The Difference Between SONET and SDH

By Alliene Turner

Synchronous Optic Network (SONET) and Synchronous Digital Hierarchy (SDH) are standards used in fiber optic networks that power large telephone and Internet networks. SONET networks are deployed in North America; SDH networks are deployed everywhere else. The data communications industry uses the concept of “backbone” to refer to a large network capable of carrying heavy loads of traffic. SONET and SDH fiber optic networks, although expensive, are ideal backbone networks, offering high speeds and reliability.

Standards

SONET adheres to standards T1.105 and T1.106 developed by ANSI or the American National Standards Institute. SDH is an ITU or International Telecommunications Union standard developed to allow it to work with SONET line rates. It is a flexible standard that connects to several interfaces and bandwidths and has bandwidth on-demand capability.

Multiplexing

Fiber optic cable is expensive and so is digging trenches to bury the cable. Telecommunications companies need each inch of the cable to pay for itself, so they divide the cable into multiple channels. In this way, data can be multiplexed -- a complex process that breaks down data into manageable pieces called frames that are then pulsed electronically onto the channels, along with information that allows the data to be combined back into their original stream order at the destination. SONET and SDH have different frame structures and methods for breaking down and restructuring data. A SONET frame is composed of 6,480 bits of data; an SDH frame has 2,430 bytes. SONET uses Synchronous Transport Signal (STS) levels to define its frame; the SDH equivalent is the Synchronous Transport Module (STM).

Line Rates

Fiber optic supports line speeds of 8,000 frames per second. Because of their different frame sizes, SONET and SDH use different schemes for determining the rate of sending their packets over fiber optic cable, but end up with the same line rate. SONET uses an algorithm that multiplies the base STS-1 rate of 51.84 Mbps by multiples of 3. SDH multiplies E1 rates -- a telecommunication standard of 2.048 Mbps -- times 1, 4, 16 and 64. The five main line rates begin at 51.85 Mbps, then step to 155.52, 622.08, 2,488.32 and 9,953.28 Mbps. Respectively, these correspond with SONET signals STS-1, STS-3, STS-12, STS-48 and STS-192, and to SDH signals STM-0, STM-1, STM-4, STM-16 and STM-64.

Topology

Network architects favor the ring network topology when designing SONET and SDH network backbones. Ring networks use two rings, each containing transmit and receive cables. One ring acts as backup in case the other fails, so ring networks are described as self-healing. Data travel long distances over fiber optic cable, so network administrators appreciate the ring topology for its fault tolerance.