transmission convergence sublayer (ATM) (TCS)
The physical layer of the ATM layer model is divided into two sublayers: The physical sublayer( PM) and the transmission adaptation sublayer, Transmission Convergence( TC). Transmission Convergence arranges the ATM cells into the transmission frames of the respective transport medium. The tasks of this sublayer include cell coding and checksum generation, HEC generation(Header Error Control).
Transmission adaptation can basically be performed in three ways: by direct cell transmission, by cell adaptation to the transmission frame, and by transmission using Physical Layer Convergence Protocol( PLCP).
In direct cell adaptation, no adaptation of the cell format to the transmission frames of the respective transmission line is performed. The cells are converted bit by bit directly into electrical or optical signals and transmitted directly on the corresponding transmission medium. Only the cellinformation field is transmitted in encrypted form to ensure unambiguous identification of the beginning and end of the respective cell header information. DSS coding is used as the encoding method. The advantage of this method is the optimum utilization of the transmission bandwidth. The ATM cell is not embedded in another transmission frame. The disadvantages of direct cell transmission are the monitoring and management functions.
In cell-matching transmission, the cell contents are encoded as in direct cell transmission, but using distributed sample scrambler (DSS). After encoding the 48 bytes of data, the cells are transmitted over the user data area VC4 of the SDH frame. In addition, transmission adaptation to existing Plesiochronous Digital Hierarchy( PDH) transmission frames has also been defined for 1.544 Mbit/s, 2.048 Mbit/s, 6.312 Mbit/s, 34.368 Mbit/s, 97.728 Mbit/s, and 139.264 Mbit/s.
The third transmission option is PLCP adaptation, which has been defined for MAN according to IEEEE 802.6 on PDH lines. In this method, the PLCP transmission frames are treated directly like ATM cells and transmitted in the user data area of the respective PDH transmission frame.