Information Technology Reference
In-Depth Information
concentrating node is customarily on the actual ring (and is dual-attach), and bene-
fits from its fault-tolerant design. The single-attach node lacks the loop back path
to its concentrator node, and so is normally used within the same building where
the fiber cuts would be rare.
The FDDI topology is an American National Standards Institute (ANSI) stan-
dard. The fiber is standard 62.5/125 mm fiber terminated in the unique duplex
FDDI connector. Both multimode and single-mode fiber are supported, although the
interface module is different, since single mode requires a laser-diode transmitter. An
FDDI single-attach connection could be supported on a TIA-568-C fiber-optic link,
but an adapter cable would have to be used to connect to the 568-SC connectors in
the workstation outlet.
Because FDDI is neither an Ethernet nor a Token-Ring protocol, those and
other protocols are usually encapsulated into an FDDI packet and decoded at the
receiving station. This method simply uses the FDDI link as a transport mechanism
that frees one from the point-to-point nature of most WAN links.
ATM
Asynchronous transmission mode (ATM) is a very high speed communications pro-
tocol that does not really imply a particular LAN topology. As a matter of fact, the
ATM protocol can be used over long distances in what is often called the wide area
network (WAN). The ATM protocol specifies a 53-byte “cell” that is somewhat
analogous to a packet or frame of conventional variable-length MAC-layer proto-
cols. Unlike those protocols, the ATM cell does not always contain source and des-
tination addresses, nor does it contain the higher-level addressing and control of
longer packets.
ATM is actually a very abbreviated point-to-point, switched protocol. An ATM
message typically sets up a virtual connection between a source and destination,
transmits cell-size chunks of the message, and ends the connection. The cells may go
through many switching points before reaching the destination. The protocol allows
cells to be received at the destination out of order and reordered before being pre-
sented to the receiving application.
ATM can be transported at many rates and over a wide variety of media. ATM
protocol is implemented over copper and fiber at rates from about 25 Mbps to over
2 Gbps. Because the protocol is not dependent on a particular connection scheme,
many ATM-based technologies have emerged under the ATM umbrella. They are not
the same, and should be treated as separate topologies that share a common link basis.
Fiber implementations of ATM often run at SONETt physical parameters, such
as 155 Mbps OC3 and 2.4 Gbps OC48. Copper implementations have been intro-
