Chapter 3: Basic Architecture¶
This chapter identifies the main architectural components of cellular access networks. It focuses on the components that are common to both 4G and 5G and, as such, establishes a foundation for understanding the advanced features of 5G presented in later chapters.
This overview is partly an exercise in introducing 3GPP terminology. For someone that is familiar with the Internet, this terminology can seem arbitrary (e.g., “eNB” is a “base station”), but it is important to keep in mind that this terminology came out of the 3GPP standardization process, which has historically been concerned about telephony and almost completely disconnected from the IETF and other Internet-related efforts. To further confuse matters, the 3GPP terminology often changes with each generation (e.g., a base station is called eNB in 4G and gNB in 5G). We address situations like this by using generic terminology (e.g., base station), and referencing the 3GPP-specific counterpart only when the distinction is helpful.
This example is only the tip of the terminology iceberg. For a slightly broader perspective on the complexity of terminology in 5G, see Marcin Dryjanski’s blog post: LTE and 5G Differences: System Complexity. July 2018.
3.1 Main Components¶
The cellular network provides wireless connectivity to devices that are on the move. These devices, which are known as User Equipment (UE), have traditionally corresponded to smartphones and tablets, but will increasingly include cars, drones, industrial and agricultural machines, robots, home appliances, medical devices, and so on.
As shown in Figure 6, the cellular network consists of two main subsystems: the Radio Access Network (RAN) and the Mobile Core. The RAN manages the radio spectrum, making sure it is used efficiently and meets the quality-of-service requirements of every user. It corresponds to a distributed collection of base stations. As noted above, in 4G these are (somewhat cryptically) named eNodeB (or eNB), which is short for evolved Node B. In 5G they are known as gNB. (The g stands for “next Generation”.)
The Mobile Core is a bundle of functionality (as opposed to a device) that serves several purposes.
Provides Internet (IP) connectivity for both data and voice services.
Ensures this connectivity fulfills the promised QoS requirements.
Tracks user mobility to ensure uninterrupted service.
Tracks subscriber usage for billing and charging.
Note that Mobile Core is another example of a generic term. In 4G this is called the Evolved Packet Core (EPC) and in 5G it is called the Next Generation Core (NG-Core).
Even though the word “Core” is in its name, from an Internet perspective, the Mobile Core is still part of the access network, effectively providing a bridge between the RAN in some geographic area and the greater IP-based Internet. 3GPP provides significant flexibility in how the Mobile Core is geographically deployed, but for our purposes, assuming each instantiation of the Mobile Core serves a metropolitan area is a good working model. The corresponding RAN would then span several dozens (or even hundreds) of cell towers.
Taking a closer look at Figure 6, we see that a Backhaul Network interconnects the base stations that implement the RAN with the Mobile Core. This network is typically wired, may or may not have the ring topology shown in the Figure, and is often constructed from commodity components found elsewhere in the Internet. For example, the Passive Optical Network (PON) that implements Fiber-to-the-Home is a prime candidate for implementing the RAN backhaul. The backhaul network is obviously a necessary part of the RAN, but it is an implementation choice and not prescribed by the 3GPP standard.
Although 3GPP specifies all the elements that implement the RAN and Mobile Core in an open standard—including sub-layers we have not yet introduced—network operators have historically bought proprietary implementations of each subsystem from a single vendor. This lack of an open source implementation contributes to the perceived “opaqueness” of the cellular network in general, and the RAN in particular. And while it is true that an eNodeB implementation does contain sophisticated algorithms for scheduling transmission on the radio spectrum—algorithms that are considered valuable intellectual property of the equipment vendors—there is significant opportunity to open and disaggregate both the RAN and the Mobile Core. The following two sections describe each, in turn.
Before getting to those details, Figure 7 redraws components from Figure 6 to highlight two important distinctions. The first is that a base station has an analog component (depicted by an antenna) and a digital component (depicted by a processor pair). The second is that the Mobile Core is partitioned into a Control Plane and User Plane, which is similar to the control/data plane split that someone familiar with the Internet would recognize. (3GPP also recently introduced a corresponding acronym—CUPS, Control and User Plane Separation—to denote this idea.) The importance of these two distinctions will become clear in the following discussion.
