Chapter 4. New Usage Models

All of the books in the world contain no more information than is broadcast as video in a single large American city in a single year. Not all bits have equal value.

—Carl Sagan

Bluetooth low energy enables a new way of using wireless technology. The main new models are based around the advertising model and include presence detection, broadcasting of data, and connectionless models. They also include gateways from devices to the Internet.

4.1. Presence Detection

The most interesting new wireless model enabled by Bluetooth low energy is presence. Presence means the state or fact of existing, occurring, or being present in a place or thing. Using the advertising model, devices can passively scan in the background for other devices that are broadcasting. The devices that are advertising might be just advertising their address, or they could be advertising some presence-based data.

Advertising is a new mode of operation defined in the Link Layer. With it, devices can periodically transmit their identity and a small amount of information. This model is possible because the modulation index of the radio has been increased and the 2.4GHz band regulations permit wider radio signals to be sent using a non-frequency-hopping radio. Because the radio doesn’t employ frequency hopping, fewer channels are needed for devices to be connectable and discoverable. This means that it is much more efficient to advertise and scan.

Scanning is possible in two modes: active and passive. Active scanning requires the scanner to request more information from advertisers, to obtain additional static data. Passive scanning just requires the scanner to listen for advertising packets. Once an advertising packet is received by the Link Layer, it can be sent to the host.

The host can use the information about what devices are nearby to determine where it is. For example, if the host discovers a car, then the host can determine that it is in or near a car and change its behavior accordingly, perhaps by connecting to the car. Similar use cases around the home, in the office, or at a café are also possible. It is this automatic determination in the background using passive scanning that allows a device to automatically change its behavior based on where it is.

Presence, as just described, is about a mobile device determining where it is. Another type of presence is about static devices being able to determine what devices are in a given location. Probably the most useful benefit of this would be to find somebody or something in a large office building, for example. The devices or people that want to be tracked would advertise infrequently, and devices in each room would monitor which devices they can detect. This information can then be communicated to a central device to determine location. You can use this to automatically route phone calls to the nearest phone or to track employees during an emergency evacuation of a building.

4.2. Broadcasting Data

The advertising model also allows a small quantity of data to be broadcast—a very small amount, just a few tens of octets of data—but the ability to broadcast this small bit of information to any device that is listening in the area is incredibly valuable. As stated earlier, the ability to determine where a device is, based on what devices are broadcasting, is a useful function in its own right. However, this relies on having some way to map the device that is broadcasting to a physical location. Data broadcasting helps with this mapping.

You can use broadcasting to transmit many different types of useful data. There are three main areas for which broadcasting data can help the user experience: initial connection setup, advertising, and broadcasting information.

To help with an initial connection setup, devices can broadcast data about what type of device they are and that they want to connect to a device with a complementary set of services or profiles. For example, when you remove a television from its box and switch it on for the first time, it starts to search for a remote control. When you install the batteries in the remote control, it starts to advertise that it is looking for the television.

The television receives this broadcast data, connects to the remote control, automatically pairs with it, and then allows the remote control to talk to it securely. This means that, from the consumer’s perspective, they turn on the television for the first time, put the batteries in the remote control, and then press buttons on the remote to control the television; no connect buttons and no pairing menus.

Advertising is a useful tool for many organizations. With it, consumers have the ability to discover real world-services from over 100 meters away. An obvious place where advertising using a free wireless technology is useful is at international airports and railway stations. The ability to advertise gate or track details for flights or trains gives travellers who don’t want to spend lots of money on roaming charges or Wi-Fi Internet access an alternative way to gather information. Simple bus stops can also advertise when the next bus will arrive and where the bus is heading.

It is also possible to broadcast information that is gathered locally by a device. For example, a temperature sensor could broadcast the temperature to any device that is currently listening for temperature information. This is most useful when information is being sensed that changes rapidly and the information is useful for multiple devices.

4.3. Connectionless Model

One of the biggest changes from Bluetooth classic to Bluetooth low energy is in the way that a connectionless model has been designed and implemented. In a connectionless model, devices do not need to maintain a connection for useful information to be exchanged quickly between them. Because the main protocols never establish a connection-oriented channel between devices, there is no cost to dropping and then reconnecting a connection when data needs to be sent. This encourages devices to only establish a connection when they need to send data, and not to maintain an expensive connection just in case some data does need to be sent. This connectionless model does impose some interesting design changes from standard wireless protocols.

In a connection-oriented channel, the state information can be established over a period of time by using the protocol. The state information, therefore, typically is not available whenever it is required, but only by remembering the state that has been implicitly created by both devices. This state information requires a long time to be established, causing delays upon the initial connection while the state information is discovered and negotiated. Protocols that are based on implicit state typically have negotiation and configuration procedures as well as feature bits and version numbers. If a connection is going to be up for a long time, and there is a lot of state information, that state-full system can be more efficient.

