Fig. 10.17. (a) Section overhead (SOH), (b) Line overhead (LOH).

Bytes A1 and A2. These two bytes indicate the beginning of an STS-1 frame. A1 and A2 are used for framing and synchronization and are called as alignment bytes.

Bytes C1. This byte carries a unique identifier for the STS-1 frame. This byte is referred as identification byte. This byte is necessary when multiple STS-1 are multiplexed to create a higher rate STS (STS-3, STS-9, STS-12 etc.).

Byte B1. This is called parity byte and it is for section bit-inter leaved parity code (BIP-8) byte. This is a even parity code used to check for transmission errors over a regenerator section. Its value is calculated over all bits of the previous STS—N frame after scrambling and then placed in the B1 byte of STS-1 before scrambling. Therefore this byte is defined only for STS-1 number 1 of an STS-N signal.

Byte E1. It is called section orderwire byte. This byte is allocated to be used as a local orderwire channel for voice communication between regenerators, hubs and remote terminal locations.

Byte F1. It is called user's byte. This byte is set side for the user's purposes. The F1 bytes in consecutive frames form a 64 kbps channel that is reserved for user needs at the section level. It terminates at all section terminating equipment within a line. It can be read and written to at each section terminating equipment in that line.

Byte D1, D2 and D3. These bytes are called management bytes or section data communication channel (DCC) bytes. These bytes together form a 192 kbps message channel providing a message based channel for OAM & P between sections. The channel is used from a central location for alarms, control, monitoring, administration and other communication needs. It is available for internally generated, externally generated or manufacturer specific messages.

Line overhead (LOH). Fig. 10.17 (b) shows the line overhead. Line overhead carries the payload pointers to specify the location of SPE in the frame and provides automatic switching (for standby equipment). It separates voice channels and provides multiplexing, line maintenance and performance monitoring. The byte descriptions of the LOH are given below.

Byte H1 and H2. These bytes are called pointer bytes. Two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STE SPE. The pointer bytes are used in STS-1s within an STS-N to align the STS-1 transport overhead in the STS-N and to perform frequency justification.

Byte H3. This is a pointer action byte and is allocated for SPE frequency justification purposes. The H3 byte is used in all STS-1s within an STS-N to carry the extra SPE byte in the event of a negative pointer adjustment.

Byte B2. It is line bit interleaved parity code (BIP-8) byte. This parity code byte is used to determine if a transmission error has occurred over a line. It is even parity and is calculated over all bits of the line overhead and STS-1 SPE of the previous STS-1 frame before scrambling.

Bytes K1 and K2. K1 and K2 bytes are called automatic protection switching (APS channel) bytes. These 2 bytes are used for protection signalling between line terminating entities for bidirectional automatic protection switching and for detecting alarm indication signal (AIS-L) and remote defect indication (RD1) signals.

Bytes D4 to D12. These bytes are called as line data communication channel (DCC) bytes. These 576 kbps message channel provides the same service as the D1-D3 bytes (OAM). They are available for internally generated, externally generated and manufacturer specific messages. A protocol analyzer is required to access the line DCC information.

Bytes Z1 and Z2. These bytes are referred as growth bytes. The Z1 byte is located in the second through Nth STS-1s of an STS-N (3 < = N < = 48) and one allocated for future growth. Z2 byte is located in the first and second STS-1s of an STS-3 and the first, second and fourth through the Nth STS-1s of an STS-N (12 < = N < = 48). These bytes are allocated for future growth.

Byte E2. This is a orderwrite byte. It provides a 64 kbps channel between line entities for an express orderwire. It is a voice channel for use by technicians and will be ignored as it passes through the regenerators.

Path overhead. Path overhead is part of SPE and contains following information. Performance monitor of synchronous Transport signal, path trace, parity check and path status. POH contains a bytes. The labels are shown in Fig. 10.17. The labels and functions are given below.

J1. Path trace byte — J1 byte sends a continuous 64 bit string to verify the connection. The choice of the string is left to the application program.

B3. Path parity byte.

C2. Path signal label byte -- It is used to identify different protocols used at higher levels (such as FDD1 or SMDS).

G1. Path status byte.

F2. Path user channel byte.

H4. Virtual tributory indicator — Indicates payloads that cannot fit into a single frame.

Z3, Z4 and Z5. Growth bytes — reserved for future use.

Virtual tributories. SONET is designed to carry broad band payloads. In order for the SONET to carry lower data rate frames such as DS-1 and DS-2, the lower data rate frame is mapped to the STS-1 payload and is called a virtual tributary (VT). A virtual tributory is a partial payload that can be inserted into an STS-1 and combined with other partial payloads to fill out the frame. The basic frame for the SONET is STS-1 with a data rate of 51.84 Mbps. The payload of STS-1 is made up of 86 columns and grows. Instead of using all 86 payload columns of an STS-1 frame for data from one source, we can subdivide the SPE and call each component a VT. Fig. 10.18 shows the VTs in the STS-1 payload.

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