SONET OC-3c / SDH STM-1 ATM Phvsical Interface

For information on testing SONET links

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Transmission of information at 155 Mbps over SONET or SDH interfaces is the transfer most associated with ATM. SONET/SDH, part of the ATM Forum UNI 3.0 specifications, is the fastest and one of the most popular ATM interfaces.

Connections to SONET/SDH lines may be via multi-mode, single-mode and UTP. The multi-mode and single-mode involve ATM links with SC-type optical connectors to multi-mode/single-mode fibers at 1300 nm. UTP links are with UTP-5 connectors.

The pin assignments of the standard UTP 155 cable are as follows:

Pin 3, 4, 5 and 6 are not used.

Both SONET (Synchronous Optical NETwork) and SDH (Synchronous Digital Hierarchy) are based on transmission at speeds of multiples of 51.840 Mbps, or STS-1. 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.

OC-3c and STM-1 rates are an extension of the basic STS-1 speed and operate at 155.520 Mbps, carrying three interleaving STS-1 frames. Thus, the OC-3c frame has nine rows and 270 columns. Nine of these columns are TOH. In ATM, the three STS-1 frames are not independent, but rather concatenated to form one larger payload. One column of the 261 columns in the payload is used for the POH.

The payload may float inside the OC-3c frame in case the clock used to generate the payload is not synchronized with the clock used to generate the overhead. Pointers in the overhead always point to the start of the payload.

Of the 270 columns, ten are used for overhead information. Thus the actual useful information rate that can be carried inside the OC-3c payload is 149.76 Mbps. Since 5 bytes out of every 53-byte cell are the header, only 135.63 Mbps carry actual ATM payload.

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.

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.

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).

For further reference, consult:

1. The ATM Forum, ATM User-Network Interface Specifications 3.0 and 3.1, Prentice-Hall, 1993 and 1994.
2. Bell Communications Research Inc. (Bellcore), "Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria", TR-NWT-000253, December 1991.
3. American National Standards Institute (ANSI), "Digital Hierarchy - Optical Interface Rates and Formats Specifications (SONET)", T1.105, 1991.