ATM Cells

ATM Cells

ATM uses fixed-size cells, consisting of a 5-octet header and a 48-octet information
field. There are several advantages to the use of small, fixed-size cells. First, the use
of small cells may reduce queuing delay for a high-priority cell, because it waits less
if it arrives slightly behind a lower-priority cell that has gained access to a resource
(e.g., the transmitter). Second, it appears that fixed-size cells can be switched more
efficiently, which is important for the very high data rates of ATM. With fixed-size
cells, it is easier to implement the switching mechanism in hardware.
Header Format Figure 3.11a shows the header format at the user-network interface.
Figure 3.11b shows the cell header format internal to the network. Internal to
the network, the Generic Flow Control field, which performs end-to-end functions, is
not retained. Instead, the Virtual Path Identifier field is expanded from 8 to 12 bits.