3.2 Radio Access Network¶
We now describe the RAN by sketching the role each base station plays. Keep in mind this is kind of like describing the Internet by explaining how a router works—a not unreasonable place to start, but it doesn’t fully do justice to the end-to-end story.
First, each base station establishes the wireless channel for a subscriber’s UE upon power-up or upon handover when the UE is active. This channel is released when the UE remains idle for a predetermined period of time. Using 3GPP terminology, this wireless channel is said to provide a bearer service. The term “bearer” has historically been used in telecommunications (including early wireline technologies like ISDN) to denote a data channel, as opposed to a channel that carries signaling information.
Second, each base station establishes “3GPP Control Plane” connectivity between the UE and the corresponding Mobile Core Control Plane component, and forwards signaling traffic between the two. This signaling traffic enables UE authentication, registration, and mobility tracking.
Third, for each active UE, the base station establishes one or more tunnels between the corresponding Mobile Core User Plane component.
Fourth, the base station forwards both control and user plane packets between the Mobile Core and the UE. These packets are tunnelled over SCTP/IP and GTP/UDP/IP, respectively. SCTP (Stream Control Transport Protocol) is an alternative reliable transport to TCP, tailored to carry signaling (control) information for telephony services. GTP (a nested acronym corresponding to (General Packet Radio Service) Tunneling Protocol) is a 3GPP-specific tunneling protocol designed to run over UDP.
As an aside, it is noteworthy that connectivity between the RAN and the Mobile Core is IP-based. This was introduced as one of the main changes between 3G and 4G. Prior to 4G, the internals of the cellular network were circuit-based, which is not surprising given its origins as a voice network.
Fifth, each base station coordinates UE handovers with neighboring base stations, using direct station-to-station links. Exactly like the station-to-core connectivity shown in the previous figure, these links are used to transfer both control plane (SCTP over IP) and user plane (GTP over UDP/IP) packets.
Sixth, the base stations coordinate wireless multi-point transmission to a UE from multiple base stations, which may or may not be part of a UE handover from one base station to another.
The main takeaway is that the base station can be viewed as a specialized forwarder. In the Internet-to-UE direction, it fragments outgoing IP packets into physical layer segments and schedules them for transmission over the available radio spectrum, and in the UE-to-Internet direction it assembles physical layer segments into IP packets and forwards them (over a GTP/UDP/IP tunnel) to the upstream user plane of the Mobile Core. Also, based on observations of the wireless channel quality and per-subscriber policies, it decides whether to (a) forward outgoing packets directly to the UE, (b) indirectly forward packets to the UE via a neighboring base station, or (c) utilize multiple paths to reach the UE. The third case has the option of either spreading the physical payloads across multiple base stations or across multiple carrier frequencies of a single base station (including Wi-Fi).
Note that as outlined in Chapter 2, scheduling is complex and multi-faceted, even when viewed as a localized decision at a single base station. What we now see is that there is also a global element, whereby it’s possible to forward traffic to a different base station (or to multiple base stations) in an effort to make efficient use of the radio spectrum over a larger geographic area.
In other words, the RAN as a whole (i.e., not just a single base station) not only supports handovers (an obvious requirement for mobility), but also link aggregation and load balancing, mechanisms that are familiar to anyone who understands the Internet. We will revisit how such RAN-wide (global) decisions can be made using SDN techniques in a later chapter.
3.3 Mobile Core¶
The main function of the Mobile Core is to provide external packet data network (i.e., Internet) connectivity to mobile subscribers, while ensuring that they are authenticated and their observed service qualities satisfy their subscription SLAs. An important aspect of the Mobile Core is that it needs to manage all subscribers’ mobility by keeping track of their last whereabouts at the granularity of the serving base station. It’s the fact that the Mobile Core is keeping track of individual subscribers—something that the Internet’s core does not do—that creates a lot of the complexity in its architecture, especially given that those subscribers are moving around.
