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Personally to me, RLC layer is one of the trickest area to understand in very detail. At the beginning it seems to be simple, but as I getting deeper and deeper into this layer I get more and more confused. Another difficulties is that I don't find any books or other training material explaining very detail on this area, whereas you would find a lot of books and materials digging very deeplyinto other layer

 

 

Followings are topics that will be described in this page. Don't get scared of the long list. Overall logic is simple. First, understand the details RLC PDU/SDU structure and go through a lot of examples.

 

 

 

Overview

 

As far as I experienced, there are three main topics we have to master on this layer to make our knowledge practical enough to the development or troubleshooting.

i) Understanding three types of RLC Mode : TM, UM, AM

ii) Segmentation (Splitting) and Concatenation (Combining) mechanism

iii) RLC Timers

 

I will not explain on these topics right now. It will be explained as you go along with this section.. but this part will be updated forever as I get more and more insight on this layer.

 

As in the section for other layer, let's start by "Reading (Verbalizing)" the pictures from the specification. The first diagram you will see in LTE RLC specification(TS 36.322) is as follows:

 

< 36.322 Figure 4.2.1-1: Overview model of the RLC sub layer >

 

What do you read from the diagram above ? First you will see the layers labeled 'upper layer' and 'lower layer' ? What does these layer specifically mean ? 'Upper layer' in this case would mean 'RRC layer' or 'PDCP Layer' or in some case (especially in the simulation or testing environment' it can be a TE port ( a kind of data Input/Output port).

Then, in which case the upper layer should be RRC Layer and in which case it should be PDCP or TE port ? These are what you have to figure out as you go along this document.

And then.. you see another layer labelled 'lower layer'. What is the lower layer in this case ? It is simple. It means 'MAC' layer in most case.

 

There are another small issues that scared me.. it was the term 'entity' which sounded too abstract to me. (it may not be the case for others). I just take the term 'entity' as a kind of 'thread' or 'task' in software terminology. It may not be a perfect analogy but this terminology change made me much more comfortable when I am reading the specs.

 

What else you can read from the diagram above ?

One thing I notice is..  There are separate entity (thread or task) for Uplink and Downlink RLC for TM and UM. But in AM, both Uplink and Downlink path are merged into a single entity. Why ? This is also what you have to figure out later.

 

Another picture that I want you to keep in mind is shown below. Most of signaling message is using RLC and each of the message is a specific types of RLC mode, TM/UM/AM. Unlike the channel mapping, the association between each message and RLC type is not so tight meaning that network can allocate RLC types a little bit different from what is shown below, but I would say the following mapping would be a kind of 'Textbook' mapping.

 

As we go through this section, you will see the detailed behavior of each RLC mode. If you try to associate this diagram and the RLC description you will have more practical understanding of RLC and will help you greatly for protocol stack implementation and troubleshooting.

 

 

Overall Data Flow for TM RLC

 

For this section as well, let's start reading diagram from the specification.

 

Let's begin with TM mode. As you see in the following diagram, TM is the simplest RLC mode. TM stands for 'Transparent Mode'. The term 'Transparent' may have many different meaning. In this case, it would mean 'the contents goes through this layer without any modification'.

What does it mean by 'modification' ? In this context, it would mean

i) It does not add or remove any header to the input data

ii) It does not split the input data into multiple segment

iii) It does not combine the multiple input data into a single big chunk

 

The only operation operation being done in this mode is a buffering operation, but even this buffering operation is also very simple. It just keeps the input data for a certain amount of time or until next input data come in, it just discard it if it does not get transmitted within a certain time frame.

 

As you see in the diagram, BCCH, PCCH, CCCH goes through this type of RLC process. In WCDMA, Voice call traffic used this RLC mode as well. It means that even some type of DTCH (voice traffic) uses this mode in WCDMA. However it is technically possible to use TM mode for DTCH as well.

< 36.322 Figure 4.2.1.1.1-1: Model of two transparent mode peer entities >

 

 

Overall Data Flow for UM RLC

 

Next, let's look into UM mode. UM stands for 'Unacknowledged Mode'. 'Unacknowledged Mode' means 'it does not require any reception response from the other party'. 'Reception response' simply mean 'ACK' or 'NACK' from the other party. (UM mode is similar to TM mode in that it does not require any ACK/NACK from the other party, but it is different from TM in that I has it's own header)

 

What is the difference between UM mode and TM mode we saw above ? It seems that UM mode is doing more operation than TM mode.

What kind of operation UM mode do ? You can just 'read' diagram for the answer. The answer would be a little bit different with transmitter side and reciever side.

 

Let's read the operation on transmitting side first. If you just read (verbalize) the diagram

    i) Buffering

    ii) Segmetation (Split a big chunk into a multiple small chunk)

    iii) Concatenation (Combine a multiple small chunk into a large single chunk)

    iv) Add RLC header (If you combine or split something, you have to be able to re-split or re-combine them into orignal chunk on the reciever side. For this, you have to put some tag(header) to those chunk)

 

Then, read the operation on recieving side. If you just read (verbalize) the diagram

    i) Buffering

    ii) Reordering (Sometimes the chunks transmitted earlier from transmitter may arrive late at the reciever. In this case you have to reorder the incoming chunks into proper order for reassembly).

    iii) Remove the RLC header (you would remember that the transmitter put the header to each of the chunk. So you have to remove this before you reassemble the data).

    iv) Reassembly

 

As you see, DTCH, MTCH. MCCH use this type of RLC process. Again, this is also a matter of choice. You can use AM or UM mode for DTCH.

< 36.322 Figure 4.2.1.2.1-1: Model of two unacknowledged mode peer entities >

 

 

Summary of RLC UM Data Flow - Transmission

    i) Receive an Higher Layer Data (SDU) from PDCP or RRC

    ii) Put the SDU into the transmission Buffer

    iii) Segment or Concatenate the SDU into RLC PDU when MAC permit transmission

    iv) Add a RLC header to the RLC PDU

    v) Send the RLC PDU to the next layer (MAC)

 

Summary of RLC UM Data Flow - Reception

    i) The MAC layer passes the received RLC PDU to the RLC layer.

    ii) The RLC layer removes the RLC header from the PDU.

    iii). The RLC layer assembles the PDUs into a upper layer SDU

    iv). Pass the assembled SDUs to the PDCP or RRC layers.

 

 

Overall Data Flow for AM RLC

 

Now let's look at AM mode which is the most complicated RLC type. 'AM' stands for 'Acknowledge Mode'. As it's name implies it requires ACK/NACK from the other party. It is more like TCP packet in IP world, whereas RLC UM is more like UDP in IP world.

 

Is it expecting the ACK/NACK for every transmission ? If it is the case, isn't it too much overhead ? Good question. Yes.. it is too much overhead. That's why we have RLC window concept (like TCP Window in IP traffic) and Polling bit concept and all sorts of ACK/NACK scheduling mechanism which makes it extremely difficult to understand full details of RLC AM operation. (This kind of detailed procedure would not be explained now.. but not sure by when I can get to the level of details -:)

 

Now just look into the diagram from the specification. If you go through the left column and the right colum, you will see the same procedures you saw in UM mode. so I don't want to verbalize that part again. What is different from UM mode lies in the middle column, namely 'Retransmission buffer' and 'RLC control' procedure.

Let's just follow the arrrows. What is coming into the retransmission buffer ? i.e, what is the input to the retransmission buffer ?

After RLC transmitter do the segmentation/concatenation process, it adds RLC header and then it creates two identical copies and transmit the one copy of the data out to lower layer (MAC) and send another copy to Retransmission buffer.

If the RLC get Nack or does not get any response from the other party for a certain period of time, the RLC packet (we call this RLC PDU) in the retransmission buffer gets transmitted again. If the RLC get ACK, the ones in retransmission buffer would be discarded.

