View source for LoRa port for TARPN node

[[File:PXL 20230820 144254009.MP.jpg|thumb|A complete TARPN 1-port node using LoRa. The smaller device is the LoRa KISS TNC Transceiver, and the larger Raspberry Pi is the TARPN node. It requires about 1.5 watts to run, just plug it into a USB power source.]]
KV4P has been experimenting with adding LoRa ports to his TARPN node. LoRa is very desirable for TARPN because LoRa transceivers are very inexpensive yet have good range, are very small and low-power, and reduce the overall setup cost of a new link.

Materials:

* Raspberry Pi Zero 2 W (~$15 when in stock), WITH gpio header. You can use any Rapsberry Pi model for this, but this is what I used.
* [https://www.adafruit.com/product/4074 Adafruit LoRa Radio Bonnet with OLED - RFM95W @ 915MHz - RadioFruit] ($32.50)
* 78mm of [https://www.amazon.com/gp/product/B07GBM6Y4Z 16AWG magnet wire] (33cm band 1/4 wave antenna), soldered to the radio bonnet antenna center conductor via (right next to the connector we won't use) (~$12 for a spool)

This assumes you already have a TARPN node. With this experimental guide, you'll be buiding a LoRa radio that shows up on your network as a TCP-based KISS TNC, which port 11 or port 12 of your TARPN node will connect to via the network.

I'm going to assume you already have the Raspberry Pi configured with Raspberry Pi OS, and connected to your local network (the same network as your TARPN node). You'll want to assign a static IP to it (rather than DHCP), so you can ensure your TARPN node will be able to find it on the network after restarts.

=== Get LoRa bonnet working ===
You can read more about these steps on [https://learn.adafruit.com/adafruit-radio-bonnets/rfm9x-raspberry-pi-setup this Adafruit page].

# sudo apt install python3-pip
# sudo pip3 install --upgrade setuptools
# sudo pip3 install --upgrade adafruit-python-shell
# wget https://github.com/adafruit/Adafruit_CircuitPython_framebuf/raw/main/examples/font5x8.bin
# wget https://raw.githubusercontent.com/adafruit/Raspberry-Pi-Installer-Scripts/master/raspi-blinka.py
# sudo python3 raspi-blinka.py '''(this will require reboot at the end)'''
# sudo pip3 install adafruit-circuitpython-ssd1306
# sudo pip3 install adafruit-circuitpython-framebuf
# sudo pip3 install adafruit-circuitpython-rfm9x
# create this test script rfm9x_check.py, which you should run ("python3 rfm9x_check.py") to prove your bonnet is properly connected and working: 
 <nowiki>
# SPDX-FileCopyrightText: 2018 Brent Rubell for Adafruit Industries
#
# SPDX-License-Identifier: MIT

"""
Wiring Check, Pi Radio w/RFM9x

Learn Guide: https://learn.adafruit.com/lora-and-lorawan-for-raspberry-pi
Author: Brent Rubell for Adafruit Industries
"""
import time
import busio
from digitalio import DigitalInOut, Direction, Pull
import board
# Import the SSD1306 module.
import adafruit_ssd1306
# Import the RFM9x radio module.
import adafruit_rfm9x

# Button A
btnA = DigitalInOut(board.D5)
btnA.direction = Direction.INPUT
btnA.pull = Pull.UP

# Button B
btnB = DigitalInOut(board.D6)
btnB.direction = Direction.INPUT
btnB.pull = Pull.UP

# Button C
btnC = DigitalInOut(board.D12)
btnC.direction = Direction.INPUT
btnC.pull = Pull.UP

# Create the I2C interface.
i2c = busio.I2C(board.SCL, board.SDA)

# 128x32 OLED Display
reset_pin = DigitalInOut(board.D4)
display = adafruit_ssd1306.SSD1306_I2C(128, 32, i2c, reset=reset_pin)
# Clear the display.
display.fill(0)
display.show()
width = display.width
height = display.height

# Configure RFM9x LoRa Radio
CS = DigitalInOut(board.CE1)
RESET = DigitalInOut(board.D25)
spi = busio.SPI(board.SCK, MOSI=board.MOSI, MISO=board.MISO)

while True:
    # Clear the image
    display.fill(0)

