Digital Structure Of Sdh Signals; Stm-1 Frame Structure - Siemens SRA 4 User Manual

1 - PRELIMINARY INFORMATION
1.4.1.3
1.4.1.4
1-12

Digital structure of SDH signals

The basic digital structure of the SDH hierarchy is formed by the frame of the so-called
synchronous transport module, first hierarchical level, indicated by the acronym STM-1.
Through a process of multiplexing and linking of the STM-1 octet, transport modules of
higher order are formed (STM-N).
The STM-1 can be arrived at by constructing previous digital structures, so-called
administrative units (AU) containing signals (TUG) which in turn have been obtained by
multiplex processes and octet linking of lower capacity digital structures. These consist
of TU's (tributary units) which in turn contain the so-called virtual containers (VC) which
transport the containers (C) with the useful signal.
Fig. 1.1 shows schematically the multiplex structure foreseen by the ETSI standard
(European telecomunications standard institute).
Fig. 1.1 shows also the transport capacity values of the single digital structures obtained
by summing the transport capacity of the inferior structure with the POH bytes, the bytes
required for allocation of pointers, and possible stuffing bytes. For AU4 and TU3
structures, further detail is reported in the following paragraphs.

STM-1 frame structure

Fig. 1.2 shows the frame structure of an STM-1 signal. A frame consists of 9 lines, each
containing 270 bytes. Each byte is composed of 8 bits.
The frame frequency (8 kHz) has been selected in such a way that 1 byte of the frame
corresponds to the transmission capacity of a 64 kbit/s channel.
The overall transport capacity is:
C = 8 x (9 x 270) x 8 x 10
STM-1
a b
a) number of bits composing each byte
b) overall number of bytes in each frame
c) frame frequency
3
= 155.520 Mbit/s
c
SRA 4 - 6/7/8/10/11/13 GHz (WB ODU) - UMN
911-381/02C0000 - Issue 3, July 2004