--- title: "PAM4 Doubles Bits per Symbol, Not Every Link Speed: Read Baud and Lane Rates" description: "Leviathan Systems — Technical content Published 2026-10-20; updated 2026-10-20." canonical: "https://www.leviathansystems.co/articles/ethernet-pam4-baud-bit-rate-and-lane-capacity" last-updated: "2026-10-20" --- # PAM4 Doubles Bits per Symbol, Not Every Link Speed: Read Baud and Lane Rates Leviathan Systems — Technical content Published 2026-10-20; updated 2026-10-20. PAM4 represents two bits per symbol; two-level NRZ represents one. At the same symbol rate and lane count, PAM4 therefore doubles the represented raw bit rate. Relating that raw count to a specified Ethernet rate requires the actual interface and encoding; delivered application payload also depends on the traffic and system. Compare GBd, Gb/s and payload only after identifying what each number counts. ## Two levels label one bit; four levels label two A symbol is a timed choice from the signal's permitted levels. Two-level nonreturn-to-zero signaling, usually abbreviated NRZ, has two choices and represents one bit per symbol. Four-level pulse amplitude modulation, PAM4, has four choices and represents two bits per symbol: there are four possible two-bit combinations. The extra bit comes from distinguishing more levels within each symbol interval. It does not come from sending two separate symbols at once. The applicable interface defines how bit pairs map to levels; a simple binary-to-level diagram is not a universal mapping for Ethernet equipment. Keysight's signal-parameter tables identify the two-versus-four levels and one-versus-two bits per symbol. Cisco's May 10, 2018 explanation independently describes the two-bit PAM4 symbol. References: [K1](https://helpfiles.keysight.com/scopes/FlexDCA-UG/Content/Topics/Quick-Start/FlexDCA-fact-6.htm), [C1](https://blogs.cisco.com/sp/pam4-for-400g-optical-interfaces-and-beyond-part-1) ## Symbols continue when the selected level repeats Baud measures symbols per second. One GBd means one billion symbols per second, whether adjacent intervals select different levels or repeat the same one. An electrical symbol can be represented by a voltage level; an optical PAM4 symbol can be represented by an optical-power level. Consider six fictional, equal-duration PAM4 symbol intervals whose levels are low, low, high, high, high, low. Low and high name two of the four available levels. There are six symbols and only two level changes among their five internal boundaries. The second low and the repeated highs still occupy symbol intervals. At two bits per PAM4 symbol, those six intervals represent twelve bit positions. They do not demonstrate twelve application-payload bits. Tektronix's application note also describes runs of consecutive same-level PAM4 symbols, corroborating why counting visible transitions would miss symbols. References: [K1](https://helpfiles.keysight.com/scopes/FlexDCA-UG/Content/Topics/Quick-Start/FlexDCA-fact-6.htm), [T1](https://www.tek.com/en/documents/application-note/analyzing-26-53-gbd-pam4-optical-and-electrical-signals) ## Calculate the raw bit rate at one interface boundary Gb/s means billions of bits per second. For the modulation calculation, multiply the symbol rate by the number of bit labels each symbol represents. A hypothetical 25 GBd NRZ lane represents 25 Gb/s; a hypothetical 25 GBd PAM4 lane represents 50 Gb/s. The numerical equality between baud and bit rate holds for this one-bit-per-symbol NRZ case, not for PAM4. For lanes using the same symbol rate and modulation, multiply again by the lane count. In the units used here, aggregate raw Gb/s = lanes × GBd per lane × bits per symbol. This is a one-direction sum across the identified lanes. Raw means the represented bit count at that signaling boundary, without claiming that every represented bit is application data. Name that boundary before adding rates. A module's host electrical lanes and its optical lanes describe different sides of the signal path. Adding the two sides would count successive representations of traffic as extra capacity. Establish each side's actual lane arrangement from its specification; do not infer that arrangement from the module's headline rate. The existing optics-selection guide develops the separate module, logical-port and host-support questions. References: [K1](https://helpfiles.keysight.com/scopes/FlexDCA-UG/Content/Topics/Quick-Start/FlexDCA-fact-6.htm), [T1](https://www.tek.com/en/documents/application-note/analyzing-26-53-gbd-pam4-optical-and-electrical-signals) ## Three fictional offers reveal what PAM4 actually doubles Suppose three offers describe the following invented lane arrangements at one identified interface boundary, in one direction. Every rate is deliberately round. None names a real Ethernet mode, module, compatible endpoint pair or payload result; encoding and FEC are left undefined. Offer A has four lanes at 25 GBd using NRZ. Its arithmetic is 4 × 25 × 1 = 100 Gb/s. Offer B has four lanes at 25 GBd using PAM4. Its arithmetic is 4 × 25 × 2 = 200 Gb/s. Between A and B, lane count and symbol rate stay fixed, so changing the bits per symbol doubles the aggregate raw bit count. Offer C has two lanes at 50 GBd using PAM4. Its arithmetic is 2 × 50 × 2 = 200 Gb/s. C uses half as many lanes as B, but each lane sends symbols twice as quickly. The two changes cancel in the aggregate. B and C therefore have equal raw sums while presenting different lane arrangements. B represents 50 Gb/s per lane; C represents 100 Gb/s per lane. A quote cannot substitute C for B merely because both totals say 200 Gb/s. Nor does C's higher baud rate make its aggregate twice B's. The lane count is part of the multiplication. If a requirement identifies four 25 GBd PAM4 lanes at that boundary, B agrees with those stipulated arithmetic fields; C describes a different interface. That limited comparison establishes no supported selection. The supplier still needs to identify the actual interface and the equipment that supports it. ## Represented bits are not all delivered application data The modulation calculation counts bit labels without deciding their purpose. On an encoded link, some represented bits can carry coding information. Forward error correction adds redundant information that the receiver uses to recover data, then removes before passing the message upward. Cisco's FEC white paper, updated May 13, 2022, explains both that redundancy and its overhead. PAM4 alone does not identify the Ethernet encoding or FEC scheme, so it cannot supply a universal efficiency factor. Do not subtract a remembered percentage from the fictional 200 Gb/s sums. Their actual protocol and coding were never specified. Equally, a nominal Ethernet rate on a product label should not silently become a measured raw line rate. Delivered application payload asks a further question: how much of the application's useful data reached its destination during the observation? A supporting result needs the exact system and operating mode, the counted payload, the interval and traffic conditions. The raw signaling sum cannot establish that result. Even a correctly stated Ethernet interface rate is not a workload-throughput measurement. Keep line-rate arithmetic, interface support and measured delivery as separate evidence. The related FEC article explains why correction counters and packet-error observations also retain their own units; neither supplies an application-throughput figure by itself. References: [C2](https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/implementation-optics-wp.html) ## More levels change the receiver's task For the same overall amplitude span, fitting four levels into the span leaves less separation between neighboring levels than using only two. The receiver must distinguish those closer alternatives. Noise and distortion can make that decision harder even though each correctly identified symbol represents more bits. Cisco's 2018 discussion identifies additional coding, equalization and receiver-design complexity. Equalization compensates for the channel's effect on the waveform; its presence does not make every channel suitable. These are qualitative design considerations, not a noise allowance or a prediction of errors for a particular link. At a fixed raw bit rate on one lane, PAM4 needs half the symbol rate of NRZ. That arithmetic does not prescribe half the analog bandwidth for every implementation, or promise twice the reach, lower power or better payload throughput. The actual transmitter, receiver, channel and supported interface determine which offer can meet the requirement. References: [C1](https://blogs.cisco.com/sp/pam4-for-400g-optical-interfaces-and-beyond-part-1), [T1](https://www.tek.com/en/documents/application-note/analyzing-26-53-gbd-pam4-optical-and-electrical-signals) ## Read