Bend Insensitive fibre also known as less bend sensitive fibre

With the rapid deployment of FTTx solutions, the need for Bend insensitive fibre is rising.  Sharp corners and bends make it impossible for standard 652D fibre to be used because any 90 degree bend can cut off the light. The G.657 fibre is special in the sense that during a bending the most of the stray light goes back into the fibre core making it possible for it to be deployed in tight spaces with small bend radius.

The variants in the bend insensitive fibre are the following:

G.657.A.1/2/3 are backward compatible with G.652.D and support Multiplexed transmission in the 1310-1625nm wavelength region and are available with bend radii of 10mm, 7.5mm and 5mm respectively.

G.657.B.1/2/3 are not designed to be backward compatible and have a specified attenuation at 1310 nm, 1550 nm and 1625 nm, but not at intermediate wavelengths. They are available in 7.5mm and 5mm radii versions.

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The Cabling Conundrum – Cable tagging

Very often we come across installations where OFC cabling is completed but not properly tagged. This creates problems after splicing with either wrong cables in the right rack/panel or the right cable in the wrong rack/panel.  How do we get over this problem –

Here are a few tips:

a. Using Labels:

Adopt a simple cable tagging code from which you can figure out the origin of the cable say  three 6F Single mode cables laid between the Main Control Room(MCR) and Remote Panel/s -1, 2, 3(RP1, 2, 3).

At the MCR, just tag  the cable RP1_SM_6F, RP2_SM_6F & RP3_SM-6F

At the Remote Panel/s just tag the cable MCR_SM_6F_1, MCR_SM_6F_2 & MCR_SM_6F_3 respectively at the three remote panels

b. Using colours:

In the above example assign colours to each of the Remote Panels say Red, Yellow, Blue for RP1, RP2 & RP3. Tape the cable ends Red, Yellow & Blue using colour insulation/duct tape.

Just deciding on any of the above tagging methods based on the number of cables to ensure easy identification will save lots of time & effort in resolving communication issues without having to reroute cables and redo splicing.

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Serial communication on optical fibre cables

There are a number of devices particularly in Plant automation and SCADA that communicate on serial protocols like RS-485/RS-422/RS-232 etc. Typically extending these serial communication signals on shielded cables has distance limitations apart from susceptibility to electromagnetic interference(EMI). To get over this limitation, we can incorporate Serial to Fibre optic converters that accept RS485/422/232 signals and convert them to optical signals for transmission over 2 Kms in the case of Multimode and 20 Kms in the case of Singlemode. The converters are transparent to the software protocols such as MODBUS-RTU, MODBUS-ASCI etc.  Here is a picture of a typical serial to optical converter:8247-Serial-to-FO-Converter

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Fibre optic Daisy chain, Ring & Star topology

There are multiple ways of extending fibre optic links that fall under the topology daisy chain, ring and star. Let’s just look at each of them briefly taking a simple 3-point network having location #A, B & C:

1.Daisy Chain:  # Location A – # Location B – # Location C . The optical signals hop from A to B to C. If there is a cable cut  between A & B the whole link goes down. If there is a cable cut between B & C then  C is cut off from the network.

2. Ring: # Location A – # Location B – # Location C – # Location A . The optical signals hop from A to B to C and then return back to A. If there is a cable cut  between A & B , then the communication happens through the path A -C-B. If there is a cable cut between B & C, the communication happens through the path A-B & A-C. So you can see that this is a better option than the daisy chain because for a single break in the link, the communication is not interrupted. However, this requires extra cable ( for the return path) and also managed switches/devices to detect break and switch communication to the available alternative path.  Most preferred for critical communication and increasingly being adopted in CCTV surveillance networks communicating on optical fibre backbone.

3. Star: Location #A – Location # B ; Location # A – Location #C. Pretty straightforward..we now have two independent links from Location #A. If there is a cable cut between A & B, then only that link goes down, communication between A & C is not affected and vice versa.

Based on the number of points to be connected and the critical nature of the application, we need to make a technically and commercially feasible choice by picking one or a combo of all the three types.

