SDH was designed as to be an answer to solve the flaws and problems of its predecessor
plesiochronous digital hierarchy (
PDH), but SDH itself will be replaced newer technologies and other demands that come about with the evolving
internetwork. Therefore, aspects of SDH that could be categorised as strengths may also be a weakness when viewed from emerging technologies.
Summarizing the techonolgy
- Some
bytes, called the
payload, of pre-defined length are grouped into a container, with each of them getting some overhead information mapping into a
virtual container (VC).
- Depending on the size of the VC, the smaller ones are grouped together, aligned, into
tributary units (TUs), who are then combined (=multiplexed) to tributary unit groups (TUGs).
- They then are multiplexed into a VC-4, the largest possible virtual container. The largest defined container, being 139264 kbits/s, goes straight into a VC-4 without the aligning and multiplexing.
- The contents of this VC-4 is poured into an Administrative Unit, and mapped into an Administrative Unit Group (AUG).
- Well, finally these AUGs go into an
STM frame having in total 9x270 bytes, racing at 155.52 Mbit/s through the
fibre (or
STS if you're thinking about
SONET. STM stands for Synchronous Transfer Module, in case you're interested.).
All this mixing, matching and combining together can make it up to an aggregate
bandwidth of
9953.280 Mbit/s with an STM-64 (= 64 STM-1 frames combined). To make a bit of sense out of such a number: that could carry 120000
telephone calls (or faxes), or 4000 low-scan video
pictures, or 64 broadcast
TV channels... nice!
General comments
SDH solved problems encountered with PDH in having defined a worldwide
standardisation as a first (of which
SONET is technically speaking a subset, although SDH evolved from SONET), enabling better integration to achieve a global network demanded by
globalisation of the economy and allowing for higher aggregate
bandwidth than PDH.
Technically, it made the
multiplex mountain of PDH obsolete with the introduction of add/drop multiplexers (
ADMs) so that a path can be extracted from a
frame and routed to another destination without the need of de- and re-multiplexing the whole frame. Third, it addressed the need for better
services (like error
performance and maintenance) by defined section and path
overhead bytes in the frame and an embedded data communications channel. This extra information and reduced amount of equipment allows for greater flexibility and more sophisticated and centralized
network management.
However, SDH is not heaven on earth. To keep the network
synchronised and to minimise
jitter, expensive high standard clocks in a
distributed timing system are required.
Because equipment costs are high, research into eliminating SDH from the network is carried out; for example
IP (over
ATM) on
WDM, which results in a simpler transmission technique with a lower overhead.
SDH still requires allocation of resources like connecting ports in a digital cross-connect (
DXC), but it would be more ideal when the management system only would have to bother with setting policies and leave the
implementation intelligence to the
network elements.
Summarizing, the advantages of SDH are network simplification, its survivability, software control, bandwidth on demand and standardisation. Disadvantages are keeping the network synchronised, high cost and management functionality not meeting the new demands.
References:
Anon. http://www.rad.com/networks/1994/sdh/doc2.htm.
Brattli, T. SDH tutorial. http://www.iec.org/online/tutorials/sdh/topic01.html to topic11. 1999.
Hawker, I., Hill, G. and Taylor, I. UK core transmission network for the new millennium. BT Technology Journal, Vol 18 No 3. July 2000.
Moss, N.. Synchronous Digital Hierarchy. Systems and Processes, T305 syllabus Block 2. 1999. pp 6-71.