Question

Explain each field of SONET STS-1 overhead in detail.

Answer 1

SONET STS-1 OVERHEAD

90 Columns Synchronous Payload Envelope (SPE) A1 JO J1 E1 B1 F1 B3 D2 D3 C2 9 Rows D1 H1 H2 H3 G1 B2 K1 K2 F2 D4 D5 D6 H4 D7 D8 D9 Z3 D12 D10 D11 Z4 M0/1 S1 E2 N1 Transport OVHT\ Payload OVH Payload overhead Section overhead Data Line Overhead

• Basic SONET STS-1 frame consisting of 9 rows and 90 columns is given in the above figure.
• SONET frame is composed of 810 octets (bytes).
• Transmission is carried out row wise from left to right and from top to bottom.
• Bits are transmitted serially.
• The STS-1 frame of SDH is composed of :
• Section overhead
• Transport overhead
• Payload overhead
• Data part.
• The frame starts with fixed A1/A2 bit pattern of 0xf628 used for bit/octet synchronization.
• SONET/SDH is referred as octet synchronous.
• The first three columns of SONET frame is referred as transport overhead.
• The next 87 columns of the frame are referred as Synchronous payload envelope (SPE).

Transport Overhead

• The first three columns of SONET frame is referred as transport overhead.
• It consists of :
  1. Framing octets (A1,A2)
  2. Section Trace (J0)
  3. Parity (B1)
  4. Order Wire (E1)
  5. Section User Channel (F1)
  6. Section Data Communication Channel (D1,D2,D3)
  7. Pointers and Pointer Action (H1, H2, H3)
  8. Line Parity (B2)
  9. Automatic Protection Switching (APS) channel (K1,K2)
  10. Line Data Communications Channel (D4-D12)
  11. Synchronization messaging (S1)
  12. STS-1 REI(M0)
  13. STS-N REI (M1)
  14. OrderWire (E2)

1.Framing octets (A1,A2) :

• These two octets are used to determine start of the SDH frame.

• A1 is 0xf6 and A2 is 0x28 hexadecimal values.

2.Section Trace (J0) :

• It is used allow connected sections to verify whether the connection is still alive and with the right terminations or not.

3.Parity (B1) :

• This parity octet is used by the receiver for bit error rate estimation.
• As this is of 8 bits, 8 parities are calculated.

4.Order Wire (E1) :

• This is not used today.
• This was used by technicians to test the system while installation.

5.Section User Channel (F1) :

• This is used by the network service provider.
• The octet is carried over from section to section within the line.

6.Section Data Communication Channel (D1,D2,D3) :

• These octets form a communication channel to send administrative messages.
• These are considered as single 192 kbps message.
• Used for maintenance, control, alarm, monitor, administration and the other need of communication between section terminating equipments.

7.Pointers and Pointer Action (H1, H2, H3) :

• These are used to point to the payload (SPE).
• They provide flags to indicate about changes to payload location and provide location for the data.

8.Line Parity (B2) :

• B2 Octet is used for bit error rate estimation.

9.Automatic Protection Switching (APS) channel (K1,K2) :

• These two octets are used for APS signalling between line level entities.
• APS stands for Automatic Protection Switching.

10.Line Data Communications Channel (D4-D12) :

• D4 to D12 octets form communication channel to send administrative messages.
• Used for line data communication and consider as single 576kbps message based channel.
• Used for maintenance, control, monitoring, administration, alarms as well as communication need between line terminating entities.

11.Synchronization messaging (S1) :

• It is used for transporting synchronization status messages and defined for STS-1 of the STS-N signal.
• Bits 5 to 8 are used for this purpose.

12.STS-1 REI(M0) :

• This octet sends no. of errors detected by B octets back to the transmitter.
• This helps in knowing line status as well as receiver status.

13.STS-N REI (M1) :

• The function is same as listed in M0 above.

14.OrderWire (E2) :

• The function is same as listed above for E1.

Payload Overhead

• The first column in synchronous payload envelope (SPE) is referred as Payload overhead (POH).

• It consists of :

  1. Path Trace (J1)

  2. Path BIP-8(B3)

  3. STS Path Signal Label (C2)

  4. Path Status (G1)

  5. Path user channel (F2)

  6. Multi-frame indicator (H4)

  7. Growth octets(Z3,Z4)

  8. N1 fields.

1.Path Trace (J1):

• It helps two ends to verify the connection status (live or not) and check whether it is connected with right terminations.
• It is used to transmit STS path Access point identifier repetitively, Hence path receiving terminal can verify its continued connection with intended transmitter.
• 64 byte frame is used for the purpose.

2.Path BIP-8(B3):

• It is used by the receiver for BER estimation.
• It is calculated over all the bits of previous STS SPE before the scrambling process.

3.STS Path Signal Label (C2):

• This indicates type of traffic carried in the payload part of the SDH frame.

4.Path Status (G1):

• It is used to convey path terminating status/performance back to the transmitter (Originating STS PTE).
• PTE stands for Path Terminating Equipment.

5.Path user channel (F2):

• It is used for user communication similar to F1 octet in transport overhead.

6.Multi-frame indicator (H4):

• It provides generalized multi-frame indicator for the payloads.
• The first purpose of this indicator is for VT structured payload.
• The second purpose is for support of virtual concatenation of STS-1 SPEs.

7.Growth octets (Z3,Z4):

• Reserved for future use.

8.N1 fields:

• This octet is used to allocate support for tandem connection maintenance and tandem connection data link.