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High-speed cell switching is at the core of ATM technology. This method is used by ATM to transmit information through a series of switch hops.
The ATM cell is 53 bytes long. The path a cell takes through a series of switch hops is determined by a virtual circuit. Call signaling procedures establish the ATM virtual circuits used to route information through the network.
Each switch in the path taken by a call creates a routing table during call setup that defines the source to destination ports and virtual circuit identifiers through each switch. After a call is established, all cell switching is based on the 5-byte header of the 53-byte ATM cell.
When a cell enters a switch, the switch extracts the 24-bit virtual circuit number contained in the cell header. The switch uses the routing table created during call setup to switch the cell to a destination port. The cell header is replaced with one containing the number of the virtual circuit for the next hop.
ATM virtual circuits deliver each cell in sequence at the destination. When a cell arrives at the destination, the adapter hardware creates a packet or byte stream. A network layer called the ATM Adapter Layer (AAL) performs the segmentation and reassembly of cells to create packet and byte streams.
There are five AAL types: AAL1 through AAL5. For example, AAL5 is a connection-oriented packet service, and AAL1 is a constant bit rate circuit emulation stream.
Software that interfaces with an ATM network is never actually aware of ATM cells. Instead, the adapters that interface with the ATM network perform the AAL functions to provide a packet or byte stream at the software interface. When using AAL5, ATM carries frames up to 64 KB and includes a CRC check. The ATM frame has no header and carries the CRC and control fields in a frame trailer. With AAL1, data has no frame structure and contains no CRC check (AAL1 cells have 8 bits of header information).
The type of AAL, along with the traffic type and bandwidth, is determined at call setup. The ATM end point determines specific data qualities for the switching network. The end station must create cells by slicing packets or byte streams and must pace the cells on to the media. Traffic contract negotiation during circuit creation determines data pacing. There are three basic data pacing contract types:
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