PPP over SONET/SDH
obsoleted by
- rfc-2615 — PPP over SONET/SDH
Extracted elements (15)
Because PPP frames are variable in length, frames are allowed to cross SPE boundaries, avoiding the complexity of padding or aligning frames to SPE boundaries.
PPP encapsulation over SONET/SDH is anticipated to yield significantly higher throughput compared to other SONET/SDH payload mappings at lower cost for line termination equipment, due to PPP's relatively low overhead.
The SDH STM series is preferred over SONET STS for higher rates because STM progresses in powers of 4 (rather than 3) and employs fewer steps, which is likely to simplify multiplexing and integration.
PPP does not require the use of control signals, but when available, control signals can allow greater functionality and performance; implications are discussed in RFC 1549 (PPP in HDLC Framing).
SONET STS-3c/STM-1 uses the same super-rate SPE payload mapping as ATM and FDDI (per ANSI T1.105.02), establishing a shared physical-layer baseline across these technologies.
Higher signal rates SHOULD conform to the SDH STM series rather than the SONET STS series as equipment becomes available.
Lower signal rates MUST use the Virtual Tributary (VT) mechanism of SONET/SDH, which maps existing signals up to T3/E3 rates asynchronously into the SPE, or uses available clocks for bit-synchronous and byte-synchronous mapping.
The Multiframe Indicator (H4) is currently unused and MUST be set to zero.
Framing for PPP over SONET/SDH uses the octet-synchronous framing described in 'PPP in HDLC Framing' (RFC 1549). PPP frames are located by row within the SPE payload.
PPP treats SONET/SDH transport as octet-oriented, full-duplex synchronous links. PPP presents an octet interface to the physical layer with no provision for sub-octets.
Standard LCP sync configuration defaults apply to SONET/SDH links. The following LCP Configuration Options are recommended: Magic Number, No Address and Control Field Compression, No Protocol Field Compression, and 32-bit FCS.
The basic rate for PPP over SONET/SDH is STS-3c/STM-1 at 155.520 Mbps. The available information bandwidth is 149.760 Mbps after removal of section, line, and path overhead — the same super-rate mapping used for ATM and FDDI.
Security issues are not discussed in this memo.
The octet stream is mapped into the SONET/SDH Synchronous Payload Envelope (SPE) with octet boundaries aligned to SPE octet boundaries. No scrambling is needed during insertion into the SPE.
The Path Signal Label (C2) byte uses the experimental value 207 (0xcf hex) to indicate that the SPE payload contains PPP.