How to calculate the SMPP SM Length field

In the SMPP protocol, the sm_length field specifies the length in bytes of the short message content contained in the short_message field of PDUs such as submit_sm, deliver_sm, and data_sm.

Field Details

  • Field Name: sm_length
  • Size: 1 byte (unsigned integer)
  • Range: 0–254 (maximum allowed by SMPP 3.4)
  • Location: Directly before the short_message field
  • Purpose: Indicates the number of bytes in the short_message payload

Key Behavior

The value of sm_length depends on the encoding used:

EncodingCharacters per SMSByte SizeNotes
GSM 7-bit Up to 160 Up to 140 Packed bits; actual byte size depends on content
UCS2 Up to 70 2 × number of characters Each character is 2 bytes
8-bit binary N/A Up to 140 Raw byte length

Example: ASCII Message

A simple message using default 7-bit encoding:

Text: "Hello"
Encoding: GSM 7-bit
Packed Bytes (Hex): C8 32 9B FD 06
Byte Length: 5
sm_length: 5

Example: UCS2 Encoded Message

A Unicode message using UCS2 encoding (data_coding = 0x08):

Text: "Hi"
UCS2 Hex: 0048 0069
Byte Array: 00 48 00 69
Byte Length: 4
sm_length: 4

SMPP PDU Example (submit_sm)

0000003B  // Command Length (59 bytes)
00000004  // Command ID: submit_sm
00000000  // Command Status
00000001  // Sequence Number
00        // service_type
01        // source_addr_ton
01        // source_addr_npi
31323334  // source_addr ("1234")
00
01        // dest_addr_ton
01        // dest_addr_npi
35363738  // destination_addr ("5678")
00
00        // esm_class
00        // protocol_id
00        // priority_flag
00        // schedule_delivery_time
00        // validity_period
00        // registered_delivery
00        // replace_if_present_flag
08        // data_coding: UCS2
00        // sm_default_msg_id
04        // sm_length: 4
00480069  // short_message: "Hi" in UCS2

Important Notes

  • sm_length refers to byte length, not character count.
  • If message content is split across multiple messages (concatenation), each part has its own sm_length.
  • Do not confuse sm_length with the total message length (PDU length).

Message Splitting & Concatenation

When a message exceeds the maximum payload size, it is split into parts using a UDH (User Data Header). The sm_length in such cases includes the UDH as well: 

Example: UDH (6 bytes) + 67 UCS2 bytes = 73
sm_length = 73

Conclusion

The sm_length field is critical in SMPP messaging as it defines how many bytes the SMSC should read for the actual message body. Understanding how encoding affects this field ensures accurate and successful message delivery.

References

  • SMPP 3.4 Specification
  • GSM 03.38 Character Set
  • Unicode UCS2 Encoding

More information