SMS glossary

Plain-language definitions of the terms used across these guides. Each term has a stable anchor you can link or cite. Machine-readable copy: glossary.json. Updated 2026-07-26.

SMS gateway: An SMS gateway bridges applications to mobile carrier networks so programs can send and receive text messages. It manages routing, encoding, and delivery across destinations. SMSRoute operates a gateway serving 149 countries with simple access and all-in pricing (US $0.0125 per message; lowest published country rate $0.004).
SMS API: An SMS API lets developers send and manage text messages through HTTP requests or similar interfaces, handling authentication, payloads, and status callbacks. It supports automation for notifications and alerts. SMSRoute provides an SMS API after email-only signup with crypto balance top-ups.
A2P messaging: A2P messaging means Application-to-Person traffic: texts originating from software or business systems to mobile users for alerts, OTPs, or campaigns. It differs from person-to-person chats and faces stricter carrier rules. Most bulk and transactional SMS, including SMSRoute traffic, is A2P.
sender ID: A sender ID is the telephone number or name shown as the source of an SMS on the recipient’s handset. Country and carrier rules dictate whether numeric or alphanumeric forms are allowed. It identifies the originator to the recipient only; SMS has no end-to-end encryption.
alphanumeric sender ID: An alphanumeric sender ID displays a custom name (usually up to 11 characters) instead of a phone number as the SMS originator. Availability and pre-registration requirements differ by country. It aids brand recognition toward the recipient while SMS itself remains unencrypted end-to-end.
GSM-7: GSM-7 is the default 7-bit character set for SMS, covering basic Latin letters, digits, and common symbols. It packs up to 160 characters into one message segment. Characters outside the set require escape codes or a switch to UCS-2 encoding.
UCS-2: UCS-2 is a 16-bit encoding used for SMS when the text includes non-GSM characters, such as many non-Latin scripts or emojis. It limits each segment to 70 characters. Multipart messages are required for longer content, with each segment billed separately.
message segment: A message segment is one SMS unit limited to 160 GSM-7 characters or 70 UCS-2 characters. Longer texts are split into multiple segments that travel and are charged independently. Carriers and gateways bill per segment, not per logical message.
concatenated SMS (UDH): Concatenated SMS uses a User Data Header (UDH) so a long message can be divided into ordered segments and reassembled by the receiving handset. Each segment counts as a separate SMS for billing and throughput. Maximum length and support vary by network.
DLR (delivery receipt): A DLR (delivery receipt) is a status report returned by the carrier or gateway indicating whether an SMS reached the handset, expired, or failed. Gateways expose DLRs via API or webhook. It confirms network delivery only; SMS has no end-to-end encryption.
HLR lookup: An HLR lookup queries a mobile network’s Home Location Register to verify whether a number is valid, active, and which operator currently serves it. Senders use it to clean lists and reduce undeliverable messages before submitting SMS traffic.
OTP: An OTP (one-time password) is a short-lived numeric or alphanumeric code delivered by SMS for login verification or transaction confirmation. It is a common A2P use case. SMS delivery of OTPs is not end-to-end encrypted and should be paired with other safeguards.
10DLC: 10DLC denotes 10-digit long codes used for A2P SMS in the United States. Brands and campaigns must be registered with carriers to obtain throughput and improve deliverability. Unregistered traffic faces filtering; compliance is required for US long-code messaging.
DLT (India): DLT (Distributed Ledger Technology) is India’s mandatory registration system for commercial SMS. Entities must pre-register headers, templates, and consents on approved platforms before traffic is accepted. Non-compliant messages are blocked by Indian operators.; These definitions are maintained as part of the SMSRoute guides and cross-link the tools and fact sheets that use each term.
E.164: E.164 is the international numbering format that begins with a plus sign and country code followed by the national significant number, maximum 15 digits. SMS APIs expect destination and source numbers in E.164 so messages route correctly across borders.
SMPP: SMPP (Short Message Peer-to-Peer) is a binary TCP protocol widely used for high-volume SMS exchange between applications, gateways, and carriers. It supports efficient bind sessions, enquire links, and delivery receipts as an alternative to HTTP-based APIs.
webhook: A webhook is a user-defined HTTPS endpoint that receives real-time push notifications, such as SMS delivery receipts or inbound messages, from the gateway. It eliminates polling and enables immediate application responses to status or MO events.
quiet hours: Quiet hours are legally or carrier-defined time windows during which promotional or non-urgent SMS may not be delivered to consumers. Restrictions vary by country and often exempt pure transactional traffic. Observing them reduces complaints and filtering risk.
opt-out (STOP): Opt-out (STOP) lets recipients revoke consent by replying with a keyword such as STOP. Regulations require senders to process these requests promptly and cease further marketing texts. Proper handling is mandatory for compliant A2P programs in most markets.
