We want to use this article to understand better the differences and consequently, typical applications between Loose Tube (LT) & Tight Buffered (TB). The general perception in the market place is LT for external and TB for Internal, there are many exceptions to this, but it is a reasonable indicator. The fibre type for like constructions is the same; it is about the cable construction.
Fundamental Differences between Loose Tube & Tight Buffered
Loose Tube – LT
Extruded semi ridged tube dry or gel-filled either single or multiple fibres laid up helically in the tube. Care is taken to manage over-length of fibre for a given length of tube. Multiple tubes can then be laid up to make a core normally SZ stranded around a Central strength Member (CSM). Typical construction would be 12 Fibre per tube, 12 tubes SZ stranded around CSM water blocking on the core with an outer sheath – 144 Fibre Cable.
Tight Buffered – TB
We take a standard 250u fibre as used in LT construction, but we then pass through an extruder and add an additional buffer taking the diameter up from 250u to 900u. This additional coating provides protection. These buffered fibres can also be used in interconnect cables simplex Duplex etc., or they can be stranded up to make cables or sub-units then stranded to make higher fibre count cables. TB cables are generally 1-24 fibre, you can get 96/144, but this is an exception.
A range of ITU (International Telecommunications Union) approved fibres can be used to manufacture either LT or TB cable construction. Fig1 below gives a simple perspective on this:
Application LT or TB
These are distinctly two different constructions with given features and benefits for particular applications, and these can be broadly defined as External & Internal cables.
Loose Tube External
External application WAN, therefore long haul, Trunk, Metro could be duct or direct buried dependent on cable construction. Typically SMF applications, uni-tube or multi-tube high fibre count constructions 12- 864 and beyond. Ribbon technology, another form of loose tube would be used in high fibre rich cable. Tubes could be gel-filled or dry water blocking.
Rugged constructions for harsh environments
Fibre is well protected within the tube
Excellent Mechanical properties
Excellent temperature properties
More tolerant of the stresses and strains associated with installation
Basic construction can be further enhanced with SWA or CST armouring
Low cost of construction.
Extended cable lengths where needed
Tight Buffered Internal
Internal application LAN, therefore short runs normally multi-mode, there are exceptions where single-mode would be used, typically where high speed/data links are needed. The tight-buffered 900u is also used extensively in Pigtails and Patch Cords.
Flexible – easy to install in a tight area with multiple bends
Easy to terminate splice & connectors
Added strength re Kevlar on simplex duplex constructions.
Better for direct termination with connectors
LSZH polymer for the sheath
Rodent protection with glass yarns
Good mechanical attributes
Good temperature performance
This article now gives us a perspective on the 2 basic constructions LT & TB; both are needed within the industry; they each have specific and unique attributes. It is imperative to understand the application so that the appropriate cable construction can be selected.