Archive 6 Dec
AVAILABILITY FROM THIN AIR:
CELLULAR IP LINK BACKS
responsible for voice communications, you know that voice is the
service that everyone expects to work all the time. That's one reason
fax transmission on voice circuits is the backup method of last resort
for transmitting certain kinds of information.
voice service, a common practice is to protect local loops against
"backhoe fade" with a second local loop. That approach can be hard at
branch sites--and expensive: diverse routings for cables, separate
entry points into the building, perhaps two local exchange carriers.
It's not always practical, and seldom is worth the expense of a second
line if both are in the same cable.
high-speed data services from cellular carriers (for example, EVDO)
offer an alternative that integrates easily with existing PBXs. This
ViewsLetter looks at how this works for a multi-site organization.
You don't want
a branch office to be isolated, unreachable by customers. This approach
provides voice and data connections from the branch to headquarters if
the branch loses its local phone lines. The key components are
branch router with voice interfaces and a high-speed wireless
larger headquarters router that complements the branch equipment to
work with branch voice connections and link them to other phones.
thanks to NSGDatacom
Note that the
cellular service used is high-speed data (often over 350 kbit/s) and
not the normal voice connection. The traffic is all IP, between the
radio voice router in the branch and the cellular network. From there
the voice packets are treated as prioritized data and delivered to the
IP address of the HQ voice router (which doesn't need a radio
central site relies on a land line, it is not protected by radio backup
in this scenario. However, protecting only one site with a diversely
routed second line is much more practical.
router doesn't dial phone number, it uses the "always on" IP link for
packetized compressed voice to the central voice router. Voice
compression (to 8 kbit/s or less) allows the cellular IP link to carry
a full T-1's worth of calls (24) and some data.
are working, there are no voice packets sent by radio. If the landlines
fail, the radio link keeps the branch connected.
calls from the branch phones pass through the radio router, which
detects when the local trunks have failed and routes calls over the
radio link to the cellular service which passes packets to
headquarters. There the router connects to the PBX--the caller can
reach other extensions or the public network. In a different topology,
a radio voice router as the last part of a trunk hunt group can provide
outbound overflow capacity.
-- Branch land
line trunks are set up in the central office to forward in-bound
calls--whenever the caller gets a busy, ring/no-answer, or failed
trunk--to the main number at headquarters. The PBX attendant transfers
those incoming calls to the extension group assigned to the branch,
which are connected to ports on the voice router. The call connection
is passed over an IP line to the cellular network for completion over
the radio data link. No lost business.
overhead and processing, the voice routers combine voice samples from
multiple conversations in one packet (standard VoIP has one
conversation per packet to permit routing each call independently,
which is not needed here). That makes the format proprietary--you need
matching boxes at both ends.
sites can share one larger voice router at headquarters, so the
solution scales up fairly well. For details on the hardware, see the
data sheets at http://www.nsgdata.com/solutions/autofailover.html
There are not
as many concerns about Voice over IP (VoIP) in this context as you'll
find on the Internet. The IP connection is point-to-point on the
cellular and back-haul infrastructure, not over the Internet. The
proprietary packet format with compression make interception very
unlikely. The radio routers prioritize voice to allow a mix of voice
and data on the link and preserve voice quality. The routers also
handle the signaling, so there's no need for a call processor server,
SIP configuration, etc.
there's a catch: all protected sites must have a high speed cellular
service. Not a lot to pay for highly available voice.
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