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Comparing low power WAN alternatives for IoT

01 Oct 2015  | Richard Quinnell

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For security the protocol uses AES-128 encryption and offers a "stealth mode" wherein end nodes can be configured to respond only to pre-approved devices.

The LoRaWAN architecture is a "star of stars" structure with gateways serving as a transparent bridge between end node devices and network servers. The wireless hop between end nodes and gateway use a proprietary chirp spread spectrum radio scheme available from Semtech and its licencees. The network structure allows three classes of end-node device. Class A (bidirectional) devices have a scheduled uplink transmission window followed by two, short downlink receive windows. Class B devices have additional, scheduled downlink windows and Class C devices have nearly continuously open receive windows. The radio scheme allows the network server to manage the data rate for each connected device via an adaptive rate algorithm to ensure optimal system performance under local radio conditions. The LoRa connections allow a trade-off between payload and range.

Security for LoRaWAN includes use of unique network, application, and device keys for encrypting data at different OSI levels.

The nWave technology is an ultra narrow band (UNB) radio technology and communications scheme that now serves as the template for the new Weightless-N standard. The nWave company offers radio modules, universal modems, and base station transceivers for developers seeking to build their own private networks, and is working with the Weightless SIG to develop similar public networks.

Weightless is a collection of three LP-WAN standards under the control of the Weightless SIG. The original Weightless-W called for use of television whitespace for the wireless link using technology originally developed by Neul. Packet size and data rates are flexible, depending on user need and link budget. Both acknowledged and unacknowledged messaging is available as is multi-cast from the access point and interrupt messaging from the end node. For security a shared secret key used by the end node and the server permit AES-128 encryption. Some private Weightless-W installations have been made, but the SIG has put public network installations on hold until international agreements on whitespace utilisation are in place.

The Weightless-N standard is based on nWave's ultra narrow band LP-WAN technology and targets low-cost applications needing only unidirectional data transmission. Weightless-N base stations can be operated by different service providers and still interoperate with devices, with each base station querying a central database to determine with which network an end node is associated. The standard was recently released and deployments have begun in London and other European cities. Weightless-N uses the same kinds of security techniques as Weightless-W.

The Weightless-P standard is under development and scheduled for release in late 2015 with hardware available in early 2016. The Weightless-P link is based on networking technology originally developed by M2 Communication and will provide fully-acknowledged bidirectional communications. Weightless P shares its MAC layer with Weightless-W, and will support fast network acquisition with hand-over of roaming end node devices across base stations.

With WiFi so popular for consumer IoT applications, it is no wonder that the IEEE is working to expand the approach to low-power wide-area networking applications. The approach being taken is to create modified PHY and MAC layers that offer support for IoT applications. The PHY layer RF link will use orthogonal frequency division multiplexing (OFDM) with either 32 or 64 tones, and is essentially a sub-GHZ variation of the IEEE 802.11ac PHY. The OFDM approach will support a variety of modulation schemes including BPSK, QPSK, and 16- to 256-QAM.

The MAC layer allows three types of stations. Traffic indication map (TIP) stations listen to access point beacons to determine when to send or receive data. Non-TIM stations negotiate with an access point to establish a transmission time allocation and can renegotiate its transmission time as needed. Unscheduled stations send poll frames to the access point to request channel access as needed. The standard is still under development, with initial release targeted for 2016.

LTE Cat. M
The 3GPP is in the midst of defining a new release for LTE cellular technology that will define a Category-M device class targeting IoT applications. It is intended as a replacement for current 2G cellular IoT system designs. Among the planned power-saving measures is extension of the end node device's sleep mode option from 2.5 seconds max to near 15 minutes, and lower data rate that current Cat-0 devices. Bidirectional communications use half-duplex operation. The standard is still in definition, however, and the details listed here are subject to change.

About the author
Rich Quinnell is an engineer, writer (threatening to become novelist) and also editor of EE Times' Industrial Control Designline and EDN's Systems Design Center. In his spare time he still review plays (part time drama critic) and still finds time to dabble in circuit design for fun.

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