Following development of the SONET standard by ANSI, the CCITT undertook to define a synchronization standard that would address interworking between the CCITT and ANSI transmission hierarchies. That effort culminated in 1989 with CCITT's publication of the synchronous digital hierarchy (SDH) standards. SDH is a world standard, and, as such, SONET can be considered a subset of SDH.

Transmission standards in the United States, Canada, Korea, Taiwan, and Hong Kong (ANSI) and the rest of the world (ITU–T, formerly CCITT) evolved from different basic-rate signals in the nonsynchronous hierarchy. ANSI time division multiplexing (TDM) combines twenty-four 64–kbps channels (DS–0s) into one 1.54–Mbps DS–1 signal. ITU TDM multiplexes thirty-two 64–kbps channels (E0s) into one 2.048–Mbps E1 signal.

The issues between ITU–T and ANSI standards-makers involved how to accommodate both the 1.5–Mbps and the 2–Mbps nonsynchronous hierarchies efficiently in a single synchronization standard. The agreement reached specifies a basic transmission rate of 52 Mbps for SONET and a basic rate of 155 Mbps for SDH.

Synchronous and nonsynchronous line rates and the relationships between each are shown in Tables 9 and 10.

SONET Signal	Bit Rate (Mbps)	SDH Signal	SONET Capacity	SDH Capacity
STS–1, OC–1	51.840	STM–0	28 DS–1s or 1 DS–3	21 E1s
STS–3, OC–3	155.520	STM–1	84 DS–1s or 3 DS–3s	63 E1s or 1 E4
STS–12, OC–12	622.080	STM–4	336 DS–1s or 12 DS–3s	252 E1s or 4 E4s
STS–48, OC–48	2,488.320	STM–16	1,344 DS–1s or 48 DS–3s	1,008 E1s or 16 E4s
STS–192, OC–192	9,953.280	STM–64	5,376 DS–1s or 192 DS–3s	4,032 E1s or 64 E4s
Note: Although an SDH STM–1 has the same bit rate as the SONET STS–3, the two signals contain different frame structures. STM = synchronous transport module (ITU–T) STS = synchronous transfer signal (ANSI) OC = optical carrier (ANSI)

Table 9. SONET/SDH Hierarchies

ANSI Rate			ITU–T Rate
Signal	Bit Rate	Channels	Signal	Digital Bit Rate	Channels
DS–0	64 kbps	1 DS–0	64–kbps	64 kbps	1 64–kbps
DS–1	1.544 Mbps	24 DS–0s	E1	2.048 Mbps	1 E1
DS–2	6.312 Mbps	96 DS–0s	E2	8.45 Mbps	4 E1s
DS–3	44.7 Mbps	28 DS–1s	E3	34 Mbps	16 E1s
	not defined		E4	144 Mbps	64 E1s

Table 10. Nonsynchronous Hierarchies

Convergence of SONET and SDH Hierarchies

SONET and SDH converge at SONET's 52–Mbps base level, defined as synchronous transport module–0 (STM–0). The base level for SDH is STM–1, which is equivalent to SONET's STS–3 (3 x 51.84 Mbps = 155.5 Mbps). Higher SDH rates are STM–4 (622 Mbps) and STM–16 (2.5 Gbps). STM–64 (10 Gbps) has also been defined.

Multiplexing is accomplished by combining or interleaving multiple lower-order signals (1.5 Mbps, 2 Mbps, etc.) into higher-speed circuits (52 Mbps, 155 Mbps, etc.). By changing the SONET standard from bit-interleaving to byte-interleaving, it became possible for SDH to accommodate both transmission hierarchies.

Asynchronous and Synchronous Tributaries

SDH does away with a number of the lower multiplexing levels, allowing nonsynchronous 2–Mbps tributaries to be multiplexed to the STM–1 level in a single step. SDH recommendations define methods of subdividing the payload area of an STM–1 frame in various ways so that it can carry combinations of synchronous and asynchronous tributaries. Using this method, synchronous transmission systems can accommodate signals generated by equipment operating from various levels of the nonsynchronous hierarchy.