Page 79 - Optical Switching And Networking Handbook

04_200023_CH03/Batesx 1/17/01 9:42 AM Page 64

64 Chapter 3

to derive its timing at stratum 3 or higher. Figure 3-6 is a reference
drawing of the clocking hierarchy used for the network and SONET
showing the way the timing is passed across the network.

The SONET Frame

SONET also defines a frame format in which to produce the basic
rate of 51.84 Mbps (STS-1). Each addition to the multiplexing rates
is a multiple of STS-1.The basic format consists of a frame that is 80
bytes (columns) wide and 9 bytes high (rows). The basic STS-1 signal
is then applied into this 810-byte frame. The frame is shown in Fig-
ure 3-7. The frame will occur 8,000 times per second. If we do the
math on this, we have

810 bytes 8 bits/byte 8000 frames/s = 51.84 Mbps

Overhead

From the 810-byte frame, overhead is enabled in several ways to
facilitate operations, administration, maintenance, and provisioning
(OAM&P) services. The first part of the overhead is defined as the
transport overhead. The transport overhead uses the first three

Stratum
Figure 3-6 Level 1 Master Clock
Clocking is passed
down the network Stratum MasterTol Office
Level 2
from the master l
clock to the end
Stratum # 4 ESS or Digital Cross # 5 ESS or
office. Level 3 DMS 100/200 Connect DMS 100

Stratum Customer Premises Multiplexer
Level 4 Channel Bank Equipment
Terminates Clock