What is the Difference Between SONET vs. SDH vs. DWDM?

When contrasting the conventional SONET or SDH with the advanced DWDM (Dense Wavelength Division Multiplexing), the latter emerges as a streamlined architecture boasting remarkable scalability, enhanced capacity add/drop capabilities, support for multiple ring terminations, versatility in accommodating various services, and the utilization of multiple fabrics. So, when pitting SONET against SDH versus DWDM, what precisely sets them apart?

 

The Basics of SONET/SDH

What Is SONET/SDH？

 

The Synchronous Optical Network (SONET) is a standardized protocol facilitating seamless digital communication between senders and receivers. Operating via optical fibers, SONET streamlines the transmission of vast data volumes across extensive distances, and it can carry multiple data streams concurrently, harnessing the efficiency of optical fibers. SONET adheres to the standards set by the American National Standards Institute (ANSI) and finds prevalent usage in the North American region.

 

In contrast, the Synchronous Digital Hierarchy (SDH) emerges as a sophisticated and globally recognized iteration of SONET. Renowned as a multiplex technology primarily employed in telecommunications, SDH enables the creation of autonomous transmission networks devoid of vendor constraints. With intricate signal structures and notable features, SDH streamlines the integration of novel network technologies and devices, albeit demanding substantial power to sustain their operations.

The Difference Between SONET vs. SDH

 

SONET/SDH is the prevailing technology entrenched in most metropolitan and long-haul networks. It encompasses a cluster of fiber optic transmission rates tailored to ferry digital signals across various capacities. Below delineates the disparities between SONET and SDH:

 

1. SONET, pioneered by ANSI, predominantly serves the United States, whereas SDH, devised by ITU-T, finds application globally.

 

2. SDH's fundamental unit is the synchronous transmission module level-1 (STM-1), while SONET's basic unit is the Optical Carrier level-1 (OC-1).

 

3. SONET boasts 27 bytes of total transport overheads, whereas SDH boasts 81 bytes.

 

4. Due to the absence of high-order multiplexing for signal transfer, SONET proffers lower transmission rates than SDH.

 

5. While SONET exclusively transmits data synchronously, SDH caters to both synchronous and asynchronous data transmission modes.

The Difference Between PDH vs. SDH/SONET

 

For years, TDM-based networks such as PDH (Plesiochronous Digital Hierarchy) and SDH/SONET have been the go-to transport platforms for cellular traffic. These networks handle bulk voice circuits with utmost reliability, minimal latency, and uninterrupted service continuity. However, PDH has several drawbacks, including a global standard for rates and optical interfaces, an intricate structure, extensive hardware requirements, high power circuitry costs, and limited flexibility. This led to the development of SDH.

 

SDH emerged as a successor to the PDH system, aiming to foster interoperability among equipment from diverse vendors. The signal hierarchy of SDH introduced several line rates, with widely adopted options including STM-1 (155 Mbps), STM-4 (622 Mbps), STM-16 (2.5 Gbps), STM-64 (10 Gbps), and STM-256 (40 Gbps).

The Basics of DWDM

 

DWDM (Dense Wavelength Division Multiplexing) is a premier technology for augmenting bandwidth across existing fiber infrastructures. It empowers users to carve out multiple "virtual fibers" over a single physical fiber by transmitting diverse wavelengths or colors of light. Initially embraced by long-distance carriers to alleviate costs associated with amplification, dispersion compensation, and regeneration in regional and national SONET networks, DWDM's popularity surged in metro networks as local exchange carriers expanded their reach. Apart from addressing fiber exhaustion concerns, the escalating traffic volume remains the primary economic driver for deploying DWDM in metro networks.

 

DWDM Channels Frequencies Introduction

 

DWDM operates within the 1530 to 1565 nm range, known as the C-band, corresponding to the optical fiber's low-loss window. This specific range aligns with the operational capabilities of the Erbium-Doped Fiber Amplifier (EDFA). According to ITU-T standards, a grid of permissible wavelengths/frequencies is established, with a central frequency of 193.1 THz or 1553.3 nm and various frequencies spaced at multiples of 25 GHz (equivalent to 0.2 nm) around this center frequency.

 

Commercial DWDM systems may offer channels at rates of 2.5 Gbps, 10 Gbps, and, recently, even 40 Gbps, with potential combinations within the same system. Higher bit rates entail more significant power budget requirements, necessitating lasers with superior signal-to-noise ratios, reduced amplifier spacing, and higher amplification. This often involves using two DWDM Optical Amplifiers in series.