    # Attempt to set up the RFM9x Module
    try:
        rfm9x = adafruit_rfm9x.RFM9x(spi, CS, RESET, 915.0)
        display.text('RFM9x: Detected', 0, 0, 1)
    except RuntimeError as error:
        # Thrown on version mismatch
        display.text('RFM9x: ERROR', 0, 0, 1)
        print('RFM9x Error: ', error)

    # Check buttons
    if not btnA.value:
        # Button A Pressed
        display.text('Ada', width-85, height-7, 1)
        display.show()
        time.sleep(0.1)
    if not btnB.value:
        # Button B Pressed
        display.text('Fruit', width-75, height-7, 1)
        display.show()
        time.sleep(0.1)
    if not btnC.value:
        # Button C Pressed
        display.text('Radio', width-65, height-7, 1)
        display.show()
        time.sleep(0.1)

    display.show()
    time.sleep(0.1)</nowiki>

=== Turn it into a TCP KISS TNC ===
See [https://github.com/IZ7BOJ/RPi-LoRa-KISS-TNC-2ndgen/blob/main/INSTALL.md the github project] for more info.
# sudo apt install git aprx python3-rpi.gpio python3-spidev python3-pil python3-smbus
# git clone https://github.com/IZ7BOJ/RPi-LoRa-KISS-TNC-2ndgen.git
# cd RPi-LoRa-KISS-TNC-2ndgen
# git clone https://github.com/mayeranalytics/pySX127x.git
# sudo mv board_config.py pySX127x/SX127x/board_config.py
# sudo mv LoRa.py pySX127x/SX127x//LoRa.py
# Replace config.py with these contents. You can/should customize the frequency to anything in the 33cm band (but at least 500kHz from the band edge):
 <nowiki>
## Config file for RPi-LoRa-KISS-TNC 2nd generation

## Lora Module selection
sx127x = True #if True, enables sx127x family, else sx126x

## Display enable and font
disp_en = False
font_size = 8

## Log enable and path
log_enable = True
logpath='/var/log/lora/lora.log' #log filename. Give r/w permission!

## KISS Settings
# Where to listen?
# TCP_HOST can be "localhost", "0.0.0.0" or a specific interface address
# TCP_PORT as configured in aprx.conf <interface> section
TCP_HOST = "0.0.0.0"
TCP_PORT = 10001

## Hardware Settings
# See datasheets for detailed pinout.
# The default pin assignment refers to PCB designed by I8FUC.
# The user can wire the module by his own and change pin assignment.
# assign "-1" if the pin is not used

# Settings valid for both SX126x and SX127x
busId = 0 #SPI Bus ID. Must be enabled on raspberry (sudo raspi-config). Default is 0
csId = 1 #SPI Chip Select pin. Valid values are 0 or 1
irqPin = 22 #DIO0 of sx127x and DIO1 of sx126x, used for IRQ in rx

# Settings valid only for SX126x. Default pin assignment refers to the PCB schematic /doc/LoRa_RPi_Companion_2022.pdf
resetPin = 6
busyPin = 4
# If txen and rxen are disabled (=-1), then DIO2 will be set as RF Switch control
txenPin = 0 #In Ebyte modules, it's used for switching on the tx pa.
rxenPin = 1 #In Ebyte modules, it's used for switching on the rx lna

# If Lora module has a TCXO, the following parameter must be True
# If True, the DIO3 line will be set as control voltage of the TCXO
tcxo=False

## LoRa Settings valid for both SX127x and SX126x modules
frequency = 910300000 #frequency in Hz
preamble = 8 #valid preable length is 8/16/24/32
spreadingFactor = 7 #valid spreading factor is between 7 and 12
bandwidth = 500000 #possible BW values: 7800, 10400,15600, 20800, 31250, 41700, 62500, 125000, 250000, 500000
codingrate = 5 #valid code rate denominator is between 5 and 8
appendSignalReport = False #append signal report when packets are forwarded to aprs server
outputPower = 17 #maximum TX power is 22(22dBm) for SX126x, and 17 (17dBm) for SX127x . Higher values will be forced to max allowed!
TX_OE_Style = False #if True, tx RF packets are in OE Style, otherwise in standard AX25
#sync_word = 0x1424 #sync word is x4y4. Es: 0x12 of 1st gen LoRa chip --> 0x1424 of 2nd gen LoRa chip
sync_word = 0x12
crc = True #defines if CRC is calculated and transmitted in the header. Note that modem works in explicit mode

