4G/LTE - Protocol Stack

 

 

 

Full Stack

 

Now that we covered each layer of LTE protocol stack, it is time to combine all these layers to a complete protocol stack. The question now is what each message needs from every layer on its way to the air. Which logical, transport and physical channel does it use? Does it get a MAC header, which RLC mode does it use, and does PDCP handle it? The sections below answer this for the attach sequence, for the downlink and uplink channel maps, and for one decoded log from a live network.

Overall Sequence and Layer Configuration

Try to follow the sequence below and combine all the knowledges you have and try to make practical sense out of it. This kind of integration is something not explanined in 3GPP specification and you have to figure out on your own.

Couple of bullet points for you to follow up are as follows :

  • i) Which of the sequence requires normal MAC header and which of them does not use any MAC header ?
  • ii) What is the practical meaning of "No MAC Header" and "Normal MAC Header" ?
  • iii) Which process use RLC TM mode ? Which Process use RLC UM mode and Which process use RLC AM mode ?
  • iv) Can you explain the detailed RLC layer behavior of each processes which is using RLC layer ?
  • v) Which process use PDCP and which does not ?

 

Step

Direction

Channel

Message

1

UE <---- NW

BCCH -> BCH,

No MAC Header,RLC TM, No PDCP

MIB

2

UE <---- NW

BCCH -> DL SCH,

No MAC Header,RLC TM, No PDCP

SIBs

3

UE ----> NW

PRACH -> RACH,

No MAC Header,No RLC, No PDCP

PRACH Preamble

4

UE <---- NW

RACH -> DL SCH,

No MAC Header,No RLC, No PDCP

RACH Response

5

UE ----> NW

UL SCH -> UL CCCH

MAC Header,No RLC, No PDCP

RRC Connection Request

6

UE <---- NW

DL CCCH -> DL SCH

MAC Header,No RLC, No PDCP

RRC Connection Setup

7

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : RRC Connection Setup Complete +

NAS : Attach Request (ESM:PDN Connectivity Request)

8

UE <---- NW

DL DCCH -> DL SCH

MAC Header,RLC AM, PDCP

RRC : dlInformationTransfer

EMM : Authentication Request

9

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : ulInformationTransfer

EMM : Authentication Response

10

UE <---- NW

DL DCCH -> DL SCH

MAC Header,RLC AM, PDCP

RRC : dlInformationTransfer

EMM : Security Mode Command

11

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : ulInformationTransfer

EMM : Security Mode Complete

12

UE <---- NW

DL DCCH -> DL SCH

MAC Header,RLC AM, PDCP

RRC : Security Mode Command

13

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : Security Mode Complete

14

UE <---- NW

DL DCCH -> DL SCH

MAC Header,RLC AM, PDCP

RRC : RRC Connection Reconfiguration

EMM : Attach Accept

ESM : Activate Default EPS Bearer Context Request

15

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : RRC Connection Reconfiguration Complete

16

UE ----> NW

UL SCH -> UL DCCH

MAC Header,RLC AM, PDCP

RRC : ulDirectTransfer

EMM : Attach Complete

ESM : Activate Default EPS Bearer Context Accept

17

UE <---- NW

DL DCCH -> DL SCH

MAC Header,RLC AM, PDCP

RRC : RRC Connection Release

 

NOTE : You can double check on these configuration in 36.331 summarized in SRB page.

Let's go through the five questions with the table above. Two rules from the specifications decide most of the answers. First, MAC leaves out its header for PCH, BCH and BCCH on DL-SCH, which 36.321 clause 6.1.4 calls transparent MAC. Second, PDCP serves only DCCH and DTCH in this sequence, as 36.323 clause 4.4 says.

  • No MAC header : the MIB on BCH and the SIBs on DL-SCH use transparent MAC, so the transport block holds only the RRC message. The preamble in step 3 is not a MAC PDU at all. The RAR in step 4 is a special case. Its MAC PDU has its own header of E/T/RAPID subheaders followed by MAC RARs, so No MAC Header in the table means that it has no normal MAC header.
  • Normal MAC header : from the RRC Connection Request on, every MAC PDU starts with subheaders. The LCID in each subheader tells the receiver which logical channel or MAC CE follows. So one transport block can carry an RRC message together with a BSR or a PHR, as the live log at the end of this page shows.
  • RLC TM, UM and AM : TM serves BCCH, PCCH and CCCH, so the MIB, the SIBs and the SRB0 messages use TM. TM adds no RLC header, which is why steps 5 and 6 say No RLC. AM serves SRB1, SRB2 and the DRBs that need retransmission. UM serves the DRBs that do not.
  • RLC AM behaviour : AM segments a large RRC message into several AMD PDUs, and the receiver answers a poll with a STATUS PDU. In the live log, the RRC Connection Setup Complete goes out in AMD PDUs with SN 0 to SN 9, and the eNB answers with ACK_SN 10.
  • PDCP : only steps 7 to 17 use PDCP, because only they run on DCCH. Steps 1 to 6 use BCCH, the random access procedure or CCCH, and none of them has a PDCP header.

