Introduction

The Quad Small Form-Factor Pluggable (QSFP) family represents a critical evolution in high-speed optical transceiver technology for data centers, telecommunications networks, and enterprise infrastructure. These hot-pluggable transceivers provide high-density, high-performance connectivity solutions for modern optical fiber communication networks.

QSFP transceivers combine four independent electrical channels into a single compact form factor, enabling scalable bandwidth from 40 Gbps to 1.6 Tbps through successive generations. The technology supports multiple protocols including Ethernet, InfiniBand, Fibre Channel, and SONET/SDH, making it versatile across various network architectures.

The QSFP family has evolved through several generations, each offering increased data rates while maintaining backward compatibility where possible. This document provides comprehensive technical information about QSFP technology, covering specifications, applications, thermal management, and implementation considerations.

QSFP Family Evolution

QSFP+ (Original Quad Small Form-Factor Pluggable)

The original QSFP+ module supports 4 lanes of 10 Gbps transmission for a total aggregate bandwidth of 40 Gbps. It was designed as a high-density alternative to parallel optics solutions, providing compact form factor suitable for switches and routers.

QSFP28 (28 Gbps per Lane)

QSFP28 increases the per-lane data rate to 25 Gbps using Non-Return-to-Zero (NRZ) modulation, achieving 100 Gbps aggregate bandwidth. This generation became the workhorse for 100 Gigabit Ethernet deployments.

QSFP56 (56 Gbps per Lane)

QSFP56 adopts PAM4 (4-level Pulse Amplitude Modulation) signaling to achieve 50 Gbps per lane, supporting 200 Gbps aggregate bandwidth while maintaining the 4-lane architecture.

QSFP-DD (Double Density)

QSFP-DD doubles the electrical interface to 8 lanes while maintaining backward compatibility with QSFP28 modules (using 4 of the 8 lanes). This architecture supports 400 Gbps and 800 Gbps transmission rates.

QSFP112 (112 Gbps per Lane)

QSFP112 returns to a 4-lane architecture but increases the per-lane rate to 100 Gbps using PAM4 modulation, achieving 400 Gbps aggregate bandwidth with reduced power consumption compared to QSFP-DD.

Comparison Table

Electrical Interface Specifications

Pin Configuration and Functions

QSFP modules utilize a high-density card-edge connector providing both high-speed differential data lanes and low-speed control/monitoring signals. The original QSFP+ employs a 38-pin configuration, while QSFP-DD extends this to 76 pins to accommodate doubled electrical interfaces.

QSFP+ / QSFP28 Pin Assignment (38-pin)

High-Speed Electrical Characteristics

The high-speed differential data signals employ CML (Current Mode Logic) signaling with 100-ohm differential impedance. All modules feature AC-coupled inputs and outputs to accommodate DC voltage differences between transmitter and receiver circuits.

Key Electrical Parameters

• Differential Impedance: 100 ohms ±10%
• Coupling: AC coupled on both transmit and receive paths
• Signaling: CML (Current Mode Logic) levels
• Data Encoding: NRZ for QSFP+/QSFP28, PAM4 for QSFP56/QSFP-DD/QSFP112
• Jitter Budget: Defined per IEEE 802.3 and application standards

Low-Speed Control Signals

Low-speed control signaling operates using LVCMOS/LVTTL logic levels at the host Vcc voltage. The module incorporates pull-up resistors on SCL and SDA lines to enable multi-drop I2C bus configurations.

Control Signal Functions

ModSelL (Module Select): When asserted low by the host, enables the module to respond to 2-wire serial interface commands. This signal allows multiple modules on a shared I2C bus with individual addressing.

ResetL (Reset): A low level held for the minimum pulse length initiates a complete module reset, returning all settings to default states. The module indicates reset completion via the IntL signal with Data_Not_Ready bit cleared.

LPMode (Low Power Mode): Controls power consumption states. When asserted high, the module may enter reduced power mode with limited functionality. Implementation varies by module type and application.

ModPrsL (Module Present): Pulled low by the module to indicate physical presence in the host connector. This signal allows hot-plug detection.

IntL (Interrupt): The module asserts this signal low to indicate various status conditions requiring host attention, as defined in the memory map.

