Technical Comparison of CoaXPress and CoaXPress over Fiber
Technical Comparison of CoaXPress and CoaXPress over Fiber
1. Overview of CoaXPress (CXP)
CoaXPress is a high-speed industrial image transmission interface standard developed by the Japan Industrial Imaging Association (JIIA), specifically designed for the machine vision field. It combines the physical simplicity of coaxial cables with advanced high-speed serial data technology, making it one of the most mainstream connection protocols between industrial cameras and frame grabbers today.
Core Features
| Feature | Specification |
|---|---|
| Physical Medium | 75Ω Coaxial Cable |
| Maximum Data Rate | CXP-12 = 12.5 Gbps (per cable) |
| Supported Rates | CXP-1/2/3/5/6/10/12 (1.25 ~ 12.5 Gbps) |
| Upstream Control Channel | 20.83 Mbps or 41.67 Mbps |
| Power Delivery Capability | PoCXP, 13W @ 24V per cable |
| Encoding Method | 8B/10B (20% bandwidth overhead) |
| Data Integrity | CRC32 Checksum |
| Trigger Accuracy | ±2ns for high-speed connections (CXP-3 and above) |
| Link Aggregation | No upper limit on cable count |
| Maximum Cable Length | CXP-12: ~15m; CXP-6: ~25m; CXP-1: 200m+ |
Protocol Architecture
A CoaXPress Link consists of one Master connection and zero or more Extension connections. Each connection provides:
- High-speed Downstream (Device→Host): Image stream data, up to 12.5 Gbps
- Low-speed Upstream (Host→Device): Trigger, control commands, up to 41.67 Mbps
- Power Delivery (Host→Device): PoCXP, up to 13W per cable
In addition, the standard allows for a dedicated high-speed upstream connection (Host→Device) for high trigger rates and camera control, but power delivery is not supported on this connection.
Link Protocol
Trigger, control data, and stream data share bandwidth and are transmitted using a priority scheme:
| Priority | Packet Type |
|---|---|
| 0 (Highest) | Trigger |
| 1 | I/O Acknowledgment |
| 2 (Lowest) | All other packets (stream data, control, heartbeat, etc.) |
High-priority packets can be inserted into low-priority packet transmission, and low-priority packet transmission resumes after high-priority packets are transmitted.
Reasons for Choosing Coaxial Cable
- No intra-pair skew — a critical performance limiting factor for differential pairs at high speeds
- Optimal bandwidth: single-ended medium loss is only half that of shielded differential pairs of the same diameter
- Excellent EMI/EMC performance
- Multiple cable options: thick cables for long distances, thin/flexible cables for short distances
- Cost-effective solution
- Simple field termination
- Relatively easy to terminate multiple coaxial cables to a common connector
- Compatible with a large amount of legacy analog cable infrastructure
Power Delivery Capability (PoCXP)
Up to 13W (nominal 24V) is available at the Device end per cable:
- Single-connector devices: 13W (suitable for most cameras)
- Dual-connector devices: 26W (up to 25 Gbps)
- Triple-connector devices: 39W
- For higher power requirements, Devices can use an independent power input
Host-side equipment is equipped with device detection and short-circuit protection to minimize the risk of damage when incorrectly connecting non-CoaXPress devices.
Data Integrity
- Control protocol (everything except data) is designed to tolerate single bit errors
- Data itself is protected by CRC32 checksum to detect data errors
- No data retransmission supported — the physical medium used by CoaXPress is designed to be nominally error-free
- Provides a link test mode to offline detect the bit error rate of the complete link (Device – Cable – Host)
2. Overview of CoaXPress over Fiber (CoF)
CoaXPress over Fiber (CoF) is a supplementary specification (CXPR-008-2021) released by JIIA in 2021, proposed jointly by Euresys and Sensor to Image. Its purpose is to use optical fiber as the transmission medium for the CoaXPress protocol to break through the physical limitations of coaxial cables in bandwidth and distance.
Core Concept
CoF does not redefine the CoaXPress protocol itself, but instead designs a CXP-PHY Bridge that maps the CoaXPress protocol to the nGMII interface of the Ethernet Physical Layer (Ethernet PHY).
