By Andee | 16 September 2025 | 0 Comments
What are differences between 40G QSFP+ DAC and 100G QSFP28 DAC Cable?
40G QSFP+ DAC (Direct Attach Copper) cable is a fixed, high-speed cable assembly used to connect two pieces of networking equipment (like switches, routers, or servers) over very short distances. It has a QSFP+ connector on each end and uses copper wires to transmit data at 40 gigabits per second (Gbps).
40G QSFP+ DAC cable works by leveraging its four internal channels:
The electrical signal from the switch's port is sent into the DAC cable.
The signal travels down the four parallel copper channels inside the cable.
Each channel operates at 10 Gbps. The four 10 Gbps channels are aggregated (combined) in the receiving end to create the total 40 Gbps data rate.
This is a very efficient and low-power way to achieve high bandwidth over short distances.
Key Characteristics
Integrated Design: The "module" ends are not optical transceivers; they are simple electrical connectors with some basic circuitry. You cannot open them or change the cable.
Short Reach: DAC cables are designed for very short distances, typically up to 5 or 7 meters (depending on the brand and quality). Beyond this, signal degradation over copper makes them unreliable.
Low Power Consumption: They use significantly less power (often less than 1 watt) than optical transceivers because they don't have lasers to power up.
Low Latency: They provide extremely fast data transmission with minimal delay because the signal doesn't need to be converted from electricity to light and back again (as with fiber optics).
Cost-Effective: They are the cheapest solution for connecting equipment over short distances within a rack or between adjacent racks.
Applications
40G QSFP+ DAC cables are primarily used in data centers for:
Top-of-Rack (ToR) Switching: Connecting a leaf switch to a spine switch in the same row.
Inter-Switch Links (ISLs): Connecting two switches with 40G ports that are very close to each other.
Server Connectivity: Connecting a high-performance server with a 40G network interface card (NIC) directly to a switch.
A Breakout DAC (or Fanout DAC) cable has a single 40G QSFP+ connector on one end that plugs into a switch, and four 10G SFP+ connectors on the other end. This allows you to connect one 40G switch port to four different devices (servers, storage units) with 10G ports.
100G QSFP28 DAC (Direct Attach Copper) cable is a fixed, high-speed cable assembly used to connect two pieces of networking equipment over very short distances. It has a QSFP28 connector on each end and uses copper wires to transmit data at 100 gigabits per second (Gbps).
How It Works
A 100G QSFP28 DAC cable achieves its high speed through one of two primary methods:
4x25G NRZ (Most Common for 100G DACs):
The cable contains four independent copper channels.
Each channel operates at 25 Gbps using NRZ (Non-Return-to-Zero) modulation, where one bit is transmitted per symbol.
The four 25 Gbps lanes are aggregated to create the total 100 Gbps data rate (4 x 25G = 100G).
2x50G PAM4 (Used in Some Designs):
A more advanced method uses two lanes instead of four.
Each lane operates at 50 Gbps using PAM4 (Pulse Amplitude Modulation 4-level) modulation. PAM4 transmits two bits per symbol, effectively doubling the data rate over the same physical lane compared to NRZ.
The two 50 Gbps lanes are aggregated to achieve 100 Gbps (2 x 50G = 100G). This design can allow for a thinner cable but requires more sophisticated (and expensive) electronics to handle the complex PAM4 signal.
Key Characteristics
High Bandwidth: Primarily designed for 100 Gigabit Ethernet applications.
Short Reach: Like all DACs, they are for very short distances, typically up to 3, 5, or 7 meters. Signal integrity degrades quickly over copper at these high frequencies.
Low Power & Latency: They consume very little power (often around ~1W or less) and provide extremely low latency because there is no electrical-to-optical conversion.
Cost-Effective: They are the most economical way to establish a 100G link for short reaches within a rack or between adjacent racks.
Backward Compatibility: A QSFP28 port is often backward compatible with QSFP+ optics and DACs. This means you can plug a 40G QSFP+ DAC into a 100G QSFP28 port if the switch supports it and you configure the port to run at 40G speed. However, a 100G DAC will not work in a 40G port.
Applications
100G QSFP28 DAC cables are the standard for modern high-speed data centers:
Spine-and-Leaf Architecture: Forming the core backbone links between spine switches and leaf (top-of-rack) switches.
