Chapter 4: RAN Internals¶
The description of the RAN in the previous chapter focused on functionality, but was mostly silent about the RAN’s internal structure. We now focus on some of the internal details, and in doing so, explain how the RAN is being transformed in 5G. This involves first describing the stages in the packet processing pipeline, and then showing how these stages can be disaggregated, distributed and implemented.
Our approach in this chapter is to incrementally build the RAN from the bottom up in the first three sections. Section 4.4 then summarizes the overall design, with a focus on how the resulting end-to-end system is architected to evolve.
4.1 Packet Processing Pipeline¶
Figure 19 shows the packet processing stages implemented by the base station. These stages are specified by the 3GPP standard. Note that the figure depicts the base station as a pipeline (running left-to-right for packets sent to the UE) but it is equally valid to view it as a protocol stack (as is typically done in official 3GPP documents). Also note that (for now) we are agnostic as to how these stages are implemented, but since we are ultimately heading towards a cloud-based implementation, you can think of each as corresponding to a microservice (if that is helpful).
The key stages are as follows.
RRC (Radio Resource Control) → Responsible for configuring the coarse-grain and policy-related aspects of the pipeline. The RRC runs in the RAN’s control plane; it does not process packets on the user plane.
PDCP (Packet Data Convergence Protocol) → Responsible for compressing and decompressing IP headers, ciphering and integrity protection, and making an “early” forwarding decision (i.e., whether to send the packet down the pipeline to the UE or forward it to another base station).
RLC (Radio Link Control) → Responsible for segmentation and reassembly, including reliably transmitting/receiving segments by implementing a form of ARQ (automatic repeat request).
MAC (Media Access Control) → Responsible for buffering, multiplexing and demultiplexing segments, including all real-time scheduling decisions about what segments are transmitted when. Also able to make a “late” forwarding decision (i.e., to alternative carrier frequencies, including Wi-Fi).
PHY (Physical Layer) → Responsible for coding and modulation (as discussed in an earlier chapter), including FEC.
The last two stages in Figure 19 (D/A conversion and the RF front-end) are beyond the scope of this book.
While it is simplest to view the stages in Figure 19 as a pure left-to-right pipeline, in practice the Scheduler running in the MAC stage implements the “main loop” for outbound traffic, reading data from the upstream RLC and scheduling transmissions to the downstream PHY. In particular, since the Scheduler determines the number of bytes to transmit to a given UE during each time period (based on all the factors outlined in an earlier chapter), it must request (get) a segment of that length from the upstream queue. In practice, the size of the segment that can be transmitted on behalf of a single UE during a single scheduling interval can range from a few bytes to an entire IP packet.
4.2 Split RAN¶
The next step is to understand how the functionality outlined above is partitioned between physical elements, and hence, “split” across centralized and distributed locations. The dominant option has historically been “no split,” with the entire pipeline shown in Figure 19 running in the base station. Going forward, the 3GPP standard has been extended to allow for multiple split-points, with the partition shown in Figure 20 being actively pursued by the operator-led O-RAN (Open RAN) Alliance. It is the split we adopt throughout the rest of this book.
This results in a RAN-wide configuration similar to that shown in Figure 21, where a single Central Unit (CU) running in the cloud serves multiple Distributed Units (DUs), each of which in turn serves multiple Radio Units (RUs). Critically, the RRC (centralized in the CU) is responsible for only near-real-time configuration and control decision making, while the Scheduler that is part of the MAC stage is responsible for all real-time scheduling decisions.
Because scheduling decisions for radio transmission are made by the MAC layer in real time, a DU needs to be “near” (within 1ms) the RUs it manages. (You can’t afford to make scheduling decisions based on out-of-date channel information.) One familiar configuration is to co-locate a DU and an RU in a cell tower. But when an RU corresponds to a small cell, many of which might be spread across a modestly-sized geographic area (e.g., a mall, campus, or factory), then a single DU would likely service multiple RUs. The use of mmWave in 5G is likely to make this later configuration all the more common.
Also note that the split-RAN changes the nature of the Backhaul Network, which in 4G connected the base stations (eNBs) back to the Mobile Core. With the split-RAN there are multiple connections, which are officially labeled as follows.
RU-DU connectivity is called the Fronthaul
DU-CU connectivity is called the Midhaul
CU-Mobile Core connectivity is called the Backhaul
One observation about the CU (which is relevant in the next chapter) is that one might co-locate the CU and Mobile Core in the same cluster, meaning the backhaul is implemented in the cluster switching fabric. In such a configuration, the midhaul then effectively serves the same purpose as the original backhaul, and the fronthaul is constrained by the predictable/low-latency requirements of the MAC stage’s real-time scheduler.
