Choosing Disaster‑Recovery Communication Links: Fiber, DWDM, and SDH Explained

Introduction

Disaster‑recovery (DR) communication link design is a critical step to ensure that users can build a DR system with reasonable communication costs. Different link types have distinct attributes—such as supported distance and bandwidth—and various DR technologies and applications impose different requirements.

Selecting a DR Communication Link

When choosing a DR solution, the selection must start from the actual DR needs of the information system, identifying risk types, analyzing the DR requirements of each business system, and defining RTO and RPO targets. Users also need to consider data characteristics and acceptable costs to pick an appropriate transmission method. Key questions include:

Distance between the production site and the DR site: regional disasters require off‑site DR, site‑level disasters may use same‑city DR, and equipment failures can be handled within the same data center.

Bandwidth: determine required bandwidth and number of links based on business applications and RTO/RPO goals.

Data change volume and transmission reliability: verify that the designed link meets expected performance.

Link Connection Methods

Industry DR solutions typically use one of the following:

Direct bare‑fiber connection to switches.

DWDM equipment connecting bare fiber.

IP‑based network connections.

Standard IP networking (ATM, E1/E3, IP) is most common. For storage‑oriented DR, options expand to include bare fiber, DWDM, SONET, SDH, or FCIP devices that leverage ATM/E1/E3/IP.

Fiber Types and Transmission Modes

Fiber is the primary transmission medium, available as single‑mode or multimode. Multimode fiber suffers from modal dispersion, limiting bandwidth over long distances (effective range 2–4 km). For distances ≥100 km, single‑mode fiber is preferred.

Traditional optical transmission standards include:

PDH (Plesiochronous Digital Hierarchy) – up to 150 Mbps.

SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) – early standards still in use, supporting 155 Mbps, 622 Mbps, 2.5 Gbps, 10 Gbps.

Wavelength Division Multiplexing (WDM) allows multiple optical signals on a single fiber. Two main variants are CWDM (coarse, 20 nm spacing) and DWDM (dense, 0.2–1.2 nm spacing). DWDM is the mainstream technology.

Typical single‑wavelength rates: 2.5 Gbps, 10 Gbps, 40 Gbps, 100 Gbps, 200 Gbps. A fiber can carry 32, 40, 80, 96, 106, or 160 wavelengths; 40‑ and 80‑wave systems are common.

Multiplexing Options

To maximize bandwidth utilization and reduce cost, multiplexing techniques split a physical channel into multiple logical channels. Common methods are:

Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

Code Division Multiplexing (CDM)

In DR fiber links, FDM and TDM are most widely used.

SDH Overview

SDH, based on SONET, is an international standard for synchronous digital transmission, multiplexing, and cross‑connection. It packages PDH, ATM, IP, etc., into STM‑N frames for transport, using synchronous and flexible mapping (essentially TDM).

WDM and DWDM Details

Before WDM, long‑distance links carried a single SDH/SONET signal per fiber pair, requiring OEO regeneration every ~80 km, which is costly. DWDM systems amplify multiple wavelengths simultaneously using EDFAs, spreading regeneration cost across channels and lowering per‑bit cost.

First‑generation DWDM supported up to ~640 km without electrical regeneration, typical for metro core networks. Third‑generation LHDWDM with ULH technology can reach 3000 km without OEO regeneration, suitable for long‑haul backbones.

Choosing the Right Link Based on Distance and Cost

Guidelines:

For DR distances around 100 km, a metro‑level WDM network is recommended.

For distances ≥1000 km, options include SDH networks, long‑haul DWDM, or hybrid SDH+WDM solutions.

DWDM System Components

A typical DWDM node consists of:

Wavelength conversion : converts services to appropriate optical wavelengths (includes transceivers).

MUX : combines multiple wavelength signals onto a single fiber.

DEMUX : separates wavelengths at the receiving end.

DCM : optional dispersion compensation module.

Optical amplifier : boosts optical power (e.g., EDFA).

Fiber : the transmission medium.

DWDM wavelengths are often called “colored” light to distinguish them from the “grey” wavelengths used in SDH or other legacy systems.