710_CTS			0x08
+#define GSM0710_DCD			0x80
+
+/* Initialize a GSM 07.10 context, in preparation for startup */
+void gsm0710_initialize(struct gsm0710_context *ctx)
+{
+	ctx->mode = GSM0710_MODE_BASIC;
+	ctx->frame_size = GSM0710_DEFAULT_FRAME_SIZE;
+	ctx->port_speed = 115200;
+	ctx->server = 0;
+	ctx->buffer_used = 0;
+	memset(ctx->used_channels, 0, sizeof(ctx->used_channels));
+	ctx->reinit_detect = 0;
+	ctx->reinit_detect_len = 0;
+	ctx->user_data = 0;
+	ctx->fd = -1;
+	ctx->at_command = 0;
+	ctx->read = 0;
+	ctx->write = 0;
+	ctx->deliver_data = 0;
+	ctx->deliver_status = 0;
+	ctx->debug_message = 0;
+	ctx->open_channel = 0;
+	ctx->close_channel = 0;
+	ctx->terminate = 0;
+	ctx->packet_filter = 0;
+}
+
+/* Determine if a channel is in use */
+static int is_channel_used(struct gsm0710_context *ctx, int channel)
+{
+	int index = channel / 32;
+	return ((ctx->used_channels[index] & (1L << (channel % 32))) != 0);
+}
+
+/* Mark a channel as used */
+static void mark_channel_used(struct gsm0710_context *ctx, int channel)
+{
+	int index = channel / 32;
+	ctx->used_channels[index] |= (1L << (channel % 32));
+}
+
+/* Mark a channel as unused */
+static void mark_channel_unused(struct gsm0710_context *ctx, int channel)
+{
+	int index = channel / 32;
+	ctx->used_channels[index] &= ~(1L << (channel % 32));
+}
+
+/* Write a debug message */
+static void gsm0710_debug(struct gsm0710_context *ctx, const char *msg)
+{
+	if (ctx->debug_message)
+		ctx->debug_message(ctx, msg);
+}
+
+/* Set the "reinitialize detect" string to "str".  When "str" is
+   encountered in the input stream, the AT+CMUX command will be
+   re-sent and the multiplexer re-started.  This is needed for
+   devices that drop out of multiplexer mode due to suspend/wakeup/etc.
+   The data at "str" must persist until the context is destroyed */
+void gsm0710_set_reinit_detect(struct gsm0710_context *ctx, const char *str)
+{
+	ctx->reinit_detect = str;
+	ctx->reinit_detect_len = (str ? strlen(str) : 0);
+}
+
+static const unsigned char crc_table[256] = {
+	0x00, 0x91, 0xE3, 0x72, 0x07, 0x96, 0xE4, 0x75,
+	0x0E, 0x9F, 0xED, 0x7C, 0x09, 0x98, 0xEA, 0x7B,
+	0x1C, 0x8D, 0xFF, 0x6E, 0x1B, 0x8A, 0xF8, 0x69,
+	0x12, 0x83, 0xF1, 0x60, 0x15, 0x84, 0xF6, 0x67,
+	0x38, 0xA9, 0xDB, 0x4A, 0x3F, 0xAE, 0xDC, 0x4D,
+	0x36, 0xA7, 0xD5, 0x44, 0x31, 0xA0, 0xD2, 0x43,
+	0x24, 0xB5, 0xC7, 0x56, 0x23, 0xB2, 0xC0, 0x51,
+	0x2A, 0xBB, 0xC9, 0x58, 0x2D, 0xBC, 0xCE, 0x5F,
+	0x70, 0xE1, 0x93, 0x02, 0x77, 0xE6, 0x94, 0x05,
+	0x7E, 0xEF, 0x9D, 0x0C, 0x79, 0xE8, 0x9A, 0x0B,
+	0x6C, 0xFD, 0x8F, 0x1E, 0x6B, 0xFA, 0x88, 0x19,
+	0x62, 0xF3, 0x81, 0x10, 0x65, 0xF4, 0x86, 0x17,
+	0x48, 0xD9, 0xAB, 0x3A, 0x4F, 0xDE, 0xAC, 0x3D,
+	0x46, 0xD7, 0xA5, 0x34, 0x41, 0xD0, 0xA2, 0x33,
+	0x54, 0xC5, 0xB7, 0x26, 0x53, 0xC2, 0xB0, 0x21,
