[VL2006] -- VL73: Issues for VoIP on Wi-Fi
ViewsLetter on VoIP
vl2006 at ViewsLetter.com
Mon Mar 30 12:23:18 EDT 2009
I didn't request being removed, however, we're getting copies of your
correspondence here at HSB.
ViewsLetter on VoIP <vl2006 at ViewsLetter.com>
Sent by: vl2006-bounces at ViewsLetter.com
03/30/2009 10:42 AM
Please respond to
circulation at viewsletter.com; Please respond to
vl2006 at ViewsLetter.com
To
<circulation at viewsletter.com>, <vl2006 at ViewsLetter.com>
cc
Subject
Re: [VL2006] -- VL73: Issues for VoIP on Wi-Fi
Can you tell me exactly which email address(es) our distributions are
being sent to so I can make sure you are removed?
Is it circulation at viewsletter.com; vl2006 at ViewsLetter.com or Bill
Flanagan?s email?
From: vl2006-bounces at ViewsLetter.com
[mailto:vl2006-bounces at ViewsLetter.com] On Behalf Of ViewsLetter on VoIP
Sent: Monday, March 30, 2009 10:25 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 publisher at viewsletter.com.
___
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Bill at Flanagan-Consulting.com
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