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For many years multimode (62.5/125 fibre optic cabling has been sold into LAN applications as a high bandwidth, future-proof computer network solution. The advent of gigabit Local Area Networks such as ATM, Gigabit Ethernet and Fibre Channel has exposed the distance and bandwidth limitations of 62.5 micron fibre. Users that have brought "data-grade" optical fibre because of its lower price could have a particularly difficult time getting high-speed backbone links to work.

On 25th June 1998, the IEEE approved the Gigabit Ethernet standard, fibre optic section, known as IEEE 802.3z. This will stimulate an explosion in the growth of gigabit backbone links. This growth is inevitable as more ad more users employ 100 Mb/s Fast Ethernet to the desk, giving an aggregate backbone load ten times larger than currently experienced. Anyone installing Fast Ethernet to the desk but leaving 100Mb/s Ethernet or FDDI in the backbone will get no more than 10Mb/s useful throughput at the desk.

The Gigabit LAN Contenders

Gigabit Ethernet

The IEEE 802.3 committee borrowed heavily from Fibre Channel technology to come up with the family of 1000 Base X proposals. IEEE 802.3z covers 1000BaseLX and 100BaseSX (Long and Short wavelength operation over fibre), plus 1000Base CX for short inter-equipment copper links. IEEE 802.3ab covers 1000BaseT, the Gigabit Ethernet standard for 4-pair copper cable.

ATM

Asynchronous Transfer Mode is a method of transport more familiar to the world of telecommunications rather than data communications and as such has been presented as much more suitable for delay sensitive traffic such as real time video and data. The ATM Forum has approved three optical interfaces, 51.84, 155.52 and 622.08 Mb/s. ATM technology lends itself to speed increases of 12, 2.4 Gb/s and beyond.

Application Data Rate Multimode Singlemode
62.5/125 50/125 Plastic
850nm 1300nm 850nm 1300nm 650nm 1300nm
 Gigabit Ethernet    220m  550m  500m  550m  -  5000m
 ATM  50Mb/s  2000m  2000m  2000m  2000m  50m  -
 155Mb/s  1000m  2000m  1000m  2000m  50m  -
 622Mb/s  300m  500m  300m  500m  -  5000m
 Fibre Channel  1.062Gb/s  175m  -  500m  -  -  10000m
 2.125Gb/s  -  -  300m  -  -  2000m
 4.25Gb/s  -  -  100m  -  -  2000m

Optical Fibre Choices

Glass

Optical fibres are comprised of a core and cladding of differing refractive indices. A core of high refractive index is surrounded by a cladding layer of lower refractive index. This difference forms a boundary, which constrains most of the light within the core by the phenomena of total internal reflection.
In general there are two types of optical fibre, Singlemode and Multimode.

Singlemode Fibre Optic

Singlemode Fibre Optic typically has a core diameter of approximately 8um. Above its cut off wavelength, a single mode is transmitted down the fibre. This approach effectively eliminated intermodal dispersion, but with 'bandwidth' is none the less limited by second-order effects such as intramodal dispersion. The combination of huge bandwidth and low attenuation makes singlemode fibre the preferred option for telecommunications systems world-wide. However, singlemode fibres require lasers, producing low numerical aperture light, in order to achieve an effective launch into the fibre. It is the high cost of these devices that has, until now, limited the use of singlemode fibre within LAN's.

Multimode Fibre Optic

Multimode fibres on the other hand, have much larger core diameters, typically 50 or 62.5 um. This effectively permits many modes to be transmitted along different paths down the fibre. Modern graded index multimode fibres have a complex optical core manufactured so that the refractive index varies in a controlled manner, from a high central axis to a lower refractive index material at the outside of the core. Careful design of this profile optimizes the transmission characteristics of the fibre. As a result, it is the most commonly used fibre in LANs and premises cabling because the larger core diameter makes if simple to terminate and ideally suited to LED (light Emitting Diodes) sources with their high numerical aperture launch conditions.