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Pigtails, Adapters & Patch cords

typical_connection_liu_pt_ad_pc_sw

Pigtails are short pieces of 0.9mm buffered fibre connectorised at one end and the other end free to be spliced with the optical fibre cable. The No. of pigtails is generally equal to no of fibres in the cable unless some fibres in the cable are left unspliced. It is a good practice to splice all the fibres with pigtails and have all fibres ready for use.You will find the pigtails inside the LIU as you can see from the pictures in the attached file.

Adapters aka Mating sleeves aka couplers are mating points of the pigtail and patchcord. The pigtail is plugged into the adapter at the end inside the LIU and the patchcord is plugged into the end outside the LIU.

Patch cords are flexible fibre optic cable assemblies that facilitate connection between the LIU and the active equipment viz. Ethernet switches and media converters. Patch cords can be simplex or duplex. Simplex means one fibre and duplex means two fibres. Generally fibre optic communication is full duplex meaning a dedicated fibre for the transmit signal and a dedicated fibre for the the receive signal. Accordingly the patch cords are more often duplex than simplex particularly for data communications.

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Splice closures

Splice closures (aka Joint closures) are weatherproof enclosures used to splice 2 or more optical fibre cables. They come in either rectangular or cylindrical construction. The splice closures have  2 -4 cable entry ports to bring the the cables in, splice trays for joining the individual fibres of the cables to each other and properly routing the same. Upon splicing the cable entry ports are sealed with heat shrinkable sleeves. There is also a mechanical design which enables re-opening of the closure if required. The closures are made out of light weight plastic. Once properly sealed the closure is water proof and air tight. Hardware used in the closure is rust proof. The closure generally conforms to IP-68 and suitable for underground direct burial, aerial or duct burial applications.

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Fibre ready Surveillance Cameras

Guys, want to share something really interesting that I came across…IP camera with a direct fibre interface. This one has an on board SM SFP  and can talk to a switch directly over fibre .

That really simplifies the connectivity by doing away with having to use media converter to establish  a copper connection besides getting over bandwidth and distance issues. Also no EMI issues either…way to go isn’t it??

Although these cameras cost a bomb today the prices will come down to reasonable levels as the benefits become evident.

Singlemode or Multimode ?

Very often we come across this question ..don’t we ? How does one make a choice especially when segment distances are under 2000m ?  Well, the choice will become clear after asking just a couple of questions:

  1. What is the total optical fibre cable(OFC) length that you are going to be using in your fibre optic network ?
  2. How many fibre optic transceivers are there in your network ?

Once you have answered the two questions above we get down to some simple number crunching:

(Total Cable length x Rate per meter of multimode cable) + (Total no. of fibre transceivers x Rate per multimode transceiver)  say X

(Total Cable length x Rate per meter of singlemode cable) + (Total no. of fibre transceivers x Rate per singlemode transceiver)  say Y

Check whether X < Y or Y< X …you have your answer there !

At data rates 100Mb/s and below with segment lengths less than 2Km, you are likely to make the right choice.

Of course the fiber of choice is singlemode if you have long segment lengths  >2 Km with total requirement of cable running into tens of kilometers and high bandwidth requirement.

Is Multimode going to be around at all is going to be my next question ? Wanna think about it ? Please do because that’s going to be subject of my next blog…happy reading folks and have a great year ahead !!

HNS

Multimode fiber – Is it going to be around ?

Let me the outset clarify that I do not hate Multimode fibre but lets accept the fact that it is really no match for singlemode fibre when it comes to distance and bandwidth.

We continue to come up with newer standards for Multimode fiber..OM4 now right..10G over 1 Km when OM3 was all about 1G over 1Km.. come on guys we need to wake up and smell the coffee unless we have companies out there having a vested interest in keeping Multimode fiber and transceivers on the ventilator.

Singlemode cable and transceiver prices have been moving down steadily over the last 4 – 5 years because of exponential rise in volumes and today we have a situation where singlemode fast ethernet media converters are cheaper than multimode.

Wouldn’t it make sense to have all our campus networking on a Singlemode fibre backbone ? It makes sense to me..how about you ? With wavelength division multiplexing you have the option of moving huge amounts of data on the same singlemode fibre at different wavelengths..Hows’s that for economy and future proofing your network ?

HNS