Procedure used in T1 transmission facilities in which each of the 24 T1 subchannels devotes one bit of every sixth frame to the carrying of supervisory signaling information. Also called 24th channel signaling. | ÙSC rSTEMS CCNA Glossary A Term Definition A B bit signaling Procedure used in T1 transmission facilities in which each of the 24 T1 subchannels devotes one bit of every sixth frame to the carrying of supervisory signaling information. Also called 24th channel signaling. AAL ATM adaptation layer. Service-dependent sublayer of the data link layer. The AAL accepts data from different applications and presents it to the ATM layer in the form of 48-byte ATM payload segments. AALs consist of two sublayers CS and SAR. AALs differ on the basis of the source-destination timing used whether they use CBR or VBR and whether they are used for connection-oriented or connectionless mode data transfer. At present the four types of AAL recommended by the ITU-T are AAL1 AAL2 AAL3 4 and AAL5. See AAL1 AAL2 AAL3 4 AAL5 CS and SAR. See also ATM and ATM layer. AAL1 ATM adaptation layer 1. One of four AALs recommended by the ITU-T. AAL1 is used for connection-oriented delay-sensitive services requiring constant bit rates such as uncompressed video and other isochronous traffic. See also AAL. AAL2 ATM adaptation layer 2. One of four AALs recommended by the ITU-T. AAL2 is used for connection-oriented services that support a variable bit rate such as some isochronous video and voice traffic. See also AAL. AAL3 4 ATM adaptation layer 3 4. One of four AALs merged from two initially distinct adaptation layers recommended by the ITU-T. AAL3 4 supports both connectionless and connection-oriented links but is primarily used for the transmission of SMDS packets over ATM networks. See also AAL. AAL5 ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5 supports connection-oriented VBR services and is used predominantly for the transfer of classical IP over ATM and LANE traffic. AAL5 uses SEAL and is the least complex of the current AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and .
