7
Preparing ATM Adapters
The Compaq Tru64UNIX operatingsystem supports Asynchronous Transfer
Mode (ATM). TruClusterServer supports the use of LAN emulation over
ATM for client access.
This chapter provides an ATM overview,an example TruCluster Server
cluster using ATM, an ATM adapter installation procedure, and information
about verifying proper installation of fiber optic cables. See the Tru64 UNIX
Asynchronous TransferMode manual for information on configuring the
ATM software.

7.1 ATM Overview

In synchronous transfer methods, time-division multiplexing (TDM)
techniques are used to divide the bandwidth into fixed-size channels
dedicated to particular connections. If a system has nothing to transmit
when its time slot comes up, that time slot is wasted. Also, if the system has
lots of information to transmit, the system can only transmit when its turn
comes up, even if other time slots are empty.
ATMeliminates the inefficiencies of TDM technology by sharing network
bandwidth among multiple logical connections. Instead of dividing the
bandwidth into fixed-size channels dedicated to particular connections, ATM
uses the entire bandwidth to transmit a steady stream of fixed-size (53-byte)
cells. Each cell includes a 5-byte header containing an address to identify
the cell with a particular logical connection.
If a connection needs more bandwidth, it is allocated more cells. When a
connection is idle, it uses no cells and consumesno bandwidth. This feature
makes ATM the ideal technology for transferring voice, video, and data
through private networks and across public networks.
ATMis a connection-oriented, cell-switching and multiplexing technology.
Cells transit ATM networks by passing through ATMswitches, which
analyze information in the header to switch the cell to the output interface
that connects the cell to the next appropriate switch as the cell proceeds to
its destination.
The ATMswitch acts as a hub in the ATM network. All devices are attached
to an ATM switch, either directly or indirectly.
Preparing ATM Adapters 7–1