#LoRa Settings valid only for SX126x
RX_GAIN_POWER_SAVING = False #If false, receiver is set in boosted gain mode (needs more power)</nowiki>

The default settings above are for maximum throughput shorter-range links. If you want to go farther (at much slower speeds), try decreasing the bandwidth to 62500 and increasing the spreadingFactor to 8 (this will concentrate more power in a narrower signal, and slightly increase the encoding spread which takes longer to transmit the signal). These are the 2 most important values (bandwidth and spreadingFactor) in terms of throughput and range.

This github project needs more modification to work with TARPN, since it was originally written only for APRS packets and will break with any other kind. It also assumed ~400MHz, whereas the Adafruit bonnet is 33cm (902-928MHz). Here is how to update it (TODO these should be in a forked github project to remove these steps):

# nano pySX127x/SX127x/board_config.py '''(Change low_band = True, to False for 33cm support)'''
# nano pySX127x/SX127x/LoRa.py '''(Search for “43” and change multiple occurences to 915MHz)'''
# nano LoraAprsKissTnc_sx127x.py '''(Ditto, there's only 1 occurence in this one)'''
# sudo mkdir /var/log/lora
# sudo touch /var/log/lora/lora.log
# Replace KissHelper.py with this contents (this extends it to handle all KISS packets not just APRS, and makes it a little more tolerant of unexpected failures rather than just stopping the process at the first weird packet):
 <nowiki>
#!/usr/bin/python3
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# 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, see <http://www.gnu.org/licenses/>.


# This program provides basic KISS AX.25 APRS frame encoding and decoding.
# Note that only APRS relevant structures are tested. It might not work
# for generic AX.25 frames.
#
# Inspired by:
# * Python script to decode AX.25 from KISS frames over a serial TNC
#   https://gist.github.com/mumrah/8fe7597edde50855211e27192cce9f88
#
# * Sending a raw AX.25 frame with Python
#   https://thomask.sdf.org/blog/2018/12/15/sending-raw-ax25-python.html
#
#   KISS-TNC for LoRa radio modem
#   https://github.com/IZ7BOJ/RPi-LoRa-KISS-TNC

import struct
import datetime
import config

KISS_FEND = 0xC0  # Frame start/end marker
KISS_FESC = 0xDB  # Escape character
KISS_TFEND = 0xDC  # If after an escape, means there was an 0xC0 in the source message
KISS_TFESC = 0xDD  # If after an escape, means there was an 0xDB in the source message

# APRS data types
DATA_TYPES_POSITION = b"!'/@`"
DATA_TYPE_MESSAGE = b":"
DATA_TYPE_THIRD_PARTY = b"}"

def logf(message):
    timestamp = datetime.datetime.now().strftime('%Y/%m/%d %H:%M:%S - ')
    if config.log_enable:
       fileLog = open(config.logpath,"a")
       fileLog.write(timestamp + message+"\n")
       fileLog.close()
    print(timestamp + message)

# from LoRa OE_Style to KISS
# Addresses must be 6 bytes plus the SSID byte, each character shifted left by 1
# If it's the final address in the header, set the low bit to 1
# If it has been digipeated, set the H bit to 1
# Ignoring command/response for simple example
def encode_address(s, final):
    H=0b00000000 #default H bit low
    if chr(s[-1])=='*': #example: WIDE1-1* or WIDE1*
        s=s[:-1]
        H=0b10000000
    if b"-" not in s:
        s = s + b"-0"  # default to SSID 0
    call, ssid = s.split(b'-')
    if len(call) < 6:
        call = call + b" "*(6 - len(call)) # pad with spaces
    encoded_call = [x << 1 for x in call[0:6]]
    encoded_ssid = (int(ssid) << 1) | H | 0b01100000 | (0b00000001 if final else 0)
    return encoded_call + [encoded_ssid]


def decode_address(data, cursor):
    (a1, a2, a3, a4, a5, a6, a7) = struct.unpack("<BBBBBBB", data[cursor:cursor + 7])
    hrr = a7 >> 5
    ssid = (a7 >> 1) & 0xf
    ext = a7 & 0x1
    addr = struct.pack("<BBBBBB", a1 >> 1, a2 >> 1, a3 >> 1, a4 >> 1, a5 >> 1, a6 >> 1)
    if ssid != 0:
        call = addr.strip() + "-{}".format(ssid).encode()
    else:
        call = addr
    return (call, hrr, ext)

def ax25parser(frame): #extracts fields from ax25 frames and add signal report do payload, if specified
    try:
        pos = 0

