4-13
Layer 3 Switching Software Feature and Configuration Guide
78-6235-04, Cisco IOS Release 12.0(10)W5(18)
Chapter4 Configuring Interfaces About the POS OC-12c Uplink Interface
Because SONET/SDH (Synchronous Digital Hierarchy) is by definitiona point-to-point circuit, PPP is
well suited for use over SONET links. The octet stream is mapped into the SONET/SDH synchronous
payload envelope(SPE) in accordance with RFC 2615, “PPP over SONET/SDH,” and RFC 2615, “PPP
in HDLC-like Framing.” Octet boundaries are aligned with the SPE octet boundaries, and the PPP
frames are located by row within the SPE payload. Because frames are variable in length, the frames
can cross SPE boundaries. Using this scheme, multiprotocol data can be encapsulated and transported
directly into SONET frames without relying on ATMto provide Layer 2 capability (for example, in IP
over ATM over SONET).
About the POS OC-12c Uplink Interface
POS technology is ideally suited for networks that are builtfor providing Internet or IP data. It provides
superior bandwidth utilization and efficiencyover other transport methods. For expensive WAN links,
POS can provide as much as 25 to 30 percent higher throughput than ATM-based networks.
Transporting frames directly into the SONET/SDH payload eliminates the overhead required in ATM
cell header, IP over ATM encapsulation, and segmentation and reassembly (SAR) functionality.
Figure 4-4 shows a typical application of the POS OC-12c uplink interface module in an enterprise
setting. Here the enterprise backbone is comprised of POS links among Catalyst 8540 campus switch
routers in each building.
Figure 4-4 POS for Enterprise Backbone Connectivity
POS
OC-12c POS
OC-12c POS
OC-12c POS
OC-12c POS
30746