SMS & Telecom Glossary

A reference glossary covering SMS messaging, telecom infrastructure, carrier compliance, industry terminology, and x402 crypto-payment concepts. Terms are listed alphabetically. Acronyms are expanded on first use.

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A

A2P (Application-to-Person)

A text message sent from a software application, API, or automated platform to an end-user's mobile phone. Carriers apply distinct routing, filtering, and pricing rules to A2P traffic compared to P2P traffic. See What Is A2P SMS? for a complete guide.

Aggregator

A company that sits between CPaaS providers (or enterprises) and mobile carriers, consolidating message traffic from multiple sources and routing it through direct carrier connections. Aggregators negotiate bulk rates, manage number provisioning, and often handle 10DLC campaign registration on behalf of downstream customers. See Short Code vs 10DLC vs Toll-Free for how aggregators fit into the messaging stack.

Alphanumeric Sender ID

A sender identifier that uses letters and numbers (e.g., "MYBANK") instead of a phone number. Supported in many countries outside the United States. Messages sent from an alphanumeric sender ID are one-way only — the recipient cannot reply directly.

B

Base (L2)

An Ethereum Layer 2 (L2) rollup built on the OP Stack, incubated by Coinbase. Base settles transactions to Ethereum mainnet while offering lower gas fees and faster confirmation times. x402 uses Base as its default settlement chain for USDC payments.

BSC (Base Station Controller)

A component in a GSM network that manages radio resources for one or more BTS (Base Transceiver Station) sites. The BSC handles handover decisions, frequency allocation, and power control within its coverage zone.

BTS (Base Transceiver Station)

The physical equipment — antennas, transceivers, and signal-processing hardware — at a cell site that communicates directly with mobile devices over the air interface. Multiple BTS units connect upstream to a single BSC.

C

CAIP-2 (Chain Agnostic Improvement Proposal 2)

A standard for identifying blockchain networks using a namespace-qualified format (e.g., eip155:8453 for Base mainnet). x402 uses CAIP-2 chain identifiers in payment headers so that facilitators and clients can unambiguously specify which network a payment targets.

CALEA (Communications Assistance for Law Enforcement Act)

A U.S. federal statute requiring telecommunications carriers and certain internet services to build lawful-intercept capabilities into their infrastructure, enabling court-authorized wiretapping. CALEA obligations extend to VoIP and broadband providers.

CAN-SPAM (Controlling the Assault of Non-Solicited Pornography and Marketing Act)

A U.S. federal law that sets rules for commercial email and, by extension, commercial electronic messaging. While primarily targeting email, CAN-SPAM provisions around deceptive headers and opt-out mechanisms are occasionally referenced in SMS compliance contexts.

Carrier

A licensed entity that operates wireless or wireline telecommunications infrastructure. In SMS, "carrier" typically refers to a Mobile Network Operator (MNO) such as AT&T, T-Mobile, or Verizon that terminates messages on its subscribers' devices.

CCaaS (Contact Center as a Service)

A cloud-delivered contact center platform that provides voice, SMS, chat, and email customer-support capabilities without requiring on-premises hardware. CCaaS platforms integrate with CPaaS APIs for programmable messaging channels.

CLEC (Competitive Local Exchange Carrier)

A telephone company that competes with the incumbent local carrier (ILEC) to provide local exchange services. CLECs often lease infrastructure from the ILEC or deploy their own fiber and switching equipment.

Concatenated SMS

A long text message that exceeds the single-segment character limit (160 characters in GSM-7, 70 in UCS-2) and is split into multiple segments by the sending platform. A UDH (User Data Header) in each segment tells the receiving device how to reassemble them into one displayed message. Each segment counts as a separate billable message. See How SMS Works for details on encoding and segmentation.

CPaaS (Communications Platform as a Service)

A cloud-based platform that exposes programmable APIs for voice, SMS, MMS, and other communication channels. Examples include Twilio, Vonage, and Sinch. CPaaS providers abstract carrier connectivity behind REST APIs and SDKs. See Short Code vs 10DLC vs Toll-Free for how CPaaS fits into the A2P ecosystem.

CSP (Campaign Service Provider)

In the 10DLC ecosystem, a CSP is the entity — typically a CPaaS provider or messaging platform — that registers brands and campaigns with The Campaign Registry (TCR) on behalf of the message sender. CSPs are responsible for vetting campaign use cases and ensuring compliance. See What Is 10DLC? for registration details.