Unfortunately, many protocols are not fully defined, with each bit of state implicitly defined as opposed to being explicitly defined. This leads to interoperability problems because each device thinks that the connection has a different state and therefore makes different assumptions about what can or should happen next. This is one of the biggest problems with connection-oriented systems. This can be solved by defining the state explicitly and also how any state machines work. A good example of this would be the Logical Link Control and Adaptation Protocol (L2CAP) Layer in Bluetooth where for Bluetooth classic, a simple state machine and configuration system are used when establishing a connection. All the state of the connection is explicitly defined, and the connection state machine is fully described. This, however, has taken over 10 years to develop to the exemplary level it now occupies.

Thus, the connectionless model solves these problems by not defining the state of a connection, but the state of the device. By exposing state through a stateless protocol, such as Attribute Protocol, it is possible to disconnect at any time and, upon reconnection, determine what the current state is directly from the other device. It is also possible to explicitly define state machines with both an exposed state and an exposed control point to persuade that state machine into different states as defined by some service. It is also possible to reestablish a connection just because some information in this state has changed and a device has registered to receive this state change information.

For example, it can be used to signal the battery level of a device. A monitoring device would connect to the battery-powered device, read the current battery level, configure the battery level to be notified when it changes, and then disconnect. When the battery level does change, the battery-powered device slowly makes itself connectable and the battery-monitoring device notices that it has something to say. The battery-powered device then connects to the battery-monitoring device. Immediately after establishing a connection, the battery-powered device can notify the monitor of the new battery level and then immediately terminate the link. This can all happen within about 3 milliseconds. For a device that was fully charged and has a battery that lasts for just one year, this would require approximately 99 reports of 3 milliseconds each, a total radio-active time of just fewer than 300 milliseconds. In contrast, just to set up a connection-oriented channel in Bluetooth classic can take a similar time, for each report.

4.4. Gateways

The most radical change in computing technology over the last few years has been the spread and pervasiveness of the Internet. It appears that everything is connected to it, from newspapers to televisions and radios. All of these are portals for media, whether it is printed words, moving pictures, or voice. Of course, there are many other uses for the Internet, including communications such as e-mail and social networking, and teleconferencing audio and video links. However, the next big challenge will be to connect hundreds more devices for each device that’s connected to the Internet today. This is a big change and the current infrastructure will probably not have the capacity to cope with all this new data initially.

The problem with the Internet is that it is built around a connection-oriented model. A TCP1 connection is a session-oriented channel that is established between two devices, which takes time to set up. The fact that devices connected to the Internet have to have an address is also session based. To obtain an address, a device must either have this address programmed into it or it has to ask another device to allocate it an address for a period of time; it can use this address for this period of time, after which it must ask for another address.

The biggest problem with the Internet as it is currently structured is that it is designed around a wired infrastructure. Wires are great; they are mostly reliable, and because the devices connected with wires aren’t moving around, they can also be connected to other wires supplying electricity. This means that energy efficiency for wired protocols is rarely considered. The fact that routers constantly check for the mapping of allocated or nonallocated Internet addresses to devices, at stochastic intervals, typically means that Internet devices need to be listening all the time. This doesn’t work when devices are constantly moving around and need ultra-low power consumption. Another approach has to be used.

The model followed in Bluetooth low energy is one that is used in most homes that have more than one computer connected to the Internet. This is the concept of a gateway using network address translation (NAT). To the outside, your typical home has a single Internet address, allocated to the gateway or router. The gateway, however, allocates a separate set of addresses for all the devices in the home that are attached to it. The key is that the gateway translates the internal addresses to the single external address, hiding the topology of the internal network from the outside world. The outside world just sees one device, and it doesn’t really care about which device is really sending or receiving the data.

The Internet Protocol itself is very expensive. For an IPv6-based network, a 128-bit source and destination address has to be included in every single packet that is transmitted. This means that the minimum size of a packet, before any other protocol overhead, is 32 bytes. This is larger than the biggest Bluetooth low energy packet. Therefore, it becomes very difficult to just use the Internet Protocols directly over low energy—even if we were to discount the fact that they were designed when everything was wired. The gateway model, however, allows us to hide the internal addressing of devices from the outside world. This internal addressing of devices could be using a separate IPv6 address space or could be using some other addressing scheme that is transparent to the outside world.

By using Bluetooth low energy gateways, tiny wireless devices not only can connect to the Internet, but they can do so using the least possible amount of power. Bluetooth low energy does this by pushing the complexity of the Internet to the gateway devices that have the resources to cope. It also allows these gateway devices to map Internet addresses to devices using any scheme they want. This could be done by allocating an individual IPv6 address to each device or by using a port number of a single Internet address to each device.

Gateways are useful because if your refrigerator needs to notify its manufacturer that the compressor pump is failing and needs to be replaced under warranty, it must have a way to get this message out to the manufacturer’s Internet server. Obviously, similar things will need to be done for all manufactured devices: washing machines, cars, and vacuum cleaners, to name just a few. The manufacturer might also want to send information to these devices; for example, they might want to upgrade washing programs. Thus, gateways provide the way for devices to interact with the Internet without being burdened by the power-hungry wired protocols that drive the Internet.

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