Wavelength Division Multiplexing

Wavelength Division Multiplexing By: y Gurudatha Pai K [email protected]

Thursday, April 17, 2008, 6:28:40

Overview ƒ

Introduction

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Popular Multiplexing Techniques

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Optical Networking ƒ

An Analogy a ogy of o Multiplexing utpe g

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SONET and SDH

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WDM

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A Small Note on CWDM and DWDM

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Introduction ƒ

Enormous bandwidth available in optical fiber.

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Implies that a single optical carrier can be base band modulated at ~25,000 Gbps, occupying 25,000 GHz s rro nding 1.55 surrounding 1 55 nm, nm

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This bit rate is impossible for present-day optical devices to achieve. ƒ given that lasers, external modulators, switches or detectors have bandwidths < 100 GHz.

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So, a single “high-speed” channel takes advantage of an extremely small portion of the available fiber bandwidth.

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Popular Multiplexing Techniques - TDM Signal A ƒ

Signal A A 100-Gbps channel probably will be a combination of many M

D E L M TI These lower-speed channels are multiplexed UL together in time to P TI L form a higher-speed channel. This is time-division multiplexing PL Signal B E E (TDM). X X E E R This can be accomplished in the electrical R or optical domain, with

l lower-speed d signals, i l U

ƒ Signal B ƒ

Signal C

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each h lower-speed l d channel h l transmitting t itti a bit/packet bit/ k t only l in i a Digital Baseband for given time slot. Transmission Signal C Generally, Digital or PAM signals are Multiplexed in Time. Results in higher bit rate, so corresponding increase in BW

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Popular Multiplexing Techniques - FDM ƒ

Analog signals,

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A number of Band Limited Base Band Signals frequency translated and sent over a single Channel

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g BW Results in higher

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Optical Networking - An Analogy of Multiplexing Packet containing stuff

Payload

Overheads

High Speed Truck 6

Optical Networking – Synchronous Transmission SONET - SDH ƒ

All Equipments synchronized to the Network Clock.

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ANSI S defined f Synchronous S O Optical Network (SONET) (SO ) for f use in U S.

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ITU standardized on Synchronous Digital Hierarchy (SDH) for worldwide usage.

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SDH & SONET Traffic Interworking is compatible.

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India uses SDH

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First level of SONET hierarchy is referred to as Synchronous Transport Signal-1 (STS-1 for an electrical signal) Or Optical Carrier-1 (OC-1 for an optical signal)

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STS-1 / OC-1 corresponds p to a bit rate of 51.84 Mbps. p

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STS-3 / OC-3 signal corresponds to STM-1 (155.52 Mbps). 7

Advantages / Applications of SDH ƒ

SDH offers higher Transmission rates,

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Offers more efficient Add / Drop / Cross connect functions.

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Incorporates powerful Network Management functions, which are g, Configuration g Software controllable like Network Planning, Management, Fault Management, Performance Management, Security Management & Accounting Management functions.

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Enables easy identification of link / node failures and centralized maintenance.

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y of Interfaces to accommodate from other networks Wide variety such as ISDN, ATM, LAN, TCP/ IP etc.

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Performs Multiplexing and Line Terminating functions.

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Allows dynamic allocation of bandwidth. 8

SONET Frame

STM-1 Frame 155 Mbps ( Synchronous Transport Module-1) ƒ

125 µsec frame

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270 x 9 bytes x 64 Kbps = STM 1

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Payload floats in SDH frame (envelop).

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y points to start of Pointer byte

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9

STM-1 frame

o v e r h e a d

Payload 270

Pointer and stuff byte

payload. ƒ

can be dropped and inserted easily

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An animation of WDM Optical Coupler

λ1

λ1

λ2

λ2

λ3

λ3

λ4

λ4 Optic Fiber Cable

Nortel's WDM System 10

Wavelength Division Multiplexing (WDM) ƒ

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Technology which multiplexes multiple optical carrier signals on a single optical Fibre by using different wavelengths (colors) off laser l light li ht tto carry diff differentt signals. i l This allows for a multiplication in capacity. Duplex communications over a strand of Fibre. Th true The t potential t ti l off optical ti l Fibre Fib is i fully f ll exploited l it d when h multiple beams of light at different frequencies are transmitted o n the same Fibre. This is a form of (FDM) but is commonly called WDM WDM. WDM system enables single fiber to carry multiple high speed d ata streams with carrier frequencies in the order of 200 THz (1 500 nm) to 300 THz (1000 nm). By using Fused biconic tapered couplers, multiple wavelength signals are combined on same fiber. Due to fiber attenuation, periodic regeneration was done; Ne w generation i E Erbium bi D Doped d Fib Fiber A Amplifiers lifi (EDFA) enable bl hi hig h-speed, long distance repeater less transmission. 11

Wavelength Demultiplexing ƒ Using an optical filter as illustrated in figure

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DWDM – Dense Wavelength Division Multiplexin g ƒ ƒ ƒ ƒ

CWDM – Coarse Wavelength Division Multiplexing Number of Wavelengths: g 4 / 8 / 16 Channel Spacing: > 200 GHz (corresponds to 1.6 nm over the usable spectrum) Applications: Cost effective Metro Access & Enterprise netw orks. orks

DWDM ƒ ƒ ƒ ƒ ƒ

As Laser and Optical Filter Technologies improved, more signa l wavelengths could be combined on a single fiber fiber. ITU has specified DWDM Band in the 1525 to 1565 nm range with suitable Channel Spacing . g 32 / 40 / 80 / 100 Number of Wavelengths: Channel Spacing: 200 / 100 / 50 GHz (corresponding to 1.6 nm / 0.8 nm / 0.4 nm over the usable spectrum) Applications: High Capacity long haul (multiple sections of 100 KMs) backbone networks etc etc. 13

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