2.5G & 5G Multi-Gig RJ45 Jacks (NBASE-T): A Selection Guide
Multi-gig Ethernet — 2.5GBASE-T and 5GBASE-T — lets a building's installed Cat5e and Cat6 carry far more than a gigabit without re-cabling. For the hardware designer that shift lands squarely on one part: the RJ45 jack (the 8P8C female modular socket, 母座) and the magnetics behind it. This guide explains what changes at 2.5G/5G, what stays the same, and how to choose a multi-gig RJ45 jack — written from the perspective of VOOHU Electronics, a manufacturer of RJ45 jacks and integrated-magnetics sockets.
What "multi-gig" actually means
When 10GBASE-T proved too demanding for older cabling, the industry defined two intermediate rates that reuse the existing twisted-pair plant. These were standardized as IEEE 802.3bz-2016 and are marketed as NBASE-T or simply "multi-gig":
- 2.5GBASE-T — 2.5 Gbit/s, designed to run on Cat5e to 100 m.
- 5GBASE-T — 5 Gbit/s, designed to run on Cat6 to 100 m.
Both are derived from 10GBASE-T (IEEE 802.3an) by down-clocking the symbol rate to one-quarter and one-half respectively, keeping the same PAM-16 modulation and LDPC forward-error-correction. All four rates — 1G, 2.5G, 5G, 10G — use all four pairs, full-duplex, with auto-negotiation, so a multi-gig port falls back gracefully to whatever the link partner and cable can support.
Speed tiers at a glance
The table compares the four BASE-T rates that a modern RJ45 jack may have to support. Standard, cabling and reach figures are from the IEEE 802.3 and ANSI/TIA-568 standards; the right-hand column reflects the data rates VOOHU offers in its RJ45 jack and LAN-magnetics ranges.
| Rate | IEEE standard | Minimum cabling (100 m) | Approx. occupied bandwidth | Modulation | VOOHU jack / magnetics availability |
|---|---|---|---|---|---|
| 1000BASE-T (1G) | 802.3ab | Cat5e | ~100 MHz | PAM-5 | Yes — standalone & integrated-magnetics |
| 2.5GBASE-T | 802.3bz (NBASE-T) | Cat5e | ~100 MHz | PAM-16 + LDPC | Yes — 2.5G Base-T range |
| 5GBASE-T | 802.3bz (NBASE-T) | Cat6 | ~200 MHz | PAM-16 + LDPC | Yes — 5G Base-T range |
| 10GBASE-T | 802.3an | Cat6A | ~400 MHz | PAM-16 + LDPC | Yes — 10G Base-T range |
Why the jack and its magnetics change at 2.5G/5G
The 8P8C contact geometry is fixed by IEC 60603-7, so a multi-gig plug mates with the same socket as a 10/100 plug. What has to improve is the analog performance through the connector:
- Return loss & insertion loss to higher frequency. 5GBASE-T pushes meaningful energy to ~200 MHz, so the jack's contact impedance and the integrated transformer must stay matched well past the 100 MHz point where a Gigabit-only part is characterized.
- Near-end crosstalk (NEXT) between the four pairs. Higher symbol rates make the receiver less tolerant of pair-to-pair coupling inside the connector — contact layout, shielding and the magnetics' inter-winding isolation all matter more.
- Isolation still required. Every BASE-T pair is transformer-coupled (1CT:1CT) for galvanic isolation; the integrated-magnetics jack keeps the IEEE 802.3 hi-pot requirement (commonly 1500 Vrms / 2250 VDC, with higher Vrms tiers available for industrial designs).
This is why you cannot assume a Gigabit magjack is "good enough" for 5G: same footprint, different qualification.
VOOHU multi-gig RJ45 jack — selectable attributes
VOOHU builds the multi-gig RJ45 socket as either a discrete jack (paired with a separate LAN transformer) or an integrated-magnetics jack. The values below are taken from VOOHU's RJ45 connector and 2.5G/5G LAN-transformer product data; connector-mechanical figures are drawn from VOOHU's integrated-magnetics RJ45 jack datasheets.