CTIA (Cellular Telecommunications Industry Association)

A U.S. trade association representing the wireless industry. CTIA publishes the Messaging Principles and Best Practices guidelines that govern A2P SMS content standards, opt-in requirements, and prohibited message categories. See SMS Compliance: TCPA & CTIA for a compliance deep-dive.

D

DCA (Direct Connect Aggregator)

An aggregator with direct connections to U.S. carrier SMSC infrastructure, bypassing intermediate aggregators. DCAs (e.g., Syniverse, Iconectiv) are authorized by carriers to register 10DLC campaigns and route A2P traffic directly.

DID (Direct Inward Dialing)

A telephone number that routes directly to a specific endpoint — a desk phone, PBX extension, or VoIP device — without going through an attendant or IVR menu. DID numbers are widely used in VoIP and CPaaS platforms as programmable phone numbers.

DLR (Delivery Receipt)

A status report returned by the carrier or SMSC indicating whether an SMS was successfully delivered to the handset, failed, expired, or was rejected. DLRs are essential for measuring deliverability and triggering retry logic. See SMS Deliverability for how DLRs factor into delivery optimization.

E

E.164

The ITU-T international numbering standard that defines the format for globally unique phone numbers: a + prefix followed by a country code and subscriber number, with a maximum of 15 digits (e.g., +14155551234). All carrier APIs and SMPP implementations expect E.164-formatted numbers.

EIP-712

An Ethereum Improvement Proposal that defines a standard for signing typed, structured data in a human-readable way. Wallets display the structured fields before the user signs, reducing the risk of blind-signing attacks. x402 payment authorizations use EIP-712 signatures.

EIP-3009 (transferWithAuthorization)

An Ethereum standard that enables gasless token transfers by allowing a token holder to sign an off-chain authorization that a third party can submit on-chain. USDC implements EIP-3009, and x402 relies on it so that API consumers can authorize payments without holding ETH for gas.

F

Facilitator

In the x402 protocol, the facilitator is a server-side component that validates payment headers, verifies EIP-712 signatures, settles transferWithAuthorization transactions on-chain, and releases the API response to the caller. The facilitator acts as a trustless intermediary between the API consumer and the resource server.

FCC (Federal Communications Commission)

The U.S. federal agency responsible for regulating interstate and international communications. The FCC enforces TCPA rules, issues carrier licenses, manages spectrum allocation, and adjudicates robocall and spam complaints.

G

Gasless Transaction

A blockchain transaction where the end-user does not need to hold or spend native gas tokens (e.g., ETH) to execute the operation. In x402, gasless payments are achieved through EIP-3009 transferWithAuthorization, where the facilitator submits the on-chain transaction and absorbs the gas cost.

GSM-7

The default 7-bit character encoding used in SMS, supporting 128 characters from the basic Latin alphabet plus common symbols. A single SMS segment encoded in GSM-7 holds up to 160 characters (or 153 when concatenation headers are present). Messages containing characters outside the GSM-7 set fall back to UCS-2 encoding. See How SMS Works for encoding details.

H

HLR (Home Location Register)

A central database in a GSM/UMTS network that stores subscriber profiles, including phone number (MSISDN), IMSI, service subscriptions, and the current VLR location of each subscriber. HLR lookups are used for number validation, portability checks, and fraud screening.

HTTP 402 (Payment Required)

An HTTP status code reserved in RFC 7231 for future use with digital payment schemes. The x402 protocol repurposes this status code: a server responds with 402 Payment Required along with a payment header describing the price, accepted token, and chain, signaling to the client that a valid payment must be attached to access the resource. See the x402 protocol documentation for implementation details.

I

ILEC (Incumbent Local Exchange Carrier)

The original, pre-deregulation telephone company in a given service area (e.g., AT&T, Verizon in their legacy territories). ILECs own the local loop infrastructure and are required by regulation to lease unbundled network elements to CLECs.

IMS (IP Multimedia Subsystem)

A carrier-grade architecture for delivering voice, video, and messaging services over IP networks. IMS is the backbone of VoLTE (Voice over LTE) and RCS messaging, replacing legacy circuit-switched infrastructure.

K

KYC (Know Your Customer)

Identity verification procedures required by regulation or platform policy before granting access to services. In the A2P SMS ecosystem, KYC is performed during 10DLC brand registration and toll-free verification to confirm the identity of the business sending messages.

