Subject: Re: [manet] RTS/CTS in wireless MAC

From: "Victoria Sudareva" <[email-masked]>
Date: Mon, 06 Jan 2003 11:10:55 +1030

Paul,

DIFS is not random and is specified for each protocol (a, b, g). 802.11 uses 
two mechanism for sensing the media: physical (actual receiving) and virtual 
(NAV). If both of them indicate clear medium and previous frame was tx/rx 
without errors then STA or AP will wait for DIFS. If no activity is detected 
during this period then the STA/AP 'owns' the medium and the frame is 
transmitted. Otherwise STA/AP goes into the backoff procedure. This is a 
simplified description but generally it holds.

Regards,

Jack Sudarev.

>Message: 1
>From: "Paul Tan" <[email-masked]>
>To: "Liaw, Yong Shyang - LIAYS001" <[email-masked]>,
>    "'Sumeeth Nagaraj'" <[email-masked]>,
>    "Sachin Ganu" <[email-masked]>,
>    "Grzegorz Banasiak" <[email-masked]>
>Cc: <[email-masked]>
>Subject: Re: [manet] RTS/CTS in wireless MAC
>Date: Sat, 4 Jan 2003 11:45:18 +0800
>
>Hi,
>
>in WLAN, collision detection is not possible due to what is know as the
>"near/far" problem. To detect a collision, a station must be able to
>transmit and listen at the same time. For this reason, 802.11 uses a
>different protocol known as Carrier Sense Multiple Access with Collision
>Avoidance (CSMA/CA) or the Distributed Coordination Function (DCF). CSMA/CA
>attempts to avoid packet collisions by using explicit packet 
>acknowledgement
>(ACK), which means that an ACK packet is sent by the receiving station to
>confirm that a packet arrived intact.
>CSMA/CA works by having the station that wishes to transmit senses the
>channel and if there is no activity detected, the station will wait an
>additional random period of time (DIFS) and if there still is no activity,
>it will transmit the data. If the packet is received intact (checksum), the
>receiving station will send and ACK frame that once it is received by the
>original sender the transmission is complete. If the ACK command is not
>received in a specified random period of time, the data packet will be
>resent, assuming that the original packet experienced a collision.
>
>By telling the card to go into "promiscuous mode", all traffic seen is
>passed to the operating system. On a shared media network, this lets a
>station to spy on every packet traversing the 'air'. Yes, you can still
>tx/rx data.
>
>Cheers,
>
>Paul Tan
>
>
>----- Original Message -----
>From: "Liaw, Yong Shyang - LIAYS001" <[email-masked]>
>To: "'Sumeeth Nagaraj'" <[email-masked]>; "Sachin Ganu"
><[email-masked]>; "Grzegorz Banasiak" <[email-masked]>
>Cc: <[email-masked]>
>Sent: Wednesday, November 06, 2002 7:30 AM
>Subject: RE: [manet] RTS/CTS in wireless MAC
>
>
> >
> > So then how can the receiver knows if there is a collosion? Does it just
>check
> > the "checksum" of the data packet?  Another question. If the wireless 
>LAN
>i/f
> > (i.e. waveLan) is put into promiscuous mode, it can receive all data
> > transmissions that reach it. But can it also send data (in promiscuous
>mode)
> > when it is not receiving data?
> >
> > Thank in advance.
> >
> >
> >
> > -----Original Message-----
> > From: Sumeeth Nagaraj [mailto:[email-masked]]
> > Sent: Wednesday, 6 November 2002 7:31
> > To: Sachin Ganu; Grzegorz Banasiak
> > Cc: [email-masked]
> > Subject: Re: [manet] RTS/CTS in wireless MAC
> >
> >
> > CSMA/ CD is used in the Ethernet, Collision Detection is not possible
> > because of the Half-Duplex nature of the nodes. The wireless nodes can
> > either transmit or receive and hence cannot detect collisions at the
> > destination(receiver). CSMA/CA is used and not CSMA/CD.
> >
> > Best Regards,
> > Sumeeth
> >
> >
> > _______________________________________________
> > manet mailing list
> > [email-masked]
> > https://www1.ietf.org/mailman/listinfo/manet

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