 

< 36.322 Figure 4.2.1.3.1-1: Model of an acknowledged mode enttiy >

 

 

Summary of RLC AM Data Flow - Transmission

    i) Receive an Higher Layer Data (SDU) from PDCP or RRC

    ii) Put the SDU into the transmission Buffer

    iii) Segment or Concatenate the SDU into RLC PDU when MAC permit transmission

    iv) Add a RLC header to the RLC PDU

    v) Make a copy of the transmission buffer for a possible retransmission

    vi) Send the RLC PDU to the next layer (MAC)

 

 

Summary of RLC AM Data Flow - Reception

    i) The MAC layer passes the received RLC PDU to the RLC layer.

    ii) The RLC layer removes the RLC header from the PDU.

    iii) If the received RLC PDU does not have any problem, mark it as positive ACK (but whether the ACK is sent now or sometime later or omitted is determined by some other RLC parameters).

    iv). The RLC layer assembles the PDUs into a upper layer SDU

    v). Pass the assembled SDUs to the PDCP or RRC layers.

 

 

< Major Functions of RLC AM >

 

I think this list can give you another aspect of reading Figure 4.2.1.3.1-1 shown above. This is one way to learn new things. Try to describe it from many different perspective. There will be many things overlapped by multiple perspectives and there will be many things that looks different depending on the perspectives. It will help you to learn it in either ways. In some case, you will learn many things because they are similar and in other case you will learn things because they are different.

  • Segmentation, concatenation, and reassembly: RLC service data units (SDUs) coming from the upper layer are segmented or concatenated to RLC protocol data units (PDUs) which has a predefined size. Each PDU is assigned its own sequence number (SN). RLC AM on reciever side will reassemble these PDUs into SDUs using the sequence number. So allocating the proper Sequence Number is very important in RLC operation.
  • Error correction by ARQ: In AM RLC, if the sender does not receive ACK from the reciever it automatically send the same data again and if the sender recieves the ACK in a predefined time frame it sends the next data. This process is called ARQ. But if RLC is using this ARQ for every transmission, it would create a lot of overhead for the data transmission. So in reality AM RLC is applying ARQ based on a special rules and this rule is defined by various factors like some special timers, polling request etc. These factors will be explained below.
  • Flow control: By changing the window size, RLC can do a kind of flow control.

 

< Polling for RLC AM ARQ >

 

As I mentioned above, one of the most important feature for RLC AM is ARQ which is designed for reliable data traffic and the key factors for RLC ARQ are various Polling mechanism.

 

This mechanism involves two main part as follows :

    i) Transmitter side : Poll Request

    ii) Receiver side : Status Report carrying ACK, NACK etc

 

Now the question is to figure out the best timing/situation for Poll Request and the best timing/situation for Satus Report. Following is the list of several important factors to trigger for Poll Request and Satus Report. Following table is based on UMTS RLC AM operation and this triggering mechanism seems to get simpler in LTE. However I would list up more complicated case (UMTS case) to give you more detailed picture.

 

Trigger for Poll

Description

Timer-based polling (timerPollPeriodic)

This configure the sender to transmit Poll periodically (once in a predefined time period)

Poll timer (timerPoll)

This is the maximum waiting time for Status Report from the other party after it sent a Poll. If Status Report does not arrive until this timer expires after the transmission of a polling request, the sender send Polling Request again.

Last PDU in buffer

(lastTransmissionPDU-Poll)

If this is configured, the poll should be sent with the last PDU in the RLC sender’s transmission buffer

Last PDU in retransmission buffer

(lastRetransmissionPDU-Poll)

If this is configured, the poll should be sent with the last PDU in the RLC sender’s retransmission buffer

Poll every N PDU (poll-PDU)

This triggers the poll for every N PDU sent

Poll every N SDU (poll-SDU)

This triggers the poll for every N SDU sent

Window-based polling

This sets the polling bit when a certain percentage of the transmission window is sent

 

 

Trigger for Status Report

Description

Polling bit

The RLC reciever should send Status Report whenever it recieves AMD PDU with Poll bit = 1 except when it is within timerStatusProhibit

Missing PDU indicator

(missingPDU-Indicator)

If this flag is set to be TRUE, RLC reciever should send Status Report whenever it finds any missing RLC sequence number except when it is within timerStatusProhibit.

timer-based STATUS transfer

(timerStatusPeriodic)

If this parameter is configured, RLC sender send Status Report with a predefined interval except when it is within timerStatusProhibit

MAC-hs reset

This is trigger from MAC layer (not from RLC) which let RLC to send Status Report whenver MAC-hs resets. This is not influenced by timerStatusProhibit.

 

 

< Major RLC AM Parameters >

 

I think it will be helpful to look into some of high level RLC AM parameters (RRC parameters) and correlate these parameters with RLC AM operation process.

    +-rrcConnectionSetup-r8 ::= SEQUENCE [0]

      +-radioResourceConfigDedicated ::= SEQUENCE [100101]

      | +-srb-ToAddModList ::= SEQUENCE OF SIZE(1..2) [1] OPTIONAL:Exist

      | | +-SRB-ToAddMod ::= SEQUENCE [11]

      | |   +-srb-Identity ::= INTEGER (1..2) [1]

      | |   +-rlc-Config ::= CHOICE [explicitValue] OPTIONAL:Exist

      | |   | +-explicitValue ::= CHOICE [am]

      | |   |   +-am ::= SEQUENCE

      | |   |     +-ul-AM-RLC ::= SEQUENCE

      | |   |     | +-t-PollRetransmit ::= ENUMERATED [ms5]

      | |   |     | +-pollPDU ::= ENUMERATED [p4]

      | |   |     | +-pollByte ::= ENUMERATED [kB25]

      | |   |     | +-maxRetxThreshold ::= ENUMERATED [t1]

      | |   |     +-dl-AM-RLC ::= SEQUENCE

      | |   |       +-t-Reordering ::= ENUMERATED [ms35]

      | |   |       +-t-StatusProhibit ::= ENUMERATED [ms35]

      | |   +-logicalChannelConfig ::= CHOICE [defaultValue] OPTIONAL:Exist

      | +-drb-ToAddModList ::= SEQUENCE OF OPTIONAL:Omit

      | +-drb-ToReleaseList ::= SEQUENCE OF OPTIONAL:Omit

      | +-mac-MainConfig ::= CHOICE [explicitValue] OPTIONAL:Exist

      | +-sps-Config ::= SEQUENCE OPTIONAL:Omit

      | +-physicalConfigDedicated ::= SEQUENCE [1111001011] OPTIONAL:Exist

 

Information Element

Description

t-PollRetransmit

This timer is used by the transmitting side of an AM RLC entity in order to retransmit a poll. Unit = ms

pollPDU

In most typical condition, the poll bit is inserted after the number transmitted RLC PDU gets larger than this value. See 36.322 5.2.2.1

pollByte

In most typical condition, the poll bit is inserted after the transmited RLC data is greater than (this value x 1000 bytes). See 36.322 5.2.2.1

maxRetxThreshold

 

t-Reordering

This timer is used by the receiving side of an AM RLC entity and receiving UM RLC entity in order to detect loss of RLC PDUs at lower layer. Unit = ms

t-StatusProhibit

This timer is used by the receiving side of an AM RLC entity in order to prohibit transmission of a STATUS PDU. Unit = ms

 

 

Example 1 : in LTE - RLC Transmission from Network (IP throughput)

    PollPDU = p128

    PollByte = kB125

    maxRetxThreshold = t32

    t-PollRetransmit=ms35

    t-Reordering=30ms

    t-StatusProhibit = 30ms

     

    Time (mm.ss.ms)

    Direction

    AMD PDU

    SN (Dec)

    # of PDUs

    Avg Size (Bytes)

    Poll Bit

    00.21.324-00.21.338

    DL

    Data

     

    27

    4586

    0

    00.21.338

    DL

    Data

     

    1

    4586

    1

    00.21.339-00.21.348

    DL

    Data

     

    17

    4586

    0

    00.21.349

    UL

    Status(ACK)

    30

    1

    2

    N/A

    00.21.349-00.21.353

    DL

    Data

     

    9

    4586

    0

    00.21.353

    DL

    Data

     

    1

    4586

    1

    00.21.354-00.21.368

    DL

    Data

     