L

LNP (Local Number Portability)

The regulatory requirement that allows subscribers to keep their phone number when switching carriers. LNP databases (managed by entities like Neustar/Iconectiv in the U.S.) track which carrier currently serves each number. Messaging platforms query LNP data to route SMS to the correct terminating carrier.

Long Code

A standard 10-digit phone number (in NANP countries) used for sending SMS. Long codes are associated with local-number sending and are subject to 10DLC registration requirements for A2P traffic. See Short Code vs 10DLC vs Toll-Free for a comparison of number types.

M

MAP (Mobile Application Part)

A protocol in the SS7 stack that enables communication between core mobile network elements such as the HLR, VLR, and MSC. MAP handles subscriber authentication, location updates, SMS delivery, and supplementary service management.

MMS (Multimedia Messaging Service)

An extension of SMS that supports sending images, video, audio, and longer text content. MMS messages are routed through the carrier's MMSC (Multimedia Messaging Service Center) and typically cost more per message than SMS.

MNO (Mobile Network Operator)

A company that owns and operates wireless network infrastructure — radio spectrum, cell towers, core network — and provides mobile services directly to subscribers. Examples: AT&T, T-Mobile, Verizon, Vodafone.

MO (Mobile Originated)

A message or call initiated by the mobile subscriber's handset. In SMS, an MO message is one sent from a user's phone to an application or another phone. MO messages trigger inbound webhooks on CPaaS platforms.

MPS (Messages Per Second)

A throughput metric indicating how many messages a number, campaign, or connection can send per second. MPS limits vary by number type: short codes support high MPS (100+), 10DLC varies by trust score (typically 1-75 MPS), and toll-free numbers fall in between.

MSC (Mobile Switching Center)

The core network element in a GSM network that handles call routing, SMS delivery, and mobility management. The MSC interfaces with the HLR, VLR, and SMSC to complete voice calls and text messages.

MT (Mobile Terminated)

A message or call delivered to the mobile subscriber's handset. In A2P SMS, MT messages are the outbound messages sent from a business application to the end-user's phone. MT delivery success rates are tracked through DLRs.

MVNO (Mobile Virtual Network Operator)

A wireless carrier that does not own physical network infrastructure but leases capacity from an MNO. MVNOs operate their own brand, billing, and customer service while traffic is carried on the host MNO's towers. Examples: Mint Mobile, Google Fi, Visible.

N

NANP (North American Numbering Plan)

The telephone numbering plan covering the United States, Canada, and 20 Caribbean and Pacific Island territories. NANP numbers follow the format +1 NPA-NXX-XXXX, where NPA is the area code and NXX-XXXX is the subscriber number. NANP is a subset of the global E.164 standard.

O

Opt-In

Explicit consent from a mobile subscriber to receive A2P SMS messages from a specific sender or campaign. U.S. regulations (TCPA, CTIA) require documented opt-in before sending marketing or promotional messages. See SMS Compliance: TCPA & CTIA for opt-in requirements.

Opt-Out

The process by which a subscriber revokes consent to receive messages. CTIA guidelines require that senders honor standard opt-out keywords (STOP, UNSUBSCRIBE, CANCEL, END, QUIT) and immediately cease messaging. Failure to process opt-outs results in carrier filtering and potential legal liability.

P

P2P (Person-to-Person)

Text messages exchanged between two individuals using their personal mobile devices and native messaging applications. Carriers distinguish P2P from A2P traffic and apply different throughput limits, filtering rules, and pricing. See What Is A2P SMS? for the full comparison.

Payment Header

In the x402 protocol, an HTTP header (X-PAYMENT) attached to a request that contains a signed payment authorization — including the amount, token address, recipient, chain ID, and EIP-712 signature. The facilitator parses this header to validate and settle the payment before allowing access to the resource.

PEWC (Prior Express Written Consent)

A TCPA requirement for certain message categories (particularly marketing and telemarketing) mandating that the sender obtain the consumer's written agreement — including a clear disclosure of what messages will be sent — before initiating contact. PEWC can be captured via web forms, paper signatures, or electronic consent flows. See SMS Compliance: TCPA & CTIA for details.