While the aggregate functionality remains largely the same as we migrate from 4G to 5G, how that functionality is virtualized and factored into individual components changes. The 5G Mobile Core is heavily influenced by the cloud’s march toward a microservice-based (cloud native) architecture. This shift to cloud native is deeper than it might first appear, in part because it opens the door to customization and specialization. Instead of supporting just voice and broadband connectivity, the 5G Mobile Core can evolve to also support, for example, massive IoT, which has a fundamentally different latency requirement and usage pattern (i.e., many more devices connecting intermittently). This stresses—if not breaks—a one-size-fits-all approach to session management.
4G Mobile Core¶
The 4G Mobile Core, which 3GPP officially refers to as the Evolved Packet Core (EPC), consists of five main components, the first three of which run in the Control Plane (CP) and the second two of which run in the User Plane (UP).
MME (Mobility Management Entity): Tracks and manages the movement of UEs throughout the RAN. This includes recording when the UE is not active.
HSS (Home Subscriber Server): A database that contains all subscriber-related information.
PCRF (Policy & Charging Rules Function): Tracks and manages policy rules and records billing data on subscriber traffic.
SGW (Serving Gateway): Forwards IP packets to and from the RAN. Anchors the Mobile Core end of the bearer service to a (potentially mobile) UE, and so is involved in handovers from one base station to another.
PGW (Packet Gateway): Essentially an IP router, connecting the Mobile Core to the external Internet. Supports additional access-related functions, including policy enforcement, traffic shaping, and charging.
Although specified as distinct components, in practice the SGW (RAN-facing) and PGW (Internet-facing) are often combined in a single device, commonly referred to as an S/PGW. The end result is illustrated in Figure 14.
Note that 3GPP is flexible in how the Mobile Core components are deployed to serve a geographic area. For example, a single MME/PGW pair might serve a metropolitan area, with SGWs deployed across ~10 edge sites spread throughout the city, each of which serves ~100 base stations. But alternative deployment configurations are allowed by the spec.
5G Mobile Core¶
The 5G Mobile Core, which 3GPP calls the NG-Core, adopts a microservice-like architecture, where we say “microservice-like” because while the 3GPP specification spells out this level of disaggregation, it is really just prescribing a set of functional blocks and not an implementation. A set of functional blocks is very different from the collection of engineering decisions that go into designing a microservice-based system. That said, viewing the collection of components shown in Figure 15 as a set of microservices is a good working model.
The following organizes the set of functional blocks into three groups. The first group runs in the Control Plane (CP) and has a counterpart in the EPC.
AMF (Core Access and Mobility Management Function): Responsible for connection and reachability management, mobility management, access authentication and authorization, and location services. Manages the mobility-related aspects of the EPC’s MME.
SMF (Session Management Function): Manages each UE session, including IP address allocation, selection of associated UP function, control aspects of QoS, and control aspects of UP routing. Roughly corresponds to part of the EPC’s MME and the control-related aspects of the EPC’s PGW.
PCF (Policy Control Function): Manages the policy rules that other CP functions then enforce. Roughly corresponds to the EPC’s PCRF.
UDM (Unified Data Management): Manages user identity, including the generation of authentication credentials. Includes part of the functionality in the EPC’s HSS.
AUSF (Authentication Server Function): Essentially an authentication server. Includes part of the functionality in the EPC’s HSS.
The second group also runs in the Control Plane (CP) but does not have a direct counterpart in the EPC:
SDSF (Structured Data Storage Network Function): A “helper” service used to store structured data. Could be implemented by an “SQL Database” in a microservices-based system.
UDSF (Unstructured Data Storage Network Function): A “helper” service used to store unstructured data. Could be implemented by a “Key/Value Store” in a microservices-based system.
NEF (Network Exposure Function): A means to expose select capabilities to third-party services, including translation between internal and external representations for data. Could be implemented by an “API Server” in a microservices-based system.
