GMSL vs MIPI Camera Interface: Key Differences and which is better?

In industries such as automotive, industrial automation, and robotics, high bandwidth, long-distance support, data integrity, higher performance, and camera reliability have always been key requirements. In these fields, commonly used camera module interfaces typically include MIPI CSI-2, USB3.0, and GigE. However, due to the distance between the camera installation location and the main control unit often exceeding 1 meter, and in some cases reaching up to 15 meters, interfaces like MIPI CSI-2 and USB3.0 struggle to meet requirements due to transmission distance limitations. This is where the advantages of Serializer/Deserializer (SerDes) technology come into play.

SerDes technology is widely adopted in telecommunications and data communication applications due to its high data rates, long-distance support, and superior performance. Even in harsh industrial and outdoor environments, this serial link technology can transmit data quickly with low latency. Data is primarily transmitted via a single coaxial cable or differential pair cables (STP, SPP, etc.), minimizing the number of input/output pins and interconnections.

Gigabit Multimedia Serial Link™ (GMSL) cameras use GMSL and GMSL2 technology (a type of SerDes technology) to transmit high-speed video, bidirectional control data, and power over a single coaxial cable.

This article will provide a detailed technical comparison of MIPI and GMSL, two of the most popular interfaces, delving into their core principles, technical specifications, advantages and disadvantages, as well as their ideal application scenarios.

What is the MIPI interface?

Before we begin exploring the differences between GMSL and MIPI interfaces, let's first try to understand what the MIPI interface is.

MIPI is a high-speed standard protocol widely used in camera modules for mobile and embedded systems, renowned for its exceptional speed and efficiency. Among these, MIPI CSI-2 is the most commonly used interface for connecting image sensors to application processors in embedded vision systems, offering a maximum bandwidth of 6 Gb per second, with actual bandwidth reaching approximately 5 Gb per second.

The CSI-2 interface controls and processes image data by connecting the image sensor to an embedded board, enabling the sensor and board to function as a collaborative camera system for image capture. It employs a layered protocol structure, including the physical layer, channel multiplexing layer, lower-layer protocol layer, pixel-to-byte conversion layer, and application layer. This layered design facilitates efficient data transmission. Its maximum cable length is 30 centimeters. Additionally, MIPI CSI-2 utilizes differential signal technology to achieve high-speed data transmission while minimizing interference and noise, ensuring signal integrity. The interface can be implemented on either the MIPI D-PHY or C-PHY physical layer, providing scalable performance.

For more information about the MIPI camera interface, visit: What is the MIPI camera interface? How does the MIPI camera interface work?

What is the GMSL interface?

GMSL (Gigabit Multimedia Serial Link) is a proprietary high-speed serial link technology developed by Maxim Integrated (now part of Analog Devices). It is an asymmetric full-duplex SerDes technology, which means it can transmit data at high speeds in the downstream direction while transmitting data at lower speeds in the upstream direction.

GMSL technology converts parallel data into a serial stream at the transmitter end and converts the serial stream back into parallel data at the receiver end for further processing. One of its key features is the ability to simultaneously transmit video, power, bidirectional control data, Ethernet, and audio over a single coaxial cable or shielded twisted-pair (STP) cable. This "power over coax" (PoC) capability simplifies cabling and reduces system complexity.

GMSL Generational Development:

• GMSL1: Launched in 2008, supporting downlink data rates of up to 3.125 Gbps.
• GMSL2: An improved version of GMSL1, offering higher data rates (up to 6 Gbps per channel), greater reliability, bidirectional communication, and PoC functionality. It supports full HD displays and cameras with up to 8 MP resolution.
• GMSL3: The latest version, introduced in 2021, features upgrades in bandwidth (up to 12 Gbps forward rate), power consumption, security, and cable length support. It supports multi-camera aggregation and multi-channel 4K display video streams transmitted over a single cable.

Differences between GMSL camera interfaces and MIPI camera interfaces

The GMSL camera interface meets the growing demand for fast data rates, high bandwidth, data integrity, and better EMI/EMC performance. We will explore the differences between GMSL and MIPI interfaces based on the following parameters:

• EMI/EMC performance
• Transmission distance
• Automatic Repeat Request (ARQ) functionality
• Virtual channel support
• Video replication
• Backward compatibility
• Compatible platforms
• Cost

EMI/EMC Performance

Electromagnetic interference (EMI) and other harmful interference have always been challenges for camera modules, as they can affect the performance of cables, PCB assemblies, and nearby electronic devices. To ensure stable operation of the equipment, EMI protection is essential. Therefore, EMC/EMI testing must be conducted to ensure that they do not cause harmful interference.

GMSL's serializer/deserializer ICs incorporate programmable spread-spectrum capabilities and high immunity mode (HIM), ensuring reliable communication in noisy environments. While MIPI also uses differential signals to suppress noise, GMSL's specific design for long cables and harsh environments gives it a significant advantage in terms of EMC tolerance.

Transmission Distance

MIPI CSI-2 was designed for on-board integration, so its transmission distance is strictly limited, typically less than 30 centimeters. In contrast, GMSL uses SerDes technology to support transmission distances of up to 15 meters over coaxial or STP cables, and even up to 30 meters with adaptive equalizers. This is the primary reason GMSL is widely adopted in automotive and industrial environments, where cameras are often located far from the main processor.