Downlink Channel Map

The diagram you saw above a kind of message flow(event diagram) in time sequence. The diagram shown below is not a time based, but it shows the channel mapping (or data flow across the full protocol stack). Pick one of the message from the diagram shown above and try to find right route for this digram and see how much details you can add.

For example, if you picked the message "RRC Connection Setup", the start point would be "RRC Message msg4".

LTE downlink channel map from physical channels through transport and logical channels to radio bearers

Read the diagram above from the bottom. PBCH carries only BCH, and BCH carries only the MIB. PDSCH carries both PCH and DL-SCH. DL-SCH then carries every other logical channel: BCCH for the SIBs, CCCH, DCCH and DTCH. PDCCH has no transport channel above it, because it carries DCI rather than data. The UE finds each PDSCH transmission through a PDCCH with the matching RNTI, which the RNTI column of the table below lists.

Following is a tabular presentation of DL Channel Map. (LCID and TrCH Number would be different depending on the network or Network Simulator)

 

RB

Lo CH

PDCP

RLC

Lo CH

LCID

MAC Hdr

HARQ

RNTI

Tr CH

 

PCCH

 

TM

PCCH

N/A

NONE

NONE

NONE

PCH

 

BCCH 0

 

TM

BCCH 0

N/A

NONE

NONE

NONE

BCH 0

 

BCCH 1

 

TM

BCCH 1

N/A

NONE

Broadcast

SI RNTI

DL SCH 0

 

RA_RES

 

TM

RA_RES

N/A

NONE

NONE

RA RNTI

DL SCH 1

SRB0

DL CCCH

NONE

TM

DL CCCH

0

NORMAL

NORMAL

T-CRNTI

DL SCH 1

SRB1

DL DCCH 0

USED

AM

DL DCCH 0

1

NORMAL

NORMAL

CRNTI

DL SCH 1

SRB2

DL DCCH 1

USED

AM

DL DCCH 0

2

NORMAL

NORMAL

CRNTI

DL SCH 1

DRB 0

DL DTCH0

USED

 UM/AM

DL DTCH0

3

NORMAL

NORMAL

CRNTI

DL SCH 1

DRB 1

DL DTCH0

USED

 UM/AM

DL DTCH0

4

NORMAL

NORMAL

CRNTI

DL SCH 1

DRB 2

DL DTCH0

USED

 UM/AM

DL DTCH0

5

NORMAL

NORMAL

CRNTI

DL SCH 1

Uplink Channel Map

The uplink map is simpler than the downlink map. The UE never sends broadcast or paging, so only two transport channels remain. RACH carries the preamble, and UL-SCH carries every logical channel from CCCH to DTCH. The diagram below shows the same radio bearers as the downlink map, on the uplink channels.

LTE uplink channel map from PRACH, PUSCH and PUCCH through RACH and UL-SCH to logical channels and radio bearers

PUCCH, like PDCCH in the downlink, has no transport channel above it. It carries UCI only, which is HARQ ACK/NACK, SR and CSI. RACH has no logical channel above it either, so the preamble goes through no MAC, RLC or PDCP processing. The first uplink MAC PDU is the RRC Connection Request on CCCH. The UE sends it on PUSCH, with the grant from the RAR.

