This document provides a simplified introduction to the encoding rules used by the 3GPP RRC protocol. It discusses how NULL, BOOLEAN, INTEGER, BIT STRING, OCTET STRING, ENUMERATED, CHOICE, SEQUENCE, and SEQUENCE OF types are encoded using the Packed Encoding Rules (PER) with some examples. It also covers how extension markers, version brackets, and length determinants are used in the encoding.

1. Quick 3GPP RRC ASN.1
Seokseong Jeon

2. DISCLAIMER
• This is a simplified introduction to encoding rule used by 3GPP RRC protocol.
The most practical examples are captured and not every single rule is treated.

3. Encoding Rule
• 3GPP uses Unaligned variant of Packed Encoding Rule (PER)
• In unaligned variant, each field is not aligned at octet boundary
• Therefore, zero-pad is used only at the end of message
• PER is insensitive to field name
• Both are considered to be same
• Reference: ITU-T X.691
sequenceType SEQUENCE{
apple APPLE
}
sequenceType SEQUENCE{
fruit APPLE
}

4. NULL, BOOLEAN and INTEGER
• NULL is zero-length type
• BOOLEAN is 1-bit type
• 0 for FALSE
• 1 for TRUE
• Constrained INTEGER
• Value range (v.r): 33 – (-30) + 1 = 64
• Field size: ⌈log2(v.r)⌉ = 8 bits
• Offset (lower bound): -30
intergerField INTEGER(-30..33)
00000011 = -27

5. BIT STRING and OCTET STRING
• Length is indicated, if required
apple BIT STRING 00000100 AAAA = 4-bit long and content follows
apple BIT STRING (SIZE(4..11)) 00000000 AAAA = 4-bit long and content follows
apple BIT STRING (SIZE(4)) AAAA
apple OCTET STRING 00000001 AAAAAAAA = 1-octet long and content follows
apple OCTET STRING (SIZE(1..8)) 00000000 AAAAAAAA = 1-octet long and content follows
apple OCTET STRING (SIZE(1)) AAAAAAAA

6. ENUMERATED and CHOICE
• Enumeration index to choose item
• Field size: ⌈log2(3)⌉ = 2
• Choice index to choose item
• Choice index field size: ⌈log2(3)⌉ = 2
enumType ENUMERATED{apple, banana, cherry}
choiceType CHOICE{
apple APPLE,
banana BANANA,
cherry CHERRY
}
10 = cherry
10 CCCC = content of cherry

7. ENUMERATED and CHOICE with Extension Marker (EM)
• Extension bit at the beginning
enumType ENUMERATED{apple, ..., cherry}
choiceType CHOICE{
apple APPLE,
...,
cherry CHERRY
}
1 0 = cherry
1 0 CCCC = content of cherry

8. SEQUENCE
• No OPTIONAL/DEFAULT fields is in definition
• Just list all content in order
• One or more OPTIONAL/DEFAULT fields in definition
• Optional bitmap indicates each OPTIONAL/DEFAULT field
• Optional bitmap size: # of OPTIONAL/DEFAULT fields = 2
sequenceType SEQUENCE{
apple APPLE OPTIONAL,
banana BANANA,
cherry CHERRY OPTIONAL
}
10 AAAA BBBB = content of apple and banana

9. SEQUENCE with EM
• Extension bit at the beginning
• Additional bitmap at the end indicates how many fields are after EM
• Bitmap shall indicate even for a mandatory field
• Length of bitmap is also indicated
• Content is included after EM. Its length (in octets) precedes
sequenceType SEQUENCE{
apple APPLE OPTIONAL,
...,
cherry CHERRY,
durian DURAIN OPTIONAL
}
1 1 AAAA 00000010 11 00000001 CCCCCCCC 00000001 DDDDDDDD
= content of apple, cherry and durian

10. SEQUENCE with Version Bracket (VB)
• The same rule of EM for SEQUENCE applies
• Each VB is considered as a single SEQUENCE
sequenceType SEQUENCE{
apple APPLE OPTIONAL,
...,
[[
cherry CHERRY,
durian DURAIN OPTIONAL
]]
}
1 1 AAAA 00000001 1 00000001 0 CCCC = content of apple and cherry

11. SEQUENCE OF
• Iteration count is indicated
apple SEQUENCE (SIZE(1..8)) OF APPLE
00000011 AAA1 AAA2 AAA3 AAA4 = 4 APPLEs

12. Length Determinant
• <= 127
• <=16383
• >= 16384
0 0001010 = 10
1 0 001010 = 2561
00000001
1 0 000100 = four 16384 (bits/octets/…)
(1 to 4 units are supported)
AAAAAAAA = content follows
LLLLLLLL = another length determinant