Cells and virtual connections
ATM is a legacy transport used in older carrier and broadband networks. Each cell is 53 bytes, with a five-byte header and 48 bytes of payload. Switches forward cells using a virtual path identifier and virtual channel identifier (VPI/VCI). That pair identifies a connection on a link and can be translated as cells cross a switch.
ATM Adaptation Layer 5 (AAL5) lets peers transport variable-length data over those fixed-size cells. The sender adds padding and a trailer to form an AAL5 unit and splits it across cell payloads. The receiver reassembles the unit and checks the trailer before delivering its data.
Try it in a lab
- On supported legacy ATM equipment or a lab trace, identify the VPI/VCI for a test connection and distinguish the cell header from the payload.
- Follow one AAL5 unit that spans multiple cells. Compare the fixed cell size with the length of the reassembled unit. A change in the VPI/VCI at a switch need not change the data delivered to the endpoint.