RAN Protocol Stack
Each layer provides the services to the layers above and consumes the services of the layers below.
PHY Layer:
- Mapping of data in transport blocks
- Responsible for efficiently transmitting over a wireless channel
- Channel coding: to protect digital information from noise and interference
- Add Modulation/demodulation techniques: to specify the no of bits that can be transmitted with a resource element
- Measures the air interface to know the channel quality
- Error correction
MAC Layer:
- Prioritization of data streams
- Error detection through HARQ
- Retransmission of Data: During the data flow from the PHY layer, errors occur. So, there is a need for fast feedback on whether the transmission was error-free or erroneous.
- Scheduling of resources
- multiplexing / demultiplexing — at gNB, SDAP maps QoS flow in the respective data bearers whereas UPF maps the IP flows into a certain QoS flow with a corresponding QFI depending on the priority.
RLC Layer:
- Robust error detection: The layer provides quite fast feedback on error detection during transmissions, but it cannot capture all the errors, so there is more robust error detection and transmission scheme available in the RLC layer.
- Concatenation, Segmentation, and reassembling: The RLC layer is responsible for breaking up the incoming data in terms of IP packets into smaller packets in a process called segmentation in the transmission site and puts them back together in a process called reassembly in that receiving site.
- Retransmission of data if necessary
- Reordering of the data packets
PDCP Layer:
- compression/ decompression of header: IP header has a lot of information that is no longer necessary, so the header of the IP packet is compressed and the information needs to be secured over the shared channel so nobody can eavesdrop(secretly listen)
- Addition of Security
- Adding sequence numbers to receive data in the correct order
- Checking duplicates and removing it
The user data has different QoS needs, so the user plane protocol stack has the SDAP layer that supports the QoS topics.
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The control plane needs protocols in place for managing the connection between the UE and 5G core networks. For this purpose, it has NAS and RRC layers.
SDAP Layer:
- Mapping QoS flows in the desired radio bearer
Control Plane Protocol Stack
NAS Layer:
- Governs the set of functions that operate between AMF in the core network and the UE.
- It includes authentication, security, and IDLE mode procedures such as paging.
- It is also responsible for allocating an IP address to the device.
RRC Layer:
- It is responsible for RAN-related control plane procedures like broadcasting system information which helps the UE learn the essential parameters about the cell, like cell id, time adjustment, etc.
- It is also responsible for transmitting paging messages, state transition
- Setting up radio bearers for RRC and NAS message exchange
- measurement configuration and reporting: UE has to do many types of measurements such as inter-frequency, intra-frequency, and internet measurement.
Service Data Unit (SDU) and Protocol Data Unit (PDU)
The data entity from or to a higher protocol layer is known as SDU.
This is the payload part of the current layer. To this payload, certain control information is added as a header and given as an output.
This output towards a lower protocol layer is called a PDU.
RLC SDU → PDCP PDU → process → PDCP SDU → compression → SDAP PDU
IP packets come from data networks → taken by SDAP → QoS flows are added to the radio bearers → SDAP SDU sent to PDCP → PDCP compress the headers and adds sequence no → PDCP SDU is formed which is sent to RLC layer → segmentation and reassembling of payload to map them in transport blocks → RLC SDU created → MAC error detection and HARQ retransmission of data → MAC layer header is distributed → mapping of payload in the transport blocks.
