STM-1

SDH C-12, VC-12, TU-12 frame format

A PDH signal stream of 2048 kbit/s, or an ATM cell stream can be mapped into a C-12, VC-12, TU-12 frame. Mapper is a digital network consisting of a flexible storage, multiplexers and a control circuit. During mapping we "copy" the resource PCM frames to be forwarded into a virtual frame named container, and complete it with appendix bytes. Bit rate and phase of the container frame are synchronized to the STM-1 frame. Size of the container frame is 144=4x32+4x4 byte, namely there are 4 PCM frames and by 4 appendix bytes in a container. Time frame is 500 us.

Generation of C-12 container, the first phase of mapping:

Location of PDH data bytes into the container. Signal stream to be forwarded gets to a flexible storage at the boundary of the two systems, and we can read bytes from here by 32-byte blocks for the location into the container.
Location may happen:
- Asynchronous mode: there is no connection between the resource frame (PCM), and the destination frame (C-12, VC-12, TU-12),
- Synchronous mode: phase of resource and destination frame is fixed (0. time gap of PCM frame always locates behind the R-bits).
In case of asynchronous mapping STM-1 does not need to know anything about the signal stream of 2 Mbit/s, except line coding, namely an unframed digital signal can be forwarded on this path, too. Forwarding framed PCM signals, however, user on the reception side have to determine the beginning of the PCM frame.
In case of synchronous mapping we should know the frame structure of PCM signal, but on the side of reception user does not have to determine the beginning of the frame.
In case of mapping of ATM cells equlization of rate is cancelled, ATM row of cells is an asynchronous signal stream.
Location of appendix bytes, rate equalization.
Meaning of appendix bytes and their bits is the following:
- D - data bit,
- R - fixed stuff bit
- O - overhead bit (reserved for the future),
- S(1,2) - justification opportunity bit, (possible space for a bit inserting)
- C(1,2) - justification control bit
Equalization of rate difference between resource and destination frame will happen with bit inserting in the following way:
- C1C1C1=000 -> S1 bit is a data bit
- C1C1C1=111 -> S1 bit is a justification bit
- C2C2C2=000 -> S2 bit is a data bit
- C2C2C2=111 -> S2 bit is a justification bit

C-12 frame (container) generated in this way consists of 4x34=136 bytes, namely we complete all the PCM frames with 2 bytes.

Second phase of mapping, generation of the VC-12 container :

serial/paralell transformation
Mapping of ATM cells. We put ATM cells to the container by a byte synchronous method. ATM cells may reach over to the next VC-12 container.
Assignment of a path identifier (Path overhead) to the container.

VC-12 frame generated in this way (virtual container) consists of 4x35=140 bytes, namely we appended to all the PCM frames 3 bytes.

Third phase of mapping, generation of TU-12 component:

Assignment of pointer to the container:
V1, V2, V3, V4 bytes are the bytes of TU-12 PTR (TU pointer).
- V1-V2 points to the V5 byte of VC-12 POH, namely it tells, where VC-12 frame begins within TU-12.
- V3
- V4

TU-12 frame (container) generated in this way, which is synchronized to the SDH either in bit rate, or frame phase, consists of 4x36=144 bytes, namely we appended to all PCM frames 4 bytes.

Frames (containers) in case of an asynchronous PDH mapping:


         C-12 keret 136 byte         VC-12 keret 140 byte        TU-12 keret  144 byte
              500 us                        500 us                      500 us
       +---------------------+     +---------------------+     +---------------------+
       |    -------------    |     |    ------------     |     |     V1  (TU pointer)| ->-+
       +---------------------+     +---------------------+     +---------------------+    |
       |    -------------    |     |         V5          |     |         V5          | -<-+
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       32 bytes      |     |       32 bytes      |     |       32 bytes      |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |    ------------     |     |     V2  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         J2          |     |         J2          |
       +---------------------+     +---------------------+     +---------------------+
       | C1 C2 O O O O R R   |     | C1 C2 O O O O R R   |     | C1 C2 O O O O R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       32 bytes      |     |       32 bytes      |     |       32 bytes      |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |    -------------    |     |     V3  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         N2          |     |         N2          |
       +---------------------+     +---------------------+     +---------------------+
       | C1 C2 O O O O R R   |     | C1 C2 O O O O R R   |     | C1 C2 O O O O R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       32 bytes      |     |       32 bytes      |     |       32 bytes      |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |   ---------------   |     |     V4  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         K4          |     |         K4          |
       +---------------------+     +---------------------+     +---------------------+
       | C1 C2 R R R R R S1  |     | C1 C2 R R R R R S1  |     | C1 C2 R R R R R S1  |
       +---------------------+     +---------------------+     +---------------------+
       |  S2 D D D D D D D   |     |  S2 D D D D D D D   |     |  S2 D D D D D D D   |
       +---------------------+     +---------------------+     +---------------------+
       |       31 bytes      |     |       31 bytes      |     |       31 bytes      |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+