    27

    4586

    0

    00.21.368

    DL

    Data

     

    1

    4586

    1

    00.21.369-00.21.381

    DL

    Data

     

    27

    4586

    0

    00.21.382

    UL

    Status(ACK)

    90

    1

    2

    N/A

    00.21.382

    DL

    Data

     

    1

    4586

    0

    00.21.382

    DL

    Data

     

    1

    4586

    1

    00.21.383-00.21.397

    DL

    Data

     

    27

    4586

    0

    00.21.397

    DL

    Data

     

    1

    4586

    1

    00.21.398-00.21.412

    DL

    Data

     

    27

    4586

    0

    00.21.412

    DL

    Data

     

    1

    4586

    1

    00.21.413-00.21.415

    DL

    Data

     

    4

    4586

    0

    00.21.415

    UL

    Status(ACK)

    153

    1

    2

    N/A

    00.21.415-00.21.426

    DL

    Data

     

    23

    4586

    0

    00.21.426

    DL

    Data

     

    1

    4586

    1

    00.21.426-00.21.441

    DL

    Data

     

    27

    4586

    0

    00.21.441

    DL

    Data

     

    1

    4586

    1

    00.21.443-00.21.446

    DL

    Data

     

    8

    4586

    0

    00.21.448

    UL

    Status(ACK)

    216

    1

    2

    N/A

 

 

Example 2 : in LTE - RLC Transmission from Network (LTE Registration)

 

    Based on Table 4.6.3-19C: RLC-Config-SRB-AM-RECONFIG of 36.508

    PollPDU = pInfinity (3GPP Default)

    PollByte = kBInfinity (3GPP Default)

    maxRetxThreshold = 3GPP Default

    t-PollRetransmit=ms50

    t-Reordering=t6

    t-StatusProhibit = t0 (3GPP Default)

     

    Time (mm.ss.ms)

    Direction

    AMD PDU

    SN (Dec)

    # of PDUs

    Avg Size (Bytes)

    Poll Bit

    Header

    00.13.297

    DL

    DATA

    0

    1

    46

    1

    A0 00

    00.13.304

    UL

    Status(ACK)

    1

    1

    2

    N/A

    00 04

    00.13.416

    DL

    DATA

    1

    1

    26

    1

    A0 01

    00.13.423

    UL

    Status(ACK)

    2

    1

    2

    N/A

    00 08

    00.13.461

    DL

    DATA

    2

    1

    10

    1

    A0 02

    00.13.468

    UL

    Status(ACK)

    3

    1

    2

    N/A

    00 0C

    00.13.522

    DL

    DATA

    3

    1

    19

    1

    A0 03

    00.13.529

    UL

    Status(ACK)

    4

    1

    2

    N/A

    00 10

    00.13.587

    DL

    DATA

    4

    1

    19

    1

    A0 04

    00.13.594

    UL

    Status(ACK)

    5

    1

    2

    N/A

    00 14

    00.13.617

    DL

    DATA

    5

    1

    10

    1

    A0 05

    00.13.624

    UL

    Status(ACK)

    6

    1

    2

    N/A

    00 18

    00.13.883

    DL

    DATA

    6

    1

    62

    0

    88 06

    00.13.884

    DL

    DATA

    7

    1

    62

    0

    98 07

    00.13.885

    DL

    DATA

    8

    1

    21

    1

    B0 08

    00.13.892

    UL

    Status(ACK)

    9

    1

    2

    N/A

    00 18

 

 

Just for your reference, I put WCDMA RLC parameters in Radio Bearer Setup message as follows.

    +-SRB-InformationSetup-r7 ::= SEQUENCE [1]

    | +-rb-Identity ::= INTEGER (1..32) [2] OPTIONAL:Exist

    | +-rlc-InfoChoice ::= CHOICE [rlc-Info]

    | | +-rlc-Info ::= SEQUENCE [1100]

    | |   +-ul-RLC-Mode ::= CHOICE [ul-AM-RLC-Mode] OPTIONAL:Exist

    | |   | +-ul-AM-RLC-Mode ::= SEQUENCE [1]

    | |   |   +-transmissionRLC-Discard ::= CHOICE [noDiscard]

    | |   |   | +-noDiscard ::= ENUMERATED [dat10]

    | |   |   +-transmissionWindowSize ::= ENUMERATED [tw128]

    | |   |   +-timerRST ::= ENUMERATED [tr550]

    | |   |   +-max-RST ::= ENUMERATED [rst1]

    | |   |   +-pollingInfo ::= SEQUENCE [010110] OPTIONAL:Exist

    | |   |     +-timerPollProhibit ::= ENUMERATED OPTIONAL:Omit

    | |   |     +-timerPoll ::= ENUMERATED [tp500] OPTIONAL:Exist

    | |   |     +-poll-PDU ::= ENUMERATED OPTIONAL:Omit

    | |   |     +-poll-SDU ::= ENUMERATED [sdu1] OPTIONAL:Exist

    | |   |     +-lastTransmissionPDU-Poll ::= BOOLEAN [TRUE]

    | |   |     +-lastRetransmissionPDU-Poll ::= BOOLEAN [TRUE]

    | |   |     +-pollWindow ::= ENUMERATED [pw50] OPTIONAL:Exist

    | |   |     +-timerPollPeriodic ::= ENUMERATED OPTIONAL:Omit

    | |   +-dl-RLC-Mode ::= CHOICE [dl-AM-RLC-Mode] OPTIONAL:Exist

    | |   | +-dl-AM-RLC-Mode ::= SEQUENCE

    | |   |   +-dl-RLC-PDU-size ::= CHOICE [fixedSize]

    | |   |   +-inSequenceDelivery ::= BOOLEAN [TRUE]

    | |   |   +-receivingWindowSize ::= ENUMERATED [rw128]

    | |   |   +-dl-RLC-StatusInfo ::= SEQUENCE [100]

    | |   |     +-timerStatusProhibit ::= ENUMERATED [tsp120] OPTIONAL:Exist

    | |   |     +-dummy ::= ENUMERATED OPTIONAL:Omit

    | |   |     +-missingPDU-Indicator ::= BOOLEAN [TRUE]

    | |   |     +-timerStatusPeriodic ::= ENUMERATED OPTIONAL:Omit

 

 

Example 3 : in LTE - RLC Bidirectional (From LTE Attach to YouTube)

 

This example would the case that you might see most often in real life communication. This example shows all the RLC traffic both downlink and uplink from power on a UE to the start of YouTube download. This log is captured by Amarisoft OTS 100. Following is the RLC configuration for SRB, DRB set in RRC Connection Setup and RRC Connection Reconfiguration message. I put the decoded RLC headers shown on Amarisoft Web based log viewer, followed by corresponding Hex string. It will be a good practice if you pull out paper and pencil, deocde each hex string by hands and compare it with the decoded result shown on the log viewer.

    srb-ToAddModList {

      {

        srb-Identity 1,

        rlc-Config defaultValue: NULL,

        logicalChannelConfig defaultValue: NULL

      }

    },

     srb-ToAddModList {

      {

        srb-Identity 2,

        rlc-Config defaultValue: NULL,

        logicalChannelConfig defaultValue: NULL

      }

    },

     drb-ToAddModList {

      {

        eps-BearerIdentity 5,

        drb-Identity 1,

        pdcp-Config {

          discardTimer infinity,

          rlc-AM {

            statusReportRequired TRUE

          },

          headerCompression notUsed: NULL

        },

        rlc-Config am: {

          ul-AM-RLC {

            t-PollRetransmit ms80,

            pollPDU p64,

            pollByte kB125,

            maxRetxThreshold t4

          },

          dl-AM-RLC {

            t-Reordering ms80,

            t-StatusProhibit ms60

          }

        },

        logicalChannelIdentity 3,

        logicalChannelConfig {

          ul-SpecificParameters {

            priority 13,

            prioritisedBitRate infinity,

            bucketSizeDuration ms100,

            logicalChannelGroup 3

          }

        }

      }

    },

Following is the screencapture of RLC traffic shown on Amarisoft Web Interface logging tool. (I removed a couple of columns between ENB and Info to make it fit to this page)

 

 

Following is the Hex string of each RLC traffic (PDU) shown above. Try decode these PDU on your own and compare it with the decoded result shown above if you want to get the detailed understanding of RLC PDU structure.

