Ethernet Clocking Versus Sonet/Sdh Clocking - Cisco ONS 15454 SDH Configuration Manual

Ethernet Clocking Versus SONET/SDH Clocking

Figure 20-10 ML-Series Card Framing and Encapsulation Options
Cisco HDLC LEX
Flag Address Control Protocol
HDLC Framing Mode
Transport Overhead
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.
Cisco ONS 15454 and Cisco ONS 15454 SDH Ethernet Card Software Feature and Configuration Guide, R8.0
20-10
GFP-Cisco HDLC
PPP BCP
Encapsulation
Payload FCS
or
SONET/SDH Payload Envelope
SONET/SDH Frame
GFP-F Frame Types
GFP-Mapped
GFP-PPP GFP-BCP
Ethernet (LEX)
Core Payload
Payload
Header Header
GFP-F Framing Mode
Chapter 20 POS on ONS Ethernet Cards
FCS