Cisco 15327 - Manual (Page 343)

20-11
Ethernet Card Software Feature and Configuration Guide, R7.2
Chapter 20 POS on ONS Ethernet Cards
Ethernet Clocking Versus SONET/SDH Clocking
Figure 20-11 ML-Series Card Framing and Encapsulation Options
Ethernet Clocking Versus SONET/SDH Clocking
Ethernet clocking is asynchronous. IEEE 802.3 clock tolerance allows some links in a network to be as
much as 200 ppm (parts or bits per million) slower than other links (0.02%). A traffic stream sourced at
line rate on one link may traverse other links which are 0.02% slower. A fast source clock, or slow
intermediate clocks, may limit the end-to-end throughput to only 99.98% of the source link rate.
Traditionally, Ethernet is a shared media that is under utilized except for brief bursts which may combine
from multiple devices to exceed line-rate at an aggregation point. Due to this utilization model, the
asynchronous clocking of Ethernet has been acceptable. Some Service Providers accustomed to loss-less
TDM transport may find the 99.98% throughput guarantee of Ethernet surprising.
Clocking enhancements on ONS Ethernet cards, excluding the E-Series cards, ensure Ethernet transmit
rates that are at worst 50 ppm slower than the fastest compliant source clock, ensuring a worst-case
clocking loss of 50 ppm - a 99.995% throughput guarantee. In many cases, the card’s clock will be faster
than the source traffic clock, and line-rate traffic transport will have zero loss. Actual results will depend
on clock variation of the traffic source transmitter.
SONET/SDH Frame
SONET/SDH Payload Envelope Transport Overhead
LEX PPP Cisco HDLC
HDLC Framing Mode GFP-F Framing Mode
Flag Address Control Protocol Payload FCS
BCP
or
Encapsulation
GFP-F Frame Types
115448
GFP-Mapped
Ethernet (LEX)
GFP-PPP GFP-BCP GFP-Cisco HDLC
Core
Header
Payload
Header
Payload FCS