System information

Placing Cells on a Physical Transport Medium
Book Title
21-440
Placing Cells on a Physical Transport Medium
After the data is packaged into 53-byte ATM cells, the cells are transferred to the physical layer,
where they are placed on a physical transport medium, such as fiber optic cable or coaxial cable. The
process of placing cells on the physical medium takes place in two sublayers: the physical medium
dependent (PMD) sublayer and the transmission convergence (TC) sublayer.
Each PMD is specific to a particular physical medium and includes definitions of proper cabling as
well as bit timing. The TC sublayer generates and receives transmission frames and performs all
overhead functions associated with the transmission frame. The TC sublayer performs a
convergence function by receiving a bit stream from the PMD and extracting cells.
Although PMD operation depends on the physical medium, the following TC functions remain
common to all physical layers:
• Cell delineation—Extraction of cells from the bit stream received from the PMD
• Cell rate decoupling—Adaptation of the speed of the ATM layer cell stream to the rate of the
physical interface
• Header error control (HEC) generation and checking—Performed when the TC sublayer checks
where each received cell starts and ends by calculating the HEC for that cell
• Various operation and maintenance (OAM) functions—ATM Forum specification for cells used
to monitor virtual circuits. OAM cells provide a virtual circuit-level loopback in which a router
responds to the cells, demonstrating that the circuit is up and the router is operational.