SONET/SDH Overview
The POS specification addresses the use of PPP encapsulation over SONET/Synchronous Digital
Hierarchy (SDH) links. SONET is an American National Standards Institute (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 SONET/SDH is by definition 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 that of 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.) For the SPE with the Cisco PA-POS-2OC3,
section, line, and path overhead are removed.
The International Telecommunications Union Telecommunication Sector (ITU-T) defines a series of
SDH transmission rates beginning at 155.520 Mbps, as follows:
Despite the name, SONET is not limited to optical links. Electrical specifications have been defined for
single-mode fiber, multimode fiber, and CATV 75-ohm coaxial cable. The PA-POS-2OC3 allows
transmission 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:
• STS-3c—155.520 Mbps (the PA-POS-2OC3 conforms to STS-3c)
• STS-12c—622.080 Mbps
• STS-48c—2,488.320 Mbps
The following references discuss concepts and specifications of POS and PPP:
• Simpson, W., Editor, The Point-to-Point Protocol (PPP), RFC 1548, Daydreamer, December 1993.
• Simpson, W., Editor, PPP in HDLC Framing, RFC 1662, Daydreamer, July 1994.
• Simpson, W, Editor, PPP Over SONET/SDH, RFC1619, May 1995.
• American National Standard for Telecommunications - Digital Hierarchy - Optical Interface Rates
and Formats Specification, ANSI T1.105-1991.
• American National Standard for Telecommunications - Synchronous Optical Network (SONET)
Payload Mappings, ANSI T1.105.02-1993 draft.
ITU-T Recommendation G.707, Synchronous Digital Hierarchy Bit Rates, June 1992.
SONET
1
1. ANSI-defined SONET specifications.
SDH Equivalent
STS-3c
2
2. Supported by the PA-POS-2OC3.
STM-1
2
STS-12c STM-4c
STS-48c STM-16c
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