Overview of the XAUI, XLAUI and CAUI: Part1

Every speed step made in the Ethernet standard has required a new front end interface to help the digital MAC framers interface to Physical layer devices and to preserve the continuity of the MAC functionality (It has to be noted that the MAC layer at 10Mbps is the same format as that used in 100Gbps). This blog tries to give a brief overview of how these interfaces have evolved and the features that have been added.

(To help with the following explanation a diagram of the Ethernet System interfaces and  functional blocks (from 10Mbps to 100Gbps) can be downloaded from the 40G Ethernet Resource Center here).

Background on 10/100/1000Mbps Ethernet system interfaces
At the very beginning we had the AUI interface for 10Mbps Ethernet that allowed a MAU to be plugged in (such a PHY that interfaced to Coax cable). The function that separated the MAC from the AUI was called the PLS which implemented the Manchester encoding along with other things. The PLS is long gone but even so the interface to the MAC itself is still referred to as the PLS service layer.

When 100Mbps arrived it was found the existing AUI was not suitable and it was replaced by the MII interface (and its derivatives RMII and SMII). The PLS function was dropped and replaced by the RS function which on one side has the MII interface and the other side the PLS service layer interface. The basic role of the RS layer is to incorporate any necessary data modifications to adapt future MII interfaces to the unchanged MAC interface. The MII would then interface to a Physical Layer device which used the 4B/5B coding scheme (taken from FDDI) plus other media specific functions, such as the MLT-3 encoding required for 100Base-T.

With the arrival of Gigabit Ethernet the GMII, and later RGMII / SGMII, were introduced along with the 8B/10B PCS coding, which was taken from Fibre Channel. At this point all GigE MAC’s integrated the PCS layer and provided a TBI which allowed direct connection to available 10bit SERDES devices (which were used for Fibre Channel). Initially the TBI was more popular and many MAC devices did not support the GMII, this required the later 1000Base-T transceivers to support both GMII and TBI on the system interface.

10G Ethernet & the XAUI
For 10GEthernet the standard MAC/RS interface is the XGMII which interfaces to a Physical layer device that supports PCS/PMA/PMD function. For optical links the PCS coding is defined as using 64B/66B coding rather than 8B/10B in order to keep the line rate down to 10.3125Gbps (rather than 12.5Gbps required if 8B/10B was used). However it was also clear that whilst the XGMII interface would be rarely offered as an external interfaces on chips, due to the pin count, the PCS/PMA/PMD functions would initially reside in an external PHY layer SERDES device – thus some type of mid range interface would be required to allow the MAC to connect to the PHY and the solution was the XAUI.

The XAUI comprises of 4 x 3.125Gbps physical lanes with each use the 8B/10B coding scheme (rather than than 64B/66B) and in order to handle the potential deskew between the four lanes a new 8B/10B control word /A/ was introduced. The /A/ control word is periodically inserted, at exactly the same time, into each XAUI stream with the idle control word in the IPG being deleted to compensate for the added bandwidth. In the receive direction the /A/ are detected and used as alignment markers to re-synchronize the four XAUI lanes.

The XAUI interface not only found it’s self as a chip to chip and backplane interface, but also as the interface to 10GBase-LX4 modules which use four WDM multiplexed lambda’s to provide 10G Ethernet over OM3 cable, a concept that would be the basis for 40/100G Ethernet.

40/100G Ethernet and the XLAUI/CAUI
As with 10Gbps Ethernet the relevant MII interfaces, XLGMII(40G) and CGMII(100G), are no longer assumed to be external interfaces but just function boundaries inside devices, the role of providing an external interface is taken by the XLAUI and CAUI. The XLAUI and CAUI interfaces follow the concept of the XAUI interface but due to the bit rate required use the 64B/66B encoding, rather than 8B/10B, to keep the line rate down to 10.3125Gbps.

As with the XAUI interface it is necessary to compensate for mis-alignment of the various lanes, due to different delays in each path, which is solved by the insertion of a special 64B/66B alignment code word. To handle the various data path widths which range from 10, 5 and 4 the concept of virtual lanes is used and the management of these virtual lanes is handled in a new function called the MLD layer.

In part 2 of the blog we’ll go into detail on the concept of virtual lanes and how the MLD function works plus take a look at recently available 40/100G Ethernet FPGA IP cores from Sarance and MorethanIP.

The 40G Ethernet Resource Center


Darn those TLA’s (and FLA’s – Four/Five Letter Abbreviations)

AUI : Attachment Unit Interface, originally connected to a MAU
MAU : Medium Attachment Unit, like a 10Base-2 transceiver.
MLT-3 : Multi Level Transmission 3; Three level line code (+1/0/-1) used by 100Base-T
XAUI : A 10G AUI, the X is the Roman numeral for 10; Data path is 4×3.125Gbps Lanes
XLAUI : A 40G AUI, XL being the Roman numeral for 40; Data path is 4×10.3125Gbps Lanes
CAUI: A 100G AUI, C (you guessed it) being the Roman numeral for 100; Data path is 10×10.3125Gbps Lanes

MII : Medium Independent Interface, 4bit wide data path.
RMII : Reduced MII, the MII but with less signals!
SMII : Serial MII, the data path is reduced to one bit.

GMII : Gigabit MII, 8bit wide data path.
RGMII : Reduced Gigabit MII.
SGMII :  Serial Gigabit MII.
TBI : Ten Bit Interface

XGMII : 10G MII (this time the G made it in).
XGXS : XGMII eXtender Sublayer.

MAC : Media Access Controller.
PLS : Physical Layer Signaling; for 10Mbps only, implemented the Manchester encoding.
RS : Reconciliation Sublayer.
PCS : Physical Coding Sublayer; e.g. 8B/10B.
MLD : Multi Lane Distribution
PMA :  Physical Medium Attachment.
PMD : Physical Medium Dependant.

IPG : Inter Packet Gap; Code words sent between valid Ethernet Frames.

4 Responses to “Overview of the XAUI, XLAUI and CAUI: Part1”

  1. Jian Meng Says:

    Looks there is a typo on page https://40gethernet.wordpress.com/2009/05/08/overview-of-the-xaui-xlaui-and-caui-part1/

    XLAUI : A 40G AUI, XL being the Roman numeral for 40; Data path is 4×10.3.125Gbps Lanes

    I think the 10.3.125Gbps should be 10.3125Gbps

  2. Patrick Says:

    minor typo “10/100/1000G” should be 10/100/1000 or 10/100/1G.

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