Next-generation Ethernet interfaces

For the first time in its history, the IEEE has released two Ethernet standards simultaneously: 40 Gigabit Ethernet (40GE) and 100 Gigabit Ethernet (100GE), both defined by the 802.3ba Task Force.

For the first time in its history, the IEEE has released two Ethernet standards simultaneously: 40 Gigabit Ethernet (40GE) and 100 Gigabit Ethernet (100GE), both defined by the 802.3ba Task Force.

40GE is expected to be initially deployed to support cost-sensitive applications such as data center switch aggregation and high-performance server interconnection, while 100GE will be used in for demanding needs such as interconnecting core routers and DWDM transport equipment. 100GE will also be used in the data center once higher performance or higher density is required and its cost decreases as deployed volumes increase.

Evolution of Ethernet

The optical interfaces for 40GE, also known as optical "Physical Media Dependent" or PMDs, are defined as the following:

* 40GBASE-SR4: Maximum link length of 100 meters on OM3-grade Multimode Fiber (MMF).

* 40GBASE-LR4: Maximum link length of 10 kilometers on Single Mode Fiber (SMF).

The PMDs defined for 100GE are the following:

* 100GBASE-SR10: Maximum link length of 100 meters on OM3-grade Multimode Fiber (MMF).

* 100GBASE-LR4: Maximum link length of 10 kilometers on Single Mode Fiber (SMF).

* 100GBASE-ER4: Maximum link length of 40 kilometers on Single Mode Fiber (SMF).

What follows is a more detailed description of the technical approaches used for these interfaces, as well as descriptions of several optical transceiver module form factors that either already exist or are being defined which can support the various 40GE and 100GE PMDs.

40G Ethernet interfaces

Two types of electrical interfaces have been defined to interconnect the 40GE optical transceiver modules and the host board. Both are based on four independent lanes, each carrying 10.3125Gbps. The first, called "40Gigabit Attachment Unit Interface", or XLAUI, assumes a retiming functionality is present inside the optical transceiver module. The second, called "40 Gigabit Parallel Physical Interface", or XLPPI, assumes no retiming in the module.

Two optical PMD interfaces have been defined for 40GE by IEEE 802.3ba, one for multimode and the other for single mode fiber optics:

* 40GBASE-SR4: This optical interface is based on four independent full-duplex 10.3125Gbps optical links, supporting a maximum distance of 100 meters over OM3-type (i.e., 2000 MHz*km bandwidth) multimode fiber optics. Four independent 850nm lasers are used as transmitters, each connected to four independent optical fibers. Similarly, the receivers consist of four photo-detectors connected to another set of four independent fibers. In a typical implementation, these eight fibers will be part of a single fiber ribbon cable, which connects to the transceiver module through a single MPO/MTP-type optical connector.

* 40GBASE-LR4: This optical interface is based on four different CWDM wavelengths in the 1300nm window, each transmitting at 10.3125Gbps and supporting a maximum distance of 10 kilometers over standard single-mode fiber. Four independent, un-cooled CWDM lasers are used as transmitters, and the four wavelengths are optically multiplexed into a single fiber. The receiver follows a similar configuration, where the four CWDM wavelengths on the incoming fiber are optically demultiplexed into four independent photo-detectors. A duplex set of single mode fibers are used on this PMD, similarly to 10GBASE-LR or 1000BASE-LX interfaces.

Additionally, the more recent IEEE 802.3bg Task Force is currently defining 40GBASE-FR, a new 40GE PMD based on a single 1550nm 40G laser, which targets multiprotocol applications in the telecom carrier environment but which is not expected to ship in significant volumes into the more traditional Ethernet applications.

There are currently two optical transceiver module form factors which can support these 40GE PMDs:

* CFP. This recent Multi-Source Agreement (MSA) was announced in March 2009, with Finisar as one of its founding members. It defines the mechanical features, MDIO-based management interface, electrical connector and pin-out of a hot-pluggable module, which can support a variety of 40G and 100G Ethernet, SONET/SDH and OTN applications. It also includes the definition of the thermal management system. More information on CFP can be obtained here. The CFP form factor can support the 40GBASE-SR4, 40GBASE-LR4 and 40GBASE-FR PMDs, with an XLAUI electrical interface to the host board.

* QSFP: This older MSA also defined the mechanical features, I2C-based management interface, electrical connector and pin-out of a hot-pluggable, high-density form factor. Originally created to support 4x2.5G interfaces, its specification has recently been updated to support 4x10G applications such as 40GE and InfiniBand QDR. The document (also known as QSFP+) is currently being maintained by the SFF Committee under SFF-8436. The QSFP form factor can only support 40GBASE-SR4 and 40GBASE-LR4 PMDs with an un-retimed XLPPI electrical interface to the host board. Due to real estate and thermal management limitations, it is not expected that serial 40G PMDs such as 40GBASE-FR or ITU-T G.693 interfaces could be supported by the QSFP form factor in the near future.