Technical Architecture
Camera (CXP) ←→ CXP-PHY Bridge (Device) ←→ Fiber ←→ CXP-PHY Bridge (Host) ←→ Frame Grabber (CXP)
↕ ↕
Ethernet PHY Ethernet PHY
(PCS/PMA/PMD) (PCS/PMA/PMD)
The CXP-PHY Bridge replaces the Reconciliation Sublayer in the Ethernet model, mapping CoaXPress 8B/10B encoded data streams to nGMII characters.
Key Specifications
| Feature | Specification |
|---|---|
| Physical Medium | Optical Fiber (Multimode/Singlemode) |
| Interface Standard | IEEE 802.3 (XGMII 10G / 25GMII 25G) |
| Equivalent Bandwidth per Fiber | 10G version XGMII ≈ CXP-12 (after removing 8B/10B overhead, 64b/66b encoding) |
| 25GMII Bandwidth | 25 Gbps (2× CXP-12) |
| Forward Error Correction (FEC) | 10G: Clause 74 (Firecode); 25G: Clause 108 (RS-FEC) |
| Transmission Distance | Several hundred meters for multimode, up to several kilometers to tens of kilometers for singlemode |
| Upstream Control | Transmitted over the same optical fiber (symmetric link, no additional cost) |
| Power Delivery | None (optical fiber is non-conductive) |
nGMII Interface
CoF uses nGMII as a unified term for XGMII and 25GMII:
- XGMII (IEEE 802.3 Clause 46): 156.25 MHz DDR, effective rate equivalent to CXP-12
- 25GMII (IEEE 802.3 Clause 106): 390.625 MHz DDR, 2.5× XGMII
Both feature a 32-bit data path + 4-bit control path, organized as 4×8-bit lanes.
nGMII Character Encoding:
| TXC/RXC | TXD/RXD | Symbol | Meaning |
|---|---|---|---|
| 0 | 0x00~0xFF | /D/ | Normal data transmission |
| 1 | 0x07 | /I/ | IDLE |
| 1 | 0xFB | /S/ | Start (valid only on lane 0) |
| 1 | 0xFD | /T/ | Terminate |
| 1 | 0xFE | /E/ | Error |
Bridge Design Principles
The CXP-PHY Bridge follows the following design objectives:
- Transparent to CoaXPress layer: Adjacent Host/Device layers require no modifications
- Version independent: As independent as possible from CoaXPress version changes (current and future)
- No protocol decoding: No CoaXPress protocol decoding is performed during bridging, only K-code detection and high-priority packet identification are required
Forward Error Correction (FEC)
- 10G Applications: IEEE 802.3 Clause 74 (Firecode FEC), capable of correcting up to 11-bit burst errors per 32×64B/66B blocks — Mandatory implementation
- 25G Applications: IEEE 802.3 Clause 108 (RS-FEC), capable of detecting up to 14 symbol errors and correcting up to 7 symbols — Mandatory implementation
CXP-PHY Packet Structure
All CXP-PHY packet formats follow: SOP + Payload + EOP
| Word Type | Abbreviation | Size | Description |
|---|---|---|---|
| Start of Packet | SOP | 4 Characters | Marks packet start |
| End of Packet | EOP | 8 Characters | Marks packet end |
| Idle Transfer | IT | 4 Characters | Inter-Packet Gap (IPG) |
| High-speed Data Payload | HDP | 4 Characters | 4 bytes of high-speed data |
| High-speed K-code Payload | HKP | 4 Characters | 4 K-code characters |
| Low-speed Payload | LSP | 4 Characters | 2 bytes control + 2 bytes data/K-code |
Multi-channel Configuration
Physical topology uses asymmetric configuration:
- Master connection: Requires 2 optical fibers (upstream + downstream)
- Extension connections: Each requires 1 optical fiber (downstream only)
For example, a 4-connection configuration requires a total of 5 optical fibers (2 + 1 + 1 + 1).