+	0x5A, 0xCB, 0xB9, 0x28, 0x5D, 0xCC, 0xBE, 0x2F,
+	0xE0, 0x71, 0x03, 0x92, 0xE7, 0x76, 0x04, 0x95,
+	0xEE, 0x7F, 0x0D, 0x9C, 0xE9, 0x78, 0x0A, 0x9B,
+	0xFC, 0x6D, 0x1F, 0x8E, 0xFB, 0x6A, 0x18, 0x89,
+	0xF2, 0x63, 0x11, 0x80, 0xF5, 0x64, 0x16, 0x87,
+	0xD8, 0x49, 0x3B, 0xAA, 0xDF, 0x4E, 0x3C, 0xAD,
+	0xD6, 0x47, 0x35, 0xA4, 0xD1, 0x40, 0x32, 0xA3,
+	0xC4, 0x55, 0x27, 0xB6, 0xC3, 0x52, 0x20, 0xB1,
+	0xCA, 0x5B, 0x29, 0xB8, 0xCD, 0x5C, 0x2E, 0xBF,
+	0x90, 0x01, 0x73, 0xE2, 0x97, 0x06, 0x74, 0xE5,
+	0x9E, 0x0F, 0x7D, 0xEC, 0x99, 0x08, 0x7A, 0xEB,
+	0x8C, 0x1D, 0x6F, 0xFE, 0x8B, 0x1A, 0x68, 0xF9,
+	0x82, 0x13, 0x61, 0xF0, 0x85, 0x14, 0x66, 0xF7,
+	0xA8, 0x39, 0x4B, 0xDA, 0xAF, 0x3E, 0x4C, 0xDD,
+	0xA6, 0x37, 0x45, 0xD4, 0xA1, 0x30, 0x42, 0xD3,
+	0xB4, 0x25, 0x57, 0xC6, 0xB3, 0x22, 0x50, 0xC1,
+	0xBA, 0x2B, 0x59, 0xC8, 0xBD, 0x2C, 0x5E, 0xCF
+};
+
+static unsigned char gsm0710_compute_crc(const unsigned char *data, int len)
+{
+	int sum = 0xFF;
+	while (len > 0) {
+		sum = crc_table[(sum ^ *data++) & 0xFF];
+		--len;
+	}
+	return (~sum & 0xFF);
+}
+
+/* Write a raw GSM 07.10 frame to the underlying device */
+static void gsm0710_write_frame(struct gsm0710_context *ctx, int channel,
+				int type, const unsigned char *data, int len)
+{
+	unsigned char *frame = alloca(ctx->frame_size * 2 + 8);
+	int size;
+	if (len > ctx->frame_size)
+		len = ctx->frame_size;
+	if (ctx->mode) {
+		int temp, crc;
+		frame[0] = 0x7E;
+		frame[1] = ((channel << 2) | 0x03);
+		frame[2] = type;
+		crc = gsm0710_compute_crc(frame + 1, 2);
+		if (type == 0x7E || type == 0x7D) {
+			/* Need to quote the type field now that crc has been computed */
+			frame[2] = 0x7D;
+			frame[3] = (type ^ 0x20);
+			size = 4;
+		} else {
+			size = 3;
+		}
+		while (len > 0) {
+			temp = *data++ & 0xFF;
+			--len;
+			if (temp != 0x7E && temp != 0x7D) {
+				frame[size++] = temp;
+			} else {
+				frame[size++] = 0x7D;
+				frame[size++] = (temp ^ 0x20);
+			}
+		}
+		if (crc != 0x7E && crc != 0x7D) {
+			frame[size++] = crc;
+		} else {
+			frame[size++] = 0x7D;
+			frame[size++] = (crc ^ 0x20);
+		}
+		frame[size++] = 0x7E;
+	} else {
+		int header_size;
+		frame[0] = 0xF9;
+		frame[1] = ((channel << 2) | 0x03);
+		frame[2] = type;
+		if (len <= 127) {
+			frame[3] = ((len << 1) | 0x01);
+			header_size = size = 4;
+		} else {
+			frame[3] = (len << 1);
+			frame[4] = (len >> 7);
+			header_size = size = 5;
+		}
+		if (len > 0) {
+			memcpy(frame + size, data, len);
+			size += len;
+		}
+		/* Note: GSM 07.10 says that the CRC is only computed over the header */
+		frame[size++] = gsm0710_compute_crc(frame + 1, header_size - 1);
+		frame[size++] = 0xF9;
+	}
+	if (ctx->write)
+		ctx->write(ctx, frame, size);
+}
+
+/* Start up the GSM 07.10 session on the underlying device.
