SDAP (Service Data Adaptation Protocol)

LTE and NR QOS:

The User-Plane protocol structure of NR is developed on similar concepts like LTE, but obviously with some differences. The major difference in User Plane Protocol Structure in LTE and NR is the introduction of a new layer in the stack called SDAP – Service Data Adaptation Protocol.

Now a question can come in mind as to why a new layer was introduced in NR. The answer to this question is the difference in the Quality of Service (QoS) concept in LTE and NR. So, before we go on to understand the importance of SDAP in NR, let us first understand the subtle difference in QoS between LTE and NR.

          Fig1: LTE and NR QoS. Source: Stefan Rommer et al

NR QoS in Detail:

Now that we have a glimpse of the difference between the QoS framework between LTE and NR we can have an understanding that to cater to this new QoS requirement there will a need of a new layer in the NR Protocol Stack. We will now take a short deep dive into the QoS flow in NR to basically understand where the new layer SDAP layer fits in to the NR protocol stack.

Fig2: QoS flow to DRB mapping. Source: https://devopedia.org/5g-quality-of-service

Let us consider the diagram Fig 2, it will give us an understanding of 5G QoS and will also set the context about why SDAP was introduced. As we can see each QoS flow is identified by as QoS Flow Identifier (QFI). Generally, we have two types of QoS flow, GBR (Guaranteed Bit Rate) and Non GBR.

One important point to note is:

  1. At Non-Access Stratum (NAS), mapping of Uplink and Downlink Packets to respective QoS flow is done by Packet Filters.
  2. At AS mapping of QoS Flow to DRB is done by the rules set in UE and gNB. This is the rule that is being taken care by SDAP.

Now let us look at Fig2: Here we can see that there are packets generated from different applications like WhatsApp Video, YouTube, Voice, Netflix etc.

Each of these applications will have a separate Service Data Flow (SDF) created and will have separate QoS requirement.  Here Internet, Netflix and IMS are three separate PDU Sessions and each application( YouTube , Voice, WhatsApp) inside the PDU sessions are SDF.

Comparison of the data packets takes place in UPF by Packet Detection rules (PDR). There are QoS Enforcement rules (QER) associated with PDR. This QER contains the QFI values which are attached to the packet header and we can say that QFI is added to SDF.

Based on requirements one or two SDF can have same QFI values, and they will be mapped to same QoS flow as we can see in Fig 2. Here WhatsApp Video and Skype Video are two SDF and both have same QFI value 2, so we can see that they are mapped to same QoS flow 2.

Now after insertion of QFI at UPF, QoS Flows reaches the NGRAN (gNB). The NG-RAN now decides how to map the QoS flow to DRB. Here comes the role of SDAP. If there are multiple QoS flow needs to be multiplexed on a single DRB, this functionality is done by SDAP.

Now if we see in Fig2: QoS flow 2 and QoS flow 3 (carrying SDF of WhatsApp Video, Skype Video and You Tube Video) are mapped to a single DRB2. The SDAP is used for QoS Flow to DRB mapping.

Important : If NR-RAN decides to set DRB per QFI flow, then there is no need to have SDAP configured.

Now that we understand the need of SDAP layer in NR protocol stack, let us see where it fits in. Fig3, illustrates it. The SDAP layer is only configured for NR UE when it is connected to 5G core in SA configuration.

 Please note if the NR UE is in NSA configuration there will be no SDAP layer.

For LTE UE when the UE is connected to a 5G core, we can have SDAP layer configured for 4G UE also.

Fig3: NR protocol stack. Source: https://devopedia.org/5g-quality-of-service  , Dahlman et al

So from the protocol stack we can see that SDAP sits on top of PDCP layer. SDAP maps the IP packets coming from application layer to specific Data Radio Bearer.

Here we can see that IP packer n and IP packet n+1 is mapped to DRB X and IP packet m is mapped to DRB Y. The SDAP packet along with header is called SDAP PDU and the data coming from top of SDAP layer is called SDU (Service Date Unit)

                                                              SDAP PDU = SDAP SDU+HEADER

The SDAP PDU is then forwarded to PDCP layer for further processing.

SDAP HEADER:

RRC protocol configures the SDAP layer, and it can operate with or without a header information. The Header for SDAP is 1 Byte. Header information is included in DL in SDAP when there is a provision of reflective QoS else there is no header information added. The adding of header information can happen at the Access Stratum layer or also at the NAS layer based on the insertion of reflective QoS.

Fig4: Downlink SDAP data with Header                                                                              
Fig 5: Downlink SDAP data without Header

RDI (Reflective QoS flow to DRB Mapping Indication): This information instructs the UE to apply reflective QoS at the Access Stratum. It is 1 bit long. When 1 QoS Flow to DRB mapping is updated by UE.

RQI (Reflective QoS indication): This information instructs the UE to apply reflective QoS at the Non-Access Stratum layer. It is 1 bit long. When 1 NAS is informed by UE that SDF to QoS flow Mapping rules have been updated

QFI: For RDI and RQI QFI is required to understand the mapping between QoS flow to DRB. It is 6 Bit long.

Few more information:

  • The PDU session and QoS flow is identified by gNB by a GTP- U header.
  • After this the SDAP layer maps it into specific DRB.
  • If header is used for reflective QoS, SDAP layer specifies the QoS flow associated with the packet. By using this information, the UE can decode the mapping between the QoS flow and DRB for uplink transmission.
  • In Downlink the QoS Flow under the PDU session is identified using identity in GTP- U Header called GTP-U tunnel End Point Identifier (TEID)

Some Brainstorming now, with a cup of Tea

Q1. Is Ciphering applied on SDAP?

Ans: No Ciphering is not applicable in SDAP layer.

Q2. In which 3GPP specification SDAP is mentioned?

Ans: 3GPP TS 37.324

Q3. If 3GPP in NR protocol, why it is mentioned in 3GPP TS 37?

Ans: SDAP protocol can be configured for EUTRAN when LTE UE is connected to 5G Core, hence it is covered under TS 37 Series.

Q4. Can SDAP be configured for EUTRAN also?

Ans: Yes, SDAP can be configured for EUTRAN, when the LTE UE is connected to 5G core.

Q5.  Is it mandatory to use SDAP is there is a one to one mapping between DRB and QoS Flow?

Ans: No , in case the NR System decides that it will go for one to one mapping between DRB and QoS flow , the system can omit the functionality of SDAP.

References:

  1. NR in Bullets.
  2. Devopedia.org

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