3. Mapping Mechanism and Overhead Analysis
CXP-to-PHY Mapping
- All 8B/10B IDLE words removed: Saves at least 1% bandwidth (CoaXPress requires at least 1 IDLE word inserted every 100 words)
- Data packet mapping:
- 4×K27.7 (SOP) + 4×Type → SOP word
- Data Words → HDP words (direct mapping)
- 4×K29.7 (EOP) → EOP word
- Stream Marker (K28.3) handling: Terminate current nGMII packet → EOP (embedded K28.3) → SOP resume transmission
- High-priority packet handling: Terminate current nGMII packet → Insert trigger/acknowledgment packet → Resume
Area Scan Camera Overhead (Mono8, 2 I/O Ack/frame, 1 connection)
| Image Size | Total Words | Mapping Overhead (Words) | Overhead Ratio |
|---|---|---|---|
| 64×64 | 1,183 | 122.2 | +10.3% |
| 128×128 | 4,383 | 218.2 | +5.0% |
| 256×256 | 16,927 | 348.7 | +2.1% |
| 512×512 | 66,591 | 364.1 | +0.5% |
| 1,024×1,024 | 264,223 | -588.2 | -0.2% |
| 2,048×2,048 | 1,052,703 | -6,425.0 | -0.6% |
| 4,096×4,096 | 4,202,527 | -33,827.1 | -0.8% |
| 8,192×8,192 | 16,793,631 | -151,546.3 | -0.9% |
Line Scan Camera Overhead (Mono8, 1 I/O Ack/line, 1 connection)
| Line Width (Pixels) | Total Words | Mapping Overhead (Words) | Overhead Ratio |
|---|---|---|---|
| 64 | 20 | 3.8 | +19.0% |
| 128 | 36 | 3.6 | +10.1% |
| 256 | 68 | 3.3 | +4.9% |
| 512 | 132 | 2.7 | +2.0% |
| 1,024 | 260 | 1.4 | +0.5% |
| 2,048 | 516 | -1.2 | -0.2% |
| 4,096 | 1,028 | -6.3 | -0.6% |
| 8,192 | 2,052 | -16.5 | -0.8% |
Key Conclusion: When the image size is sufficiently large, the bandwidth saved by removing 8B/10B IDLE words exceeds the EOP/SOP segmentation overhead, resulting in CoF having higher effective bandwidth than original CoaXPress.
4. Application Scenario Comparison
| Scenario | CXP (Coaxial) | CoF (Fiber) |
|---|---|---|
| Short-distance Industrial Inspection (<15m) | ★★★★★ Best Choice | ★★★ Over-engineering |
| Medium-distance Production Lines (15~40m) | ★★★★ Requires rate reduction | ★★★★★ Full rate without pressure |
| Long-distance Transmission (>100m) | ★★ Only CXP-1/2 reachable | ★★★★★ Easily supported |
| High-bandwidth Area Scan Cameras (>12.5 Gbps single stream) | ★★★ Requires multi-cable aggregation | ★★★★★ Single 25G fiber sufficient |
| High-speed Line Scan Cameras | ★★★★ Suitable | ★★★★★ Suitable with lower overhead |
| Multi-camera Topology | ★★★ Independent cable set per camera | ★★★★ Can reuse Ethernet switching infrastructure |
| High Electromagnetic Interference Environment | ★★★★ Good (coaxial shielding) | ★★★★★ Best (fiber immune to EMI) |
| Cameras Requiring Power Delivery | ★★★★★ PoCXP 13W per cable | ★ Not supported, requires independent power |
| Moving/Rotating Equipment | ★★★★★ Good coaxial flexibility | ★★★ Fiber bend radius limitations |
| Existing Ethernet Infrastructure | ★ Cannot be reused | ★★★★★ Directly reuse standard components |
| Camera Size Constraints | ★★★★★ Small connectors | ★★★ Slightly larger optical module size |
| Outdoor/Extreme Environments | ★★★ Requires protection | ★★★★★ Good fiber weather resistance |
5. Advantages and Disadvantages Comparison
CXP (Coaxial Cable)
Advantages
- Integrated Power Delivery: PoCXP provides 13W per cable, most industrial cameras do not require additional power supply, simplifying wiring
