RE: [WISPA] PAcket loss with CSMA/CA
Imho, packet loss on your system is happening when the latency or retrans is exceeding the tcp timeout ..? Gino A. Villarini [EMAIL PROTECTED] Aeronet Wireless Broadband Corp. tel 787.273.4143 fax 787.273.4145 -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom DeReggi Sent: Thursday, December 28, 2006 8:11 PM To: WISPA General List Subject: [WISPA] PAcket loss with CSMA/CA I just installed a PTP 900Mhz Atheros SR9 StarOSV3 link that had 5% packet loss that I could not get rid of. (Set 12mbps modulation, and averaged greater than 20db SNR.) In theory, CSMA/CA should not get PAcket loss, like a TDD system might, as the CSMA waits for acknowledment and re-transmits if it does not get it, Wifi's built-in native ARQ. I was not surprices to see Latency skyrocket, or retransmisson to sky rocket, but I was surprised to see uncorrectable 5% packetloss. Any ideas on why it occured. Meaning why 802.11 MAC didn't self correct the packet loss with its native re-transmission? Tom DeReggi RapidDSL & Wireless, Inc IntAirNet- Fixed Wireless Broadband - Original Message - From: "Charles Wu" <[EMAIL PROTECTED]> To: "'WISPA General List'" Sent: Thursday, December 28, 2006 4:47 PM Subject: RE: [WISPA] Alvarion Comnet Radios have arrived - regardinginterference - Part 1 I go to see Mickey Mouse for a few days and look where this thread has gone...wow So, my 2 cents... One of the largest concerns in the license-exempt world is the question of a system's interference robustness. However, before we can get into further detail on the pros and cons of Alvarion VL vs Canopy, CSMA/CA vs GPS, etc -- it is necessary to realize that interference as a term is extremely broad and vague, and can mean just about anything to anyone. Heck, all radios in the market have some sort of "interference robustness / avoidance capability" -- the trick to understanding a system's capabilities is knowing what TYPE of interference the system can actually handle. Read on...I'll talk more about each particular platform when I get some time to write Part 2 =) WHAT IS INTERERENCE? In the wireless world, interference, by definition, is a situation where unwanted radio signals operate in the same frequency channels or bands - i.e. they mutually "interfere," disrupt or add to the overall noise level in the intended transmission. Interference can be divided into two forms, based on whether it comes from your own network(s) or from an outside source. If the interfering RF signals emanate from a network under your control, whether it is on the same tower or several miles away, it is termed "self-interference." If the opposing signals come from a network, device or other source that is not under your control, it is termed "outside interference." Thus, the definition of what type of interference is being combated is not based on technology, but ownership. In licensed bands, where spectrum is relatively scarce (due to high costs) self-interference alone must be taken into account; however given a more or less known operating environment (the radio spectrum will only have signals transmitting that are under control by a single entity) proper product design and network deployment can reduce these interferes to a level where they do not impact network performance. Self-interference is not a phenomenon that is confined to licensed band operations; license-exempt bands must address the same issues. The techniques and design elements of a given product that serve to reduce and tame self-interference in licensed band operations can be applied directly to license-exempt systems. THE LICENSE-EXEMPT CHALLENGE OF INTERFERENCE In the license-exempt bands, not only must self-interference be accounted for, but, given the nature of the regulations governing these bands, external interference must be designed for as well. This can be extremely challenging, as there is no way of knowing in advance where these outside signals may be or will be sourced from, or even how strong the interfering transmissions will be relative to the desired transmission. This aspect of the license-exempt bands represents the possible "downside" of license-exempt network operation. Yet as potentially damaging and unpredictable as external interference can be in license-exempt networks, a properly designed and implemented broadband wireless system can make a significant difference in the performance of a network under siege from unwanted external radio transmissions. DEALING WITH COCHANNEL INTERFERENCE: PHY LAYER 1. Modulation & the C/I Ratio At the most fundamental level, an interfering RF source disrupts the digital transmission by making it too difficult for the receiving station to "decode" the signal. How much noise or interference a digital RF transmission can tolerate depends on the modulation used. Fundamentally, modulation is the method whereby zeros and ones are communicated by varying one of three
