Why a 1 Gbps Ethernet Doesn’t Reach 125 MB/s and How Jumbo Frames Boost Speed

When transmitting data over a network, units like Mbps are common, and Gigabit Ethernet (1 GbE) is often called a “gigabit card.” Disk sizes are usually described with GB or MB (decimal gigabytes/megabytes), while file sizes often use GiB or MiB (binary prefixes).

Decimal prefixes (KB, MB, GB, TB) follow the short‑scale system (1 billion = 1 000 million). To avoid confusion with the long‑scale system, the International Organization for Standardization defined a standardized set of prefixes based on powers of 10³.

In binary, prefixes are based on powers of 2, and the suffix “bi” is added (e.g., gibibyte, tebibyte). Thus, a megabyte (MB) becomes a mebibyte (MiB) in binary notation.

For a 1000 Mbps network (Gigabit Ethernet), the theoretical maximum is 1 000 000 000 bits per second, which equals 125 megabytes per second (since 1 byte = 8 bits). However, actual transfer speed is lower because data must be encapsulated into network packets, adding header overhead.

Ethernet frames have a maximum transmission unit (MTU) of 1500 bytes. Including the 38‑byte Ethernet overhead, the efficiency is 1500 / (1500 + 38) ≈ 97.53 %. Adding a 4‑byte VLAN tag reduces it to 1500 / (1500 + 38 + 4) ≈ 97.28 %.

Because of this overhead, a gigabit Ethernet typically achieves around 975 Mbps of usable bandwidth, or about 121.9 MB/s after accounting for protocol overhead such as SMB.

To approach the theoretical limit, one can increase the MTU using jumbo frames. Jumbo frames allow an MTU up to 9000 bytes, giving an efficiency of 9000 / (9000 + 38) ≈ 99.58 %, which is higher than the standard Ethernet efficiency.

However, when small packets are transmitted over a network using jumbo frames, the overhead becomes significant relative to the payload, potentially reducing performance. The common practice is to separate traffic: use jumbo‑frame‑enabled switches for large data transfers and standard‑MTU switches for small control packets.

This separation often involves dedicated storage networks (e.g., iSCSI) or fiber‑channel switches, keeping storage traffic distinct from regular business traffic.