PSTN (Public Switched Telephone Network)

The global network of circuit-switched telephone networks, including landline, cellular, and undersea cable systems. The PSTN carries traditional voice calls and SMS traffic, interconnected through SS7 signaling and increasingly migrating to IP-based architectures.

R

RCS (Rich Communication Services)

A carrier-grade messaging protocol designed to replace SMS and MMS with richer features: read receipts, typing indicators, high-resolution media, group chat, and interactive cards. RCS adoption depends on carrier and device support, and it does not yet have universal reach comparable to SMS.

S

SCP (Service Control Point)

An SS7 network element that hosts databases and service logic for advanced call features — toll-free number routing, local number portability lookups, calling-name delivery, and fraud screening. SCPs respond to queries from SSPs and STPs.

SHAFT

An acronym for content categories restricted or prohibited in A2P SMS: Sex, Hate, Alcohol, Firearms, and Tobacco (sometimes expanded to include cannabis and gambling). Carriers and The Campaign Registry flag campaigns involving SHAFT content for additional scrutiny or outright rejection. See SHAFT: Restricted SMS Categories for the full breakdown.

Short Code

A 5- or 6-digit number used for high-volume A2P SMS messaging. Short codes offer the highest throughput (100+ MPS), strong deliverability, and carrier pre-approval, but require a lengthy provisioning process (8-12 weeks) and significant monthly fees. See Short Code vs 10DLC vs Toll-Free for a detailed comparison.

SIP (Session Initiation Protocol)

A signaling protocol used to establish, modify, and terminate voice, video, and messaging sessions over IP networks. SIP is the foundation of modern VoIP and IMS-based communications, replacing legacy SS7 signaling for IP-connected endpoints.

SMPP (Short Message Peer-to-Peer)

A binary protocol used to exchange SMS messages between ESMEs (External Short Messaging Entities) and SMSCs. SMPP is the industry-standard interface for high-volume A2P messaging, providing fine-grained control over message submission, delivery receipts, and flow control. See How SMS Works for how SMPP fits into the SMS delivery pipeline.

SMS (Short Message Service)

A text messaging standard defined in the GSM specification that allows sending messages of up to 160 characters (in GSM-7 encoding) between mobile devices. SMS is universally supported across all carriers and handsets and remains the most reliable channel for reaching mobile subscribers. See How SMS Works for a technical deep-dive.

SMSC (Short Message Service Center)

The carrier-side network element responsible for storing, routing, and forwarding SMS messages. The SMSC receives messages from the sending platform (via SMPP or SS7/MAP), determines the recipient's serving carrier and current location, and delivers the message to the handset. If the recipient is unreachable, the SMSC queues the message for later retry.

SS7 (Signaling System No. 7)

A set of telephony signaling protocols used by carriers worldwide to set up calls, route SMS, manage number portability, and exchange billing information across network boundaries. SS7 operates over a dedicated signaling network separate from voice/data channels. Security vulnerabilities in SS7 have been widely documented, spurring migration toward Diameter and SIP-based signaling.

SSP (Service Switching Point)

An SS7 network element — typically a telephone switch or MSC — that originates and terminates signaling messages. SSPs query SCPs for routing decisions and relay call-setup and SMS-delivery messages through STPs.

Stablecoin

A cryptocurrency designed to maintain a stable value relative to a reference asset, typically the U.S. dollar. USDC (USD Coin) is a regulated stablecoin issued by Circle and is the payment token used in x402 transactions.

STOP Keyword

A reserved opt-out keyword that, when texted by a subscriber to an A2P sender, must immediately halt all future messaging to that number. CTIA guidelines mandate support for STOP along with UNSUBSCRIBE, CANCEL, END, and QUIT. See SMS Compliance: TCPA & CTIA for opt-out handling rules.

STP (Signal Transfer Point)

An SS7 network node that routes signaling messages between SSPs, SCPs, and other STPs. STPs act as the routers of the signaling network, performing Global Title Translation to direct queries to the correct database or switch.

T

TCAP (Transaction Capabilities Application Part)

A protocol layer in the SS7 stack that provides transaction-oriented communication between network elements. TCAP carries MAP operations (subscriber lookups, SMS delivery) and INAP (Intelligent Network Application Part) queries between SSPs and SCPs.

TCR (The Campaign Registry)

A centralized registration system for 10DLC messaging in the United States. TCR stores brand identities, campaign use cases, and message samples, and assigns trust scores that determine per-campaign throughput limits. CSPs submit registrations to TCR on behalf of message senders. See What Is 10DLC? for the full registration workflow.

