Cellular communications technology never sits still — there is always more to learn. Additionally, there are decades of existing technology underpinning today’s updates — and tomorrow’s.
To help you make sense of the changing landscape of cellular IoT, we’ve produced this glossary. Unsure of the meaning of a term? Then click its initial from the list below and you’ll be on you way to gaining a better understanding of the technologies, systems, processes and jargon behind this dynamic industry.
And if you can’t find assistance with an item here — maybe we haven’t added it yet — contact our support team and we’ll post an update.
The term is also used for the actual code an end-user will enter to enable this binding. The code can be presented as a key-in text string, or encoded as a scannable QR code.
This value is recorded on a mobile device and used to locate the correct Packet-data network Gateway (PGW) — the device’s Internet entry point.
The APN comprises a carrier-specific label — often also referred to as the APN as most end-users only ever see this part — and a domain name on the global cellular IP backbone.
For Super SIM, the label is
super and this is appended to a domain name of the form
and which is provided when the device attaches by the Home Subscriber Service (HSS) to which the basestation is linked. The values
<yyy> in the domain name are the SIM’s actual Mobile Country Code (MCC) and Mobile Network Code (MNC) values. They are derived from the SIM’s IMSI — or current IMSI, in the case of Super SIM which contains multiple IMSIs.
A basestation’s Mobility Management Entity (MME) unit uses the full APN to do a DNS lookup that will yield the numerical IP address of the PGW on the IP backbone. The MME passes this address to the basestation’s Serving Gateway (SGW), which uses it to contact the PGW and negotiate the establishment of an IP tunnel through which the device will access the Internet.
An IoT-centric version of the existing LTE Category 1 cellular Low-Power Wide Area Network (LPWAN) specification. It offers the same data rates as Cat 1 (10Mbps downlink, 10Mbps uplink) but via a single receive and transmit antenna rather than two at minimum. This reduces device complexity and therefore cost. Cat M1 also uses a single antenna, but offers much lower data rates — but better power utilization.
LTE Cat M1 — the ‘Cat’ is short for ‘Category’ — is a cellular Low-Power Wide Area Network (LPWAN) specification developed for IoT projects. It was designed for applications that transfer low to medium amounts of data, to do so at a long range from the cell basestation to which the IoT device is attached, and to consume minimal amounts of power — handy for battery powered units.
Typically, Cat M1 runs on a 1.4MHz spectrum with a transmit power of -20Bm, with data throughput in the between 200-400Kbps range. It operates with a single antenna.
Super SIM supports Cat M1.
Sometimes also called a “Decor”, a DCN is a component of a network topology that an operator may choose to deploy in order to better support very different subscriber use cases. Rather than build out a single, cross-function mobile core, the operator uses multiple parallel cores — the DCNs — that are each configured with a specific use case or subscriber type in mind. When devices connect, their traffic is routed through the most appropriate DCN. The goal is to deliver the best performance for that use case without penalizing other uses cases, who are routed to DCNs more suitable to their characteristics.
It’s a choice operators have to make: better server a selection of very specific uses cases through a DCN-based network, or build a generic network that is flexible and can deliver good performance to all users, whatever their use case.
This is the “authentication, authorization, and accounting” (AAA) protocol used by LTE networks, and binds together the data flows between mobile core and basestation components. For example, a basestation Mobility Management Entity (MME) talks to the core’s Home Subscriber Service (HSS), which in turn communicates with the core’s Authentication Server Function (AUSF), Policy and Charging Rules Function (PCRF), and Online Charging System (OCS) to determine how the user of the SIM in a connecting device should be billed for the services they use — and which services they are permitted to use.
Diameter messages comprise sequences of “attribute-value pairs” (AVPs), each of which has a code defined by the standard, an optional vendor ID, some data, and a flag byte that provides message metadata. Standard codes include values for, for example, location information, quality of service settings, network usage limitations, and authentication data. By adding a vendor ID, carriers can extend the basic set of attributes with their own.
This standard cellular communications power-saving technique allows application developers to specify a period during which a device will stay in a low-power sleep state before waking to see if it has data pending. The device can listen for pending data indications without having to establish a full network connection, which uses less power than a full network connection does, and the time required to check for pending data is much shorter (just 1ms) than the time it takes to establish a full network connection.
For guidance on making use of eDRX in your application, see Low-power Optimization for Cellular Modules.
This is the formal LTE term for a basestation, often abbreviated further to “eNB”.
According to the standard, eNodeBs must maintain separate radio access technologies for the uplink — the path from the mobile device to the basestation and through backbone radio links connecting eNodeBs to the core — and the downlink — the path in the opposite direction.
