Thanks for the info, Patrick.  I sure hope these systems have all the 
attributes that Tom & Brad were describing, and they're real and can be 
evaluated live now.  I presume they have OFDM and TDD.  Can anyone comment if 
they have "everything" in Brad's wish list?

>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!

As Patrick says, they're available ... try'em out.  Doubt they're $300, but I 
think Tom commented that such a radio would be worth more than $300 to him.  If 
anyone has trialed them can they comment to the list?

>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.

Don't know what this meant, as it's real Alvarion WiMAX product that Patrick is 
describing.  I'm sure there's other brands also available now as well.  Maybe 
it meant no product "like it" yet available in UL 900 / 2.4 / 5??? Dunno.  A 
little help please?

  ----- Original Message ----- 
  From: Patrick Leary 
  To: WISPA General List 
  Sent: Thursday, December 28, 2006 8:34 PM
  Subject: RE: [WISPA] Alvarion Comnet Radios 
havearrived-regardinginterference- Part 1

  Alvarion's got actual WiMAX gear Rich. Our WiMAX-certified BreezeMAX 3500 is 
being deployed in over 100 commercial networks along with about 120 trials. In 
the U.S. we are selling and deploying early BreezeMAX 2500 and BreezeMAX 2300 
to a handful of operators. These are TDD 802.16e-ready solutions and they will 
be certified when the WiMAX Forum opens up .16e certification testing.

  Some call BreezeACCESS pre-WiMAX, but that is only true to the extent that it 
uses OFDM and has a host of other features that some might call "WiMAX-like." I 
am personally not fond of pre/like/kinda, etc. UNLESS the system is real WiMAX 
and just awaits the certification process, such as is the case with BreezeMAX 
2300 and BreezeMAX 2500. BreezeMAX 3500 is already certified. Anything called 
"BreezeMAX" was designed from the ground up to support WiMAX profiles and will 
ultimately be WiMAX-certified. Anything in our line NOT called BreezeMAX will 
not ever be WiMAX-certified.
  Patrick Leary
  AVP WISP Markets
  Alvarion, Inc.
  o: 650.314.2628
  c: 760.580.0080
  Vonage: 650.641.1243
  -----Original Message-----
  From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Rich Comroe
  Sent: Thursday, December 28, 2006 5:31 PM
  To: WISPA General List
  Subject: Re: [WISPA] Alvarion Comnet Radios have 
arrived-regardinginterference- Part 1

  Again, I think they're already being made, aren't they? for 3.5GHz.  Doesn't 
have to be final WiMAX ... I presume that all the pre-WiMAX products are OFDM 
and TDD.  I've yet to hear of one at 900, 2.4, or 5.  Anyone?  Am I all wet on 
what the pre-WiMAX products are?  I could very well be all wet, as I am only 
talking from what I've picked up from reading here ... and I've not had any 
first-hand experience with real available pre-WiMAX gear that's out there.  
Alvarion's got pre-WiMAX gear ... maybe Patrick can confirm, or alternatively 
slap me back to reality!   :-)

    ----- Original Message ----- 
    From: Brad Belton 
    To: 'WISPA General List' 
    Sent: Thursday, December 28, 2006 6:16 PM
    Subject: RE: [WISPA] Alvarion Comnet Radios have 
arrived-regardinginterference - Part 1

    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!



    -----Original Message-----
    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


    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 

    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'" <>
    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

    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 =)


    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.


    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.


    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.

    <outbind://93-00000000A1BC9C82EE27CA43A85AEBB3458E5835C444CD00/#_ftn1> [1]


    (20 MHz Channel)

    Rx Sensitivity

    C/I (dB)


    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


    [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.


    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.


    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

    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.


    [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

    [2] Reference Layer-2 Frame Loss Concealment Discussion in our 3.5 GHz
    Wireless DOCSIS Review for a more detailed analysis of this phenomenon

    [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.

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