        # DST
        (dest_addr, dest_hrr, dest_ext) = decode_address(frame, pos)
        pos += 7
        print("DST: ", dest_addr)

        # SRC
        (src_addr, src_hrr, src_ext) = decode_address(frame, pos)
        pos += 7
        print("SRC: ", src_addr)

        # REPEATERS
        ext = src_ext
        rpt_list = b""
        while ext == 0:
            rpt_addr, rpt_hrr, ext = decode_address(frame, pos)
            if rpt_hrr==b'111': # H bit high-->packet has been digipeated
               rpt_addr+=b'*'
            rpt_list += b","+rpt_addr
            print("RPT: ", rpt_addr)
            pos += 7

        # CTRL
        ctrl = frame[pos]
        pos += 1
        if (ctrl & 0x3) == 0x3:
            #(pid,) = struct.unpack("<B", frame[pos])
            try:
                pid = frame[pos]
            except Exception:
                pid = 240
            print("PID: "+str(pid))
            pos += 1
        elif (ctrl & 0x3) == 0x1:
            # decode_sframe(ctrl, frame, pos)
            logf("SFRAME")
            return None,None,None,None,None
        elif (ctrl & 0x1) == 0x0:
            # decode_iframe(ctrl, frame, pos)
            logf("IFRAME")
            return None,None,None,None,None
        payload=frame[pos:]
        print("payload: ", payload)
        try:
            dti=payload[0]
        except Exception:
            dti=":"
        logf("Extracted AX25 parameters from AX25 Frame")
        logf("From: "+repr(src_addr)[2:-1]+" To: "+repr(dest_addr)[2:-1]+" Via: "+repr(rpt_list)[3:-1]+" PID: "+str(hex(pid))+" Payload: "+str(payload)[str(payl
oad).find("'"):-1])

        return src_addr,dest_addr,rpt_list,payload,dti
    except Exception:
        return None,None,None,None,None

def encode_kiss_AX25(frame,signalreport): #from Lora to Kiss, Standard AX25

    src_addr,dest_addr,rpt_list,payload,dti=ax25parser(frame) #only for logging

    # Escape the packet in case either KISS_FEND or KISS_FESC ended up in our stream
    if config.appendSignalReport and str(dti) != DATA_TYPE_MESSAGE:
        frame += b" "+str.encode(signalreport,'utf-8')

    packet_escaped = []
    for x in frame:
        if x == KISS_FEND:
            packet_escaped += [KISS_FESC, KISS_TFEND]
        elif x == KISS_FESC:
            packet_escaped += [KISS_FESC, KISS_TFESC]
        else:
            packet_escaped += [x]

    # Build the frame that we will send to aprx and turn it into a string
    kiss_cmd = 0x00  # Two nybbles combined - TNC 0, command 0 (send data)
    kiss_frame = [KISS_FEND, kiss_cmd] + packet_escaped + [KISS_FEND]

    try:
        output = bytearray(kiss_frame)
    except ValueError:
        logf("Invalid value in frame.")
        return None
    return output

def encode_kiss_OE(frame,signalreport): #from Lora to Kiss, OE_Style
    # Ugly frame disassembling
    if not b":" in frame:
        logf("Can't decode OE LoRa Frame")
        return None
    path = frame.split(b":")[0]
    payload = frame[frame.find(b":")+1:]
    dti = payload[0]
    src_addr = path.split(b">")[0]
    digis = path[path.find(b">") + 1:].split(b",")
    dest_addr = digis.pop(0)

