Understanding the Ethernet Nomenclature – Data Rates, Interconnect Mediums and Physical Layer

By: Rita Horner, Senior Technical Marketing Manager, Synopsys

Utilization of the Ethernet protocol for connectivity is widespread in a broad range of things or devices around us. Historically, Ethernet has been used in local area networks (LANs) and metropolitan area networks (MANs), and now markets such as storage and automotive are adopting it due to its popularity and numerous benefits like its massive ecosystem and growing economies of scale. Integrated circuit (IC) designers are looking to integrate Ethernet functionality into their designs with an Ethernet IP solution that best serves their target applications.

However, because of its unique nomenclature, Ethernet may be a confusing standard to decipher. Serial interfaces such as PCI Express, Serial ATA (SATA) and USB have a single specification for each of their respective data rates, but Ethernet has many different specifications for the same data rate. For example, 10GBASE-ER and 10GBASE-KR are 10 Gbps Ethernet specifications but they describe different interconnect medium interfaces. As of 2016, there are at least twenty different types of one Gigabit Ethernet or Gigabit Ethernet and close to 30 different 10 Gigabit Ethernet specifications that have been defined by the IEEE 802.3 standard. As more Ethernet interfaces are deployed, it is important for designers to understand the Ethernet specification nomenclature. This article uses the Gigabit and 10 Gigabit references to define Ethernet nomenclature and help designers select the right specification for their target application.

Ethernet Data Rates

Serial data communication consists of data bits transmitting one at a time over an interconnect medium. The data rate is the number of bits transmitted per second (bits/s or bps), so if the bit time is 1 nanosecond (ns), then the data rate is 1000 million bits per second (1000 Mbps or 1 Gbps). Bit rates are typically defined as actual data rates; however, in serial transmission, the data rate is a subset of the total transmitted bits. To achieve the target data rate, line rates or physical layer gross data rate are increased. In Ethernet, to achieve the effective 1 Gbps throughput, the actual line rate is 1.25 Gbps, and in a 10 Gigabit Ethernet throughput, the line rate is 10.3125 Gbps.

Ethernet speeds are the actual data throughput rate without the data overhead, which are control bits, source address, destination address, and other non-data bits. The actual data throughput rate is also the operating rate of the Ethernet controller, which is also known as the media access control (MAC) or Ethernet MAC. 

Ethernet Interconnect Mediums

Figure 1 shows the five major Ethernet interconnect mediums. An Ethernet medium may consist of only pairs of printed circuit board (PCB) traces connecting each PHY in the two ICs at each end, or it may include additional devices such as connector(s), cable (optical or copper), and transceivers. 

Figure 1: Example of Ethernet interconnect mediums 

The medium between two Ethernet PHYs can either be mechanical and electrical or optical. The PHYs driving each medium may have the same throughput data rate, but have different specifications, depending on the medium interface.

The mechanical and electrical medium is copper-based cables (twisted pair or twin axial) or backplanes with multiple connectors. Due to differences in cable, connector and backplane characteristics, the PHY specification for each of the interfaces may be required to be medium-dependent with different specifications.

The optical medium utilizes optical transceivers for electrical signal conversion to and from light signals and electrical signals at the two ends of a fiber optic cable. The two major optical transceiver types are single mode fiber (SMF) and multi-mode fiber (MMF), each supporting multiple different wavelengths (λ), fiber types, and cable reaches.

Ethernet Physical Layer

The IEEE 802.3 standard defines a Gigabit or 10 Gigabit PHY as a combination of three building blocks:

1. Physical medium dependent (PMD)
2. Physical medium attachment (PMA)
3. Physical coding sublayer (PCS)

The PHY connects to the interconnect medium through the Media Dependent Interface (MDI) and connects to the MAC in the data link layer, through the media independent interface (MII), as shown in Figure 2.

Figure 2: Gigabit and 10 Gigabit Ethernet physical and data link layers

Speed-specific MII is an optional interface that provides an architectural implementation for different PHY entities, especially when the MAC is connected to an off-chip PHY. The MII interface is a chip-to-chip interface without a mechanical connector. Gigabit MAC or a repeater can be connected to a Gigabit PHY through the Gigabit Medium Independent Interface (GMII), and the 10 Gigabit MAC can connect to a 10 Gigabit PHY through the optional 10 Gigabit MII (XGMII).

Ethernet Nomenclature

Ethernet nomenclature is based on the interconnect data rate (R), modulation type (mTYPE), medium lengths (L), and a reference to the PHY’s PCS coding (C) scheme. When multiple lanes are aggregated, there is additional information on the number of aggregated lanes (n). In the absence of a reference to the number of lane(s), a single lane interface is assumed. The R mTYPE - L C n parameters used in Ethernet nomenclature are defined as:

For example, 10GBASE-KR is a 10 Gbps (10G) data rate baseband (BASE) specification, with a backplane (K) medium, using a 64B/66B (R) coding scheme, in a single lane configuration. This is purely an electrical specification that fully defines the features and characteristics of a compliant Ethernet PHY.

10GBASE-KX4 is also a 10 Gbps baseband specification for a backplane; however, it uses 8B/10B (X) coding, in an aggregated 4-lane configuration. Even though both 10GBASE-KX4 and 10GBASE-KR are 10 Gbps electrical interfaces, they describe different PHYs. A 10GBASE-KX4 PHY operates at 1/4 rate of the 10GBASE-KR across 4 lanes to achieve the same throughput.

Similarly, although 10GBASE-ER is a 10 Gbps baseband specification, it is not an electrical description like 10GBASE-KR or 10GBASE-KX4. 10GBASE-ER is an extra-long reach (E) single mode optical transceiver specification that utilizes 64B/66B (R) coding, capable of 40 km fiber optic cable reach. 10GBASE-ER mainly describes the requirements of an optical transceiver and does not provide the electrical requirements of a PHY that could drive the transceiver. 

Therefore, it is important to distinguish the differences between the optical transceiver specification and electrical specifications defined in the IEEE 802.3 standard.