5G Mobile Networks - Larry Peterson Synthesis Lectures on Network Systems

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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.

      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.

      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.

      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 that 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 running on commodity servers. In our experience, NFV is a stepping

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