19.3
SYNCHRONOUS DIGITAL HIERARCHY
501
Figure 19.15
An ADM in a terminal configuration.
technician interfaces and to connect to required and optional operation links that permit
maintenance, provisioning, and testing.
Figure 19.15 shows an example of an ADM in the terminal mode of operation with
DS1 interfaces. In this case, the ADM multiplexes up to Nx(28 DS1)
7
or equivalent
signals into an OC-N bit stream. Timing for this terminal configuration is taken from
either an external synchronization source, the received OC-N signal (called loop timing),
or its own local clock depending on the network application.
19.3
SYNCHRONOUS DIGITAL HIERARCHY
19.3.1
Introduction
SDH was a European/CCITT development, whereas SONET was a North American devel-
opment. They are very similar. One major difference is their initial line rate. STS-1/OC-1
has an initial line rate of 51.84 Mbps; and SDH level 1 has a bit rate of 155,520 Mbps.
These rates are the basic building blocks of each system. SONET's STS-3/OC-3 line rate
is the same as SDH STM-1 of 155.520 Mbps.
Another difference is in their basic digital line rates. In North America it is at the
DS1 or DS3 lines rates; in SDH countries it is at the 2.048-, 34-, or 139-Mbps rates (see
Chapter 6). This has been resolved in the SONET/SDH environment through the SDH
administrative unit (AU) at a 34-Mbps rate. Four such 34-Mbps AUs are "nested" (i.e.,
joined) to form the SDH STM-1, the 155-Mbps basic building block. There is an AU3
used with SDH to carry a SONET STS-1 or a DS3 signal. In such a way, a nominal
50-Mbps AU3 can be transported on an STM-1 SDH signal.
19.3.2
SDH Standard Bit Rates
The standard SDH bits rates are shown in Table 19.2. ITU-T Rec. G.707 (Ref. 5) states
"that the first level digital hierarchy shall be 155,520 kbps
. . . and . . . higher syn-
chronous digital hierarchy rates shall be obtained as integer multiples of the first level
bit rate."
7
This implies a DS3 configuration. It contains 28 DS1s.