    # destination address
    packet = encode_address(dest_addr.upper(), False)
    # source address
    packet += encode_address(src_addr.upper(), len(digis) == 0)
    # digipeaters
    for digi in digis:
        final_addr = digis.index(digi) == len(digis) - 1
        packet += encode_address(digi.upper(), final_addr)
    # control field
    packet += [0x03]  # This is an UI frame
    # protocol ID
    packet += [0xF0]  # No protocol
    # information field
    logf("Extracted AX25 parameters from OE LoRa Frame")
    logf("From: "+repr(src_addr)[2:-1]+" To: "+repr(dest_addr)[2:-1]+" Via: "+str(digis)[1:-1].replace("b","").replace("'","")+" Payload: "+repr(payload)[2:-1])
    packet += payload
    if config.appendSignalReport and str(dti) != DATA_TYPE_MESSAGE:
        #some SW (es OE5BPA) append newline character at the end of packet. Must be cut for appending signal report
        if chr(packet[-1])=="\n":
          packet=packet[:-1]
        packet += b" "+str.encode(signalreport,'utf-8')

    # Escape the packet in case either KISS_FEND or KISS_FESC ended up in our stream
    packet_escaped = []
    for x in packet:
        if x == KISS_FEND:
            packet_escaped += [KISS_FESC, KISS_TFEND]
        elif x == KISS_FESC:
            packet_escaped += [KISS_FESC, KISS_TFESC]
        else:
            packet_escaped += [x]

    # Build the frame that we will send to Dire Wolf and turn it into a string
    kiss_cmd = 0x00  # Two nybbles combined - TNC 0, command 0 (send data)
    kiss_frame = [KISS_FEND, kiss_cmd] + packet_escaped + [KISS_FEND]
    try:
        #print(bytearray(kiss_frame).hex())
        output = bytearray(kiss_frame)
    except ValueError:
        logf("Invalid value in frame.")
        return None
    return output

def decode_kiss_OE(frame): #From Kiss to LoRa, OE_Style

    if frame[0] != 0xC0 or frame[len(frame) - 1] != 0xC0:
        logf("Kiss Header not found, abort decoding of Frame: "+repr(frame))
        return None
    frame=frame[2:len(frame) - 1] #cut kiss delimitator 0xc0 and command 0x00

    src_addr,dest_addr,rpt_list,payload,dti=ax25parser(frame) #only for logging

    #build OE_style frame piece by piece
    result = src_addr.strip()+b">"+dest_addr.strip()+rpt_list+b":"+payload
    return result

def decode_kiss_AX25(frame): #from kiss to LoRA, Standard AX25
    result = b""

    if frame[0] != 0xC0 or frame[len(frame) - 1] != 0xC0:
        logf("Kiss Header not found, abort decoding of Frame: "+repr(frame))
        return None
    frame=frame[2:len(frame) - 1] #cut kiss delimitator 0xc0 and command 0x00

    src_addr,dest_addr,rpt_list,payload,dti=ax25parser(frame) #only for logging

    return frame


class SerialParser():
    '''Simple parser for KISS frames. It handles multiple frames in one packet
    and calls the callback function on each frame'''
    STATE_IDLE = 0
    STATE_FEND = 1
    STATE_DATA = 2
    KISS_FEND = KISS_FEND

    def __init__(self, frame_cb=None):
        self.frame_cb = frame_cb
        self.reset()

    def reset(self):
        self.state = self.STATE_IDLE
        self.cur_frame = bytearray()

    def parse(self, data):
        #Call parse with a string of one or more characters
        for c in data:
            if self.state == self.STATE_IDLE:
                if c == self.KISS_FEND:
                    self.cur_frame.append(c)
                    self.state = self.STATE_FEND
            elif self.state == self.STATE_FEND:
                if c == self.KISS_FEND:
                    self.reset()
                else:
                    self.cur_frame.append(c)
                    self.state = self.STATE_DATA
            elif self.state == self.STATE_DATA:
                self.cur_frame.append(c)
                if c == self.KISS_FEND:
                    # frame complete
                    if self.frame_cb:
                        self.frame_cb(self.cur_frame)
                    self.reset()

if __name__ == "__main__":
    # Playground for testing

    kissframe = b"\xc0\x00\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90p\x88\x92\x8e\x92@@f\x88\x92\x8e\x92@@e\x03\xf0!4725.51N/00939.86E[322/002/A=001306 Batt=3.99V
\xc0"

