Understanding 10 Gigabit Ethernet Physical Layer (10GbE PHY)

There are various physical layers in 10 Gigabit Ethernet based on the medium the data has to be transported, such as copper, fiber optic, etc.

The physical layer subgroups are:

• 10GBASE-R: Developed for optical fiber medium.
• 10GBASE-X: Developed for copper and optical fiber.
• 10GBASE-T: Developed for twisted pair cable.
• 10GBASE-W: Developed for OC-192 SONET.

In this article, we’ll use 10GBASE-R as an example to understand the 10GbE PHY layer. As shown in the figure, 10GbE consists of sublayers, which include:

• XGMII (10 Gbps Media Independent Interface)
• PCS (Physical Coding Sublayer)
• PMA (Physical Medium Attachment)
• PMD (Physical Medium Dependent)
• MDI (Medium Dependent Interface)

We will delve into the functions of these sublayers used in 10GBASE-R.

Following are the functions of 10 Gigabit ethernet PHYSICAL Layer:

• It should support full-duplex ethernet MAC layer.
• It provides 10 Gbps at the XGMII sublayer.
• It should support LAN PMD sublayer at 10 Gbps.
• It should support WAN PMD sublayer which operates at SONET/SDH rates.
• It should support network extension up to a distance of 40 Km.
• It should support copper as well as twinaxial cable as the medium.
• A Bit Error Rate (BER) of 10^-12 should be achievable.

10GbE PHY Layers: Reconciliation, XGMII, PCS, PMA, PMD

Let’s understand the 10GBASE-R Ethernet physical layer type, which has both transmitter and receiver functionalities. We will explore all the sublayers of this Ethernet physical layer.

Reconciliation Sublayer

This sublayer provides a mapping between the signals available at the XGMII sublayer and the MAC layer. The RS adapts bit serial protocols of the MAC layer to parallel encodings of 10 Gbps PHY sublayers. Essentially, the RS sublayer converts between the MAC serial data stream and parallel data paths of XGMII.

The RS sublayer generates continuous data/control characters for the transmit direction. It also expects continuous data/control characters in the receive direction.

XGMII

This sublayer supports 10 Gbps operation through its 32-bit wide transmit and receive data paths.

The XGMII sublayer has the following features:

• Data and delimiters are synchronous to the clock reference.
• It provides independent 32-bit wide transmit and receive data paths.
• It supports full-duplex operation.
• It supports signal levels as ASIC compliant.

Physical Coding Sublayer (PCS)

The 10GBASE-R PCS serves the XGMII sublayer and performs the following functionalities:

• Encoding of 8 XGMII data bytes (i.e., 64 bits) to 66-bit blocks.
• Decoding of 8 XGMII data bytes from 66-bit blocks.
• Transfer of encoded data (16-bit transfer mode) in the transmit direction from XGMII to PMA sublayer.
• Transfer of encoded data from PMA to XGMII in the receive direction.

Physical Medium Attachment (PMA)

This PMA sublayer provides a medium-independent means for the PCS sublayer to support the use of a range of physical media.

The PMA sublayer in the physical layer type 10GBASE-R performs the following functions:

• Mapping of transmit and receive data streams between the PCS or WIS and PMA via the PMA service interface.
• Serialization (deserialization) of bits for transmission (reception) on the underlying serial PMD.
• Recovery of clock from the received data stream.
• Mapping of transmit and receive bits between the PMA and PMD via the PMD service interface.
• Optionally provides data loopback at the PMA service interface.

The ordering of bits and octets for the case when no WIS (WAN Interface Sublayer) is present between the PCS and PMA for the case of 10GBASE-R Physical layer.

Physical Medium Dependent (PMD)

There are several PMDs designed and developed to meet various medium and distance requirements. Table 1 mentions the maximum distance supported by various fiber optic cables.

Table 1: Fiber Optic Cable Distances

Table 2 mentions various copper cable types and maximum distances supported.

Table 2: Copper Cable Distances

Medium Dependent Interface (MDI)

This sublayer is the cable connector between the signal transceivers and the link.

Medium

The medium is the physical medium carrying the data, either fiber or copper. In the receiver, all the reverse operations will take place to the one outlined in the transmitter part.