What Do OM, PAM4, and PAM8 on an Optical Spec Sheet Actually Mean?

A module spec sheet says “OM4, PAM4, 800GBASE-SR8” — these terms each specify a different thing: OM specifies the fiber, PAM4 specifies the modulation format, and SR8 specifies the lane count and reach. Get any one of them wrong and the module won’t reach its nominal distance — buying the wrong fiber (using OM3 as OM4) or the wrong module (using SR as DR) surfaces the problem on site, and the cost of re-cabling or swapping modules far exceeds the ten minutes spent checking the spec during selection.

This article answers one question: what do OM, PAM4, and PAM8 on a spec sheet actually mean? The answer has three layers: OM is the five-class multimode-fiber rating (OM1–OM5), set by core diameter and bandwidth; PAM4 and PAM8 are modulation formats, differing in bits per symbol (PAM4 carries 2, PAM8 carries 3); and the spec name (e.g., 800GBASE-SR8) layers on total rate, lane count, and applicable reach.

Figure 1 | The three layers of the spec sheet, using OM4 + PAM4 + 800GBASE-SR8 as an example. Click for full size.

1. OM: the five multimode-fiber classes

OM (Optical Multimode) is the ISO 11801 classification of multimode fiber, set by core diameter and the 850 nm bandwidth^[3][5]. The five classes:

Three points:

• Core diameter: OM1 is 62.5 µm; OM2 and later are all 50 µm — a smaller core means fewer modes and less modal dispersion^[5]
• Bandwidth unit is MHz·km: this is the “bandwidth × distance” product, not an RF-style bandwidth. A larger number means a longer reach at the same rate^[3]
• Laser-optimized: OM3/OM4/OM5 have a precisely controlled index profile and pair with an 850 nm VCSEL; OM1/OM2 use LEDs or early lasers^[5]

OM5 and OM4 have the same 850 nm bandwidth (3500 MHz·km) — OM5’s difference is its 850–953 nm wideband response, which lets it run WDM on multiple wavelengths (short-wave WDM), not “faster than OM4”^[1][3].

2. NRZ, PAM4, PAM8: bits per symbol

The modulation format sets “how many bits one symbol carries.” Three formats:

• NRZ (Non-Return-to-Zero): 2 levels (0, 1), 1 bit/symbol
• PAM4 (4-level Pulse Amplitude Modulation): 4 levels, 2 bits/symbol
• PAM8 (8-level): 8 levels, 3 bits/symbol

Data rate = baud rate × bits per symbol. At the same baud rate, PAM4’s data rate is 2× NRZ’s and PAM8’s is 3× NRZ’s. The baud rate is limited by the electrical side (SerDes) and the optical components (laser, modulator, photodetector) — pushing the baud rate higher doubles the bandwidth demand on all of these, so the industry prefers “more bits per symbol” over “unlimited baud rate”^[4]. Current standard baud rates^[2][1]:

An 800G module is 8 lanes × 53.125 GBd PAM4 (8 × 100 Gb/s)^[1].

Figure 2 | The levels and bit count of the three formats. Click for full size.

The price is signal-to-noise ratio: more levels means a smaller amplitude gap between adjacent levels, so noise misjudges levels more easily. At the same baud rate and peak power, PAM4’s minimum level gap is 1/3 of NRZ’s, so it needs 3² = 9× the signal power (about 9.5 dB, from 10·log₁₀(9)); PAM8’s gap is 1/7 of NRZ’s, needing 7² = 49× (about 16.9 dB, from 10·log₁₀(49))^[6].

PAM8 is not the standard modulation for any published rate. IEEE 802.3dj’s 200 Gb/s per lane (for 1.6T) chose PAM4 @ 106.25 GBd, not PAM8^[1] — PAM8 remains a candidate format at the research/proposal stage.

