SONET Frames and SPE's

STS-1 Frame  vs  STS-1 SPE

STS-1 Frame

The STS-1 frame is 90 columns x 9 rows, for a total of 810 bytes
As with ALL STS-n data rates,  Frame Rate is Constant at 8000 fps

STS-1  SPE

each SONET STS-1 frame is 90 bytes wide, and 9 bytes tall

each SPE is 87 bytes wide, and 9 bytes tall

Order of byte Transmission (reads like a book)– the model that uses columns and rows to depict a SONET or SDH frame, transmits the bytes in the same order as if you were reading like a book – the top leftmost byte is transmitted first, then the 2^nd byte, and so on, until you reach the rightmost byte of row 1.  The it starts with the leftmost byte of Row 2, and continues downward.

SPE (Synchronous Payload Envelope)

The SPE is supplied by the User !!!  It can arrive at the SONET framing device at any time.  Therefore it is almost always embedded across two STS-1 Frames, since the chances of it arriving exactly at the time of the first byte of the SONET frame is very remote.

Since SONET is “plesiosynchronous” (almost synchronous), the SPE can begin anywhere in the SONET Frame and always begins with the first byte of the POH (Path OverHead) . . . the J1 byte.  The placement of J1 varies and therefore a pointer is needed.  This pointer has 3 bytes (H1, H2, and H3) and is the 4^th row of the Transport Overhead.  The position of J1 is referenced by the H1 and H2 bytes in the pointer, which contain an offset value:

• the byte directly to the right of H3 (which is always the 274^th byte of the frame) is the reference byte position, and is referenced as zero offset

• if J1 is before the reference byte, then H1 and H2 contain a decrement offset

• if J1 is after the reference byte, then H1 and H2 contain an increment offset.

NOTE:  diagrams that show the SPE all by itself, do not show the SPE start point as being  anywhere in the in the payload since the payload structure is omitted.  Some diagrams show the SPE with the Transport columns included – others show the SPE without the Transport columns they show the SPE with the POH as column 1, which is confusing.. 

Major components of the SONET STS-1 Frame:

SONET Frame – the all-inclusive unit of data, which is always depicted as a 2D array of bytes - 90 columns x 9 rows, for a total of 810 bytes per frame.  The 90 columns are comprised of 3 columns of Transport overhead and 87 columns of Payload.  

• Transport Overhead - columns 1,2, and 3 (3 columns x 9 rows = 27 bytes per frame) – the first 3 columns (at the left of the frame) contain information about the sections and lines

• Payload – columns 4 through 90 of the SONET frame (87 columns x 9 rows = 783 bytes per frame).  The payload is just that portion of the SONET frame containing SPE’s (Synchronous Payload Envelope).  The SPE’s can float around within the payload columns, since they are created by the end user’s equipment.  Therefore the SPE’s do not conform to staring at any particular column !!!  However, they cannot use the first 3 columns, which are reserved for the Transport Overhead.
  
  

• SPE (Synchronous Payload Envelope) – same size as the payload, 87 columns x 9 rows = 783 bytes, but it can begin at any column and any row, depending on when the user’s equipment began transmitting the data.  The SPE “floats” within the payload structure of the SONET frames.  The SPE includes 86 columns of user data and 1 column of path overhead (POH).
  
  

• Path OverHead (POH) - one column (whose position can be anywhere in the frame from the 4^th column to the 90^th column – see the above diagram) is used to control and monitor the path.

Why do they call 87 of the columns (column 4-90 = 783 bytes), the “Payload”?  This includes the path overhead column as payload!   Isn’t the payload actually 86 columns, or 774 bytes?  Because the Equipment views it as payload.  The basic SONET frame itself is defined without reference to the path overhead.  The equipment along the path (the amps, regens, and mux’s) work only with the transport of the SONET frames.  They use only the 3 transport overhead columns because they contain info about the sections and lines.  So the equipment “sees” the SPE as pure payload.  The SPE is embedded within the payload columns, and is in fact named as payload (Synchronous Payload Envelope).  But the equipment does not need to reference the POH column, which is only needed by the end-user equipment (such as an OC48 card on an MSPP – MultiService Provisioning Platform)

It is true, however, that the actual “user data” is 86 columns x 9 rows = 774 bytes.  The User Data = SPE-POH (POH = Path OverHead).  Note that you should not use the term, “user payload” – because it may be confused with the word payload in SPE)

NOTE:  this is similar to an IP packet,, which views the two portions as IP Headers (overhead), and Data (payload)  - the data does contain headers from the upper layers (Transport, Session, etc) but those headers are invisible to the IP layer and are seen only as data that is wrapped within the IP header.