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Add packet demodulator
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Jon Beniston
@@ -0,0 +1,213 @@
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///////////////////////////////////////////////////////////////////////////////////
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// Copyright (C) 2020 Jon Beniston, M7RCE //
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// //
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// This program is free software; you can redistribute it and/or modify //
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// it under the terms of the GNU General Public License as published by //
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// the Free Software Foundation as version 3 of the License, or //
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// (at your option) any later version. //
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// //
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// This program is distributed in the hope that it will be useful, //
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// but WITHOUT ANY WARRANTY; without even the implied warranty of //
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the //
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// GNU General Public License V3 for more details. //
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// //
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// You should have received a copy of the GNU General Public License //
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// along with this program. If not, see <http://www.gnu.org/licenses/>. //
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///////////////////////////////////////////////////////////////////////////////////
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#include <QDebug>
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#include "ax25.h"
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// CRC is assumed to be correct (checked in packetdemodsink)
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bool AX25Packet::decode(QByteArray packet)
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{
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int i, j;
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char destAddress[7];
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unsigned char destSSID;
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char sourceAddress[7];
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unsigned char sourceSSID;
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char repeaterAddress[7];
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unsigned char repeaterSSID;
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unsigned char ssid;
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unsigned char control;
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// Check for minimum size packet. Addresses, control and CRC
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if (packet.size() < 7+7+1+2)
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return false;
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// Address - ASCII shifted right one bit
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for (i = 0; i < 6; i++)
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destAddress[i] = (packet[i] >> 1) & 0x7f;
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destAddress[6] = NULL;
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destSSID = packet[6];
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for (i = 0; i < 6; i++)
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sourceAddress[i] = (packet[7+i] >> 1) & 0x7f;
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sourceAddress[6] = NULL;
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sourceSSID = packet[13];
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// From = source address
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m_from = QString(sourceAddress).trimmed();
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ssid = (sourceSSID >> 1) & 0xf;
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if (ssid != 0)
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m_from = QString("%1-%2").arg(m_from).arg(ssid);
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// To = destination address
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m_to = QString(destAddress).trimmed();
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ssid = (destSSID >> 1) & 0xf;
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if (ssid != 0)
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m_to = QString("%1-%2").arg(m_to).arg(ssid);
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// List of repeater addresses for via field
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m_via = QString("");
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i = 13;
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while ((packet[i] & 1) == 0)
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{
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i++;
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for (j = 0; j < 6; j++)
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repeaterAddress[j] = (packet[i+j] >> 1) & 0x7f;
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repeaterAddress[j] = NULL;
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i += 6;
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repeaterSSID = packet[i];
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ssid = (repeaterSSID >> 1) & 0xf;
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QString repeater = QString(repeaterAddress).trimmed();
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QString ssidString = (ssid != 0) ? QString("%2-%3").arg(repeater).arg(ssid) : QString(repeater);
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if (m_via == "")
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m_via = ssidString;
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else
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m_via = QString("%1,%2").arg(m_via).arg(ssidString);
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}
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i++;
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// Control can be 1 or 2 bytes - how to know if 2?
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// I, U and S frames
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control = packet[i++];
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if ((control & 1) == 0)
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m_type = QString("I");
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else if ((control & 3) == 3)
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{
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// See figure 4.4 of AX.25 spec
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switch (control & 0xef)
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{
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case 0x6f:
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m_type = QString("SABME");
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break;
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case 0x2f:
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m_type = QString("SABM");
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break;
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case 0x43:
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m_type = QString("DISC");
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break;
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case 0x0f:
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m_type = QString("DM");
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break;
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case 0x63:
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m_type = QString("UA");
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break;
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case 0x87:
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m_type = QString("FR");
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break;
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case 0x03:
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m_type = QString("UI");
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break;
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case 0xaf:
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m_type = QString("XID");
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break;
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case 0xe3:
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m_type = QString("TEST");
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break;
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default:
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m_type = QString("U");
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break;
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}
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}
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else
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m_type = QString("S");
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// APRS packets use UI frames, which are a subype of U frames
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// Only I and UI frames have Layer 3 Protocol ID (PID).
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if ((m_type == "I") || (m_type == "UI"))
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m_pid = QString("%1").arg(((unsigned)packet[i++]) & 0xff, 2, 16, QLatin1Char('0'));
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else
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m_pid = QString("");
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int infoStart, infoEnd;
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infoStart = i;
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infoEnd = packet.size()-2-i;
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QByteArray info(packet.mid(infoStart, infoEnd));
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m_dataASCII = QString::fromLatin1(info);
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m_dataHex = QString(info.toHex());
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return true;
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}
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bool AX25Packet::ssid(QByteArray& b, int i, int len, uint8_t& ssid)
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{
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if (b[i] == '-')
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{
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if (len > i + 1)
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{
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ssid = b[i+1] - '0';
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if ((len > i + 2) && isdigit(b[i+2])) {
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ssid = (ssid*10) + (b[i+2] - '0');
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}
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if (ssid >= 16)
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{
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// FIXME: APRS-IS seems to support 2 letter SSIDs
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// These can't be sent over RF, as not enough bits in AX.25 packet
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qDebug() << "AX25Packet::ssid: SSID greater than 15 not supported";
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ssid = 0;
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return false;
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}
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else
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{
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return true;
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}
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}
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else
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{
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qDebug() << "AX25Packet::ssid: SSID number missing";
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return false;
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}
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}
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else
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return false;
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}
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QByteArray AX25Packet::encodeAddress(QString address, uint8_t crrl)
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{
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int len;
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int i;
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QByteArray encodedAddress;
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QByteArray b;
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uint8_t ssid = 0;
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bool hyphenSeen = false;
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len = address.length();
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b = address.toUtf8();
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ssid = 0;
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for (i = 0; i < 6; i++)
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{
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if ((i < len) && !hyphenSeen)
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{
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if (b[i] == '-')
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{
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AX25Packet::ssid(b, i, len, ssid);
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hyphenSeen = true;
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encodedAddress.append(' ' << 1);
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}
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else
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{
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encodedAddress.append(b[i] << 1);
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}
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}
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else
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{
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encodedAddress.append(' ' << 1);
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}
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}
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if (b[i] == '-')
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{
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AX25Packet::ssid(b, i, len, ssid);
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}
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encodedAddress.append(crrl | (ssid << 1));
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return encodedAddress;
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}
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