ATM LAYERING AND B-ISDN
SAR-PDU format for AAL-1. This figure shows the content of a cell that contains an
SAR-PDU. [From CCITT Rec. I.363, Figure 1/I.363, page 3 (Ref. 10).]
. Initially, in ITU-T Rec I.363 (Ref. 10), there were two separate AALs, one for
connection-oriented variable bit rate data services (AAL-3) and one for connectionless
service. As the specifications evolved, the same procedures turned out to be necessary
for both of these services, and the specifications were merged to become the AAL-3/4
standard. AAL-3/4 is used for ATM transport of SMDS, CBDS (connectionless broadband
data services, an ETSI initiative), IP (Internet protocol) and frame relay.
AAL-3/4 has been designed to take variable-length frames/packets and segment them
into cells. The segmentation is done in a way that protects the transmitted data from
corruption if cells are lost or mis-sequenced. Figure 20.10 shows the cell format of an
AAL-3/4 cell. These types of cells have only a 44-octet payload, and additional overhead
fields are added to the header and trailer.
These carry, for example, the BOM, COM, and
EOM indicators (carried in segment type [ST]) as well as a MID (multiplexing identifier)
so that the original message, as set up in the convergence sublayer PDU (CS PDU),
SAR-PDU format for AAL-3/4. [From ITU-T Rec. I.363, page 13, Figure 6/I.363 (Ref. 10).]
Trailer consists of overhead fields added to the end of a data frame or cell. A typical trailer is the CRC parity
field appended at the end of a frame.