A second observation about the CU shown in Figure 20 is that it encompasses two functional blocks—the RRC and the PDCP—which lie on the RAN’s control plane and user plane, respectively. This separation is consistent with the idea of CUPS introduced in Chapter 3, and plays an increasingly important role as we dig deeper into how the RAN is implemented. For now, we note that the two parts are typically referred to as the CU-C and CU-U, respectively.
For more insight into design considerations for interconnecting the distributed components of a Split RAN, see RAN Evolution Project: Backhaul and Fronthaul Evolution. NGMN Alliance Report, March 2015.
4.3 Software-Defined RAN¶
We now describe how the RAN is implemented according to SDN principles, resulting in an SD-RAN. The key architectural insight is shown in Figure 22, where the RRC from Figure 19 is partitioned into two sub-components: the one on the left provides a 3GPP-compliant way for the RAN to interface to the Mobile Core’s control plane, while the one on the right opens a new programmatic API for exerting software-based control over the pipeline that implements the RAN user plane.
To be more specific, the left sub-component simply forwards control packets between the Mobile Core and the PDCP, providing a path over which the Mobile Core can communicate with the UE for control purposes, whereas the right sub-component implements the core of the RRC’s control functionality. This component is commonly referred to as the RAN Intelligent Controller (RIC) in O-RAN architecture documents, so we adopt this terminology. The “Near-Real Time” qualifier indicates the RIC is part of 10-100ms control loop implemented in the CU, as opposed to the ~1ms control loop required by the MAC scheduler running in the DU.
Completing the picture, Figure 23 shows the Near-RT RIC implemented as an SDN Controller hosting a set of SDN control apps. The RIC maintains a RAN Network Information Base (R-NIB)–a common set of information that can be consumed by numerous control apps. The R-NIB includes time-averaged CQI values and other per-session state (e.g., GTP tunnel IDs, QCI values for the type of traffic), while the MAC (as part of the DU) maintains the instantaneous CQI values required by the real-time scheduler. Specifically, the R-NIB includes the following state.
NODES: Base Stations and Mobile Devices
Base Station Attributes:
PHY resource usage
Mobile Device Attributes:
LINKS: Actual between two nodes and Potential between UEs and all neighbor cells
SLICES: Virtualized RAN Construct
MAC RRM Configuration
RRM Control Configuration
The example Control Apps in Figure 23 include a range of possibilities, but is not intended to be an exhaustive list. The right-most example, RAN Slicing, is the most ambitious in that it introduces a new capability: Virtualizing the RAN. It is also an idea that has been implemented, which we describe in more detail in the next chapter.
The next three (RF Configuration, Semi-Persistent Scheduling, Cipher Key Assignment) are examples of configuration-oriented applications. They provide a programmatic way to manage seldom-changing configuration state, thereby enabling zero-touch operations. Coming up with meaningful policies (perhaps driven by analytics) is likely to be an avenue for innovation in the future.
The left-most four example Control Applications are the sweet spot for SDN, with its emphasis on central control over distributed forwarding. These functions—Link Aggregation Control, Interference Management, Load Balancing, and Handover Control—are currently implemented by individual base stations with only local visibility, but they have global consequences. The SDN approach is to collect the available input data centrally, make a globally optimal decision, and then push the respective control parameters back to the base stations for execution. Realizing this value in the RAN is still a work-in-progress, but products that take this approach are emerging. Evidence using an analogous approach to optimize wide-area networks over many years is compelling.
While the above loosely categorizes the space of potential control apps as either config-oriented or control-oriented, another possible characterization is based on the current practice of controlling the mobile link at two different levels. At a fine-grain level, per-node and per-link control is conducted using Radio Resource Management (RRM) functions that are distributed across the individual base stations. RRM functions include scheduling, handover control, link and carrier aggregation control, bearer control, and access control. At a coarse-grain level, regional mobile network optimization and configuration is conducted using Self-Organizing Network (SON) functions. These functions oversee neighbor lists, manage load balancing, optimize coverage and capacity, aim for network-wide interference mitigation, centrally configure parameters, and so on. As a consequence of these two levels of control, it is not uncommon to see reference to RRM Applications and SON Applications, respectively, in O-RAN documents for SD-RAN.
For an example of how SDN principles have been successfully applied to a production network, we recommend B4: Experience with a Globally-Deployed Software Defined WAN. ACM SIGCOMM, August 2013.
4.4 Architect to Evolve¶
We conclude this description of RAN internals by re-visiting the sequence of steps involved in disaggregation, which as the previous three sections reveal, is being pursued in multiple tiers. In doing so, we tie up several loose ends, including the new interfaces disaggregation exposes. These interfaces define the pivot points around which 5G RAN is architected to evolve.
