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Important Consideration for the FTTH Design

Important Consideration for the FTTH Design: The purpose of writing this article is to introduce and explain the design method of excavation small Trenches to be implemented in FTTH Network. Small trenching is designed for placing ducts for pulling fiber optic in preparation for application & future use with new technologies & FTTH installations.

Ducts are designed to ultimate capacity of forecasted cables plus spare ducts for unseen future demand and possibility of renting to other parties. Take into account that the course design is to serve any GSM tower located at a distance of 300 meters in the design of the route of new ducts.

Important Consideration for the FTTH Design network
FTTH Design Tips

What should be the important consideration for the FTTH design

There are some important ideas for FTTH Design which play a key role in creating FTTH Network. Infrastructures and some materials like- Mini/small trenches, Manhole (MM), Hand-hole (HH), types of ducts, FDT, FAT, FTB, feeder, distribution and drop to the customer end together create a FTTH network, each component has its own importance in build up the network.

Important point should be considered, prior to the actual design reference must be made to the specifications of the municipality to ascertain their conformity of the proposed design.

Duct preparation and laying is also a very important part in the FTTH design.The ducts are to be laid in bends not less the 2 meters in diameter.The ducts shall be arranged on top of other by suitable spacers that will keep the ducts in place during backfilling and pouring cemented materials. Spacers shall be placed every two meters.

Cover head over the ducts in small trench shall be (270 – 300) mm from top surface – Asphalt or ground level (GL) in un-asphalted areas.The level of the bottom of the small trench shall be higher than the level of bottom of HH / MH by (125 – 300) mm.

Trenching in FTTH Feeder Routes

In all feeder routes for FOC, use mini manholes or 2-cover HH in all civil works & trenching; as necessary & needed and as per designer estimation and best choice. The span distance between manholes in the primary/ feeder routes shall not exceed 300 meters.

The trench in the primary/ feeder routes shall be 200 (-0/+2) mm wide & 500 mm deep from ground level (GL), 600 mm from asphalt (Parameters can varies according to the requirement of telecom industry).

The ducts in the primary/ feeder routes in streets wider than 30 meters shall be 20 ducts 32 mm in diameter + 16x20 mm mini ducts. The same applies for Long distance (LD) and junction routes; or fiber optical cables on the highways or major roads inside cities (Telecom industry follow different parameters in Asia regions).

For streets less than 30 meters, the ducts arrangement shall be 16 ducts 32 mm in diameter + 16x20 mm mini ducts. Mostly telecom industries uses the three color of ducts.The main ducts shall be of three colors: Black, Orange and Orange with Black Stripes or blue for easy identification differentiation during application & use.

When design ducts for fiber optic cables on the highways outside the cities (Long Distance) direct buried, use small MHs with 1500-3000 meters span in standard trench 300 wide and 1000 mm depth.
Trench and duct details in FTTH network design
Trench & duct details in FTTH Design

FTTH Distribution / Secondary Routes

In the secondary/ Distribution routes use one-cover & tow-cover hand-holes with small trenching. Two-cover HHs are used in front of the cabinets; and in locations involving cable jointing. The one-cover HH is used in front of DPs, FATs & in commercial and key account customer buildings.

The trench in the secondary/ distribution routes shall be (200 -0/ +2) mm wide; and 600 mm deep from asphalt / 500 mm from ground level (GL). For drop to the customer the trench shall be (130 -0/+2) mm wide & 500 mm from the asphalt / 400 mm from GL.

Ducts in secondary & distribution routes shall be 12 ducts 32 mm in diameter + 23x 20 mm mini ducts between the (FDT) & (FAT); and 25x20 mm mini ducts between the FAT) & the client. The ducts arrangements in the trench shall be as shown in the below figure.
Placing & Arranging Ducts In Distribution Network
FTTH distribution network details

Ducts for Drops

Trenching for drop cables shall be 130 mm wide & 400 mm deep from ground level (500 mm from asphalt). The ducts for drop cables shall be 20 mm HDPE ducts; will be arranged in the trench as shown in the below figure. The number of ducts can be 2 and the length should not be greater than 300 meters.

The duct location shall be in the nearest boundary line outside the customer's building.
Arranging Ducts in small trenches for Drop Cables
Drop duct for the customer

Ducts of Drops for Blown Fiber

When using blown fiber for the drops use two twin ducts or one 50 mm diameter duct with 4 subducts as shown in the below figure. The width of the trench shall be 130 mm, and the depth shall be 500 mm from the asphalt level (400 mm from GL).
Ducts Arrangement for Blown Fiber Drops in FTTH network
FTTH Blown fiber drop arrangement

Back-filling, Asphalting & Surface Reinstatement

The back-filling shall provide mechanical force similar as much as possible to the soil adjacent to the trench, and meet the following requirements-
Stability of filling material and fill the whole trench.
Adhesion to the trench wall & no decline in the trench.
Coherence of materials so that asphalting possible within 24 hours.
Capable of carrying traffic load. Easy to remove.
Providing mechanical protection for cable and ducts.
A layer of sand is placed at the bottom of the trench before placing the ducts; and a layer on top of the ducts before pouring mix concrete.
After completing trench excavation all remnants shall be removed.
Steel Warning Tapes shall be placed & embedded in the backfilling (200 -300) mm below the top level.
Before asphalting, a width of old asphalt may be milled on both sides of trench then filled with asphalt and joined to old asphalt as usually done.

In the very short, I want to explain the few points FTTH related information here.

Mini/small trenched: The trench with maximum dimensions 13-20 cm width; and depth of 50/60 cm.

Manhole: It is the collect &distribution point for cable duct routes, it is the safeties location for pulling cables, splicing & maintenance for OSP (copper & FOC).

Hand-hole: A hand-hole (HH) is a small enclosure underground used for pulling, and sometimes jointing, and distributing of cables (copper & FOC). There are two types of hand-holes: 2-cover & 1-cover handěžole.

Fiber Distribution Terminal (FDT): A hub where the fiber from central office (exchange) is split and distributed to the end users.

Fiber Access Terminal (FAT): A distribution point that distributes fibers to each customer.

Fiber Termination Box (FTB): Customer Lead-In Unit, installed at the customer outer wall for termination of incoming & indoor fiber.

Feeder: We can say that it is equivalent to the primary cables in the copper network. The optical fiber cables that connect the exchange to the FDT or a splice point in manhole that serve as a cross connect to Distribution network in FTTH.

Distribution: We can say that it is equivalent to the secondary cables in copper network. It is the cable between the FDT & FAT.

Drops: The fiber from the last distribution point (FAT) up to customer’s location.

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