High-Performance Computing (HPC) Clusters: Connecting compute nodes that require massive bandwidth for low-latency communication.
Data Center Inter-Switch Links (ISLs): Connecting switches within the same rack or adjacent racks.
Connecting to 100G Servers/Storage: Linking high-performance servers and all-flash storage arrays to the network.
This is a critical function, just like with 40G DACs.
A 100G Breakout DAC cable has a single 100G QSFP28 connector on one end and four 25G SFP28 connectors on the other end. This allows a single 100G switch port to connect to four different 25G devices (servers, NICs, storage).
The primary differences between a 40G QSFP+ DAC and a 100G DAC cable boil down to bandwidth, technology, physical design, application, and cost.
Here’s a detailed breakdown of the key differences:
1. Bandwidth and Data Rate
40G QSFP+ DAC: Designed to support a total aggregate bandwidth of 40 Gbps. It typically uses 4 lanes of 10 Gbps (10Gbps x 4) to achieve this.
100G DAC: Designed to support a total aggregate bandwidth of 100 Gbps. This is achieved using more advanced signaling, commonly 4 lanes of 25 Gbps (25Gbps x 4) or 2 lanes of 50 Gbps (PAM4 modulation) for newer standards like QSFP28 and QSFP56.
2. Connector and Module Standard
40G QSFP+ DAC: Uses a QSFP+ (Quad Small Form-factor Pluggable Plus) connector on both ends. The term "Quad" refers to its four electrical lanes.
100G DAC: Primarily uses a QSFP28 connector. While it's the same physical size and shape as QSFP+, it's specifically engineered to handle the higher electrical signals required for 25/50 Gbps per lane. For 200G/400G, even denser connectors like QSFP-DD are used, but 100G is dominated by QSFP28.
3. Internal Construction and Signaling
This is the most crucial technical difference.
40G QSFP+ DAC: Contains four twinaxial copper channels (8 conductors). Each channel operates at 10 Gbps using NRZ (Non-Return-to-Zero) modulation, a simple encoding scheme where one bit is transmitted per symbol.
100G DAC (QSFP28): Also contains four twinaxial copper channels. However, each channel operates at a higher 25 Gbps. To achieve even higher rates (like 100G over 2 lanes or 400G), many modern 100G/400G DACs use PAM4 (Pulse Amplitude Modulation 4-level) modulation. PAM4 transmits two bits per symbol, effectively doubling the data rate over the same number of wires but requiring more sophisticated (and expensive) electronics to handle the increased signal integrity challenges.
4. Physical Appearance and Gauge
While the connectors are the same size, the cable itself can often be a clue.
40G QSFP+ DAC: Often uses a slightly thicker and stiffer copper cable, especially for longer runs (e.g., 5m or 7m), because it's harder to maintain 10Gbps signal integrity over copper.
100G QSFP28 DAC: Due to the higher frequencies involved (25Gbps), the cable is often thinner and more flexible (using better shielding and materials) to minimize signal loss and attenuation. However, their maximum length is typically shorter than 40G DACs for the same gauge due to the higher signal integrity requirements.
5. Application and Use Case
40G QSFP+ DAC: Used for 40G Ethernet links. Common uses include:
Connecting top-of-rack (ToR) switches to aggregation or spine switches.
Interconnecting servers with 40G NICs to the network.
Breakout Applications: A key use case is splitting one 40G port into four 10G ports. A 40G QSFP+ DAC can connect to a breakout cable that splits into four SFP+ connectors on the other end, linking a 40G switch port to four 10G servers.
100G QSFP28 DAC: Used for 100G Ethernet links, which is the current standard for high-speed spine switches and high-performance computing (HPC). Common uses include:
Spine-and-leaf network architecture core links.
Connecting high-performance storage arrays and servers.
Breakout Applications: Similarly, a 100G port can often be split into four 25G ports or two 50G ports using a specialized breakout DAC.
6. Compatibility and Interoperability
They are not directly interchangeable. You cannot plug a 40G DAC into a 100G QSFP28 port and expect it to work, or vice-versa.
The electrical characteristics, data rates, and firmware are completely different. The switch or NIC will not recognize the incompatible module.
However, some ports are rate-selectable. A QSFP28 port on a modern switch can often be configured to run in 40G mode, but you would still need a 40G DAC to connect to another 40G device.
