SMS Relay -- An Idea for Fault-Tolerant Communications
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Automatic discovery and message delivery
A telephone or pager in this system will normally send and receive messages through the nearest base station. When the phone loses its ability to communicate with a base station, either because it is out of range or because the network has failed, it switches to its backup mode.
Whenever the device is turned on, it will listen for short messages from other devices capable of relaying messages and, in turn, the devices they are able to communicate with. The device uses this information to learn how messages can be routed to a base station, using other phones or pagers as intermediate relay points when direct communication is not possible. The device does this silently and automatically.
A typical status message might look something like the following example:
<route> <id>4155552222</id> <dnstream>4155551234,4155552345,4155553456</dnstream> <upstream>4155559876</upstream> </route>
In this example, the phone 415-555-2222 is telling other devices that it can relay messages to 415-555-1234, 415-555-2345, 415-555-3456 and 415-555-9876. It is also saying that it can relay messages upstream, to other devices and the outside world, via 415-555-9876.
When the user travels out of range, or the network goes dark, the device will attempt to relay messages via other handheld devices instead of trying to communicate with a base station directly. Messages can make many hops between endpoints; for example, hopping across a half dozen cellphones in their journey to the nearest operating base station. This will be invisible to the user. Even if a message traverses many devices in its trip, the cumulative delay will be barely noticeable, typically a few seconds.
Likewise, the terrestrial network can use the same technique to discover how to reach users who cannot communicate directly with a base station. Just as handheld devices will listen for status messages about who can communicate with whom, so too will the base stations. With this information, the terrestrial side of the network can maintain a constantly-updated table of the best routes to individual users, including routes that involve bouncing messages off of several handheld devices en route.
When relaying messages on behalf of other users, a phone or pager will be playing the electronic equivalent of hot potato. Upon receiving a message that needs to be relayed, the device will be tasked with finding the nearest base station or another device that has an upstream connection as quickly as possible. In a worst-case scenario, the device will store and queue the message for delivery as soon as a path opens up. All of this will happen automatically, so a user might be unaware that his phone is being used to relay messages to and from people in a blacked out area.
Store and relay
What is interesting about this approach is that, in addition to making efficient use of scarce resources (base stations), it is also capable of operating in a worst-case scenario, where a large region is completely isolated by a service outage; for example, by a large hurricane. In such a scenario, hundreds of thousands of people could be cut off due to widespread damage to terrestrial facilities. There would be no network to talk to across a large area, and what facilities were still online would be swamped with traffic.
Users who needed to communicate with other people within this blackout area would be able to do so without ever touching the terrestrial network, although it might take some time for a message to reach its destination via a "scenic route." Users who needed to communicate with people outside the blackout area could also do so, as their messages would be passed along, in bucket brigade fashion, until they eventually found a point of entry to the outside world (not unlike the characters in The Matrix).
One can even envision a situation where a message literally hitches a ride with another user. Imagine that someone is driving through a disaster area. There is no network service whatsoever in the affected area. Several people have attempted to send messages out of the area. Their phones discover this passerby's phone and push copies of their queued messages onto it. A couple hours later, this traveler finds his way out of the affected area. His phone has queued copies of several dozen messages for delivery and faithfully forwards the mail now that it can talk to the network again. It's certainly not "instant messaging" by any stretch of the imagination, but it's better for the message to arrive late than never. In this extreme situation, queued messages literally walk or drive out of the affected area until they can find a point of entry to the terrestrial network.
Other advantages
This type of system is not intended to replace existing cellular and paging networks, but rather to serve as a backup during service outages and peak demand periods. This technique may be particularly useful for cellular networks, as part of a strategy to clear non-emergency voice and data traffic from the primary network when necessary, and to provide reliable text-only communication in the event of a widespread outage or emergency situation.
In addition, such a system could also prove useful outside of disasters and wartime. A decentralized text messaging network could reduce the need for terrestrial facilities, or at least be used to queue and reroute messages around isolated equipment failures (a common event, compared to large-scale outages). This approach could also be used to create location-based messaging services, since phones and pagers could infer the distance to nearby users based on signal strength, number of hops, and so on. Besides improving overall reliability, it should encourage the use of text messaging, which is becoming an important source of revenue for carriers, especially among younger users.
Most interesting, perhaps, is the minimal cost of creating such a system, compared to other telecom infrastructure projects. All of the infrastructure hardware needed to provide this capability is already in place. Creating such a distributed messaging system is primarily a matter of defining standards and writing new software based on those standards. It should not require a massive investment, such as switches, in new facilities. The biggest obstacle to creating such a system is bureaucracy, both within the carriers and in standards agencies.
In fact, something very similar to this system already exists. Cybiko has already created a peer-to-peer instant-messaging system and PDA aimed at the youth market. This device operates independently of cellular telephone networks, and uses the same communication frequencies as cordless telephones. The system allows for line-of-sight transmission, and for one-hop message forwarding. Although it does not interact with cellular networks, and does not have a Gnutella-like client discovery mechanism, the system offers a glimpse of what the future holds, since it would be fairly straightforward to meld something like Cybiko with the text/instant messaging services offered by cellular and paging service providers.
If used in conjunction with other low-cost strategies for controlling the use of the telephone network (such as forcing cellphones to block redialing attempts to non-emergency numbers, encouraging users to send voicemail or voice email in lieu of live calls), carriers can build wireless communication systems that can stand up to the worst sort of abuse from Mother Nature, enemies and their own customers. Hopefully we won't be needing this any time soon, but you never know when the next natural or man-made disaster will arise.
It's something to ponder when you live two miles from the San Andreas fault.
Brian McConnell is an inventor, author, and serial telecom entrepreneur. He has founded three telecom startups since moving to California. The most recent, Open Communication Systems, designs cutting-edge telecom applications based on open standards telephony technology.
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