Chapter 1: Introduction
Mobile networks, which have a 40-year history that parallels the Internet’s, have undergone significant change. The first two generations supported voice and then text, with 3G defining the transition to broadband access, supporting data rates measured in hundreds of kilobits per second. Today, the industry is transitioning from 4G (with data rates typically measured in the few megabits per second) to 5G, with the promise of a tenfold increase in data rates.
But 5G is about much more than increased bandwidth. 5G represents a fundamental rearchitecting of the access network in a way that leverages several key technology trends and sets it on a path to enable much greater innovation. In the same way that 3G defined the transition from voice to broadband, 5G’s promise is primarily about the transition from a single access service (broadband connectivity) to a richer collection of edge services and devices. 5G is expected to provide support for immersive user interfaces (e.g., Augmented Reality, Virtual Reality), mission-critical applications (e.g., public safety, autonomous vehicles), and the Internet of Things (IoT). Because these use cases will include everything from home appliances to industrial robots to self-driving cars, 5G will support not only humans accessing the Internet from their smartphones, but also swarms of autonomous devices working together on their behalf.
There is more to supporting these services than just improving bandwidth or latency to individual users. As we will see, a fundamentally different edge network architecture is required. The requirements for this architecture are ambitious, and can be illustrated by three classes of capabilities:
To support Massive Internet of Things, potentially including devices with ultra-low energy (10+ years of battery life), ultra-low complexity (10s of bits per second), and ultra-high density (1 million nodes per square kilometer).
To support Mission-Critical Control, potentially including ultra-high availability (greater than 99.999% or “five nines”), ultra-low latency (as low as 1 ms), and extreme mobility (up to 100 km/h).
To support Enhanced Mobile Broadband, potentially including extreme data rates (multi-Gbps peak, 100+ Mbps sustained) and extreme capacity (10 Tbps of aggregate throughput per square kilometer).
These targets will certainly not be met overnight, but that’s in keeping with each generation of the mobile network being a decade-long endeavor.
On top of these quantitative improvements to the capabilities of the access network, 5G is being viewed as a chance for building a platform to support innovation. Whereas prior access networks were generally optimized for known services (such as voice calls and SMS), the Internet has been hugely successful in large part because it supported a wide range of applications that were not even thought of when it was first designed. The 5G network is designed with this same goal: enabling future applications beyond those we fully recognize today. For an example of the grand vision for 5G, see the whitepaper from one of the industry leaders.
Qualcomm Whitepaper. Making 5G NR a Reality. December 2016.
The 5G mobile network, because it is on an evolutionary path and not a point solution, includes standardized specifications, a range of implementation choices, and a long list of aspirational goals. Because this leaves so much room for interpretation, our approach to describing 5G is grounded in three mutually supportive principles. The first is to apply a systems lens, which is to say, we explain the sequence of design decisions that lead to a solution rather than fall back on enumerating the overwhelming number of acronyms or individual point technologies as a fait accompli. The second is to aggressively disaggregate the system. Building a disaggregated, virtualized, and software-defined 5G access network is the direction the industry is already headed (for good technical and business reasons), but breaking the 5G network down into its elemental components is also the best way to explain how 5G works.
The third principle is to illustrate how 5G can be realized in practice by drawing on specific engineering decisions made in an open source implementation. This implementation leverages best practices in building cloud apps, which is an essential aspect of 5G evolving into a platform for new services. This implementation also targets enterprises that are increasingly deploying 5G locally, and using it to help automate their manufacturing, retail, and business practices—a trend that has been dubbed Industry 4.0. Such enterprise-level deployments are known as Private 5G, but there is nothing about the technical approach that couldn’t be adopted throughout the more traditional “public mobile network” that comes to mind when you think about your cell service today. The only difference is that private deployments are more aggressively embracing the cloud practices that will ultimately distinguish 5G from earlier generations.
