Understanding Channel Operating Margin (COM) for Signal Integrity Analysis
- Signal Edge Solutions
- 4 hours ago
- 6 min read
What is Channel Operating Margin (COM)?
You might be more familiar with Channel Operating Margin (COM) as it relates to IEEE 802.3 and OIF-CEI standards that use Phase-Amplitude-Modulation. For example, PAM-4 encodes four symbols per unit interval (UI), whereas NRZ modulation encodes one symbol per UI. A key advantage of PAM is that deployed hardware and existing PCB technology bins can achieve higher data-throughput rates by enhancing the modulation capability of PHY designs in silicon. Currently, state-of-the-art IEEE 802.3 and OIF-CEI specifications introduce advanced statistical methods of symbol recovery from Communications Systems Theory, including MLSE and multi-packet FEC.
In fact, before capabilities such as PAM were introduced, COM was already established as a best practice for communication channels designed for NRZ-modulated signals. This was introduced in 2014 by the IEEE 802.3 working group as part of the 802.3bj Annex 93A specification for physical layer Ethernet. In this specification, Ethernet throughput increased from 10 Gbps (symbols-per-second with NRZ encoding, equivalent to 10 Gbit/s) to 25 Gbps NRZ.
COM defines a valid channel in mathematical terms that include multiple design factors from Electrical and Communications Engineering. This includes characteristics of a physical channel, and also characteristics of a nominal transmitter and receiver for a given specification. For example, the equalization capabilities of a transmitter and the symbol recovery capabilities of a receiver, including its sensitivity and equalization capabilities. In addition, statistical metrics such as jitter and noise factors can be evaluated as part of COM analysis.
Employing COM during physical-layer channel design enables manufacturing of viable hardware at scale, proves support for compliant transmitter and receiver silicon designs in a multi-vendor industry, enables cost-optimization, and plans generational enhancements to hardware designs deployed across large-scale markets such as AI, Internet-datacenter, and aerospace applications.
What is included in COM analysis?
COM allows the definition of a valid communications channel by a specification such as those from IEEE 802.3 or OIF-CEI standards. A COM specification defines a valid channel in mathematical terms - as a Signal-to-Noise Ratio (SNR) with a margin requirement or recommendation expressed in dB units. The simplified equation appears as follows (Eq. 93A-1, IEEE 802.3bj):
The parameters of this equation include As and Ani, which break down into numerical terms that account for respective signal components and noise components in its calculation for channel compliance. These include items such as:
Channel Insertion Loss
Channel Return Loss or Equivalent Return Loss (defined as ERL for COM*)
Metrics such as ICR, NEXT, FEXT
Metrics for Transmitter and Receiver, such as:
Tx FFE taps
Rx CTLE settings
Rx DFE
*Note that in IEEE 802.3ck (112Gsps/lane), the equation for Equivalent Return Loss (ERL) supersedes classical return-loss masks for channel and s-parameter compliance.
Summary points:
COM was introduced for 25/28G NRZ channels by the IEEE 802.3bj specification
Defines a valid channel in mathematical terms as an SNR in dB units
Accounts for Signal and Noise components, as well as Tx and Rx capabilities
COM parameters differ for each specification by IEEE 802.3 and OIF-CEI
Equivalent Return Loss (ERL) for COM was introduced in the IEEE 802.3ck specification
Where can designers learn more about COM?
COM is discussed and documented as part of the IEEE 802.3 task group, and information is also available as part of OIF-CEI specifications. Much of the information is publicly available, including illustrative slide deck presentations to go with each iteration as the 802.3 specification has grown over time.
How does COM analysis actually operate?
COM analysis is provided as a set of equations by various IEEE 802.3 specification annexes. Therefore, analysis can be run in a mathematics EDA tool or the machine language of your choice. Imputed data consists of S-parameter Touchstone files representing a channel model, along with system metrics for the transmitter and receiver. Additionally, impairments such as crosstalk (NEXT and FEXT) can be evaluated. Typically, a full channel design can be analyzed to evaluate or sweep a gamut of the solution space to assess actual in-situ performance. Keysight provides an example COM analysis with ADS distributions. In the example, you can see how a full channel can be implemented and evaluated for COM:
A COM table can be translated to spreadsheet format, such as a .csv file, and some reference spreadsheets are provided by the IEEE 802.3 or OIF-CEI task groups. Meanwhile, you can also transfer a COM Table within those respective specifications to an input file format that meets your own EDA workflow requirements.
