lol...gotta love it! I'd argue it doesn't have to be only $300 to sell. I'd pay two or three times that for such a product.
But honestly that isn't that much to ask as many products are already so close...Alvarion VL being one of the closest, but still no cigar. I like what you said about developing Trango products and agree they are way past due to "leapfrog" back to the front of the pack. Oh those were the days when Sunstream/Trango was the undisputed leader with the début of the M5800 and then the M5830. <sigh> Maybe they can do it again! Best, Brad -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Tom DeReggi Sent: Thursday, December 28, 2006 6:05 PM To: WISPA General List Subject: Re: [WISPA] Alvarion Comnet Radios have arrived -regardinginterference - Part 1 Charles, WOW! Great Post! That covers about everything. It increases the understanding of the complexity, but it doesn't answer the ultimate question, "What to use". What we really want is an efficient OFDM system, with a strong TDD w/ARQ MAC, RFThreshold, Good Noise Filtering, Packet aggregating/compressing, adeqaute CPU processing, Quality narrow beam diversity antennas, all pre-packaged in a system/box under $300. But that product does not exist today. So why doesn't a manufacturer just make it, so we can stop debating what is best, and just deploy radios! 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/