- Cost Advantage: Coaxial cables and connectors (DIN 1.0/2.3, BNC, Micro-BNC) are low-cost, simple to terminate, and field-operable
- Mature Ecosystem: Large number of JIIA registered products, complete GenICam compatible ecosystem, abundant camera and frame grabber options
- Simple and Reliable: Point-to-point topology, no protocol bridging required, deterministic and extremely low latency
- Extremely High Trigger Accuracy: ±2ns for high-speed connections (CXP-3 and above), ±4ns fixed latency for low-speed connections
- Legacy Compatibility: Can reuse existing analog coaxial cable infrastructure (e.g., upgrading from analog to digital cameras)
- Link Testing: Built-in link test mode for quick self-test and diagnosis of cable/connector quality
Disadvantages
- Bandwidth Limitation: Maximum 12.5 Gbps per cable (CXP-12), higher rates require multi-cable aggregation
- Distance Limitation: CXP-12 only ~15m, CXP-6 ~25m, rate is inversely proportional to distance
- High Cost for High-speed Upstream: Implementing symmetric high-speed upstream on coaxial medium requires additional hardware cost
- Scalability Bottleneck: Cable count and cost rise sharply for extremely high bandwidth requirements (>50 Gbps)
- EMI Sensitivity: Although coaxial has shielding advantages, it may still be affected in extreme electromagnetic environments
CoF (Optical Fiber)
Advantages
- Extremely High Bandwidth: 10G/25G per fiber, linear scalability with multiple fibers, far exceeding coaxial limits
- Ultra-long Distance: Several hundred meters for multimode, several kilometers or even tens of kilometers for singlemode, no rate-distance trade-off
- EMI Immunity: Optical fiber is completely immune to electromagnetic interference, suitable for high-noise environments such as welding, motors, and high-frequency equipment
- Ethernet Ecosystem Reuse: Utilizes mature standard components such as Ethernet PHY, SFP/QSFP optical modules, and switches
- Symmetric Upstream/Downstream: Fiber is inherently symmetric, high-speed upstream has almost zero additional cost (high cost on coaxial)
- Negative Overhead Potential: For large image frames, bandwidth saved by removing 8B/10B IDLE words exceeds mapping overhead
- Future Scalability: 25GMII already supported, higher rate (40G/100G) Ethernet PHY can be directly benefited
- Lighter and Thinner Cables: Optical fibers are lighter and thinner than coaxial cables, beneficial for dense wiring
Disadvantages
- No Power Delivery Capability: Optical fiber is non-conductive, Device end requires independent power supply
- Bridging Complexity: Requires FPGA to implement CXP-PHY Bridge IP, increasing development and verification costs
- Additional Latency: Bridging introduces packet segmentation/reassembly latency (but minimal impact on most applications)
- Inconvenient Fiber Termination: Field termination of fiber connectors is more complex than coaxial, usually requiring professional tools
- Ecosystem Maturity: Product registration quantity and industrial maturity are still growing
- Small Image Overhead: Mapping overhead can reach 10%+ for small frames (e.g., 64×64)
6. Technical Detail Comparison
Physical Layer Comparison
| Dimension | CXP | CoF |
|---|---|---|
| Medium | 75Ω Coaxial Cable | Optical Fiber (OM3/OM4 multimode or OS2 singlemode) |
