Cell Relay Archive[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index][Thread Index][Author Index][Subject Index] Re: SMDS & ATM
Hi! In the original specs, AAL3 and ALL4 were defined for Service Classes C (connection-oriented, variable bit rate, time-insensitive; i.e., c.o. data) and D ((connectionless, variable bit rate, time-insensitive; i.e., c.l. data), respectively. At some point, someone looked closely at the definitions of AAL 3 and 4, and realized that they differed by about one paragraph (literally). The two definitions were merged and we get AAL3/4. This is the late-1980s, the same time frame as the SMDS definition. So, SMDS is built on IEEE 802.6 DQDB, which waits about a year so that it can be brought into alignment with the emerging ATM specifications. So, SMDS/DQDB is AAL3/4 based. *Later*, AAL5 was introduced, as the Simple and Efficient Adaptation Layer (SEAL). AAL3/4 has indeed fallen out of favor because it accomplished too much!! AAL5 is simpler and more efficient. But that's why SMDS isn't based on AAL5; purely history. /kess At 09:58 AM 1/22/99 -0500, you wrote: >Ok, so what is aal3/4 used for ? I know aal1 is CBR, and aal5 is UBR, aal2 is VBR ? >So I didn't think aal3/4 were used that much if at all... > > > ---- On Jan 21 "Gary Kessler" <g.kessler@hill.com> wrote: >> Hi! >> >> SMDS has the same cell format as ATM's AAL3/4. In both, there is a 53-byte cell, 5 header plus 48 payload. Within the payload, the first 2 and last 2 bytes are header and trailer, respectively, yielding 44 bytes of effective payload/cell. >> >> Think of SMDS as connectionless-ATM. >> >> Later! >> /kess >> >> At 02:40 PM 1/21/99 -0800, mjy wrote: >> >Someone told me that SMDS is a 'great' WAN transport as the cells can >> >carry ATM traffic w/o segmentation, as SMDS cells are 53 bytes and so >> >are ATM's. >> >However, doesn't SMDS have 44 byte data payload whereas ATM has 48 ? >> >Seems like we would segment in one direction at least... >> > >> >Please reply via email too...so many newsgroups, so little time. >> > >> > >> > > >----------------------------------------------- >FREE! The World's Best Email Address @email.com >Reserve your name now at http://www.email.com > ================================================================= Gary Kessler +1 802-655-0940 (main office) Hill Associates +1 802-655-8659 (direct line) 17 Roosevelt Highway +1 802-655-7974 (fax) Colchester, VT 05446 kumquat@hill.com http://www.hill.com http://www.together.net/~kessler/gck PGP Public Key: http://www.hill.com/PGP/gck_pubkey.html |
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