Ran Protocol Stack ( SDAP, PDCP, RLC) Part-1

There are two different types of Ran Protocol Stack as follow

1) User-Plane Protocol Stack

2) Control-Plane Protocol Stack

Lets us discuss with the help of diagram

USER-PLANE PROTOCOL STACK

It supports in carrying the user data network between different applications of user equipment and in the network.

Each layers shown in above diagram provides the services to the layer above and consumes the services of the below layer.

Protocols :

1) SDAP — includes function for Quality of Service.

2) PDCP — includes function for Header Compensation, Ciphering & Integrity Protection, Duplicate Removal.

3) RLC — includes function for ARQ, Segmentation.

4) MAC — includes function for Retransmission, Multi/Demulti-plexing, Scheduling

5) PHY — includes function for Efficient Wireless Communication.

CONTROL-PLANE PROTOCOL STACK

It supports in carrying the control informations between the user equipment and gNodeB and the core-network.

Each layers shown in above diagram provides the services to the layer above and consumes the services of the below layer.

Protocols :

1) NAS — includes function for Authentication, Security, Idle mode procedure (Paging). It has no relation with Radio Access Network.

2) RRC — includes function for System Information, Radio Bearer, Measurement Configuration.

3) PDCP — includes function for Header Compensation, Ciphering & Integrity Protection, Duplicate Removal.

4) RLC — includes function for ARQ, Segmentation.

5) MAC — includes function for Retransmission, Multi/Demulti-plexing, Scheduling

6) PHY — includes function for Efficient Wireless Communication.

PROTOCOLS

1. SDAP (Service Data Adaptation Protocol)

It is the technology that manages the data trafficc to achieve the desired performance.

5G uses QoS Flows (QFI) to make it performance better in different data traffic.

QFI provides different Flow ID to each services so that each services has difference between them and does not affect each other services through which they will achieve the desired performance.

Radio Bearer — They are used only for the transmission of RRC and NAS messages.

N3 tunnel — All QoS Flows of a UE is bundled inside a N3 GTP-U tunnel between the gNB and the UPF in a 5G core network.

Types of QoS Flow

1) GBR :- This is used in application which requires guaranteed bit rate.

2) Non-GBR :- This is used for the traffic having bursty in nature.

3) Delay Critical :- This is used for the mission critical application.

Note: SDAP on gNB is responsible for mapping the QoS Flow to teh desired Radio-Bearer. So that, it provides same radio-bearer for whatsapp video call and zoom video call and different radio-bearer for the youtube as it is related to streaming. If in some case there is calling using 5G then it will creates its own PDU-Session.

2. PDCP (Packet Data Convergence Protocol)

The functions of PDCP are as follows:-

1) Header Compression

2) Ciphering and Integrity Security

3) Routing and Duplicating of Split-Bearer

4) In-Sequence Delivery

Header Compression

The length of header will increase according to the data packets such as if it was IPv4 their size will be 40 bytes and if IPv6 their size will be 60 bytes. So as to decrease/reduce of header PDCP is used.

Therefore PDCP compresses the length of header up to couple of bytes before wireless transmission and decompresses the header at receiving sides before transmitting to the IP-Protocol in wired network.

This compression is based on scheme known as ROHC.

Ciphering and Integrity Security

There are two types of security that PDCP looks for :-

1. Prevent Eavesdropping — To counter this we use PDCP Ciphering.

2. Data Origin — To verify data is originated from right source for this purpose it performs Integrity Protection.

Routing and Duplicating of Split-Bearer

Suppose multiple cell are connected to the User-Equipment then in this case we use split and duplicate the data so as to maintain the performance of services.

In-Sequence Delivery

In this , we provide the sequence number to each packets and also stores the PDU’s in the buffer until all the previous PDU’s is received .

3. RLC (Radio Link Control)

The functions of RLC are as follows :-

1. Segmentation

2. ARQ-Retransmission

There are three different RLC Modes :-

Transparent Mode — No retransmission, No duplicate detection, No segmentation/reassembly. This mode are useful for control-plane broadcast channels.

Unacknowledgement Mode — Segmentation but no retransmission. This is used when error free delivery is not required.

Acknowledgement Mode — Retransmission, Segmentation, Duplicate removal. This is used for downlink/uplink shared channel.

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