# Physical coding sublayer

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{{Short description|Networking protocol sublayer}}
The '''physical coding sublayer''' ('''PCS''') is a networking protocol sublayer in the [Fast Ethernet](/source/Fast_Ethernet), [Gigabit Ethernet](/source/Gigabit_Ethernet), and [10 Gigabit Ethernet](/source/10_Gigabit_Ethernet) standards. It resides at the top of the [physical layer](/source/physical_layer) (PHY), and provides an interface between the '''physical medium attachment''' ('''PMA''') sublayer and the [media-independent interface](/source/media-independent_interface) (MII). It is responsible for data encoding and decoding, [scrambling and descrambling](/source/Scrambler), alignment marker insertion and removal, block and symbol redistribution, and lane block synchronization and deskew.<ref name="Ethernet: The Definitive Guide">{{cite book | last = Spurgeon | first = Charles | title = Ethernet: The Definitive Guide | publisher = O'Reilly | year = 2014 | pages = 198 | isbn = 978-1449361846 }}</ref>

== Description ==
The Ethernet PCS sublayer is at the top of the Ethernet [physical layer](/source/physical_layer) (PHY). The hierarchy is as follows:

* [Data link layer](/source/Data_link_layer) (Layer 2)
** [Logical link control](/source/Logical_link_control) (LLC) sublayer
** [Medium access control](/source/Medium_access_control) (MAC) sublayer
*** Reconciliation sublayer (RS){{dash}}This sublayer processes PHY local/remote fault messages and handles DDR conversion
* [PHY Layer](/source/physical_layer) (Layer 1)
** Physical coding sublayer (PCS){{dash}}This sublayer determines when a functional link has been established, provides rate difference compensation, and performs coding such as [64b/66b encoding](/source/64b%2F66b_encoding) and <math>x^{58}+x^{39}+1</math> [scrambling/descrambling](/source/Scrambler)
** Physical medium attachment (PMA) sublayer{{dash}}This sublayer performs PMA framing, octet synchronization/detection, and <math>x^{7}+x^{6}+1</math> scrambling/descrambling
** [Physical medium dependent](/source/Physical_medium_dependent) (PMD) sublayer{{dash}}This sublayer consists of a transceiver for the physical medium

== Specifications ==

===10&nbsp;Mbit/s Ethernet===
* Classic Ethernet uses [Manchester code](/source/Manchester_code) in the [Physical signaling sublayer](/source/physical_layer) (PLS), encoding each bit as a high-low (logical zero) or low-high transition (logical one).<ref>IEEE 802.3 ''1.4 Definitions''</ref>

===Fast Ethernet ===
* 100BASE-X for fiber (100BASE-FX) and twisted pair copper (100BASE-TX) encodes data [nibble](/source/nibble)s to five-bit code groups ([4B5B](/source/4B5B)).<ref>IEEE 802.3 Clause 24.1.4.1</ref>

=== Gigabit Ethernet ===
*1000BASE-X for fiber and 150&nbsp;Ω balanced copper (twinaxial) uses [8b/10b encoding](/source/8b%2F10b_encoding) with a [symbol rate](/source/symbol_rate) of 1.25 GBd.<ref>IEEE 802.3-2012 Clause 36</ref>
*1000BASE-T for twisted pair copper splits the data into four lanes and uses four-dimensional, five-level (quinary) [trellis modulation](/source/trellis_modulation) with [PAM-5](/source/PAM-5) and a symbol rate of 125 MBd.<ref>IEEE 802.3-2012 Clause 40</ref>

===2.5 and 5 Gigabit Ethernet===
* 2.5GBASE-T and 5GBASE-T use the same encoding as 10GBASE-T slowed by a factor of four or two, respectively.

=== 10 Gigabit Ethernet ===
*10GBASE-R (LAN) is the serial encoded PCS using [64b/66b encoding](/source/64b%2F66b_encoding) that allows for Ethernet framing at a rate of 10.3125&nbsp;Gbit/s. This rate does not match the rate 9.953&nbsp;Gbit/s used in [SONET](/source/SONET) and SDH and is not supported over a WAN based on SONET or SDH.
*10GBASE-X (LAN/WAN) uses 8b/10b encoding over four lanes at 3.125 GBd each and is used for 10GBASE-LX4 (single-mode and multi-mode fiber), 10GBASE-CX4 (twinax), and 10GBASE-KX4 (backplane).<ref>IEEE 802.3 ''48. Physical Coding Sublayer (PCS) and Physical Medium Attachment (PMA) sublayer, type 10GBASE-X''</ref>
*10GBASE-W (WAN) defines WAN encoding for 10GbE. It uses 64/66b encoding and lowers the MAC rate to 9.95&nbsp;Gbit/s, so that is compatible with SONET STS-192c data rates and SDH VC-4-64 transmission standards when wrapped into a SONET frame.
*10GBASE-T for twisted pair copper splits the data into four lanes and uses 64B/65B encoding, scrambling, and 128 double-square (DSQ128) checkerboard encoding with PAM-16 generated at 800&nbsp;MBd.<ref>IEEE 802.3 Clause 55.3.2</ref>

=== 25 Gigabit Ethernet ===
* 25GBASE-R uses the same 64b/66b encoding as 10GBASE-R with a speed-up to 25.78125&nbsp;GBd.<ref>{{Cite web|url=http://www.ieee802.org/3/by/public/Jan15/baden_3by_01b_0115.pdf |title=IEEE 802.3by 25G Ethernet TF, A BASELINE PROPOSAL FOR RS, PCS, AND FEC |date=2015-01-12 |access-date=2016-08-06}}</ref>

=== 40/100 Gigabit Ethernet ===
* 40GBASE-R and 100GBASE-R use 64b/66b encoding over multiple lanes of 10.3125 GBd or 25.78125 GBd each. These lanes – four for 40&nbsp;Gbit/s, four or ten for 100&nbsp;Gbit/s per direction – are either transmitted separately over short distance or together with coarse [wavelength division multiplexing](/source/wavelength_division_multiplexing) on long distance fiber (-LR).<ref>IEEE 802.3 Clauses 82-89</ref>

==See also==
* [PHY-Level Collision Avoidance](/source/PHY-Level_Collision_Avoidance)

== References ==
{{reflist}}
*{{cite journal|last=Barbieri |first=Alessandro |title=10 GbE and Its X Factors |journal=Packet: Cisco Systems Users Magazine |volume=17 |issue=3 |pages=25–28 |url=http://www.cisco.com/asiapac/campaigns/metroethernet/files/10ge_po_pack_mag_arti.pdf |access-date=2007-12-31 |url-status=dead |archive-url=https://web.archive.org/web/20110608032208/http://www.cisco.com/asiapac/campaigns/metroethernet/files/10ge_po_pack_mag_arti.pdf |archive-date=2011-06-08 }}

== External links ==
* [https://grouper.ieee.org/groups/802/3/ae/public/mar00/figueira_1_0300.pdf IEEE 802.3 Meeting]
* [https://web.archive.org/web/20110716191855/http://www.xilinx.com/systemio/1gbsx_phy/basics.htm Ethernet 1000BASE-X PCS/PMA Technology Basics]

{{Ethernet}}

Category:Ethernet

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Adapted from the Wikipedia article [Physical coding sublayer](https://en.wikipedia.org/wiki/Physical_coding_sublayer) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Physical_coding_sublayer?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.