+   If "send_cmux" is non-zero, then send the AT+CMUX command.
+   Otherwise the underlying device is assumed to already be
+   in multiplexing mode.  Returns zero if the AT+CMUX failed */
+int gsm0710_startup(struct gsm0710_context *ctx, int send_cmux)
+{
+	int channel;
+	char command[64];
+
+	/* Discard any data in the buffer, in case of restart */
+	ctx->buffer_used = 0;
+
+	/* Send the appropriate AT+CMUX command */
+	if (send_cmux) {
+		int speed;
+		switch (ctx->port_speed) {
+		case 9600:
+			speed = 1;
+			break;
+		case 19200:
+			speed = 2;
+			break;
+		case 38400:
+			speed = 3;
+			break;
+		case 57600:
+			speed = 4;
+			break;
+		case 115200:
+			speed = 5;
+			break;
+		case 230400:
+			speed = 6;
+			break;
+		default:
+			speed = 5;
+			break;
+		}
+		sprintf(command, "AT+CMUX=%d,0,%d,%d",
+					ctx->mode, speed, ctx->frame_size);
+		if (!ctx->at_command || !ctx->at_command(ctx, command)) {
+			gsm0710_debug(ctx,
+				"could not initialize multiplexing with AT+CMUX");
+			return 0;
+		}
+	}
+
+	/* Open the control channel */
+	gsm0710_write_frame(ctx, 0, GSM0710_OPEN_CHANNEL, NULL, 0);
+
+	/* Open previously-used channels if this is a reinit.
+	   Send "ERROR" on re-opened channels, to cause higher
+	   layers to abort pending AT commands */
+	for (channel = 1; channel <= GSM0710_MAX_CHANNELS; ++channel) {
+		if (is_channel_used(ctx, channel)) {
+			gsm0710_write_frame(ctx, channel,
+						GSM0710_OPEN_CHANNEL, NULL, 0);
+			if (ctx->deliver_data)
+				ctx->deliver_data(ctx, channel,
+							"\r\nERROR\r\n", 9);
+		}
+	}
+	return 1;
+}
+
+/* Shut down the GSM 07.10 session, closing all channels */
+void gsm0710_shutdown(struct gsm0710_context *ctx)
+{
+	static const unsigned char terminate[2] = { GSM0710_TERMINATE_BYTE1,
+						GSM0710_TERMINATE_BYTE2 };
+	if (!ctx->server) {
+		int channel;
+		for (channel = 1; channel <= GSM0710_MAX_CHANNELS; ++channel) {
+			if (is_channel_used(ctx, channel)) {
+				gsm0710_write_frame(ctx, channel,
+						GSM0710_CLOSE_CHANNEL, NULL, 0);
+			}
+		}
+		gsm0710_write_frame(ctx, 0, GSM0710_DATA, terminate, 2);
+	}
+	memset(ctx->used_channels, 0, sizeof(ctx->used_channels));
+}
+
+/* Open a specific channel.  Returns non-zero if successful */
+int gsm0710_open_channel(struct gsm0710_context *ctx, int channel)
+{
+	if (channel <= 0 || channel > GSM0710_MAX_CHANNELS)
+		return 0;	/* Invalid channel number */
+	if (is_channel_used(ctx, channel))
+		return 1;	/* Channel is already open */
+	mark_channel_used(ctx, channel);
+	if (!ctx->server)
+		gsm0710_write_frame(ctx, channel,
+					GSM0710_OPEN_CHANNEL, NULL, 0);
+	return 1;
+}
+
+/* Close a specific channel */
+void gsm0710_close_channel(struct gsm0710_context *ctx, int channel)
+{
+	if (channel <= 0 || channel > GSM0710_MAX_CHANNELS)
+		return;		/* Invalid channel number */
+	if (!is_channel_used(ctx, channel))
+		return;		/* Channel is already closed */
+	mark_channel_unused(ctx, channel);