K. Schwab. The Fourth Industrial Revolution. World Economic Forum.
What this all means is that there is no simple definition of 5G, any more than there is for the Internet. It is a complex and evolving system, constrained by a set of standards that purposely give all the stakeholders many degrees of freedom. In the chapters that follow, it should be clear from the context whether we are talking about standards (what everyone must do to interoperate), trends (where the industry seems to be headed), or implementation choices (examples to make the discussion more concrete). By adopting a systems perspective throughout, our intent is to describe 5G in a way that helps the reader navigate this rich and rapidly evolving system.
1.1 Standardization Landscape
As of 3G, the generational designation corresponds to a standard defined by the 3rd Generation Partnership Project (3GPP). Even though its name has “3G” in it, the 3GPP continues to define the standards for 4G, 5G, and so on, each of which corresponds to a sequence of releases of the standard. Release 15 is considered the demarcation point between 4G and 5G, with Release 17 having been completed in 2022.
In addition to 3GPP-defined standards, national governments establish how the radio spectrum is used locally. Unlike Wi-Fi, for which there is international agreement that permits anyone to use a channel at either 2.4 or 5 GHz (these are unlicensed bands), governments have auctioned off and licensed exclusive use of various frequency bands to service providers, who in turn sell mobile access service to their subscribers. The use of licensed spectrum brings certain benefits such as greater control over the quality of service delivered, while also imposing costs both in terms of paying for licenses and in the complexity of the systems needed to manage access to the spectrum. We will explore how these costs and benefits play out in subsequent chapters.
There is also a shared-license band at 3.5 GHz, called Citizens Broadband Radio Service (CBRS), set aside in North America for cellular use. Similar spectrum is being set aside in other countries. The CBRS band allows 3 tiers of users to share the spectrum: first right of use goes to the original owners of this spectrum (naval radars and satellite ground stations); followed by priority users who receive this right over 10MHz bands for three years via regional auctions; and finally the rest of the population, who can access and utilize a portion of this band as long as they first check with a central database of registered users. CBRS, along with standardization efforts to extend mobile cellular networks to operate in the unlicensed bands, opens the door for private cellular networks similar to Wi-Fi. This is proving especially attractive to enterprises looking to establish a Private 5G service.
The specific frequency bands that are licensed for cellular networks vary around the world, and are complicated by the fact that network operators often simultaneously support both old/legacy technologies and new/next-generation technologies, each of which occupies a different frequency band. The high-level summary is that traditional cellular technologies range from 700-2400 MHz, with new mid-spectrum allocations now happening at 6 GHz, and millimeter-wave (mmWave) allocations opening above 24 GHz.
While the specific frequency band is not directly relevant to understanding 5G from an architectural perspective, it does impact the physical-layer components, which in turn has indirect ramifications on the overall 5G system. We identify and explain these ramifications in later chapters, keeping in mind that ensuring the allocated spectrum is used efficiently is a critical design goal.
Finally, in addition to the long-established 3GPP standards body and the set of national regulatory agencies around the world, a new organization—called the Open-RAN Alliance (O-RAN) —has recently been established to focus on “opening up the Radio Access Network”. We’ll see specifically what this means and how the O-RAN differs from the 3GPP in Chapter 4, but for now, its existence highlights an important dynamic in the industry: 3GPP has become a vendor-dominated organization, with network operators (AT&T and China Mobile were the founding members) creating O-RAN to break vendor lock-in.
1.2 Access Networks
The mobile cellular network is part of the access network that implements the Internet’s so-called last mile. (Another common access technology is Passive Optical Networks (PON), colloquially known as Fiber-to-the-Home.) These mobile access networks have historically been provided by both big and small Mobile Network Operators (MNOs). Global MNOs such as AT&T run access networks at thousands of aggregation points of presence across a country such as the US, along with a national backbone that interconnects those sites. Small regional and municipal MNOs might run an access network with one or two points of presence, and then connect to the rest of the Internet through some large operator’s backbone.
As illustrated in Figure 1, access networks are physically anchored at thousands of aggregation points of presence within close proximity to end users, each of which serves anywhere from 1,000-100,000 subscribers, depending on population density. In practice, the physical deployment of these “edge” locations vary from operator to operator, but one possible scenario is to anchor both the cellular and wireline access networks in Telco Central Offices.
