Date:   Mon, 13 Aug 2018 00:37:24 +0800
From:   Jian-Hong Pan <starnight@...cu.edu.tw>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Andreas Färber <afaerber@...e.de>,
        netdev@...r.kernel.org,
        "<linux-arm-kernel@...ts.infradead.org\\" 
        <linux-arm-kernel@...ts.infradead.org>,
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        Jiri Pirko <jiri@...nulli.us>,
        Marcel Holtmann <marcel@...tmann.org>,
        "David S. Miller" <davem@...emloft.net>,
        Matthias Brugger <mbrugger@...e.com>,
        Janus Piwek <jpiwek@...oweurope.com>,
        Michael Röder <michael.roeder@...et.eu>,
        Dollar Chen <dollar.chen@...ec.com>,
        Ken Yu <ken.yu@...wireless.com>,
        Konstantin Böhm <konstantin.boehm@...ud.de>,
        Jan Jongboom <jan.jongboom@....com>,
        Jon Ortego <Jon.Ortego@...t.de>,
        "linux-kernel@...r.kernel.org>," <contact@...otlab.com>,
        Ben Whitten <ben.whitten@...rdtech.com>,
        Brian Ray <brian.ray@...k-labs.com>, lora@...balsat.com.tw,
        Alexander Graf <agraf@...e.de>,
        Michal Kubeček <mkubecek@...e.cz>,
        Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org,
        Steve deRosier <derosier@...il.com>,
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        Pieter ROBYNS <pieter.robyns@...sselt.be>,
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        Daniele Comel <dcomel@...ot.com>, shess@...sware.de,
        Xue Liu <liuxuenetmail@...il.com>
Subject: Re: [RFC net-next 00/15] net: A socket API for LoRa

Alan Cox <gnomes@...rguk.ukuu.org.uk> 於 2018年8月10日 週五 下午11:57寫道：
>
> > Except saving power, mitigating the wireless signal conflict on the
> > air is one of the reasons.
>
> If the device level is always receiving when not transmitting it has no
> effect on this. The act of listening does not harm other traffic.

My friend had tested practically:
If he changes the LoRa interface to RX mode after TX completes
immediately, he will receive the signals like reflection echo some
times.
That is interesting!

There is a paper "Exploring LoRa and LoRaWAN A suitable protocol for
IoT weather stations?" by Kristoffer Olsson & Sveinn Finnsson
http://publications.lib.chalmers.se/records/fulltext/252610/252610.pdf
In chapter 3.2 Chirp Spread Spectrum, it describes the reflection echo
phenomenon.
I think that is why LoRaWAN places the RX delay time which avoids
receiving the reflection noise.

> > The sleep/idle/stop mitigate the unconcerned RF signals or messages.
>
> At the physical level it's irrelevant. If we are receiving then we might
> hear more things we later discard. It's not running on a tiny
> microcontroller so the extra CPU cycles are not going to kill us.

According different power resource, LoRaWAN defines Class A, B and C.
Class A is the basic and both Class B and C devices must also
implement the feature of Class A.
If the end device has sufficient power available, it can also
implement the Class C: Continuously listening end-device.
Here are the descriptions in LoRaWAN spec. for Class C:
- The Class C end-device will listen with RX2 windows parameters as
often as possible.
- The end-device listens on RX2 when it is not either (a) sending or
(b) receiving on RX1, according to Class A definition.
- 1. It will open a short window on RX2 parameters between the end of
the uplink transmission and the beginning of the RX1 reception window.
(*)
  2. It will switch to RX2 reception parameters as soon as the RX1
reception window is closed; the RX2 reception window will remain open
until the end-device has to send another message.

According to the LoRaWAN Regional Parameters, the DataRate (including
spreading factor and bandwidth) and frequency channel of RX1 and RX2
windows may be different.(*)

So, yes!  Class C opens the RX windows almost all the time, except the TX time.
And uses different channel to avoid the reflection noise (*).

However, Class C must also implements Class A and C is more complex than A.
I think starting from the simpler one and adding more features and
complexity in the future will be a better practice.

> > > How do you plan to deal with routing if you've got multiple devices ?
> >
> > For LoRaWAN, it is a star topology.
>
> No the question was much more how you plan to deal with it in the OS. If
> for example I want to open a LORA connection to something, then there
> needs to be a proper process to figure out where the target is and how to
> get traffic to them.
>
> I guess it's best phrased as
>
> - What does a struct sockaddr_lora look like

According to LoRaWAN spec, the Data Message only has the device's
DevAddr (the device's address in 4 bytes) field related to "address".
The device just sends the uplink Data Message through the interface
and does not know the destination.  Then, a LoRaWAN gateway receives
the uplink Data Message and forwards to the designated network server.
So, end device does not care about the destination.  It only knows
there is a gateway will forward its message to some where.
Therefore, only the DevAddr as the source address will be meaningful
for uplink Data Message.

> - How does the kernel decide which interface it goes out of (if any), and
>   if it loops back

There is the MAC Header in the Data Message which is one byte.
Bits 5 to 7 indicate which kind of type the message is.
000: Join Request
001: Join Accept
010: Unconfirmed Data Up
011: Unconfirmed Data Down
100: Confirmed Data Up
101: Confirmed Data Down
110: RFU
111: Proprietary

So, end device only accepts the types of downlink and the matched
DevAddr (the device's address) in downlink Data Message for RX.

> remembering we might only be talking to a hub, or we might even be a
> virtualized LORA interface where we are pretending to be some kind of
> sensor and feeding it back.
>
> Long term yes I think Alexander is right the inevitable fate of all
> networks is to become a link layer in order to transmit IP frames 8)

Yeah, maybe.  It will be easier for life.
But I have not seen the formal standard for that yet or I missed it.
If the standard appears, we can try to implement it.

Jian-Hong Pan

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