Plastic Fibre Optic

Plastic fibre has long held the promise of very low cost and easy termination. To date, however, nobody has been able to demonstrate a plastic fibre, at an affordable price, with the distance and bandwidth performance of Category 5 copper cable, let alone any silica glass fibre.

Cost and Performance Trade-off's

There are three operational wavelengths, long established as the basis for fibre optic data transmission:

850nm

The dominant operating (short) wavelength for most data transmission systems.

1300nm (long wavelength)

Used for higher speed multimode data communications requirements (such as FDDI) and telecoms (with singlemode fibre).

1550nm

Very low attenuation, hence used for telecommunications.

Bandwidth

Singlemode fibre optic cabling offers the greatest bandwidth. The additional complication of intermodal dispersion limits multimode bandwidth, being progressively more of an issue with increasing core diameter.

Cost

Without the need to manufacture a graded index profile and helped by the economies of scale of the telecommunications market, singlemode fibre is significantly cheaper to manufacture. As far as multimode fibre is concerned, 50/125 is a lower cost solution than 62.5/125.

Attenuation

Singlemode fibre offers significantly lower attenuation, making it the preferred choice for long haul telecommunications. Multimode fibre, on the other hand is designed with short haul data communication in mind where attenuation is not generally a limiting factor.

Coupled Power

Highlights the huge advantage of multimode fibre where it is far easier to achieve an efficient launch of light into the fibre.

In essence, short haul networks with a high degree of interconnectivity, demands the use of larger core multimode fibre where attenuation and bandwidth limitations can be tolerated.

Limitations on Bandwidth in Multimode Fibre

The bandwidth of fibre is limited by dispersion. . That is, pulse spreading of the digital signal as it travels down the fibre. Dispersion in fibre comes form several different factors:

Modal Dispersion

Because the modal rays of light travel different path lengths down the fibre some will reach the receiver before others. This effect is negated to some extent by using graded index cores in multimode fibres. The refractive index at the centre of the core is higher. The higher the refractive index, the slower to lights travels. This means that the rays of light travelling the shortest distance (i.e. down the centre) travel slowest, and the ones taking the longest route travel the fastest.

Chromatic Dispersion

The speed of light in the glass depends upon the refractive index which is also dependant upon the wavelength of the light. A laser, and especially and LED, even though their output is centred upon a particular wavelength, still transmit a spectrum of light. The pulse spreads out over the length of fibre because all the different chromatic elements within the original pulse of light are travelling at different speeds.

The advent of using lasers over multimode fibre has exposed another form of dispersion:

Differential Mode Dispersion (DMD)

This effect is most pronounces when driving lasers into 62.5/125 fibre, where the size of the cone of light entering the fibre is smaller than the core. It is caused by small variations in the refractive index profile of the core causing a differential delay depending upon which part of the core the light is travelling. Larger sources such as LEDs and VCSELs 'overfill' the core with light and tend to cancel the DMD effect out.

An additional complication, when using the 1300mm operating window on multimode to try to get longer distance, is the need to use a special offset launch connector to overcome the Differential Mode delay problem.

Latest Standards

In recognition of the problems likely to be encountered when using gigabit transmission speeds over the existing base of installed multimode fibre, the latest draft of the Gigabit Ethernet specification allows for separate bandwidth cells.

Latest Standards

Fibre Measurement 850nm 1300nm
 62.5/125  Bandwidth  160MHz/km  500MHz/km
 Distance  220m  550m
 62.5/125  Bandwidth  200MHz/km  500MHz/km
 Distance  275m  550m
 50/125  Bandwidth  400MHz/km  400MHz/km
 Distance  500m  500m
 50/125  Bandwidth  500MHz/km  500MHz/km
 Distance  550m  550m
 Singlemode  Distance  5500m


According to published figures, 82% of installed 62.5/125 multimode fibre has a bandwidth of 160MHz.km at 850nm. The 220m Gigabit Ethernet length limits this implies may be a severe restriction on large sites such as airports, universities and large industrial plants.

