Let's say you write an e-mail message and then send it to your colleague to inform them about an urgent meeting. Within moments it's in the recipient's inbox. The entire process might look very simple but the story inside is not.

Every time we use internetbased services we in fact send and receive data. Internet is called network of networks and data travels through the network in a discrete form called packet. In order to ensure an uninterrupted communication link, satellites and undersea cables (optical fibre) are used to connect geographically separated places.

Nowadays optical fibre has won popularity for long haul communications. Telecommunications sector has adopted this technology for its huge bandwidth. It has also dramatically reduced international call charge.

The main drawback of satellite communication is its expensive bandwidth. Moreover, people experience propagation delay during voice communication. Despite these hitches, satellite is still now the only solution to establish communication in remote places.

This article reveals the inside story of optical fibre technology and gives a concise idea for its implementation in Bangladesh as well. Optical fibre is wafer-thin, human hair like entity that is used to transmit light signals over long distances. In general, these are arranged in bundles called optical cables. When these cables are laid across the seabed they are called submarine cables.

Optical fibre consists of three main parts: core, cladding and buffer coating. Core is the centre part of the entire set-up; fibres reside in this area. Cladding is an optical material that circumvents the core and enables light signals to reflect back into the core.

Buffer coating is the plastic coating that protects the fibre from damage and moisture. Hundreds or thousands of these optical fibres are arranged in bundles in optical cables. The bundles are protected by the cable's outer most protective shield called jacket.

Depending on the mode of light, optical fibres come in two

types: single-mode fibre and multi-mode fibre. Single-mode fibre uses laser (Light Amplification by Stimulated Emission of Radiation) as a source of light and it has very small core, near about 3.5 X Winches or 9 microns in diameter. Multi-mode fibre has relatively large core near about 2.5X 103inches or62.5 microns in diameter and transmits infrared light from LED (Light Emitting Diode).

In a fibre optic cable when light signals travel through the core they follow zigzag paths. Due to

the principle of total internal reflection, light signals are not able to come out from the core. Naturally, light signals travel long distances without significant changes in signal quality.

But in many cases signal quality depends on the purity of the glass and the wavelength of the transmitted light (for example, 850 nm = 60 to 75 percent/km; 1,300 nm = 50 to 60 percent/km; 1,550 nm is greater than 50 percent/km). Some premium optical fibres show much less signal degradation -- less than 10 per-

cent/km at1,550nm.

A fibre optic relay system consists of four components such as transmitter, optical regenerator and optical receiver. Transmitter originates and encodes the light signal and focuses it on the optical fibre through a lens. Optical regenerator is essential to regain the signal strength. When light signals travel through long distances, optical regenerator is installed along the cable to boost the signal strength. Optical receiver uses a photocell or photodiode to detect the light and

then convert it into electrical signal for general purposes.

Bangladesh has finally been connected with the long-awaited global fibre optic network through SEA-ME-WE-4 (South East Asia-Middle East-Western Europe 4) submarine cable. SEA-ME-WE-4 is the fourth project in the SEA-ME-WE series. On March 27, 2004, a consortium of sixteen international telecoms companies singed construction and maintenance agreements for the new optical fibre submarine cable system linking South East Asia to Europe via the Indian subcontinent and Middle East with terminal stations in Singapore, Malaysia, Thailand, Bangladesh, India, Sri Lanka, Pakistan, United Arab Emirates, Saudi Arabia, Egypt, Italy, Tunisia, Algeria and France.

SEA-ME-WE-4 system is based on hi-tech DWDM (Dense Wavelength Division Multiplexing) technology to achieve ultra high-speed (terabit per second) connectivity. The total length of this cable is approximately 20,000km and it is capable of supporting telephone, internet, multimedia and various broadband data applications.

In order to spread the benefits of SEA-ME-WE-4 submarine cable, we need a strong optical fibre network in the country. Metronet is playing a significant role to that end by creating fibre optic network in the country. Currently, Metronet services are available in Dhaka metropolitan area, Narayanganj, Gazipur, Ashulia and Savar EPZ.

Metronet provides services which include point-to-point optical fibre connectivity, point-to-multipoint optical fibre connectivity, real time DVD quality video conferencing, real time office/factory video surveillance system, telemedicine and others. In a friendly conversation with StarTech, Almas Kabir, COO, Metronet informed that they are planning to extend their services into Chittagong. He, however, expressed his concern about 15 per cent VAT and said, "the government can take proper steps to bring down this amount in order to facilitate cheap telecoms services to general people".