SMS Terminology & Glossary: The Meaning of Commonly Used Words
When you start working with business SMS, you’ll run into a lot of technical words and may feel overwhelming.
This guide explains every important SMS term in simple, plain language so you always know what you’re dealing with.
The basic terminology of SMS
Before we get into the technical terms, let’s start with the basics.
SMS (Short Message Service) – The standard way of sending short text messages between mobile phones or application to phones. SMS has been around since the early 1990s and is still one of the most reliable ways to connect with customers. The recipient doesn’t need an internet connection or any app or high-end model phone to receive SMS.
Character Limit – A standard SMS can carry up to 160 characters when written in English. If you add an emoji, a special symbol, or text in a regional language like Hindi or Telugu, the limit drops to 70 characters per SMS. Messages longer than these limits are automatically split into parts and each part is charged separately.
Concatenated SMS – When a message is longer than the character limit, it gets split into multiple parts. These parts are joined back together on the recipient’s phone, so it looks like one long message to them. However, each part is sent and billed separately. These are called concatenated SMS.
GSM-7 Encoding – The standard character format used for SMS. It supports English letters, numbers, and basic punctuation. Messages using GSM-7 can be up to 160 characters long. If your message includes emojis, special characters, or regional language text, it automatically switches to Unicode, which reduces the limit to 70 characters.
Unicode / UCS-2 Encoding – A character format that supports almost every language and symbol in the world, including emojis. It’s per-message limit is 70 characters.
Infrastructure SMS terminology
These are the systems that carry your messages from SMS gateway to customer’s phone.
SMS Gateway – When your app or platform wants to send a text message, it goes through the SMS gateway first. The gateway takes care of routing and operator connections.
SMSC (Short Message Service Centre) – The carrier’s internal server that stores and forwards SMS messages. When you send a text, it doesn’t go directly to the recipient’s phone – it passes through the SMSC first. If the recipient’s phone is off or unavailable, the SMSC holds the message and retries delivery until the validity period expires (usually 24–72 hours).
SMPP (Short Message Peer-to-Peer) – A protocol used for fast, direct connections between SMS platforms and mobile networks. Most large-scale SMS gateways use SMPP because it handles very high message volumes quickly.
API (Application Programming Interface) – Think of an API like a waiter in a restaurant. You place order, the waiter takes it to the kitchen, and brings back your food. In the same way, an SMS API (given by the SMS provider) takes the message from your software, carries it to the SMS gateway, and gets it delivered to the customer’s phone.
Aggregator – A company that has direct connections with major telecom operators and provides SMS services to businesses. When you sign up with a provider like Digimiles, you are working with an aggregator – so you don’t need to deal with Airtel, Jio, Vi, and BSNL separately. We handle all of that for you.
SS7 (Signalling System No. 7) – The telecom protocol that mobile networks use to route SMS messages. You never deal with SS7 directly – your SMS gateway runs through it background.
Types of SMS messages
Not all SMS messages are the same. Depending on what you’re sending – a marketing offer or an OTP – there are different rules when it can be sent and who it can be sent to.
Transactional SMS – Messages that are sent in response to an event triggered by the customer – OTPs, order confirmations, payment alerts, booking reminders, account notifications, etc. These are informational, not promotional. In India, transactional SMS can be sent at any time with 6-alphabetic sender ID, including to DND numbers.
Promotional SMS – Marketing messages sent to customers like sale offers, new product announcements, discount codes, event invitations, etc. In India, promotional SMS can only be sent between 9 AM and 9 PM. They cannot be sent to numbers registered on the DND list.
OTP SMS (One-Time Password) – A temporary 4 or 6 digit code sent to a user’s phone to verify their identity. It is valid for a few minutes and can only be used once, which is why it’s called a one-time password. It falls under transactional SMS.
Bulk SMS – Sending the same message or a personalized version of it to a list of contacts at once using SMS gateway like Digimiles.
Flash SMS – A message that appears as a pop-up directly on the recipient’s screen, without saving to their inbox. Flash SMS is sometimes used for urgent alerts by the Government.
Two-Way SMS – SMS where the recipient can reply and your system receives that response. This is useful for appointment confirmations, surveys, feedback collection, or letting customers reach your support team. You need a dedicated long code or short code to enable this.
Sender ID and routing terminology
This section covers what the recipient sees as the sender name, and how your messages are directed to the right network.
Sender ID (Sender Name / Header) – The name or number that appears as the sender when someone receives a transactional message. This is usually a 6-letter code – for example, DMILES or HDFCBK. For promotional SMS, it is usually a random 6-digit number. Sender IDs must be registered on the DLT platform before you can use them.
Short Code – A 5/6 digit number used to receive SMS. They are commonly used for campaigns where customers can reply to the messages sent by the business. Example: Text OFFER to 56789 to get 20% off your next order.
Long Code (Virtual Mobile Number / VMN) – A regular 10-digit mobile number used to receive SMS. Example: A customer texts HELP to 9876543210 and your system automatically replies with your support details or connects them to your team
Grey Route – An unofficial path used to route SMS messages, often used by very cheap providers to cut costs. Grey route messages skip proper carrier agreements and regulatory requirements, which means they are unreliable, often blocked, and can get you into legal trouble. If a provider’s pricing seems unusually low, grey routing is usually the reason.