+	if (!ctx->server)
+		gsm0710_write_frame(ctx, channel,
+					GSM0710_CLOSE_CHANNEL, NULL, 0);
+}
+
+/* Determine if a specific channel is open */
+int gsm0710_is_channel_open(struct gsm0710_context *ctx, int channel)
+{
+	if (channel <= 0 || channel > GSM0710_MAX_CHANNELS)
+		return 0;	/* Invalid channel number */
+	return is_channel_used(ctx, channel);
+}
+
+/* Process an incoming GSM 07.10 packet */
+static int gsm0710_packet(struct gsm0710_context *ctx, int channel, int type,
+					const unsigned char *data, int len)
+{
+	if (ctx->packet_filter &&
+			ctx->packet_filter(ctx, channel, type, data, len)) {
+		/* The filter has extracted and processed the packet */
+		return 1;
+	}
+	if (type == 0xEF || type == 0x03) {
+
+		if (channel >= 1 && channel <= GSM0710_MAX_CHANNELS &&
+					is_channel_used(ctx, channel)) {
+			/* Ordinary data packet */
+			if (ctx->deliver_data)
+				ctx->deliver_data(ctx, channel, data, len);
+		} else if (channel == 0) {
+			/* An embedded command or response on channel 0 */
+			if (len >= 2 && data[0] == GSM0710_STATUS_SET) {
+				return gsm0710_packet(ctx, channel,
+							GSM0710_STATUS_ACK,
+							data + 2, len - 2);
+			} else if (len >= 2 && data[0] == 0xC3 && ctx->server) {
+				/* Incoming terminate request on server side */
+				for (channel = 1; channel <= GSM0710_MAX_CHANNELS; ++channel) {
+					if (is_channel_used(ctx, channel)) {
+						if (ctx->close_channel)
+							ctx->close_channel(ctx, channel);
+					}
+				}
+				memset(ctx->used_channels, 0,
+						sizeof(ctx->used_channels));
+				if (ctx->terminate)
+					ctx->terminate(ctx);
+				return 0;
+			} else if (len >= 2 && data[0] == 0x43) {
+				/* Test command from other side - send the same bytes back */
+				unsigned char *resp = alloca(len);
+				memcpy(resp, data, len);
+				resp[0] = 0x41;	/* Clear the C/R bit in the response */
+				gsm0710_write_frame(ctx, 0, GSM0710_DATA,
+								resp, len);
+			}
+		}
+
+	} else if (type == GSM0710_STATUS_ACK && channel == 0) {
+
+		unsigned char resp[33];
+
+		/* Status change message */
+		if (len >= 2) {
+			/* Handle status changes on other channels */
+			channel = ((data[0] & 0xFC) >> 2);
+			if (channel >= 1 && channel <= GSM0710_MAX_CHANNELS &&
+						is_channel_used(ctx, channel)) {
+				if (ctx->deliver_status)
+					ctx->deliver_status(ctx, channel,
+							data[1] & 0xFF);
+			}
+		}
+
+		/* Send the response to the status change request to ACK it */
+		gsm0710_debug(ctx,
+				"received status line signal, sending response");
+		if (len > 31)
+			len = 31;
+		resp[0] = GSM0710_STATUS_ACK;
+		resp[1] = ((len << 1) | 0x01);
+		memcpy(resp + 2, data, len);
+		gsm0710_write_frame(ctx, 0, GSM0710_DATA, resp, len + 2);
+
+	} else if (type == (0x3F & 0xEF) && ctx->server) {
+
+		/* Incoming channel open request on server side */
+		if (channel >= 1 && channel <= GSM0710_MAX_CHANNELS) {
+			if (!is_channel_used(ctx, channel)) {
