Interesting thing is Mikrotik is seeing the same problem on 3.0beta4. They say it'll be corrected when beta 5 is released. No date on t that yet....
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mark Koskenmaki Sent: Thursday, December 28, 2006 9:53 PM To: WISPA General List Subject: [WISPA] STar-OS and 900 mhz Tom, I haven't got time at the moment, but I have some experience with this combination... I'll write more later... but I believe there's a driver or radio problem that causes this. Mark +++++++++++++++++++++++++++++++ neofast.net - fast internet for North East Oregon and South East Washington email me at mark at neofast dot net 541-969-8200 Direct commercial inquiries to purchasing at neofast dot net ----- Original Message ----- From: "Tom DeReggi" <[EMAIL PROTECTED]> To: "WISPA General List" <wireless@wispa.org> Sent: Thursday, December 28, 2006 4:10 PM 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'" <wireless@wispa.org> > 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 aspects of radio signal. The three > portions of an RF signal that can be changed or modulated are phase, > frequency and amplitude. Shirting the properties of any of these parameters > can be used to communicate different "states." These states, in turn, are > translated to zeros and ones for binary communications. > > For example, with frequency modulation, if the sine wave is at frequency > one, it is decoded as a zero. If the sine wave is shifted slightly to > frequency two, this is decoded as a one. This type of modulation is > referred to as Binary Frequency Shift Keying (BFSK). In this example, a > system must only be able to tell the difference between one of two states or > phases. More complex modulations, such as 16 QAM (Quadrature Amplitude > Modulation), attempt to differentiate among 16 different possible states of > an incoming signal. > > The advantage to higher order modulation schemes, like 16QAM, is that > compared to BPSK, 16QAM conveys more information per bandwidth (more > bits/Hz). The disadvantage of 16QAM lies in the fact that, in order to > distinguish among the 16 different states, the signal must be very clean and > very strong relative to background noise and/or interference. > > The ability of a receiving station to decode an incoming signal at the most > basic physical layer is dependent on a factor called the "carrier to > interference ratio," or C/I. This term means exactly what it says: how > strong the desired signal (the carrier) is relative to the unwanted signals > (the interference). C/I ratios are based primarily on the modulation used, > with more complex modulations requiring higher C/I numbers than more robust > modulations, such as BFSK. > > > > > > > Modulation > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn1> [1] > > Throughput > > (20 MHz Channel) > > Rx Sensitivity > > C/I (dB) > > Co-Channel > > > BPSK ½ > > 6 Mb > > -86 dBm > > 6 dB > > > BPSK ¾ > > 9 Mb > > -85 dBm > > 10 dB > > > QPSK ½ > > 12 Mb > > -85 dBm > > 12 dB > > > QPSK ¾ > > 18 Mb > > -82 dBm > > 15 dB > > > 16 QAM ½ > > 24 Mb > > -80 dBm > > 17 dB > > > 16 QAM ¾ > > 36 Mb > > -76 dBm > > 21 dB > > > 64 QAM 2/3 > > 48 Mb > > -70 dBm > > 29 dB > > > 64 QAM ¾ > > 54 Mb > > -66 dBm > > 31 dB > > > > _____ > > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftnref1> > [1] RedLine Communications 5.8 GHz AN-50 System Specifications > > > > Due to the large amount of available license-exempt spectrum (a 500+ Mhz > chunk in 5 GHz compared to 6 Mhz slivers in 3.5 GHz) one can easily justify > sacrificing bandwidth efficiency in favor of more "interference resistant" > lower-order modulation products. For example, the Motorola Canopy product > uses BFSK for modulation, and is able to operate properly with an error rate > of 1E-4 at a 3 dB C/I; i.e., the wanted signal need be only 3 dB higher in > power than the unwanted interferes. A system operating with 16 QAM at these > levels would require a C/I ratio closer to 20 dB. > > > > Putting this into perspective, with every 3 dB of additional signal > strength, the power of a signal is doubled. This means that a system with a > 3 dB C/I ratio can tolerate an interfering signal that is many times more > powerful than a 16QAM system and still operate at the specified error rate. > Whether the interference is from another cell site on the network or another > network completely, systems employing lower-order modulations systems (like > BFSK) will tolerate substantially higher levels of interference before the > communication stream becomes impacted. All other PHY layer techniques are > designed to improve this most fundamental measurement of network robustness > and operational effectiveness by sustaining the necessary C/I level. > > > > 2. Another Method: Receiver Threshold Dampening > > > > In some instance, in order to avoid a "race to the bottom," certain > license-exempt systems retain high modulation bandwidth efficiencies in > interference heavy environments with a method known as receiver threshold > dampening. Trango Broadband uses a RF Rx Threshold command to specify the > receiver sensitivity of the radio. It is a powerful tool for a higher > modulation radio operating in a noisy environment, as it allows the radio to > block out and ignore signals received below