CCNA/CCNP preparation notes

by Andreas Ryf

Main distinction:

• Multimode fibres (MM)
• Singlemode fibres (SM)

Singlemode fibres are sometimes also called "Monomode", but this can lead to confusion when using a short form "MM". Therefore the term Singlemode is preferable.

Singlemode fibres

• Expensive
• perform much better than Multimode
• used for long distances (WAN) & DWDM
• Transmitters & receivers much more expensive than for Multimode

Multimode fibres

• considerably cheaper than Singlemode
• mainly used for LAN applications, sometimes in the past MAN
• Transmitters and receivers considerably cheaper than for Singlemode
• Can carry 10GBit-Ethernet and some CWDM-applications in LAN range (few kilometers).

Usual wavelengths used for laser diodes are

• 850 nm
• 1300 nm
• 1550 nm

These are the so-called optical windows, where attenuation is lower and/or where prices for laser diodes are cheaper.

Modal Bandwidth

Like a quality indication for MM fibres, the higher the Modal Bandwidth, the further or faster data can be transmitted over the fibre.

Common modal bandwidth for MM fibres range from 160 to 2000 MHz*km.

A doubeling of the modal bandwidth means that a given transmitter can transmit over the double fibre length.

Core diameters

Common for MM fibres are 62,5 and 50 microns.

Single mode fibres usually have a core diameter of 9 microns.

62.5 micron MM fibres are more widespread, in the past they were preferred since they could also be used with LEDs instead of Laser diodes.

Nowadays 50 microns are becoming more popular, as better modal bandwidths can be achieved.

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