NB-IoT

NB-IoT, short for Narrowband IoT, is a mobile technology for small devices that send a little data now and then. It was added to the mobile standards in 2016, alongside LTE-M. It uses a very thin slice of radio spectrum, which lets operators fit it inside or beside their 4G networks.

What it is good at

NB-IoT suits devices that stay in one place, send a few bytes or a few hundred bytes at a time, and must last for years on a battery. Water and gas meters, parking sensors and building monitors are typical examples.

Its strength is reach. The network can repeat each message many times, so a signal gets through to meters in basements and cupboards where a phone would show no bars. Devices can also sleep for long periods between reports using PSM and eDRX.

What it is not good at

• Speed and response time. NB-IoT is slow, and a reply can take seconds. It is not suited to anything interactive, to large firmware files or to voice.
• Moving devices. A device can switch masts while it is idle. If it moves during a transfer, the connection drops and the device has to connect again on the new mast. For trackers and vehicles, LTE-M is the better fit.
• Text messages. The standard allows SMS over NB-IoT, but many operators have not switched it on.

Availability and roaming

Many networks do not offer NB-IoT, and roaming on it is much rarer than roaming on LTE-M. A home operator has to agree NB-IoT roaming with each visited network specifically, and far fewer of those agreements exist. Some operators have also closed their NB-IoT networks. AT&T in the US shut its NB-IoT network in 2025, after it stopped selling NB-IoT in 2024 and moved its customers to LTE-M.

Before choosing NB-IoT, check that it is live, and that roaming on it is allowed, in every country where your devices will be. Many modules support both NB-IoT and LTE-M, which gives a fallback where one is missing. See choosing a radio technology for a side-by-side comparison.

The table below shows which radio technologies each DataSIM SIM profile supports.

The standard

NB-IoT is a 3GPP radio access technology introduced in Release 13, specified within the LTE family of standards (TS 36.300 for the overall description). It reuses LTE’s numerology and core network but has its own physical channels, a much simpler device and a carrier one LTE resource block wide, 180 kHz. Release 13 defines UE category NB1. Release 14 adds category NB2 and further features.

Deployment modes

An operator can place the 180 kHz carrier in three ways, all defined in Release 13.

• In-band. One resource block inside a normal LTE carrier.
• Guard-band. In the unused guard band at the edge of an LTE carrier.
• Standalone. In its own spectrum, for example a refarmed GSM channel.

A roaming device has to work in whichever mode the visited network uses, and GSMA recommends that every NB-IoT device can attach in all three.

Categories and data rates

Each transport block is spread over several milliseconds, and control, data and acknowledgement are sent in turn with gaps between them. With one HARQ process and half-duplex operation, NB1 peak rates are in the tens of kbit/s. NB2’s larger blocks and second HARQ process raise peaks to the low hundreds of kbit/s. These are peaks in good coverage. Deep-coverage devices use many repetitions and see a small fraction of them.

Release 14 also adds multicast to groups of devices in a cell, based on LTE’s single-cell point-to-multipoint (SC-PTM), and OTDOA positioning using a narrowband positioning reference signal. GSMA reports little commercial use of either. NB2 itself is now widely supported by modules and network equipment.

Latency

NB-IoT was designed for traffic that tolerates delay. The Release 13 study, TR 45.820, set a target of 10 seconds or less for an exception report from a device at the coverage limit. Even in good coverage, random access, scheduling gaps and half-duplex turnaround add up, so round trips of a second or more are common. Do not build an NB-IoT product that needs answers in under a second.

Coverage

NB-IoT reaches a maximum coupling loss of about 164 dB, about 20 dB more than GPRS at 144 dB. It gets there by repeating transmissions and by concentrating the device’s transmit power in a narrow uplink tone. The network defines three coverage enhancement levels, each with its own repetition settings. More repetitions mean more air time and more battery used per message, so a device deep indoors pays for its reach.

Mobility

Idle-mode cell reselection works. A device moving between cells while idle reselects and, if needed, updates its tracking area, as any LTE device does.

Connected-mode handover is not supported in Release 13. A device that moves during a connection suffers radio link failure, returns to idle, reselects a cell and connects again. Data in flight may need resending. Later releases add RRC connection re-establishment for the control plane optimisation, which lets the MME keep the session and retransmit NAS messages after the device reconnects on a new cell. That reduces the cost of moving but is still not a handover. NB-IoT is a poor fit for devices that move while transferring.

Data over NAS

Release 13 introduced the control plane CIoT EPS optimisation (TS 23.401). Small amounts of user data, IP or non-IP, travel inside NAS signalling messages to the MME, which forwards them to the gateway. No user-plane radio bearer is set up, which saves signalling and energy for short transfers. GSMA recommends that operators support IP over the control plane for NB-IoT as the minimum for roaming. User-plane data over NB-IoT is not supported by most NB-IoT operators.

SMS

SMS over NB-IoT is standardised. Messages are carried in NAS transport messages (TS 24.301) between the device and the MME, which reaches the SMS centre either through an MSC over SGs or directly over SGd. Support is the operator’s choice, and GSMA notes that many operators enable SMS on LTE-M but not on NB-IoT. A device in PSM cannot receive a mobile-terminated SMS until it wakes. See SMS for IoT and SMS API.

Power saving

NB-IoT supports PSM and eDRX. eDRX cycles reach about 2.9 hours on NB-IoT, longer than on LTE-M. The network decides the values it grants, and visited networks often grant less or none.

Availability and roaming

NB-IoT roaming needs NB-IoT roaming agreed between the home and visited operators, and the visited network to have NB-IoT live. Such agreements are far fewer than for LTE-M. GSMA’s Mobile IoT deployment map shows where NB-IoT has launched, but a launch does not mean roaming is open to a given home operator.

Networks also close. AT&T shut down its NB-IoT network in early 2025 (planned for end of March 2025), and moved its customers to LTE-M. NTT Docomo closed its NB-IoT service on 31 March 2020. Design for that risk with modules that support both NB-IoT and LTE-M and firmware that can use either.

Practical points

• Use NB-IoT for static, low-data, delay-tolerant devices that need deep indoor reach and long battery life.
• Keep payloads small and expect seconds of latency.
• Confirm in each country that NB-IoT is live, roaming on it is allowed, and whether SMS, PSM and eDRX are supported.
• Prefer dual-mode NB-IoT and LTE-M modules, and see choosing a radio technology for the comparison.