the preset RF Rx Threshold. > > > > By creating an artificial receiver threshold below which no RF signals are > processed, the Receiver Threshold Dampening allows for the rejection of > distance interferences and reduces co-location interference at the expense > of a reduced coverage radius. > > > > PHY: DEALING WITH INTRA-SYSTEM INTERFERENCE > > > > TDD Synchronization > > > > License-Exempt systems that use Time Division Duplexing (TDD) for separating > upstream and downstream communications are ideally suited for dynamically > changing asymmetric traffic, like data. The ability to adjust the amount of > bandwidth dedicated for upstream and downstream communications without > changing hardware is a powerful feature. > > > > TDD systems operate by transmitting downstream (from the AP to the SU) for a > period of time - 1 ms for example. Following a short guard time, the SMs > then transmit on the same frequency in the upstream. For a cell site with > more than one radio operating in TDD mode, it is important that all the > sectors of the cell transmit and receive at precisely the same time. > Otherwise, if sector 1 is transmitting when sector 2 is receiving, sector 2' > incoming transmission can be interfered with because the sector 1 signal is > so close that it is strong enough to "flood" or overwhelm the "front end" in > sector 2. > > > > When deploying a TDD system in a cellular topology, it is desirable to be > able to use the same frequency in each cell site even though those cell > sites are possibly several miles away. This means that sector 1 from AP A > may interfere with sector 1 of AP B. The frequency planning diagram below > shows how such signals might interfere. In this case, inter-cellular > synchronization is required, making sure that all the sectors in all the > cell sites are properly timed and synchronized in terms of downstream and > upstream communications > > > > Delivering tight synchronization across potentially hundreds of square miles > can be a challenge. Some systems utilize TDD synchronization via GPS to > solve this issue. These precise satellite signals are used for timing and > ultimately, transmit/receive synchronization, thus tying all sectors within > the system to the same "clock." > > > > Another Method: Antenna F/B Ratio & ATPC > > > > When a BWA signal is followed from end to end, it leaves the radio and > travels first through a transmitting antenna, over the air to a receiving > antenna, and into the radio. The antenna, an important component in the RF > chain, can also have an impact on how well the network tolerates > interference, both internal and external. > > > > Antenna performance is specified in a variety of ways, but for purposes of > this discussion, we focus on the front-to-back ratio. The front-to-back > ratio of an antenna indicates how much of an incoming signal will be > absorbed coming into the front of the antenna as compared to how much of a > signal arriving at the back of the antenna is absorbed. > > > > When deploying networks in a cellular topology, the performance of the > antenna in rejecting unwanted signals from behind is an important feature. > In some cases, metal RF shields can be mounted on an antenna, augmenting and > further increasing an antenna's F/B ratio. > > > > Automatic Transmit Power Contrrol (ATPC) is a feature that allows the system > to self-optimize the transmit power and provide for the best overall link > performance. The ATPC function automatically will adjust the output power > level of remote-end systems to match a pre-specified signal strength value. > When ATPC is enabled, the system will attempt to establish the wireless link > and exchange performance information. Once the wireless link is > established, the master-end system will dynamically adjust the remote-end > systems transmit power to maintain optimum link characteristics while > minimizing power output. In short, ATPC optimizes the transmission power > for best operation, while minimizing excess power and interference with > other devices. > > > > DEALING WITH INTERFERENCE: MAC LAYER > > > > Frame/Slot Size > > > > A typical MAC frame for a TDD system is shown below. As can be seen, the > upstream and downstream portions of the frame are divided into slots, each > slot carrying what can be termed a "radio data packet," or RDP. The > original data, an IP packet datagram, for example, is segmented into packets > that fit into a RDP. > > > > Despite all the best system deployment designs, there will be instances > where interference will overcome these measures and corrupt a MAC frame or a > portion of a MAC frame. When this happens, the corrupted data must be sent > again. If the MAC frame is designed for large RDPs on the order of several > hundred bytes, the entire slot must be re-transmitted even if only a small > amount of this packet is damaged. > > > > The impact on network throughput as a result can be large, with a few bytes > in error causing hundreds of bytes to be re-sent. By using a smaller RDP > size, the re-transmission can be contained to only those bytes that were > damaged, thus avoiding the re-send of large chunks of valid data. However, > as RDP size decreases, the slot header which is fixed becomes a more > significant portion of the packet data, hence