Packet over SONET (POS) is a method of carrying packet traffic, such as IP datagrams, over SONET or SDH synchronous circuits. In the standards-defined form, the packets travel in Point-to-Point Protocol (PPP) frames, and those frames use HDLC-like framing inside the SONET/SDH payload. The specification is RFC 2615, “PPP over SONET/SDH,” which obsoletes RFC 1619 (RFC 2615).
What POS is, and what it is not
SONET (North America) and SDH (international) are the synchronous transport systems. They define signal rates, frame structure and the payload container. POS is not a rival physical medium. It describes how packet-layer data is placed into that payload using a data-link method.
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The simplest way to separate the pieces:
- SONET/SDH supplies the synchronous transport and payload structure.
- PPP supplies a point-to-point link protocol and identifies which network-layer protocol each frame carries.
- HDLC-like framing marks where each frame begins and ends on the synchronous link.
PPP and HDLC are not interchangeable terms. PPP can use HDLC-like framing, and Cisco documentation notes that some POS implementations offer other encapsulation choices as well (Cisco ASR 9000 POS commands; Cisco POS troubleshooting). Cisco’s troubleshooting material specifically warns that PPP and Cisco HDLC are not identical, so a link must be configured with the same encapsulation at both ends.
Standards and history
| Document | Date | Role |
|---|---|---|
| RFC 1619, “PPP over SONET/SDH” | May 1994 | Original specification; superseded |
| RFC 2615, “PPP over SONET/SDH” | June 1999 | Later specification; obsoletes RFC 1619 |
RFC 1619 opens with the idea that PPP “provides a standard method for transporting multi-protocol datagrams over point-to-point links,” and applies it to SONET/SDH circuits (RFC 1619). RFC 2615’s change history names payload scrambling as the major change, along with updated terminology and specified SONET/SDH payload mappings (RFC 2615).
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Line rates
RFC 1619 describes SONET rates as integral multiples of 51.840 Mbps; STS-3, for example, is 155.520 Mbps (RFC 1619, 1994). Cisco’s older IOS interface guide likewise documents a POS OC-3 interface at 155.520 Mbps (Cisco serial interface guide; publication date not stated in the retrieved page). These are link-rate examples, not throughput, adoption or performance claims.
Where POS sits in equipment
Cisco’s SONET overview describes packet-over-SONET cards as path-terminating equipment, distinct from line-terminating equipment that handles the SONET ring (Cisco SONET overview). In practice, the POS endpoint terminates the SONET payload and applies the data-link encapsulation. Some integrated platforms combine roles, so a POS router should not be assumed to connect directly to every SONET ring.
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Vendor implementation and legacy cautions
Cisco’s ASR 9000 IOS XR command reference describes POS as carrying packets in SONET or SDH frames, with PPP in HDLC-like framing for Layer 2 encapsulation. It says hardware performs the real-time POS functions while software configures interfaces and gathers statistics. The page was updated June 9, 2026, and syntax and options vary by platform and release (Cisco ASR 9000 POS commands).
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- Encapsulation configured: PPP, Cisco HDLC or another documented option.
- SONET/SDH signal and payload mapping.
- Interface rate.
- Whether the device is a path-terminating endpoint or also handles line termination.
- Platform and software-release support.
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