Figure 8 illustrates how signals are carried in SONET. First, SONET packages a signal into containers. It then adds the section overhead so that the signal and the quality of transmission are all traceable. The containers have two names depending on size: virtual tributary (VT) or a synchronous payload envelope (SPE). The path overhead contains data to control the facility (end to end) such as for path trace, error monitoring, far-end error, or virtual container (VC) composition.

Figure 8. Signal Transfer in SONET

SONET traffic is packaged in VCs and transported in synchronous transport vehicles or signals (STS). An STS exists on each section (i.e., the link between two nodes). An STS is made up of the payload and extra information called the line or section overhead. This line/section overhead contains data to control the node-to-node transmission (protection switching, error monitoring), and, in addition, provides extra channels (network management and maintenance phone link).

The SONET line transmission rates are 51, 155, and 622 Mbps; 2.5 Gbps; and 10 Gbps. Line signals can be optical or electrical. Electrical signals are used for short distances (e.g., between shelves in a rack) for interconnections of equipment on the same site. Electrical signals are sent on copper wires, while optical signals used for longer distances (from 40 km) require optical fibers.

SONET is an American National Standards Institute (ANSI) standard for North America, while synchronous digital hierarchy (SDH) is the standard for the rest of world. With many customers requiring international circuits, it is not uncommon to come across references to SDH. Table 1 demonstrates the various rates (note the different names used at transport level for SDH and SONET). First common transport rate is at 155 Mbps; SONET transport starts at 51 Mbps (28 T1).

Bit Rate (Mbps)	PDH North America		PDH Europe		SDH		SONET
	Name	Bit Rate (Mbps)	Name	Bit Rate (Mbps)	Container	Transport	Container	Transport
10,000						STM–64		STS/OC–192
2,500						STM–16		STS/OC–48
622						STM–4		STS/OC–12
155						STM–1		STS/OC–3
140			E4	140	VC4
51								STS/OC–1
45	DS–3/T3	45				STS–1 SPE
34			E3	34	VC3
8			E2	8
6	DS–2/T2	6				VT6—not really used
2			E1	2	VC12
1.5	DS–1/T1	1.5					VT1.5
0.064	DS0/T0	0.064	E0	0.064

Table 1. Functional Components