Power Requirements

QSFP modules operate from a single +3.3V supply with tight regulation requirements. Power consumption varies significantly based on module type, reach, and technology generation.

Host Board Power Supply Filtering

Proper power supply decoupling and filtering are critical for reliable high-speed operation. The host board should implement multi-stage filtering with both bulk and high-frequency capacitors placed close to the module connector.

• Bulk capacitance: 100-220 µF electrolytic or tantalum capacitors
• High-frequency decoupling: 0.1 µF and 0.01 µF ceramic capacitors
• Placement: Capacitors should be located within 10mm of connector pins
• Power planes: Separate analog and digital power planes recommended for sensitive designs

Optical Interfaces and Fiber Types

Connector Types

QSFP modules employ various optical connector types depending on the application and reach requirements. The mechanical interface must ensure proper fiber alignment and minimize insertion loss.

MPO/MTP Connectors (Parallel Optics)

Multi-fiber Push-On (MPO) connectors, also known as MTP (a performance-enhanced variant), provide high-density parallel optical interfaces. These connectors house multiple fibers in a single ferrule, enabling simultaneous transmission across all lanes.

• MPO-12: 12-fiber connector used in 40G and 100G applications
• MPO-16: 16-fiber connector for higher lane count applications
• BiDi MPO-12: Bidirectional transmission using different wavelengths
• Polarity Management: Type A, Type B, and Type C polarity methods ensure proper transmit-receive fiber mapping

LC Duplex Connectors

LC (Lucent Connector or Little Connector) duplex assemblies provide separate transmit and receive fibers in a compact form factor. These connectors are widely used in single-mode applications and offer excellent optical performance.

• Standard LC: Physical Contact (PC) polish for general applications
• UPC LC: Ultra Physical Contact polish for improved return loss
• APC LC: Angled Physical Contact (8-degree angle) for demanding coherent applications

CS Optical Connector

The CS (Consorzio) connector is an emerging high-density optical interface designed for next-generation transceivers. It provides space-efficient connections for advanced applications requiring multiple fiber pairs in compact form factors.

Fiber Types and Applications

Multimode Fiber Applications

Multimode fiber supports short-reach applications within data centers and enterprise networks. The larger core diameter (50 µm or 62.5 µm) enables use of lower-cost VCSEL (Vertical Cavity Surface Emitting Laser) technology.

Single-Mode Fiber Applications

Single-mode fiber enables long-reach transmission for data center interconnect, metro, and wide-area networks. The smaller core diameter (9 µm) requires more sophisticated laser sources but provides minimal dispersion and attenuation.

Wavelength Division Multiplexing

WDM technology combines multiple optical wavelengths onto a single fiber pair, enabling higher capacity without additional fiber infrastructure. QSFP modules implement various WDM approaches.

CWDM (Coarse Wavelength Division Multiplexing)

CWDM uses wider channel spacing (approximately 20nm) to multiplex 4-8 wavelengths. This approach requires less precise temperature control and utilizes uncooled lasers, reducing power consumption and cost.

LAN-WDM (Local Area Network WDM)

LAN-WDM employs four wavelengths spaced at approximately 4.5nm intervals around 1310nm. This specification balances cost and performance for 10km reach applications.

DWDM (Dense Wavelength Division Multiplexing)

DWDM provides the finest wavelength spacing (typically 50 GHz or 100 GHz ITU grid), enabling the highest capacity per fiber. Coherent QSFP-DD modules implement DWDM for data center interconnect and service provider applications.

Thermal Management

Operating Temperature Classifications

QSFP modules are specified for various operating temperature ranges to accommodate different deployment environments. The case temperature measurement point is typically located near the hottest internal component.

ASHRAE-Based Classifications (Data Center Environments)

For tightly controlled data center environments, alternative temperature classifications align with ASHRAE thermal guidelines. These specify both functional and performance temperature ranges.

Thermal Design Considerations

System-Level Thermal Management

The host system is responsible for controlling module case temperature within specified limits through proper airflow design and heat removal mechanisms. Several factors influence thermal performance:

• Module Density: Up to 36 QSFP-DD modules can be deployed in 1U rack space, requiring careful thermal planning
• Airflow Direction: Front-to-back or side-to-side airflow patterns impact cooling effectiveness
• Ambient Temperature: Inlet air temperature directly affects achievable case temperatures
• Elevation: Higher elevations reduce air density and cooling capacity

QSFP-DD Thermal Innovations

QSFP-DD introduces enhanced thermal management capabilities through riding heat sinks that can be optimized independently of the optical module design.

