Finding a Wi-Fi network to provide the correct service that your smartphone or IoT device is looking for has always been a problem. When you bring your new device into the office or home, it seems to always involve connecting to a nearby Wi-Fi network to try out the available features. As IoT devices advance and proliferate, this may become an issue for first-time adopters of IoT devices, especially when they don’t have user interfaces.
To assist with this type of issue, IEEE recently published IEEE 802.11aq—“Pre-Association Service Discovery”—the latest standard in the IEEE 802.11 family. This article will explain a little more as to what it provides.
Wi-Fi is the premier local-area-network (LAN) technology for high-speed internet access for laptops, smartphones, and tablets, built using IEEE 802.11 standards. Most of us are familiar with the new faster versions of IEEE 802.11 that use the 2.4-, 5-, and 60-GHz industrial, scientific, and medical (ISM) bands, such as IEEE 802.11ac and the current IEEE 802.11ax under development. In addition, there are also updates, such as IEEE 802.11aq providing simple Wi-Fi service discovery, together with some MAC Address privacy enhancements.
Pre-association Discovery (PAD)
Let’s start by looking at PAD, which is the primary feature of IEEE 802.11aq. It enables a protocol designed to discover services on a Wi-Fi network by an end-user device. PAD is an interworking function provided by a Wi-Fi network to allow a device, prior to association, to discover information concerning services that might be available to that device, once it is associated with the Wi-Fi network.
PAD provides methods for the device to gather information to aid in the decision to select a Wi-Fi network with which to associate. It’s important to realize that PAD doesn’t provide an actual connection to a service, before association; just the information about the service’s availability.
IoT Service Discovery
Your new IoT device requires a specific network service to control its power on and off feature. However, in your office environment, several Wi-Fi networks are available. Although the IoT device is smart, there’s no user interface to assist an initial connection. How does that IoT device find a Wi-Fi network that has the specific service needed by the IoT device? These devices don’t want to connect (associate) to every Wi-Fi network in range and then try to discover the service it requires. Indeed, without a user interface this may not be possible. The IoT device just wants to connect and connect in a manner that’s seamless.
Broadcast Service Information
The first PAD method allows the device to receive broadcast advertisements from an access point (AP) (Fig. 1). The unsolicited PAD advertisements contain information about the services supported by the wireless local-area network (WLAN) to which the AP is connected. The device needn’t send any uplink request for these advertisements.
1. Pre-association Discovery (PAD) advertisement frames are transmitted over Wi-Fi in an unsolicited manner. The Service Information Registry contains information about supported network services.
When the device receives the advertisements, it passes information about supported Wi-Fi network services to the Service Information Client (SIC) that interfaces with higher layer applications. An application on the device that requires a service will then look in the SIC to determine if any Wi-Fi networks support the service that it requires.
Request/Response Service Discovery
The next PAD method allows devices to perform a request of “what services can be reached” in a Wi-Fi network, before connecting (associating) (Fig. 2). This enables the devices, in a pre-associated state, to request solicited PAD advertisements about a specific service. The request can also contain a more detailed query to discover additional information about that specific service. This method allows for more functionality than the unsolicited PAD advertisement described earlier, but also makes it possible for the device to operate a more directed search.
2. Pre-association Discovery (PAD) supports a query/response protocol that allows a Service Information Client (operating at the device) to determine detailed information about available services in the SIR via an Access Network Query Protocol (ANQP) server.
A device creates a service request message in the SIC, containing some information about the service such as a keyword or a well-known service hash, which is then sent from the device to the access point. From here, the request is forwarded to a Service Information Registry, via an Access Network Query Protocol (ANQP) server.
The SIR creates a service response message, containing more detailed information about any available services that match those of the request. If no service is available, this is also detailed in the service response message. The SIR relays the service response message back to the AP, which is then transmitted back to the device. The request and response are allowed to transit from the AP to the SIR even though the device isn’t associated to the Wi-Fi network.
The service request and response messages can be considered as containers that enable requests based on upper layer service discovery protocols (e.g., UPnP, mDNS) to be transported between the device and AP.
Provisioning and Configuration of Services
By introducing the SIR, PAD allows the provisioning and configuration of services within the Wi-Fi network. An AP can use the SIR as a proxy server that stores the service capability of the Wi-Fi network to which the access point is connected. Available services in the Wi-Fi network can be registered within the SIR, so that devices using PAD can then query for them at some later point. Therefore, the access point can quickly respond to “Service Discovery” queries without flooding the WLAN with network service discovery requests.
For example, a smart speaker is already connected to the Wi-Fi network. It has a guest mode that allows any device to play music. A new device is brought in range and wishes to play music. Using PAD, the new device correctly discovers the Wi-Fi network that supports the smart speaker service for devices. Using the PAD methods, outlined above, it can be done in a seamless way without any user intervention.
MAC Address Randomization
By enabling and encouraging devices to transmit service discovery requests, before those devices have associated to Wi-Fi network, there’s an increased possibility that the MAC addresses of those devices can be tracked. Users with IoT devices walking through a shopping mall or a sports stadium can have the MAC address of that IoT device logged, so that the same device can be recognized in the future. This type of information can then be sold to marketers and other third parties.
To address this privacy concern, IEEE 802.11aq also focuses on pre-association privacy. If a country was to adopt a requirement to provide privacy of device addresses, IEEE 802.11aq provides a standardized way to achieve this.
When an IEEE 802.11aq device connects to, or attempts to discover services, on a Wi-Fi network pre-association, it defines the addressing of its link layer for the particular connection. One such addressing scheme is to periodically and randomly change its MAC addresses, reset counters, and seeds prior to association. The device sets its MAC address in accordance with the policy of the network it’s configured to connect.
Benefits of IEEE 802.11aq
- Enhance installation by allowing IoT devices to discover Wi-Fi networks with the services that are required.
- Increase the value of enterprise and public Wi-Fi networks to IoT home users, installers, and operators through a better experience.
- Improve the IoT ecosystem by enabling the operator to offer additional services.
- Provide enhancements to device privacy through the use of MAC address randomization.
Stephen McCann is Chair of the IEEE 802.11aq Task Group for the IEEE Standards Association.