+				mark_channel_used(ctx, channel);
+				if (ctx->open_channel)
+					ctx->open_channel(ctx, channel);
+			}
+		}
+
+	} else if (type == (0x53 & 0xEF) && ctx->server) {
+
+		/* Incoming channel close request on server side */
+		if (channel >= 1 && channel <= GSM0710_MAX_CHANNELS) {
+			if (is_channel_used(ctx, channel)) {
+				mark_channel_unused(ctx, channel);
+				if (ctx->close_channel)
+					ctx->close_channel(ctx, channel);
+			}
+		}
+
+	}
+	return 1;
+}
+
+/* Function that is called when the underlying device is ready to be read.
+   A callback will be made to ctx->read to get the data for processing */
+void gsm0710_ready_read(struct gsm0710_context *ctx)
+{
+	int len, posn, posn2, header_size, channel, type;
+
+	/* Read more data from the underlying serial device */
+	if (!ctx->read)
+		return;
+	len = ctx->read(ctx, ctx->buffer + ctx->buffer_used,
+			sizeof(ctx->buffer) - ctx->buffer_used);
+	if (len <= 0)
+		return;
+
+	/* Update the buffer size */
+	ctx->buffer_used += len;
+
+	/* Check for the re-initialization detection string */
+	if (!ctx->server && ctx->reinit_detect_len &&
+				len >= ctx->reinit_detect_len &&
+			!memcmp(ctx->buffer, ctx->reinit_detect, ctx->reinit_detect_len)) {
+		gsm0710_startup(ctx, 1);
+		return;
+	}
+
+	/* Break the incoming data up into packets */
+	posn = 0;
+	while (posn < ctx->buffer_used) {
+		if (ctx->buffer[posn] == 0xF9) {
+
+			/* Basic format: skip additional 0xF9 bytes between frames */
+			while ((posn + 1) < ctx->buffer_used &&
+						ctx->buffer[posn + 1] == 0xF9) {
+				++posn;
+			}
+
+			/* We need at least 4 bytes for the header */
+			if ((posn + 4) > ctx->buffer_used)
+				break;
+
+			/* The low bit of the second byte should be 1,
+			   which indicates a short channel number */
+			if ((ctx->buffer[posn + 1] & 0x01) == 0) {
+				++posn;
+				continue;
+			}
+
+			/* Get the packet length and validate it */
+			len = (ctx->buffer[posn + 3] >> 1) & 0x7F;
+			if ((ctx->buffer[posn + 3] & 0x01) != 0) {
+				/* Single-byte length indication */
+				header_size = 3;
+			} else {
+				/* Double-byte length indication */
+				if ((posn + 5) > ctx->buffer_used)
+					break;
+				len |= ((int)(ctx->buffer[posn + 4])) << 7;
+				header_size = 4;
+			}
+			if ((posn + header_size + 2 + len) > ctx->buffer_used)
+				break;
+
+			/* Verify the packet header checksum */
+			if (((gsm0710_compute_crc(ctx->buffer + posn + 1,
+					header_size) ^ ctx->buffer[posn + len + header_size + 1]) & 0xFF) != 0) {
+				gsm0710_debug(ctx,
+					"*** GSM 07.10 checksum check failed ***");
+				posn += len + header_size + 2;
+				continue;
+			}
+
+			/* Get the channel number and packet type from the header */
+			channel = (ctx->buffer[posn + 1] >> 2) & 0x3F;
+			type = ctx->buffer[posn + 2] & 0xEF;	/* Strip "PF" bit */
+
+			/* Dispatch data packets to the appropriate channel */
+			if (!gsm0710_packet(ctx, channel, type,
+					ctx->buffer + posn + header_size + 1, len)) {
