HGSCHULZ@cs.umass.edu (Henning Schulzrinne) (02/22/90)
I have a number of questions regarding SONET, the Bellcore-standard Synchronous Optical NETwork. 1. Are there any readily accessible papers (i.e., not just some standard) containing details on SONET, beyond the paper in the March 1989 issue of the IEEE Communications Magazine? I am especially interested in motivations of certain design decisions, not just "that's how it is and there is nothing you can do about it". 2. Why was the row size set to 90 bytes? As it is, ATM packets will have to be broken across rows. 3. How do ATM and SONET interact? What gets switched where? 4. What is the advantage of interleaving ``header'' information throughout the frame, rather than concentrating it at the beginning of a frame? Why are the payload pointers put a number of rows after the beginning of the frame, so that I have to wait until I can determine where the payload begins? 5. Why was the path overhead made part of the payload rather than the header? 6. What is the implementation status of SONET? Thanks for any help or pointers to people who might know. If there is enough interest, I will summarize to the Digest. Henning Schulzrinne (HGSCHULZ@CS.UMASS.EDU) Department of Electrical and Computer Engineering University of Massachusetts at Amherst Amherst, MA 01003 - USA === phone: +1 (413) 545-3179 (EST); FAX: (413) 545-1249
goldstein@carafe.enet.dec.com (Fred R. Goldstein) (02/24/90)
In article <4265@accuvax.nwu.edu>, HGSCHULZ@cs.umass.edu (Henning Schulzrinne) writes... >1. Are there any readily accessible papers (i.e., not just some >standard) containing details on SONET, beyond the paper in the March >1989 issue of the IEEE Communications Magazine? I am especially >interested in motivations of certain design decisions, not just >"that's how it is and there is nothing you can do about it". >2. Why was the row size set to 90 bytes? As it is, ATM packets will >have to be broken across rows. You have to remember that SONET and ATM are only distantly related. SONET predates ATM; it did not anticipate ATM. ATM does not require SONET; the B-ISDN crowd simply took pieces that the ATM fanatics wanted and pieces that the SONET fanatics wanted and glommed them together. They don't fit together particularly well. >3. How do ATM and SONET interact? What gets switched where? SONET is simply the physical medium that carries ATM (or other things). It's synchronous. The row size is based on a compromise between the US and Europe. Originally the US ran 50.02 Mbps (I think) STS-1 with 13 rows, but that's meaningless to the Europeans (whose hierarchy is different) so the compromise was to "meet" at STS-3 (STM-1 to CCITT), with 270 columns and 9 rows. The compromise fit together that way. ATM cell size is controversial. The Aussies pushing DQDB (802.6) had 69-octet cell silicon and the Americans agreed with that size (64 octet payload, 5 octet header). The French did the Prelude experiment in '82 using 18 octet (16+2) cells, and figured that they could go as high as 32 octet payloads without needing echo cancellers for voice. (Echo cancellers are needed if your packetization and propagation delays are excessive. In a country the size of the US, 32 octets of packetization delay, or 4 milliseconds, would have been excessive. So we Gringos are pretty much resigned to using echo cancellers over ATM.) CCITT struck a compromise last summer that nobody really liked: Split the difference and have a 48-octet payload (48+5 cell). Dividing 53 octets into the 260-column STM-1 (after 10 columns of overhead are subtracted) does not leave an integer, but you can hardly blame SONET for that! >4. What is the advantage of interleaving ``header'' information >throughout the frame, rather than concentrating it at the beginning of >a frame? Why are the payload pointers put a number of rows after the >beginning of the frame, so that I have to wait until I can determine >where the payload begins? In practice, I think you'll have to buffer a frame anyway; putting the overhead throughout the frame (actually, in the first columns) allows the rest of the columns to be used as virtual tributaries, undisturbed. It makes sense to me. >5. Why was the path overhead made part of the payload rather than the >header? SONET is layered. It requires a section layer overhead. The path is layered above section, as a path may run over many concatenated sections. fred (ANSI T1S1 rep) Fred R. Goldstein goldstein@carafe.enet.dec.com or goldstein@delni.enet.dec.com voice: +1 508 486 7388 opinions are mine alone. sharing requires permission.