The following shows the distribution of optical fibre run lengths in UK building backbones. This highlights that 23% of all backbones are over 200m indicating that a significant proportion of the installed base may experience problems when trying to support Gigabit Ethernet. It is, of course, reasonable to assume that a larger proportion of campus or inter-building links will be longer and again this existing infrastructure may be unsuitable if low bandwidth 62.5/125 fibre has been installed.

UK Building Backbone Fibre Optic Survey

Graph 1: Survey Results

Although the standards imply 220m length limit for the 62.5/126 fibre optics, the performance of MillenniuM Brand-Rex product under test using VCSEL (the latest generation of low cost lasers) in our MillenniuM Applications Research Centre, and shown below, has exhibited typically superior performance.

Graph 2: Transmission Speed / Length

The 50/125 Issue

The price/performance characteristics of 50/125 fibre have now become more significant. It clearly makes sense to consider 50/125 fibre, which will give:
  • Support for centralised fibre optic cabling requiring longer link lengths and higher bandwidth


  • Higher bandwidth, particularly at 850nm


  • Compatibility with all 62.5/125 transmission equipment


  • Lower price


  • Short Term Bandwidth 'Fixes'


There are two methods of getting more bandwidth out of existing optical cable plant:

Wavelength Division Multiplexing

This is an optical filter that allow different wavelengths to be used simultaneously on the same fibre, e.g. 850 and 1300nm. This does nothing to solve the distance issue at gigabit speeds as the inherent bandwidth/distance limitation is unchanged. The cost of the devices is also significant compared to the price if new cable.

Bandwidth Enhancing Devices

A special kind of patch cord can strip out some of the higher order optical modes. This increases the attenuation and the bandwidth, up to a point. These units had to be 'engineered' to fit and then each fibre retested for bandwidth to see if it worked.

Obviously anyone with significant existing cable plant is interested in maximising its potential, but, for any new fibre requirements it makes sense to buy the appropriate grade fibre for all future requirements.

Delivering the Fibre Optic to the Point of Use

The fibre has to be protected in a suitable construction according to the environments it will be used in. Future-proofing can be achieved in two ways:

Composite Cable

One approach to the increasing requirement to satisfy future bandwidth demand is to consider a composite cable utilising a mixture of fibre for today's needs together with spare fibre for tomorrow's needs. There has been a steadily increasing demand for these cables deployed in the backbone with combinations of multimode and single mode fibres giving excellent future expandability at low cost.

Blown Fibre

For the lowest cost and greatest flexibility blow the fibre into pre-installed blown fibre ducts using Brand-Rex Blolite system. Blolite consists of low price empty tubes installed around the site, within or in-between buildings. Future decisions and expenditure, on fibre type, can therefore be deferred. The fibre is simply blown in when required. Additionally, old fibre can be blown out and the ducts used again for the installation of new upgraded optical fibre.

Conclusion

The Gigabit Ethernet standard for fibre optics from IEEE 802.3z has now been approved. This puts more emphasis on the performance of fibre optics than any other preceding standard.

Multimode fibres still offer the best combination of price and performance when total system costs are taken into consideration. However, the benefits of the 50/125 product offers significant performance advantages over the current 62.5/125 LAN standard fibre.

There is a wide range in performance of optical fibres available on the market today ranging from the low cost, low bandwidth so-called 'data grade' fibre up to the high quality, well proven product such as the MillenniuM brand. There is a significant risk that lower grade products will not be capable of providing the performance envelope demanded by the latest standard. Extensive testing in the Brand-Rex MillenniuM Application research Centre, however, has proven the value of quality fibre deployed in the backbone.

Brand-Rex offers a comprehensive range of fibre optic cabling and fibre types for LAN's. Thorough standards participation, a thorough understanding of the new and emerging technologies and our customers requirement, Brand-Rex can provide product meeting all existing and emerging requirements.


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