Telindus 1031, User And Reference Manual

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Telindus 1031 Router Chapter 6
User manual Configuring the WAN encapsulation protocols
87
6.4.1 Introducing ATM
What is ATM?
ATM is a cell-switching and multiplexing technology that combines the benefits of circuit switching (guar-
anteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency
for intermittent traffic). It provides scalable bandwidth. Because of its asynchronous nature, ATM is more
efficient than synchronous technologies, such as time-division multiplexing (TDM).
With TDM, each user is assigned to a time slot, and no other station can send in that time slot. If a station
has much data to send, it can send only when its time slot comes up, even if all other time slots are
empty. However, if a station has nothing to transmit when its time slot comes up, the time slot is sent
empty and is wasted. Because ATM is asynchronous, time slots are available on demand with informa-
tion identifying the source of the transmission contained in the header of each ATM cell.
What is VPI and VCI?
ATM networks are fundamentally connection-oriented, which means that a virtual channel must be set
up across the ATM network prior to any data transfer. (A virtual channel is roughly equivalent to a Per-
manent Virtual Circuit or PVC.)
Two types of ATM connections exist:
• virtual paths, which are identified by Virtual Path Identifiers (VPIs).
• virtual channels, which are identified by the combination of a VPI and a Virtual Channel Identifier
(VCI).
A virtual path is a bundle of virtual channels, all of which are switched transparently across the ATM net-
work based on the common VPI. All VPIs and VCIs, however, have only local significance across a par-
ticular link and are remapped, as appropriate, at each switch.
How does ATM switching work?
The basic operation of an ATM switch is straightforward:
The cell is received across a link on a known VCI or VPI value. The switch looks up the connection value
in a local translation table to determine the outgoing port (or ports) of the connection and the new VPI/
VCI value of the connection on that link. The switch then retransmits the cell on that outgoing link with
the appropriate connection identifiers. Because all VCIs and VPIs have only local significance across a
particular link, these values are remapped, as necessary, at each switch.