 quality). To fulfill such wide requirements, ATM was designed for transporting small fixed length messages (or "packages") of 48 octets of user data and a 5-octet header (compared to the varia- ble and large length of most other types of switched packages). This message type was called cell, and the ATM switching technique is therefore called cell switching. To be able to support media streams, an ATM network performs almost no nor- mal link layer functions, such as flow control, error detection, and resending of 6.4 ATM Cell Switching packages. Such functions (when needed), were to be handled end-to-end by ATM adaptation layer (AAL) functions in ATM terminals. ATM also takes advantage of the development in physical transmission technol- logies (optical fibers and SDH cross connection), and can therefore offer considera- bly higher transfer rates than PSTN/ISDN and X.25. The maximum transfer rates that are available for end user services (such as data, voice, video, and multimedia services) depend on the type of core network and access connections, and the type of media encoding. An ATM switching stratum that relies on SDH for physical transport in accesses and in core can switch user data up to over 500 Mbps. Con- trary to ISDN, the actual transfer rate for an ATM service need not be selected from a predefined set of levels (64 kbps, 128 kbps, etc.), but could be negotiated for each ATM connection on a continuous scale. However, in order to simplify the realiza- tion of ATM switching strata, the standard still defines discrete levels of transfer rates. When the ITU-T standardized the ATM switching stratum about 10 years ago, it was obvious that the raw cell services provided by this stratum had to be adapted to different types of media service needs, including connectivity services for control signaling. A number of AALs were therefore defined that could be placed in between the ATM switching stratum and using strata (including service controlling strata) and offer a number of different connection types. Thus, 