9.3 SONET/SDH
SONET, more commonly known as SDH outside North America, supports longer network distances than the previously outlined optical transport technologies. SONET/SDH is typically used for city-to-city or country-to-country communications.
Figure 6. SONET/SDH
SONET/SDH provides the following benefits to a wide range of applications within the infrastructure:
• SONET/SDH are readily available in many areas.
• Spans longer network distances than CWDM or DWDM at a lower cost.
• Provides robust network management and troubleshooting capabilities. In SONET/SDH, the optical signal is converted to an electronic signal at all network access and regeneration points. Although this conversion introduces some latency, it allows much more robust monitoring.
• SONET/SDH provides a variety of protection systems, including n+1, a more cost effective protection system then 1+1. DWDM and CWDM do not offer an n+1 protection capability
n+1 refers to one spare circuit providing redundancy for multiple active circuits. When the first active circuit fails, the standby circuit is enabled and used. In DWDM and CWDM, 1+1 protection is a redundant set of fiber used to provide failover if the primary set fails.
In summary, SONET/SDH has the following key design considerations:
• SONET/SDH supports longer distances than WDM
• Most typically used over short and intermediate distances
• Used when dark fibre is not available
• Robust network management and troubleshooting
• Significant installed infrastructure required
• Variety of protection systems offered
• Can be combined with DWDM to increase capacity or redundancy
