Access points and "hidden transmitter syndrome" !?
Alfred.Reynolds at dsto.defence.gov.au
Wed Jan 16 09:43:21 EST 2002
The term for this problem in the networking world is the "hidden station
problem". It is solved by 802.11b by using the RTS/CTS handshake at the
beginning of a transmission.
In your situation, if A send a RTS to B then C would also hear the RTS and
would backoff on any transmissions. This media access control method is
called "MACA". To make it collision free you can add data frame acks and it
is then called MACAW (multiple access with collision aviodance).
802.11 also used carrier detection to avoid collisions on a channel.
> -----Original Message-----
> From: Dent, Mike [mailto:m.dent at lancaster.ac.uk]
> Sent: Tuesday, January 15, 2002 11:23 PM
> To: 'wireless at lists.samba.org'
> Subject: Access points and "hidden transmitter syndrome" !?
> Hi, I come from a ham radio and packet radio background so I
> realise the
> effects of the so called "hidden transmitter syndrome".
> This is where station A, can hear client B and client C but
> alas client B
> cannot hear client C and visa versa.
> So it is possible for client B and C to both transmit packets
> to station A
> simultaneously, station A does not recognise the clashing
> packets and ignores
> What I would like to know is if the 802.11b protocol in the
> Access Points takes
> any of this in to account, or does it simply use CSMA with CA
> and hope all
> its clients can communicate with each other? I realise
> clients can be set to
> use infrastructure/BSS mode to only communicate through the
> AP but this does
> take in to account more than 1 client trying to access the AP
> at the same
> time..... or does it??
> I'm setting up a small community network at home, I am the
> access point and two
> neighbours the clients but they cannot hear each other.
> Thanks for any info.
> > --
> > Mike Dent,
> > Chief Technician,
> > ISS,
> > Computer Centre,
> > Lancaster University, Lancaster, LA1 4YW.
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