The transport protocol used on the telephone network is ATM . The DSL modem encapsulates PPP packets inside ATM cells and sends them over the WAN . There are several encapsulation methods.
12.
The process is conceptually simple : an incoming packet from another protocol to be transmitted across the ATM network is chopped up into segments that fit into 48-byte chunks carried as ATM cell payloads.
13.
The chassis features the X-Cell architecture, a fully-distributed, non-blocking ATM cell backplane, so it can be used as a small ATM switch, with up to 16 ports of ATM OC-3 or 48 ports of ATM25.
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The ATM cell transfer layer is classified as " machine-to-machine " because the layering principle applies from one machine to the next ( e . g ., between a host and a switch or between two switches ).
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TDMoIP allows concatenation of any number of AAL1 cells into a packet ( note that these are AAL1 cells and not ATM cells, i . e . they do not include the five-byte " cell tax " ).
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However more realistically, the typical DSL / ATM overhead is either zero or 53 bytes as in case ( ii ) where the ATM last-cell payload maximum of 48-8 bytes is exceeded, so requiring an additional 53 byte ATM cell.
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The AAL5 on the receiving side knows how many cells comprise a packet because the sending AAL5 uses the low-order bit of the " PAYLOAD TYPE " field of the ATM cell header to mark the final cell in a packet.
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This replacement of the process with an RTC is possible because ATM cells have a fixed length ( 53 bytes ), thus " T " is always a constant, and the calculation of the new bucket level ( or of " TAT " ) does not involve any multiplication or division.
19.
"' Packet over SONET / SDH "', abbreviated "'POS "', is a communications protocol for transmitting ATM cells over SONET / SDH . However, scrambling can optionally be disabled to allow a node to be compatible with another node that uses the now obsoleted RFC 1619 version of Packet over SONET / SDH which lacks the scrambler.
20.
The use of Asynchronous Transfer Mode ( ATM ) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM . This adaptation layer defines how to segment and reassemble higher-layer packets into ATM cells, and how to handle various transmission aspects in the ATM layer.