 

No

RLC PDU

1

88 00 00 20 30 00 80 23  80 84 17 3a a0 26 49 05  

07 41 02 0b f6 00 f1 10  00 80 23 c1 00 45 67 05

2

b0 01 00 26 02 01 d0 11  27 20 80 80 21 10 01 00

00 10 81 06 00 00 00 00  83 06 00 00 00 00 00 0d

3

00 08

4

88 00 00 08 00 18 3a a8  08 00

5

b0 01 00 00 00

6

00 08

7

a0 02 01 48 02 22 f6 54  b6 b1 c0 c0 ea c1 01 22  

02 02 06 4a 8e d3 00 00  00 00 00

8

00 0c

9

a0 02 01 08 01 20 3a 90  05 b2 a3 b1 d4 2e 09 ff

e6 81 f3 71 fb 65 17 73  50 80 4e 0c a2 cc 5b cc

10

00 0c

11

a0 03 02 48 03 22 f4 2e  33 18 a0 e0 ea 62 16 c8

aa 81 38 32 8b 31 6b 14  f8 90 94 0f e0 12 a0 00

12

00 10

13

a0 03 02 08 00 89 bb ea  00 ec b8 00 3a e8 08 00

2f 83 86 02 03 86 08 00  00 00 00

14

00 10

15

a0 04 03 48 02 68 f3 8b  f7 73 a0 00 eb c4 61 26

72 a0 20 a0 c0 04 80 5e  a0 00 00 00 00

16

00 14

17

a0 04 03 30 00 10 84 90  80 b1

18

00 14

19

a0 05 04 28 00 06 04 bf  29

20

00 18

21

a0 05 04 38 00 00 3e a3  1a bb  

22

00 18

23

88 06 05 38 01 08 09 dc  99 80 05 0c 20 b0 c0 41

05 8b a8 ff f1 7f 1f fe  2b a3 ff c5 fc 7f f8 ab

24

b0 07 fc 7f f8 bf 8f ff  17 f1 ff e2 fe 3f fc 5f

c7 ff 00 26 00 00 c8 37  f8 00 00 00 1f f0 00 00

25

00 20

26

a0 06 05 20 06 02 a1 3f  39 fd 02 80 08 3a 10 17

00 30 00 07 88 80 00 08  01 22 90 0e 08 08 48 40

27

00 1c

28

a0 08 06 10 00 d3 10 a9  14  

29

00 24

30

a0 00 00 48 01 a4 e4 cc  6c a2 20 20 e8 60 00 6a

40 18 40 50 32 70 40

31

00 04

32

a0 07 06 08 01 91 3c b9  1a 8f 30 10 3b 0a 18 84

0e 0b b2 c4 f4 f5 fe 6b  a4 85 9a 65 fd ff 2b 5a

33

00 20

34

a0 00 80 00 45 00 00 45  2a a0 40 00 40 11 3d 4e

c0 a8 02 02 08 08 08 08  1e 02 00 35 00 31 00 d2

35

00 04

36

a0 00 80 00 45 00 01 0a  88 5d 00 00 36 11 28 cc

08 08 08 08 c0 a8 02 02  00 35 1e 02 00 f6 7b da

37

00 04

38

a0 01 80 01 45 00 00 3c  61 ac 40 00 40 06 bb 29

c0 a8 02 02 36 e6 24 56  9f a2 00 50 90 2d f7 5e

39

00 08  

40

a0 01 80 01 45 00 00 3c  00 00 40 00 35 06 27 d6

36 e6 24 56 c0 a8 02 02  00 50 9f a2 66 b7 2f 53

41

00 08

42

8c 02 03 60 80 02 45 00  00 34 61 ad 40 00 40 06

bb 30 c0 a8 02 02 36 e6  24 56 9f a2 00 50 90 2d

43

b0 03 74 72 61 32 2e 62  69 6e 20 48 54 54 50 2f

31 2e 31 0d 0a 41 63 63  65 70 74 3a 20 2a 2f 2a

44

00 10

45

a0 02 80 02 45 00 00 34  f2 c8 40 00 35 06 35 15

36 e6 24 56 c0 a8 02 02  00 50 9f a2 66 b7 2f 54

46

8c 03 5a 20 80 03 45 00  05 a0 f2 ca 40 00 35 06

2f a7 36 e6 24 56 c0 a8  02 02 00 50 9f a2 66 b7

 

 

 

RLC Data Structure

 

Like any data structure, you would not completely understand this structure until you really construct this structure or analyze the structure with real data either in programming or in manually. Of course my recommendation is to analyze manually.. I will put some of real RLC data example later, but for now let's just review the diagram from 3GPP specification and just try to be familiar with them.

 

< TMD Structure >

 

The first structure is as follows. It is data structure for TMD (TM mode data) structure. What kind of structure you see from the following diagram ?

 

< 36.322 Figure 6.2.1.2-1: TMD PDU >

 

It is Data only structure, not even with Header. Most of the data structure for any data communication has it's own header. IP packet has IP header. UDP packet has UDP header. TCP header has TCP header. ICMP packet has ICMP header, etc. But this packet does not have any header. As you know, the most important role of header is to carry the information as follows :

i) Who is the sender ? (sender address, sender id etc)

ii) Who is the reciepient (reciepient address, id etc)

iii) What is the size of the data ?

iv) etc..

 

But this data structure has none of these information. It means that the TMD packet does not add any additional (header) to the input data and does not split or combine the data coming into the RLC entity.

Then how the size of this TMD PDU is determined ? It is automatically set to be the same size as MAC PDU size.

What if the incoming data is bigger than the RLC PDU size ? It just take the initial part of data that can fit into its size and discard the rest of the data.

What if the incoming data is smaller than the RLC PDU size ? It just take the whole data and pass it to MAC layer and MAC layer add padding data at the end.

 

 

< UMD Structure >

 

Following is a data structure for UMD (UM mode data) structure. What are the meaning of Fl, E, SN ? You should refer to other sets of diagrams later. If you remember the diagram in previous section (Fig 4.2.1.2.1-1), RLC UM entity has capability of segmentation/concatenation. It means ... if the incoming data is greater than the RLC size, it segment the data into multiple chunks and passthem one by one. In this case, you have to put some tag (number) to each of the chunks.. otherwise the receiving side cannot combine them in proper sequence. This tag (number) is SN (Sequence Number). As you see in the following diagram, there are two different types of SN. One is 5 bits and the other one is 10 bits.

 

< 36.322 Figure 6.2.1.3-1: UMD PDU with 5 bit SN (No LI) >

 

< 36.322 Figure 6.2.1.3-2: UMD PDU with 10 bit SN (No LI) >

 

What if the a lot of small sized data packets are coming into the RLC entity ? The simplest way is to pack the small chunk as it is (as in TM mode), but it is waste of space.

 

Then what do we have to do in this case ? The simplest way is to put the multiple small chunk into a single RLC packet. In this case, you need special information for each of the small chunk within the RLC packet, except for the last one. LI (Length Indicator) is the length value for each of the small chunks within a RLC PDU, except for the last. For example, if you have 5 chunks in a single RLC packet, you would have four E fields and 4 LI fields since you don't need E and LI field for the last one (5th one in this case)

 

< 36.322 Figure 6.2.1.3-3: UMD PDU with 5 bit SN (Odd number of LIs, i.e. K = 1, 3, 5, …) >

< 36.322 Figure 6.2.1.3-4: UMD PDU with 5 bit SN (Even number of LIs, i.e. K = 2, 4, 6, …) >

 

< 36.322 Figure 6.2.1.3-5: UMD PDU with 10 bit SN (Odd number of LIs, i.e. K = 1, 3, 5, …) >

 

< 36.322 Figure 6.2.1.3-6: UMD PDU with 10 bit SN (Even number of LIs, i.e. K = 2, 4, 6, …) >

 

 

< AMD Structure >

 

The last type of RLC packet type is AMD (AM mode data) structure. Overall structure is very similar to UMD structure except that it has a couple of additional field D/C, RF, P.

