# [Intern] 30/08/2022 RLC transmission modes ###### tags: `BMW-Lab`, `Intern` :::success **Goal:** Understand about RLC transmission modes. ::: :::info **Question List:** 1. How many transmission modes has RLC? 2. Procedures of RLC transmission modes? ::: :::success **References** - [5G Wikipedia](https://wiki.patras5g.eu/radio-equipment/g-node-bs) - [RLC transmission modes](https://www.sharetechnote.com/html/5G/5G_RLC.html#TM_Mode_Procedure) - [RLC](https://docs.google.com/presentation/d/1aG4JHkcXnJkXmmxBJmhfLz1QZNmiBq7T/edit?usp=sharing&ouid=109366638509638462838&rtpof=true&sd=true) ::: ## Background RLC (radio link control) is a layer 2 Radio Link Protocol used in UMTS, LTE and 5G on the Air interface. This protocol is specified by 3GPP in TS 25.322 for UMTS, TS 36.322 for LTE and TS 38.322 for 5G New Radio (NR). RLC is located on top of the 3GPP MAC-layer and below the PDCP-layer. # How many transmission modes has RLC? ![](https://imgur.com/5hsQ3rh.png) RLC (radio link control), which has three different transmission modes: transparent mode (TM), unacknowledged mode (UM) or acknowledged mode (AM), which are determined by the specific type of service. ## Procedures of RLC transmission modes? # TM Mode / Procedure ![](https://imgur.com/J73k0dK.png) As you see in the following illustration. TM mode would mean 'almost no processing to RLC data'. The only thing it does is to buffer data on Tx side. There is no RLC header, No reordering, no segmentation, no reassembly is happening in this layer. Because of this 'no data processing' nature of TM mode, if you compare the RLC input and RLC output data of TM mode, you would see no difference between the two. One important thing to keep in mind is that you need to pay attention to MAC/PHY resource allocation. Even if MAC/PHY resource is allocated smaller than the RLC packet, the RLC wouldn't care. It would just forward whatever it has to MAC/PHY. So those RLC data bigger than MAC/PHY resource may be chopped off or discarded. # UM Mode / Procedure ![](https://imgur.com/vDgM8Tq.png) 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 ? Let's read the operation on transmitting side first. If you just read (verbalize) the diagram i) Buffering the data and generate RLC Header. ii) Segmetation (Split a big chunk into a multiple small chunk) and Modify RLC Header (Some field in RLC header should be changed based on the segmentation status) iii) Add RLC header # AM Mode / Procedure ![](https://imgur.com/IVT4ZnM.png) '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. 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. What is different from UM mode lies in the middle column, namely 'Retransmission buffer' and 'RLC control' procedure. 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.