Medium-dependent interface
A medium-dependent interface (MDI) is the mechanical, electrical, or optical connection point in an Ethernet network that links the physical layer device—such as a media access unit (MAU) or physical layer (PHY) transceiver—to the transmission medium, as specified in the IEEE 802.3 standard.[1] This interface ensures compatibility between the network hardware and various media types, including twisted-pair copper cables, fiber optics, or backplanes, by defining the precise signaling and connector requirements for data transmission.[2] In the IEEE 802.3 Ethernet architecture, the MDI forms a critical component of the physical layer (PHY), sitting between the physical medium dependent (PMD) sublayer—which handles medium-specific signaling—and the actual interconnect medium.[2] It supports a range of Ethernet speeds and variants, from 10 Mbps (e.g., 10BASE-T over unshielded twisted pair) to multi-gigabit rates (e.g., 10GBASE-T), adapting to the electrical or optical characteristics of the medium to maintain signal integrity and link establishment.[3] The MDI's design allows for standardized interoperability, enabling devices like network interface cards (NICs) in end stations to connect reliably without custom adaptations.[4] For twisted-pair Ethernet implementations, which dominate local area networks (LANs), MDI ports are conventionally used on end devices such as computers or servers, where transmit signals are sent over pins 1 and 2, and receive signals are expected on pins 3 and 6 of an RJ-45 connector, following TIA/EIA-568-B wiring conventions.[4] In contrast, switches and hubs typically employ MDIX (medium-dependent interface crossover) ports, which reverse the transmit and receive pin assignments to support direct connections via straight-through cables, eliminating the need for crossover cables in MDI-to-MDIX links.[4] This distinction arose from early Ethernet deployments to simplify cabling topologies in star-wired networks.[3] To address configuration challenges, modern Ethernet PHYs incorporate auto-MDI/MDIX functionality, which automatically detects the connected device's port type and swaps transmit/receive pairs as needed during link negotiation, a feature standardized in IEEE 802.3ab for Gigabit Ethernet and later amendments.[3] This innovation reduces installation errors and enhances flexibility in diverse network environments, from enterprise data centers to automotive Ethernet applications where MDI specifications ensure robust performance over harsh media.[5] Overall, the MDI remains foundational to Ethernet's scalability, supporting ongoing evolutions like single-pair Ethernet for industrial and IoT uses.[6]Fundamentals
Definition
A medium-dependent interface (MDI) is the standardized physical, electrical, and optical interface that connects a network device, such as a media access unit (MAU) or physical layer (PHY) entity, to the transmission medium in Ethernet networks. It serves as the point of attachment where signals are transmitted and received, ensuring compatibility between the device's physical layer and the medium. This interface is defined in the IEEE 802.3 standard as the mechanical and electrical or optical connection between the transmission medium and the PHY or MAU, facilitating reliable data exchange across various cabling types.[7] Key components of an MDI include specific connectors and signal specifications tailored to the medium. For twisted-pair copper cabling, the MDI typically uses an 8-pin RJ-45 connector (per IEC 60603-7), with designated pins for transmit (e.g., pins 1 and 2) and receive (e.g., pins 3 and 6) pairs, supporting differential signaling without built-in crossover. In coaxial cable systems, such as early 10BASE5 implementations, a BNC connector provides the 50 Ω impedance-matched interface for baseband transmission. For optical media, connectors like SC, ST, or MPO are employed, accommodating multimode or single-mode fiber with defined optical power levels and wavelengths (e.g., 850 nm for short-range multimode). These components ensure the MDI handles both directions of communication over the medium while adhering to electrical isolation, return loss, and attenuation requirements specified in IEEE 802.3 clauses.[8][3] The term MDI was formalized in the IEEE 802.3 standards, beginning with the 10BASE-T specification in IEEE Std 802.3i-1990, to standardize Ethernet attachments to twisted-pair media. Over time, it has evolved to support diverse examples, including Category 5e or higher twisted-pair copper for Gigabit Ethernet, coaxial cables in legacy broadband setups, and multimode fiber for high-speed optical links like 10GBASE-SR. This evolution reflects the standard's adaptation to increasing data rates and media varieties while maintaining interoperability.[7][8]Purpose in Networking