• Riding Heat Sinks: External heat sinks mount on top of modules, sized according to thermal requirements
• Inter-Module Heat Sinks: Shared heat sinks between adjacent modules improve heat spreading
• Module Type Variants:
  • Type 1: Standard form factor (72.4mm length)
  • Type 2: Extended length (87.4mm) for additional cooling surface
  • Type 2A: Integrated nose heat sink for enhanced cooling
  • Type 2B: High-performance heat sink for modules up to 25W

Touch Temperature Requirements

External surfaces must comply with applicable touch temperature standards to prevent user injury during module handling. For surfaces exceeding short-term touch temperature limits, protective handles or guards must prevent direct contact with hot surfaces during removal.

Thermal Testing and Validation

Modules undergo rigorous thermal testing to ensure reliable operation across the specified temperature range. Testing includes thermal cycling, high-temperature operational life testing, and thermal shock to validate mechanical and optical stability.

Management Interface

2-Wire Serial Interface (I2C)

QSFP modules implement a 2-wire serial interface based on I2C protocol for configuration, monitoring, and diagnostic functions. This interface provides access to a comprehensive memory map containing module identification, operating parameters, and real-time diagnostic data.

Interface Specifications

• Protocol: I2C-compatible 2-wire serial interface
• Clock Frequency: Up to 400 kHz (Fast Mode) for QSFP+/QSFP28; 1 MHz maximum for newer generations
• Addressing: Individual modules selected via ModSelL signal
• Pull-up Resistors: Host provides pull-up to Vcc on SCL and SDA lines

Timing Parameters

Memory Map Architecture

The module memory map provides standardized access to identification, configuration, monitoring, and control functions. Different QSFP generations implement compatible but evolved memory structures.

QSFP+ / QSFP28 Memory Map (SFF-8636)

The legacy QSFP memory map organizes information into lower and upper page structures:

• Lower Page (Addresses 00h-7Fh): Base identification, real-time monitoring, and control functions
• Upper Page 00h: Extended module identification and capabilities
• Upper Page 01h-02h: Threshold settings and control functions
• Upper Page 03h: User-writable EEPROM for custom data

CMIS (Common Management Interface Specification)

QSFP-DD, QSFP56, and QSFP112 modules adopt CMIS, a unified management interface specification supporting advanced features and multiple form factors.

Key CMIS Features

• Unified Memory Map: Common structure across QSFP-DD, OSFP, and other next-generation form factors
• Enhanced Diagnostics: Expanded monitoring capabilities including per-lane parameters
• Advanced Configuration: Support for tunable parameters, application selection, and firmware management
• Module Firmware Update: Standardized mechanism for in-system firmware updates
• Application Selection: Host-configured module applications for multi-rate operation

Digital Diagnostic Monitoring (DDM)

QSFP modules provide extensive real-time monitoring of operational parameters, enabling proactive network management and fault diagnosis.

Monitored Parameters

Alarm and Warning Thresholds

Each monitored parameter includes configurable high and low alarm and warning thresholds. When measured values exceed thresholds, the module asserts appropriate flags and may generate interrupt signals to the host.

• High Alarm: Parameter exceeds critical high threshold
• Low Alarm: Parameter below critical low threshold
• High Warning: Parameter approaching high limit
• Low Warning: Parameter approaching low limit

Applications and Use Cases

Data Center Applications

Spine-Leaf Architectures

Modern data center networks employ spine-leaf topologies requiring high-radix switches with hundreds of ports. QSFP technology enables dense, cost-effective connectivity between spine and leaf switches.

• Leaf-Spine Links: 100G/400G uplinks using QSFP28/QSFP-DD transceivers
• Server Connectivity: Direct-attach copper or short-reach optical links to servers
• Storage Networks: High-bandwidth, low-latency connections to storage arrays

Data Center Interconnect (DCI)

Connecting geographically distributed data centers requires transceivers combining high capacity with extended reach. QSFP-DD coherent modules enable efficient DCI implementations.