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SONET Tutorial: Transport Overhead

By K. Surya Prakash

 

 

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Transport Overhead: 

Let us go through each and every byte of Transport Overhead (TOH). I made sure that by the end of this section  you will have fairly good idea of overheads in a SONET frame. Transport Overhead is divided into two parts -Section Overhead and Line Overhead. The first three rows inside the TOH constitute Section Overhead and the remaining six rows of TOH constitute Line Overhead (See figure-2).

Section Overhead:

Framing Bytes (A1 and A2): These bytes are used to indicate the start of SONET/SDH frame. A1 byte is 1111 0110 and A2 byte is 0010 1000. These values remain the same in all STS-1s in an STS-N. SDH uses the same values for framing.

Section Trace (J0)/Section Growth: This byte formerly defined as STS-1 ID (C1), is redefined as Section Trace byte.  It allows two connected sections to verify the connections between them by transmitting a sixteen-byte message. This message is transmitted in sixteen consecutive frames with first byte carried in first frame, second byte in second frame and so on. If no such section trace message is defined or being transmitted, then in STS-48 or lower bit rate the, J0 and each Z0 shall be set corresponding to its order of appearance in the STS-N frame (i.e. J0 shall be set to 000000001, first Z0 to 0000010, second Z0 to 00000011 etc.) Where as in STS-192 frame each Z0 byte is set to the fixed pattern ‘11001100’.

Section BIP-8 (B1): B1 byte indicates bit error rate to the receiving terminal. This byte is known as Bit Interleaved Parity (BIP-8). The first bit in all the bytes in the previous frame are taken and then B1 is set so that the parity is even. Similarly all the other bits in B1 are set. The parity is calculated after scrambling and placed before scrambling. Scrambling is explained in later sections. The parity represented by this octet is the parity of the previous frame. It is used to estimate the bit error rate (BER) on the line. Note that the B1 parity is computed over all the bytes in the frame, no matter how large the frame. Because of this, the B1 byte does not provide a good BER estimation for large frames (perhaps STS-48 and larger) under adverse error conditions. SDH uses this byte for the same purpose.

Orderwire (E1): The E1 byte is located in the first STS-1 of an STS-N. It is called Local Orderwire (LOW). The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is used for a voice channel between two technicians as they installed and tested an optical link. It has a bit rate of 64kb/s. SDH uses this octet for the same purpose.

Section User Channel (F1): The F1 byte is located in the first STS-1 of an STS-N, and is used by the network provider. The corresponding byte locations in the second through Nth STS-1s are currently undefined. This byte is passed from Section to Section within a Line and can be read, written, or both at each Section Terminating Equipment (STE) in that line. The use of this function is optional. SDH also uses this byte for the same purpose.

Section Data Communication Channel (D1, D2 and D3): These are the bytes, which form communication channel. These bytes are defined only for first STS-1 of an STS-N frame. These three bytes are considered as one 192-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and other communication needs between STE. This channel is used for internally generated, externally generated and supplier-specific messages. SDH uses this channel for the same purpose.

Line Overhead:

Pointers (H1 and H2): The processing of H1 and H2 bytes in SONET and SDH is a beautiful concept. Consider figure-2 which describes the STS-1 frame. The SPE can be floating in a SONET frame. It can start in one frame and end in the next frame. Now these two bytes are allocated to a pointer that indicates the offset in bytes between the pointer and the first byte of the STS SPE. The pointer bytes are used in all STS-1s within an STS-N to align the STS-1 Transport Overheads in the STS-N, and to perform frequency justification. You need not worry about this concept as of now as I am going to explain it much more detail in later sections. SDH handles these pointer bytes in the same way.

Pointer Action Byte (H3): This byte is used when negative frequency justification is carried out. The value carried by H3 is not defined when there is no negative frequency justification. SDH handles this byte in the same way.

Line BIP-8 (B2): The operation of this B2 byte is same as that of B1 byte in the SOH except that B2 is calculated over Line Overhead and Synchronous Payload Envelope of the previous frame before scrambling and placed in the current STS-1 frame before scrambling. SDH uses this byte for the same purpose.

Automatic Protection Switching (APS) Channel (K1, K2): In the beginning of this article, I told that a set of fibers is used for protection. These K1 and K2 are the bytes, which are transmitted over these protection channels for Automatic Protection Switching (APS) signaling between line level entities. These bytes are defined only for first STS-1 of an STS-N. In the remaining STS-1s it is undefined. These bytes are used to indicate a number of defects, alarms etc. detected at the receiving terminal back to the corresponding transmitting terminal through protection channels. SDH  uses these bytes for the same purpose. There is lot more explanation to be done on this concept of APS. I’ll discuss this in later sections.

Line Data Communication Channel (D4-D12): These bytes form a communication channel to send administrative messages just as D1 to D3. These nine bytes are considered as one 576-kb/s, message-based channel for alarms, maintenance, control, monitoring, administering and other communication needs. This channel is available for internally generated, externally generated and supplier-specific messages. These bytes are defined only for STS-1 number 1 of an STS-N signal. SDH uses these bytes for the same purpose but with additional codes.

Synchronization Status (S1): This byte is allocated for transporting synchronization status messages. S1 is defined only for first STS-1 of an STS-N signal. Currently only bits 5-8 of S1 are used to transport synchronization status messages. Bits 1-4 are undefined.  These messages contain clock quality labels that allow a SONET NE to select the most suitable synchronization reference from the set of available references. The purpose of these messages is to allow SONET NEs to reconfigure their synchronization references autonomously while avoiding the creation of timing loops. Let me give a few examples for bits 5-8 in S1. Bits 5-8 are 0001 for stratum 1 traceable, 0111 for stratum 2 traceable, 0000 Synchronized traceability unknown etc. SDH uses this byte for the same purpose

Growth (Z1): Z1 byte is located in second through Nth STS-1s of an STS-N. This byte is undefined.

STS-1 REI (M0): The M0 byte is defined only for the STS-1 in an OC-1 or STS-1 electrical signal. Bits 5 through 8 of the M0 byte are allocated for a Line Remote Error Indication function (REI-L), which conveys the error count detected by LTE (using the B2 code) back to its peer LTE. Bits 1 through 4 of the M0 byte are currently undefined. The error count shall be a binary number from zero (i.e., ‘0000’) to 8 (i.e., ‘1000’). The remaining seven values represented by the four REI-L bits (i.e., ‘1001’ through ‘1111’) shall not be transmitted, and shall be interpreted by receiving LTE as zero errors. Since there is no rate in SDH equivalent to STS-1, SDH does not define an M0 value for this byte.

STS-N REI (M1): The M1 byte is located in the third STS-1 in an STS-N (N³ 3) and is used for Line Remote Error Indication (REI-L). The entire M1 byte is used to convey the count of errors detected by the Line BIP-8 (B2) byte. This count has (8 times N) + 1 legal values, namely 0 to 8N errors. For rates below STS-48, the remaining possible 255-(8 times N) values are interpreted as zero errors. For the STS-48 and STS-192 rates, if the line BIP-8 detects greater than 255 errors, the line REI will relay a count of 255 errors. SDH uses this byte for the same purpose.

Growth (Z2): These bytes are allocated for future growth, and their use is currently undefined. Note that STS-1 signal does not contain a Z2 byte.

Orderwire (E2): This byte has the same purpose for line entities as the E1 byte has for section entities. It is called Express Orderwire (EOW) channel. The corresponding bytes in the second through the Nth STS-1s of an STS-N frame are currently undefined. SDH uses this byte for the same purpose.

 

 

 

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