the supported interface before defining physical work When comparing offers, locate the interface each number describes: host electrical side or optical side, per lane or aggregate, and one direction or a stated bidirectional total. Read the lane count, exact symbol rate, modulation, encoding and FEC from the applicable interface specification. Ask the supplier to identify an omitted field rather than completing it from PAM4 or a rounded marketing label. Then relate that interface to the exact host, module and operating mode. Use the optics-selection guide for the full compatibility review and the LPO-versus-retimed guide where signal-conditioning architecture differs. If an offer claims delivered performance, ask for evidence covering the proposed combination and the metric actually required. Matching raw arithmetic answers only the arithmetic question. Once the equipment and channel decisions are resolved, Leviathan's structured cabling service is relevant to the surrounding physical installation: agreed routing, dressing, connections, labeling, physical checks and handoff records. For a project inquiry, share the site, selected equipment and assemblies, approved endpoints and routes, material responsibilities and requested physical handoff. Scope and availability are confirmed for the engagement. This signaling explanation supplies neither a field procedure nor link qualification. References: [L1](https://www.leviathansystems.co/services/structured-cabling) ## Primary sources - [K1 — Keysight FlexDCA Essentials: Understanding Signal Parameters](https://helpfiles.keysight.com/scopes/FlexDCA-UG/Content/Topics/Quick-Start/FlexDCA-fact-6.htm). Checked 2026-10-20. Use the NRZ/PAM4 bits-per-symbol and 10 GBd tables for the rate relationship. Nearby unit slips and transition-count wording are not adopted; simplified level mapping is not an Ethernet mapping specification. - [C1 — Cisco: PAM4 for 400G Optical Interfaces and Beyond, Part 1](https://blogs.cisco.com/sp/pam4-for-400g-optical-interfaces-and-beyond-part-1). Checked 2026-10-20. May 10, 2018 explanation of two-bit symbols and qualitative coding, equalization and receiver complexity. Dated NRZ-feasibility, universal bandwidth and numerical SNR claims are not applied. - [T1 — Tektronix: Analyzing 26 to 53 GBd PAM4 Optical and Electrical Signals](https://www.tek.com/en/documents/application-note/analyzing-26-53-gbd-pam4-optical-and-electrical-signals). Checked 2026-10-20. Sections 1–2 corroborate two-bit symbols and voltage/power levels; section 3.4 describes consecutive same-level symbols. Useful for the signaling distinction only; test methods, historical limits and numerical performance claims are not adopted. - [C2 — Cisco: Understanding FEC and Its Implementation in Cisco Optics](https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/implementation-optics-wp.html). Checked 2026-10-20. Updated May 13, 2022. What is FEC? and Trade-offs explain redundant bits and coding overhead. No particular code, efficiency, product setting or acceptance limit is assigned to the hypothetical lanes. - [L1 — Leviathan Systems: Structured Cabling & Fiber Installation](https://www.leviathansystems.co/services/structured-cabling). Checked 2026-10-20. Physical routing, connections, labeling, agreed checks and documentation are relevant after equipment and channel selection. Actual tasks, material responsibilities and availability depend on the engagement. ## Related Leviathan resources - [Structured cabling and fiber installation](/services/structured-cabling) - [Discuss physical cabling for the selected equipment](/contact?service=structured-cabling&from=/articles/ethernet-pam4-baud-bit-rate-and-lane-capacity) - [400G, 800G and 1.6T: read the complete interface specification](/articles/400g-800g-1600g-optics-selection) - [LPO or retimed optics: establish host and channel support](/articles/gpu-network-lpo-versus-retimed-optics) - [FEC counters and fiber reports: keep the evidence units](/articles/gpu-ethernet-fec-counters-vs-fiber-test-reports) ## Define the physical cabling scope for your selected interfaces Share the site, selected equipment and assemblies, approved endpoints and routes, and required physical handoff. We can discuss scope, requirements and availability. [Discuss the scope](/contact?service=structured-cabling&from=/articles/ethernet-pam4-baud-bit-rate-and-lane-capacity)