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What is Cat M?LTE Cat M (also known as LTE-M) is a low-power wide-area (LPWA) technology designed to support the “massive Internet of Things” through cellular technology, which encompasses billions (!) of IoT devices. Cat M primarily refers to Cat M1, as the adoption of Cat M2 will take a few more years.LTE radio technology uses “categories” to differentiate the capabilities of each device connected to the LTE network. For example, Cat 1 refers to devices that can support download speeds of up to 10 Mbps, while Cat 4 refers to devices that can use carrier aggregation and support download speeds of up to 150 Mbps. Cat M1 refers to a class of devices that operate on a 1.4 MHz narrow channel, with observed download speeds in the range of 589 Kbps and upload speeds of 1.1 Mbps (3GPP Release 14). Older Cat M modules have lower speeds (300 Kbps downlink / 375 Kbps uplink).The Main Advantages of Cat MCat M offers significant benefits for both mobile network operators (MNOs) and IoT builders.The main advantage for MNOs is that Cat M devices operate in a smaller frequency band (1.4 MHz). This allows them to pack more devices into the available spectrum at each cellular site. A typical LTE cellular site can support ten times more active Cat M devices compared to active conventional LTE devices.The primary advantage for IoT manufacturers is that Cat M modems use less power, enabling power-constrained devices to operate for months or years on small batteries with advanced energy-saving features. Cat M modems also have faster startup speeds during cold starts compared to Cat 1 modems, reducing connection times. Another key advantage is that Cat M modems often cost less than conventional LTE modems, helping to lower hardware costs.Cat M Energy-Saving FeaturesCat M provides several power-saving features that allow IoT devices to operate in low-power modes and extend battery life.

Power Saving Mode (PSM)

PSM functionality allows IoT devices to remain in a long sleep state without being woken up by the network. Typical cellular devices actively switch between two modes – IDLE and ACTIVE. When a device is not sending/receiving traffic, it enters an idle state, which positively impacts battery life. Assuming there are IP packets to be delivered to the device and the device’s network page, the device must respond to the paging and switch to ACTIVE mode to receive traffic. This can impact power-constrained IoT devices. PSM allows these IoT devices to negotiate with the network to extend sleep times (for hours or days) and avoid being paged during that sleep period. If any traffic arrives at the device during sleep, it will be buffered in the network (at least the last 100 bytes) and delivered when the device becomes ACTIVE.

Extended Discontinuous Reception (eDRX)

eDRX is a Cat M feature that allows devices to remain in a long sleep state and wake up at fixed intervals (also known as eDRX cycles) to send and receive traffic. Devices enabled with eDRX need to negotiate the eDRX cycle with the network to avoid being paged during these sleep periods. eDRX allows devices to use a paging cycle of, for example, 10.24 seconds, instead of the 1.28 seconds used by conventional LTE devices.PSM and eDRX are complementary and can both be used by Cat M devices. eDRX helps devices extend sleep times, waking up at fixed intervals, and generally reduces the “chatter” between the device and the network. PSM helps devices extend sleep times for hours or days.Depending on the use case, you may only need to use eDRX, PSM, or both. Note that not all Cat M networks support eDRX and PSM, but most networks today do. We expect the situation to improve continuously as more networks enable these two key features to support power-constrained devices.Constraints of Cat M NetworksWhile Cat M offers tremendous opportunities for power-constrained IoT devices, there are also some drawbacks.

1. Cat M provides low bandwidth. Although Cat M supports peak upload speeds of around 1 Mbps, our tests show that typical speeds are about 300 Kbps.

2. Cat M support is not yet widely available worldwide. Many countries have not yet launched Cat M networks. For the foreseeable future, M class coverage is a subset of conventional LTE coverage.

3. In congested situations, devices using Cat M may be pushed off the network (preempted). When a cellular site is congested, the network may force Cat M devices that have been connected for a long time to disconnect and reconnect later.

Cat M Use CasesIt should now be clear that Cat M is best suited for power-constrained IoT devices that exchange small amounts of traffic with application clouds. Use cases include asset tracking (e.g., waste management), smart metering (typically any sensors that regularly report temperature, air quality, etc.), and smart irrigation.In general, any device that operates on small batteries (which cannot be easily replaced or recharged) can benefit significantly from using Cat M.LTE Cat 1 vs. Cat MIf your IoT use case involves maintaining a “always-on” connection between your device and application cloud, then an LTE Cat 1 (or Cat 3/4) modem is more suitable for your device. In “always-on” mode, your device does not enter sleep mode, and you do not take advantage of any power-saving features offered by Cat M, but you also face low bandwidth connections and the risk of being forced to disconnect when the local cellular site you are connected to is congested.Conventional LTE coverage is far superior to Cat M coverage. Some countries do not offer Cat M, and even when available, not all mobile network operators in that country support Cat M. Therefore, when you use Cat M for IoT devices, it automatically translates to fewer options related to the available connection providers.If your device frequently transmits large amounts of data to/from the network, a conventional LTE modem may be a better choice than a Cat M modem. The low bandwidth provided by Cat M connections means that devices must remain online longer to transmit large files.If you are using PPP (Point-to-Point Protocol), some of the low-power modes of Cat M are not supported. When using PPP, these modems prevent entering low-power modes.

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