SONET or SDH frame provides data communications channel (DCC) bytes for network element operations such as administration, maintenance, and provisioning.

Each SONET or SDH frame includes two DCCs—Section DCC (SDCC or RS-DCC) and Line DCC (LDCC or MS-DCC). The section and line DCCs are used for transporting management messages between Network Elements (NEs) and between NEs and Network Management System (NMS).

Each SONET or SDH frame uses these DCC bytes in overhead of the frame to carry management information for SONET or SDH networks, or Add or Drop multiplexers (ADMs). These DCC bytes traverse through hop-by-hop between ADMs and perform a path discovery and end-to-end provisioning in SONET or SDH network.

The behavior without DCC tunneling is that the NMS discovers the connected ADM but could not reach the ADM at the remote end of the TDM circuit.

With the introduction of DCC tunneling, the NMS discovers the existing topology and the insertion of ASR nodes are transparent to the DCC bytes. These ASR nodes help to tunnel DCC bytes and the NMS connectivity remains intact.

In the following diagram, the ADM-1 reaches the ADM-2 through transparent overhead (TOH) tunnel that is established between provider edges (PE1 and PE2).

The DCC bytes from ADM-1 reach ADM-2 through the TOH tunnel. Therefore, the NMS could reach the ADM-2 and the topology across ADMs remains intact.

TOH tunneling DCC is supported on the OCx ports for following CEM interface modules:

• 1-port OC-48/STM-16 or 4-port OC-12/OC-3 / STM-1/STM-4 + 12-port T1/E1 + 4-port T3/E3 module

• 1-port OC-192 or 8-port low rate module

• NCS 4200 1-port OC-192 or 8-port low rate CEM 20G bandwidth module

The transparent overhead tunnel supports the following DCC types that are embedded in the transport overhead of STS-1 frame:

• Section DCC—Supports 3 bytes, D1, D2, and D3 responsible for message-based administration, monitoring, alarm maintenance, and other communication requirements. The section DCC provides a bandwidth of 192 Kbit/s between each pair of SONET section termination equipment. Any SONET equipment that can extract these 3 bytes from the STS-1 frame overhead and process them is considered to support a DCC interface.

• Line DCC—Supports D4 to D12 bytes with bandwidth of 576 Kbit/s.

In the following diagram, STS-1 Transport and Path Overhead are displayed.

The following table describes the transparent overhead tunnel segments with the supported tunnel types and the respective tunnel header bytes.

Before creating transparent overhead tunnel, ensure that you perform the following steps:

• Set mode under the STS path for SONET before enabling the overhead tunnel.

  The following example details on how to set STS-1 mode on SONET.

  platform enable controller MediaType 0/3/2 oc3
  controller MediaType 0/3/2
   mode sonet
  controller SONET 0/3/2
   rate OC3
   no ais-shut
   alarm-report all
   clock source internal
   !
   sts-1 1 
    clock source internal
    mode vt-15
  

• Set mode under the AU-4 path on SDH before enabling the overhead tunnel.

  The following example details on how to set AU-4 mode on SDH.

  platform enable controller MediaType 0/3/2 stm1
  controller MediaType 0/3/2
   mode sdh
  controller SDH 0/3/2
   rate STM1
   no ais-shut
   alarm-report all
   clock source internal
   aug mapping au-4
   au-4 1
    clock source internal
    channel-group 0
    mode vc4 
  

• Mode and TOH type must be same on both PEs in an end-to-end setup.

  For example, if the PE-1 device has mode that is configured as VT-15 and TOH type as SDCC, then the same mode and TOH type must be configured on the PE-2 device.

  If the mode and TOH type configurations differ, then the TOH tunneling does not work.

This section provides information about configuring the transparent overhead tunnel.