KYC: KYC (Know Your Customer) is identity verification that many SMS providers perform to meet anti-fraud and regulatory obligations. It commonly involves government ID or business documents before full account access or higher tiers are granted.
no-KYC signup: No-KYC signup means creating an account without submitting identity documents; SMSRoute offers email-only registration. Users still must obey applicable messaging laws. Any reduced friction applies to account opening; anonymity exists only toward message recipients, and SMS has no end-to-end encryption.
crypto top-up: Crypto top-up is the process of adding prepaid balance with cryptocurrency so SMS traffic can be sent. SMSRoute accepts crypto payments to fund usage at its published all-in rates (example $0.0125 per message, floor $0.004) across supported destinations.
MCC / MNC: MCC (Mobile Country Code) and MNC (Mobile Network Code) are the paired numeric identifiers that uniquely name a mobile operator’s network. A sender meets them in practice when routing or looking up a handset: the destination IMSI (and thus the serving network) is selected by MCC+MNC, which drives interconnect path, signalling, and delivery handling for the SMS.
SMSC: The SMSC (Short Message Service Center) is the network element that accepts, stores, and forwards SMS messages between senders and recipients in a mobile network. A sender encounters it in practice because every SMS is first submitted to an SMSC, which then attempts delivery to the destination handset and may hold the message until the device is reachable.
Fake DLR: A Fake DLR is a delivery receipt that reports an SMS as delivered to the handset when no such delivery occurred. Senders encounter it in practice when an intermediate network or platform accepts the message, drops or blocks it, and still returns a success status report instead of a real handset confirmation.
DLR latency: DLR latency is the time that elapses between submission of an SMS to the network and the return of its delivery report indicating final handset status. A sender encounters it in practice whenever delivery receipts are requested, as the interval reflects downstream routing, handset reachability, and network processing before the report is generated and sent back.
MO test (two-way test): An MO test (two-way test) checks that a sender’s number or short code can accept mobile-originated SMS from handsets as well as deliver outbound messages. A sender meets it in practice by successfully receiving an inbound message from a mobile device and handling or replying to it, confirming bidirectional traffic works.
Sender-ID preservation: Sender-ID preservation is the end-to-end retention of an SMS message’s original originating address so the recipient handset displays exactly the Sender ID supplied by the originator. A sender meets it in practice when every SMSC, gateway and interconnect on the route forwards that address unchanged rather than substituting another numeric or alphanumeric value.; These definitions are maintained as part of the SMSRoute guides and cross-link the tools and fact sheets that use each term.
Validity period: The validity period is the time window set on an SMS during which the network may keep trying to deliver it if the handset is unreachable. A sender meets it in practice by including or accepting a TP-VP value so the message expires after that interval instead of being held indefinitely, after which delivery attempts stop and a failure can be reported.
MSISDN: MSISDN is the Mobile Station International Subscriber Directory Number: the full E.164 phone number (up to 15 digits) that identifies a mobile subscriber. In SMS it is the destination (or source) address a sender must supply so the message can be routed to the correct handset. Operators and platforms use it in signaling and delivery receipts to match the message to that subscriber.
Route quality: Route quality is the measured reliability and consistency of the network path an SMS takes from the originating sender through intermediary gateways and carriers to the recipient handset. It reflects successful delivery signaling, correct routing of GSM-compliant messages (including proper 160/153-character segment handling for GSM-7 or 70/67 for UCS-2), and valid E.164 addressing up to 15 digits. A sender meets it in practice by submitting well-formed traffic over stable, standards-aligned connections that maintain intact delivery receipts and minimize transit failures.
Grey route: A grey route is an SMS delivery path that reaches handsets by blending into a mobile network’s local or wholesale access instead of using the operator’s official international interconnect. Senders meet grey routes in practice when they buy lower-priced international termination that the supplying network has obtained through SIM-based or other indirect access rather than a bilateral carrier agreement. Because the traffic is not authorised under the receiving operator’s interconnect rules, filtering or blocking can occur without notice.
A2P: Application-to-Person SMS: traffic originated by an application or service toward subscriber handsets, distinct from handset-to-handset chat.
