SDH Multiplexing Hierarchy

From a Tributary Stream to an STM-N Signal

The basic SDH transport unit is the STM-1 frame, with a line rate of 155.520 Mbit/s. The multiplexing hierarchy defines how tributaries are mapped into the STM-1 payload and how several STM-1 structures form a higher-order signal. STM-4, STM-16, STM-64, and STM-256 use fixed multiples of the STM-1 rate.

The process has two distinct stages:

1. Tributary mapping: Placing lower-rate signals such as PDH E1, E3, and E4 into the appropriate containers and payload structures of an STM-1.
2. Synchronous multiplexing: Combining 4, 16, 64, or 256 STM-1 structures into STM-4, STM-16, STM-64, or STM-256 by byte interleaving.

Part 1: Mapping Tributaries into the STM-1 Payload

A tributary signal must first be placed in the STM-1 payload. The process of taking external signals, known as tributaries, and formatting them to fit inside the STM-1 frame is called mapping. The procedure adds path overhead and pointer-bearing units around a sequence of logical containers.

The mapping structure hierarchy

Each layer has a defined role: the container carries client data, the VC adds path overhead, and a TU or AU locates the VC within its parent structure.

• 1
  Container (C): A bit field assigned to a particular tributary type. A C-11 container carries a T1 or DS1 signal at 1.544 Mbit/s. Mapping adapts the client clock to the network clock and, in the relevant variants, uses justification bits.
• 2
  Virtual Container (VC): A container with path overhead added. The Path Overhead (POH) carries payload type, quality, and alarm information along the path. The container field and its POH together form VC-n.
• 3
  Tributary Unit (TU) / Administrative Unit (AU): A virtual container with a pointer. A low-order VC is placed in a TU, such as VC-12 in TU-12, while a high-order VC is placed in an AU, such as VC-4 in AU-4.
• 4
  Tributary/Administrative Unit Group (TUG/AUG): TUs are byte-interleaved into TUG structures. Three TU-12s form a TUG-2, seven TUG-2s form a TUG-3, and three TUG-3s fill the VC-4 payload. In the AU-4 variant, one AU-4 forms AUG, which is placed in the STM-1 payload.

Example Mapping Path: 63 E1 Signals

This example follows the complete low-order mapping path used in European SDH. The goal is to place 63 independent E1 signals at 2.048 Mbit/s into one STM-1 signal at 155.520 Mbit/s.

1. Each E1 stream is mapped into a C-12 container.
2. Path overhead is added to each C-12, creating a VC-12 (Virtual Container-12).
3. A pointer is associated with each VC-12, creating a TU-12 (Tributary Unit-12).
4. Three TU-12s are byte-interleaved to form a TUG-2 (Tributary Unit Group-2).
5. Seven TUG-2s are byte-interleaved to form a TUG-3 (Tributary Unit Group-3). This structure contains $7 \times 3 = 21$ E1 streams.
6. Three TUG-3s are multiplexed into the payload area of a VC-4 (Virtual Container-4). The total is $3 \times 21 = 63$ E1 streams.
7. An AU-4 pointer is associated with the VC-4, creating an AU-4. One AU-4 forms AUG, which is placed in the STM-1 frame payload.
8. In the North American variant, four TU-11s form a TUG-2, and seven TUG-2s carry 28 DS1 streams in a TUG-3 structure.
9. When the AU-3 path is used, the TUG-3 forms a VC-3 and then an AU-3; three AU-3s are byte-interleaved into an AUG-1.

This hierarchy lets equipment such as an ADM extract an individual VC-12 and terminate it as E1 without unpacking all 63 paths in the STM-1 frame. The operation works at VC level, leaving the other paths in the transport structure.

Part 2: Synchronous Multiplexing to Higher-Order Levels

Once STM-1 has been formed, higher levels are built by multiplexing structures with the same frame duration. The standardized factors N are 4, 16, 64, and 256, producing STM-4, STM-16, STM-64, and STM-256.

The process uses byte interleaving:

• To create an STM-4 signal, a multiplexer takes four independent STM-1 signals.
• For each frame position, it takes the corresponding byte from STM-1 number 1, then from number 2, number 3, and number 4.
• The interleaving continues for all bytes. The four input structures produce one STM-4 frame with the same row and column order within each constituent structure.
• The STM-4 frame has 9 rows and 1080 columns, four times the STM-1 width. Its frame duration remains 125 µs and its line rate is 622.080 Mbit/s.

Interleaving combines frame structures while preserving pointers and path overhead. Each constituent keeps its VC relationship to a TU or AU, while section overhead is generated and monitored for the new STM-N signal.

Standardized SDH levels and SONET equivalents

Every level keeps the 125 µs frame duration and increases the structure width to 270N columns. Moving to a higher level requires compatible equipment, an appropriate optical interface, and a suitable power budget and transmission margin. The existing fiber may remain usable, but that outcome is not guaranteed for every upgrade.