SUBJECT D15)ATM Physical Level Questions.
SUBJECT D15-1)Question:Whats the difference between SONET and SDH?
Answer:SONET and SDH are very close, but with just enough differences that they don't really interoperate. Probably the major difference between them is that SONET is based on the STS-1 at 51.84 Mb/s (for efficient carrying of T3 signals), and SDH is based on the STM-1 at 155.52 Mb/s (for efficient carrying of E4 signals). As such, the way payloads are mapped into these respective building blocks differ (which makes sense, given how the European and North American PDHs differ). Check the September 1993 issue of IEEE Communications Magazine for an overview article on SONET/SDH.
The following table shows how the US STS and the European STM levels compare:
US Europe Bit Rate (total) STS-1 -- 51.84 Mb/s STS-3 STM-1 155.52 Mb/s STS-12 STM-4 622.08 Mb/s STS-24 STM-8 1244.16 Mb/s STS-48 STM-16 2488.32 Mb/s STS-192 STM-64 9953.28 Mb/sFrom a formatting perspective, however, OC-3/STS-3 != STM-1 even though the rate is the same. SONET STS-3c (i.e., STS-3 concatenated) is the same as SDH STM-1, followed by STS-9c = STM-3c, etc.
There are other minor differences in overhead bytes (different places, slightly different functionality, etc), but these shouldn't provide many problems. By the way, most physical interface chips that support SONET also include a STM operation mode. Switch vendors which use these devices could then potentially support STS-3 and STM-1 for example. For anyone interested, there is an ANSI T1 document which reports on all the differences between SONET and SDH, and proposals to overcome them. (Document T1X1.2/93-024R2). It's available at ftp.tele.fi in the directory /atm/ansi, files sonet-sdh-1.ps and sonet-sdh-2.ps
Answer: On finding boundaries between cells, called "cell delineation" in the stds docs: in addition to a Header Error Check scan to search for valid CRCs, some physical layers cells have a known relationship to the PHY structure. With some PHY's, the cell's are byte-aligned with the underlying structure, with others, the alignment may be nibble or even bit (i.e., no alignment at all). The so-called TAXI phy, now fading towards the sunset, does use special codes in a 4B/5B encoding to mark beginning of cell, etc, but it's the exception.
In any case, since with most PHY's, cells are continuously arriving back to back (idle or unassigned cells are filled in by the transmitter if there is no data-carrying cell in the slot), it only takes a few cell times to sync up, and it's not too hard to maintain "cell sync" at the receiver.
Most of the PHY specs are online at the ATM Forum's web site. The first few PHY (SONET/SDH, DS-3, TAXI) specs were included in the UNI 3.0/3.1 spec; later ones (and there's a lot of them!) are in their own docs.
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