7. Cost
100G QSFP28 DACs are more expensive than 40G QSFP+ DACs due to the more advanced transceivers and higher-quality cable required to maintain signal integrity at 25 Gbps per lane.
40G QSFP+ DAC cable works by leveraging its four internal channels:
The electrical signal from the switch's port is sent into the DAC cable.
The signal travels down the four parallel copper channels inside the cable.
Each channel operates at 10 Gbps. The four 10 Gbps channels are aggregated (combined) in the receiving end to create the total 40 Gbps data rate.
This is a very efficient and low-power way to achieve high bandwidth over short distances.
Key Characteristics
Integrated Design: The "module" ends are not optical transceivers; they are simple electrical connectors with some basic circuitry. You cannot open them or change the cable.
Short Reach: DAC cables are designed for very short distances, typically up to 5 or 7 meters (depending on the brand and quality). Beyond this, signal degradation over copper makes them unreliable.
Low Power Consumption: They use significantly less power (often less than 1 watt) than optical transceivers because they don't have lasers to power up.
Low Latency: They provide extremely fast data transmission with minimal delay because the signal doesn't need to be converted from electricity to light and back again (as with fiber optics).
Cost-Effective: They are the cheapest solution for connecting equipment over short distances within a rack or between adjacent racks.
Applications
40G QSFP+ DAC cables are primarily used in data centers for:
Top-of-Rack (ToR) Switching: Connecting a leaf switch to a spine switch in the same row.
Inter-Switch Links (ISLs): Connecting two switches with 40G ports that are very close to each other.
Server Connectivity: Connecting a high-performance server with a 40G network interface card (NIC) directly to a switch.
A Breakout DAC (or Fanout DAC) cable has a single 40G QSFP+ connector on one end that plugs into a switch, and four 10G SFP+ connectors on the other end. This allows you to connect one 40G switch port to four different devices (servers, storage units) with 10G ports.
100G QSFP28 DAC (Direct Attach Copper) cable is a fixed, high-speed cable assembly used to connect two pieces of networking equipment over very short distances. It has a QSFP28 connector on each end and uses copper wires to transmit data at 100 gigabits per second (Gbps).
How It Works
A 100G QSFP28 DAC cable achieves its high speed through one of two primary methods:
4x25G NRZ (Most Common for 100G DACs):
The cable contains four independent copper channels.
Each channel operates at 25 Gbps using NRZ (Non-Return-to-Zero) modulation, where one bit is transmitted per symbol.
The four 25 Gbps lanes are aggregated to create the total 100 Gbps data rate (4 x 25G = 100G).
2x50G PAM4 (Used in Some Designs):
A more advanced method uses two lanes instead of four.
Each lane operates at 50 Gbps using PAM4 (Pulse Amplitude Modulation 4-level) modulation. PAM4 transmits two bits per symbol, effectively doubling the data rate over the same physical lane compared to NRZ.
The two 50 Gbps lanes are aggregated to achieve 100 Gbps (2 x 50G = 100G). This design can allow for a thinner cable but requires more sophisticated (and expensive) electronics to handle the complex PAM4 signal.
Key Characteristics
High Bandwidth: Primarily designed for 100 Gigabit Ethernet applications.
Short Reach: Like all DACs, they are for very short distances, typically up to 3, 5, or 7 meters. Signal integrity degrades quickly over copper at these high frequencies.
Low Power & Latency: They consume very little power (often around ~1W or less) and provide extremely low latency because there is no electrical-to-optical conversion.
Cost-Effective: They are the most economical way to establish a 100G link for short reaches within a rack or between adjacent racks.
Backward Compatibility: A QSFP28 port is often backward compatible with QSFP+ optics and DACs. This means you can plug a 40G QSFP+ DAC into a 100G QSFP28 port if the switch supports it and you configure the port to run at 40G speed. However, a 100G DAC will not work in a 40G port.
Applications
100G QSFP28 DAC cables are the standard for modern high-speed data centers:
Spine-and-Leaf Architecture: Forming the core backbone links between spine switches and leaf (top-of-rack) switches.
High-Performance Computing (HPC) Clusters: Connecting compute nodes that require massive bandwidth for low-latency communication.
Data Center Inter-Switch Links (ISLs): Connecting switches within the same rack or adjacent racks.
Connecting to 100G Servers/Storage: Linking high-performance servers and all-flash storage arrays to the network.
