Optical networking: An overview

BANGALORE, INDIA: Optical networks, which been the transport backbones for telephony and communications, have seen many developments over the past three decades. 

Also Read: Undersea cables: The information arteries

Optical Networks: An Overview

The 1980s were the age of microwave, when almost all the long distance telephony happened over microwave links.

The 1990s saw the launch of INSAT satellites and STD,ISD. However, the first decade of this millenium has truly been the age of optical fibres. Optic fibre backbones carry a dominant chunk of communication traffic these days.

There are several reasons for the overwhelming success of OFC (optic fiber cables) for communication.

The first and foremost is that light propagates with little attenuation, and that implies that repeaters/regenerators can be put further apart in each link. Besides these signals do not suffer from electrical disturbances unlike copper or radio/microwave medium.

Second is that each wave of light/infrared carries much more information than traditional radio and microwave frequencies. Typical channel capacity is 10Gbps which can go up to 40Gbps or even 100Gbps.

Third reason is that multiple channels/waves can be carried over the same fiber. Systems with 80 waves per fiber are common and we’re beginning to see equipment supporting 320 waves in the market. 

Thus an 80 wave, 10Gbps per channel system can send up to 800Gbps of data through a single optical fiber. On top of that, there can be multiple cores or fibres within a single optic fiber cable. 

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Thus the capacity of an OFC is way more than any other technology at present.

Thus, optic fibers were the most preferred technology for capacity enhancements in the last decade. It is estimated that India has a total of about 670,000 route-km of optical fiber laid throughout the country by major telecom operators like BSNL, Airtel, Reliance and Railtel.

And this process continues as more fiber is laid to add more and more capacity and to expand the telecom revolution to rural India.

Typically once an optical fiber is laid, the telecom operator chooses a combination of three technologies to carry voice and data traffic over the optical fiber.

The first one is DWDM. DWDM stands for Dense Wave Division Multiplexing. DWDM technology enables multiple channels (or waves) to be carried over the same fiber. Commonly available systems support up to 80 waves per fiber, and 320 waves per fiber are just coming up.

The second technology is SDH. SDH stands for Synchronous Digital Hierarchy. SDH is optimised for carrying voice traffic and provides a hierarchy of trunks or circuits. The interfaces vary from STM1 (155Mbps) to STM4 (622Mbps), STM16 (2.5Gbps), STM64 (10Gbps) and STM64 (40Gbps). The trunks or circuits that can be configured vary from E1 (2Mbps) all the way up to 10Gbps.

Each SDH link would use one channel from DWDM, and hence a combination of DWDM and SDH can support up to 80 links of SDH between any two points, each of them running on STM64.

The third technology is Ethernet. Ethernet is a packet switching technology that was developed for LAN environments in 1973. Since then it has become very successful at Layer 2 and is naturally suitable for carrying data traffic.

The current boom in communications is driven more by Internet and data applications than by voice. Even voice is moving to packet based transport in the form of VOIP. Most of IP routers hand of traffic on Ethernet ports.

Ethernet is supported in transport networks in two ways. Through enhancements to SDH (like VCAT, GFP, LCAS) it can either be carried as Ethernet over SDH, or as Ethernet over Fiber/DWDM.

The author is CTO at Tejas Networks Ltd.