the IEEE 802.11 standard places specifications on the parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, can use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and calls for operation in the 2.4 - 2.4835 GHz frequency band (in the case of spread-spectrum transmission), which is an unlicensed band for industrial, scientific, and medical (ISM) applications, and 300 - 428,000 GHz for IR transmission. Infrared is generally considered to be more secure to eavesdropping, because IR transmissions require absolute line-of-sight links (no transmission is possible outside any simply connected space or around corners), as opposed to radio frequency transmissions, which can penetrate walls and be intercepted by third parties unknowingly. However, infrared transmissions can be adversely affected by sunlight , and the spread-spectrum protocol of 802.11 does provide some rudimentary security for typical data transfers.
The MAC layer is a set of protocols which is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random "backoff factor" which determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is clear, the transmitting node decrements its backoff counter. (When the channel is busy it does not decrement its backoff counter.) When the backoff counter reaches zero, the node transmits the packet. Since the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet, cannot be used for the radio frequency transmissions of IEEE 802.11. The reason for this is that when a node is transmitting it cannot hear any other node in the system which may be transmitting, since its own signal will drown out any others arriving at the node. > -----Original Message----- > From: Horacio Castellini [SMTP:[EMAIL PROTECTED]] > Sent: Monday, February 25, 2002 11:19 AM > To: [EMAIL PROTECTED] > Subject: Re: [LUG.ro] Redes libres > > --- Sebasti�n D. Criado <[EMAIL PROTECTED]> > escribi�: > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA1 > > > > El Monday 25 February 2002 09:49, Mauro J. > > Jaskelioff escribi�: > > > 5 o 7 antenas para ieee802.11? > > > > > > �Cu�l es el alcance que tendr�a una antena seg�n > > tus c�lculos? > > > > > > 2 km ; ) con bridge. > > Se necesitar�an 2 tipos de antena por nodo. Una para > > enlazar los distintos > > puntos y otra para que se conecten quienes est�n > > cerca. ; )) > > > Ja como la vez fasil vos, si son micro-ondas como creo > que banda queres usar? para crear celdas emisoras > esfericas planas, lo podes hacer con dos dipolos y un > tercero para evitar el "fading", pero como dije antes > en micro-ondas debes pedir permiso al CONFER, sino > estas por izquierda.... > > > > > Saludos.- > > > > - -- > > - -- > > Sebasti�n D. Criado - [EMAIL PROTECTED] > > L.U.G.R.o - http://www.lugro.org.ar > > GNU/Linux Registered User # 146768 > > - > > > ------------------------------------------------------------------- > > "Si el Universo fuera un programa estar�a hecho en > > C, y correr�a sobre > > un sistema UNIX" > > > > An�nimo. > > -----BEGIN PGP SIGNATURE----- > > Version: GnuPG v1.0.6 (GNU/Linux) > > Comment: For info see http://www.gnupg.org > > > > > iD8DBQE8ejZS8hmHQ8ZCg0IRAmGLAKCegvm4nsD4vbRmHdqzS+cb80Yw2wCeObF/ > > ZNkuAghJ1+c8tT38XOl/WU8= > > =JtuC > > -----END PGP SIGNATURE----- > > Conectate a Internet GRATIS con Yahoo! Conexi�n: > http://conexion.yahoo.com.ar