 

< 36.322 Figure 6.2.1.4-1: AMD PDU (No LI) >

 

< 36.322 Figure 6.2.1.4-2: AMD PDU (Odd number of LIs, i.e. K = 1, 3, 5, …) >

 

< 36.322 Figure 6.2.1.4-3: AMD PDU (Even number of LIs, i.e. K = 2, 4, 6, …) >

 

We saw many of small fields in RLC header and now let's try understanding the meaning of each of these fields.

 

D/C Field : This field indicate whether the PDU is for RLC control or Data .

 

< 36.322 Table 6.2.2.9-1: D/C field interpretation >

 

RF Field : This field indicate tye type of the AMD PDU.

 

< 36.322 Table 6.2.2.10-1: RF field interpretation >

 

P Field : This field indicate the PDU requires a status report (RLC ACK or NACK) from the other party or not.

 

< 36.322 Table 6.2.2.11-1: P field interpretation >

 

FI Field : This field indicate the relative location of the RLC PDU within a RLC SDU. For example, '01' indicate the PDU is the first segment of the SDU and '10' indicates the PDU is the last segment of the SDU, and '11' indicate the PDU is in between the first and last segment. In a SDU, there is only one '01' type PDU and only one '10' type PDU. There can be one or more '11' type PDU. If FI is 00, it means the RLC PDU is same as RLC SDU. When RLC recieves '01' type, the layer start buffering the PDU within RLC memory and only when it recieves '10' PDU, RLC reassembles all the recieved PDU and transfer it to PDCP.

 

< 36.322 Table 6.2.2.6-1: FI field interpretation >

 

< 36.322 Table 6.2.2.8-1: LSF field interpretation >

 

E Field : This field indicate whether this field is the end of the header or another extention follows or not.

 

< 36.322 Table 6.2.2.4-1: E field interpretation (for E field in the fixed part of the header) >

 

 

< 36.322 Table 6.2.2.4-2: E field interpretation (for E field in the extension part of the header) >

 

 

Following is the structure of STATUS PDU and I recommend you to be familiar with this structure because this would be very useful for various troubleshoot case.

 

< Figure 6.2.1.6-1: STATUS PDU >

 

 

CPT Field : This field applies when PDU type is Control PDU. It indicates whether the PDU is STATUS or not.

 

< 36.322 Table 6.2.2.13-1: CPT field interpretation >

 

E1 Field : This field applies when PDU type is Control PDU. It indicates whether E1 and E1 follows or not.

 

< 36.322 Table 6.2.2.15-1: E1 field interpretation >

 

< 36.322 Table 6.2.2.17-1: E2 field interpretation >

 

SOstart and SOend fields indicated the corresponding NACK_SN stands for 'detected as lost' which means a portion or the whole RLC PDU is lost. According to 36.322, section 6.2.2.18, 6.2.2.19 descrbes these fields as follows.

 

SOstart (15 bits) : The SOstart field (together with the SOend field) indicates the portion of the AMD PDU with SN = NACK_SN (the NACK_SN for which the SOstart is related to) that has been detected as lost at the receiving side of the AM RLC entity.

Specifically, the SOstart field indicates the position of the first byte of the portion of the AMD PDU in bytes within the Data field of the AMD PDU. The first byte in the Data field of the original AMD PDU is referred by the SOstart field value "000000000000000", i.e., numbering starts at zero

 

SOend (15 bits) : The SOend field (together with the SOstart field) indicates the portion of the AMD PDU with SN = NACK_SN (the NACK_SN for which the SOend is related to) that has been detected as lost at the receiving side of the AM RLC entity.

Specifically, the SOend field indicates the position of the last byte of the portion of the AMD PDU in bytes within the Data field of the AMD PDU. The first byte in the Data field of the original AMD PDU is referred by the SOend field value "000000000000000", i.e., numbering starts at zero. The special SOend value "111111111111111" is used to indicate that the missing portion of the AMD PDU includes all bytes to the last byte of the AMD PDU

 

 

RLC Decoding Tips

 

When you don't have automatic RLC decoder and you have to manually decode RLC header, you can get a lot of hints just by the first 4 bits of RLC PDU as listed below. Check this table with the examples.

 

Binary

Hex

Description

0000

0

AMD Control PDU, Status PDU

100x

8 or 9

AMD Data PDU, Polling Bit = 0

101x

A or B

AMD Data PDU, Polling Bit = 1

 

 

 

Examples for RLC AMD PDU

 

Followings are a couple of RLC PDU example. In RLC PDU case, especially AMD PDU case, you should anlayze several consecutive PDUs in both direction for you to get overal picture.

With these examples, you would learn and get familiar to how to utilize all the tables/diagrams shown above.

 

DL AMD : 88 06 06 20 06 03 41 39 40 5A D9 80 18 3A 10 17 00 30 00 07 88 80 00 08 01 A2 90 0E 08 08 48 40 3B 0B 73 93 4B A3 9B A8 28 0E 05 40 20 0A E8 08 01 80 71 1C 90 54 9C B2 03 43 A0 02 03 F8

 

88 06 = 10001000 00000110

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 01 = First byte of the Data field corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the LI field following this E field

SN = 0000000110

 

 

DL AMD : 98 07 00 00 01 90 1C 09 38 44 00 00 68 26 05 40 08 12 80 5F B0 07 88 84 00 08 00 00 00 00 08 98 07 88 80 00 01 18 2F A0 00 00 00 00 BF 02 CF 03 20 08 0B 53 E0 FA 81 BC 0F A8 E0 60 78 E6

 

98 07 =  10011000 00000111

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 11 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the LI field following this E field

SN = 0000000111

 

 

DL AMD : B0 08 3E 9D 52 D2 E8 00 FB 50 65 03 FF 04 07 E8 10 55 13 20 49

 

B0 08 =  10110000 00001000

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 1 = Status PDU is requested

FI  = 10 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field corresponds to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the LI field following this E field

SN = 0000001000

 

 

DL AMD : 8C 00 DD CD DC 5D C0 80 00 6D 1B 72 6F 06 97 7E 70 DB 6C 8D C5 F5 88 B3 F3 5D 7F E4 44 B4 FF E9 3F 54 30 67 02 19 EA 57 A2 5A 33 B4 7D B5 6C A5 4B B5 ...

 

8C 00 DD CD DC 5D = 10001100 00000000 11011101 11001101 11011100 01011101

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 01 = First byte of the Data field corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN = 0000000000

E   = 1

LI1 = 10111011100 (1500 Dec)

E   = 1

LI2 = 10111011100 (1500 Dec)

 

RLC PDU = 8C 00 DD CD DC 5D C0 80 00 6D 1B 72 6F 06 97 7E 70 DB 6C 8D C5 F5 88 B3 F3 5D 7F E4 44 B4 FF E9 3F 54 30 67 02 19 EA 57 A2 5A 33 B4 7D B5 6C A5 4B B5...

 

 

DL AMD : 9C 01 D8 DD DC 5D C0 EB E2 5F BC 87 1E CA CD 1D F6 65 19 E6 3F B2 E8 F1 A8 87 C7 DC 1E 1A 01 54 91 F6 80 48 72 53 AC C3 43 6A 6B D6 9E DF 43 D6 A4 5C 03  ...

 

9C 01 D8 DD DC 5D = 10011100 00000001 11011000 11011101 11011100 01011101  

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 11 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN = 0000000001

E   = 1

LI1 = 10110001101 (1421 Dec)

E   = 1

LI2 = 10111011100 (1500 Dec)

 

 

DL AMD : 9C 02 D3 ED DC 5D C0 E4 D4 DE 42 97 6B C5 3F DD 14 D8 31 CA F6 4F 16 D8 70 8D 4A 8B 0B 4C 07 CB D0 DC 0B BD 9A FD C3 24 07 3D 79 AA 02 0E E0 23 29 D0 ...  

