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
>
>
>
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