| Connectors | DIN 1.0/2.3, BNC, Micro-BNC | LC, SC and other standard fiber connectors |
| Encoding | 8B/10B | 64B/66B (Ethernet PCS) |
| Encoding Efficiency | 80% | ~97% |
| SerDes | Dedicated CXP SerDes | Standard Ethernet SerDes |
| FEC | None | Clause 74 (10G) / Clause 108 (25G) |
| Clock Recovery | Built-in with 8B/10B | Built-in with Ethernet PCS |
Link Layer Comparison
| Dimension | CXP | CoF |
|---|---|---|
| Packet Start Marker | 4× K27.7 (SOP) | nGMII /S/ + SOP word |
| Packet End Marker | 4× K29.7 (EOP) | nGMII /T/ + EOP word |
| IDLE | 8B/10B IDLE word (K28.5 K28.1 K28.1 D21.5) | IT word (4× /I/) |
| Stream Marker | 4× K28.3 | EOP (embedded K28.3) + SOP |
| Upstream Multiplexing | Independent physical connection | Multiple low-speed connections multiplexed over one fiber |
| Device Discovery | Standard CXP discovery process | Transparently supported (via low-speed channel) |
Bandwidth Efficiency Comparison (Taking CXP-12 as an Example)
| Item | CXP (Coaxial) | CoF 10G (XGMII) | CoF 25G (25GMII) |
|---|---|---|---|
| Raw Line Rate | 12.5 Gbps | 10.3125 Gbps | 25.78125 Gbps |
| Encoding Efficiency | 80% (8B/10B) | ~97% (64B/66B) | ~97% (64B/66B) |
| Effective Data Rate | 10 Gbps | ~10 Gbps | ~25 Gbps |
| Mapping Overhead | — | -0.9% ~ +10% | -0.9% ~ +10% |
| Net Effective Rate | ~10 Gbps | ~10 Gbps | ~25 Gbps |
Note: The effective rate of CoF 10G (XGMII) is almost equal to CXP-12 — the 64B/66B encoding efficiency of XGMII compensates for the lower line rate.
7. Selection Recommendations
Scenarios Recommended for CXP
- Standard industrial vision inspection with distance <15m
- Compact cameras requiring PoCXP power delivery
- Cost-sensitive projects
- Existing coaxial cable infrastructure
- Need for the most mature ecosystem and widest product selection
- Extremely strict latency requirements (nanosecond-level determinism)
Scenarios Recommended for CoF
- Long-distance transmission (>50m), such as large production lines, cross-workshop transmission
- Ultra-high bandwidth requirements (>12.5 Gbps single stream), such as ultra-high resolution area scan or ultra-high speed line scan
- High electromagnetic interference environments (welding workshops, near motors, beside high-frequency equipment)
- Existing fiber/ethernet infrastructure
- Need for symmetric high-speed upstream/downstream (e.g., high-speed trigger + high-speed imaging)
- Distributed deployment of multiple cameras requiring Ethernet switching utilization
Hybrid Solution
A hybrid approach can be used in practical projects:
Camera ←Coaxial→ CXP-PHY Bridge (Device) ←Fiber→ CXP-PHY Bridge (Host) ←Coaxial→ Frame Grabber
That is, the Camera end connects to the CoF Bridge using a short coaxial cable, the Bridge transmits over fiber over long distances, and the Host-side Bridge connects to the frame grabber via coaxial cable. This approach leverages the advantages of both technologies.
8. Future Evolution: CoaXPress v3.0
25G Coaxial Solution
In October 2025, the CoaXPress Committee announced progress on the v3.0 draft at IVSM (Haikou). The core upgrade of CXP v3.0 is to double the coaxial cable rate from 12.5 Gbps to 25 Gbps, while officially incorporating CoaXPress over Fiber into the main standard.
