2-8 Cisco BPX 8600 Series Reference
Functional Description
Figure 2-7 ATM Cell Format
ATM Cell Headers
There are two basic header types defined by the standards committees, a UNI header and a NNI
header; both are quite similar. Cisco has expanded on these header types to provide additional
features beyond those proposed for basic ATM service. Usage of each of the various cell header
types is described as follows:
• The UNI header (see Figure 2-8) must be specified for each User-to-Network Interface. A UNI
is any interface between a user device, such as an ATM router, and an ATM network.
• The NNI header (see Figure 2-9) must be specified for each Network-to-Network Interface. This
is used, for example, at the interface between a user’s private ATM network and a service
provider’s public ATM network.
• The STI header (see Figure 2-10) is an extension of these two header types and is used between
Cisco switching nodes to provide advanced network features, including ForeSight, that improve
performance, efficiency, and congestion control.
H81
