deploying and managing ip over wdm networks - Index of

STM1/ATM
interface
requirements. For IP traffic, which is by essence connectionless, the unspecified
bit rate (UBR) traffic contract is mainly used within ATM networks. Nevertheless,
if IP applications require a particular QoS, especially for real-time constraints,
it is possible to use other ATM transfer capability (ATC) such as
constant bit rate (CBR) or variable bit rate–real time (VBR-rt). However, mapping
variable length IP-packets onto fixed length ATM cells imposes additional
overhead if fragmentation of the packet is necessary. This is sometimes called cell
tax. Differences in size may also raise the need to pad empty space in the cells,
which also gives additional overhead. One solution to prevent padding is to put
packets directly one after another, but this means a potential risk of losing two
consecutive packets in case of cell loss. IP over ATM can also be implemented
through the use of MPLS. In this situation, the PVCs are not set up by intervention
of the ATM management system, but dynamically by the MPLS protocol.
For MPLS based on ATM, the label can be stored in the ATM VCI.
5.2.2 IP Over ATM Directly on WDM
IP Over WDM Integration Mechanisms 75
OADM
OADM
32 λ WDM ring
STM16c
interfaces
ATM
switch
OADM
OADM
STM16c/ATM
interface
STM16c/ATM interface
IP
router
Figure 5.1 Example of IP over ATM over SDH encapsulation for transport over a WDM
network.
It is possible to have a scenario where ATM cells are transported directly on a
WDM channel. From an architectural point of view, this scenario is the same as
the previous one but the ATM cells are not encapsulated into SDH frames.