Overview of ATM

ATM, or Asynchronous Transfer Mode, is a widely deployed networking technology. ATM has many applications, including both wide area and campus area backbones. It has a lot of flexibility because it is designed for both constant bit rates services such as voice and variable bit rate services such as data.

ATM devices are connected together via point-to-point links. These links can consist of different media and can operate at different transmission speeds. The most common transport for ATM is a SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy) network. Both network types are very similar. They use OC (Optical Carrier) standards. OC standards specify a range of properties and speeds, starting at OC1 at 51.84 MBps ranging up to OC192 at 9953.28 MBps. The most common are OC3 and OC12.

ATM networks are connection-oriented. They use virtual circuits to transfer the data. The end-to-end virtual connection is made from a Virtual Channel Connection (VCC) and a Virtual Path Connection (VPC). A VCC or VPC is really made up of multiple links between the end points of the connection. The individual links are called Virtual Channel Links (VCL) or Virtual Path Links (VPL). Each VCL or VPL has a unique number assigned: the Virtual Channel Identifier or Virtual Path Identifier (VCI/VPI). Each link along the path may well have different VCI/VPI numbers. The switches in the path make the end-to-end connection by routing cells between the links that make up the path.

ATM virtual circuits come in two basic flavors:

• Permanent Virtual Circuits (PVC)

• Switched Virtual Circuits (SCV)

PVCs are built by manually configuring each link between each ATM switch in the end-to-end path. SVCs are built using a signaling protocol between the ATM switches.

The data in an ATM circuit rides in 48-byte units. There is a 5-byte header that contains the VPI/VCI and other information. The 48-byte payload and the 5-byte header together are known as a cell. The way data gets from the higher layers down to the ATM cell layer is through an ATM adaptation layer (AAL). There are different adaptation layers for different types of data. AAL1 is used for constant bit rate services such as voice. AAL3/4 and AAL5 are used for data. AAL5 is a simplified version of AAL3/4 and is virtually the only scheme used. This chapter concentrates on data for the ATM and the AAL5 layer.

A data payload is presented to the ATM AAL5 layer for transport on an ATM link. The convergence sub-layer appends some padding and a trailer so the payload is now an integer multiple of 48 bytes. The trailer contains the length of the frame and a CRC. The data, padding, and trailer form the Common Part Convergence Sub-layer (CPCS) protocol data unit (PDU). The CPCS PDU passes through the AAL5 layer to the Segmentation and Reassembly (SAR) sub-layer, where it is segmented into 48-byte units, which are the SAR PDUs. Now, a 5-byte header is placed at the front of each SAR PDU and this unit makes up the 53-byte ATM cell (see Figure 14-1 for details).

Figure 14-1. AAL5 Cell Preparation