Following is a tabular presentation of UL Channel Map. (LCID and TrCH Number would be different depending on the network or Network Simulator)

 

RB

Lo CH

PDCP

RLC

Lo CH

LCID

MAC Hdr

HARQ

RNTI

Tr CH

 

RA_PRE

 

TM

RA_PRE

N/A

NONE

NONE

NONE

UL SCH 0

SRB0

UL CCCH

NONE

TM

UL CCCH

0

NORMAL

NORMAL

T-CRNTI

UL SCH 0

SRB1

UL DCCH 0

USED

AM

UL DCCH 0

1

NORMAL

NORMAL

CRNTI

UL SCH 0

SRB2

UL DCCH 1

USED

AM

UL DCCH 0

2

NORMAL

NORMAL

CRNTI

UL SCH 0

DRB 0

UL DTCH0

USED

 UM/AM

UL DTCH0

3

NORMAL

NORMAL

CRNTI

UL SCH 0

DRB 1

UL DTCH0

USED

 UM/AM

UL DTCH0

4

NORMAL

NORMAL

CRNTI

UL SCH 0

DRB 2

UL DTCH0

USED

 UM/AM

UL DTCH0

5

NORMAL

NORMAL

CRNTI

UL SCH 0

 

The two maps and their tables lead to a few rules that hold for every LTE call.

  • DL-SCH and UL-SCH carry almost everything : only the MIB on BCH, paging on PCH and the preamble on RACH use other transport channels.
  • PDCCH and PUCCH carry no transport channel : they carry DCI and UCI, the control information for the shared channels.
  • SRB0 has no PDCP : CCCH uses RLC TM, and 36.323 clause 4.4 keeps PDCP to DCCH and DTCH.
  • LCID 0 is CCCH : LCID 1 and 2 are SRB1 and SRB2, and the DRBs start from LCID 3.

Channel Mapping Table throughout Call Processing

This is only an example case and Mapping (especiall LoCH No) can vary depending on situations. The point is that it will be really helpful for your troubleshooting or test case creation if you create this kind of table for your case.

 

Message

RB

Lo CH

LoCH No

LCID

MIB

-

BCCH

0

-

SIB 1

-

BCCH

1

-

SIB 2

-

BCCH

1

-

RRC : PRACH Preamble

-

-

-

-

RRC : RACH Response

-

-

-

-

RRC : RRC Connection Request

SRB0

UL CCCH

0

0

RRC : RRC Connection Setup

SRB0

DL CCCH

0

0

RRC : RRC Connection Setup Complete + NAS : Attach Request + ESM : PDN Connectivity Request

SRB1

UL DCCH

0

1

RRC : DL Information Transfer + NAS : Authentication Request

SRB1

DL DCCH

0

1

RRC : UL Information Transfer + NAS : Authentication Response

SRB1

UL DCCH

0

1

RRC : DL Information Transfer + NAS : Security Mode Command

SRB1

DL DCCH

0

1

RRC : UL Information Transfer + NAS : Security Mode Complete

SRB1

UL DCCH

0

1

RRC : Security Mode Command

SRB1

DL DCCH

0

1

RRC : Security Mode Complete

SRB1

UL DCCH

0

1

RRC : RRC Connection Reconfiguration

SRB1

DL DCCH

0

1

RRC : RRC Connection Reconfiguration Complete

SRB1

UL DCCH

0

1

RRC : UL InformationTransfer + NAS : Attach Complete + NAS : Activate Default EPS Bearer 

SRB2

UL DCCH

1

2

RRC : UL Information Transfer + ESM : PDN Connectivity Request

SRB2

UL DCCH

1

2

 

Note : Refer to TS 36.331 - 9.1.1 Logical channel configurations

Notice where SRB2 appears in the table above. SRB1 carries RRC messages and the NAS messages piggybacked on them, and it carries NAS messages alone until SRB2 exists. E-UTRAN configures SRB2 only after security activation, in the RRC Connection Reconfiguration of the attach. The Attach Complete is therefore the first message that can use SRB2. 36.331 v19.3.0 clause 4.2.2 also gives SRB2 a lower priority than SRB1.

The LCID values of SRB1 and SRB2, 1 and 2, are the default SRB configurations of 36.331 clause 9.1.2. The LoCH numbers and the DRB LCIDs are the choices of the network or the network simulator, so expect other values in your own logs.

  • SRB0 for connection setup : RRC Connection Request and RRC Connection Setup, on CCCH with LCID 0.
  • SRB1 until security is active : all RRC messages, and NAS messages wrapped in DL and UL Information Transfer.
  • SRB2 after security activation : NAS messages with a lower priority than SRB1.

Live Network Examples in Full Stack

This can a good example in which you can practice on very details of MAC to PDCP in detail. This is a segment of logs between a UE and a live network, meaning it shows what's happening in real network. I captured a log using Intellijudge and Wavejudge from SanJole and reformated so that it is easier to read. (First understand the details of RACH, MAC, PDCP in detail especially about the data structure of each layer and then revisit this and see how much you can make sense out of this).

