SMS Relay -- An Idea for Fault-Tolerant Communications

by Brian McConnell

This article examines the task of creating a wireless communication system that can survive a catastrophic failure, and still provide basic communication services to its users. Specifically, the author suggests that the mobile carriers borrow a page from Gnutella and enable mobile phones to relay their short text messages to one another until they reach land lines.

While wireless communication systems appear to be inherently resistant to damage in natural or man-made disasters, they are not virtual systems that reside entirely in the ether. Like landline networks, they depend on physical facilities: short- and long-haul interconnections to the rest of the telephone network, antennas, and switching facilities. Although the last mile may be a wireless connection, much of the time calls are traveling through terrestrial facilities that can be damaged or knocked offline.

The recent events in New York and Washington, D.C. exposed two key weaknesses in wireless communication systems. First, unlike two-way radios, cellular telephones depend on centralized facilities to function. Knock those facilities offline or swamp them with high volumes of call traffic, and the handsets become little more than battery-operated paperweights. Even if the base stations are functioning, the network can be overwhelmed when a large number of users try to use the system simultaneously. Most telephone networks are designed with the assumption that only a fraction of the total number of users will attempt to place calls at any given moment. This assumption fails in an emergency situation such as a natural disaster or the September 11 attacks.

Second -- and this factor is unique to the United States -- there is no uniform standard for digital cellular networks. The U.S. cellular telephone system is a balkanized patchwork of several different networks, each using different and incompatible technologies. As a result, a handset designed to work on one carrier's network can only talk to that type of network. In other regions of the world, carriers commonly use the GSM standard, which enables phones to automatically roam across different carrier networks, even within the same city. Because of the balkanized nature of the U.S. system, a bad situation was made even worse because carriers could not pick up the slack for each other. If the government had not abdicated its responsibility and had forced carriers to use a common standard (just as it has done for TV and radio broadcasters), subscribers could have automatically placed calls through alternate carriers if their primary carrier was knocked offline.

This wasn't a problem when cellular telephones were viewed as a luxury item or an expensive business tool. The loss of service, while inconvenient, was not viewed as a critical event. But in recent years, cellular telephones have become an essential tool, and in emergency situations, a vital lifeline. Cellular dialtone is no longer a luxury; it is just as important as other essential utilities. This trend will only accelerate in the wake of recent events.

Every phone a relay point

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Fortunately, the cellular carriers and handset manufacturers already have the tools at hand to create a nearly indestructible wireless communication network. This network can be overlaid onto existing systems, and does not require the use of new technologies, beyond a software upgrade.

Nearly every digital cellular phone sold today is a computer. It may look like a phone, but it is really a computer designed for making and receiving voice calls and text messages. By making minor hardware changes and updating the software used in cellular phones, vendors could produce phones that operate in a limited capacity even when the cellular network is rendered inoperable.

The key to doing this is to redesign the way cellular phones handle text messaging (also known as SMS or short messaging service). This service enables phone users to send and receive short text messages via their phones. It's not as good as a live phone call, but it's better than nothing, essentially a cross between instant messaging and email. In the current system, the phone needs to be able to communicate with a base station to send and receive text messages, just as it does with a voice call.

"Every phone or pager could act as a relay point for forwarding short text messages. This would enable a user who is out of range from a functioning base station to bounce his or her message off of one or more other telephones that act as intermediate relay agents."

If you alter the phones so that each handset can itself relay text messages on behalf of other phones, you can build a text-messaging network that can continue operating even in the face of a severe and widespread outage. The trick is to apply techniques used in peer-to-peer computing (and in Gnutella in particular) to create a parallel system that can relay text messages without central coordination. In this system, every phone or pager could act as a relay point for forwarding short text messages. This would enable a user who is out of range from a functioning base station to bounce his or her message off of one or more other telephones that act as intermediate relay agents. This system can also be implemented in a way that is automatic and invisible to users.

While this technique is not appropriate for voice phone calls (the quality of service would be too unpredictable), it could work quite well for text messages. Text messages do not need to be delivered instantly. A delay of several seconds, while intolerable for voice, is barely perceptible for a text message. It is much easier to build a store-and-forward system than it is to support real-time full duplex voice communication.

Text messages also require much less bandwidth than a phone call, so even a severely-compromised network can carry a large volume of text messages. A text message requires one kilobyte, or less, on average, whereas a compressed digital phone call requires about 10 Kbps upstream and downstream. A one-minute phone call therefore requires 120 Kbytes of bandwidth, compared to a single kilobyte or less for a text message.

The goal in creating such a system is to build a network that can fail gracefully, and to clear the airwaves for emergency phone calls by providing a reliable way to send non-urgent messages during widespread outages and periods of high system demand.

Peer-to-peer messaging

This basic scheme has already been widely used in PC file sharing applications, as well as PC-based instant messaging services. One of the best examples to date is the Gnutella file-sharing system. Many of the techniques used in this system can be directly applied to the task of creating an ultra-reliable text-messaging system for cellular phones and pagers.

Gnutella is a completely decentralized, or peer-to-peer, file-sharing system. Unlike Napster, there is no centralized server that acts as a broker in processing search requests, matching users with each other. Gnutella clients automatically seek out other Gnutella clients elsewhere on the Internet. Each time a Gnutella client discovers another client, it asks that computer for a list of the other clients it has already discovered. It then repeats this process for each of the computers in this list, and does so ad infinitum. Within a matter of minutes, the program typically discovers thousands of other computers that are running the Gnutella system, and can communicate with any one of them.

"Gnutella's designers were less concerned with creating a network that could withstand a physical attack than they were with creating a system that could withstand legal attacks from censors and attorneys."

This design makes the system very reliable and difficult to disable. Gnutella's designers were less concerned with creating a network that could withstand a physical attack than they were with creating a system that could withstand legal attacks from censors and attorneys. The decentralized nature of the system makes it impossible to shut the system down by disabling a single node or small group of nodes.

When applied to wireless text messaging, this technique can be used to build a network that automatically routes around outages by enabling messages to bypass terrestrial facilities in the affected areas. There are three keys to building a system like this.

  • The first is to enable cell phones and pagers to exchange data with each other directly, without using a base station.
  • The second is to program the devices to automatically discover nearby devices that are capable of relaying messages.
  • The third is to assign a small band of unlicensed communication spectrum for peer-to-peer text messaging, available for all mobile communication devices to use. This last point is important; there needs to be a uniform standard for the transmittal of short text messages that is supported by all carriers.

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