+				/* Session has been terminated */
+				ctx->buffer_used = 0;
+				return;
+			}
+			posn += len + header_size + 2;
+
+		} else if (ctx->buffer[posn] == 0x7E) {
+
+			/* Advanced format: skip additional 0x7E bytes between frames */
+			while ((posn + 1) < ctx->buffer_used &&
+					ctx->buffer[posn + 1] == 0x7E) {
+				++posn;
+			}
+
+			/* Search for the end of the packet (the next 0x7E byte) */
+			len = posn + 1;
+			while (len < ctx->buffer_used &&
+						ctx->buffer[len] != 0x7E) {
+				++len;
+			}
+			if (len >= ctx->buffer_used) {
+				/* There are insufficient bytes for a packet at present */
+				if (posn == 0 && len >= (int)sizeof(ctx->buffer)) {
+					/* The buffer is full and we were unable to find a
+					   legitimate packet.  Discard the buffer and restart */
+					posn = len;
+				}
+				break;
+			}
+
+			/* Undo control byte quoting in the packet */
+			posn2 = 0;
+			++posn;
+			while (posn < len) {
+				if (ctx->buffer[posn] == 0x7D) {
+					++posn;
+					if (posn >= len)
+						break;
+					ctx->buffer[posn2++] = (ctx->buffer[posn++] ^ 0x20);
+				} else {
+					ctx->buffer[posn2++] = ctx->buffer[posn++];
+				}
+			}
+
+			/* Validate the checksum on the packet header */
+			if (posn2 >= 3) {
+				if (((gsm0710_compute_crc(ctx->buffer, 2) ^
+						ctx->buffer[posn2 - 1]) & 0xFF) != 0) {
+					gsm0710_debug(ctx,
+							"*** GSM 07.10 advanced checksum "
+							"check failed ***");
+					continue;
+				}
+			} else {
+				gsm0710_debug(ctx,
+						"*** GSM 07.10 advanced packet "
+						"is too small ***");
+				continue;
+			}
+
+			/* Decode and dispatch the packet */
+			channel = (ctx->buffer[0] >> 2) & 0x3F;
+			type = ctx->buffer[1] & 0xEF;	/* Strip "PF" bit */
+			if (!gsm0710_packet(ctx, channel, type,
+						ctx->buffer + 2, posn2 - 3)) {
+				/* Session has been terminated */
+				ctx->buffer_used = 0;
+				return;
+			}
+
+		} else {
+			++posn;
+		}
+	}
+	if (posn < ctx->buffer_used) {
+		memmove(ctx->buffer, ctx->buffer + posn,
+			ctx->buffer_used - posn);
+		ctx->buffer_used -= posn;
+	} else {
+		ctx->buffer_used = 0;
+	}
+}
+
+/* Write a block of data to the the underlying device.  It will be split
+   into several frames according to the frame size, if necessary */
+void gsm0710_write_data(struct gsm0710_context *ctx, int channel,
+						const void *data, int len)
+{
+	int temp;
+	while (len > 0) {
+		temp = len;
+		if (temp > ctx->frame_size)
+			temp = ctx->frame_size;
+		gsm0710_write_frame(ctx, channel, GSM0710_DATA, data, temp);
+		data = ((const unsigned char *) data) + temp;
+		len -= temp;
+	}
+}
+
+/* Set the modem status lines on a channel */
+void gsm0710_set_status(struct gsm0710_context *ctx, int channel, int status)
+{
+	unsigned char data[4];
+	data[0] = GSM0710_STATUS_SET;
+	data[1] = 0x03;
+	data[2] = ((channel << 2) | 0x03);
+	data[3] = status;
+	gsm0710_write_frame(ctx, 0, GSM0710_DATA, data, 4);
+}