Decoded log between a UE and a live network, captured. Each shaded row gives the SFN and subframe and the event, and the row under it holds the decode. Field values are from a live capture, not from the specification.

 

SFN : 402.4

RACH Preamble

 

 
    RNTI = None
    Timing Offset = 2
    Logical Root = 219
    Preamble index = 33
    NC Configuration = 12
    Set Type = Unrestricted
    Logical Root = 215
    Preamble Format = 0
    RbStart = 2

SFN : 402.8

MAC RA Response

 

 
MAC : 61 00 B0 C0 4C 2C 09
    E = 0(False)
    T = 1
    RAPID = 33
    Timing Advanced = 11
    Hopping Flag 0 = False
    Fixed Size Resource Block Assignment = 96 (RB Start = 46, RB Length = 2)
    MCS = 2, I_TBS = 2, rv = 0
    TPC Command for PUCCH 3 = 0
    UL Delay 0 = False
    CQI Request = False
    T_CRNTI = 11273

SFN : 403.4

PUSCH - RRC Connection Request

 

 
MAC : 20 06 1F 5C 2C 04 B2 AC F6
    Sub Header 0
        R = OK
        E = 1
        LCID = 0 (CCCH)
        F = 0 (False)
        L = 6
    Sub Header 1
        R = OK
        E = 0
        LCID = 31 (Padding)

CCCH-RLC : 5C 2C 04 B2 AC F6 (RRC Connection Request)

SFN : 403.8

PHICH-ACK

 

 

SFN : 404.7

PDCCH (DCI Format 1) + PDSCH (RRC Connection Setup)

 

 
CCE Start = 0
CCE Length = 8

DCI Format 1A (Hex : 47D01E2)
    Format = 1
    Distributed VRB flag = 0 (Local)
    Resource Allocation = 500 (RB Start = 0, RB Length = 11)
    MCS = 0 (I_TBS = 0)
    HARQ Process Number = 7
    NDI (New Data Indicator) = 1 (True)
    RV = 0
    TPC Command for PUCCH = 1

MAC : 3C 20 1A 1F 5C 2C 04 B2 AC F6 60 12 98 08 FD 4E .....
    Sub Header 0
    R = OK
    E = 1
    LCID = 28 (UE Contention Resolution Identity)

    Sub Header 1
        R = OK
        E = 1
        LCID = 1 (CCCH)
        F = 0 (False)
        L = 26

    Sub Header 2
        R = OK
        E = 0
        LCID = 31 (Padding)

    UE Contention Resolution Identity
        UE Contention Resolution Identity= 5C 2C 04 B2 AC F6

SFN : 405.1

PUCCH - UCI HARQ ACK

 

PUCCH Format 1 A

n PUCCH = 16

SFN : 406.2

PUCCH - UCI SR

 

N PUCCH RB = 2

SFN : 406.6

PDCCH - DCI Format 0

 

 
PDCCH DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 406.7

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 406.8

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 406.9

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.0

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.0

PUSCH - RRC Connection Setup Complete (First Segment)

 

 
MAC = 3A 3D 01 22 10 88 00 00 20
    Sub Header 0
    R = OK
    E = 1
    LCID = 26 (Power Headroom Report)

    Sub Header 1
    R = OK
    E = 1
    LCID = 29 (Short Buffer Status Report)

    Sub Header 2
    R = OK
    E = 0
    LCID = 1 (identity)

    Power Headroom
    R = OK
    Power Headroom --> 11 dB <= PH <= 12 dB

    Short Buffer Status Report
    LCG ID = 0
    Buffer Size 16 --> 91 < BS <= 107

RLC AMD = 88 00 00 20
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 1 (First Byte of the Data Field corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 0

PDCP-CP-SRB = 00 20

SFN : 407.1

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 01 20 80 01 00 59 17
    Sub Header 0
    R = OK
    E = 0
    LCID = 1 (identity)
RLC AMD = 98 01 20 80 01 00 59 17
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU.
    Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 1
PDCP-CP-SRB = 20 80 01 00 59 17

SFN : 407.1

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.1

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 02 39 45 E5 34 0B 07
    Sub Header 0
    R = OK
    E = 0
    LCID = 1 (identity)
RLC AMD = 98 02 39 45 E5 34 0B 07
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 2
PDCP-CP-SRB = 39 45 E5 34 0B 07

SFN : 407.2

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.3

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 02 41 02 0B F6 03 02
    Sub Header 0
        R = OK
        E = 0
        LCID = 1 (identity)

RLC AMD = 98 03 41 02 0B F6 03 02
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 3
PDCP-CP-SRB = 41 02 0B F6 03 02

SFN : 407.3

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180540)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 1 (True)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.4

PHICH ACK

 

....