Communication between eNodeB and the mobile device employs techniques called Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) for the uplink. Both are ways of transmitting more digital data through the analog radio link by maintaining many parallel data streams.
Traditional SIMs, whether they are cards or embedded chips, contain connectivity settings for a single connectivity provider. These settings are together called a ‘profile’. The provider may be a mobile network operator such as AT&T, or a company like Twilio that leverages networks from all over the world. The SIM’s profile is baked in at manufacture and can’t be changed.
eSIMs — aka eUICC SIMs — are not bound to specific profiles, and the profiles they contain are not fixed in place. On the contrary: eSIMs, unlike traditional SIMs, can contain multiple profiles, can switch between them, and downloaded further profiles over the air. However many profiles an eSIM contains, only one can be active at any given time.
You can think of each eSIM profile as a virtual SIM card. Switching from one profile to another is the digital equivalent of swapping SIM cards.
An eSIM is implemented in a physical device called an Embedded Universal Integrated Circuit Card (eUICC). For this reason, the term ‘eUICC’ is often used interchangeably with ‘eSIM’. Here we use the latter form.
To find out more how Super SIM supports eSIM use cases, see Get Started with Super SIM eSIM Profiles for eUICCs.
A Super SIM API object which represents an eSIM profile. Creating an eSimProfile resource starts the process of reserving the represented eSIM profile on a Subscription Manager Data Preparation Platform (SM-DP+) server. Once the profile is ready for use, the eSimProfile’s
status is set to
available and a Sim resource is created to allow you to manage the eSIM.
A Super SIM API object which defines common settings that will be applied to any Super SIMs assigned to it using the Sim resources that represent them. This includes the list of global networks they can connect to — via the Fleet’s Network Access Profile (NAP) — and functional settings, including monthly data transfer capacity limits, and whether SMS Commands and/or IP Commands are enabled.
For more details, see the Fleet resource reference.
The GUTI is used to authorize device re-attachments to the same cell basestation’s Mobility Management Entity (MME) in place of the SIM’s IMSI. This is for privacy and security reasons: using the GUTI minimizes the number of times the IMSI has to be sent over the air, which makes it harder for the device to be identified by its IMSI and therefore more difficult for it to be tracked by a third-party snooping the radio traffic. It also speeds the device’s attachment the next time the device tries to connect.
The GUTI is a value comprising:
- Globally Unique MME Identifier (GUMMEI), formed from:
- An MME Temporary Subscriber Identity (M-TMSI).
The GUTI is long so a shorter form is used to identify the UE in situations where the MME Group ID won’t have changed, such as service requests and device pages. The shorter form is called S-TMSI and it comprises the MMEC and the M-TMSI so that it’s unique only within MMEs of the same ID, ie. the same carrier.
Also known more simply as ‘Home Network’, this is the network owned by the carrier which issued a device’s SIM. All other networks to which the SIM may be able to connect to — for example, when it is roaming in another country, or in the home country in a region without coverage by the prime carrier — are Visited Networks, or VPLMNs (Visited Public Land Mobile Network).
Super SIM has no home network, so any network it can connect to is by definition a visited network.
This is the mobile core-hosted service that manages users’ accounts and assigns their access permissions.
If your device is in another country and has roaming enabled, for example, the HSS instructs the visited network’s Mobility Management Entity (MME) to let your device attach.
This is a unique 15-digit code that identifies a device to cellular networks. The first 14 digits are defined by GSM Association. The last digit is a checksum generated by the Luhn algorithm.
You can view the IMEI on any phone by entering the sequence *#06# on its keypad.
This is a unique 14- or 15-digit code that identifies a single user to cellular networks. When a device connects to a cellular network, that network uses the IMSI, which is provided by the device’s SIM, to check with the SIM’s home network to query what privileges it should allow the device: whether it can use data, whether it can use SMS — and even whether it’s allowed to attach to that network at all.
The IMSI comprises two numerical components:
- A Public Land Mobile Network (PLMN) ID which indicates the SIM’s home network.
- A nine-digit Mobile Subscription Identification Number (MSIN) which identifies the SIM to its home network.
The PLMN ID itself comprises two parts:
- A three-digit ID Mobile Country Code (MCC) — the SIM’s home territory.
- A two- or three-digit Mobile Network Code (MNC) — the network the SIM belongs to.
The IMSI’s PLMN ID tells the cell basestation’s Mobility Management Entity (MME) which Home Subscriber Service (HSS) it needs to contact to determine what network resources the SIM is permitted to use. Specifically, the MME has a database of MCCs and MNCs in which it looks up the IP Backbone address of the target HSS, or the gateway the target network uses to route communications to a particular HSS.
