RE: [smartBridges] RTS / CTS

[Seeni Mohamed]

Hi Brian, Multipath is a form of RF interference that occurs when a radio signal has more than one path between the receiver and the transmitter. Multipath propagation occurs when RF signals take different paths when traveling from a source to a destination. A portion of the signal might go directly to the destination, while another part might bounce off an obstruction or ceiling, then on to the destination. As a result, some of the signal encounters delay and travels longer paths to the destination. If the delays are great enough, bit errors in the packet occur. The receiver can't distinguish the packets and interpret the corresponding bits correctly. The receiving station detects the error in the packets.  It means, the cyclic redundancy check (CRC) checksum does not compute correctly, indicating that there is error in the packet. In response to the bit errors, the receiving station does not send an acknowledgment to the sending station. The sender eventually retransmits the signal after regaining access to the medium. Because of the re-transmissions, users encounter lower throughput when Multipath interference is significant. Due to multiple re-transmissions, the default value (RTS/CTS) may not able to deliver the packet with original size. In such condition, adjusting or reducing the RTS/CTS value will improve the efficiency in the data transmission. You can use the RTS/CTS adjustment to overcome the effects of HIDDEN NODE. The default value works great in the environment where all the clients can hear each other such as indoor wireless LAN. However, in an outdoor environment, not all the clients can hear from one each other. If the clients located far away, neither one can hear the other. For example, if the clients are uploading/downloading large file size at a same time. That means data being transmitted simultaneously, which results in a COLLISION. Whenever there is a collision, both clients have to re-transmit at a same time. That is the reason the hidden node is a problem when there is many clients talking to the same AP at same time. None of them knows when the others are talking, so it will end of with collisions all over the place. A lot of collisions results in a lot of re-transmissions which will reduce the overall throughput at the CPE end. Due to multiple re-transmissions, the RTS/CTS adjustment is necessary to overcome the packet loss and throughput problem. I hope above info helps to understand the RTS/CTS adjustment for the Multipath interference and Hidden node issues.     Kind regards, Seeni sB Tech Support     -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Brian Winter Sent: Wednesday, November 26, 2003 7:30 PM To: [EMAIL PROTECTED] Subject: Re: [smartBridges] RTS / CTS   Hi Seeni,   Could you please explain how rts/cts combats multipath or any other interference (ie not my or other users real traffic).  I thought it was used to tell the far end (AP), and subsequently other in range clients (by cts) to shut up while I get on with my transmission and hear the ack.  I understood that it temporarily stops real traffic, useful on a busy contentious system, - not interference.  Efficiency is only improved as collisions are reduced.   Am I missing something ? ----- Original Message ----- From: Seeni Mohamed To: [EMAIL PROTECTED] Sent: Wednesday, November 26, 2003 10:35 AM Subject: RE: [smartBridges] RTS / CTS   The RTS/CTS adjustment is required ONLY in the CPE devices when there is poor performance due to MULTIPATH INTERFERENCE and many HIDDEN NODES which will result in packet loss. Sometimes when we deal with higher interference then we need to lower down the RTS/CTS values to improve the efficiency in the packet transmission which avoids the packet loss. Normally, RTS/CTS adjustment is NOT required in the AP side. RTS/CTS only comes into play when a client is transmitting and it does nothing for the receive traffic.   Kind regards, Seeni sB Tech Support   -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of TJ Burbank Sent: Wednesday, November 26, 2003 9:18 AM To: [EMAIL PROTECTED] Subject: [smartBridges] RTS / CTS   I am curious what the best values for a network with 30 bridges per AP, and a 1.5MB backbone would be.    I guess I am not certain if changing the RTS / CTS values on customer bridges will help in increasing netork load and capacity.   If you change the RTS / CTS values on the CPE is is neccesary to change them on the AP?   Thanks,   -TJ Last Mile Wireless