The medium-dependent interface (MDI) serves as a critical standardization mechanism in networking, ensuring compatibility between device interfaces and various physical media by defining consistent signal polarity and wire pair assignments. In twisted-pair Ethernet implementations, for instance, MDI designates pins 1 and 2 for transmit signals and pins 3 and 6 for receive signals, allowing devices to reliably encode and decode data over balanced cabling without ambiguity in signal direction. This standardization, rooted in IEEE 802.3 specifications, promotes interoperability across hardware from different manufacturers by enforcing uniform electrical and mechanical characteristics at the connection point.[9][7] Without proper MDI adherence, signal mismatches can occur, such as transmit signals from one device feeding into the transmit path of another, leading to loopback or collisions that disrupt communication and cause network failure, particularly in half-duplex environments where shared media requires collision detection. By rigidly specifying these interface details, MDI prevents such issues, enabling stable data transmission even in point-to-point or multi-drop topologies. This role is essential for maintaining signal integrity over media like unshielded twisted pair (UTP), where differential signaling relies on precise pair isolation to minimize crosstalk and noise.[10][9] In the broader context of networking, MDI functions as the boundary element of the OSI model's physical layer (Layer 1), bridging the hardware components of a network device—such as the physical coding sublayer (PCS) and physical medium attachment (PMA)—to the cabling infrastructure. It facilitates both half-duplex operations, where devices alternate between transmitting and receiving to avoid collisions, and full-duplex modes, which use separate pairs for simultaneous bidirectional communication, thereby supporting higher throughput without the need for carrier sensing. As a prerequisite for basic connectivity, MDI enables end-user devices like computers to interface directly with diverse media types, such as coaxial or fiber optics in earlier standards, without requiring custom adapters or complex reconfiguration in straightforward setups.[7][10]Ethernet Implementations
MDI Configuration
The Medium-Dependent Interface (MDI) configuration is the standard setup for Ethernet ports on end-user devices, serving as the default interface for transmitting and receiving data over twisted-pair cabling.[9] This configuration is commonly implemented on network interface cards (NICs), personal computers (PCs), routers, and workstations, where it functions as a "straight-through" port designed for direct connection to multi-port devices like switches.[9][11] In the MDI configuration for 10/100BASE-T Ethernet, the pin assignments on the RJ-45 connector follow a specific scheme without internal crossover, dedicating pairs for transmit and receive functions as defined in IEEE 802.3 standards. Pins 1 and 2 (typically the orange/white-orange pair in T568B wiring) handle transmit (TX+) and transmit (TX-) signals, while pins 3 and 6 (green/white-green pair) manage receive (RX+) and receive (RX-) signals; the remaining pins (4, 5, 7, and 8) are unused for data in this mode.[9][11][12] The primary connector type for MDI ports in twisted-pair Ethernet is the 8P8C (8-position 8-contact) modular jack, commonly referred to as RJ-45, which supports unshielded twisted-pair (UTP) cabling such as Category 5.[9][11] This setup ensures compatibility with straight-through cables when connecting to MDI-X ports on switches, maintaining signal integrity over distances up to 100 meters.[12][11] Operationally, MDI ports assume a connection to a multi-port device such as a switch, enabling half- or full-duplex communication at speeds up to 100 Mbps in legacy 10/100 Ethernet implementations, with baseband signaling over the dedicated twisted pairs.[9] This contrasts with the inverted pin assignments on MDI-X ports typically found on switches.[9]| Pin | Signal | T568B Color Pair |
|---|---|---|
| 1 | TX+ | White/Orange |
| 2 | TX- | Orange |
| 3 | RX+ | White/Green |
| 6 | RX- | Green |
| 4,5,7,8 | Unused | - |