Many Signal Integrity EDA tools provide a UX-based wizard to run COM analysis by offloading the data to external execution using reference code. COM reference code is currently available in MATLAB or Python. In the case of Keysight ADS, COM analysis is available via both methods.
Implementing the various equations for COM analysis is a non-trivial task of sufficient scale to be regarded as a Software Engineering project in its own right. The industry has made available reference code to short-circuit this process. Meanwhile, there is always the option to render an in-house solution with your organization's language of choice.
The IEEE 802.3 task group provides MATLAB code as a reference for each COM version. The codebase has been migrated to GitHub as an open-source project called 802-COM.
A Python implementation of COM analysis is also available via GitHub. Background and details of this Python implementation are outlined in the article "COM: A Link Designer's Field Guide," in Signal Integrity Journal (David Banas and Keysight Technologies Inc., June 2024).
What are some Advantages of using COM?
Advantages:
Defines pass vs. fail and margin as a "figure of merit (FOM)" in dB units
Includes parameters for Transmitter:
Diff peak-peak Voltage for victim and NEXT/FEXT aggressors
Define signal levels in NRZ or PAM4 (or PAMn)
Level separation mismatch
Nose spectral density
FFE tap count with min/max values
Includes parameters for Receiver:
3dB bandwidth
CTLE characteristics: DC gain and pole-zero frequencies
DFE length in UI, coefficient values, and ranges, and magnitude min/max values
Termination resistance
What are some Pitfalls of using COM?
Pitfalls:
Analysis must use the appropriate COM table from each specific 802.3 or OIF-CEI specification for a given channel design
Data files that implement newer keywords for COM are incompatible with older reference-code releases
Translating COM table Tx/Rx metrics to real Tx/Rx silicon or model taps is not always straightforward
DFE is idealized during COM calculations: Real DFE silicon will first train and then hunt for cursor values
Some later 802.3nn specs have no plots for ILD or RL, but ILD and RL can be reasons a channel design fails to meet COM margin requirements
Later versions of COM (802.3ck) introduce equivalent return loss (ERL) as FOM in dB units
Conclusion
Channel Operating Margin is an inherently model-agnostic way to evaluate a communications channel for compliance with IEEE 802.3 or OIF-CEI specifications. COM is discussed and documented within the IEEE 802.3 task group, and information is also available in the OIF-CEI specifications. COM reference code is currently available in MATLAB or Python via GitHub. Advantages of COM analysis include defining margin as a figure of merit in dB units, while pitfalls include the implicit requirement to only use the appropriate COM table from each specific 802.3 or OIF-CEI specification for a given channel design.
At Signal Edge Solutions, we specialize in modeling high-speed interconnects, ensuring simulation fidelity through rigorous evaluation, including COM, as well as S-parameter characterization (including verification of causality, passivity, and reciprocity). And when measurement correlation is involved in a design, we use efficient workflows (like test measurement automation) that ensure consistent results delivered on time.
If you're developing systems that specify Channel Operating Margin for Signal Integrity, reach out to us at info@signaledgesolutions.com and ask about our Signal Integrity design and compliance services.
References
Keysight Technologies, Advanced Design System (ADS) Documentation, https://keysight.com.
Keysight Technologies, Example AMI and COM Flow for IEEE 802.3_93a, 100GBASE-KP4 with Keysight ADS SIPro.
"COM: A Link Designer's Field Guide," Signal Integrity Journal, David Banas and Keysight Technologies Inc., June 2024.
"IEEE Standard for Ethernet," IEEE Std 802.3-2022 (Revision of IEEE Std 802.3-2018), July 2022, doi: 10.1109/IEEESTD.2022.9844436.
Optical Internetworking Forum - Common Electrical Interface (OIF-CEI) 5.0 specification, https://www.oiforum.com/.
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