There have been numerous different form factors and optics types introduced since 10 gigabit Ethernet has been launched years ago. XENPAKS, the oldest, is very popular as the install base is large, while the newest SFP+ offers a much smaller form factor and the ability to offer 1G/10G combo ports on hardware for the first time. Now there are four standard modules/form-factors available.

Main module types and optical standards

XENPAK

• Used primarily by Cisco and Juniper. The 4-port Cisco module WS-X6704-10GE is still very popular
  
• Very large module that includes some PHY logic in the package (example – WAN PHY capability on a XENPAK card is placed on the optic, while this capability on an XFP or SFP+ based interface is placed on the linecard)
  
• Uses SC connector for fiber or CX4 (Infiniband) for copper
  

X2 

• Reduced-size evolution of the XENPAK
  
• Used almost exclusively by Cisco and HP
  
• Same general capabilities as XENPAK
  
• 30% reduced footprint allowed 10G port densities to increase by 2X and then 4X in Cisco modules
  
• Introduced a converter module to convert X2 modules to 2X1G or 1XSFP+ (please note the TwinGig converters have many compatibility restrictions)
  
• Uses SC connector for fiber or CX4 (Infiniband) for copper
  
• X2 form factor is the ONLY option for pluggable 10GbE-T
  

XFP

• Small form factor optic, no PHY logic
  
• Moving the PHY logic to the linecard gave made it possible to make 10G/OC192 dual-rate optics
  
• Uses LC fiber connector
  
• XFP is currently the smallest form factor tunable DWDM optic that is available
  
• Used mostly in WAN applications, due to dual-rate 10G/OC192 and Tunable DWDM offerings
  

SFP+ 

• Same form factor as SFP
  
• SFP+ ports are often 1G/10G, offering huge flexibility in deploying mixed environments
  
• (Note: some chassis have restrictions on which ports are 10G-only and which are 1G/10G)
  
• All major vendors are moving towards SFP+ for common 10GbE applications, especially in the Data Center
  

Within these form factors are many different types of optical and electrical specifications; the only requirement is that the optics type match. It is perfectly acceptable to connect an X2 to an SFP, or a XENPAK to an SFP+, or any other combination.

Optical Standards

Electrical/Copper

10GBase-CX4

CX4 modules use Infiniband 4X cabling, and have a maximum distance of 15 meters.  CX4 is an early copper standard, and due to the physical size of the connector, is not available in SFP+ form, or in XFP form from Cisco (Force10, though, offers a CX4 XFP).  CX4 was designed as a drop-in replacement for legacy Infiniband switching hardware – the existing Infiniband cable plant can be reused in a CX4 based network.

10GBase-CX1/10GBase-CU

10GBase-CX1 is the SFP+ copper standard.  The standard has a maximum distance of 10 meters, though Cisco currently only offers lengths up to 5m.  This is actually a cable with SFP+ ends, not a module with a separate cable.

10GBase-LX4

To overcome the distance limitations of SR optics, the 10GBase-LX4 standard was developed. LX4 uses 4 lasers, each operating at a different wavelength, at a 2.5Gbps data rate. This results in a range of 240-300 meters, depending on cable grade. However, due to the complex laser assembly, it is not possible to get LX4 optics in XFP or SFP+ versions. With the ready availability of OM3 fiber and newer standards that provide long reach over multimode with a single laser, LX4 is rapidly becoming obsolete.

10GBase-LRM

The replacement to LX4, 10GBase-LRM will reach up to 220m over standard multimode fiber, but without the complexity of the LX4 optics.  Instead, a single laser operating at 1310nm is used.  This allows LRM optics to be packaged in XFP and SFP+ form factors.

Single mode Fiber

10GBase-LR

10GBase-LR can reach up to 10km over singlemode fiber. There is no minimum distance for LR, either, so it is suitable for short connections over single mode fiber as well.

10GBase-ER

10GBase-ER can reach up to 40km over singlemode fiber.  Due to the laser power, attenuation is required for links less than 20km long.

10GBase-ZR

ZR optics can reach up to 80km over singlemode fiber.  Due to the very high transmit power, significant attenuation is needed for shorter links.  Use of ZR optics should be preceded with an optical power test of the fiber span in question to ensure a problem-free deployment. Interestingly, 10GBase-ZR is actually not an IEEE standard, though most vendors offer a ZR option.

10GBase-LW

10GBase-LW optics use the same laser, and have the same specifications as the 10GBase-LR optics. However, the LW optics present SONET/SDH physical signaling, allowing LW-equipped devices to interface directly with an existing OC192 transport infrastructure. LW optics are only available in XENPAK and X2 form factors. XFP-based cards move this functionality from the transicever to the card itself, so make sure that if this functionality is needed, that the card itself has this support. The primary example is the as in the SPA-1X10GE-L-V2 vs. the SPA-1X10GE-LW-V2.

10G over DWDM

With 10GbE, it is possible to get optics modules that output at DWDM wavelengths, allowing for much simpler DWDM deployments, and with this optics no additional transponder hardware is required.  A select few 10GbE cards even offer fixed, tunable DWDM interfaces for maximum flexibility. As one of the most confusing aspects of choosing and deploying 10Gig fiber switches, I hope this information helps. And a nod to our excellent presales engineers (thanks chris!) for putting this together for me for this article.

Quick Review of Optics Form Factors

Optics Specs

Cisco DWDM SFP+ Module Overview

The Cisco 10GBASE DWDM SFP+ Modules (Figure 1) are fiber line cards for a wide variety of Cisco switches, routers, and other equipment. They allow enterprises and service providers to provide scalable and easy-to-deploy 10-Gbps LAN, WAN, and optical transport network (OTN) services in their networks.

Features and Benefits

The Cisco 10GBASE Dense Wavelength-Division Multiplexing SFP+ Module offer the following features and benefits:

• Supports 10-Gigabit data rates from 9.9G to 11.1G (LAN, WAN, and OTU2/OTU2e) to accommodate different applications
  
• Smallest SFP+ module footprint in the industry
  
• Hot-swappable input/output device plugs into an Ethernet SFP+ port of a Cisco switch or router to link the port with the network
  
• Support for a “pay-as-you-grow” model for investment protection
  
• Digital optical monitoring capability for enhanced diagnostics and troubleshooting
  
• DWDM fixed module supports 40 non-tunable ITU 100-GHz wavelengths
  
• DWDM tunable module supports 96 tunable ITU 50-GHz wavelengths
  
• Supports the Cisco quality identification (ID) feature, which enables a Cisco switch or router to identify whether or not the module is an SFP+ module certified and tested by Cisco
  

Platform Support

The Cisco DWDM SFP+ modules are supported across a variety of Cisco switches, routers, and optical transport devices. For more details, refer to the Cisco 10-Gigabit transceivers compatibility matrix at

http://www.cisco.com/en/US/docs/interfaces_modules/transceiver_modules/compatibility/matrix/OL_6974.pdf.

Connectors and Cabling

• Equipment: standard SFP+ interface
  
• Network: dual LC/PC connector
  

Note:    Only connections with patch cords with PC or UPC connectors are supported. Patch cords with APC connectors are not supported. All cables and cable assemblies used must be compliant with the standards specified in the standards section.