100G Ethernet interfaces

Two types of electrical interfaces have also been defined to interconnect 100GE optical transceiver modules and the host board. Both are based on 10 independent lanes, each carrying 10.3125Gbps. The first, called "100Gigabit Attachment Unit Interface", or CAUI, assumes a retiming functionality is present inside the optical transceiver module, while the second, called "100 Gigabit Parallel Physical Interface", or CPPI, assumes no retiming in the module.

Three optical PMD interfaces have been defined for 100GE, one for multimode and the other two for single mode fiberoptics:

* 100GBASE-SR10: This optical interface is based on 10 independent full-duplex 10.3125Gbps optical links, supporting a maximum distance of 100 meters over OM3-type (i.e., 2000 MHz*km bandwidth) multimode fiber optics. Ten independent 850nm lasers are used as transmitters, each connected to ten independent optical fibers. Similarly, the receivers consist of 10  photo-detectors connected to another set of 10 independent fibers. In a typical implementation, these 20 fibers will be part of a single fiber ribbon cable, which connects to the transceiver module through a single MPO/MTP–type optical connector.

* 100GBASE-LR4: This optical interface is based on four different LAN-WDM wavelengths in the 1300-nm window, each transmitting at 25.8Gbps and supporting a maximum distance of 10 kilometers over standard single-mode fiber. Four independent, cooled LAN-WDM lasers are used as transmitters, which are optically multiplexed into a single fiber. The receiver follows a similar configuration, where the four LAN-WDM wavelengths on the incoming fiber are optically demultiplexed into four independent photo-detectors. A duplex set of single mode fibers are used on this PMD, similar to 40GBASE-LR4 interfaces. This PMD requires the implementation of a Serdes (or "Gearbox") IC inside the optical transceiver module, in order to convert the 10x10G CAUI electrical interface to 4x25G.

* 100GBASE-ER4: This optical interface is based on four different LAN-WDM wavelengths in the 1300-nm window, each transmitting at 25.8Gbps and supporting a maximum distance of 40 kilometers over standard single-mode fiber. Four independent, cooled LAN-WDM lasers are used as transmitters, which are optically multiplexed into a single fiber. The receiver follows a similar configuration, where the four LAN-WDM wavelengths on the fiber coming into the transceiver module go through an optical amplifier before they are optically demultiplexed into four independent photo-detectors. A duplex set of single mode fibers are used on this PMD, similar to 40GBASE-LR4 interfaces. This PMD also requires a Serdes IC in the module.

There are currently also two optical transceiver module form factors which can support these 100GE PMDs:

* CFP: The CFP form factor described above can support the 100GBASE-SR10, 100GBASE-LR4 and 100GBASE-ER4 PMDs, with a CAUI electrical interface to the host board. As previously stated, the 100G LR4 and ER4 interfaces require a Serdes IC inside the CFP module.* CXP: This is also a new form factor, which has been defined by the InfiniBand Trade Association (IBTA) for High-Performance Computing (HPC) applications. The CXP Specification defines the mechanical features, I2C-based management interface, electrical connector and pin-out of this hot-pluggable, high-density form factor. Created to support 12x10Gbps applications such as Infiniband QDR, it can also support 10x10.3125Gbps applications such as the 100GBASE-SR10 PMD, with an un-retimed CPPI electrical interface to the host board. The CXP form factor cannot support the 10km or the 40km single-mode PMDs.

Future form factors

Advancements in photonics integration as well as the standardization of 25Gbps electrical I/O (under the OIF CEI-28G-VSR) are expected to enable a higher-density form factor for 100GBASE-LR4 applications in the foreseeable future, called CFP2. This new form factor, which still needs to be defined by the MSA, is expected to have approximately half the width of a CFP, as well as significantly lower power dissipation.

On the multimode front (MMF), the InfiniBand market is expected to define a new form factor called QSFP2 very soon, which will support 4x25G parallel optics applications using 25Gbps 850nm VCSELs over multimode fiber. It is very likely that the IEEE will define a PMD based on this architecture, which would be called "100GBASE-SR4" and which could be implemented using a QSFP2. Looking much further into the future, enabled by the subsequent generation of photonics integration, the QSFP2 form factor may also be able to support the 100GBASE-LR4 PMD.

Read more about lan and wan in Network World's LAN & WAN section.

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Christian Urricariet

Network World
Topics: metro Ethernet, Networking, Ethernet Switch, LAN & WAN
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