In the first tier of disaggregation, 3GPP standards provide multiple options of how horizontal RAN splits can take place. Horizontal disaggregation basically splits the RAN pipeline shown in Figure 19 into independently operating components. Figure 24 (a) illustrates horizontal disaggregation of the RAN from a single base station into three distinct components: CU, DU and RU. The O-RAN Alliance has selected specific disaggregation options from 3GPP and is developing open interfaces between these components. 3GPP defines the N2 and N3 interfaces between the RAN and the Mobile Core.
The second tier of disaggregation is vertical, focusing on control/user plane separation (CUPS) of the CU, and resulting in CU-U and CU-C shown in Figure 24 (b). The control plane in question is the 3GPP control plane, where the CU-U realizes a pipeline for user traffic and the CU-C focuses on control message signaling between Mobile Core and the disaggregated RAN components (as well as to the UE). The O-RAN specified interfaces between these disaggregated components are also shown in Figure 24 (b).
The third tier follows the SDN paradigm by carrying vertical disaggregation one step further. It does this by separating most of RAN control (RRM functions) from the disaggregated RAN components, and logically centralizing them as applications running on an SDN Controller, which corresponds to the Near-RT RIC shown previously in Figures 22 and 23. This SDN-based vertical disaggregation is repeated here in Figure 24 (c). The figure also shows the additional O-RAN prescribed interfaces.
The interface names are cryptic, and knowing their details adds little to our conceptual understanding of the RAN, except perhaps to re-enforce how challenging it is to introduce a transformative technology like Software-Defined Networking into an operational environment that is striving to achieve full backward compatibility and universal interoperability. That said, we call out two notable examples.
The first is the A1 interface that the mobile operator’s management plane—typically called the OSS/BSS (Operations Support System / Business Support System) in the Telco world—uses to configure the RAN. We have not discussed the Telco OSS/BSS up to this point, but it safe to assume such a component sits at the top of any Telco software stack. It is the source of all configuration settings and business logic needed to operate a network. Notice that the Management Plane shown in Figure 24 (c) includes a Non-Real-Time RIC functional block, complementing the Near-RT RIC that sits below the A1 interface. We return to the relevance of these two RICs in a moment.
The second is the E2 interface that the Near-RT RIC uses to control the underlying RAN elements. A requirement of the E2 interface is that it be able to connect the Near-RT RIC to different types of RAN elements. This range is reflected in the API, which revolves around a Service Model abstraction. The idea is that each RAN element advertises a Service Model, which effectively defines the set of RAN Functions the element is able to support. The RIC then issues a combination of the following four operations against this Service Model.
Report: RIC asks the element to report a function-specific value setting.
Insert: RIC instructs the element to activate a user plane function.
Control: RIC instructs the element to activate a control plane function.
Policy: RIC sets a policy parameter on one of the activated functions.
Of course, it is the RAN element, through its published Service Model, that defines the relevant set of functions that can be activated, the variables that can be reported, and policies that can be set.
Taken together, the A1 and E2 interfaces complete two of the three major control loops of the RAN: the outer (non-real-time) loop has the Non-RT RIC as its control point and the middle (near-real-time) loop has the Near-RT RIC as its control point. The third (inner) control loop, which is not shown in Figure 24, runs inside the DU: It includes the real-time Scheduler embedded in the MAC stage of the RAN pipeline. The two outer control loops have rough time bounds of >>1sec and >10ms, respectively, and as we saw in Chapter 2, the real-time control loop is assumed to be <1ms.
This raises the question of how specific functionality is distributed between the Non-RT RIC, Near-RT RIC, and DU. Starting with the second pair (i.e., the two inner loops), it is important to recognize that not all RRM functions can be centralized. After horizontal and vertical CUPS disaggregation, the RRM functions are split between CU-C and DU. For this reason, the SDN-based vertical disaggregation focuses on centralizing CU-C-side RRM functions in the Near-RT RIC. In addition to RRM control, this includes all the SON applications.
Turning to the outer two control loops, the Near RT-RIC opens the possibility of introducing policy-based RAN control, whereby interrupts (exceptions) to operator-defined policies would signal the need for the outer loop to become involved. For example, one can imagine developing learning-based controls, where the inference engines for these controls would run as applications on the Near RT-RIC, and their non-real-time learning counterparts would run elsewhere. The Non-RT RIC would then interact with the Near-RT RIC to deliver relevant operator policies from the Management Plane to the Near RT-RIC over the A1 interface.
Finally, you may be wondering why there is an O-RAN Alliance in the first place, given that 3GPP is already the standardization body responsible for interoperability across the global cellular network. The answer is that over time 3GPP has become a vendor-dominated organization, whereas O-RAN was created more recently by network operators. (AT&T and China Mobile were the founding members.) O-RAN’s goal is to catalyze a software-based implementation that breaks the vendor lock-in that dominates today’s marketplace. The E2 interface in particular, which is architected around the idea of supporting different Service Models, is central to this strategy. Whether the operators will be successful in their ultimate goal is yet to be seen.