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.
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 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 three 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 10 MHz bands for 3 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.
1.2 ACCESS NETWORKS
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 U.S., 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.
1.3 EDGE CLOUD
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 begs 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 U.S., 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 rearchitecting 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 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.
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