eSIM, eUICC and MFF2

Three terms are often mixed up when people talk about SIMs for devices. They describe different things. One is the chip’s ability, one is the system that loads it, and one is the shape it comes in.

UICC and eUICC

A SIM card is, in the standards, a UICC. It is a small secure computer that holds an operator’s details and keys, so the network can recognise the device.

A traditional UICC holds one set of operator details, called a profile, put there at the factory. An eUICC can hold several profiles, and new ones can be loaded onto it over the air. One is in use at a time in most devices, and it can be swapped for another remotely. The eUICC is hardware with software on it. It is a chip, the same as any SIM.

What eSIM means

eSIM is the everyday name for the GSMA’s system for loading and managing profiles remotely, and for SIMs that use it. The GSMA has published three variants of that system, one for machine-to-machine devices, one for phones and one for IoT devices. They are explained in eSIM standards SGP.02, SGP.22 and SGP.32.

An eSIM is a SIM whose operator details can be changed. That alone gives a device no extra networks. Which networks a device can use still depends on the profile it has and that operator’s agreements.

What MFF2 means

MFF2 is a package. It is a small chip, 6 mm by 5 mm, soldered onto the circuit board instead of pushed into a slot, so it cannot be removed.

An MFF2 can contain an eUICC or an ordinary single-profile SIM. And an eUICC can come in an ordinary plastic SIM card too, or be built into the device’s main processor, which is often called iSIM. So the package tells you how the SIM is fitted, and nothing about whether it can take new profiles.

Why it matters

When you choose SIMs for a product, ask two separate questions. How will it be fitted: a card in a slot, a soldered chip, or built into the processor? And can its profile be changed remotely, and by whom? The answers decide whether you can change operator after the devices are in the field, and how much work that would be. Each eUICC also has its own number, the EID, which stays the same whatever profile is loaded.

Three separate properties

A SIM has a platform, a provisioning method and a physical form. The terms eUICC, eSIM and MFF2 each describe one of these, and they combine freely.

The eUICC

The GSMA architecture specifications define the eUICC as a discrete or integrated tamper-resistant component of hardware and software, able to host applications and cryptographic data securely (SGP.01 EUICC1-A, SGP.21). To the device it behaves like an ordinary UICC. The difference is internal. It contains an issuer security domain for the platform (ISD-R) and one security domain per profile (ISD-P), so profiles can be downloaded, enabled, disabled and deleted under the control of a remote server, while each operator’s keys stay isolated from the others.

A profile is the set of data and applications an operator provisions on the eUICC, such as the USIM with its IMSI and keys. Each profile has its own ICCID. In the machine-to-machine architecture only one profile is enabled at a time (SGP.01 EUICC4). The consumer architecture in SGP.21 version 3.1 allows a non-removable eUICC to support Multiple Enabled Profiles (EUICC64) in devices built to use them.

The eUICC itself is identified by the EID. GSMA SGP.29 sets it at 32 digits and requires it to identify one eUICC uniquely. Most MFF2 eUICCs are rated −40 °C to +105 °C; the +125 °C class is rarer. The EID belongs to the chip, so it stays the same whichever profile is enabled. The ICCID changes with the profile. Device makers are expected to give the owner a way to read the EID, and SGP.01 recommends printing it on the device. See SIM identifiers for how the EID, ICCID and IMSI relate.

eSIM

eSIM is the GSMA’s name for its remote SIM provisioning ecosystem: the eUICC, the servers that prepare and deliver profiles, and the interfaces between them. It is used loosely for any SIM provisioned this way. Three architectures exist, for M2M (SGP.01 and SGP.02), consumer devices (SGP.21 and SGP.22) and IoT (SGP.31 and SGP.32). eUICCs, devices and servers are built and certified to particular specifications, so check which apply before choosing a provisioning platform. eSIM standards SGP.02, SGP.22 and SGP.32 compares them.

Remote provisioning changes who can be the operator. Roaming works as it does with any SIM. A profile gives the device the networks its operator has agreements with. A device with an eSIM profile abroad is roaming on that profile’s agreements, and the country rules in permanent roaming rules still apply. Loading a local profile is one way to deal with them.

MFF2

ETSI TS 102 671 adds form factors to those in TS 102 221. MFF2 is an 8-pin leadless package with a body of 6.00 ± 0.15 mm by 5.00 ± 0.15 mm, contacts on a 1.27 mm pitch and an exposed heat pad underneath. It is reflow-soldered to the board. Contacts C4 and C8 (for the TS 102 600 high-speed interface) and C6 (for the TS 102 613 single wire protocol) are left unbonded when those functions are not supported. The same specification also defines MFF1, and the smaller MFF3 and MFF4 packages.

TS 102 671 also defines environmental classes that may be applied to any UICC form factor, including temperature ranges up to −40 °C to +125 °C. A part’s datasheet states which classes it meets.

The package says nothing about the silicon inside. An MFF2 part can carry a single-profile UICC with one operator’s profile fixed at manufacture, or an eUICC. SGP.21 states that a discrete eUICC may be removable or non-removable, and that a removable eUICC is packaged in a standardised ETSI form factor, so 2FF, 3FF and 4FF cards can be eUICCs too. See SIM form factors for the dimensions of each.

Integrated eUICC

An integrated eUICC runs on a tamper-resistant element inside the device’s system-on-chip, which may use memory outside the SoC (SGP.21). ETSI specifies this element as the integrated Smart Secure Platform (iSSP) in TS 103 666-2, and the GSMA covers its security evaluation in SGP.08. The common name is iSIM. It removes the separate SIM component from the board.

Practical points for IoT

• Specify the platform and the package separately. “MFF2” on a quotation does not mean the part is an eUICC.
• For an eUICC, ask which GSMA architecture it follows, which servers can manage it, and whether it has been certified against that specification.
• Record the EID at manufacture alongside the IMEI. The EID is the key every eSIM platform uses to address a device.
• A soldered SIM cannot be swapped in the field. If you might need a different operator later, an eUICC keeps that option open.