SFN : 407.4

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0180440)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 96
    MCS 2, RV 0
    NDI = 0 (False)
    TPC = 1
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.4

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 04 27 80 01 00 D0 CC
    Sub Header 0
        R = OK
        E = 0
        LCID = 1 (identity)
RLC AMD = 98 04 27 80 01 00 D0 CC
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 4
PDCP-CP-SRB = 27 80 01 00 D0 CC

SFN : 407.5

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 3D 01 0E 98 05 71 51 04 E0
    Sub Header 0
        R = OK
        E = 1
        LCID = 29 (Short Buffer Status Report)
    Sub Header 1
        R = OK
        E = 0
        LCID = 1 (identity)
    Short Buffer Status Report
        LCG ID = 0
        Buffer Size 14 --> 67 < BS <= 78
RLC AMD = 98 05 71 51 04 E0
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 5
PDCP-CP-SRB = 71 51 04 E0

SFN : 407.5

PHICH ACK

 

........

SFN : 407.5

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 0246280)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 145
    MCS 17, RV 0
    NDI = 0 (False)
    TPC = 2
    Cyclic Shift for DMRS = 0
    CQI Requested = 0 (False)

SFN : 407.6

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 06 E0 C0 40 00 21 02
    Sub Header 0
        R = OK
        E = 0
        LCID = 1 (Identity)
RLC AMD = 98 06 E0 C0 40 00 21 02
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 6
PDCP-CP-SRB = E0 C0 40 00 21 02

SFN : 407.6

PHICH ACK

 

....

SFN : 407.7

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 07 03 D0 11 D1 27 1A
    Sub Header 0
        R = OK
        E = 0
        LCID = 1 (Identity)
RLC AMD = 98 07 03 D0 11 D1 27 1A
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 7
PDCP-CP-SRB = 03 D0 11 D1 27 1A

SFN : 407.7

PHICH ACK

 

.....

SFN : 407.8

PHICH ACK

 

.....

SFN : 407.8

PUSCH - RRC Connection Setup Complete (Mid Segment)

 

 
MAC = 01 98 08 80 80 21 10 01 00
    Sub Header 0
        R = OK
        E = 0
        LCID = 1 (Identity)
RLC AMD = 98 08 80 80 21 10 01 00
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 0 (Status Report Not Requested)
    Fl = 3 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field does not corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 8

PDCP-CP-SRB = 80 80 21 10 01 00

SFN : 407.9

PUSCH - RRC Connection Setup Complete (Last Segment)

 

 
MAC = 3E 21 36 1F 00 00 00 B0 09 00 10 81 06 00 00 00 00 83 06 00 00 00 00 ....
    Sub Header 0
        R = OK
        E = 1
        LCID = 30 (Long Buffer Status Report)
    Sub Header 1
        R = OK
        E = 1
        LCID = 1 (identity)
        F = 0 (False)
        L = 54
    Sub Header 2
        R = OK
        E = 0
        LCID = 31 (Padding)
    Long Buffer Status Report
        Buffer Size #0 = 0 (BS = 0)
        Buffer Size #1 = 0 (BS = 0)
        Buffer Size #2 = 0 (BS = 0)
        Buffer Size #3 = 0 (BS = 0)
RLC AMD = B0 09 00 10 81 06 00 00 00 00 83 06 00 00 00 00 ....
    D/C = 1 (Data PDU)
    RF = 0 (AMD PDU)
    P = 1 (Status Report Requested)
    Fl = 2 (First Byte of the Data Field does not corresponds to the first byte of a RLC SDU. Last byte of Data field corresponds to the last byte of a RLC PDU)
    E = 0 (False)
    SN = 9
PDCP-CP-SRB = 00 10 81 06 00 00 00 00 83 06 00 00 00 00 ....

SFN : 407.9

PHICH ACK

 

.....

SFN : 408.0

PHICH ACK

 

.....

SFN : 408.1

PHICH ACK

 

.....

SFN : 408.2

PHICH ACK

 

.....

SFN : 408.3

PHICH ACK

 

.....

