reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Transport Technologies tains cell sequence and synchronization information that is required to provision constant bit rate, fully sequenced service. AAL1 provides cir- cuit emulation service without dedicating a physical circuit, which explains the need for an end-to-end timing relationship between the transmitter and the receiver. AAL5, on the other hand, is designed to provide both Class C and D services, and while it was originally proposed as a transport scheme for connection-oriented data services, it turns out to be more efficient than AAL3/4 and accommodates connectionless services quite well. To guard against the possibility of errors, AAL5 has an eight-octet trailer appended to the user data, which includes a variable-size pad field used to align the payload on 48-octet boundaries, a two-octet control field that is currently unused, a two-octet length field that indicates the number of octets in the user data, and, finally, a four-octet CRC that can check the integrity of the entire payload. AAL5 is often referred to as the "simple and easy adaptation layer," or SEAL, and it may find an ideal application for itself in the burgeoning Internet arena. Recent studies indicate that TCP/IP transmissions produce comparatively large num- bers of small packets that tend to be around 48 octets long. That being the case, AAL5 could well transport the bulk of them in its user data field. Furthermore, the maximum size of the user data field is 65,536 octets; coincidentally, the same size as an IP packet. ATM Forum Service Classes The ATM Forum looks at service definitions slightly differently than the ITU-T does (see Figure 7-11). Instead of the A-B-C-D services, the ATM Service Descriptors Delay Bandwidth Feedback Figure 7-11 ATM Forum PCR, CDVT service VBR-RT PCR, CDVT, definitions SCR, MBS PCR, CDVT, SCR, MBS PCR, CDVT PCR, CDVT, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.c