Page 104 - Optical Switching And Networking Handbook

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Synchronous Digital Hierarchy 89

specification. Further, it is one of two options for the user-to-network
interface (UNI) to support the B-ISDN.

The Multiplexing Scheme

When trying to understand the SDH architecture, it is important to
remember that the North Americans have always done things one
way and the Europeans (actually the rest of the world) have done
things a different way. The two entities never seem to become harmo-
nized. As a result, the way the mapping and multiplexing were
arranged differs from the way the standards and multiplexing tech-
niques work in the North American communities (ANSI and SONET).
The language differs from what is used in SONET specifications.
As the framing and formatting begins with the STM-1 frame,
using the equivalent of the OC-3 specifications, the language begins
to shift in a simple yet confusing manner.The STM-1 frame is shown
in Figure 4-2 again, but with a few differences. First, the frame does
consist of 270 columns and 9 rows, creating a frame of 2430 bytes.
Second, there will be 8,000 frames per second (the frame time is
125 = micro(s). This creates an overall throughput of 155.52 Mbps,
the same as SONET.

270 Columns
Figure 4-2 9 Columns
Size of the STM-1
frame Repeater
3 rows overhead
section

1 row AU-4 Pointer
9 rows

5 rows Multiplexer
section
overhead

261 Columns
125 sec.