Chip manufacturers such as Microchip are advancing 25G coaxial SerDes solutions, with key features including:
- Still using 8B/10B encoding (instead of switching to more efficient encoding such as 64B/66B)
- Line rate up to 31.25 Gbps (25 Gbps × 10/8 = 31.25 Gbps raw line rate)
- Candidate release expected at the end of 2025, official approval at IVSM (Prague) in April 2026
Pros and Cons Analysis of Continuing 8B/10B Encoding
| Dimension | Evaluation |
|---|---|
| Backward Compatibility | ★★★★★ Biggest advantage. Existing CXP 1.x/2.x cameras, frame grabbers, IP Cores, and test equipment can be smoothly upgraded or directly interoperate with v3.0, protecting huge existing investments |
| Protocol Consistency | No link layer modifications required — definitions of SOP/EOP/Stream Marker/IDLE word remain completely unchanged, existing CXP IP Cores only need SerDes rate update |
| Trigger Enhancement | v3.0 supports trigger shared distribution across all connections, 4-connection systems can achieve 4× trigger rate |
| Encoding Overhead | ★★ Main disadvantage. 25 Gbps effective rate requires 31.25 Gbps line rate, with encoding overhead as high as 20% (6.25 Gbps wasted on encoding redundancy) |
| Comparison with CoF | CoF uses Ethernet 64B/66B encoding with only ~3% overhead. For the same 25 Gbps effective rate, CoF only requires ~25.78 Gbps line rate, saving ~5.5 Gbps line bandwidth compared to coaxial |
| Equalizer Challenges | Transmission of 31.25 Gbps over coaxial cable places higher requirements on equalizers and signal integrity, potentially further reducing maximum cable length |
Efficiency Comparison: Coaxial 25G vs Fiber 25G
| Metric | CXP v3.0 Coaxial 25G | CoF 25G (25GMII) |
|---|---|---|
| Line Rate | 31.25 Gbps | 25.78 Gbps |
| Encoding Method | 8B/10B | 64B/66B |
| Encoding Efficiency | 80% | ~97% |
| Effective Data Rate | 25 Gbps | ~25 Gbps |
| Encoding Waste | 6.25 Gbps (20%) | 0.78 Gbps (3%) |
| FEC | None | RS-FEC (Clause 108) |
| Maximum Distance | Expected <10m | Several hundred meters (multimode) ~ several kilometers (singlemode) |
| Power Delivery | PoCXP supported | Not supported |
Other Key Features of CXP v3.0
- CoF incorporated into main standard: CoaXPress over Fiber upgraded from independent supplementary specification to part of the CXP standard
- Fiber FEC support: RS-FEC for fiber links officially incorporated into the standard to improve long-distance reliability
- Multi-connection trigger sharing: Triggers can be distributed to all connections, N-connection systems achieve N× trigger rate
- Backward compatibility: v3.0 devices fully compatible with v1.x/v2.x devices (rate negotiation)
Evolution Roadmap Summary
CXP v1.0 (2010) → CXP v2.0 (2017) → CXP v2.1 (2021) → CXP v3.0 (2026)
6.25 Gbps 12.5 Gbps 12.5 Gbps 25 Gbps
8B/10B 8B/10B 8B/10B 8B/10B (Coaxial)
+ CoF (Independent) 64B/66B (Fiber)
+ CoF (Incorporated into main standard)
Core Trade-off: CXP v3.0's choice to continue 8B/10B encoding is a "ecosystem continuity first" strategy — trading 20% encoding overhead for seamless upgrade of the entire existing ecosystem (chips, IP Cores, cameras, frame grabbers, test tools). This is a commercially correct choice, but from a technical efficiency perspective, coaxial 25G is significantly behind fiber 25G in bandwidth utilization.
9. Summary
CoaXPress and CoaXPress over Fiber are not substitutes but complements:
- CXP has obvious advantages in short-distance, power-required, cost-sensitive scenarios with high ecosystem maturity requirements
- CoF is irreplaceable in long-distance, ultra-high bandwidth, high-interference environments with existing Ethernet infrastructure
With CXP v3.0 introducing 25 Gbps to both coaxial and fiber media and incorporating CoF into the main standard, the integration trend between the two will become more obvious. Coaxial 25G protects the existing ecosystem with backward compatibility advantages, while fiber 25G meets incremental demand with higher encoding efficiency and longer transmission distance. In the long run, CoF's application scope will continue to expand with the popularization of Ethernet PHY and the reduction of optical module costs; however, CXP will maintain its dominant position in medium and short-distance industrial vision applications for a long time due to advantages such as PoCXP power delivery, simplicity and reliability, and mature ecosystem.
Reference Documents:
- JIIA CXP-001-2021 — CoaXPress Standard Version 2.1
- JIIA CXPR-008-2021 — CoaXPress over Fiber Bridge Protocol Version 1.0
- Active Silicon — Update on CoaXPress v3.0 (IVSM Fall 2025, Haikou)
- Microchip CoaXPress Technology — https://www.microchip.com/en-us/products/high-speed-networking-and-video/data-and-video-transceivers/coaxpress-technology