increasing the MAC layer > overhead. > > > > > > Automatic Retransmission Request > > > > In wireless broadband systems, small amounts of interference can have large > impacts on end-to-end network performance. This is tied to the way TCP/IP > networks were designed to operate in the wired world. > > > > TCP/IP was designed to operate over wire, where interference was assumed to > be negligible. The protocol design calls for positive acknowledgement sent > from the receiving station to the sending station for every IP packet sent > out. If the sending station does not receive the TCP ACK in a certain > amount of time, it is assumed that the cause was congestion of the network - > not an error resulting from transmission impediments. When encountering > congestion, TCP responds by dramatically slowing down the transmission and > then increasing transmission speed slowly. > > > > In a BWA network, a lost or corrupted packet can occur from interference > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn1> [1]. > However, the TCP protocol has no knowledge or ability to account for higher > error rates and responds by slowing down the end-to-end data rates. This is > a phenomenon that can multiply a small amount of RF interference into > significant network degradation. > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn2> [2] > > > > In most license-exempt broadband wireless systems, this is not a factor. > Automatic Retransmission request (ARQ) is a feature that addresses this > issue. ARQ inspects the RDPs that come in and looks for errors. If an > error is detected, the system will send a request to the sending entity to > re-send the RDP. All of this is accomplished two layers below TCP in the > protocol stack. The net effect is that as far as TCP is concerned, it never > receives a packet of data with an error as a result of the wireless portion > of the network, thus preventing TCP from invoking the slow start algorithm > and keeping the end-to-end data rates at the peak or just slightly below > peak operational rates. > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn3> [3] > > > > Centralized Transmission Control > > > > The popularity and cost-economics of the IEEE 802.11 standards make it a > potentially attractive BWA solution. However, it is worth noting that the > IEEE 802.11 MAC operates in what is known as a distributed control manner > (CSMA/CA). In this "contention-based" scenario, each SU has the ability to > send a packet at its own discretion. Typically in this scenario the SU will > "listen" and if it does not hear any transmission, it will assume that the > channel is clear and will send its data. > > > > In high density or high traffic deployments, the problem arises due to the > fact that (1) license-exempt wireless communications are typically half > duplex and (2) the sending SU typically cannot hear other SUs in the system > "hidden node" problem. In this instance, two or more SUs may send a packet > at the same time, corrupting both and causing a retransmission. Unlike > contention-based Ethernet networks (CSMA/CD), the half-duplex nature of > license-exempt wireless communications precludes the sending of an "error > notice" until initial transmission is complete, further delaying the > correction. In addition to this "self-induced" interference, external > sources can also block SUs from hearing each other to the same effect. > > > > > _____ > > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftnref1> > [1] It is also worth noting that ARQ is also applicable in licensed bands, > as atmospheric effects, such as multipath, fading and ducting effects, are > also a cause for lost or corrupted packets within BWA network operations > > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftnref2> > [2] Reference Layer-2 Frame Loss Concealment Discussion in our 3.5 GHz > Wireless DOCSIS Review for a more detailed analysis of this phenomenon > > <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftnref3> > [3] It is worth noting that not all ARQ implementations are created equally. > For example, 802.11 implements a form of ARQ known as the stop and wait > protocol. Essentially, the transmitter sends the packet, and then waits for > an ACK from the receiver before it tries to send the next packet. If the > transmitting station is the AP, this delay will impact all of the SUs > because they will have to wait to send any transmission destined for the AP. > If the stop and wait ARQ is combined with the RTS/CTS protocol, even each > ACK will have to be preceded by an RTS and followed by a CTS, thus slowing > down the network significantly. > > > > > > > > ------------------------------------------- > WiNOG Wireless Roadshows > Coming to a City Near You > http://www.winog.com > > > > -- > WISPA Wireless List: wireless@wispa.org > > Subscribe/Unsubscribe: > http://lists.wispa.org/mailman/listinfo/wireless > > Archives: http://lists.wispa.org/pipermail/wireless/ > > -- > WISPA Wireless List: wireless@wispa.org > > Subscribe/Unsubscribe: > http://lists.wispa.org/mailman/listinfo/wireless > > Archives: http://lists.wispa.org/pipermail/wireless/ -- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/ -- WISPA Wireless List: wireless@wispa.org Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/