Historically, the Central Office—officially known as the PSTN (Public Switched Telephone Network) Central Office—anchored wired access (both telephony and broadband), while the cellular network evolved independently by deploying a parallel set of Mobile Telephone Switching Offices (MTSO). Each MTSO serves as a mobile aggregation point for the set of cell towers in a given geographic area. For our purposes, the important idea is that such aggregation points exist, and it is reasonable to think of them as defining the edge of an operator-managed access network. For simplicity, we sometimes use the term “Central Office” as a synonym for both types of edge sites.
Finally, one aspect of the mobile network that may not be obvious from Figure 1 is that it supports global connectivity, independent of the Internet (which is technically just one of many available backbone technologies). That is, the cellular network supports a universal addressing scheme, similar in principle (but significantly different in details) from the Internet’s universal IP-based addressing scheme. This addressing scheme makes it possible to establish a voice call between any two cell phones, but of course, IP addresses still come into play when trying to establish a data (broadband) connection to/from a cell phone or other mobile device. Understanding the relationship between mobile addresses and IP addresses is a topic we will explore in later chapters.
1.3 Managed Cloud Service
The previous section gives a decidedly Telco-centric view of the mobile cellular network, which makes sense because Telcos have been the dominant MNOs for the past 40+ years. But with 5G’s focus on broadening the set of services it supports, and embracing general platforms that can host yet-to-be-invented applications, the mobile cellular network is starting to blur the line between the access network and the cloud.
The rest of this book explains what that means in detail. As an overview, thinking of 5G connectivity as a cloud service means that instead of using purpose-built devices and telephony-based operational practices to deliver mobile connectivity, the 5G network is built from commodity hardware, software-defined networks, and cloud-based operational practices. And, just as with familiar cloud applications, the end result is a system that increases both feature velocity and operational uniformity. These advantages are available to legacy MNOs, but whether they will fully embrace them is yet to be seen, so we do not limit ourselves to existing stakeholders or business models. In particular, this book focuses on how enterprises can be their own MNOs, or alternatively, acquire private 5G connectivity as a managed cloud service from non-traditional MNOs.
To this end, Figure 2 depicts a simplified Private 5G deployment that the rest of this book works toward. At a high level, the figure shows a wide range of enterprise use cases that might take advantage of 5G connectivity, with the data plane of the 5G service running on-prem (on an edge cloud running within the enterprise), and the control plane of the 5G service running off-prem (in the global cloud).1 Enterprise administrators control their service through a management console, much in the same way they might log into an AWS, GCP, or Azure console to control a cloud-based storage or compute service. Finally, applications are distributed across both edge and centralized clouds, taking advantage of what is commonly referred to as a hybrid cloud.
We use the term “data plane” in the generic sense in this description. As we’ll see in Chapter 2, the 5G architecture refers to it as “user plane”.
Hosting a 5G connectivity service on an edge cloud is perfectly aligned with one of the most pronounced trends in cloud computing: moving elements of the cloud from the datacenter to locations that are in close proximity to end users and their devices. Before looking at how to realize 5G on an edge cloud, we start by considering why edge clouds are gaining momentum in the first place.
The cloud began as a collection of warehouse-sized datacenters, each of which provided a cost-effective way to power, cool, and operate a scalable number of servers. Over time, this shared infrastructure lowered the barrier to deploying scalable Internet services, but today, there is increasing pressure to offer low-latency/high-bandwidth cloud applications that cannot be effectively implemented in remote datacenters. Augmented Reality (AR), Virtual Reality (VR), Internet of Things (IoT), and Autonomous Vehicles are all examples of this kind of application. Such applications benefit from moving at least part of their functionality out of the datacenter and towards the edge of the network, closer to end users.
The idea of such deployments is to first collect operational data on assets and infrastructure, from sensors, video feeds and telemetry from machinery. It then applies Machine Learning (ML) or other forms of analysis to this data to gain insights, identify patterns and predict outcomes (e.g., when a device is likely to fail). The final step is to automate industrial processes so as to minimize human intervention and enable remote operations (e.g., power optimization, idling quiescent machinery). The overall goal is to create an IT foundation for continually improving industrial operations through software.
