Why does 5G require higher-capacity backhaul than 4G?

A single 5G NR macro cell with 100 MHz of bandwidth in band n78 and 64T64R massive MIMO generates roughly 25 Gbps of peak fronthaul data over CPRI (or 10 Gbps over eCPRI with functional split Option 7-2x). A typical 4G LTE cell with 20 MHz and 2x2 MIMO generates only about 2.5 Gbps CPRI. With three sectors and multiple frequency layers, a 5G site can require 50 to 100 Gbps of aggregate optical transport, driving the need for 25G SFP28 or 100G QSFP28 optics and DWDM wavelength multiplexing on the fronthaul fiber.

Optical & Transport

BONE

Q: What is the difference between fronthaul, midhaul, and backhaul?

Fronthaul connects the Radio Unit (RU) to the Distributed Unit (DU), carrying time-critical baseband IQ samples or partially processed data over CPRI or eCPRI with a latency budget of 100 to 250 microseconds one way. Midhaul connects the DU to the Centralized Unit (CU), carrying higher-layer protocol data over Ethernet with a latency budget of 1 to 5 milliseconds. Backhaul connects the CU to the core network and can tolerate 5 to 10 milliseconds of latency. BONEs may serve any or all three segments depending on their placement in the transport network.

Q: How does a BONE differ from a standard optical switch?

A BONE is optimized for mobile transport requirements: it supports CPRI and eCPRI traffic with strict latency and jitter guarantees (less than 1 microsecond jitter for CPRI), provides IEEE 1588 PTP or SyncE synchronization distribution to the RAN, and implements OAM functions specific to mobile backhaul like performance monitoring per cell site. Standard optical switches focus on enterprise or datacenter traffic without the synchronization or latency features. Many BONE platforms also integrate small-cell aggregation, PON interfaces, and microwave backhaul termination in a single chassis.

/bohn/ — Backhaul Optical Network Element

A fiber-optic transport node that aggregates, multiplexes, and switches traffic between RAN sites and the mobile core network. BONEs handle CPRI, eCPRI, and Ethernet fronthaul, midhaul, and backhaul traffic over DWDM or OTN optical layers, providing the high-capacity, low-latency transport fabric that 4G LTE and 5G NR base station architectures depend on for connecting distributed radio units to centralized processing and the packet core.

Category: Optical & Transport

Capacity: 10G to 400G per wavelength

Latency: < 100 µs (fronthaul)

Understanding BONE

The disaggregation of the 5G RAN into Radio Unit (RU), Distributed Unit (DU), and Centralized Unit (CU) created three distinct transport segments, each with different bandwidth, latency, and synchronization requirements. A BONE sits at the aggregation point where multiple cell site fibers converge, multiplexing their traffic onto shared trunk fiber using wavelength-division multiplexing. At a typical urban deployment, one BONE aggregates 10 to 40 cell sites onto a single fiber pair carrying 40 to 80 DWDM wavelengths, each at 10G, 25G, or 100G depending on the RAN functional split and traffic load.

Unlike generic Ethernet switches, BONEs must maintain sub-microsecond jitter for CPRI transport (which carries digitized radio samples that must arrive at the RU within a tight timing window) and distribute IEEE 1588 Precision Time Protocol (PTP) or SyncE frequency synchronization to every connected cell site. The timing accuracy required for 5G NR TDD operation is plus or minus 1.5 microseconds at the air interface, and the transport network (including the BONE) typically consumes no more than 100 to 200 nanoseconds of this budget.

Transport Segment Requirements

CPRI Data Rate (per antenna carrier):
R_CPRI = f_s × N_bits × N_ant × 2 (I+Q) × (16/15) (line coding)

Example: 20 MHz LTE, 2x2 MIMO:
R = 30.72 Msps × 15 bits × 2 ant × 2 × (16/15) = 2,457.6 Mbps (CPRI Option 7)

eCPRI Bandwidth Savings (Split 7-2x):
R_eCPRI ≈ R_CPRI / 3 to R_CPRI / 10 (depending on traffic load)

eCPRI sends frequency-domain IQ data after the FFT, reducing bandwidth by 3 to 10 times compared to time-domain CPRI.

Transport Segment Comparison

Segment	Connects	Protocol	Latency Budget	Typical Rate	Sync Requirement
Fronthaul	RU to DU	CPRI / eCPRI	100 to 250 µs	10G to 25G per link	± 1.5 µs (TDD)
Midhaul	DU to CU	Ethernet / F1	1 to 5 ms	10G to 100G	± 3 µs
Backhaul	CU to Core	IP / MPLS / OTN	5 to 10 ms	10G to 400G	Relaxed (NTP ok)

Common Questions

Frequently Asked Questions

Why does 5G need more backhaul capacity than 4G?

A single 5G NR macro cell with 100 MHz bandwidth in band n78 and 64T64R massive MIMO generates roughly 25 Gbps of peak CPRI fronthaul (or 10 Gbps eCPRI with split Option 7-2x). A typical 4G cell with 20 MHz and 2x2 MIMO produces only about 2.5 Gbps. With three sectors and multiple frequency layers, a 5G site can require 50 to 100 Gbps of aggregate optical transport, driving the need for 25G SFP28 or 100G QSFP28 optics and DWDM multiplexing.

What is the difference between fronthaul, midhaul, and backhaul?

Fronthaul connects the RU to DU, carrying time-critical IQ samples over CPRI/eCPRI with 100 to 250 microsecond latency. Midhaul connects DU to CU with higher-layer data over Ethernet at 1 to 5 ms latency. Backhaul connects CU to the core network at 5 to 10 ms latency. BONEs may serve any or all three segments depending on their network placement.

How does a BONE differ from a standard optical switch?

BONEs support CPRI/eCPRI with strict jitter guarantees (under 1 microsecond for CPRI), provide IEEE 1588 PTP or SyncE synchronization distribution, and implement mobile-specific OAM like per-cell-site performance monitoring. Standard optical switches lack these synchronization and latency features. Many BONE platforms also integrate small-cell aggregation, PON interfaces, and microwave backhaul termination.

Related Terms

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