Advantages of SDH transmission compared to PDH:
* High enough transmission rates for current mobile core data network needs.
* Synchronous network provides accurate timing.
* More simple add/drop functionality compared to PDH.
* Reliable in node and network level.
* Global standardization and interconnectivity.
* Support of many client protocols (with NG-SDH), future proof.
The basic transmission format for SDH is STM-1 (Synchronous Transport Module,
Level 1). Each SDH rate is an exact multiple of the lower level signal. STM-1 frame is
transmitted in 125
s time period. The corresponding signal in SONET is STS-3c (Synchro-
nous Transport Signal, Level 3, concatenated) or OC3c (Optical Carrier, Level 3,
concatenated). Basic rate in SONET is called STS-1/OC-1, 51.840/50.112Mbit/s, that is
sometimes called also STM-0. Overhead is 3.4% of data rate.
There are three ways of mapping 1544 and 2048 kbit/s primary rate signals into the VC-11
and VC-12, respectively, as defined in ITU-T Recommendation G.707: asynchronous, bit
synchronous and byte synchronous.
Incoming PDH signal in Figure 5.2 is synchronized to SDH rate and stuffing bits are used.
Pointers are used to adjust varying PDH rates to SDH rate. In SDH multiplexing stuffing is not
needed. PDH rates are mapped to STM frames by fixed size containers (C). To carry a
container over a SDH path overhead is needed for alarm and management information. If the
stream is slower then the rest of the container is stuffed with extra bits. This filling is called
mapping. Path Overhead (POH) is used to define a path between edges of SDH network. A
container together with POH forms a virtual container (VC) that travels over the SDH network
to the point where (PDH) data is extracted. Administrative Unit (AU) is the next step in SDH
framing hierarchy when AU-pointer is added to the frame. AU-pointer indicates the location of
the first VC byte and allows the VC 'float' inside STM-frame. This turns VC into AU.
Tributary Units (TU) are used to multiplex lower order PDH signals to VC-4. Each TU-n has a
corresponding VC-n that is then mapped to VC-4 with the help of TU-n pointer. Adminis-
trative Unit Group (AUG) is used to multiplex a STM-signal either from the same level AU or
from several lower level AUs.
Here 'c' refers to concatenated or clear channel. This implies that the entire payload rate
may be used by a single data stream. It is not tied to E- or T-rates. The rest of the transmission
Table 5.2 STM, SONET and Optical Carrier data rates.