To learn how Super SIM makes use of a number of IMSIs, review Super SIM’s Multi-IMSI Applet.
Often also known as the IP Backbone, this is a global, private Internet that exists to connect different carriers’ own IP networks. It allows phones to call customers of other carriers, to reach landlines, and to break out to the Internet.
This is a so-called “integrated SIM”, an eSIM built into a device‘s cellular module or into its microcontroller. Today, eSIMs are separate components and require work to be done by the designer to add them to a product. They require power lines, data connections to the cellular module, and room on the circuit board. Integration into the module or microcontroller silicon will deliver a big reduction in host device complexity, and reduces the device’s power overhead too. However, many of their mooted security benefits arise simply because they are also eSIMs.
Module or microcontroller? It depends on who you talk to. Whichever of these products a manufacturer makes, that’s the place where they insist iSIMs should be integrated. As yet there are no standards, only rival manufacturers attempting to stake their claims on an as-yet-unrealised market. What can be said is that miniaturisation will undoubtedly lead to SoCs contain all three components — eSIM, cellular module, and microcontroller — in the not-too-distant future.
This is on-device software which manages eSIM profiles stored on a device’s eUICC and provides an interface through which the user can swap the current profile for another to switch between cellular connectivity providers.
The LPA may be part of a device’s operating system or a separate application, or it may be built into the eUICC itself. Many recent Android and iOS devices that are eSIM compatible integrate an LPA. IoT devices’ modems may or may not have one built into their eUICC.
An umbrella term for wireless technologies that have been designed for or are inherently suited to IoT applications, which generally don’t require the speed and bandwidth demanded by consumer cellular devices, such as phones and tablets. The GSM Association has defined a set of standards which meet the needs embodied in the LPWAN concept, among them LTE Cat-M1 and NB-IoT.
Super SIM supports the LPWAN technology Cat-M1, plus the more general Wireless Wide Area Network (WWAN) technology LTE.
LTE is the current series of global cellular communications technologies maintained by the 3GPP, an organization which manages cellular standards. LTE incorporates 4G and 5G cellular technologies whose components are organized into speed/use case categories, such as Cat M1.
|LTE Category||Max Download Speed
|Max Upload Speed
LTE categories are not merely speed bands, but are enabled by technologies introduced over time by the 3GPP. These technologies arrive in 3GPP ‘releases’:
|3GPP Release||Categories Enabled|
|12||Cat 13-16, Cat 0|
|13||Cat M1, Cat NB1 (aka ‘NB-IoT’)|
This is the component of 4G/LTE cell basestations with which mobile devices communicate to request access to a carrier’s cellular network. This takes place when the device tries to attach to the cell.
A SIM’s IMSI contains a home network identifier, the PLMN ID, which tells the MME which Home Subscriber Service (HSS) it needs to contact to determine which network resources the SIM is permitted to use. If the SIM connects to the same basestation again, it identifies itself with a Globally Unique Temporary Identifier (GUTI) for security reasons.
In the case of Super SIM, the MME is directed to one of a number of Twilio partners called “sponsors” who route the MME’s requests to Twilio’s HSS. It’s the sponsors who provide the PLMN ID in Twilio’s IMSIs.
All major carriers, such as AT&T, Deutsche Telecom, Vodafone, and Telefónica, maintain networks of cell basestations. Each basestation is connected over its carrier’s own IP network to its mobile core. It’s the part of the network which doesn’t directly involve itself in supporting device connection technologies. The core is where all the real smarts of the cellular network are located: it comprises all the systems which manage SIM access authorization and traffic routing.
Some carriers do not have their own mobile cores. Instead they rent access to these resources — they are the so-called MVNOs. Twilio isn’t an MVNO. It doesn’t have any basestations — instead it leverages all of those other networks’ cell towers. But it does have its own mobile core, connected to the global IP Backbone.
MCCs are standard three-digit country identifiers used by cellular networks to indicate the region that a mobile subscriber calls home.
You can get a full listing of MCCs from this website.
MNCs are standard 2-3 digit carrier identifiers used by cellular networks to indicate the network to which a mobile subscriber has signed up with.
You can get a full listing of MNCs from this website.
This is a cellular network provider which has no cellular network or network management technology — the mobile core — of its own but instead rents capacity from other carriers’ infrastructure.
Twilio does not maintain its own network of cells and basestations, but neither is it an MVNO in that it operates is own mobile core.