Re: [WISPA] PAcket loss with CSMA/CA
Tom, After making several retransmission attempts and still not getting a packet through, the radio will discard the packet and move on to sending the next packet. In other words, the packet is lost. Often, depending on the radio or the operating system, the number of retransmission attempts made before the packet is discarded is software-configurable. One typical default value for the number of retransmission attempts is eight. jack Tom DeReggi wrote: I just installed a PTP 900Mhz Atheros SR9 StarOSV3 link that had 5% packet loss that I could not get rid of. (Set 12mbps modulation, and averaged greater than 20db SNR.) In theory, CSMA/CA should not get PAcket loss, like a TDD system might, as the CSMA waits for acknowledment and re-transmits if it does not get it, Wifi's built-in native ARQ. I was not surprices to see Latency skyrocket, or retransmisson to sky rocket, but I was surprised to see uncorrectable 5% packetloss. Any ideas on why it occured. Meaning why 802.11 MAC didn't self correct the packet loss with its native re-transmission? Tom DeReggi RapidDSL & Wireless, Inc IntAirNet- Fixed Wireless Broadband - Original Message - From: "Charles Wu" <[EMAIL PROTECTED]> To: "'WISPA General List'" Sent: Thursday, December 28, 2006 4:47 PM Subject: RE: [WISPA] Alvarion Comnet Radios have arrived - regardinginterference - Part 1 I go to see Mickey Mouse for a few days and look where this thread has gone...wow So, my 2 cents... One of the largest concerns in the license-exempt world is the question of a system's interference robustness. However, before we can get into further detail on the pros and cons of Alvarion VL vs Canopy, CSMA/CA vs GPS, etc -- it is necessary to realize that interference as a term is extremely broad and vague, and can mean just about anything to anyone. Heck, all radios in the market have some sort of "interference robustness / avoidance capability" -- the trick to understanding a system's capabilities is knowing what TYPE of interference the system can actually handle. Read on...I'll talk more about each particular platform when I get some time to write Part 2 =) WHAT IS INTERERENCE? In the wireless world, interference, by definition, is a situation where unwanted radio signals operate in the same frequency channels or bands - i.e. they mutually "interfere," disrupt or add to the overall noise level in the intended transmission. Interference can be divided into two forms, based on whether it comes from your own network(s) or from an outside source. If the interfering RF signals emanate from a network under your control, whether it is on the same tower or several miles away, it is termed "self-interference." If the opposing signals come from a network, device or other source that is not under your control, it is termed "outside interference." Thus, the definition of what type of interference is being combated is not based on technology, but ownership. In licensed bands, where spectrum is relatively scarce (due to high costs) self-interference alone must be taken into account; however given a more or less known operating environment (the radio spectrum will only have signals transmitting that are under control by a single entity) proper product design and network deployment can reduce these interferes to a level where they do not impact network performance. Self-interference is not a phenomenon that is confined to licensed band operations; license-exempt bands must address the same issues. The techniques and design elements of a given product that serve to reduce and tame self-interference in licensed band operations can be applied directly to license-exempt systems. THE LICENSE-EXEMPT CHALLENGE OF INTERFERENCE In the license-exempt bands, not only must self-interference be accounted for, but, given the nature of the regulations governing these bands, external interference must be designed for as well. This can be extremely challenging, as there is no way of knowing in advance where these outside signals may be or will be sourced from, or even how strong the interfering transmissions will be relative to the desired transmission. This aspect of the license-exempt bands represents the possible "downside" of license-exempt network operation. Yet as potentially damaging and unpredictable as external interference can be in license-exempt networks, a properly designed and implemented broadband wireless system can make a significant difference in the performance of a network under siege from unwanted external radio transmissions. DEALING WITH COCHANNEL INTERFERENCE: PHY LAYER 1. Modulation & the C/I Ratio At the most fundamental level, an interfering RF source disrupts the digital transmission by making it too difficult for the receiving station to "decode" the signal. How much noise or interference a digital RF transmission can tolerate depends on the modulation used. Fundamentally, modulation is the method where
