User Data Header

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User Data Header (UDH) is a binary structure which may be present at the start of a short message in the Short Message Service in GSM. It does not contain any text, but it specifies how the message should be formatted and processed.

UDH can be used to form Concatenated SMS, to enrich the content of the message with colors, text formatting, small pictures and animations and simple music which are used in Enhanced Messaging Service (EMS), to convey port numbers which may cause start of an application in the mobile phone, which is used for Multimedia Messaging Service (MMS) or to indicate using of national language shift tables.[1]

UDH is defined in 3GPP 23.040 (originally GSM 03.40).[2]

Technical Realization[edit]

Presence of User Data Header is indicated by the TP-UDHI (Transfer Layer Protocol User Data Header Indicator) bit - 6th bit of the first octet of the GSM 03.40 or 3GPP 23.040 message.

If UDH is present, it is at the beginning of the TP-UD (TP-User Data) field and it always starts with an UDHL (UDH Length) octet. The rest of UDH is a sequence of Tag-Length-Value triplets. The Tag is named Information-Element-Identifier (IEI) and is always 1 octet long, the Length octet is always 1 octet long and it specifies the length of the Information Element Data.

UDH uses space intended for the message payload. The longer the UDH, the less space remains for the message text or other contents. When TP-UD contains seven-bit data, the length of TP-UD is given in septets (but UDHL is still in octets, so length in septets is (UDHL + 1) * 8 / 7) and the UDH is filled to a septet boundary with 0-6 zero bits. The next table shows how a TP-UD of a message in the GSM 7 bit default alphabet containing text 'Hi' with a predefined sound 'Chord high' will be encoded:

TP-UD Octet 1 2 3 4 5 6 7
Value (hex) 04 0B 02 00 08 20 D3
Meaning UDH Length IEI: Predefined sound IE Length Position Chord high 6 lower bits of the 1st char + 2 fill bits 7 lower bits of the 2nd char + 1 upper bit of the first char

The total message length TP-UDL will be 8. (UDH has 8 * (1 + 4) = 40 bits, 2 bits is the padding as (40 + 2) / 7 = 6 without remainder, 2 * 7 = 14 bits for text; total TP-UD length is 40 + 2 + 14 = 56 bits = 7 octets = 8 septets.)

UDH Information Elements[edit]

IEI (hex) Meaning Classification Length May repeat
00 Concatenated short messages, 8-bit reference number SMS Control 3 no
01 Special SMS Message Indication SMS Control 2 yes
02 Reserved N/A N/A yes
03 Not used to avoid misinterpretation as <LF> character N/A N/A yes
04 Application port addressing scheme, 8 bit address SMS Control 2 no
05 Application port addressing scheme, 16 bit address SMS Control 4 no
06 SMSC Control Parameters SMS Control 1 no
07 UDH Source Indicator SMS Control 1 yes
08 Concatenated short message, 16-bit reference number SMS Control 4 no
09 Wireless Control Message Protocol SMS Control 1-255 yes
0A Text Formatting EMS Control 3-4 yes
0B Predefined Sound EMS Content 2 yes
0C User Defined Sound (iMelody max 128 bytes) EMS Content 2-129 yes
0D Predefined Animation EMS Content 2 yes
0E Large Animation (16*16 times 4 = 32*4 =128 bytes) EMS Content 129 yes
0F Small Animation (8*8 times 4 = 8*4 =32 bytes) EMS Content 33 yes
10 Large Picture (32*32 = 128 bytes) EMS Content 129 yes
11 Small Picture (16*16 = 32 bytes) EMS Content 33 yes
12 Variable Picture EMS Content 4-255 yes
13 User prompt indicator EMS Control 1 yes
14 Extended Object EMS Content 7-255 yes
15 Reused Extended Object EMS Control 3 yes
16 Compression Control EMS Control 3-255 no
17 Object Distribution Indicator EMS Control 2 yes
18 Standard WVG object EMS Content 1-255 yes
19 Character Size WVG object EMS Content 1-255 yes
1A Extended Object Data Request Command EMS Control 0-255 no
1B Reserved for future EMS features N/A 0-255 yes
1C Reserved for future EMS features N/A 0-255 yes
1D Reserved for future EMS features N/A 0-255 yes
1E Reserved for future EMS features N/A 0-255 yes
1F Reserved for future EMS features N/A 0-255 yes
20 RFC 822 E-Mail Header SMS Control 1 no
21 Hyperlink format element SMS Control 0-255 yes
22 Reply Address Element SMS Control 1-255 no
23 Enhanced Voice Mail Information SMS Control 0-255 no
24 National Language Single Shift SMS Control 1 no
25 National Language Locking Shift SMS Control 1 no
26 – 6F Reserved for future use N/A 0-255 N/A
70 – 7F (U)SIM Toolkit Security Headers SMS Control 0-255 ?
80 – 9F SME to SME specific use SMS Control 0-255 ?
A0 – BF Reserved for future use N/A 0-255 ?
C0 – DF SC specific use SMS Control 0-255 ?
E0 – FF Reserved for future use N/A 0-255 ?

Use in Other Protocols[edit]

The UDH is used in other protocols for short message transport - Short Message Peer-to-Peer (SMPP), Universal Computer Protocol (UCP) and in WEMT teleservice in CDMA2000.

See also[edit]


  1. ^ "Alphabets and language-specific information". Retrieved 2021-07-19.{{cite web}}: CS1 maint: url-status (link)
  2. ^ 3rd Generation Partnership Project; Technical realization of the Short Message Service (SMS)