Backhaul and fronthaul have come a long way through the generations. When I started out, fronthaul was literally the length of industrial coax cable that you could easily observe running from the bottom of a cell phone tower to the top. Backhaul was always where the excitement was—in all the signal processing smarts surrounding transcoding. This technology existed to condense almighty 64k bit/sec. pipes down to a 16k bit/sec. ones and vice versa.
Today, the story has moved on and converged quite a bit, but the uncoordinated shaping of these spaces in those early days has resulted in two worlds or two heterogeneous technology silos, one each for fronthaul and backhaul. This is not going to fly in 5G, and a fusion is coming that will bring these two worlds together.
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Looking at various research and standardization forums leading the development of 5G, such as 5GPPP, NGMN, 3GPP, IEEE, ITU, IETF and ETSI, all seem to align on the vision that fronthaul and backhaul will converge into what some in research and standards communities refer to as crosshaul or Xhaul. The 5G requirements are driving this opinion.
We are, however, past discussing the requirements in 5G. Commonly converging opinion now tells us that 5G will bring a unified, programmable and shareable infrastructure that can deliver the raft of services envisioned flexibly, speedily and efficiently.
What will that mean for fronthaul and backhaul?
First for backhaul, which is the path connecting the base stations to the core network. Bandwidth-wise, there is a clear need for bigger backhaul pipes (tens of Gbps optical and wireless, for example, at mmWave frequencies). Latency-wise, backhaul needs fast and resilient forwarding, as well as intelligent leveraging of the edge networking and computing infrastructure.
Cost-wise, there is a need to use less fiber optics where possible (e.g. DWDM and/or fiber-like wireless), as well as to move away from specialized switching hardware and towards virtualized switches running over COTS servers. Network function virtualization (NFV) is, therefore, a key technology for backhaul to embrace.
Programmability of the backhaul control and its centralization become essential to add flexibility and agility for shorter service deployment times and traffic-aware backhaul network management so that one can achieve higher (energy) efficiency. Software-defined networking (SDN) is therefore an indispensable technology on this front.
Fronthaul is the path connecting the (radio) cell site unit (also known as Remote Radio Head or Radio Unit) of the base station to its digital unit (also known as Baseband Unit) residing centrally in a data center.
Bandwidth-wise, the high-capacity and data rate KPIs targeted imply the need for a massive number of antenna elements (e.g., massive MIMO) and large spectrum (e.g., mmWave) in the access. That, in turn, raises the need for much bigger pipes in the fronthaul (several tens of Gbps optical and wireless), as well as ways to limit the required fronthaul capacity. Without that, it becomes impossible to scale through compression and/or new less-demanding functional splits (compared to current CPRI-based methods). These new functional splits are often referred to as the Next Generation Fronthaul Interface (NGFI), which is a hot area of specification in various forums.
Latency-wise, even more stringent than backhaul, fronthaul needs very fast and resilient forwarding, as well as intelligent leveraging of the edge networking and computing resources.
Cost-wise, there is a need to lower the dependency of fronthaul on fiber and on proprietary CPRI, as well as to move away from specialized BBU hardware towards virtualized, central and remote units where network functions can be moved in between seamlessly. NFV is therefore a key technology for fronthaul to embrace. To control the fronthaul switching network, programmability and centralization become necessary to enable flexibility and agility. SDN is clearly the paradigm to embrace for controlling fronthaul switches.
It becomes clear that for backhaul and fronthaul to meet the 5G KPIs, they must evolve using a common set of enabling technologies. Those include:
- Gbps optical and wireless transmission
- Fast, flexible and resilient packet-based switching (with time-sensitive networking)
- SDN-based control
- Edge networking and computing
This common set of technologies is dissolving the boundaries of what has been traditionally backhaul and fronthaul and is driving us towards a full fusion.
In 5G we will see the emergence of an integrated crosshaul network where the discussion of fronthaul and backhaul will likely be transformed to one of traffic classes transported and multiplexed over the same packet-based switching fabric under the same SDN-based control. A unified set of crosshaul networking resources and functions will therefore be exposed to intelligent NFV-enabled orchestrators that take advantage of computational resources available in the infrastructure (including at the edge) and will coordinate its decisions with the core network.
A fusion in fronthaul and backhaul is coming, and it will shape a big part of the 5G network that we will see tomorrow.
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