Thursday, October 22, 2015

Single-mode vs. Multi-mode Transceiver

We are quite familiar with difference between single-mode fiber (SMF) and multi-mode fiber (MMF), but we know little about single-mode and multi-mode transceiver. So what are they? And how to choose between them. Actually single-mode and multi-mode fiber are the transport media of a transceiver module. Besides this, here are certain things you need to know before making that decision which can prevent additional costs in the future.
single-mode vs.multi-mode fiber transceiver
The key differences between single-mode and multi-mode transceiver lie in the following expects—distance and cost, speed, how they work and compatibility issue. The following text will explain them one by one.
Distance and Cost
The single-mode fiber based transceivers work mainly in 1310nm and 1550nm wavelength and have a transmission distance of 2 km to 120 km. While multi-mode fiber based transceivers work in 850nm wavelength and are only used for short distance defined from 150 meters to 5 kilometers. The difference in transmission distance is owing to different transmitter power, receiver sensitivity and the wavelength in operation.
The cost difference between single-mode and multi-mode transceiver mainly arises from the cost difference between single-mode fiber and multi-mode fiber. It is known that the optic for single-mode is nearly twice the cost of multi-mode fiber. But the cost of a single-mode transceiver is negligible when installed as part of a project. Therefore, multi-mode transceivers are typically used when the distance involved is not long.
Both single-mode transceiver and multi-mode transceiver can transmit 10G speed. You just need to consider the distance requirement. Multi-mode can only support 300 to 400 meters. Single-mode transceiver can support link lengths up to 120 km. But prices go up accordingly.
How They Work
Single-mode fiber has a smaller core (9 micron) and narrower wavelength, which means that SMF has the capability for higher bandwidth and much longer distances in transmission. And because of its smaller core, single-mode fiber has less light diffraction over distance than multi-mode fiber (50, 62.5 micron). That’s the reason why single-mode transceiver can support link length up to 120 km. Figure 1 shows a compatible Cisco GLC-LX-SM-RGD and a single-mode patch cable.
 Single-mode SFP and SMF
In contrast, multi-mode fiber has a much bigger core and uses a longer wavelength of light, resulting in MMF having higher numerical apertures which means they are better at collecting light than single-mode fibers. In fact, this refers to the optics is cheaper. Due to the modal dispersion in the fiber, multi-mode fiber has higher pulse spreading rates than single mode fiber, limiting multi-mode fiber’s information transmission capacity. Thus multi-mode transceiver can only support shorter distance. Figure 2 shows a compatible Cisco GLC-SX-MMD and a multi-mode patch cable.
Multi-mode SFP and MMF
Compatibility Issue
Can I use single-mode fiber over multi-mode transceiver or vice versa? This is the question we have heard so many times from our customers. The answer is not that easy.
We all knew that single-mode fiber and multi-mode fiber are not compatible. The main reason is that they possess distinct wavelength of the laser and core size of the fiber. Take Cisco SFP as an example, there is a type of SFP transceiver module (GLC-LX-SM-RGD) which can support both single-mode and multi-mode fiber. But a mode conditioning patch cable is always required when using single-mode transceiver (like GLC-ZX-SMD) with 62.5-micron MMF, and only at distances that would have worked using multi-mode transceivers. It does not work when we are exceeding the distance specified for multi-mode fiber. The result could be an elevated bit error rate.
When we choose between single-mode transceiver and multi-mode transceiver, we must take all the above factors into consideration. Budget of course is the number one factors considered by many users. To save more, we are able to choose the compatible modules without sacrificing any quality or reliability but only with a low cost. Fiberstore supplies compatible fiber optic transceiver modules of many brands, such as Cisco, HP, Juniper, Brocade, Finisar etc. You are welcomed to contact us for more information.

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