Information Technology Reference
In-Depth Information
duced at 25, 52, 100, and 155 Mbps. The 25 and 52 Mbps speeds are intended to
operate over Category 3 twisted-pair cabling. The current ATM UNI-155 standard
requires a full Category 5 link for operation.
You should be aware that the ATM speeds shown are rounded off from the
actual data rates. For example, UNI-155 and OC3 actually run at 155.52 Mbps. To
get the exact data rate, multiply 51.84 Mbps by the multiple. For example, OC12
would be 51.84 Mbps
12, or 622.08 Mbps.
Not to be outdone by fiber optics, copper-based 622 Mbps technologies are
under development at this very moment. Watch your mailbox for details! Copper
proponents claim that 1 GHz bandwidths are possible for twisted pair, although it
is doubtful that this could be achieved with the present complement of connectors
and cabling.
Unlike the standard LAN topologies, ATM is a switched protocol. It is not
really suitable for standard bus or shared star (repeater-coupled) topologies. It does
not imply a collision detection or a token passing mechanism. When used for a
LAN, it is switched, with the physical switch being at the hub of a star of worksta-
tion legs. Often, LAN protocols such as Ethernet or Token-Ring are transported
over ATM links via a method of LAN emulation.
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