ATM Cell and Physical Layer Questions
What is the basic ATM cell format?ATM cells are fixed length, with the following layout:
The payload layout is determined by the adaption layer type. More information on adaption layer can be found in the AAL entry of the Cell Relay dictionary and the ATM FAQ topic Adaption Layers and Interoperability. The header layout is as follows:
GFC - generic flow control bits, present only in UNI cells VPI - virtual path identifier, NNI uses GFC bits for 12 total bits for VPI VCI - virtual circuit identifier PT - payload type information CLP - cell loss priority HEC - header error control, checksum for header information only Why are ATM cells 53 bytes?ATM cells are standardized at 53 bytes because it seemed like a good idea at the time! As it turns out, during the standardization process a conflict arose within the CCITT as to the payload size within an ATM cell. The US wanted 64 byte payloads because it was felt optimal for US networks. The Europeans and Japanese wanted 32 payloads because it was optimal for them. In the end 48 bytes was chosen as a compromise. So 48 bytes payload plus 5 bytes header is 53 bytes total.The two positions were not chosen for similar applications however. US proposed 64 bytes taking into consideration bandwidth utilization for data networks and efficient memory transfer (length of payload should be a power of 2 or at least a multiple of 4). 64 bytes fit both requirements. Europe proposed 32 bytes taking voice applications into consideration. At cell sizes >= 152, there is a talker echo problem. Cell sizes between 32-152 result in listener echo. Cell sizes <= 32 overcome both problems, under ideal conditions. For several years the *near* consensus was 64 octets. France wanted 32 because they figured with 4 ms. cell fill time, they could *just* scrape by from one end of the country to the other without echo cancellers, while in the US we need em 'anyway. So France held its breath, took a few smaller European countries with them, and demanded that 64 be lowered. Hence the "split the difference" 48 size. This was at a CCITT SG XVIII meeting ca. 1989. CCITT chose 48 bytes as a compromise. As far as the header goes, 10% of payload was perceived as an upper bound on the acceptable overhead, so 5 bytes was chosen. Are ATM cells delivered in order?Yes. The ATM standards specify that all ATM cells will be delivered in order. Any switch and adaptation equipment design must take this into consideration.How does a receiver know where the boundaries between cells are?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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