Nortel Networks 8000 - Manual (Page 63)

4 POS and CPOS interface configuration
Nortel Secure Router 8000 Series
Configuration -WAN Access
4.1 Introduction
This section covers the following topics you need to know before you configure a POS or
CPOS interface:
• SONET
• SDH
• POS
• CPOS
4.1.1 SONET
4.1.2 SDH
The Synchronous Optical Network (SONET) is a synchronous optical transmission system
defined by the ANSI.
The SONET supports a suite of transmission rates, which includes OC-1 (51.84Mbit/s), OC-3
(155Mbit/s), OC-12 (622Mbit/s), and OC-48 (2.5Gbit/s). The SONET can multiplex multiple
synchronous signals because they are synchronous signals.
Synchronous Digital Hierarchy (SDH) is the widely used synchronous data transmission
system defined by ITU-T based on the concept of the SONET.
Frame structure of SDH
In Synchronous Digital Hierarchy (SDH), you can add low speed tributary signals to or drop
from the SDH signals with the synchronous multiplex mode and the flexible mapping
structure.
SDH reduces signal consumption and investment on devices, without using the multiplexing
or demultiplexing devices.
Low speed signals multiplex the SDH signal, which are also known as channels.
Using the features of SDH mechanism, the channelized POS (CPOS) interface can perform
the following functions:
• Perform the fine division of bandwidth.
• Reduce the requirement quantity of low speed physical ports on a router in networking.
• Enhance the convergence capability of low speed ports of a router.
• Improve the dedicated line access capacity of a router.
The following describes the SDH signal - frame structure of STM-N to help you understand
the content.
To extract or insert low speed signals from or into high speed signals, distribute low speed
signals in the frame regularly and in order. The ITU-T defines that STM-N frames adopt the
structure of rectangle blocks in the unit of byte, as shown in Figure 4-1.
4-2 Nortel Networks Inc. Issue 5.3 (30 March 2009)