 

9C 02 D3 ED DC 5D = 10011100 00000010 11010011 11101101 11011100 01011101

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 11 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN = 0000000010

E   = 1

LI1 = 1010011 1110 (1342 Dec)

E   = 1

LI2 = 10111011100 (1500 Dec)

 

 

DL AMD : 94 12 84 ED DC 5D C0 BB 9E AE E2 26 8C 70 1D 50 FB 35 B2 B4 1A E1 96 52 5B BE 0A A1 D2 DF C1 FF 46 F4 00 54 06 AD CB 14 8F 56 7A DE 43 A6 D1 F9 C8 ...

 

94 12 84 ED DC 5D = 10010100 00010010 10000100 11101101 11011100 01011101

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 0 = Status PDU is not requested

FI  = 10 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field corresponds to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN = 00 00010010

E   = 1

LI1 = 00001001110 (78 Dec)

E   = 1

LI2 = 10111011100 (1500 Dec)

 

 

DL AMD : BC 1B B6 4D DC 5D C0 C7 33 AF D4 1D FB CC 72 17 B0 35 91 F3 6F 37 38 66 EE 07 D2 8A B1 2C 15 73 80 57 22 CD 3D A5 D8 4A 4B 0A AF 31 C2 2B D6 BD

 

BC 1B B6 4D DC 5D = 10111100 00011011 10110110 01001101 11011100 01011101  

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 1 = Status PDU is requested

FI  = 11 = First byte of the Data field does not corresponds to the first byte of a RLC SDU,

              Last byte of the Data field does not corresponds to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN = 0000011011 (27 Dec)

E   = 1

LI1 = 01101100100 (868 Dec)

E   = 1

LI2 = 10111011100 (1500 Dec)

 

 

DL AMD : A0 00 00 08 01 20 3A 90 05 1E F0 63 69 B1 F1 86 1B 25 20 3C 78 DF C6 AB B8 82 F3 93 5A B5 A7 64 80 0D 1E 79 72 F3 93 5E 35 A8 00 00 00 00

 

A0 00 = 10100000 00000000

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 1 = Status PDU is requested

FI  = 00 = First byte of the Data field corresponds to the first byte of a RLC SDU,

              Last byte of the Data field corresponds to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the LI field following this E field

SN  = 0000000000

RLC PDU = A0 00 00 08 01 20 3A 90 05 1E F0 63 69 B1 F1 86 1B 25 20 3C 78 DF C6 AB B8 82 F3 93 5A B5 A7 64 80 0D 1E 79 72 F3 93 5E 35 A8 00 00 00 00

 

UL AMD  : 00 04 = 00000000 00000100

 

D/C = 0   = Control PDU

CPT = 000 = Status PDU

ACN_SN = 0000000001 (This is ACK for DL RLC SN 0)

E1  = 0

 

UL AMD : 00 78 = 00000000 01111000

 

D/C = 0   = Control PDU

CPT = 000 = Status PDU

ACN_SN = 0000011110 (This is ACK for DL RLC SN 29)

E1  = 0

 

 

 

UL AMD : A0 01 01 48 01 60 EA 61 14 79 E5 CB CE 4D 6A D6 80 00 00 00 00

 

A0 00 = 10100000 00000001

 

D/C = 1 = Data PDU

RF  = 0 = AMD PDU

P   = 1 = Status PDU is requested

FI  = 00 = First byte of the Data field corresponds to the first byte of a RLC SDU,

              Last byte of the Data field corresponds to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the LI field following this E field

SN  = 0000000001

RLC PDU = A0 01 01 48 01 60 EA 61 14 79 E5 CB CE 4D 6A D6 80 00 00 00 00

 

DL AMD : 00 08 = 00000000 00001000

 

D/C = 0   = Control PDU

CPT = 000 = Status PDU

ACN_SN = 0000000010 (This is ACK for UL RLC SN 1)

E1  = 0 (It means that Octat 2 is the last Octat for this RLC PDU).

 

DL AMD : 00 22 00 40 0C 01 C0 20 = 00000000 00100010 00000000 01000000 00001100 00000001 11000000 00100000

 

D/C = 0   = Control PDU (1 bit)

CPT = 000 = Status PDU (3 bits)

ACN_SN = 0000001000 (10 bits, This is ACK for UL RLC SN 7)

E1  = 1 (1 bit, It means that this is not the end of RLC PDU).

 

NACK_SN = 0000000000 = 0 (10 bits)

E1 = 1 (1 bit, It means that this is not the end of RLC PDU)

E2 = 0 (1 bit, a set of SOstart and SOend does not follow for this NACK_SN)

 

NACK_SN = 0000000001 = 1 (10 bits)

E1 = 1 (1 bit, It means that this is not the end of RLC PDU)

E2 = 0 (1 bit, a set of SOstart and SOend does not follow for this NACK_SN)

 

NACK_SN = 0000000011 = 3 (10 bits)

E1 = 1 (1 bit, It means that this is not the end of RLC PDU)

E2 = 0 (1 bit, a set of SOstart and SOend does not follow for this NACK_SN)

 

NACK_SN = 0000000100 = 4 (10 bits)

E1 = 0 (1 bit, It means that this is the end of RLC PDU)

E2 = 0 (1 bit, a set of SOstart and SOend does not follow for this NACK_SN)

 

 

Examples for RLC UMD PDU

 

 

This example data are captured from a RLCs for IP throughput test (in this case, video streaming test)

 

DL UMD PDU 1: 1C E1 06 8F CC 48 23 E9 90 DA 18 D2 2B 1B A3 D9 5C 72 E8 D7 71 A9 51 A9 82 78 17 80 DC 9B 18 76 17 99 18 48 7F 77 22 B9 8E 79 DA A9 58 5A C8 6E 1C 93 F9 1D 64 C4 58 56 46 0C B5 A3 0C 5E B3 24 49 8F F0 51 FC D7 A0 F6 29 A3 51 73 C2 31 9E F8 4B 0F 07 34 B1 06 A8 E1 79 E0 E0 DD C7 C8 FC 23 52 08 D8 F4 48 ED 9A 70 10 EF DD 1C 80 54 45 00 02 50 0E 62 00 00 80 11 95 C9 01 01 C9 37 01 01 C9 38 04 E7 04 D2 02 3C 9B 62 47 00 44 11 E6 F0 B0 1C 80 33 A5 2C 12 C2 C8 35 16 92 61 68 12 07 0D 81 CC C7 29 C9 21 A1 B9 94 6C 25 22 E3 28 78 1A 6A E3 5A 50 1B 13 83 33 C6 57 11 4D 5B 19 55 08 31 26 75 FA 4B 42 FB 84 FF 2B 1B E2 22 B3 9C 67 07 20 AC 24 07 0D 89 6B 4D 07 24 57 BC 94 CF

 

 

1C E1 = 00011100 11100001

 

R1  = 0

R1  = 0

R1  = 0

FI  = 11 = First byte of the Data field does not correspond to the first byte of a RLC SDU,

           Last byte of the Data field does not correspond to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN  = 0011100001 = 225

 

06 8F = 00000110 10001111

 

E  = 0

LI = 0000110 1000 = 104

Padding = 1111

 

SDU 1:

CC 48 23 E9 90 DA 18 D2 2B 1B A3 D9 5C 72 E8 D7 71 A9 51 A9

82 78 17 80 DC 9B 18 76 17 99 18 48 7F 77 22 B9 8E 79 DA A9

58 5A C8 6E 1C 93 F9 1D 64 C4 58 56 46 0C B5 A3 0C 5E B3 24

49 8F F0 51 FC D7 A0 F6 29 A3 51 73 C2 31 9E F8 4B 0F 07 34

B1 06 A8 E1 79 E0 E0 DD C7 C8 FC 23 52 08 D8 F4 48 ED 9A 70

10 EF DD 1C

 

SDU 2: (The part in color is IP address of the data going through this RLC. This IP is nothing to do with RLC itself, but it will be a good example that multiple IP packets are distributed among multiple RLC PDUs)

 