Automatic Repeat Request (ARQ) Function

GMSL integrates the Automatic Repeat Request (ARQ) feature, a powerful error control and data packet recovery method that ensures reliable data transmission by retransmitting unacknowledged data packets. GMSL2 further enhances robustness by combining Cyclic Redundancy Check (CRC). While MIPI CSI-2 includes error correction code (ECC) functionality within its packets, the explicitly mentioned ARQ functionality in GMSL highlights its unique advantages in critical data links.

Virtual Channel Support

SerDes architecture enables multi-camera capture through virtual channels. The MIPI CSI-2 and CSI-3 specifications support virtual channels. For a dual 4-channel MIPI CSI-2 configuration, a GMSL deserializer (e.g., MAX9296) can effectively decode up to 16 virtual channel IDs. The deserializer merges all input video streams and outputs them via the CSI-2 interface (using packets identified by virtual channel IDs). Frame synchronization locking must be established here.

Video replication

GMSL supports separator mode and aggregation mode. Separator mode is specifically defined as one serializer connected to two deserializers, while aggregation mode is two serializers connected to one deserializer. These two modes enable flexible video routing and efficient multi-camera capture. This is particularly useful for systems requiring 360° panoramic views or multiple camera streams.

MIPI CSI-2 supports up to 32 virtual channels to accommodate image sensors with various data types, enabling multi-exposure and multi-range sensor fusion, and can utilize multiple channels for multi-camera configurations. However, the explicit "separator" and "aggregator" modes are unique features of GMSL.

Backward Compatibility

GMSL1 and GMSL2 interfaces support backward compatibility modes. This GMSL1 backward compatibility mode allows GMSL2 SerDes components to operate compatibly with the previous generation GMSL1. The GMSL2 backward compatibility mode operates similarly. However, there are some operational differences between GMSL1 and GMSL2 modes. However, certain GMSL2 features may not be available when operating in GMSL1 mode.

Compatible Platforms

GMSL supports existing NVIDIA® Jetson™ development kits as well as Connect Tech platforms based on the Jetson Xavier™ NX, including Rogue, Rudi-AGX, and Rudi NX. These enable rapid prototyping and deployment of products supporting visual development. The UART interface is compatible with all GMSL devices, enabling commands to be sent between devices via multiple UART data packets. On the other hand, the MIPI® CSI interface is widely supported across a range of development kits, including Jetson™ Nano, TX2, and AGX Xavier.

Cost

For long-distance applications, GMSL cameras with longer cables can offer better performance, but this is not necessarily the case for short-distance applications. Since GMSL camera modules use longer cables for transmission and must maintain high performance, they are more expensive than MIPI and USB camera modules. However, using coaxial cables instead of STP cables for SerDes links offers a cost advantage. Coaxial cables are cheaper, lighter, more flexible, and have lower loss at high frequencies. In MIPI, the introduction of C-PHY℠ has reduced system costs. However, in some cases, additional development costs may arise if there are no MIPI camera drivers available.

In addition to MIPI and GMSL, USB is also one of the most popular camera interfaces. If you want to compare MIPI cameras with USB cameras, please refer to the detailed comparison of MIPI cameras and USB cameras.

To learn how to select the right interface for embedded vision systems, visit How to Select the Right camera module for Embedded Vision Systems.

To better understand the key differences between GMSL, MIPI, and USB camera interfaces, the table below provides a comprehensive feature comparison.

Embedded vision applications typically prioritize GMSL interfaces

As mentioned above, interfaces such as MIPI or USB are insufficient for applications that require long-distance transmission of large amounts of image or video data. Below, we will explore some common camera-based applications where GMSL interfaces are preferred over other interfaces.

Robotics

Autonomous mobile robots (AMRs) are commonly used in various automated tasks such as warehousing and manufacturing. They can meet a variety of imaging needs. In AMRs, high frame rates and low exposure are key factors to consider when selecting a camera module. GMSL camera modules are well-suited for this purpose. As mentioned earlier, they can transmit data up to 15 meters away from the host.

Intelligent Transportation Systems

Intelligent transportation systems utilize sensors, cameras, routers, and other technologies to improve traffic management and enhance safety. This application requires long-distance transmission of captured image and video data. Therefore, the GMSL interface, equipped with a 15-meter cable and multi-camera capture functionality, can be used to reliably transmit data to the host platform.

Advanced Driver Assistance Systems

ADAS systems help improve road safety. They also enhance the driving experience through features such as lane positioning, parking assistance, and collision avoidance. With the advantages discussed in this article, GMSL SerDes technology meets the basic requirements of ADAS systems, such as high reliability and flexibility, enabling uncompressed camera signal transmission.

Muchvision's GMSL Camera Modules

Muchvision offers a variety of camera modules suitable for various applications. They feature HDR, IP66/67-rated enclosures, global shutter and rolling shutter, and LED flicker suppression. They are also compatible with NVIDIA Jetson development kits. Of course, this includes GMSL camera modules. If your project involves selecting GMSL cameras, feel free to browse our product list or contact us directly.