But precisely where this edge is physically located depends on who you ask. If you ask a network operator that already owns and operates thousands of Central Offices, then their Central Offices are an obvious answer. Others might claim the edge is located at the 14,000 Starbucks locations (for example) across the US, and still others might point to the tens of thousands of cell towers spread across the globe. Our approach is to be location agnostic, but to make the discussion concrete, we use enterprises as our exemplar deployment.
At the same time cloud providers started pursuing edge deployments, network operators began to re-architect their access network to use the same commodity hardware and best practices in building scalable software as the cloud providers. Such a design, which is sometimes referred to as CORD (Central Office Re-architected as a Datacenter), supports both the access network and edge services co-located on a shared cloud platform. This platform is then replicated across hundreds or thousands of operator sites, including Central Offices.
Traditional network operators did this because they wanted to take advantage of the same economies of scale and feature velocity as cloud providers. CORD gave them a general architecture to work towards, but also an open source Kubernetes-based reference implementation to model their solutions on. That original implementation of CORD is the direct predecessor to the Aether platform that we use as a reference implementation in this book.
L. Peterson, et al. Central Office Re-architected as a Datacenter.. IEEE Communications, October 2016.
A.D. Little Report. Who Dares Wins! How Access Transformation Can Fast-Track Evolution of Operator Production Platforms. September 2019.
An important takeaway from this discussion is that to understand how 5G is being implemented, it is helpful to have a working understanding of how clouds are built. This includes the use of commodity hardware (both servers and bare-metal switches), horizontally scalable microservices (also referred to as cloud native), and Software-Defined Networks (SDN). It is also helpful to have an appreciation for how cloud software is developed, tested, deployed, and operated, including practices such as DevOps and Continuous Integration / Continuous Deployment (CI/CD). We recommend two companion books to help fill the gaps in your understanding of these foundational technologies.
1.4 Beyond 5G
From the moment MNOs started rolling out 5G in 2019, people started talking about what comes next. The obvious answer is 6G, but it’s not at all clear that the decadal generations of the past 40 years will continue into the future. Today, you hear alternatives like “NextG” and “Beyond 5G” more often than 6G, which could be a sign that the industry is undergoing a fundamental shift. And there is an argument that we’re in the midst of a sea change that will render the generational distinction largely meaningless. There are two complementary reasons for this, both at the heart of what’s important about Private 5G.
The first factor is that by adopting cloud technologies, the mobile cellular network is hoping to cash in on the promise of feature velocity. This “agility” story was always included in the early 5G promotional material, as part of the case for why a 5G upgrade would be a worthwhile investment, but the consequence of those technologies now finding their way into the mainstream is that new features can be introduced rapidly and deployed continuously. At some point, the frequency of continual improvements renders generational distinctions irrelevant.
The second factor is that agility isn’t only about cadence; it’s also about customization. That is, these changes can be introduced bottom-up—for example by enterprises and their edge cloud partners in the case of Private 5G—without necessarily depending on (or waiting for) a global standardization effort. If an enterprise finds a new use case that requires a specialized deployment, only its Private 5G deployment needs to adopt the necessary changes. Reaching agreement with all the incumbent stakeholders will no longer be a requirement.
It’s anyone’s guess where this will take us, but it will be interesting to see how this dynamic impacts the role of standardization: what aspects of the mobile network require global agreement and what aspects do not because they can evolve on a case-by-case basis. While standards often spur innovation (TCP and HTTP are two great examples from the Internet experience), sometimes standards actually serve as a barrier to competition, and hence, innovation. Now that software is eating the mobile cellular network—with Private 5G deployed in enterprises likely setting the pace—we will learn which standards are which.
In summary, that 5G is on an evolutionary path is the central theme of this book. We call attention to its importance here, and revisit the topic throughout the book. We are writing this book for system generalists, with the goal of helping bring a community that understands a broad range of systems issues (but knows little or nothing about the cellular network) up to speed so they can play a role in its evolution. This is a community that understands both feature velocity and best practices in building robust scalable systems, and so has an important role to play in bringing all of 5G’s potential to fruition.