Frames in case of synchronous PDH mapping:


         C-12 keret 136 byte         VC-12 keret 140 byte        TU-12 keret  144 byte
              500 us                        500 us                      500 us
       +---------------------+     +---------------------+     +---------------------+
       |    -------------    |     |    ------------     |     |     V1  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    -------------    |     |         V5          |     |         V5          |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 0   |     |       Time Slot 0   |     |       Time Slot 0   |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 1 to 15  |     | Time Slots 1 to 15  |     | Time Slots 1 to 15  |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 16  |     |       Time Slot 16  |     |       Time Slot 16  |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 17 to 31 |     | Time Slots 17 to 31 |     | Time Slots 17 to 31 |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |    ------------     |     |     V2  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         J2          |     |         J2          |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 0   |     |       Time Slot 0   |     |       Time Slot 0   |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 1 to 15  |     | Time Slots 1 to 15  |     | Time Slots 1 to 15  |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 16  |     |       Time Slot 16  |     |       Time Slot 16  |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 17 to 31 |     | Time Slots 17 to 31 |     | Time Slots 17 to 31 |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |    -------------    |     |     V3  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         N2          |     |         N2          |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 0   |     |       Time Slot 0   |     |       Time Slot 0   |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 1 to 15  |     | Time Slots 1 to 15  |     | Time Slots 1 to 15  |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 16  |     |       Time Slot 16  |     |       Time Slot 16  |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 17 to 31 |     | Time Slots 17 to 31 |     | Time Slots 17 to 31 |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |   ---------------   |     |     V4  (TU pointer)|
       +---------------------+     +---------------------+     +---------------------+
       |    --------------   |     |         K4          |     |         K4          |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 0   |     |       Time Slot 0   |     |       Time Slot 0   |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 1 to 15  |     | Time Slots 1 to 15  |     | Time Slots 1 to 15  |
       +---------------------+     +---------------------+     +---------------------+
       |       Time Slot 16  |     |       Time Slot 16  |     |       Time Slot 16  |
       +---------------------+     +---------------------+     +---------------------+
       | Time Slots 17 to 31 |     | Time Slots 17 to 31 |     | Time Slots 17 to 31 |
       +---------------------+     +---------------------+     +---------------------+
       |   R R R R R R R R   |     |   R R R R R R R R   |     |   R R R R R R R R   |
       +---------------------+     +---------------------+     +---------------------+

Frames in case of mapping ATM cells:


              VC-12 keret 140 byte        TU-12 keret  144 byte
                     500 us                      500 us
            +---------------------+     +---------------------+
            |    ------------     |     |     V1  (TU pointer)| ->-+
            +---------------------+     +---------------------+    |
            |         V5          |     |         V5          | -<-+
            +---------------------+     +---------------------+
            |     34 D bytes      |     |     34 D bytes      |
            +---------------------+     +---------------------+
            |    ------------     |     |     V2  (TU pointer)|
            +---------------------+     +---------------------+
            |         J2          |     |         J2          |
            +---------------------+     +---------------------+
            |     34 D bytes      |     |     34 D bytes      |
            +---------------------+     +---------------------+
            |    -------------    |     |     V3  (TU pointer)|
            +---------------------+     +---------------------+
            |         N2          |     |         N2          |
            +---------------------+     +---------------------+
            |     34 D bytes      |     |     34 D bytes      |
            +---------------------+     +---------------------+
            |   ---------------   |     |     V4  (TU pointer)|
            +---------------------+     +---------------------+
            |         K4          |     |         K4          |
            +---------------------+     +---------------------+
            |     34 D bytes      |     |     34 D bytes      |
            +---------------------+     +---------------------+

VC-12 Path OverHead (POH)

Bytes of VC-12 POH are located in VC-12/TU-12 frames as mentioned above. The first byte of TU-12 frame (V1) is TU-12 PTR, which points V5 byte of VC-12 POH. During the multiplexing the first byte of TU-12 frame gets to a given position of VC-4 frame.

LOVC POH (VC-12 POH):

Label Description Values
V5 TU-12 PTR points here Error checking BIP-2 bit1-2, REI bit3, RFI bit4, signal label bit5-7

000 Unequipped
010 Async
011 Bit sync
100 Byte sync
, RDI bit8
J2 LOW PATH trace (low order path access point identifier)
N2 Network operator byte
K2 Automatic Protection Switching (APS) channel