When the cell is carrying segmented network layer packets, the overhead is higher
because the ATM 48-byte data payload might not map very well to other packet sizes
(64-byte IP packets, for example). A typical ATM line needs almost 20 percent greater
bandwidth than Frame Relay to carry the same volume of network layer data.
Like Frame Relay, ATM is implemented using VCs that can be either PVC or SVC. With
ATM, the data is divided into small 53-byte cells before it is transmitted. The ATM cell
header contains a ?¬?eld called the virtual path identi?¬?er/virtual channel identi?¬?er (VPI/
VCI) that indicates to which VC an ATM cell belongs. At the physical layer, ATM can run
over a variety of physical media, including ?¬?ber optics using Synchronous Digital
Hierarchy (SONET)/Synchronous Digital Hierarchy (SDH) framing and coaxial cable
using DS3.
An ATM network includes ATM switches, which are responsible for cell forwarding.
The ATM switch receives the incoming cell from an ATM endpoint or another ATM switch.
The ATM switch then uses the incoming VPI/VCI to map to the outgoing interface and new
VPI/VCI to be used on the next link toward its destination. The ATM cell-switching process
is extremely fast and can be programmed in hardware.
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