ellson%sdcsvax@ucsd.edu (02/24/90)
In article <4265@accuvax.nwu.edu>, HGSCHULZ@cs.umass.edu (Henning Schulzrinne) writes: > I have a number of questions regarding SONET, the Bellcore-standard > Synchronous Optical NETwork. > 1. Are there any readily accessible papers (i.e., not just some > standard) containing details on SONET, beyond the paper in the March > 1989 issue of the IEEE Communications Magazine? I am especially > interested in motivations of certain design decisions, not just > "that's how it is and there is nothing you can do about it". As a long-time contributor to the development of the Sonet standard I can probably answer your questions as far as the technical motivations behind the design decisions in Sonet. > 2. Why was the row size set to 90 bytes? As it is, ATM packets will > have to be broken across rows. ATM was not a consideration when the 9 by 90 frame structure of the Sonet STS-1 signal was decided, in fact the ATM cell size was not agreed upon until well after the first release of Sonet. The 9 row structure was chosen to best accomodate both US and CEPT digital hierarchies. 3 columns (27 bytes) carries a 1.544Mb signal and 4 columns (36 bytes) carries a 2.048 Mb signal. The 90 byte row in the STS-1 signal is sized such that the 9 by 90 frame carries the next major signal in the US hierarchy, the DS3 at 44.736Mb. The CCITT STM-1 signal, which is equivalent to 3 times the STS-1, carries the next major signal in the CEPT hierarchy at 139.264Mb (also DS4-NA in the US). Sonet was designed to reasonably accomodate the all existing digital hierarchies so that Sonet equipment could be introduced, globally, in an evolutionary, rather than revolutionary manner. ATM is expected to be the first global payload application of Sonet. The ATM mapping uses the H4 byte to contain an offset indicator to the next ATM header. This allows ATM cell alignment to be obtained within 125us of aquiring Sonet frame. This mechanism would have worked for any ATM cell size. I suspect that the availability of this mechanism was a factor in finally reaching agreement on the cell size; because the ability of Sonet to carry a particular size was not a factor. The bytes of the ATM cells arrive sequentially even though they are broken across rows and frames. An ATM demapper would simply use a gapped clock to clock the cell bytes into an ATM queue buffer. > 3. How do ATM and SONET interact? What gets switched where? Sonet is a circuit switching format, ATM is packet switching. The ATM mapping in Sonet permits ATM nodes to be interconnected using facilities that are shared with the existing digital network. Sonet makes extensive provisions for the maintenance of those facilities thus relieving ATM of the responsibility. > 4. What is the advantage of interleaving ``header'' information > throughout the frame, rather than concentrating it at the beginning of > a frame? Why are the payload pointers put a number of rows after the > beginning of the frame, so that I have to wait until I can determine > where the payload begins? I am not quite sure which "header" information you are refering to. The ATM cell headers are associated with each cell so that each can be routed independently. Remember that adjacent cells may belong to completely different virtual circuits. The Section, Line, Path overhead is distributed to minimize the gaps in the recovered payload clocks that must be smoothed out with buffering, at least for DS1 and DS3 signals. (Some would say that the overhead was not distibuted enough!) > 5. Why was the path overhead made part of the payload rather than the > header? The path overhead is not part of the payload, it belongs to the container that carries the payload end-to-end. Path overhead is responsible for ensuring that the payload is succesfully carried end-to-end across the circuit switched network. Conversely, Line overhead only monitors the signal node-to-node, where a node is a cross-connect or a multiplexer. In the case of ATM, a Sonet "Path" corresponds to a circuit between adjacent ATM nodes. > 6. What is the implementation status of SONET? Sonet equipment is available now from a number of manufacturers. John Ellson // ellson@ontap.stsusa.com // 602-395-5281 Siemens Transmission Systems, 8620 N 22nd Ave, Phoenix AZ 8502