 

Typically, a setup with 64 DWDM channels at 10 Gbps can achieve a maximum distance of approximately 1,500 km, with amplifier spacing close to 100 km. Advanced and more expensive systems are commercially available for long-distance transmissions up to 4,500 km.

The Application of DWDM Technology

 

The DWDM layer exhibits protocol and bit rate neutrality, simultaneously accommodating ATM (Asynchronous Transfer Mode), SONET, and IP packets. WDM technology finds application in Passive Optical Networks (PONs), which serve as access networks where all transport, switching, and routing occur optically. Recent advancements include the integration of 3R (reshape, retime, retransmit) devices internally within the DWDM system, enabling the construction of circuits solely utilizing DWDM equipment that can span entire countries. These devices feature enhanced performance monitoring capabilities to facilitate link maintenance and repairs. By employing DWDM as the transmission method, the bandwidth of existing fiber infrastructure is fully optimized.

 

Compare The SONET/SDH vs. DWDM

 

The difference between SONET/SDH vs DWDM is shown in the following table:

 

 

The need for SONET granularity arises from the demand for low-speed services such as DS1/DS3 and the limited sub-wavelength grooming capabilities within DWDM platforms. DWDM aims to carry the same traffic using minimal topological structures, such as rings. However, this may result in some data traveling longer distances than SONET.

 

For instance, an optimal SONET design might entail multiple topologically distinct OC-48 and OC-192 rings, whereas a cost-effective DWDM transport network might consist of just one DWDM ring.

 

Traditionally, Internet traffic flows through IP, which typically rides on ATM and SONET/SDH, or IP over SDH, before reaching the optical layer. The misconception that IP operates solely on ATM/SDH stems from the belief that IP traffic is small and requires bundling with other services for cost-effective delivery.

 

Contrary to this belief, IP over DWDM accommodates voice, video, and data traffic, reserving the remainder for high-speed data. Streamlining layers (such as SONET/ATM) simplifies network management and reduces costs.

 

In the realm of application, SDH stands out as a dependable and coordinated conveyance system for both voice and data streams within core networks, with a critical focus on facilitating seamless connectivity between diverse systems. Its adaptability shines incredibly bright in accommodating older infrastructures and networks with diverse protocol and format needs.

 

Conversely, DWDM amplifies the throughput and capability of fiber optic networks, highlighting its prowess in scalability, adaptability, and operational efficiency. This technology finds its prime utility in burgeoning infrastructures and networks demanding rapid and extensive data transmission capacities.

Backbone Technology of The Past: SONET

 

As anticipated, SONET setups offer a lower initial cost. When traffic volume remains modest, opting for a SONET framework proves significantly more cost-effective than DWDM setups. According to FS's analysis, constructing a SONET overlay network to accommodate OC-3, OC-12, OC-48, and Gigabit Ethernet demands is ideal when the design necessitates fewer than 4-10 OC-192 rings.

Leading Choice For Present And Future Network Needs: DWDM

 

As traffic volume escalates, DWDM emerges as the ultimate choice in network technology. The timing of this transition hinges on factors such as span distances, pricing dynamics, and interface density. Variances in demand primarily stem from the comparative design efficiency of interface cards between these two technologies, specifically in density and cost.

 

QSFPTEK's research indicates that span distances often trigger additional requirements for regenerators, optical amplifiers, and dispersion compensation modules (DCMs) along routes. Longer span distances favor DWDM architectures due to their adept utilization of fibers and optical bypass capabilities at intermediary nodes.

 

Moreover, higher fiber costs and instances where fiber limitations are enforced prompt more significant consideration for DWDM over SONET. DWDM proves immensely beneficial in conserving fiber within optical networks. While DWDM systems can be designed for many channels, a pay-as-you-grow strategy can also be adopted, adding channels based on demand. It's crucial to calculate the amplifier distance and overall power budget of the system right from the outset to effectively determine the final quantity of channels.

Conclusion

This intriguing study explores various alternatives and their economic implications in designing identical networks. SONET point-to-point configurations exhibit even better performance. While these findings may not universally apply, they suggest that in extensive network designs, the most efficient solution may only sometimes entail a singular architecture. Different network sections may adopt distinct approaches, with some employing ring structures while others utilize point-to-point setups. Typically, the core segment of the network would find justification for adopting a DWDM architecture.