80 54 45 00 02 50 0E 62 00 00 80 11 95 C9 01 01 C9 37 01 01

C9 38 04 E7 04 D2 02 3C 9B 62 47 00 44 11 E6 F0 B0 1C 80 33

A5 2C 12 C2 C8 35 16 92 61 68 12 07 0D 81 CC C7 29 C9 21 A1

B9 94 6C 25 22 E3 28 78 1A 6A E3 5A 50 1B 13 83 33 C6 57 11

4D 5B 19 55 08 31 26 75 FA 4B 42 FB 84 FF 2B 1B E2 22 B3 9C

67 07 20 AC 24 07 0D 89 6B 4D 07 24 57 BC 94 CF

 

 

DL UMD PDU 2 : 18 E2 05 D2 2D 55 82 D9 82 51 91 A9 B8 BE C0 42 23 87 1B B6 1B CA B1 B6 3A 09 0E B9 64 6C B0 72 BA 74 53 A4 9D 8D 23 5C 9B 18 23 78 87 55 02 F4 07 E4 48 27 F0 8D 4A 3A 7E 09 80 8B 29 13 1A B2 7E 3C 8C 2B 9F 7F 0C AD E0 38 7E ED 40 B1 E8 73 46 62 4F 12 C8 FB 17 FC 70 FC A2 CF 85 52 A6 C3 40 4A 92 79 42 DC FE B0 47 00 44 12 E4 2A 94 21 8B DF 37 69 2F 5E D9 8A 3F 65 E8 73 D4 9A 00 EE 49 D0 AC 8E 39 7E 23 F0 F9 43 F8 1B 1E FF 0A F4 5E 7C 8C B0 F2 B3 C3 80 29 E1 3A 9E 2E A0 38 DF 3C EE 1D DF 06 BE FD 82 37 62 13 A5 F1 40 F2 D5 17 6D 99 9E 5A BF D6 AB 1D 7B 96 31 1F 31 0A 1C 24 AA 35 3D 13 00 D5 B1 97 1A BC C4 7C 7A A3 C0 9E AB AC A3 75 1D 3B 2D D5 04 AA 3F 73 73 36 1F

 

18 E2 = 00011000 11100010

 

R1  = 0

R1  = 0

R1  = 0

FI  = 11 = First byte of the Data field does not correspond to the first byte of a RLC SDU,

           Last byte of the Data field does not correspond to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the fixed part of the header

SN  = 0011100010 = 226

 

 

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

 

18 E3 = 00011000 11100011

 

R1  = 0

R1  = 0

R1  = 0

FI  = 11 = First byte of the Data field does not correspond to the first byte of a RLC SDU,

           Last byte of the Data field does not correspond to the last byte of a RLC SDU

E   = 0  = Data field follows from the octet following the fixed part of the header

SN  = 0011100011 = 227

 

 

DL UMD PDU 4: 1C E4 02 2F 92 F1 62 09 15 25 34 19 DC E4 3D 46 DE 9A 44 A9 2B 3F CF 44 1A 36 BA B9 89 D6 14 54 F0 B5 F0 60 7C 7F 80 55 45 00 02 50 0E 63 00 00 80 11 95 C8 01 01 C9 37 01 01 C9 38 04 E7 04 D2 02 3C 6C 19 47 00 44 14 29 50 78 3D EE C9 0C 23 9E CF A4 41 4E F1 E5 6B 81 72 B2 1A 57 CD FC 39 55 29 4F 84 CF 52 79 CC A5 03 5A 1A 34 E6 30 39 C2 3F B1 E3 BE 13 99 B8 50 63 4D 45 4E 69 F8 5B 9A 46 4B A6 23 9C 8E 12 99 CD 4C B0 73 49 78 33 C6 8C 6B C1 94 9C D2 20 F3 87 61 0B EC D1 E3 FD CC 0E D1 9D 2D 20 EE 60 34 6E F6 4E 5E 41 C1 3B 06 B9 72 20 32 B2 35 30 6B 63 E8 BC 3B 27 AB D6 F5 70 B2 0D 4F 09 34 06 69 96 59 1B 37 0D C3 5B D8 2C D4 6A FA 9F 3D 5E E3 5B AF AF F0 D8 43 0B

 

1C E4 = 00011100 11100100

 

R1  = 0

R1  = 0

R1  = 0

FI  = 11 = First byte of the Data field does not correspond to the first byte of a RLC SDU,

           Last byte of the Data field does not correspond to the last byte of a RLC SDU

E   = 1  = A set of E field and LI field follows from the octet following the fixed part of the header

SN  = 0011100100 = 228

 

 

02 2F = 00000010 00101111

 

E  = 0

LI = 0000010 0010 = 34

Padding = 1111

 

 

SDU 1:

92 F1 62 09 15 25 34 19 DC E4 3D 46 DE 9A 44 A9 2B 3F CF 44

1A 36 BA B9 89 D6 14 54 F0 B5 F0 60 7C 7F

 

SDU 2: (The part in color is IP address of the data going through this RLC. This IP is nothing to do with RLC itself, but it will be a good example that multiple IP packets are distributed among multiple RLC PDUs)

 

80 55 45 00 02 50 0E 63 00 00 80 11 95 C8 01 01 C9 37 01 01

C9 38 04 E7 04 D2 02 3C 6C 19 47 00 44 14 29 50 78 3D EE C9

0C 23 9E CF A4 41 4E F1 E5 6B 81 72 B2 1A 57 CD FC 39 55 29

4F 84 CF 52 79 CC A5 03 5A 1A 34 E6 30 39 C2 3F B1 E3 BE 13

99 B8 50 63 4D 45 4E 69 F8 5B 9A 46 4B A6 23 9C 8E 12 99 CD

4C B0 73 49 78 33 C6 8C 6B C1 94 9C D2 20 F3 87 61 0B EC D1

E3 FD CC 0E D1 9D 2D 20 EE 60 34 6E F6 4E 5E 41 C1 3B 06 B9

72 20 32 B2 35 30 6B 63 E8 BC 3B 27 AB D6 F5 70 B2 0D 4F 09

34 06 69 96 59 1B 37 0D C3 5B D8 2C D4 6A FA 9F 3D 5E E3 5B

AF AF F0 D8 43 0B

 

 

Examples from real life (Live Network)

 

Refer to Live Network Examples in Fullstack. You may need to put some more effort to find out RLC layer log from this example, but you will see more diverse RLC PDU/SDU structure and see the relationship between RLC and other layers.

 

 

Variables, constants and timers for RLC

 

You may noticed that the overall data flow of RLC sounds relatively easy to understand. But understanding the RLC layer mechanism in such a detail that would help you with troubleshooting and RLC performance optimization would not be a simple task. For the level of understanding like this, you need to understand the details of 3GPP 36.322 section 7  (Variables, constants and timers) and 3GPP 36.322 section 5 (Procedures). I would suggest to study section 7 first and then the section 5.

 

Variable Name

Category

Description

VT(A)

State Variable

Acknowledgement state variable

This state variable holds the value of the SN of the next AMD PDU for which a positive acknowledgment is to be received in-sequence, and it serves as the lower edge of the transmitting window.

It is initially set to 0, and is updated whenever the AM RLC entity receives a positive acknowledgment for an AMD PDU with SN = VT(A).

VT(MS)

State Variable

Maximum send state variable

VT(MS) = VT(A) + AM_Window_Size

it serves as the higher edge of the transmitting window

VT(S)

State Variable

Send state variable

This state variable holds the value of the SN to be assigned for the next newly generated AMD PDU.

It is initially set to 0, and is updated whenever the AM RLC entity delivers an AMD PDU with SN = VT(S)

VR(R)

State Variable

Receive state variable

This state variable holds the value of the SN following the last in-sequence completely received AMD PDU

it serves as the lower edge of the receiving window. It is initially set to 0, and is updated whenever the AM RLC entity receives an AMD PDU with SN = VR(R).

VR(MR)

State Variable

Maximum acceptable receive state variable

VR(MR)=VR(R) + AM_Window_Size

it holds the value of the SN of the first AMD PDU that is beyond the receiving window and serves as the higher edge of the receiving window.

