SDAP (Service Data Adaptation Protocol) in 5G-NR

LTE and NR QOS:

The way data moves in NR (that's like super-fast internet) is based on ideas similar to LTE (kind of like fast internet), but there are some differences. One big difference between how data is handled in LTE and NR is that NR has a new layer in its setup called SDAP – which stands for Service Data Adaptation Protocol.

Now, you might wonder why they added this new layer in NR. The reason has to do with how they make sure the quality of your service (QoS) is good in LTE and NR. So, before we talk about why SDAP is important in NR, let's first understand the small differences in how they make sure your service is good between LTE and NR:

• NR's Structure: NR is built similarly to LTE but has its own changes.
• SDAP Introduction: NR has SDAP, a new layer, that LTE doesn't have.
• QoS Difference: The way they make sure your service is good is a bit different in LTE and NR.
• Why SDAP in NR: They added SDAP in NR because they manage service quality differently.
• Understanding QoS: Before we talk about SDAP's importance in NR, let's grasp the slight differences in how they manage service quality in LTE and NR.

NR QoS in Detail:

Having looked at the variations in the Quality of Service (QoS) approach between LTE and NR, we can recognize the necessity for a fresh layer within the NR Protocol Stack to address these updated QoS demands. Let's now take a brief and more detailed exploration of the QoS process in NR. This will help us gain insight into how the new SDAP layer is integrated into the NR protocol stack.

Taking a look at Figure 2 in the diagram can provide us with insights into 5G Quality of Service (QoS) and help us grasp the rationale behind the introduction of SDAP. In this visual representation, each QoS flow is distinguished by a unique QoS Flow Identifier (QFI). Typically, there are two main types of QoS flows: Guaranteed Bit Rate (GBR) and Non GBR.

One important point to note is:

1.????Within the Non-Access Stratum (NAS), the assignment of Uplink and Downlink Packets to their respective QoS flows is achieved through Packet Filters.

2.????In the Access Stratum (AS), the alignment of QoS Flows with Data Radio Bearers (DRBs) is governed by the rules established within User Equipment (UE) and gNB. It's important to note that this particular rule is managed by SDAP.

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Looking at Figure 2, we observe the presence of packets generated by different applications such as WhatsApp Video, YouTube, Voice, and Netflix.

• Distinct Service Data Flows (SDFs): Each of these applications triggers the establishment of a separate Service Data Flow (SDF), each having distinct Quality of Service (QoS) requirements.
• PDU Sessions and SDFs: In the context of this illustration, we find three separate PDU Sessions: Internet, Netflix, and IMS. Within each of these sessions, applications like YouTube, Voice, and WhatsApp are categorized as individual SDFs.
• Packet Comparison and PDR: The process of analysing the data packets occurs within the User Plane Function (UPF) through the utilization of Packet Detection Rules (PDR).
• QoS Enforcement Rules (QER): Associated with the PDRs are QoS Enforcement Rules (QERs), which encompass Quality of Service Indicator (QFI) values attached to the packet headers. This effectively integrates QFI information into the respective SDFs.
• Shared QFI Values: Depending on specific requirements, one or more SDFs might share the same QFI value. In such instances, these shared-QFI SDFs are mapped to the same QoS flow. For instance, both WhatsApp Video and Skype Video are distinct SDFs, yet they share the QFI value 2, leading to their mapping to the same QoS flow labeled as QoS flow 2.
• QFI Insertion: After the QFI is embedded within the UPF, the QoS Flows are transmitted to the Next Generation Radio Access Network (NG-RAN), embodied by the gNB.
• Mapping QoS Flows to DRBs: Within the NG-RAN, the determination of how to map the QoS flows onto Data Radio Bearers (DRBs) takes place.

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Role of SDAP: This is where the Service Data Adaptation Protocol (SDAP) becomes pivotal. In scenarios where multiple QoS flows require multiplexing onto a single DRB, SDAP fulfils this critical function.

In accordance with Figure 2, a notable observation is that both QoS flow 2 and QoS flow 3—carrying SDFs associated with WhatsApp Video, Skype Video, and YouTube Video—are jointly mapped to a singular DRB2.

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The SDAP is used for QoS Flow to DRB mapping.

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Note : 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.?

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)

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???????????????????????????????????????????????SDAP PDU = SDAP SDU+HEADER

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

SDAP HEADER:

• SDAP Layer Configuration: The RRC protocol is responsible for configuring the SDAP layer, which has the flexibility to function with or without header information.
• Header Size: The SDAP layer's header is 1 Byte in size.
• Header Inclusion: In the Downlink (DL) transmission through SDAP, the header information is incorporated when there exists a reflective Quality of Service (QoS) provision. Conversely, if no reflective QoS is present, header information isn't added.
• Header Addition Points: The addition of header information can occur at either the Access Stratum (AS) layer or the Non-Access Stratum (NAS) layer, contingent upon the presence of reflective QoS.

• RDI (Reflective QoS flow to DRB Mapping Indication): This 1-bit information signals the User Equipment (UE) to implement reflective Quality of Service (QoS) at the Access Stratum layer. When set to 1, it signifies that the QoS Flow to Data Radio Bearer (DRB) mapping has been updated by the UE.
• RQI (Reflective QoS indication): This 1-bit data instructs the UE to apply reflective QoS at the Non-Access Stratum layer. A value of 1 indicates that the UE has informed the Non-Access Stratum (NAS) layer about updates in the Service Data Flow (SDF) to QoS flow Mapping rules.?
• QFI (QoS Flow Identifier): A 6-bit value that is integral for both RDI and RQI. It facilitates the comprehension of the mapping between QoS flows and Data Radio Bearers (DRBs).

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)

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Some Brainstorming now, with a cup of?

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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