Internet Explorer no es un explorador compatible con TI.com. Para disfrutar de una mejor experiencia, utilice otro navegador.

VIDEO SERIES

Precision labs series: Ethernet

TI Precision Labs (TIPL) es la sala de clases en línea más completa para los diseñadores de cadenas de señales analógicas. La serie de interfaces ofrece formación técnica sobre los protocolos más utilizados en aplicaciones industriales, automotrices y más. Inicie la serie "Ethernet" para obtener una perspectiva general de los transceptores, retemporizadores y recontroladores Ethernet PHY y aprender a optimizar la integridad de la señal y el alcance de la red.

Video Player is loading.
Current Time 0:00
Duration 0:00
Loaded: 0%
Stream Type LIVE
Remaining Time 0:00
 
1x
  • Chapters
  • descriptions off, selected
  • subtitles off, selected

      Presenter(s)

      Welcome to this Precision Labs presentation on ethernet. In this session, we'll spend a few minutes looking at ethernet physical layer transceivers, also called PHYs. In this video, we'll cover the following topics. How a PHY is connected in a typical application circuit, a breakdown of the PHY into common sub-functions, and the capabilities of each sub-function.

      This diagram shows a basic system level implementation of an ethernet physical layer connection. The PHY sits between the media access controller device, or MAC, and the network connection. The MAC device can be either a microcontroller, processor, FPGA, or ethernet switch. The connection to the MAC layer is called the media independent interface, or MII. Different versions of this interface are defined in the IEEE 802.3 standard and offer either reduced number of signal lines, higher data speeds, or both.

      The supported data speeds are 10, 100, 1,000 megabits per second and higher. The term XMII is used to collectively refer to multiple versions of the MAC interface. To support the higher speeds while easing clock requirements, multiple data lanes maybe be used.

      The MAC interface may also include transmit and receive clocks and other status signals, again, depending on the version. There may also be another connection between the PHY and the MAC device, what we show as SMI in the diagram, which stands for station management interface. This is a two-wire communications port specified in the IEEE 802.3 standard that allows a system controller or the MAC to manage the PHY by accessing its internal registers.

      The PHY's network connection is called the media dependent interface, or MDI. Unlike the MII, the signaling characteristics of the MDI depends on the nature of the physical channel, which could be either copper or fiber. The PHY usually provide general purpose input output pins or GPIO, which typically can be used to either drive status LEDs or to provide access to other internal signals. When used as LED drivers, they can give you visual indications of link conditions, such as speed or status.

      The PHY requires an external clock source, which can be used as a 25 megahertz crystal or a 15 megahertz clock supplied from another device. Both of these must meet a minimum frequency accuracy of plus minus 100 PPM, accounting for all error sources. Some of the details on clocking XMII, SMI, and MDI will be covered in other Precision Lab sessions about ethernet.

      The IEEE 802.3 standard defines three major functional blocks or sublayers that make up the PHY. These layers are the physical coding sublayer, or PCS. The primary function of the PCS is to encode and decode data. For example, a 100BASE-TX, four bits of data received from the MAC interface to be transmitted over the network are encoded to 5-bit block code. Hence, the actual data rate transmitted over the channel becomes 125 megabits per second.

      The use of block codes helps with clock recovery at the receiver and provides additional codes that are used for linked management. The physical medium attachment sublayer, or PMA, provides functions for bit to symbol mapping, determining link status, clock recovery, and detecting error events. The physical medium dependent sublayer, or PMD, implements the functions that support the PHY's physical connection to the network, whether it is copper or fiber.

      Each layer contains both transmit and receive paths. The PCS block is responsible for receiving data from and transmitting data to the back. While the PMA function is always present in the PHY, the PMD may not be. This is determined by the versions of the Ethernet standard supported by the MAC. In some versions of the standard, the PMD functions are included in the PMA. Regardless, these blocks are the interface to the physical medium. And they provide services that convert electrical or optical signals to bits in the receive path and bits to electrical or optical signals in the transmit path.

      Though the diagram here shows separate transmit and receive channels in the physical medium, the physical medium may actually be shared between RX and TX. In this case, hybrid and echo canceling functions hosted in the PMA, PMD allow the PHY to successfully receive data over the link at the same time it is transmitting.

      Finally, the physical medium may consist of multiple parallel physical channels that enable the transmission and reception of data at higher rates. For example, 1000BASE-T uses four twisted pair channels, A, B, C, and D, each supporting 250 megabits per second to achieve a total data rate of 1 gigabit per second.

      The physical coding sublayer, or PCS, provides different services to the MAC layer and PMA. It is primarily responsible for encoding and decoding data passed over the link. The PCS exchanges transmit and receive data and other control signals from the MAC layer through the XMII interface.

      The widths of the transmit and receive data paths between the PHY and the MAC here labeled as TXDn0 and RXDn0 may have values of 1, 2, 4, or 8-bits, depending on the total data speed. The PCS also contains functions that monitor both the transmit and receive paths to determine when the network interface is active. And if there are collisions, signal the MAC layer the occurrence of a collision using the COL signal. It supports low power mode if implemented by processing idle code groups.

