SONET is not a simple replacement for asynchronous digital hierarchy. It is a network in its own right (i.e., reconfigurable, with embedded switches and centralized management that allow automated processes). See Figure 10.

Figure 10. SONET Network Architecture

SONET benefits are delivered through functionality. A transport network is hierarchical as a road network, comprised of small roads, medium-sized roads, and highways for long distance:

• Collector rings provide the network interface for all access applications, including local offices, private automatic branch exchange (PABX), access multiplexers, wireless base stations, and ATM terminals. In some instances, a SONET multiplexer is located in the customer premises and provides direct service (T1 leased line, for instance).
• The bandwidth-management function routes, grooms, and consolidates traffic between the collectors and backbone networks. It ensures that backbone synchronous transport signal (STS–1) switch processing element (SPE) are filled to the maximum.
• The high-speed backbone transport function provides reliable and economical long-distance transport.

SONET Features

• network management
• protection
• bandwidth management
• network simplification
• mid-fibre meet

SONET Benefits

• increased revenues
  • improved services
  • differentiated services
• survivable network
• reduced operating cost
  • centralized management
• reduced capital investment