Direct Carrier Connection – A formal, official connection between an SMS provider and a mobile network operator. Messages sent through direct connections are faster, more reliable, and fully compliant.
Delivery, reporting, and status terminology
Once a message is sent, these terms tell you whether it reached the recipient and what happened along the way.
DLR (Delivery Receipt / Delivery Report) – A confirmation sent back to your platform once a message has been delivered to the recipient’s phone. A DLR showing “delivered” means the message reached the device. A “failed” or “undelivered” status means something went wrong, and the error code helps identify where the issue occurred.
Delivery Rate – It is the percentage of messages that were successfully delivered out of the total you sent. A good bulk SMS provider should deliver above 90% of messages in most cases. However, this rate also depends on invalid numbers, carrier filtering, or network issues.
Latency – In simple, it’s the time take for an SMS to reach the recipient’s phone. A reliable gateway typically delivers within 5 to 7 seconds for most routes in India. Slow delivery is usually caused by network congestion or too many systems in the routing path.
Validity Period – How long the SMSC will keep trying to deliver an SMS if the recipient’s phone is switched off or unavailable. The default is usually 24 to 72 hours. After this period, the message will be failed.
TPS (Transactions Per Second) – How many messages a gateway can send per second. Good gateways support atleast 10,000 SMS per second. Low throughput can cause delays, especially during peak hours.
Opt-Out / Unsubscribe – When a customers asks to stop receiving messages, usually by replying STOP or writing an email to the business.
Compliance and regulation terms (India)
India has clear rules around business SMS, set by TRAI. If you’re sending SMS to any Indian mobile numbers, these are the terms you need to be familiar with.
TRAI (Telecom Regulatory Authority of India) – The government body that regulates the Indian telecom industry, including commercial SMS. Overall, TRAI sets all the rules for telecom operators as well as sms providers. They must comply with TRAI’s Telecom Commercial Communications Customer Preference Regulations (TCCCPR).
DLT (Distributed Ledger Technology) – Before sending any business SMS, you must register your company, your sender IDs, and your message templates on a DLT platform (like Vodafone, Jio). Any message that does not match a registered template will be blocked by the network operator. A good SMS provider will walk you through the DLT registration process.
DND (Do Not Disturb) – A registry maintained by TRAI where mobile users can register to block unsolicited commercial messages. If a number is on the DND list, promotional SMS cannot be delivered to it. However, Transactional SMS messages are exempt from DND restrictions.
SMS Template – It is nothing but the SMS content. It must be approved on the DLT portal in order to send to customers. If the SMS content you send is different from the one you register on DLT portal, then carriers will block it.
PE ID (Principal Entity ID) – Think of it as your business’s identity number in the DLT portal. You get one PE ID per company, and it’s linked to all your registered sender IDs and templates. Without a PE ID, you cannot send bulk SMS in India.
Scrubbing – The process of cleaning your contact list before sending a campaign – removing DND-registered numbers, invalid numbers, duplicates, and contacts who have opted out.
Quick reference table
Here is a summary of every term covered in this guide, all in one place.
| Term | What it means (in simple way) |
|---|---|
| SMS | Short text message sent over mobile networks, no internet needed |
| Character limit | 160 (GSM-7) or 70 (Unicode) characters per SMS part |
| Concatenated SMS | Long message split into multiple parts, stitched together on delivery |
| GSM-7 | Standard encoding for English text – 160 chars per SMS |
| Unicode | Encoding for emojis and non-Latin scripts – 70 chars per SMS |
| SMS Gateway | Platform connecting your software to mobile networks |
| SMSC | Carrier’s server that stores and forwards messages |
| SMPP | Protocol for high-speed, direct carrier connections |
| HTTP API | Standard web-based way to connect to an SMS gateway |
| Aggregator | Company with carrier connections that resells SMS capacity |
| Transactional SMS | Triggered messages – OTPs, alerts, confirmations |
| Promotional SMS | Marketing messages – offers, campaigns, announcements |
| OTP SMS | One-time password for authentication |
| Bulk SMS | Sending messages to a large list at once |
| Flash SMS | Pop-up message that appears directly on screen, not saved in inbox |
| Two-Way SMS | SMS where recipients can reply and you receive the response |
| Sender ID | The name/number the recipient sees as the sender |
| Short Code | 5–6 digit number used for business SMS |
| Long Code / VMN | Standard 10-digit number for conversational SMS |
| Grey Route | Unofficial routing path – cheap but unreliable and non-compliant |
| DLR | Delivery receipt – tells you if a message was delivered or failed |
| Delivery Rate | Percentage of messages successfully delivered |
| Latency | Time between sending and receiving a message |
| Validity Period | How long SMSC retries an undelivered message |
| Throughput / TPS | Messages a gateway can send per second |
| TRAI | India’s telecom regulator |
| DLT | India’s blockchain registry for commercial SMS compliance |
| DND | Do Not Disturb – blocks promotional messages to opted-out numbers |
| Message Template | Pre-approved message format required for DLT compliance |
| PE ID | Your business’s unique ID on the DLT platform |
| Scrubbing | Cleaning your list by removing DND, invalid, and opted-out numbers |