      The received path of the PCS takes in data from the PMA interface and basically reverses the processing that the PCS and the sending node apply to the data. It decodes the received code groups to the original packet bits. Based on either the success or failure of proper data decode and recovery, the received block would either assert an RX DV signal to indicate it is forwarding valid data to the MAC or a certain RXER signal to indicate it has detected an error in the data.

      The PHY's clock recovery function is used to synchronize its receiver bit clock to the transmitting PHY's bit clock. This is vital to understand when performing loopback testing. If all is decoded and encoded properly, the same number of received and transmitted packets should be the same with no errors. In some versions of IEEE 802.3 standard, additional scrambling is applied to the data.

      The main function of the physical medium attachment sublayer, or PMA, is to convert the encoded transmitted bit stream from the PCS sublayer to the appropriate data symbol for transmission on the network medium and convert incoming data symbols from the network into bits that are then passed to the PCS sublayer. The PMA sublayer also performs carrier detection by looking for a link transition from the idle state in the received bitstream RXK. When this transition is detected, it then looks for the start of stream delimiter, SSD.

      Once the ladder is detected, it indicates carrier detection to the PCS sublayer. It also monitors the signal status reported by the PMD sublayer if present. If the auto-negotiation capability has been implemented in the PHY, the PMA sublayer will coordinate with the ladder and set the link status indication to the appropriate value pending the state of auto-negotiation and pass this to the PCS. Otherwise, it sets the link status according to the signal status indicated by the PMD.

      If the PHY does not support auto-negotiation, it may instead include an optional far-end fault detect capability. If the PHY detects a physical error condition in the receive channel, it will generate a fault indication and insert this in the TX channel stream. If it receives a foreign fault indication in the RX channel, which is a special signal pattern, it will begin transmitting an idle symbol pattern to insist with reestablish of normal communications. Note that far-end fault detect is not supported if the PHY is capable of auto-negotiation.

      Some optional functionality that the PMA performs also include generating indications and carrier areas from the PMD if present and sensing receive channel failures, then transmitting or detecting far-end fault indication. This is useful for any debugging on the PHY. Proper functioning of the PMA is important as it needs to pass compliance tests.

      Finally, note that for versions of the standard that use multiple twisted pairs, such as 1000BASE-T, there will be a one to end mapping of the symbol stream. For example, 1000BASE-T uses four twisted pair designated as channels A, B, C, and D. The physical medium dependent sublayer, or PMD, if implemented in the PHY, is primarily responsible for converting the TX symbols to the appropriate physical signals used for the network medium. It also converts the received path signals to RX symbols in the receive path.

      The PMD may not always be part of the PHY. The use of the PMD is defined by the specific version of the standard implemented by the PHY, the standard defined systems that accommodate certain mediums and types of cabling. In some amendments to IEEE 802.3 standard, for example, IEEE 802.3bw, which is also called 100BASE-T1, the PMD is not defined because only a single twisted pair copper is supported.

      In this case, the PMA handles the task of conversion between symbols and signaling. This is also true for 1000BASE-T. 1000BASE-TX defines PMD functionality, but 1000BASE-T does not. The PMD is useful for debugging the medium as it is the physical layer responsible for interfacing to the physical media.

      Considering the only network interfaces that use twisted pair copper for the moment, the electrical format of the signaling is differential. Some examples of the types of signaling used include 100BASE-TX using MLT3, 1000BASE-T using PAM5, and 100BASE-T1 using PAM3. Examples of MLT3 and PAM5 are shown here as ideal signals.

      The symbol values illustrated here are relative values not actual voltage levels, which are defined in the respective IEEE 802.3 amendments. The actual voltages are also a differential value. For example, V diff of T is equal to TX positive minus TX negative. More information and details on the different types of signaling will be explained in future videos. If the network uses copper cable, the differential signals are usually driven into a twisted pair differential impedance of 100 ohms.

      The IEEE 802.3 specification clauses listed here contain details on the PCS, PMA, and PMD functions in the PHY, broken down by link speed. As used here, the notation of 100 BASE-X represents both copper 100 BASE-TX and fiber 100 BASE-FX versions of the 100 megabits per second specification. The PCS and PMA functions are identical for either copper or fiber because they are medium independent and are described in section 2 clause 24 of the standard.

      Because the PMD depends on the physical medium, the copper and fiber versions are described in different clauses, clause 25 for copper and clause 26 for fiber. Section 3 of IEEE 802.3 covers of various versions of 1 gigabit per second ethernet. 1000BASE-T is described in clause 40, while long and short wavelength fiber versions, as well as short haul copper, are described in clauses 36 through 39.

      Thank you for viewing this Precision Lab session on ethernet physical layer transceivers. To find more ethernet technical resources and to search TI's portfolio of ethernet PHY products, visit ti.com/ethernet.

      View series

      Precision labs series: Ethernet