GRF IP switch offer IP over SONET i/f, part II
Virtually disconnected
The cell tax is not the only reason for eliminating ATM
from the wide-area equation, however. ATM is by
definition a connection-oriented protocol, requiring virtual
circuits to be set up for each flow.
IP by its nature is connectionless and doesn't require
multitudes of virtual circuits, as does ATM. Setting up
these virtual circuits takes huge amounts of processing
power and adds to the latency of ATM transmissions.
There is also a lot of redundancy in running IP over ATM.
The most common transport for IP is TCP, a control
mechanism that has a provision for reassembling
out-of-order packets. ATM also has this function, which
proves useless when combined with TCP/IP.
Another concern is the anemic 53-byte cell size in ATM.
The minimum 64-byte IP data packet must be split into
two ATM cells for transport. If either of these cells is
dropped, the entire IP packet must be re-sent.
The idea of removing ATM from a network transmission
scheme might sound appealing, but the migration to IP
over SONET is difficult, because the management
infrastructure required for SONET is completely different
from ATM's tools.
The traditional data network can be traced from end to
end with time-tested tools, through the different
topologies of a switched Ethernet network to an ATM
network running LAN emulation services. Once SONET is
introduced, the landscape changes completely, and
well-known tools no longer suffice.
In the SONET world, such things as SNMP are
nonexistent. Instead, most carriers use the TL-1 protocol,
with a planned migration to the Common Management
Information Service Element. |
