A protocol stack is a prescribed hierarchy of software layers, starting from the application layer at the top (the source of the data being sent) to the data link layer at the bottom (transmitting the bits on the wire). Every communication technology requires a competent protocol suite to function as required and the upcoming generations of wireless technology are no different. Imbibed in the architecture of 4G as well as 5G are their protocol stacks which are alike to some extent. Let’s analyze these 4G and 5G protocol stacks in detail with similarities and differences.
Given below is the image of LTE (4G) Protocol Stack:
The LTE (4G) Protocol Stack comprises of Layer 1, 2 and 3. Each Layer is explained in brief below:
- Physical Layer
Physical Layer carries all information from the MAC transport channels over the air interface. It takes care of the link adaptation (AMC), power control, cell search (for initial synchronization and handover purposes) and other measurements (inside the LTE system and between systems) for the RRC layer.
- Medium Access Layer (MAC)
MAC layer is responsible for Mapping between logical channels and transport channels, multiplexing of MAC SDUs from one or different logical channels onto transport blocks (TB) to be delivered to the physical layer on transport channels, de-multiplexing of MAC SDUs from one or different logical channels from transport blocks (TB) delivered from the physical layer on transport channels, scheduling information reporting, error correction through HARQ, priority handling between UEs by means of dynamic scheduling, priority handling between logical channels of one UE, and Logical Channel prioritization.
- Radio Link Control (RLC)
RLC operates in 3 modes of operation: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). It is responsible for transfer of upper layer PDUs, error correction through ARQ (only for AM data transfer), concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer). It is also responsible for re-segmentation of RLC data PDUs (only for AM data transfer), reordering of RLC data PDUs (Only for UM and AM data transfer), duplicate detection (only for UM and AM data transfer), RLC SDU discard (only for UM and AM data transfer), RLC re-establishment, and protocol error detection (only for AM data transfer).
- Packet Data Convergence Control (PDCP)
PDCP layer is responsible for Header compression and decompression of IP data, Transfer of data (user plane or control plane), maintenance of PDCP Sequence Numbers (SNs), in-sequence delivery of upper layer PDUs at re-establishment of lower layers, duplicate elimination of lower layer SDUs at re-establishment of lower layers for radio bearers mapped on RLC AM, ciphering and deciphering of user plane data and control plane data, integrity protection and integrity verification of control plane data, timer based discard, duplicate discarding, PDCP is used for SRBs and DRBs mapped on DCCH and DTCH type of logical channels.
- Radio Resource Control (RRC)
The main services and functions of the RRC sublayer include broadcast of System Information related to the non-access stratum (NAS), broadcast of System Information related to the access stratum (AS), paging, establishment, maintenance and release of an RRC connection between the UE and E-UTRAN, security functions including key management, establishment, configuration, maintenance and release of point-to-point Radio Bearers.
- Non-Access Stratum (NAS) Protocols
The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. They support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW.
Similar to 4G (LTE), with addition to the “SDAP” layer in 5G, depending upon the direction of data to be processed, 5G radio protocol stack has two categories:
- Control Plane (C-Plane) Stack – if the type of data is signaling or controlling message, then it is sent/forwarded through the control plane.
- User Plane Stack – user data is sent/ forwarded through the user plane.
Given below is an image depicting the layers in the User Plane Stack and the Control Plane Stack for 5G. We can see that the layers up to the PDCP layer are same in both the stacks, the difference is in the layers beyond the PDCP layer.
Given below is the image of LTE (4G) Protocol Stack:
Since it is similar to LTE, Layer 1 is the Physical layer. Layer 2 consists of the MAC, RLC and PDCP layers plus the additional SDAP layer on the User Plane Protocol Stack which isn’t in the Control Plane Protocol Stack. Layer 3 is in the Control Plane Protocol Stack and consists of the RRC and NAS layers.
The additional SDAP layer stands for Service Data Adaptation Protocol in the 5G Protocol Stack. It handles the new QoS framework of the 5G System (in the 5G Core). The introduction of SDAP enables end-to-end QoS framework that works in both directions. In particular, SDAP will map a specific QoS flow within a PDU Session to a corresponding Data Radio Bearer (which has been established with the appropriate level of QoS). In addition, SDAP will mark the transmitted packets with the correct QFI (QoS Flow ID), ensuring that the packet receives the correct forwarding treatment as it traverses the 5G System. It applies also to LTE when connected to the 5G Core.
In essence, the protocol stacks of 4G (LTE) and 5G NR are more or less alike with the exception of one extra layer in the latter, i.e., SDAP layer in the top most section of Layer 2 of 5G NR.
Note: At Expertlancing, our team of experts has expertise in protocol stacks and other telecom– related technologies. Please reach out to us for any kind of technology analysis related to telecom SEPs.
Written by – Yavnica & Ankit Puri