Virtual Containers in SDH
A Virtual Container (VC) combines a client payload with Path Overhead (POH). The payload can be an E1, T1, E3, DS3, or another defined mapping. POH remains associated with the VC along the path and carries the fields used for path monitoring.
A Virtual Container has two defined functions:
- To provide a defined byte structure sized for a specific client data stream.
- To attach a set of management and monitoring data (the ) that travels with the payload from its source to its destination. The fields identify the path and report defects at its endpoints.
From Container (C) to Virtual Container (VC)
A Virtual Container is formed from a basic Container (C) that carries a defined tributary signal. Adding the Path Overhead creates the VC structure.
For example, the high-capacity VC-4 is formed by taking a C-4 container, which is a block of 9 rows by 260 columns of bytes, and prepending one additional column for the 9-byte VC-4 Path Overhead. This creates the final VC-4 structure of 9 rows by 261 columns.
Path Overhead (POH) bytes
POH is a set of monitoring and management bytes carried with the VC from the path entry point to the path exit point. Its functions apply to the path independently of the individual transport sections.
High-order POH bytes, including VC-4 and VC-3
- J1, path trace: J1 is the first byte of VC-4 or VC-3 and carries a repeated path access point identifier in a 16-byte frame. The receiver compares the identifier to detect an incorrect connection between the path source and destination.
- B3, Bit Interleaved Parity (BIP-8): B3 detects bit errors on the path. The sender calculates the parity of the previous VC frame and places the result in B3 of the current frame. The receiver performs the same calculation and compares it with the received byte.
- C2, signal label: The C2 value identifies the signal carried in the VC. The receiver uses it when interpreting the payload, for example when it contains a , a packet-over-SONET/SDH stream, or an older PDH signal.
- G1, path status: Carries status information from the path end back to the source equipment. It has two functions:
- REI (Remote Error Indication): Informs the sender about errors detected through B3 monitoring.
- RDI (Remote Defect Indication): Informs the sender about a defect that prevents correct path reception.
- H4, multiframe and position indicator: H4 indicates the multiframe phase when the VC carries smaller multiplexed structures. TU-11, TU-12, and TU-2 use a four-frame multiframe lasting 500 µs.
The complete high-order POH contains J1, B3, C2, G1, F2, H4, F3, K3, and N1. J1 is the first byte of VC-4 or VC-3, while the other fields occupy defined positions in the POH column. The fixed layout lets equipment monitor the path without changing the payload format.
Lower-order Virtual Containers, including VC-11 for T1 and VC-12 for E1, use lower-order POH bytes V5, J2, N2, and K4. V5 carries BIP-2, REI, signal label, and RDI fields, while the RFI function depends on the VC type.
V5 is the first byte of lower-order POH, and the remaining POH bytes appear in successive multiframe positions. VC-12 has 140 bytes distributed over four frames, so its position is described by the TU-12 pointer rather than by a fixed index in one STM-1 frame.
SDH Virtual Container types
SDH defines Virtual Container sizes for standard tributary signals. The table pairs common SONET payload structures with their SDH counterparts.
VC-3 has a 9-by-85 byte structure, while VC-4 has 9-by-261 bytes including its POH column. The nominal VC-12 rate is 2.240 Mbps with 2.176 Mbps payload capacity, VC-3 is 48.960 and 48.384 Mbps, and VC-4 is 150.336 and 149.760 Mbps. The difference accounts for path overhead and alignment fields.
| SONET Virtual Container (VT/STS) | Equivalent SDH Container | Designed to Carry | Typical Application |
|---|---|---|---|
| VT-1.5 | VC-11 | 1 x DS1 / T1 (1.544 Mbps) | Used for individual voice or data channels. |
| VT-2 | VC-12 | 1 x E1 (2.048 Mbps) | Used for carrying European E1 signals. |
| STS-1 SPE | VC-3 | 1 x DS3 (44.736 Mbps) or 28 x DS1 | Basic SONET payload unit. Carries a DS3 signal or a group of T1s. |
| STS-3c SPE | VC-4 | ~150 Mbps (e.g., ATM, packet streams) | High-speed payload for modern data services, directly maps into an OC-3/STM-1. "c" stands for concatenated. |
*SPE stands for Synchronous Payload Envelope, which is the SONET term for a Virtual Container.
Adapting VCs into TUs and AUs
A VC with its payload and POH is a path structure. Before it is placed in an STM or STS frame, it is combined with a structure containing a pointer that identifies the VC position.
- Low-Order VCs (like VT-1.5) are adapted into Tributary Units (TU).
- High-Order VCs (like STS-1 SPE or STS-3c SPE) are adapted into Administrative Units (AU).
The pointer identifies the VC position in the higher-order structure and supports compensation for frequency differences between the tributary signal and the network clock. Add/drop equipment can read or insert the indicated VC without demultiplexing the complete STM signal. Pointer operation is described in a separate topic.
VC-12 is combined with a TU-12 pointer, while VC-4 is combined with an AU-4 pointer. AUG-1 can contain one AU-4 or three AU-3s. The pointer does not change VC contents; it reports the VC position and phase in the higher-order structure.
The pointer also separates the VC path format from the transport frame phase, so the same VC remains identifiable after section-level processing.