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 at 4G (supporting data rates typically measured in the few megabits-per-second) and transitioning 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., AR/VR), 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 won’t just support 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 very much being designed with this same goal of enabling all sorts of future applications beyond those we fully recognize today.
Further Reading
For an example of the grand vision for 5G from one of the industry leaders, see Making 5G NR a Reality. Qualcomm Whitepaper, 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 two 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. It also helps to illustrate how 5G might evolve in the future to provide even more value.
Evolutionary Path
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.
What this all means is that there is no single, comprehensive 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.
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 and 5G, 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 scheduled for 2021. Complicating the terminology, 4G was on a multi-release evolutionary path referred to as Long Term Evolution (LTE). 5G is on a similar evolutionary path, with several expected releases over its lifetime.
While 5G is an ambitious advance beyond 4G, it is also the case that understanding 4G is the first step to understanding 5G, as several aspects of the latter can be explained as bringing a new degree-of-freedom to the former. In the chapters that follow, we often introduce some architectural feature of 4G as a way of laying the foundation for the corresponding 5G component.
Like Wi-Fi, cellular networks transmit data at certain bandwidths in the radio spectrum. Unlike Wi-Fi, which 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.
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 cellular networks to operate in the unlicensed bands, open the door for private cellular networks similar to Wi-Fi.
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. Ensuring that the allocated spectrum is used efficiently is also a critical design goal.
The cellular network is part of the access network that implements the Internet’s so-called last mile. Other access technologies include Passive Optical Networks (PON), colloquially known as Fiber-to-the-Home. These access networks are provided by both big and small network operators. Global network operators like AT&T run access networks at thousands of aggregation points-of-presence across a country like the US, along with a national backbone that interconnects those sites. Small regional and municipal network operators 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.
In either case, 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 the operator-managed access network. For simplicity, we sometimes use the term “Central Office” as a synonym for both types of edge sites.
Because of their wide distribution and close proximity to end users, Central Offices are also an ideal place to host the edge cloud. But this raises the question: What exactly is the edge cloud?
In a nutshell, 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 centralized datacenters. Augmented Reality (AR), Virtual Reality (VR), Internet-of-Things (IoT), and Autonomous Vehicles are all examples of this kind of application. This has resulted in a trend to move some functionality out of the datacenter and towards the edge of the network, closer to end users.
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 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 it is worth pointing out that the cloud’s migration to the edge coincides with a second trend, which is that network operators are re-architecting the 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 sites (including, but not limited to, Central Offices). So while we shouldn't limit ourselves to the Central Office as the only answer to the question of where the edge cloud is located, it is becoming a viable option.
Further Reading
To learn about the technical origins of CORD, which was first applied to fiber-based access networks (PON), see Central Office Re-architected as a Datacenter, IEEE Communications, October 2016.
To understand the business case for CORD (and CORD-inspired technologies), see the A.D. Little report Who Dares Wins! How Access Transformation Can Fast-Track Evolution of Operator Production Platforms, September 2019.
When we get into the details of how 5G can be implemented in practice, we use CORD as our exemplar. For now, the important thing to understand is that 5G is being implemented as software running on commodity hardware, rather than embedded in the special-purpose proprietary hardware used in past generations. This has a significant impact on how we think about 5G (and how we describe 5G), which will increasingly become yet another software-based component in the cloud, as opposed to an isolated and specialized technology attached to the periphery of the cloud.
Keep in mind that our use of CORD as an exemplar is not to imply that the edge cloud is limited to Central Offices. CORD is a good exemplar because it is designed to host both edge services and access technologies like 5G on a common platform, where the Telco Central Office is one possible location to deploy such a platform.
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 white-box 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 like DevOps and Continuous Integration / Continuous Deployment (CI/CD).
Further Reading
If you are unfamiliar with SDN, we recommend a companion book: Software-Defined Networks: A Systems Approach. March 2020.
If you are unfamiliar with DevOps—or more generally, with the operational issues cloud providers face—we recommend Site Reliability Engineering: How Google Runs Production Systems.
One final note about terminology. Anyone who has been paying attention to the discussion surrounding 5G will have undoubtedly heard about Network Function Virtualization (NFV), which involves moving functionality that was once embedded in hardware appliances into VMs (or, more recently, containers) running on commodity servers. In our experience, NFV is a stepping stone towards the fully disaggregated and cloud native solution we describe in this book, and so we do not dwell on it. You can think of the NFV initiative as mostly consistent with the approach taken in this book, but making some specific engineering choices that may differ in detail from that described here.
While equating NFV with an implementation choice is perfectly valid, there is another interpretation of events that better captures the essence of the transformation currently underway. When Telcos began the NFV initiative, they imagined incorporating cloud technologies into their networks, creating a so-called Telco Cloud. What is actually happening instead, is that the Telco's access technology is being subsumed into the cloud, running as yet another cloud-hosted workload. It would be more accurate to refer to the resulting system now emerging as the Cloud-based Telco. One reading of this book is as a roadmap to such an outcome.