This document answers the question of whether a Synchronous Optical
Network (SONET) link can support SingleMode Fiber (SMF) on one end and
MultiMode Fiber (MMF) on the other end of an optical link between Cisco
routers. This document also explains the difference between SMF and MMF and the
current interface modules that support them. At the end of this document, you
must be able to identify the interface type and configure the interface.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Technical Tips Conventions for more information on document
In order to understand how to interconnect modes, you first need to
define a mode. There are two typical definitions of a mode, as explained here:
Bundles of light rays that enter the fiber at a particular angle.
Paths that light rays travel through the fiber. These paths can have
different lengths and transmission delays as the light travels through the
MMF allows multiple modes of light to propagate through the fiber.
Multiple modes of light that propagate through the fiber travel different
distances, based on the entry angles. The differences in travel speeds cause
the modes to arrive at the destination at different times. MMF typically uses
Light-Emitting Diodes (LEDs) to launch the optical signal.
SMF allows only one mode of light to propagate through the fiber. SMF
uses lasers to launch light in a more concentrated fashion. A laser transmitter
couples light into only a fraction of the existing modes or optical pathways
present in the fiber-optic cable. Therefore, SMF is capable of higher bandwidth
and greater cable run distances than MMF.
Figure 1 illustrates the transmission
differences between MMF and SMF.
Figure 1 – Transmission Differences Between MMF and
Section 4 of the
GR-253 Specification for SONET Transmission Systems
defines "a small set
of application categories and corresponding sets of optical interface
This table lists these categories, which generally describe the power
level and theoretical distance of the transmitted signal:
0 dB and 4 or 7 dB.
0 dB and 11 or 12 dB.
10 dB to 22, 24 or 28 dB, depending on the bit rate.
Up to 33 dB. (Defined at Optical Carrier-192 (OC-192) bit rates
Within the MMF category, only Short Reach (SR) is available. Within the
SMF category, two types of transmission are defined:
Intermediate Reach (IR)
Long Reach (LR)
Typically, POS and Asynchronous Transfer Mode (ATM) over SONET hardware
is available in MMF and SMF versions. Here is an example that shows the use of
the PA-POS adapter for the 7x00 series.
PA-POS-OC3SMI - SMF, IR
PA-POS-OC3MM - MMF, SR
In most cases, the output of the show diag
command indicates the mode type and reach of the optical hardware. The mode
type for the PA-POS adapter for the 7x00 Series appears in the
show diag command output in a future release of
Cisco IOS® Software. As a workaround, look for MM for MultiMode or IR
(Intermediate Reach) for SingleMode on the faceplate to determine the model and
Cisco SONET interfaces support interconnection of SMF and MMF optics.
In other words, an MMF receiver at one end, and an SMF receiver at the other
end. However, this mismatch of mode types is not officially supported by Cisco
Technical Assistance Center (TAC). The reason is that when an unconditioned
laser source designed for operation on an SMF cable is directly coupled with an
MMF cable, Differential Mode Delay (DMD) can occur. DMD can degrade the modal
bandwidth of the fiber-optic cable. This degradation causes a decrease in the
link span (the distance between the transmitter and the receiver) that can be
reliably supported. In addition, when you interconnect the two modes, take
extra care to ensure that the SMF transmitter is attenuated sufficiently to
avoid an impact and overdrive of the multimode receiver optics.
Here is a list of third-party vendors who offer devices for converters
to interconnect SMF and MMF optics:
Alternatively, you can use an intermediate switch or device with an SMF
interface and an MMF interface, which then creates two segments and effectively
converts between the nodes.