Concatenated SMS: A payload split across multiple SMS parts because it exceeds one segment. Each part fits 140 octets (153 septets or 67 UCS-2 chars with UDH); the handset reassembles via concatenation headers.
Flash SMS: Class 0 short message shown on the handset display immediately; it may not be stored in the inbox depending on the device.
Intermediate DLR: Delivery receipt that reports a non-terminal message state (for example enroute). It indicates progress before a final outcome is known. The full state list lives in the DLR status-codes reference.
P2P: Person-to-Person SMS: messages exchanged between subscriber handsets over the operator SMS service rather than from an application platform.
SMPP bind: An authenticated SMPP session between ESME and SMSC. Bind type is transmitter, receiver, or transceiver, which controls whether the client may submit messages, receive them, or both.
Throughput (TPS): Messages handled per second on a bind or platform. In SMS stacks it usually counts submit_sm or deliver_sm operations the session or system can process under load.
deliver_sm: SMPP operation the SMSC uses to push an inbound MO message or a delivery receipt to the ESME over a receiver or transceiver bind.
enquire_link: SMPP PDU sent on an idle bind to test that the peer is still reachable. Missing enquire_link_resp usually means the session is dead and should be rebound.
submit_sm: SMPP operation an ESME uses to hand a short message to the SMSC for routing toward a handset or other endpoint.; These definitions are maintained as part of the SMSRoute guides and cross-link the tools and fact sheets that use each term.
Final DLR: A final DLR is the terminal delivery receipt for an SMS, reporting an end-state outcome such as delivered, failed, or expired after any intermediate updates. It closes the message lifecycle from the network's perspective. The full state list lives in the DLR status-codes reference.
Long code: A long code is a full-length MSISDN used as an SMS sender or recipient address, expressed in E.164 (max 15 digits), as opposed to a short code.
Message state: Message state is the SMPP v3.4 enumerated status of an SMS in its lifecycle (e.g. ENROUTE, DELIVERED, EXPIRED, DELETED, UNDELIVERABLE, ACCEPTED, UNKNOWN), carried in delivery receipts and query responses. The full state list lives in the DLR status-codes reference.
National numbering plan: A national numbering plan is a country's official scheme for structuring telephone numbers, defining length, area or network codes, and subscriber number format inside that nation's public switched or mobile network.
Septet: A septet is a 7-bit unit in the GSM 03.38 default alphabet. Up to 160 septets fit in the 140-octet user data of one SMS; concatenated segments typically carry 153 septets each after UDH overhead.
Trunk prefix: A trunk prefix is the digit or digits dialed before the national significant number for domestic calls (often 0). It is normally omitted when the number is written in international E.164 form.
Quiet hours: Time windows - set by local regulation or carrier practice in some markets - during which promotional SMS should not be delivered. The rules differ by country and by message class, and transactional messages are commonly treated differently from marketing. Scheduling marketing sends inside the destination's local daytime is the safe default.
PDU: Protocol Data Unit - the binary-encoded form of an SMS (SMS-SUBMIT, SMS-DELIVER, SMS-STATUS-REPORT) exchanged with a service centre, specified in 3GPP TS 23.040. Everything an SMS is - addresses, encoding, timestamps, payload - travels inside it as hex octets.
TP-DCS: TP-Data-Coding-Scheme: the one-octet PDU field (3GPP TS 23.038) that selects the alphabet (GSM 7-bit or UCS-2), message class and related flags. It is why one emoji re-encodes a whole message.
SMSC address: The E.164 address of the service centre that stores and forwards a message. In a submitted PDU it precedes the message itself; it identifies infrastructure, not the sender the recipient sees.
IMSI: International Mobile Subscriber Identity: the internal subscription identifier (MCC + MNC + serial, per ITU-T E.212) on the SIM. Networks route with it internally; SMS APIs never address it.
submit_sm_resp: The SMPP response to a submit_sm: carries the command status and the message id that later delivery receipts reference - the correlation key for matching DLRs to sends.
SAR TLVs: SMPP's segmentation-and-reassembly parameters (message reference, total segments, sequence number) - an alternative to UDH concatenation that exists only on the SMPP link and must be rewritten to UDH before the air interface.
DCS message classes: The class bits inside TP-DCS: class 0 displays immediately (flash), classes 1-3 target the device, SIM or terminal respectively. Class affects display behaviour, not billing.
TP-PID: TP-Protocol-Identifier: a one-octet PDU field signalling interworking type (default 0x00 for plain SMS). Most application traffic never changes it.