Sonet/Sdh Overview; Atm Overview - Cisco 7603 - Modular Expansion Base Installation Manual

OSM Technology Overview

SONET/SDH Overview

The Packet over SONET (POS) specification defines the use of PPP encapsulation over
SONET/Synchronous Digital Hierarchy (SDH) links. SONET is an ANSI standard (T1.1051988) for
optical digital transmission at hierarchical rates from 51.840 Mbps (STS-1) to 2.5 Gbps (STS-48) and
greater. SDH is the international standard for optical digital transmission at hierarchical rates from
155.520 Mbps (STM-1) to 2.5 Gbps (STM-16) and greater.
Because a SONET/SDH link is considered to be a point-to-point circuit, PPP is well-suited for use over
these links. PPP was designed as a standard method of communicating over point-to-point links.
SONET is an octet-synchronous multiplex scheme that defines a family of standard rates and formats.
The basic rate for POS is STS-3c/STM-1, which is 155.520 Mbps. The available information bandwidth
is 149.760 Mbps, which is the STS-3c/STM-1 Synchronous Payload Envelope (SPE), the payload
portion of the SONET frame into which the octet-oriented user data is mapped. (Octet boundaries are
aligned with the SPE octet boundaries.)
The International Telecommunications Union Telecommunication Sector (ITU-T) defines a series of
SDH transmission rates beginning at 155.520 Mbps as follows:
SONET
STS-3c
STS-12c
STS-48c
1. ANSI-defined SONET specifications.
SONET is not limited to optical links. Electrical specifications have been defined for single-mode fiber,
multimode fiber, and CATV 75-ohm coaxial cable. OSMs currently allow transmission only over
single-mode and multimode optical fiber. Transmission rates are integral multiples of 51.840 Mbps,
which can be used to carry T3/E3 bit-synchronous signals.
The following transmission multiples are currently specified and commonly used:
•
•
•

ATM Overview

ATM uses cell-switching and multiplexing technology that combines the features of circuit switching
(constant transmission delay and guaranteed capacity) with the features of packet switching (flexibility
and efficiency for intermittent traffic).
ATM is a connection-oriented environment. All traffic to or from an ATM network is prefaced with a
virtual path identifier (VPI) and virtual channel identifier (VCI). A VPI/VCI pair is considered a single
virtual circuit. Each virtual circuit is a private connection to another node on the ATM network. Each
virtual circuit is treated as a point-to-point mechanism to another router or host and is capable of
supporting bidirectional traffic.
Book Title
1-40
1
SDH Equivalent
STM-1
STM-4c
STM-16c
STS-3c—155.520 Mbps
STS-12c—622.080 Mbps
STS-48c—2,488.320 Mbps
Chapter 1 Product Overview
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