Consultant Programme Article 09 / 2006

OCTOBER 2006

Specifying Optical fibre for high speed LANs To get the most cost effective solution for a

The horizontal axis is the distance limitation,

communications project the correct optical

e.g. up to 300 metres, 500 metres and 2000

fibre has to be selected that reflects the

metres. These links would then be known as

distance and speed requirements of the Local

OF300, OF500 or OF2000.

Area Network, LAN. The vertical axis gives the speed in megabits Since 2002 optical fibre selection has been

per second, and the scale goes from 10 Mb/s

made much easier by ISO/IEC and CENELEC

to 10 Gb/s. One then selects the appropriate

adopting a simplified nomenclature for optical

fibre according to the requirements.

fibre.

The next generation of American

example if we need to transmit ten gigabit

cabling standards, i.e. TIA/EIA 568-C, also

Ethernet up to 300 metres then an OM3 fibre

looks likely to adopt this convention.

is required. For a gigabit Ethernet link of 1800

For

metres, for example, singlemode, OS1, fibre The

naming

method

identifies

three

would be required.

multimode fibres, known as OM1, OM2 and OM3 and one single mode fibre, known as

Although the fibres go up in terms of speed

OS1.

It then comes down to a case of

capability their pricing doesn’t follow the same

selecting the correct fibre according to the

pattern and hence a total system cost-

speed and distance expectations from the

engineering approach needs to be taken.

following chart.

Distance, Metres

Speed, Mb/s

300

500

2000

10

OM1

OM1

OM1

100

OM1

OM1

OM1

1000

OM1

OM2

OS1

10 000

OM3

OS1

OS1

Table 1

©2006 Connectix Limited

Consultant Programme Article 09 / 2006

The best performing fibre is of course

OM3 fibre is the most expensive of the fibres

singlemode, which has a capability of sending

listed above. It is a multimode fibre developed

tens of gigabits of data over tens of

especially for the gigabit and ten gigabit LAN

kilometres.

market.

Surprisingly it is also the

However, despite being the most

cheapest. So why not use OS1 all the time

expensive fibre it works with the low-cost

then?

VCSEL laser, so the overall cost for ten-

The answer lies in the cost of the

gigabit links, up to 300 m, is the lowest for the

transmission equipment.

OM3/VCSEL combination. The core size of singlemode fibre is so small, around 9 microns, that only an expensive telecommunications

grade

laser

can

successfully launch light into it. This makes the transmission equipment very expensive. For links measured in tens of kilometres the cost of the transmission equipment takes a very small percentage of the total installation cost, however if the link is measured in tens or hundreds of metres then the cost of laserbased

equipment

would

start

to

look

prohibitive.

Although cheaper forms of singlemode lasers have been under development for many years they are still not with us yet and so for shorter distances we have the choice of the much lower priced Light Emitting Diodes, LEDs, and the special form of laser known as a VCSEL. VCSEL lasers tend to be used where gigabit speeds are required up to distances of hundreds of metres, but they still only launch into multimode fibre.

We thus have a combination of costs in a project and the cost of the fibre has to be added to the cost of the transmission equipment to get the real system cost.

©2006 Connectix Limited

Consultant Programme Article 09 / 2006

Fibre Types

Singlemode fibres also have a family of variants as is seen in the world of

The multimode fibres are usually expressed in

telecommunications. However for LAN use,

terms of core size relative to cladding size,

the Standards only define OS1, which is a

e.g. 50/125 or 62.5/125.

basic

The first number

and

general

purpose

telecommunications grade optical fibre.

denotes the core size in microns and the second number gives the overall cladding diameter.

Table 2 gives the full definitions of the family

As already stated, singlemode

would be called 9/125 in this terminology.

of optical fibres.

According to the Standards (ISO 11801:2002)

We can see that OM3 has two different

OM1 and OM2 can be either 50/125 or

definitions of bandwidth. This came about a

62.5/125, but the market seems to have

few years ago when people discovered that

decided that OM1 always means 62.5/125 and

the bandwidth of multimode fibre wasn’t

OM2 always means 50/125. This is not really

always the same when using LEDs or

the case and the Standards merely describe

VCSELs as the transmitting source.

them in terms of available bandwidth in MHz.km. OM3 is always 50/125 as only that

This gave many users a problem when they

smaller core size could offer the extra

tried to upgrade their exiting multimode plant

bandwidth required. OM3 is a very high quality

to try and use ten gigabit Ethernet and

fibre offering exceptionally high bandwidth and

imposed severe length restrictions upon

is engineered so that it is optimised to work at

them.

the same operating wavelength as the VCSEL,

minimum guaranteed performances when

i.e. 850 nanometres.

both methods are used.

