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Architecture to Implement the Reassembly Function for Asynchronous Transfer Mode

IP.com Disclosure Number: IPCOM000110633D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 4 page(s) / 159K

Publishing Venue

IBM

Related People

Patel, BV: AUTHOR [+3]

Abstract

Disclosed is an apparatus to perform the reassembly of incoming ATM (Asynchronous Transfer Mode) cells into AAL PDUs (ATM Adaptation Layer Protocol Data Units), conforming to SAR (Segmentation And Reassembly) sublayer specification defined by the ATM standards [*].

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 47% of the total text.

Architecture to Implement the Reassembly Function for Asynchronous Transfer Mode

       Disclosed is an apparatus to perform the reassembly of
incoming ATM (Asynchronous Transfer Mode) cells into AAL PDUs (ATM
Adaptation Layer Protocol Data Units), conforming to SAR
(Segmentation And Reassembly) sublayer specification defined by the
ATM standards [*].

      A format for segmentation and reassembly of AAL messages into
ATM cells is defined in the SAR sublayer of ATM, as shown in Fig. 1.
According to this format, in addition to the ATM header, each ATM
cell carries 44 bytes of data and 2 bytes of segment header and
segment trailer each.  There are four possible segment types
indicating which part of the AAL frame is being carried in a segment:
(1) Beginning of Message (BOM), (2) Continuation of Message (COM),
(3) End of Message (EOM), and (4) Single Segment Message (SSM).  The
sequence number ranges between 0 and 15 and cycles through to number
consecutive segments of a packet; the value is incremented relative
to the previous value for a particular VCI/MID value.  The MID
(Multiplexing ID) is the identifier used in combination with the VCI
(Virtual Circuit ID) for reassembly of the packets.  For
connection-oriented service (e.g., AAL 3), each connection itself
represents segments arriving in the order of transmission, and,
therefore, all cells transferred using a VCI must arrive in order.
Since cells are not multiplexed for connection-oriented service, only
the VCI is used for connection and reassembly.  For connection less
datagram service (e.g., AAL 4), the cells from many sources may be
multiplexed into one connection.  Therefore, a MID value is used to
demultiplex them  during reassembly.  Both the VCI and MID values
have to be used for reassembly of messages for the datagram service.

      The block diagram of the reassembly apparatus is shown in Fig.
2.  Upon reception of a cell, it is screened for transmission errors
by checking the HEC (Header Error Check) and CRC.  The cell is
discarded in case of an error.  A lookup table, the Reassembly Lookup
Table (RLT), is used to store the destination address for the next
arriving cell and its expected sequence number to perform reassembly.
For every cell received, the target address for the cell payload in
the packet  memory (PM) and the expected sequence number of the cell
are read from the RLT using the VCI/MID.
o   connection.address = RLT[VCI/MID].address
o   connection.sequence-no = RLT[VCI/MID].sequence-no

      A small payload FIFO buffer is used to hold the ATM payload
while the RLT lookup is performed.  After the RLT lookup, the
following actions are taken based on the segment type (listed above).
     SSM.  For a single segment, the RLT is not used because there is
no reassembly required.  The payload is directly transferred to the
PM.
     BOM.  When the first cell of a new message is received,
successful reassembly of the previous mes...