Archive 1 Nov 2007 #64
HIGH AVAILABILITY FROM THIN AIR:
CELLULAR BACKUP FOR DATA LINKS
By William Flanagan, Publisher
We always get a chuckle from stories of cable cuts where the back up
link was in the same bundle as the main link. Without diverse routing,
which can be even more expensive, redundancy alone may not be worth the
price of the second line. It depends on what you want to protect and
Cellular networks offer connectivity that's always diverse from a land
line. This form of wireless hasn't been exploited much in the past,
perhaps because of the limited speeds, relatively high per-minute usage
charges, and the need to use multiple channels to get the top throughput.
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The latest cellular standards set up dedicated data capability separate
from the dialed voice channels--allowing carriers to drop the price of
bandwidth significantly. Examples of services include EVDO, HSDPA,
EDGE, and GPRS. Recent generations of cellular equipment boost the
speed limit to more than a T-1 (in the direction from cell base station
to end point). Upstream the services clock data at 800 kbit/s, but
practical throughput tops out at half that. Work proceeds on services
offering symmetrical throughput with uploads as fast as downloads. To
keep the players straight you need more of a scorecard than we have room
To make things easy on you, the basic networks are all IP, today's
lingua franca of communications.
WAY TO GO
Specialized routers are the way to take advantage of cellular data to
back up your applications. You can get small routers with a variety of
data interfaces: Ethernet, of course, and serial ports for all the
legacy protocols (frame relay, SCADA, SDLC, ALC, X.25, etc.).
Routers with serial interfaces either encapsulate the legacy frames in
IP or spoof the protocol of the local device. Either way, the traffic
becomes IP and therefore is routable on the Internet. The router's
forwarding table typically makes the land line the primary or preferred
route. If the primary is unavailable, packets take the secondary route
via the cellular interface. No intervention by an operator is needed.
Thus the remote site is protected from loss of local terrestrial
connectivity, improving availability.
The cellular antenna on the router is quite small, and may work fine.
If the router is in a commercial wiring closet, it may be shielded by
metal studs or lathe. The antenna alone may be relocated into a
stronger signal by adding a thin coax cable between it and the router.
One vendor offers a 6-foot extension as standard; runs up to 20 feet
What about the central site? Because the cellular service connects
directly to the Internet, no special equipment is needed at the central
site. If the remote sites are "pure IP" then the central site is
probably a server running an appropriate application, with Internet
access. Connectivity over Ethernet does the job at both ends. You can
see Fig. 64-1 at http://viewsletter.com/VLHAhtml/vlha64Fig1.html. Note
that this scenario protects the remote site from loss of the data land
line; it doesn't provide redundant access at headquarters--that's a
Legacy protocols offer "more interesting" scenarios--and may see even
quicker paybacks on the investment. Where the remote router spoofs the
protocol, the central site probably needs to "unspoof" it or convert it
back to the original format for the central computer. The drawing at
www.viewsletter.com/VLHAhtml/vlha64Fig2.html shows this for electric
power distribution. Note that the solution does not require any change
to either the central computer or remote SCADA equipment.
Companies monitor and control switches, transformers, and other
equipment using the decades-old Supervisory Control and Data Acquisition
(SCADA) protocol. The central computer polls the remote sites, sending
messages over serial interfaces designed for multi-drop modem links. A
central router or data conversion unit appears to the computer as
multiple leased or dial-up lines. Information actually moves over the
IP network to the remote site, where it is converted back to SCADA format.
The last major blackout increased the awareness of control functions for
power transmission and triggered new regulations to improve reliability.
SCADA was designed for modems and analog lines, which are quite
reliable but vulnerable to a break in the line, either by weather or
sabotage. Cellular can back up that land line with a connection that's
much harder to bring down. If the cellular router is within range of
more than one cell site, even better.
Some "accessibility challenged" locations are hard to provision with any
form of land line. Those sites benefit immediately from cellular
connectivity, where available, as the primary or only connection.
As cellular coverage spreads and data connections increase in speed, the
service becomes more appealing for applications that formerly may have
seen satellite as the optimum solution. Examples include point of sale
devices such as credit card verification, lottery ticket sales, ATM
(cash) machines, and updating of prices on gas pumps.
Your feedback is always welcome: Publisher@ViewsLetter.com.
How Can Flanagan Consulting Help You?
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We have current experience in litigation support for attorneys
involved with patents or contracts related to networking or
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