TCPA (Telephone Consumer Protection Act)

A U.S. federal law enacted in 1991 that regulates telemarketing calls, pre-recorded messages, auto-dialed calls, and text messages. TCPA requires prior express consent for non-emergency calls/texts to mobile phones and prior express written consent for marketing messages. Violations carry statutory damages of $500-$1,500 per message. See SMS Compliance: TCPA & CTIA for compliance requirements.

10DLC (10-Digit Long Code)

A registration framework introduced by U.S. carriers (via TCR) that assigns trust scores and throughput limits to A2P messaging campaigns sent from standard 10-digit phone numbers. 10DLC registration requires brand verification, campaign use-case declaration, and sample messages. See What Is 10DLC? for a complete guide.

TFV (Toll-Free Verification)

A carrier-mandated verification process for A2P messaging on toll-free numbers (800, 888, 877, 866, 855, 844, 833). TFV requires businesses to submit use-case descriptions, sample messages, opt-in mechanisms, and business details. Unverified toll-free numbers are subject to aggressive carrier filtering. See Short Code vs 10DLC vs Toll-Free for comparison details.

Throughput

The rate at which messages can be sent from a given number or campaign, measured in MPS (messages per second). Throughput varies by number type, trust score, and carrier. Short codes offer the highest throughput; 10DLC throughput scales with the trust score assigned by TCR.

Toll-Free

A phone number (in the 800, 888, 877, 866, 855, 844, or 833 prefixes in NANP) where the called party, not the caller, pays for the call. In A2P SMS, toll-free numbers serve as a middle ground between long codes and short codes in terms of cost, throughput, and registration requirements. See Short Code vs 10DLC vs Toll-Free for a full comparison.

transferWithAuthorization

The EIP-3009 function on USDC (and other compliant ERC-20 tokens) that allows a spender to transfer tokens using an off-chain signature from the token holder, rather than requiring the holder to submit the transaction directly. This enables gasless payment flows — the holder signs, and a third party (such as an x402 facilitator) submits the transaction on-chain and pays the gas.

Trust Score

A numeric score (0-100) assigned by The Campaign Registry to a registered 10DLC brand based on factors such as business age, size, reputation, and registration completeness. Higher trust scores unlock higher MPS throughput limits. See What Is 10DLC? for how trust scores affect messaging capacity.

U

UCaaS (Unified Communications as a Service)

A cloud-delivered platform that bundles voice, video conferencing, team messaging, and sometimes SMS into a single subscription. UCaaS platforms (e.g., RingCentral, Zoom, Microsoft Teams) differ from CPaaS in that they target end-user productivity rather than developer APIs.

UCS-2

A 16-bit character encoding used for SMS messages that contain characters outside the GSM-7 set — including emoji, CJK characters, Arabic, and many accented letters. UCS-2 reduces the per-segment character limit to 70 (or 67 with concatenation headers), significantly increasing segment count for longer messages. See How SMS Works for encoding comparisons.

UDH (User Data Header)

A binary header prepended to the SMS payload that carries metadata for concatenated messages, including a reference number, total segment count, and current segment index. UDH consumes payload space (6 bytes per segment), reducing usable characters per segment from 160 to 153 (GSM-7) or from 70 to 67 (UCS-2).

USDC (USD Coin)

A regulated, dollar-pegged stablecoin issued by Circle and available on multiple blockchains including Ethereum, Base, and Solana. USDC is the payment token used in x402 transactions. It implements EIP-3009 (transferWithAuthorization), enabling gasless, signature-based payment flows.

V

VLR (Visitor Location Register)

A temporary database in a mobile network that stores subscriber information for devices currently roaming in the area served by a particular MSC. The VLR caches data from the HLR to reduce signaling overhead during call routing and SMS delivery.

X

x402

An open protocol that enables machine-to-machine payments for API access using the HTTP 402 status code. When a server returns 402 Payment Required, the client constructs a payment authorization (an EIP-3009 transferWithAuthorization signed via EIP-712), attaches it as a payment header, and retries the request. A facilitator validates and settles the payment on-chain (typically in USDC on Base) before releasing the API response. x402 eliminates API keys, subscriptions, and billing integrations in favor of per-request, cryptographic payment.