NB-IoT — short for ‘Narrownand-IoT’ and more formally known as LTE Cat-NB1 — is a cellular Low-Power Wide Area Network (LPWAN) specification developed for IoT projects. It was devised to support the efficient two-way transmission of small amounts of data.
Super SIM does not support NB-IoT.
For more details, see the NAP resource reference.
The state in which any SIM is said to have entered if it has been roaming on a visited network for a period that depends on the network and the telecommunications regulator in the network’s territory. The period is usually between one and three months. A small number of countries impose limits on how often a SIM may permanently roam in their jurisdictions. Some even prohibit it entirely.
Super SIM is a product that always roams, so you should be aware of any roaming limits set by the countries in which your Super SIM-connected devices will be used. To find out more about permanent roaming, which countries impose restrictions upon it, and how Super SIM deals with this, see Prepare for Production Deployments with Super SIM.
This is the point at which the mobile core touches the public Internet. When an attached device wants to connect to the Internet, a data tunnel is set up and maintained between the cell basestation’s Serving Gateay (SGW) and the core’s PGW using the GPRS Transport Protocol (GTP), which is used even when the network is 3G or above. When the tunnel is in place, the device’s data flows out through the PGW.
The Twilio Mobile Core currently has a primary Internet point of presence in the US. However, Twilio is adding further PGWs around the globe so that devices, using their stored Access Point Names (APNs), can talk to the most geographically proximate PGW and so minimize the number of hops over which packets need to be routed as they travel between device and server.
This is a standard power-saving technique a modem can request that the basestation it is connected to will enable. Both then agree times at which the mobile device can sleep and the basestation will not page it to check that it is still present. It it needs to, the device can awake at any time within the agreed time and send data without having to reattach to the network.
For guidance on making use of PSM in your application, see Low-power Optimization for Cellular Modules.
A formal term for any network operator, aka a carrier.
This term covers those elements of an operator’s network that deal with the maintenance of radio communications and connecting individual devices to to the network. The architecture of a given RAN depends very much on the RATs (Radio Access Technologies) that the network supports. A basestation may contain infrastructure to support multiple RATs (Radio Access Technologies) by hosting a number of RANs side by side.
This is a catch-all term for a cellular technology standard that might be supported by a mobile device and/or a mobile core through its basestations. Examples include GSM (2G) GPRS (2.5H), UMTS (3G), and LTE (4G).
Short for “reduced capacity”, this refers to a 5G technology aimed at top-tier IoT applications: those that don’t require full broadband capacity data services, but need more than the ability to connect occasionally or continuously but with low data rates. It’s envisaged that RedCap will target devices above those for which Cat-M1 was created. The LTE technology is expected to continue in service for many years to come, to evolve, and to be aimed at what’s called ‘eMTC’ (enhanced Machine-Type Communication) applications. Inevitable many applications will lie in the gray area between these two technologies.
RedCap is more formally known as 5G NR-Light — “NR” is New Radio, the core technology underpinning 5G. It supports downlink and uplink speeds of 150Mbps and 50Mbps, respectively — equivalent to LTE Cat 4. Target applications include wearables, industrial wireless sensors, and CCTV cameras.
A modem determines which tower to connect to on the basis of this value, which is the measured power of the LTE reference signals spread across the broadband and narrowband portions of the spectrum. RSRP values, presented in dBm, are always negative, and the higher the number, ie. the closer to zero it is, the higher the power of the signal.
With RSRP values for all the nearby cell basestations, a modem chooses the basestations with the best RSRP. If two basestations’ RSRP values are too close to call, the modem uses Reference Signal Received Quality (RSRQ) as the basis for its choice.
To learn more about how cellular signals are measured, see How to Determine Good Cellular Signal Strength.
This is the ratio of the carrier power to the interference power: essentially it’s a signal-noise ratio measured using a standard signal. A connection with a high RSRQ should be good, even if the Reference Signal Received Power (RSRP) is low: the modem is able to extract the information in the weak signal because of minimal noise.
To learn more about how cellular signals are measured, see How to Determine Good Cellular Signal Strength.
This is a measurement of the power present in a received radio signal. It’s measured in decibels relative to a reading a 1mW of power (dBm). Values are usually negative; the higher the value (closer to zero) the value, the higher the RSSI.
RSSI is sometimes used as a crude guide to signal strength. However, RSSI values by, say, user equipment — the cellular modules within devices — may be measured in subtly different ways, at different stages in the communication process. Consequently, RSSI values presented by different devices, even in the same place, may not be comparable.
To learn more about how cellular signals are measured, see How to Determine Good Cellular Signal Strength.
