[Ns-developers] Mac layer 802.11n

Mathieu Lacage mathieu.lacage at sophia.inria.fr
Wed Jan 7 03:46:04 PST 2009

On Wed, 2009-01-07 at 12:25 +0100, Mirko Banchi wrote:

> Use of A-MSDU or A-MPDU should be chosen according to type of traffic.
> The idea is to implement the possibility of choose first, second or
> both.

Ok. Is it the responsability of the MAC layer to choose whether to use
A-MPDUs or is the PHY layer expected to be able to make its own choices
on its own without interference from the MAC ?

> 1)I agree with you about EdcaTxopN class. Maybe the better way to
> reuse
> also DcfManager to implement Edca mechanism is write an interface
> DcaTxop and inherit from it.

You don't need anything special to reuse DcfManager: You should be able
to instance one DcfManager, 4 DcfState, 4 EdcaTxop and associate each
DcfState to one EdcaTxop.

> 2)About BlockAck, you suggest to move storage of packets for which an
> ack hasn't been received yet in DcaTxop and i agree with you.

I think that it should be the EdcaTxop class which should be responsible
for keeping a copy of each outgoing packet to perform a retransmission
if needed. I merely suggested that the MacLow be responsible for sending
a block of packets all in one go. I would expect the MacLow to forget
about each packet as soon as it is sent on the medium, regardless of
whether the packet is acked later or not.

> > a) change the signature of MacLowTransmissionListener::GotAck to
> include
> > the sequence number of the packet we got an ack for:
> >  virtual void GotAck (double snr, WifiMode txMode, uint16_t seq) =
> 0;
> > This method will be invoked by the MacLow class to notify the
> EdcaTxop
> > class of each packet whose bit is set in a received BlockAck.
> Changing the signature of
> virtual void MacLowTransmissionListener::GotAck(double snr, WifiMode
> txMode, uint16_t seq) = 0;
> involves calling it for every corretly received packet that is
> acknowledged with the BlockAck frame. Maybe is better a function like
> this?
> virtual void MacLowTrasmissionListener::GotBlockAck(double snr,
> WifiMode
> txMode, uint16_t *p) = 0;
> where p array contains sequence numbers indicated in the blockack
> bitmap.

Yes, you could do something like this but the one I suggested:
  - allows you to not have to deal with an array of uint16_t (hence, not
have to allocate/deallocate it)
  - allows you to not pass around a pointer

I think that both options have probably about the same performance cost
(in one case, you have to do a dynamic allocation and perform a single
function call, in the other case, you don't have to do dynamic
allocation and you perform multiple function calls) but the one I
suggested seems more simple.

> > b) add MacLowTransmissionListener::MissedAck to notify a listener
> that a
> > specific packet has been missed.
> >  virtual void MissedAck (uint16_t seq) = 0;
> If we implement the function above there isn't need to add thid
> method.

It seems easier for EdcaTxop to have this method too because it saves
EdcaTxop from having to infer which packets must be retransmitted to
prepare the retransmission.

> > add MacLowTransmissionParameters::AddPacket and m_packets:
> > void AddPacket (Ptr<const Packet> packet,
> >                 const WifiMacHeader *hdr);
> > std::list<std::pair<Ptr<const Packet>, WifiMacHeader> > m_packets;
> Sorry but i have some problems here :( How is used m_packets? When is
> AddPacket called ?

Sample code:

EdcaTxop::Send (void)
  MacLowTransmissionParameters params;
  params.AddPacket (p1, h1);
  params.AddPacket (p2, h2);
  m_low->StartTransmission (params, m_listener);

MacLowTransmissionParameters contains all the parameters to apply to a
set of packets to transmit. This, of course, will work only if all
packets share the same tx parameters. Will it be the case for you ?

Finally, I wanted to point out that if you really care about merging
your work in ns-3 itself, it is very important to proceed in small steps
and not try to merge/implement all features at once: I would suggest
that you first define a couple of clear milestones with subsets of the
full functionality and that we try to merge each of these incrementally.


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