Flexibility in network control and distribution has created the need for separating the functions between wireless and wired network equipment.
To deliver on the promise of 5G, which sometimes seems like everything to everyone, network operators will need to augment their existing network architectures, addressing some flexibility brought into existence with the new 5G technologies and designs. Overall, 5G brings numerous changes to the mobile network, its architecture, and the equipment used to construct it. 5G is so much more than a wireless technology. Indeed, it’s producing significant changes on the wired network, particularly in the fronthaul, the portion of the network closest to the radio.
Most of us are familiar with backhaul, that network connection pulling traffic from RAN (radio access network) into the mobile core network. The rapid growth of data rich mobile services (video chat, augmented maps / directions, rich web applications, streaming video, etc.) and the raw number of connected devices, drove increasing requirements on these connections, including throughput and latency requirements. Most of these connections are deployed over dedicated fiber optical links or microwave links. Operators have also deployed additional links, upgraded connections, and where necessary, installed additional equipment.
5G requires operators to review strategies and plan for evolution in the mobile transport networks. First, the 5G new radio (NR) will use increasingly higher frequencies (moving from sub-1GHz to above 6 GHz and beyond) and wider spectrum bandwidths (channels). These changes will drive a need to deploy more radio units (RU). More RUs are needed because as the frequency of the radio carrier is increased, its range will decrease (i.e. higher radio frequencies can only travel a shorter distance). This increase in RU deployment has commonly been referred to as the “5G densification”, and is part of the 5G strategy to deliver on the promise of increased throughput to the user equipment (UE), a.k.a. your mobile device. That’s because high radio frequencies and wider spectrum/channel usage can deliver higher throughput.
Second—though not specific to 5G—there is a move to the disaggregation of network equipment. The disaggregation of a networking device is a reengineering process, where a monolithic device is separated into constituent components. These components can then be deployed into different network locations to gain efficiencies in deployment, operation, and management of those devices.
In 5G, the need for the fronthaul network materializes when we couple these two topics together, increasing the number of radio units, along with disaggregation of the 5G network devices, specifically the radio units, distributed units (DUs), and control units (CUs). The fronthaul network will come into existence between the radio unit and the distributed unit. Figure 1 shows the evolution from existing deployments without any fronthaul networks or disaggregated components to the fully disaggregated deployment scenario. Looking from the bottom scenario, the RU, DU, and CU are collocated, with a single backhaul connection to the mobile core network. Moving up the figure, as we separate the components, the fronthaul link is added between the radio unit and the distributed unit.
One motivation for operators to move to the disaggregated scenario comes from gains in spectrum utilization. This is where the distributed unit responsible for coordination of multiple radio units (not shown in Fig. 1) can best determine the utilization over all those units, maximizing the usage of the scarce resources (i.e. the radio spectrum). With gains, however, comes complexity and requirements. In the case of the fronthaul networking, requirements will exist not only on the capacity of the connection, but also on the maximum latency and tolerable packet jitter. The exact values of these requirements will depend on how the radio units are disaggregated from the distributed unit and how that unit is disaggregated from the central unit. In general terms, these requirements will be fairly “tight,” because of the coordination required between the radio units to both gain the efficient usage of the spectrum, but also to deal with coordination as subscribers move quickly from radio unit to radio unit (i.e. a roaming event). Keep in mind, as the number of radio units increases and the “radius” of the “cell” decreases, the number of handoffs between radios will increase, both in count and in regularity.
There may be a tendency to view the fronthaul/backhaul “links” in Fig. 1 as dedicated connections between the equipment installations (i.e a piece of fiber). In deployment practice, however, those cases will likely be fewer and farther between; operators won’t be able to capitalize on other technologies such as software defined networking to help manage and control the transport network(s) providing the fronthaul and backhaul services.
Software control and management of the transport network will become a requirement, as network slicing capabilities are introduced into the 5G network as well. Network slicing will let network/equipment operators “carve” out resources/allocations within the network, providing those resources to different tenants, typically with dedicated or specific service parameters, such as assured latency, minimum guaranteed throughput, etc. The tenants will use their slice to provide services that need these assured parameters, like vehicle collision avoidance or telemedicine.
At this time, 3GPP and the Broadband Forum are working together to develop the new architecture and specifications for the mobile transport networks, specifically the requirements of both fronthaul and backhaul for 5G. The new architectures and requirements will address the stricter tolerances on latency and jitter, as well as the increases in throughput. The first specification to be released from the Broadband Forum will be the TR-521, “5G Transport Networks.” Preceding TR-521 the BBF has recently published a short overview of the architecture, MR-521.1: 5G Network Architecture – Overview, and a more in depth tutorial on the 5G transport architectures is also in the works as well. As the work ramps up, other projects are underway to address how the 5G Fixed Line convergence can be aligned with the existing deployment requirements in the broadband network, including the customer premises equipment provided to the end users.
The complete picture of 5G promises to provide operators with incentives to scale their networks and gain efficiencies through convergence and sharing of some network resources, especially scarce resources such as radio spectrum. To deliver on these incentives, operators will need to update and augment their existing mobile transport networks, introducing front haul links into their deployment, as the number of radio units increases and their control and coordination functions are disaggregated from the radio units. The architectures and technologies used to construct the new fronthaul networks will need to account for future flexibility and requirements as network slicing and multiple network tenants also become the new norm in mobile network operations.
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