SFN : 408.4

PDCCH - DCI Format 2A / PDSCH - RLC ACK

 

 
DCI Format 2A (Hex : 080005C0802)
    Resource Allocation Type = 0
    Resource Allocation = 65536
    TPC Command for PUCCH = 2
    HARQ Process Number = 7
    Transport block to codeword swap flag = 0
    Transport Block 1
        MCS = 0
        New Data Indicator = 1
        Redundancy Version = 0
    Transport Block 2
        MCS = 0
        New Data Indicator = 1
        Redundancy Version = 0
    Codeword 0 = Transport block 1

MAC : 21 02 1F 00 28 00 00
    Sub Header 0
        R = OK
        E = 1
        LCID = 1 (Identity)
        F = 0 (False)
        L = 2

    Sub Header 1
        R = OK
        E = 0
        LCID = 31 (Padding)
RLC-STS-PDU : 00 28
        D/C = 0 (Control PDU)
        CPT = 0 (Status PDU)
        ACK_SN = 10
        E1 = 0 (False)
        Padding OK

SFN : 408.5

PDCCH - DCI Format 0

 

 
DCI Format 0 (Hex : 023DE84)
    Format 0
    Hopping Flag = 0 (False)
    RB Allocation of 1st Slot in UL subframe = 143
    MCS 15, RV 0
    NDI = 0 (False)
    TPC = 2
    Cyclic Shift for DMRS = 0
    CQI Requested = 1 (True)

SFN : 408.8

PUCCH - UCI HARQ ACK

 

....

SFN : 408.9

UCI + PUSCH

 

 
UCI
    RI = 2
    Higher layer configured subband CQI
    Wideband CQI Codeword = index 10, modulation 64QAM, efficiency = 2.7305
    Subband differential CQI = 0,1,2,1,3,3,3,3,0
    CRC
PUSCH - MAC : 3E 1F 00 00 00 00 ....
    Sub Header 0
        Reserved = OK
        E = 1 (True)
        LCID = 30 (Long Buffer Status Report)
    Sub Header 1
        Reserved = OK
        E = 0 (False)
        LCID = 31 (Padding)
    Long Buffer Status Report
        Buffer Size #0 : 0 --> BS = 0
        Buffer Size #0 : 1 --> BS = 0
        Buffer Size #0 : 2 --> BS = 0
        Buffer Size #0 : 3 --> BS = 0

 

Let's trace the start of the attach in the log above, layer by layer. The same bytes appear in more than one row, and following them is the quickest way to connect the layers.

  • Preamble index 33, RAPID 33 : the RAR at SFN 402.8 starts with 61, which is E = 0, T = 1 and RAPID 33, so it answers this preamble. It assigns T_CRNTI 11273, which is 2C09 in hex.
  • RRC Connection Request on CCCH : the MAC PDU starts with 20 06, a subheader for LCID 0 with L = 6, and ends with a padding subheader. The six CCCH bytes carry no RLC or PDCP header.
  • Contention resolution : in the RRC Connection Setup, the UE Contention Resolution Identity MAC CE, LCID 28, echoes the six bytes 5C 2C 04 B2 AC F6 that the UE sent.
  • SRB1 in ten AMD PDUs : the RRC Connection Setup Complete is sent in AMD PDUs with SN 0 to SN 9, and the FI field marks the first, middle and last segments. The first segment carries the PDCP header 00, SN 0, and then 20, the first byte of the UL-DCCH message.
  • Poll and status : the last segment sets P = 1. The eNB answers at SFN 408.4 with an RLC STATUS PDU with ACK_SN 10, so all ten segments arrived.
  • MAC CEs share the transport block : the segments travel with PHR, short BSR and long BSR MAC CEs, LCID 26, 29 and 30.

One label in the capture disagrees with the bytes. In the RRC Connection Setup at SFN 404.7, the decoder shows the second subheader as LCID = 1 (CCCH). That byte is 20 in hex, which is E = 1 and LCID = 0, and LCID 0 is CCCH in the DL-SCH table of 36.321 clause 6.2.1. The row title also says DCI Format 1, while the decode below it is DCI Format 1A. The capture is left as it was recorded.

Reference

[1] 3GPP TS 36.321 v19.3.0 - clauses 6.1.4, 6.1.5 and 6.2.1, transparent MAC, RAR and LCID values

[2] 3GPP TS 36.322 v19.0.0 - clause 4.2.1, TM, UM and AM RLC entities

[3] 3GPP TS 36.323 v19.0.0 - clause 4.4, PDCP functions and the logical channels it serves

[4] 3GPP TS 36.331 v19.3.0 - clause 4.2.2, signalling radio bearers, and clauses 9.1.1 and 9.1.2, default configurations