[VL2006] -- VL73: Issues for VoIP on Wi-Fi
ViewsLetter on VoIP
vl2006 at ViewsLetter.com
Mon Mar 30 10:57:20 EDT 2009
I have no idea why I'm getting these emails. I am not sending these.
Pat Donohue
At 01:56 PM 3/30/2009, you wrote:
>Bill,
>
>Here’s another:
>
>
>From: vl2006-bounces at ViewsLetter.com
>[mailto:vl2006-bounces at ViewsLetter.com] On Behalf Of ViewsLetter on VoIP
>Sent: Monday, March 30, 2009 7:25 AM
>To: circulation at viewsletter.com; vl2006 at viewsletter.com
>Subject: Re: [VL2006] -- VL73: Issues for VoIP on Wi-Fi
>
>Please remove me from this e-mail.
>
>
>----------
>From: vl2006-bounces at viewsletter.com
>[mailto:vl2006-bounces at viewsletter.com] On Behalf Of ViewsLetter on VoIP
>Sent: Monday, March 30, 2009 10:09 AM
>To: circulation at viewsletter.com; vl2006 at viewsletter.com
>Subject: Re: [VL2006] -- VL73: Issues for VoIP on Wi-Fi
>PLEASE REMOVE ME FROM THIS EMAIL
I HAVE ASKED
>MULTIPLE TIMES. NEXT STEP WILL BE TO REPORT TO
>ATT SECURITY AND YOU WILL BE BLACKLISTED FROM ATT AND REPORTED AS SPAM.
>
>From: vl2006-bounces at viewsletter.com
>[mailto:vl2006-bounces at viewsletter.com] On Behalf Of ViewsLetter on VoIP
>Sent: Sunday, March 29, 2009 11:52 PM
>To: vl2006 at viewsletter.com
>Subject: [VL2006] -- VL73: Issues for VoIP on Wi-Fi
>
>[]
>
>We've moved; see footer for new address. Phones remain the same.
>
>Configuration Issues for VoIP on Wi-Fi
>
>By William Flanagan
>
>The designs of VoIP and wireless LANs originated
>separately and at first didn't consider each other as part of a total network.
>802.11b, first to deploy, was kept simple by focusing on data applications.
>VoIP protocols ignored the possibility of a
>radio link (not to mention Network Address Translation).
>
>VoIP and Wi-Fi kept bumping into each other in
>the wiring closet and data center until each
>technology had to acknowledge the other. The
>result was enhanced protocols and added features:
>VoIP: encryption in IP phones countered the
>weakness of the WEP, first air-side algorithm for Wi-Fi.
>Wi-Fi: not only improved encryption (WPA2 uses
>AES), vLANs, and authentication, but also added
>capabilities (with 802.11e) to assign priority to voice packets.
>
>Now vendors offer VoIP mobile handsets with
>Wi-Fi radios (some with cellular too, but that's
>for another issue). Wireless Access Points can
>give priority to the phones, ahead of the PC's
>data--if you configure them all properly. In
>addition to any cellular service setup, there's
>the Wi-Fi configuration. Just for starters:
>Service Set Identifier (Extended SSID when
>configured on an Access Point); some APs support multiple SSIDs
>virtual LAN ID (in the 802.11Q header)
>Priority class assignments in both IP stack and Wi-Fi
>Encryption protocol and algorithm
>RF channel assignments
>Authentication method (registering the MAC
>address of a device, requiring a name/password
>log in to get on the network, knowing the SSID, etc.)
>Voice encoding algorithm, most often G.711 (PCM) or G.729 (ACELP)
>
>That last item, the choice of codec, can
>generate a lot of discussion related to
>bandwidth usage and the number of simultaneous
>voice channels a link can carry. G.711 encodes
>at 64,000 bit/s while G.729 compresses the
>digital stream to 8,000 bit/s. On a satellite
>link, when using header compression to replace
>the RTP/UDP/IP headers with four bytes or less,
>the difference can be significant. In the local
>area, without header compression, the situation is not as clear.
>
>Voice is not only real-time, it is constant,
>equal in both directions, and demands low
>latency from the network. To minimize delay at
>the source, an IP phone seldom accumulates more
>than 10 ms of sound before sending the digital information in a new packet.
>
>G.711 generates 640 bits or 80 bytes in 10 ms,
>where G.729 outputs 10 bytes. The catch is that
>either payload requires the same set of headers,
>which take 74 bytes for the LAN headers. Encryption adds another header.
>
>On modern wired LANs, voice usually represents a
>small fraction of total traffic. If prioritized
>ahead of data (with absolute priority) voice
>packets "see" a lightly loaded network,
>encounter minimum delay, and have no packets
>dropped due to congestion. Granting top priority
>to voice under these conditions can't block or
>starve data traffic because each voice channel
>has only a fixed and relatively low throughput.
>An IP phone doesn't burst to high volume the way a file transfer can.
>
>Wi-Fi adds overhead in the form of time delays
>to allocate air time to different stations and a
>necessary wait time between packets on the
>radio. With the headers and overhead dominating
>the air interface, it's often the large number
>of small packets that limits the number of voice
>channels on an AP and not the encoded bandwidth of the voice payloads.
>
>In their book Wi-Fi Telephony (Newnes/Elsevier,
>2007) Chandra and Lide calculate the number of
>telephone connections one 802.11b access point
>can support without recent improvements in a/g/n
>Wi-Fi versions or 802.11 enhancements. Even at
>the shortest range (highest data rate) only 12
>calls fit on the 11 Mbit/s channel. The
>interesting point is that the difference between
>G.711 and G.729 is just one connection--10 vs.
>11 for one set of conditions--confirming that
>codec selection has little impact on capacity.
>
>You mileage probably varies. Particularly if you
>have deployed the later releases of the Wi-Fi
>standards and radios, you should be able to
>support more simultaneous calls per AP. What do
>you see in your environment? Let us know and
>we'll share the aggregate info (or not yours, if
>you prefer). Email
><mailto:publisher at viewsletter.com>publisher at viewsletter.com.
>
>___
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Patricia A. Donohue
Wesleyan University
(860) 685-2022
pdonohue at wesleyan.edu
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