Understanding the network.pdf - Back to Home

Because both the VPI and VCI are used in combination to make up the
LCID, two types of forwarding can be performed. This circumstance
has resulted in two different types of ATM switches: VP switches and
VC switches. VP switches make forwarding decisions based only on
the VPI value. VC switches alternatively forward cells based on both
the VPI and VCI values. VP switches forward cells at a faster rate than
VC switches, because only the VPI is considered when making
forwarding decisions. Most public ATM carrier-class switches are
VP-type switches, because the granularity afforded by VC-type
switches is not required to transport cells across a carrier's ATM cloud.
However, the additional examination of the VCI header is required at
both ends of the public/private transit path and with most private ATM
backbone implementations. Consequently, most private-class ATM
switches are VC-type switches.
The label swapping approach works well for all packet- and
cell-switching connection technologies because of their
connection-oriented natures. As for the VPI and VCI, values are
mostly arbitrary and only relevant to the local switch. Therefore, they
can be either statically or dynamically allocated using signaling when
ATM PVC or SVC connections are established.
LANE
With its fast operating rate and its optimization for carrying
multimedia data (specifically, voice and streaming video), ATM is very
attractive as a LAN backbone and high-speed server connection
technology. It was for this kind of application that LANE was
developed. Today, with Gigabit Ethernet and switched port trunking,
the use of ATM as a LAN backbone solution is not nearly as attractive
as it was a few years ago. Although ATM's popularity as a LAN
backbone technology is fading, it is growing as a WAN backbone
technology. The bandwidth limitations of Frame Relay are being
realized with more network-intensive applications.
Before we look at how LANE works, a brief description of why it is
needed is required. The major operational incompatibility between
ATM and traditional LAN protocols is not specific to ATM only. This
incompatibility exists with all Layer 2 WAN transmission protocols,
and it is the inability to handle Layer 3 broadcast and multicast
messages. This is not because of some operational limitation, but
mainly because there are drastically different operational paradigms
that govern the design of these protocols.