VR(X)

State Variable

t-Reordering state variable

This state variable holds the value of the SN following the SN of the RLC data PDU which triggered t-Reordering

VR(MS)

State Variable

Maximum STATUS transmit state variable

This state variable holds the highest possible value of the SN which can be indicated by 'ACK_SN' when a STATUS PDU needs to be constructed. It is initially set to 0

VR(H)

State Variable

Highest received state variable

This state variable holds the value of the SN following the SN of the RLC data PDU with the highest SN among received RLC data PDUs. It is initially set to 0.

VT(US)

State Variable

This state variable holds the value of the SN to be assigned for the next newly generated UMD PDU.

It is initially set to 0, and is updated whenever the UM RLC entity delivers an UMD PDU with SN = VT(US).

VR(UR)

State Variable

UM receive state variable

This state variable holds the value of the SN of the earliest UMD PDU that is still considered for reordering. It is initially set to 0

VR(UX)

State Variable

UM t-Reordering state variable

This state variable holds the value of the SN following the SN of the UMD PDU which triggered t-Reordering

VR(UH)

State Variable

UM highest received state variable

This state variable holds the value of the SN following the SN of the UMD PDU with the highest SN among received UMD PDUs, and it serves as the higher edge of the reordering window. It is initially set to 0

POLL_SN

State Variable

Poll send state variable

This state variable holds the value of VT(S)-1 upon the most recent transmission of a RLC data PDU with the poll bit set to '1'. It is initially set to 0.

PDU_WITHOUT_POLL

Counter

This counter is initially set to 0. It counts the number of AMD PDUs sent since the most recent poll bit was transmitted

BYTE_WITHOUT_POLL

Counter

This counter is initially set to 0. It counts the number of data bytes sent since the most recent poll bit was transmitted

RETX_COUNT

Counter

This counter counts the number of retransmissions of an AMD PDU. There is one RETX_COUNT counter per PDU that needs to be retransmitted.

AM_Window_Size

Constant

AM_Window_Size = 512

UM_Window_Size

Constant

UM_Window_Size = 16 when SN size = 5 bits

UM_Window_Size = 512 when SN size = 10 bits

UM_Window_Size = 0 when UM RLC is for MTCH or MCCH

t-PollRetransmit

Timer

used by the transmitting side of an AM RLC entity in order to retransmit a poll

t-Reordering

Timer

This timer is used by the receiving side of an AM RLC entity and receiving UM RLC entity in order to detect loss of RLC PDUs at lower layer. only one t-Reordering per RLC entity is running at a given time.

t-StatusProhibit

Timer

This timer is used by the receiving side of an AM RLC entity in order to prohibit transmission of a STATUS PDU

maxRetxThreshold

Configurable parameters

This parameter is used by the transmitting side of each AM RLC entity to limit the number of retransmissions of an AMD PDU

pollPDU

Configurable parameters

This parameter is used by the transmitting side of each AM RLC entity to trigger a poll for every pollPDU PDUs

pollByte

Configurable parameters

This parameter is used by the transmitting side of each AM RLC entity to trigger a poll for every pollByte bytes

sn-FieldLength

Configurable parameters

This parameter gives the UM SN field size in bits

 

 

Process of RLC UM Transmission

 

This section is about the RLC UM transmission procedure marked in yellow below. You can see the overall description from Overall Data Flow for UM RLC

 

<  36.322 Figure 4.2.1.2.1-1: Model of two unacknowledged mode peer entities >

 

The detailed procedure is described in 36.322 5.1.2.1 Transmit operations. (To understand this description, you have all of the variable description from Variables, constants and timers for RLC)

 

The process is simple. If I describe it in a psuedo code, it will be as follows.

i) VT(US) = SN (Sequence Number of the current PDU)

ii) VT(US) = VT(US) + 1

 

 

Process of RLC UM Reception

 

This section is about the RLC UM reception procedure marked in yellow below. You can see the overall description from Overall Data Flow for UM RLC

 

<  36.322 Figure 4.2.1.2.1-1: Model of two unacknowledged mode peer entities >

 

The reception process described in 36.322 5.1.2.2 Receive operations has three different steps.

 

< Step 1 : Actions when an UMD PDU is received from lower layer >

 

This process is described in 36.322 5.1.2.2.2 Actions when an UMD PDU is received from lower layer. I will summarise the process as follows.

 

if ( ( VR(UR) < x < VR(UH) ) AND (SN is overlapping with any one of previous PDUs) )

     OR ((VR(UH) ?UM_Window_Size) <= x < VR(UR) )

{

    discard the received UMD PDU;

} else {

    place the received UMD PDU in the reception buffer;

};

 

 

< Step 2 : Actions when an UMD PDU is placed in the reception buffer >

 

This process is described in 36.322 5.1.2.2.3 Actions when an UMD PDU is placed in the reception buffer. I will summarise the process as follows.

 

if ( SN is Not within the reordering window)

{

    i) VR(UH) = SN + 1;

    ii) reassemble RLC SDUs from any UMD PDUs with SN that falls outside of the reordering window, remove RLC headers when doing so and deliver the reassembled RLC SDUs to upper layer in ascending order of the RLC SN if not delivered before

          if (VR(UR) is Not within the reordering window) {

                    VR(UR) = (VR(UH) ?UM_Window_Size);

        }

}

 

if (the reception buffer contains an UMD PDU with SN = VR(UR) )

{

    i) VR(UR) = (the SN of the first UMD PDU with SN > current VR(UR) that has not been received);

    ii) reassemble RLC SDUs from any UMD PDUs with SN < updated VR(UR), remove RLC headers when doing

    so and deliver the reassembled RLC SDUs to upper layer in ascending order of the RLC SN if not delivered

    before;

}

 

if(t-Reordering is running) {

     if (VR(UX) <= VR(UR) ) OR (VR(UX) falls outside of the reordering window) AND (VR(UX) != VR(UH) ) {

             stop and reset t-Reordering;

     }

} else {

     if (VR(UH) > VR(UR) ) {

            i) start t-Reordering;

            ii) VR(UX) = VR(UH).

    }

}

 

 

Process of RLC AM Transmission

 

:: Coming soon

 

 

Process of RLC AM Reception

 

:: Coming soon

  

 

Some issues with RLC Verification/Validation

 

I think one of the biggest issues with RLC test would be that most of the test equipt does not provide much flexibility in terms of RLC layer testing. Most of the equipment seems to think of RLC as just an intermediate blocks between PHY/MAC and RRC/NAS and think that just providing a normal RLC functionality as a big chunk with very limited user configuration would be good enough, and most of the customer of those equipment vendor does not put many strong requests for it. It may be OK and I haven't seen many cases where RLC layer is causing any serious issues, but sometimes this layer is really causing some problem and once the problem happens in this layer it tend to be really tricky to pin point out the root cause of the problem.

 

Some of the typical issues which may caused by RLC layer would be

    i) When a bunch of RLC PDU/SDUs coming into RLC reciever with out-of sequence, the reciever fails to properly reorder it in correct sequence.

    ii) When a bunch of RLC PDU/SDUs coming into RLC reciever with some missing sequence number, the receiver fails to detect the exact missing sequence number and send proper RLC status report to the other party.

    iii) When a RLC sender send a RLC packet with Poll Bit Set, the reciever failed to reply with correct RLC status report or the status report within the specified time frame.

 

Probably this kind of problem would tend to occur in WCDMA low data rate configuration (e.g, R99) because there would be higher degree of fragmentation for a single C-plane or U-Plane user data packet. In this theory, LTE would have less problems since LTE can have very large data pipe that can carry a whole C-Plane/U-Plane packet with single transmission. But still there would be some possibility of RLC layer issues especially related to fragmentation and timing issues due to following reasons :

    i) In real network (live network), it is not guaranteed that Network would allocate enough resource for the UE so that it can transmit a whole U-plane/C-plane packet without fragmentation.

    ii) Even though the network would allocate the big enough resources for the UE, you cannot guarantee that it would allocate the resources before RLC timer get expired. (this would be less probable, but possible in theory).