| Attribute | VOOHU options (sourced) |
|---|---|
| Data rate | 10/100M, 100/1000M, 2.5G, 5G, 10G, 18G Base-T (incl. +SPD protected variants) |
| Integrated magnetics | Yes / No (magjack or plain socket) |
| Mounting | Through-hole DIP and SMT (low-height SMT available) |
| Ports / layout | 1×1 up to 2×8; tab Up / Down |
| PoE current | non-PoE through 4PPoE, up to ~3000 mA tiers (350/600/720/850/900/1000/1200/1500 mA …) |
| Magnetics isolation (Vrms) | 1500 / 2000 / 2500 / 3000 / 4000 / 4500 / 4800 / 5000 |
| Integrated-jack hi-pot | 1500 VAC / 2250 VDC (6 s, 1 mA) on the Gigabit magjack family |
| Housing | PBT, UL94 V-0 |
| Shield | Brass C2680, nickel-plated |
| Contacts / plating | Phosphor bronze C5210; 6 µ″ gold on contact area (thicker gold on request) |
| Durability | ≥ 750 mating cycles |
| Mating force | ≤ 23 N |
| Operating temperature | Connector grades 0~+70 °C, -10~+85 °C, -40~+85 °C, -40~+105 °C; multi-gig magnetics to -55~+150 °C |
| LED indicators | Bi-color link/activity, e.g. Green ~568 nm / Yellow ~585 nm, 20 mA; many color combinations |
| Compliance | RoHS, REACH (ISO9001 / ISO14001 manufacturer) |
How to choose: a practical decision path
- Fix the rate to the cable. Installed Cat5e → specify 2.5G. Cat6 → 5G is realistic to 100 m. Only commit to 10G where Cat6A is in place.
- Decide integrated vs discrete magnetics. Choose an integrated-magnetics (magjack) socket to save board area and simplify routing; choose a plain jack plus a separate LAN transformer when you need layout flexibility or a specific magnetics part. See our integrated vs discrete guide.
- Add PoE early. If the port carries PoE/PoE++/4PPoE, pick the matching current tier and a temperature grade with headroom — PoE current adds heat that erodes magnetics margin.
- Match mounting and footprint. Confirm SMT vs DIP, tab orientation and the PCB hole pattern against the datasheet so the part drops into your layout.
- Verify at the right frequency. Ask for return-loss / NEXT data to 200 MHz for 5G (400 MHz for 10G), not just the Gigabit band.
Common mistakes
- Reusing a Gigabit-only magjack at 5G. Identical footprint, but the magnetics may not be characterized past ~100 MHz. Specify a part qualified for the target NBASE-T rate.
- Running 5GBASE-T on long, bundled Cat5e. Alien crosstalk can cap 5G on Cat5e; move to Cat6/Cat6A, or keep Cat5e for 2.5G. (See Cat6A vs Cat8.)
- Ignoring PoE heating. Multi-gig plus PoE++ raises insertion loss and temperature; under-rating the current tier or temperature grade shows up as field failures. (See PoE failure prevention.)
- Forgetting connector-level crosstalk and plating. At 200 MHz the contact plating and shield design measurably affect NEXT and return loss; thin or worn plating degrades the link. (See materials & plating.)
- Pinout/footprint mismatch on a swap. Tab-up and tab-down variants mirror the pad order; always check the PCB hole pattern and panel cutout before substituting a multi-gig jack.
FAQ
What is a multi-gig (NBASE-T) RJ45 jack?
It is an 8P8C RJ45 socket designed to carry 2.5GBASE-T or 5GBASE-T, the rates defined by IEEE 802.3bz (NBASE-T). It is the same modular interface as a Gigabit jack, but the socket and its magnetics must hold return loss and crosstalk over a wider band — roughly 100 MHz for 2.5G and 200 MHz for 5G. VOOHU offers both plain multi-gig sockets and integrated-magnetics (magjack) versions.
Can I run 2.5G and 5G over existing Cat5e or Cat6?
Yes — that is the entire point of NBASE-T. 2.5GBASE-T is specified to 100 m over Cat5e, and 5GBASE-T to 100 m over Cat6. On long, tightly bundled Cat5e, 5GBASE-T can be limited by alien crosstalk, so Cat6/Cat6A is the safer choice for 5G. 10GBASE-T needs Cat6A.
Does a multi-gig RJ45 jack need different magnetics than a Gigabit jack?
Yes. Both use a 1CT:1CT isolation transformer per pair, but a 2.5G/5G jack's magnetics must keep insertion loss, return loss and crosstalk in spec to ~200 MHz rather than the ~100 MHz band 1000BASE-T relies on. Reusing a Gigabit-only magjack at 5G can degrade the link, so select a connector qualified for the target rate.
Does VOOHU offer integrated-magnetics 2.5G/5G RJ45 jacks with PoE?
Yes. VOOHU's RJ45 jack range covers 2.5G, 5G and 10G Base-T in SMT and through-hole (DIP) mounting, single- and multi-port, with PoE current options up to 4PPoE and magnetics isolation tiers from 1500 to 5000 Vrms. Email olivia@voohuele.com for the part list and samples.
2.5GBASE-T5GBASE-TNBASE-T RJ45 jackintegrated magneticsIEEE 802.3bz