This is a critical function, just like with 40G DACs.
A 100G Breakout DAC cable has a single 100G QSFP28 connector on one end and four 25G SFP28 connectors on the other end. This allows a single 100G switch port to connect to four different 25G devices (servers, NICs, storage).
The primary differences between a 40G QSFP+ DAC and a 100G DAC cable boil down to bandwidth, technology, physical design, application, and cost.
Here’s a detailed breakdown of the key differences:
1. Bandwidth and Data Rate
40G QSFP+ DAC: Designed to support a total aggregate bandwidth of 40 Gbps. It typically uses 4 lanes of 10 Gbps (10Gbps x 4) to achieve this.
100G DAC: Designed to support a total aggregate bandwidth of 100 Gbps. This is achieved using more advanced signaling, commonly 4 lanes of 25 Gbps (25Gbps x 4) or 2 lanes of 50 Gbps (PAM4 modulation) for newer standards like QSFP28 and QSFP56.
2. Connector and Module Standard
40G QSFP+ DAC: Uses a QSFP+ (Quad Small Form-factor Pluggable Plus) connector on both ends. The term "Quad" refers to its four electrical lanes.
100G DAC: Primarily uses a QSFP28 connector. While it's the same physical size and shape as QSFP+, it's specifically engineered to handle the higher electrical signals required for 25/50 Gbps per lane. For 200G/400G, even denser connectors like QSFP-DD are used, but 100G is dominated by QSFP28.
3. Internal Construction and Signaling
This is the most crucial technical difference.
40G QSFP+ DAC: Contains four twinaxial copper channels (8 conductors). Each channel operates at 10 Gbps using NRZ (Non-Return-to-Zero) modulation, a simple encoding scheme where one bit is transmitted per symbol.
100G DAC (QSFP28): Also contains four twinaxial copper channels. However, each channel operates at a higher 25 Gbps. To achieve even higher rates (like 100G over 2 lanes or 400G), many modern 100G/400G DACs use PAM4 (Pulse Amplitude Modulation 4-level) modulation. PAM4 transmits two bits per symbol, effectively doubling the data rate over the same number of wires but requiring more sophisticated (and expensive) electronics to handle the increased signal integrity challenges.
4. Physical Appearance and Gauge
While the connectors are the same size, the cable itself can often be a clue.
40G QSFP+ DAC: Often uses a slightly thicker and stiffer copper cable, especially for longer runs (e.g., 5m or 7m), because it's harder to maintain 10Gbps signal integrity over copper.
100G QSFP28 DAC: Due to the higher frequencies involved (25Gbps), the cable is often thinner and more flexible (using better shielding and materials) to minimize signal loss and attenuation. However, their maximum length is typically shorter than 40G DACs for the same gauge due to the higher signal integrity requirements.
5. Application and Use Case
40G QSFP+ DAC: Used for 40G Ethernet links. Common uses include:
Connecting top-of-rack (ToR) switches to aggregation or spine switches.
Interconnecting servers with 40G NICs to the network.
Breakout Applications: A key use case is splitting one 40G port into four 10G ports. A 40G QSFP+ DAC can connect to a breakout cable that splits into four SFP+ connectors on the other end, linking a 40G switch port to four 10G servers.
100G QSFP28 DAC: Used for 100G Ethernet links, which is the current standard for high-speed spine switches and high-performance computing (HPC). Common uses include:
Spine-and-leaf network architecture core links.
Connecting high-performance storage arrays and servers.
Breakout Applications: Similarly, a 100G port can often be split into four 25G ports or two 50G ports using a specialized breakout DAC.
6. Compatibility and Interoperability
They are not directly interchangeable. You cannot plug a 40G DAC into a 100G QSFP28 port and expect it to work, or vice-versa.
The electrical characteristics, data rates, and firmware are completely different. The switch or NIC will not recognize the incompatible module.
However, some ports are rate-selectable. A QSFP28 port on a modern switch can often be configured to run in 40G mode, but you would still need a 40G DAC to connect to another 40G device.
7. Cost
100G QSFP28 DACs are more expensive than 40G QSFP+ DACs due to the more advanced transceivers and higher-quality cable required to maintain signal integrity at 25 Gbps per lane.
Leave a Reply
Your email address will not be published.Required fields are marked. *