The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are a set of related standards for synchronous data transmission over fiber optic networks that are often used for framing and synchronization at the physical layer. SONET is the United States version of the standard published by the American National Standards Institutue (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU).

SONET/SDH can be used in an ATM or non-ATM environment. Packet Over SONET/SDH (POS) maps IP datagrams into the SONET frame payload using Point-to-Point Protocol (PPP). In the ATM enviroment, connections to SONET/SDH lines may be via multi-mode, single-mode or UTP.

SONET is based on transmission at speeds of multiples of 51.840 Mbps, or STS-1 and SDH is based on STM-1 which has a data rate of 155.52 Mbps, equivalent to STS-3. The following table lists the hierarchy of the most common SONET/SDH data rates:

Other rates such as OC-9, OC-18, OC-24, OC-36, OC-96 and OC-768 are referenced in some of the standards documents but were not widely implemented. Higher rate maybe defined for future implementations.

Protocol Structure

The frame structure of STS and STM is different. We only display the details of the STS-1 frame structure here. The STS-1 frame is composed of octets which are nine rows high and 90 columns wide. The first three columns are used by the Transport Overhead (TOH) and contain framing, error monitoring, management and payload pointer information. The data (Payload) uses the remaining 87 columns, of which the first column is used for Path Overhead (POH). A pointer in the TOH identifies the start of the payload, which is referred to as the Synchronous Payload Envelope or SPE.

SOH- Section Overhead.

• A1, A2- Frame alignment. These octets contain the value of 0xF628. The receiver searches for these values in the incoming bit stream. These bytes are not scrambled.
• C1- STS-1 identification. Since OC-3c and STM-1 contain three STS-1 streams, the three C1 bytes contain 0x01, 0x02 and 0x03, respectively.
• B1- Section error monitoring. Contains BIP-8 of all bits in the previous frame using even parity, before scrambling.

LOH- Line Overhead

• B2- Line error monitoring. Contains BIP-24 calculated over all bits of the line overhead of the previous frame with even parity.
• H1 (bits 1-4)- New data flag (specifies when the pointer has changed), path AIS.
• H1 and H2 (bits 7-16)- Pointer value, path AIS. These bytes specify the offset between the pointer and the first payload byte. A change in this value is ignored until received at least three consecutive times.
• H1* and H2*- Concatenation indication, path AIS.
• H3- Pointer action (used for frequency justification), path AIS.
• K2 (bits 6-8)- Line AIS, line FERF, removal of line FERF.
• Z2- Line FEBE. This contains the number of B2 (BIP-24) errors detected in the previous interval.

POH- Path Overhead

• J1- STS path trace. This byte is used repetitively to transmit a 64-byte fixed string so that the receiving terminal in a path can verify its continued connection to the transmitter. Its contents are unspecified.
• B3- Path error monitoring. Path BIP-8 over all bits of the payload of the previous frame, using even parity before scrambling.
• C2- Path signal level indicator. Contains one of two codes:
  Code 0: indicates STS payload unequipped: no path originating equipment.
  Code 1: indicates STS payload equipped: nonspecific payload for payloads that need no further differentiation.
• G1 (bits 1-4)- Path FEBE. Allows monitoring of complete full-duplex path at any point along a complex path.
• G1 (bit 5)- Path yellow alarm, path RDI (Remote Defect Indicator).

Related Protocols: ATM, STS, STM

Sponsor Source: SONET is an ANSI standard defined in documents T1.105.xx and T1.119.xx and SDH is defined by ITU-T in documents G.707, G.781, G.782, G.783 and G.803.

Reference: http://www.iec.org/online/tutorials/sonet/ : Synchronous Optical Network (SONET)