3. How to read the spec sheet: SR/DR/LR/ZR, lane count, and reach

A full spec name (e.g., 800GBASE-SR8) splits into four parts: total rate (800 Gb/s), medium/reach code (SR), lane count (8). The reach codes^[4]:

• SR (Short Reach): multimode fiber, 850 nm, VCSEL
• DR: single-mode fiber, 1310 nm, 500 m
• FR: single-mode, 2 km (CWDM multi-wavelength)
• LR: single-mode, 10 km (CWDM)
• ZR: single-mode, 40 km (coherent)

The verified reach table^[1][2]:

Next generation: 1.6T (1600 Gb/s) is 8 lanes × 200 Gb/s per lane, still PAM4 @ 106.25 GBd (802.3dj, in development, targeted for completion in July 2026); 1.6TBASE-DR8 reaches 500 m on single-mode fiber, and the DR8-2 variant reaches 2 km. The 802.3ds project adds 200/400/800/1600 Gb/s multimode PHYs (30 m and 50 m)^[1]. 3.2T (3200 Gb/s) is the step after 1.6T, with two main routes: 16 lanes × 200 Gb/s (extending 802.3dj’s 200G/lane PAM4) or 8 lanes × 400 Gb/s (requiring PAM8 or a higher baud rate) — 3.2T is not yet a published standard; the per-lane rate and modulation format are still under standardization discussion^[1].

Single-mode long-haul (coherent DCI): DR/FR/LR/ER use IM-DD (direct detection) + CWDM multi-wavelength; longer links switch to coherent detection + DP-16QAM, where a single wavelength carries 400 Gb/s and beyond^[1]. IEEE’s 400GBASE-ZR (single wavelength, 80 km, DP-16QAM) project was canceled in 2024, and coherent long-haul moved to OIF standardization (the 400ZR / 800ZR family), mainly for data-center interconnect (DCI), not in-rack short reach^[1].

A key observation: at the same lane count, a higher per-lane rate means a shorter multimode reach. 100GBASE-SR4 (25 Gb/s per lane) reaches 100 m on OM3, while 400GBASE-SR4 (100 Gb/s per lane) reaches only 60 m on OM3 — a higher per-lane rate raises the baud rate and shortens the reach that modal dispersion allows^[1][2].

The SR variant uses MPO parallel fiber: 400GBASE-SR4 uses MPO-12 (4 lanes × duplex = 8 fibers + spare), 800GBASE-SR8 uses MPO-16 (8 lanes × duplex = 16)^[1]. Fiber count directly sets the patch-panel cabling density — the 800G-SR8 patch panel is twice the 400G-SR4’s. On the package side, 400G/800G modules use QSFP-DD or OSFP (8 lanes), 100G uses QSFP28 (4 lanes)^[4].

The difference between DR and SR is the fiber and laser: DR uses single-mode fiber + a DFB laser (1310 nm); single-mode fiber has no modal dispersion, so the same 100 Gb/s per lane reaches 500 m, far longer than SR’s 60–100 m; the price is that a DFB laser costs more than a VCSEL and the module draws more power^[4].

4. Common misunderstandings and self-check

Misunderstanding 1: PAM4 lets the same fiber run 2× the rate. PAM4 doubles the data rate at the same baud rate; it doesn’t make the fiber itself faster. The price is a 9.5 dB SNR demand, so the actual reach can be shorter than NRZ.

Misunderstanding 2: OM5 is faster than OM4. OM5’s 850 nm bandwidth equals OM4’s (3500 MHz·km); OM5’s value is its 850–953 nm wideband for multi-wavelength WDM, not a higher single rate^[1][3].

Misunderstanding 3: PAM8 is the standard for 200 Gb/s per lane. 802.3dj’s 200 Gb/s per lane uses PAM4 @ 106.25 GBd; PAM8 is still at the proposal stage^[1].

Self-check (you should be able to answer after reading):

1. What are the 850 nm bandwidths of OM3 and OM4? (1500 and 3500 MHz·km)
2. How many bits does PAM4 carry per symbol? What is the baud rate for 100 Gb/s per lane? (2 bits; 53.125 GBd)
3. Why does 800GBASE-SR8 reach 100 m on OM4 while 100GBASE-SR4 reaches 125 m on OM4? (The former is 100 Gb/s per lane, the latter 25 Gb/s per lane; a higher per-lane rate shortens the multimode reach)

Back to the opening question: OM on a spec sheet specifies the fiber class, PAM4/PAM8 specify the modulation format (bits per symbol), and SR/DR/LR/ZR plus the lane count specify reach and channel count — three layers, each governing one thing. Scope: the reach and bandwidth numbers here come from the published IEEE 802.3 PHYs (802.3ba/bs/df/dj objectives), for the current 100G–1.6T parallel optical PHYs. Unresolved limits: PAM8’s actual adoption awaits standardization results after 802.3dj; OM5’s WDM benefit depends on whether the transceivers support multi-wavelength.