[riot-devel] CSMA MAC-layer, Open Issues

Hauke Petersen hauke.petersen at fu-berlin.de
Mon Mar 23 19:09:18 CET 2015


Hi Jonas,

On 20.03.2015 18:02, Jonas Remmert wrote:
>
> 1. Both, the upper layer and the current implementations of the 
> ng_netdev radio drivers use task messages to signalize an event 
> (Packet to be sent or Packet to receive waiting). Both message 
> mechanisms send their messages to the mac_pid and therefore use the 
> same message queue. If the mac-layer and the transceiver is busy with 
> sending a message out or waiting for an ACK it might wait for a signal 
> from the driver (RX-ready, ACK-received...). In the same time there 
> might also be messages from the upper layers that can not be handled 
> until the previous packet is sent out.
>
> - I handled this problem in using the drivers call-back function in 
> combination with different task lock mechanisms for upper layer 
> (msg_receive) and for driver events 2x(mutex_lock), which causes in 
> TASK_BLOCKED state. That method worked very well, even if I fired the 
> mac-layer continously with upper layer messages.
Though this probably works fine, this is not quite how the concept was 
meant. The problem is of course there: when the MAC layer is getting 
spammed by upper layer messages, the reaction time for device driver 
messages might be not as fast... Though the messages should not be los 
(as they are queued...). The concept is based on the assumption, that 
the time needed for handling incoming messages in the MAC layer is very 
short. Also so far we basically assumed, that the MAC layer has a higher 
priority then the upper layers. This way, the MAC layer will always 
finish processing all it's queued messages before the upper layers get 
scheduled again.
>
> 2. The drivers send function builds the IEEE 802.15.4 MAC header on 
> its own. The alternative would be to let the MAC layer do this job. 
> This would avoid code duplication and make it easier to implement new 
> radio drivers. Is there any reason to do this in the driver 
> implementation?
I don't really understand the code duplication issue here... The MAC 
layer should actually not know anything about the radio frame format or 
the like, as the idea behing the MAC layer in the current form is to 
make them interchangable and usable with different network devices (not 
only 802.15.4 devices)... So to answer your question: yes!
>
> 3. Introduction of PHY dependant constants: The constants (e.g. 
> SYMBOL_LENGTH used in backoff intervall, MAX_BE, MIN_BE, 
> TURNAROUND_TIME, MAC_ACK_WAIT_DURATION, MAX_RETRIES..). As these 
> constants are different for each type of PHY (2,5 Ghz, subGhz and also 
> Modulation specific) we could put them in a struct that would be 
> linked to the device descriptor.
Actually, these constants should not be constants :-) The way I would 
implement them is by naming these defines something like 
xx_DEFAULT_MAX_BE, xx_DEFAULT_TURNAROUND_TIME, etc. The you include 
fields in your device descriptor for these values and initialize them 
with these values. This way you can change them during runtime and 
define different values if you run more then 1 device concurrently...
>
> 4. Generally, a successfull TX of a packet is not signalized to upper 
> layers. But how do we handle a packet that could not be sent to the 
> channel (e.g. channel busy)? Should upper layer be informed about the 
> failure?
So far the packets are just silently dropped. The plain netapi_send() 
does not support feedback to upper layers on the success of the send 
process - this we did to avoid deadlock scenarios and synchronization 
issues...

Cheers,
Hauke


>
> A WIP state of the ng_kw2xrf-driver that I used for testing can be 
> found in my repo [2].
>
> a nice Weekend,
> Jonas
>
>
> [1] - https://github.com/RIOT-OS/RIOT/pull/2467
> [2] - https://github.com/jremmert-phytec-iot/RIOT/tree/wip%40kw2xrf_ng
>
>
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