    #test decode KISS->OE
    print(decode_kiss_OE(kissframe))

    #test decode KISS->AX25
    print(decode_kiss_AX25(kissframe))

    #test encode OE->KISS
    OE_frame = b"OE9TKH-8>APRS,digi-3,digi-2:!4725.51N/00939.86E[322/002/A=001306 Batt=3.99V\n"
    signalreport="Level:-115dBm, SNR:0dB"
    print(encode_kiss_OE(OE_frame,signalreport))

    #test encode AX25->KISS
    ax25_frame = b"\x82\xa0\xa4\xa6@@`\x9e\x8ar\xa8\x96\x90p\x88\x92\x8e\x92@@f\x88\x92\x8e\x92@@e\x03\xf0!4725.51N/00939.86E[322/002/A=001306 Batt=3.99V\n"
    print(encode_kiss_AX25(ax25_frame,signalreport))
</nowiki>

NOTE: The above KissHelper.py has not been optimized at all, it was just made to work with all KISS packets and made fault-tolerant so it doens't crash. The OE-based encoding can be removed, and many simplifications of the code can be made! This is the quick and dirty version.

Start/test your TCP-based KISS TNC with this command (you should see it print out the config values and say it's listening for connections, with no errors listed):

* sudo python3 Start_lora-tnc.py

This is what you should see:

 <nowiki>
########################
#LORA KISS TNC STARTING#
########################
#Lora parameters:
frequency= 910300000
preamble= 32
spreadingFactor= 7
bandwidth= 500000
codingrate= 5
APPEND_SIGNAL_REPORT= False
outputPower= 17
TX_OE_Style= False
sync_word= 0x12
crc= True
########################
2023/08/15 00:27:18 - KISS-Server: Started. Listening on IP 0.0.0.0 Port: 10001

2023/08/15 00:27:18 - LoRa radio initialized. Waiting for LoRa Spots...
</nowiki>

When you run the TNC for long-term use, you'll want to run it with "nohup sudo python3 Start_lora-tnc.py &" which will disassociate it with your linux account and run it in the background until manually terminated. In the future, we should wrap this in a systemd service.

=== TARPN configuration ===

TARPN only supports serial TNCs, so first we need to "fake" a serial TNC that actually connects to our TCP-bsaed LoRa KISS TNC using the "socat" command.

* sudo apt install socat
* sudo nano lora-port-up.sh
* Use these contents but replace the IP address to your LoRa KISS TNC's address:
 <nowiki>
#!/bin/bash
printf "Bringing up LoRa port as /dev/ttyS0\n"
while true
    do
      sudo socat pty,link=/dev/ttyS0,b115200,raw,echo=0,mode=777 tcp:192.168.1.90:10001,forever,interval=10
      printf "LoRa port /dev/ttyS0 disconnected, waiting 1 second and bringing it up again.\n"
      sleep 1
    done
</nowiki>

* Start the fake serial device with "./lora-port-up.sh &" and it will show up as /dev/ttyS0 like a serial TNC would be. (You might want to do this in crontab or something, so it always runs on node restart.)

Then you can edit your TARPN node.ini to configure port 11 as follows:

 <nowiki>
usb-port11:ENABLE
portdev11:/dev/ttyS0
speed11:115200
txdelay11:5
frack11:2000
neighbor11:KV4P-3
</nowiki>

Of course, assign "neighbor11" to the callsign/ssid of the node you plan to connect to via LoRa on your new port 11.

=== Performance ===
With the above config.py settings, a throughput test on my own TARPN node with a "bench setup" (2 LoRa transceivers right next to each other), I achieved 135bytes/sec. That's about as good as a very solid 9600 baud link with a NinoTNC and high quality mobile radios.

It can probably be made to go much faster with additional optimization. It's unclear if the KISS service running in Python represents a cap on the throughput, or if it's just the settings.

Although the distance of this link hasn't been tested yet, the LoRa module claims to work between 4km and 40km (from city to perfectly flat ideal conditions). 33cm has the nice property of easily going through windows and walls, so it should be especially good for suburban links, or between nodes with a fairly clear shot.