NRF (NF Repository Function): A means to discover available services. Could be implemented by a “Discovery Service” in a microservices-based system.
NSSF (Network Slicing Selector Function): A means to select a Network Slice to serve a given UE. Network slices are essentially a way to partition network resources in order to differentiate service given to different users. It is a key feature of 5G that we discuss in depth in a later chapter.
The third group includes the one component that runs in the User Plane (UP):
UPF (User Plane Function): Forwards traffic between RAN and the Internet, corresponding to the S/PGW combination in EPC. In addition to packet forwarding, it is responsible for policy enforcement, lawful intercept, traffic usage reporting, and QoS policing.
Of these, the first and third groups are best viewed as a straightforward refactoring of 4G’s EPC, while the second group—despite the gratuitous introduction of new terminology—is 3GPP’s way of pointing to a cloud native solution as the desired end-state for the Mobile Core. Of particular note, introducing distinct storage services means that all the other services can be stateless, and hence, more readily scalable. Also note that Figure 15 adopts an idea that’s common in microservice-based systems, namely, to show a message bus interconnecting all the components rather than including a full set of pairwise connections. This also suggests a well-understood implementation strategy.
Stepping back from these details, and with the caveat that we are presuming an implementation, the main takeaway is that we can conceptualize the Mobile Core as a graph of services. You will sometimes hear this called a Service Graph or Service Chain, the latter being more prevalent in NFV-oriented documents. Another term, Service Mesh, has taken on a rather specific meaning in cloud native terminology—we’ll avoid overloading that term here. 3GPP is silent on the specific terminology since it is considered an implementation choice rather than part of the specification. We describe our implementation choices in later chapters.
3.4 Security and Mobility¶
We now take a closer look at two unique features of the cellular network—its support for security and mobility—both of which differentiate it from WiFi. The following also serves to fill in some details about how each individual UE connects to the network.
We start with the security architecture, which is grounded in two trust assumptions. First, each Base Station trusts that it is connected to the Mobile Core by a secure private network, over which it establishes the tunnels introduced in Figure 11: a GTP/UDP/IP tunnel to the Core’s User Plane (Core-UP) and a SCTP/IP tunnel to the Core’s Control Plane (Core-CP). Second, each UE has an operator-provided SIM card, which uniquely identifies the subscriber (i.e., phone number) and establishes the radio parameters (e.g., frequency band) needed to communicate with that operator’s Base Stations. The SIM card also includes a secret key that the UE uses to authenticate itself.
With this starting point, Figure 16 shows the per-UE connection sequence. When a UE first becomes active, it communicates with a nearby Base Station over a temporary (unauthenticated) radio link (Step 1). The Base Station forwards the request to the Core-CP over the existing tunnel, and the Core-CP (specifically, the MME in 4G and the AMF in 5G) initiates an authentication protocol with the UE (Step 2). 3GPP identifies a set of options for authentication and encryption, where the actual protocols used are an implementation choice. For example, Advanced Encryption Standard (AES) is one of the options for encryption. Note that this authentication exchange is initially in the clear since the Base Station to UE link is not yet secure.
Once the UE and Core-CP are satisfied with each other’s identity, the Core-CP informs the other components of the parameters they will need to service the UE (Step 3). This includes: (a) instructing the Core-UP to initialize the user plane (e.g., assign an IP address to the UE and set the appropriate QCI parameter); (b) instructing the Base Station to establish an encrypted channel to the UE; and (c) giving the UE the symmetric key it will need to use the encrypted channel with the Base Station. The symmetric key is encrypted using the public key of the UE (so only the UE can decrypt it, using its secret key). Once complete, the UE can use the end-to-end user plane channel through the Core-UP (Step 4).
There are three additional details of note about this process. First, the secure control channel between the UE and the Core-CP set up during Step 2 remains available, and is used by the Core-CP to send additional control instructions to the UE during the course of the session.
Second, the user plane channel established during Step 4 is referred to as the Default Bearer Service, but additional channels can be established between the UE and Core-UP, each with a potentially different QCI value. This might be done on an application-by-application basis, for example, under the control of the Mobile Core doing Deep Packet Inspection (DPI) on the traffic, looking for flows that require special treatment.
