GSM, APN, IMSI & USSD Explained: A Traveler's Network Glossary

GSM, APN, IMSI, USSD — eSIM shopping and troubleshooting throws these terms around constantly, usually with zero explanation. None of them are complicated once you see them side by side. Here's the traveler's glossary.

Key Takeaways

- GSM (Global System for Mobile Communications) is the network standard most of the world's carriers run on — it's the reason a phone bought in one country can often connect to networks in another.

- APN (Access Point Name) is the specific setting that tells your phone how to reach the internet through your carrier's network; a wrong APN is a common cause of "signal but no data."

- IMSI is a 15-digit number stored on your SIM that identifies your subscription to the network — different from the IMEI, which identifies your phone.

What does GSM actually mean?

GSM stands for Global System for Mobile Communications — the network standard the large majority of the world's mobile carriers use. Practically, it's the reason a phone bought in one country can connect to a network in another: both sides are speaking the same underlying protocol. When an eSIM provider says a plan works on "GSM networks," they're describing this broad compatibility, not a specific feature.

What is an APN, and why does it matter for eSIM?

An APN (Access Point Name) is the gateway setting that tells your phone exactly how to reach the internet through your specific carrier's network — think of it as an address book entry your phone needs to actually use its data connection. Most travel eSIM profiles configure this automatically when you install them, but an incorrect or missing APN is one of the most common causes of "signal shows, but data won't load." Our cellular data troubleshooting guide walks through checking and fixing it directly.

What is an IMSI, and how is it different from an IMEI?

The IMSI (International Mobile Subscriber Identity) is a 15-digit number stored on your SIM or eSIM that identifies your specific subscription to a network — not your device. That's the key distinction from the IMEI, which identifies the physical phone. Two different SIMs in the same phone have two different IMSIs; the phone's IMEI stays the same either way. See our IMEI, EID and ICCID guide for the full set of device and SIM identifiers.

What is a USSD code?

USSD (Unstructured Supplementary Service Data) is the system behind those short codes you dial starting with and ending with # — like #06# to display your IMEI. It works without an internet connection, communicating directly with the carrier's network in real time, which is why it still works even on a phone with no data connection at all.

What does ISP mean, and is my eSIM provider an ISP?

ISP stands for Internet Service Provider — broadly, any company that gives you access to the internet. A mobile carrier providing cellular data technically fits that definition, though in everyday use "ISP" more often refers to home broadband providers, while "carrier" or "eSIM provider" is the more precise term for mobile data specifically.

Frequently asked questions

Do I need to understand any of this to use a travel eSIM?

No — a well-designed eSIM install handles APN and network configuration automatically. This glossary is mainly useful when something goes wrong and a support article or provider tells you to check a setting you don't recognize.

Is GSM the only network standard eSIMs use?

GSM-family networks (including modern 4G/5G, which evolved from the same standards lineage) cover the large majority of eSIM travel plans. A small number of networks, mostly legacy ones in parts of North America, historically used a separate standard called CDMA, which works differently.

Why does *#06# show me multiple numbers?

That code surfaces both your IMEI (device) and EID (eSIM chip) at once on eSIM-capable phones — see our IMEI vs EID vs ICCID guide for how to tell them apart.

Can I look up my carrier from my IMSI?

Yes, technically — the first several digits of an IMSI encode the country and network operator, which is part of how networks route your connection to the right carrier automatically.

Sources

• GSMArena, APN (Access Point Name) definition, retrieved 2026-07-08, https://www.gsmarena.com/glossary.php3?term=apn
• Wikipedia, International mobile subscriber identity, retrieved 2026-07-08, https://en.wikipedia.org/wiki/International_mobile_subscriber_identity