ATM Physical Layer

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The ATM Physical Layer (PHY) is divided into two sublayers. PHY corresponds to the OSI Physical Layer. The upper sublayer of the PHY is called the Transmission Convergence (TC) sublayer, which is primarily responsible for common Physical Layer functions. The term “convergence” refers to the fact that although ATM links can vary from DS-1/E-1 to SONET/SDH to unshielded twisted pair, they all transport ATM cells when used in this manner. That is, they all converge on ATM cell transport.

Below are functions of the TC sublayer.

  • Cell rate decoupling adapts the rate of cell transmission to the actual rate of the transmission facility by inserting idle cells when necessary. For example, consider a bursty LAN application running over a 100 Mbps Ethernet connected to an ATM switch with an OC-3c/STM-1 interface running at 155 Mbps. Even at peak rate, the ATM switch must generate at least 55 Mbps of idle cells to match the cell rate to the link speed. This function was originally defined at the Physical Layer by the ITU-T, but it is supported at the ATM Layer by the ATM Forum UNI.
  • Cell delineation enables the receiver to determine cell boundaries. This function was originally defined at the Physical Layer by the ITU-T, but it is supported at the ATM Layer by the ATM Forum UNI.
  • Transmission frame generation adapts the cell flow according to the payload structure of the transmission system (e.g., maps ATM cells to the SONET/SDH or DS-1/E-1 frame structure). This is necessary because most transports are framed. The transmission frame structure is distinct from OSI RM Layer 2 “frame” PDUs, which means that although a DS-1/E-1 frame might contain ATM cells instead of voice samples, an ATM device must still generate a DS-1/E-1 frame. Without this transport frame structure, DS-1/E-1 repeaters and cross-connects cannot handle the bit stream and would require modification to understand ATM cells directly.

The lower sublayer of the PHY is called the Physical Medium (PM) sublayer. It is primarily responsible for medium-dependent and transmission-system-dependent functions such as connector types. The PM sublayer is responsible for ensuring that the bit timing, transmitted signals, line code, electrical and optical characteristics, and frame format match the specific transmission system.