This is a basestation unit which communicates with a mobile core’s Packet-data network GateWay (PGW) to establish a secure data tunnel between them. The device’s Internet communications will then pass through this tunnel while the device is attached.
This is the element within a mobile device that identifies a subscriber to cellular network services. It is used to validate a device’s access to a network by telling the network who should be billed for the device’s use of that network.
Originally, SIMs were data storage devices, but they have evolved to gain the processing power to run applets which deliver their functionality to the host device.
This is an online resource which stores a carrier’s eSIM profiles and which makes these profiles available to devices containing an eSIMs. The SM-DP+ can also hold profile activation and installation confirmation codes. Its IP Backbone address must be programmed into the eSIM or installed into it by an application capable of reading QR codes.
The eUICC in which the eSIM profile is stored is managed by a Local Profile Assistant (LPA), an applet or OS component which contacts the SM-DP+ at the specified address, sends over the host’s eUICC ID (EID) and, in return, receives an eSIM profile reserved exclusively for that EID. Alternatively, the LPA may send a ‘Matching ID’ if one has been provided via an Activation Code.
The LPA installs the profile, and this allows the device to attach to the cellular network and connect to the Internet.
TDD is method for organizing communication when the uplink (device-to-basestation) and the downlink (basestation-to-device) use the same radio frequency. Time is organized into slots which are allocated to the uplink or the downlink so that when, say, the radio is being used to carry data from the device, it is not being used to send data to the device. The available slots are also spread across all the devices connected to a given basestation, so the more devices, the narrower the time slots. Time slot width is a multiple of a base, standard-specified duration.
Depending on network utilization and use cases, networks may be configured to allocate more slots to the downlink than the uplink — or vice versa. For example, smartphone and tablet users streaming video files need much less uplink bandwidth than they do on the downlink, and networks can adjust their slot allocation accordingly.
LTE supports the use of TDD.
Twilio doesn’t have basestations of its own. Instead it leverages other networks’ cell towers. These communicate over the IP Backbone with Twilio’s own mobile core. Unlike many carriers’ cores, the Twilio Mobile Core is distributed: it is deployed to multiple data centers around the world. This ensures that devices get a consistent connectivity experience, whether they’re deployed in Los Angeles, Berlin, Taipei, or Nairobi. The result: less device downtime, and a real choice of which networks to use in any given region.
UDP is a standard way of exchanging data over the Internet. It emphasizes speed over reliability: data packets are transmitted to the destination without any checks to see if the packet was received. This ‘fire and forget’ approach reduces the time required to transmit the bulk data from of which the packet is a part because the server doesn’t have to wait to hear from the receiver whether the packet was received or needs to be sent again. This makes UDP very useful for video or audio streaming, where a little corrupt data or a missing packet, isn’t going to trouble the viewer much, but not for applications relying on data integrity.
That said, if the data being sent is small and within a sequence of transmission of the same information — the temperature recorded by a remote sensor, for example — packet loss may be entirely acceptable. If you don’t get a temperature reading at one interval, chances are you’ll get it at the next.
Super SIM’s IP Commands leverage UDP. For more information on IP Commands, see Get Started with Super SIM IP Commands and the Raspberry Pi.
A generic cellular engineer’s term for any device — IoT gadget, phone, tablet, computer — which allows an end-user to access the cellular network.
Also known as USIM (Universal Subscriber Identity Module), this is the element with a device that identifies a subscriber to cellular network services. It is used to validate a device’s access to a network. It tells the netwokrk who should be charged for use of the device.
The SIM is an application running on a UICC (Universal Integrated Circuit Card). It is accessed by the host device’s cellular module when the device wants to connect to cellular services. The key value stored within the SIM is the IMSI. With USIMs
UICC cards come in several form-factors. The best-known type is the plastic card, originally credit card-sized, but not also available in small, micro, and nano form-factors. Additionally, some UICCs come embedded in chips — the ‘MFF2’ form-factor — for integration directly onto the device’s main circuit board.
Also known more simply as ‘visited network’, this is any network a device can attempt to attach to other than its home network — i.e., the network operated by the carrier that supplied the device’s SIM.
Super SIM has no home network, so any one of the many networks it can connect to is by definition a visited network.
A VPN is an encrypted connection between devices — typically a user device and a server — over the Internet. Strong encryption ensures privacy from snooping. Additionally, because devices’ requests to web servers and other Internet-hosted services break out from the VPN at the server, it is not possible to determine devices’ physical locations from their IP addresses. All requests come from the server IP.
Super SIM’s IP Commands feature delivers a VPN-like experience for IoT devices. For more information on IP Commands, see Get Started with Super SIM IP Commands and the Raspberry Pi.