OM3 fibre is engineered to give

Minimum modal bandwidth MHz.km

Fibre type

Core diameter, microns

Overfilled launch bandwidth Effective modal bandwidth 850 nm

1300 nm

850 nm

OM1

50 or 62.5

200

500

Not Specified

OM2

50 or 62.5

500

500

Not Specified

OM3

50

1500

500

2000

OS1

singlemode

N/s

N/s

Not Specified

Table 2

©2006 Connectix Limited

Consultant Programme Article 09 / 2006

New projects The new naming system means that it is easy to define a n optical link in terms of its distance versus speed capability. Defining a link as, for example, OM3/OF300 completely defines the capability of that link in terms of its bandwidth and attenuation, and means that anybody can simply read off a list as to which protocols will always be guaranteed to work Modal bandwidth in effect means the

over such a link.

available bandwidth when using a laser source and ‘overfilled’ means the bandwidth

New projects that require ten gigabit speeds

when using an LED source. The older OM1

up to 300 m should specify OM3 optical fibre.

and

Links up to 500 m should specify Connectix

OM2

fibres

have

no

guaranteed

bandwidth when using a laser source.

premium grade fibre and links over 500 m need to specify singlemode, OS1.

Like all optical fibres we also have to remember that the performance of optical

Coping with old fibre

fibres changes according to the wavelength of light used.

Hence fibre performances are

Older multimode fibre might not even make it

always quoted at one or more of the three

to the OM1 and OM2 starting grid.

operating ‘windows’, 850 nm, 1300 nm and

example old ‘FDDI’ grade 62.5/125 fibre is

1550 nm.

limited in transmission distance to under thirty

For

metres when trying to upgrade to 10GBASE-

Premium grade fibres

SR ten gigabit Ethernet.

Although OM3 is a very high quality fibre,

Two technologies are available to rescue

manufacturers such as Connectix can offer an

some of the intrinsic bandwidth of old fibre.

even better version which takes the 10-gigabit

The first is known as coarse wavelength

transmission distance from 300 m up to 500

division multiplexing, or CWDM. This is also

m. This would certainly be very cost effective

referred to as 10GBASE-LX4. This means

for sites with distances up to 500 m as it

splitting the ten-gigabit data stream into four

moves the point at which singlemode fibre

streams of 2.5 Gb/s and then sending them

would have to be utilised.

over

one

fibre

but

at

four

separate

©2006 Connectix Limited

Consultant Programme Article 09 / 2006

Conclusion Using the OMx/OFy naming convention makes designing, specifying, procuring and testing optical fibre LAN links simple and unambiguous. LAN capabilities in terms of speed and distance are guaranteed.

New projects that require ten gigabit speeds up to 300 m should specify OM3 optical fibre. Links up to 500 m should specify Connectix premium grade fibre and links over 500 m wavelengths or colours of light. This will drag

need to specify singlemode, OS1.

the distance capability back up to 300 metres for most fibres but at a considerable cost as now instead of one cheap VCSEL laser it will now need four telecom grade lasers, four receiving circuits plus the multiplexing and demultiplexing circuitry.

It is possible to drag old fibres into the ten gigabit arena using technologies such as CWDM and 10GBASE-LRM, but at cost usually significantly higher than installing new optical fibre, and would only be considered where the costs of recabling are unusually

Another technique, known as 10GBASE-

excessive.

LRM, is to use a single telecommunications grade laser with a special offset patchcord. This means that the two fibres are deliberately misaligned by a few tens of microns which prevents the small laser beam being aimed straight down the potentially troubling centre point in the core of the old fibre. It is claimed to work up to 220 metres on any fibre but once again at the expense of a sophisticated laser and receiver circuit and the management overhead of enforcing the computer room discipline to always use the correct type of patchcord.

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©2006 Connectix Limited