SMS glossary

SMS #

SMS (Short Message Service) is the standard for short text messages between mobile devices, delivered over the operator's network without requiring an internet connection.

Because SMS works at network level rather than through an app, a message reaches any mobile phone — including older devices and situations with a weak or missing data connection.

GSM-7 #

GSM-7 is the default character set of SMS, in which one message holds 160 characters.

The set contains the common Latin letters, digits and punctuation marks. A few characters (such as {, }, [, ], ~, ^, \ and the euro sign) count as two positions.

Unicode (UCS-2) #

Unicode is the alternative character set for SMS, in which one message holds 70 characters instead of 160.

As soon as a message contains a single character that does not fit GSM-7 — an emoji, a Cyrillic letter, certain accents — the entire message switches to Unicode and the limit drops to 70 characters per part.

Concatenation (multipart SMS) #

Concatenation is the splitting of a long message into multiple SMS parts that the recipient sees again as a single message.

Each part is billed separately. Splitting also sacrifices characters to the linking information: a split message holds 153 characters per part in GSM-7 and 67 in Unicode, not 160 and 70.

SMS gateway #

An SMS gateway is a service that converts messages from software or a web interface into SMS and delivers them to the recipient's operator.

The gateway handles routing to the right network, billing and delivery-status feedback, so the sender does not need a direct agreement with every operator.

Bulk SMS #

Bulk SMS is sending the same message — personalised or not — to a large group of recipients in a single operation.

Sender ID #

A sender ID is the sender the recipient sees: a phone number, a shortcode or an alphanumeric name of up to 16 characters.

Whether an alphanumeric name is allowed differs per country; some countries also require prior registration. An alphanumeric sender cannot be replied to — that requires a number.

Virtual number #

A virtual number is a mobile number managed online without a physical SIM card or device.

It serves as both sender and receiving point, so messages can flow in both directions without a phone having to sit around somewhere.

Shortcode #

A shortcode is a shortened number of four to six digits used as sender and receiving point for SMS.

A shared shortcode is used by multiple senders, each with their own keyword; a dedicated shortcode is reserved for a single organisation.

Keyword #

A keyword is the first word of an incoming message, used to determine which sender or campaign that message is meant for.

Keywords make a shared shortcode usable: the message "SALE" and the message "STOP" arrive on the same number but are assigned to different recipients or processes.

Two-way SMS #

Two-way SMS is SMS communication in which the recipient can reply to the message and that reply reaches the original sender.

Mail2SMS #

Mail2SMS is a method by which an email is automatically converted into an SMS message.

Useful for systems that can send email but cannot call an API, such as many monitoring, accounting and ERP packages.

DLR (delivery report) #

A DLR or delivery report is the network's feedback on what happened to a sent SMS.

Among other things it distinguishes Sent (handed over to the operator) from Delivered (arrived on the device). Only the latter confirms delivery.

Buffered #

Buffered means the network has accepted a message but cannot deliver it yet, usually because the device is switched off or out of range.

The network keeps retrying for as long as the validity period lasts. If that fails, the status eventually becomes Expired.

Validity period #

The validity period is the time during which the network keeps trying to deliver a message before it expires.

For time-sensitive messages such as verification codes, a short validity period is desirable: a code that still arrives hours later is no longer usable and can be confusing.

Webhook #

A webhook is an HTTP call that a service makes on its own initiative to an address you specify whenever something happens, so your system does not have to keep polling.

HLR (Home Location Register) #

The HLR is a mobile operator's central database that records which numbers belong to it and where a device is located.

An HLR lookup queries that database to determine whether a number exists, is active and which operator it belongs to at that moment.

MCC / MNC #

MCC (Mobile Country Code) and MNC (Mobile Network Code) are the numeric codes identifying a country and a mobile network within that country, respectively.

Together they form the MCCMNC, the key used to determine rates and routes per network.

Number portability (porting) #

Number portability is a subscriber's right to take their phone number along to another operator.

As a result the number range no longer says anything about the current operator: only an HLR lookup tells you conclusively where a number really belongs.

E.164 (international format) #

E.164 is the international standard for phone numbers, in which a number consists of the country code followed by the subscriber number without the leading zero.

A Belgian mobile number such as 0496 12 34 56 becomes 32496123456 in that format. Writing numbers internationally prevents confusion when sending abroad.

SMPP #

SMPP (Short Message Peer-to-Peer) is the protocol used by SMS platforms and operators to exchange high-throughput messages over a permanent connection.

For most applications a REST API is sufficient; SMPP is mainly relevant for very large, continuous volumes.

OTP (one-time password) #

An OTP is a one-time code used to verify a user, which expires after use or after a period of time.

Two-factor authentication (2FA) #

Two-factor authentication is a login method that requires a second proof in addition to a password, for example a code sent by SMS.

Transactional SMS #

A transactional SMS is a message tied to a service or order the recipient requested themselves, such as a confirmation or an appointment reminder.

The distinction from promotional SMS is legally relevant: commercial messages to consumers require opt-in, operational messages do not.

Opt-in #

Opt-in is a recipient's prior, explicit consent to receive commercial messages.

Always keep a record of when and how that consent was obtained. This is general information, not legal advice on a specific situation.

Opt-out #

Opt-out is the recipient's ability to unsubscribe from further commercial messages, in practice often by replying STOP.

Credit #

A credit is the accounting unit in which sends are billed; how many credits a message costs depends on the destination and the length.

One credit is therefore not automatically one SMS: an expensive destination network or a message split into multiple parts costs more.