# SMS encoding reference

Use this reference to look up SMS character encodings, the number of
characters that fit in single and concatenated messages, how each SMS trunk
type delivers multipart messages, and how SMS fragments are billed.

## GSM-7 and UCS-2

The encoding of an SMS message determines which characters it can contain and
how many characters fit in one message.

| Encoding | Description |
| --- | --- |
| GSM-7 | The standard 7-bit SMS alphabet. It covers Latin letters, digits, and common punctuation, and is used for most SMS. |
| UCS-2 (Unicode) | A 16-bit encoding required for characters outside the GSM-7 alphabet, such as Chinese, Arabic, or Cyrillic script. UCS-2 reduces the number of characters per message. |

When you send SMS through an HTTP OUT trunk, DIDWW selects the encoding
automatically. GSM-7 is used when every character in the message is supported
by the GSM-7 alphabet. Otherwise, UCS-2 is used. Over SMPP, the `data_coding` parameter of each
message specifies the encoding. See [Data Coding Scheme (DCS)](smpp-specification.html#smpp-encoding).

Note

- A single character outside the GSM-7 alphabet, such as an emoji, makes
  the whole message use UCS-2.
- Most emoji are encoded as two 16-bit units (a surrogate pair). Each such
  emoji counts as two characters toward the UCS-2 limits.
- Some characters, such as `€`, `[`, `]`, `{`, `}`, `\`,
  `^`, `~`, and `|`, belong to the GSM-7 extension table. Each of
  them uses the space of two characters.

## Single-part and concatenated SMS limits

A message that exceeds the single-SMS limit is sent as a concatenated
(multipart) SMS. A User Data Header (UDH) is added to each part so the
recipient's device can reassemble the message. The header uses part of the
space in each fragment, so each part holds fewer characters than a single
SMS.

| Encoding | Bits per character | Characters in a single SMS | Characters per part of a concatenated SMS |
| --- | --- | --- | --- |
| GSM-7 | 7 | 160 | 153 |
| UCS-2 (Unicode) | 16 | 70 | 67 |

Up to 255 parts can be combined to form one long message.

## Reassembly by transport

How an inbound multipart message reaches your system depends on the trunk
type that delivers it.

| Trunk type | Multipart message delivery |
| --- | --- |
| HTTP IN | DIDWW reassembles concatenated messages by using the UDH before it delivers them to your endpoint. Information about fragmentation is not transmitted. The maximum size of an inbound SMS message is 64 KB. See [Technical details](http-specification.html#sms-in-spec-technical). |
| SMS to Email | DIDWW reassembles the parts and delivers the message as one email. |
| SMPP ESME | DIDWW delivers each part in a separate `deliver_sm` with a UDH in the `short_message` field. Your application must process the UDH and reassemble the parts. See [SMPP message flow](smpp-specification.html#smpp-pdu-flow). |
| SMPP SMSC | DIDWW submits each part to your SMSC in a separate `submit_sm` with a UDH in the `short_message` field. Your SMSC must process the UDH and reassemble the parts. See [SMPP message flow](smpp-specification.html#smpp-pdu-flow). |

## Fragment billing

SMS billing is based on fragments, not on the message as submitted. When a
long message is split into several fragments, each fragment is billed
separately. Inbound SMS, outbound P2P SMS, and outbound A2P SMS are all
charged per message fragment.

The number of fragments depends on the message length and its encoding. The
following examples show how messages of different lengths are split and
billed:

| Example message | Encoding | Fragments (characters per fragment) | Billed as |
| --- | --- | --- | --- |
| 160 GSM-7 basic-alphabet characters | GSM-7 | 1 | 1 SMS |
| 200 GSM-7 basic-alphabet characters | GSM-7 | 2 (153 + 47) | 2 SMS |
| 70 Cyrillic characters | UCS-2 | 1 | 1 SMS |
| 100 Cyrillic characters | UCS-2 | 2 (67 + 33) | 2 SMS |

To review the fragments of sent messages, see the **Billed Fragments** and
**Charged (USD)** fields in the [Outbound SMS logs](../../../logs-analytics/sms-logs/outbound-logs.html#outbound-logs),
or
the `fragments_sent` attribute of the [status
callback](../../outbound-sms-api/status-callbacks-and-delivery-receipts.html#sms-out-spec-callbacks). For current rates,
see [Download
pricelists](../../../phone-numbers/buy-numbers/download-pricelists.html#services-coverage-pricing-access-pricelists).

## Related resources

- [Data Coding Scheme (DCS)](smpp-specification.html#smpp-encoding) - Look up the SMPP
  `data_coding` values and their character limits.
- [Concatenated messages over SMPP](smpp-specification.html#smpp-concatenated-messages) -
  Review how SMPP splits long messages into parts.
- [Outbound SMS API reference](../../outbound-sms-api/outbound-sms-api-reference.html#outbound-sms-api-reference) - Review the
  `content` attribute and other request attributes.
- [Outbound SMS logs](../../../logs-analytics/sms-logs/outbound-logs.html#outbound-logs) - Review billed fragments and
  charges for sent messages.
- [DIDWW pricing](../../../billing/billing-and-prices/didww-pricing.html) - Review
  how SMS services are priced and billed.