The use of Ethernet technology for fronthaul (the links between centralized basebands and remote radio heads in a virtualized RAN) has been hailed as the change that will make vRAN viable. While operators used to talk about deploying vRAN even before 5G came along, in reality, 5G is being deployed in the conventional way as the MNOs wait for vRAN to mature.
Ethernet, as an alternative to the semi-proprietary CPRI fronthaul protocol, will certainly address some of the performance and economic shortcomings of that technology. But it has drawbacks of its own, many of which are finally being mitigated by a new IEEE standard called 802.1CM.
A few chips, notably Broadcom’s Monterey high performance Ethernet offering, have already supported 802.1CM before its full ratification, but that has now been agreed, opening the way to a broader ecosystem, and further improvement to vRAN economics.
The standard addresses time-sensitive networking for fronthaul over a bridged IEEE 802.3 Ethernet link. There are two main Ethernet options available – the packet-based implementation of CPRI, called eCPRI, which supports Ethernet or IP; and IEEE’s 1914.3 Radio over Ethernet (RoE). The latter is more fully open, but the latter, of course, offers better coexistence for companies which have already deployed some CPRI fronthaul to link remote radio heads to basebands at the bottom of the tower, or centralized in a base station hotel.
However, the big challenges of Ethernet fronhaul relate to timing and synchronization. Ethernet was originally not defined for low-latency applications like fronthaul. It was unable to support synchronization and jitter requirements for CPRI-to-Ethernet bridging and so was confined to relatively small vRAN designs. And it needed to address the strict latency requirement – a maximum of 100 microseconds one-way delay between baseband and radio head, which includes both delays through the fiber or air (5 microseconds/km for fiber) and any delays caused by intermediate switches.
Hence the 802.1CM standard, which IEEE developed in collaboration with the CPRI Cooperation, as well as Study Group 15 of ITU’s Telecommunication Standardisation Sector (ITU-T). The latter contributed its synchronization solutions.
“As we see carriers and operators pursuing 5G technologies, and a wide range of new possibilities for next generation mobile applications, IEEE 802.1CM represents a key pioneering step for providing reliable packet transport within 5G networks,” said Glenn Parsons, chair of the IEEE 802.1 Working Group.
The new standard is critical, because it gives operators the option of an end-to-end Ethernet architecture for cellular networks, for the first time, lowering costs by enabling the use of common components.