Third, while the resulting user plane channels are logically end-to-end, each is actually implemented as a sequence of per-hop tunnels, as illustrated in Figure 17. (The figure shows the SGW and PGW from the 4G Mobile Core to make the example more concrete.) This means each component on the end-to-end path terminates a downstream tunnel using one local identifier for a given UE, and initiates an upstream tunnel using a second local identifier for that UE. In practice, these per-flow tunnels are often bundled into an single inter-component tunnel, which makes it impossible to differentiate the level of service given to any particular end-to-end UE channel. This is a limitation of 4G that 5G has ambitions to correct.
Support for mobility can now be understood as the process of re-executing one or more of the steps shown in Figure 16 as the UE moves throughout the RAN. The unauthenticated link indicated by (1) allows the UE to be known to all Base Station within range. (We refer to these as potential links in later chapters.) Based on the signal’s measured CQI, the Base Stations communicate directly with each other to make a handover decision. Once made, the decision is then communicated to the Mobile Core, re-triggering the setup functions indicated by (3), which in turn re-builds the user plane tunnel between the Base Station and the SGW shown in Figure 17 (or correspondingly, between the Base Station and the UPF in 5G). One of the most unique features of the cellular network is that the Mobile Core’s user plane (e.g., UPF in 5G) buffers data during the handover transition, avoiding dropped packets and subsequent end-to-end retransmissions.
In other words, the cellular network maintains the UE session in the face of mobility (corresponding to the control and data channels depicted by (2) and (4) in Figure 16, respectively), but it is able to do so only when the same Mobile Core serves the UE (i.e., only the Base Station changes). This would typically be the case for a UE moving within a metropolitan area. Moving between metro areas—and hence, between Mobile Cores—is indistinguishable from power cycling a UE. The UE is assigned a new IP address and no attempt is made to buffer and subsequently deliver in-flight data. Independent of mobility, but relevant to this discussion, any UE that becomes inactive for a period of time also loses its session, with a new session established and a new IP address assigned when the UE becomes active again.
Note that this session-based approach can be traced to the cellular network’s roots as a connection-oriented network. An interesting thought experiment is whether the Mobile Core will continue to evolve so as to better match the connectionless assumptions of the Internet protocols that typically run on top of it.
3.5 Deployment Options¶
With an already deployed 4G RAN/EPC in the field and a new 5G RAN/NG-Core deployment underway, we can’t ignore the issue of transitioning from 4G to 5G (an issue the IP-world has been grappling with for 20 years). 3GPP officially spells out multiple deployment options, which can be summarized as follows.
Standalone 4G / Stand-Alone 5G
Non-Standalone (4G+5G RAN) over 4G’s EPC
Non-Standalone (4G+5G RAN) over 5G’s NG-Core
The second of the three options, which is generally referred to as “NSA“, involves 5G base stations being deployed alongside the existing 4G base stations in a given geography to provide a data-rate and capacity boost. In NSA, control plane traffic between the user equipment and the 4G Mobile Core utilizes (i.e., is forwarded through) 4G base stations, and the 5G base stations are used only to carry user traffic. Eventually, it is expected that operators complete their migration to 5G by deploying NG Core and connecting their 5G base stations to it for Standalone (SA) operation. NSA and SA operations are illustrated in Figure 18.
One reason we call attention to the phasing issue is that we face a similar challenge in the chapters that follow. The closer the following discussion gets to implementation details, the more specific we have to be about whether we are using 4G components or 5G components. As a general rule, we use 4G components—particularly with respect to the Mobile Core, since that’s what’s available in open source today—and trust the reader can make the appropriate substitution without loss of generality. Like the broader industry, the open source community is in the process of incrementally evolving its 4G code base into its 5G-compliant counterpart.
For more insight into 4G to 5G migration strategies, see Road to 5G: Introduction and Migration. GSMA Report, April 2018.