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Understanding SONET: Pointer Processing

By K. Surya Prakash

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       I already said in one of the previous sections that the STS SPE would be floating and the pointers give the position from where it is going to start. The phase differences between Transport Overhead and SPE must be accommodated. Also SONET/SDH systems are synchronous systems, i.e., all the clocks are supposed to be same, but in reality they are not. There shall always be small differences.  Even when all the clocks are same there can be jitter, which must also be accommodated. Data can come into a device slower or faster than it is transmitted out at the other side.  So something has to be done to adjust the differences between the transmit and receive clocks. This is where pointer action bytes H1, H2, and H3 come in.

H1 and H2 bytes are called pointer bytes. Consider H1 and H2 as shown in figure-6. Bits 1 to 4 are called New Data Flag (NDF) bits represented by Ns. It is set to 0110 for normal operation. Bits 5 and 6 are currently undefined in SONET but are used in SDH, which we will study when we go through SDH.  Bits 7 to 16 carry the pointer value.

Figure-6 

The value of the bits 7 to 16 can vary from 0 to 782.  A value of 0 indicates that SPE starts at the first byte immediately after H3 byte. If the pointer value is 1 the payload starts at the second byte after H3 etc. Figure 6 shows the layout of H1 and H2 pointer. For the time being ignore the meaning of �I� and �D� labels.

Figure 7 below shows locations of SPE for different values of pointer. 

Figure 7

Now let us see why and how H3 is used. Suppose the incoming clock is faster than the outgoing clock. Then an extra byte is accumulated in our receive buffer, compared to what we can transmit. Now this extra byte is put into H3 location. So when we transmit one SONET frame of 810 bytes, we actually transmit 784 bytes of payload (86 columns times 9 rows, plus one H3 octet), rather than 783 bytes of payload.

A similar problem occurs if the incoming clock is slower than the outgoing clock. Then there will be a deficit in the receiver buffer. To overcome this problem a stuff byte in the location after the H3 byte is sent. Moving of SPE backwards is called negative justification and moving it forward is called positive justification.

In figure-6 pointer bits are labeled as IDIDIDIDID. �I� indicates increment and �D� indicates decrement. With the help of these I and D bits we can make positive or negative justification. Let us go through how these positive and negative justifications are done.

Sonet equipment will have a register using which it compares the present pointer bits with the previous bits. If �I� bits are inverted then pointer value is incremented (positive justification) or if �D� bits are inverted then pointer value is decremented (negative justification). By changing the NDF value also new pointer value can be introduced. We shall discuss all this in more detail.

Positive Justification:

Figure-8

 Figure-9

Consider Figures 8 and 9 for the explanation that follows. Let the pointer value be 214.  Its decimal equivalent is 0011010110 which is shown in first row in figure-8. Now at the frame where positive justification should be done the I bits are inverted which is shown in the second row in figure-8. It is in this frame that the byte after the H3 is stuffed. As soon as the receiver detects that the I bits are inverted, it discards the byte after H3 as the content is meaningless-but it usually contains all 0s and also the pointer value is incremented at the receiver side to point to the adjusted payload.

In the next frame the receiver receives the incremented pointer value. This new pointer value is repeated for the next frame as well as for the fourth frame. The new pointer value received in the fourth frame is available for pointer adjustment again where as that received in the previous two frames is not available for adjustment. This is because a new pointer value is accepted only when it is received for three consecutive frames. 

Negative Justification:

Figure-10

 Figure-11

Consider Figures 10 and 11 for the explanation that follows. It is very much similar to positive justification. Let the pointer value be 214.  Its decimal equivalent is shown in first row in figure-8. Now at the frame where negative justification should be done the �D� bits are inverted which is as shown in the second row in figure-10. It is in this frame that the byte after the H3 is used to carry a data byte. As soon as the receiver detects that the �D� bits are inverted, it immediately takes up H3 as it contains some relevant data  and also the pointer value is decremented at the received SONET equipment to point to the adjusted payload. 

In the next frame the receiver receives the decremented pointer value. This new pointer value is repeated for the next frame as well as for the fourth frame. The new pointer value received in the fourth frame is available for pointer adjustment again.  

Pointer Adjustment Using NDF: 

 Figure-12            

Till now we have discussed incrementing or decrementing of pointer value only by 1. What if we want to adjust pointer value by more than 1. This can be achieved using NDF.  To make adjustments using NDF,  it's bits must be set to1001. The frame in which NDF is set new pointer value is introduced immediately. Here there is no need for inversion of �I� or �D� bits. Pointer adjustments using NDF cannot occur more often than every fourth frame in SONET.  This is so because the new pointer value is accepted only if it is received for three consecutive frames. Another reason for the limit of every fourth SONET frame is because of two special cases.

Special Cases:

In the positive pointer adjustment case suppose the pointer is changed from 782 to zero. Here you can see that the pointer in the frame following the frame with the NDF inverted will point to the same SPE as the (implied) pointer of the frame with the inverted NDF. This means that the system has to ignore the pointer for one SONET frame and the first valid pointer will be the third SONET frame (the frame with the NDF inverted is one, the next frame is two, and the third frame has a valid pointer).

 A similar special case occurs when the pointer rolls over backwards from zero to 782. The zero value pointer is pointing at the byte after the H3 byte. The next frame has the NDF set and the H3 byte is used for data. The resultant pointer is pointing to the H3 byte. The beginning of the next SPE is the last byte in row 3 of the SONET frame after the frame with the NDF inverted. However, the pointer in that frame cannot point backwards in the frame, so the SONET equipment has to handle this situation as a special case. The pointer in that SONET frame points to the last byte in row 3 of the next (third) SONET frame. This can be confusing but if you draw pictures similar to Figure 9 and Figure 11 you shall see how it works.

 We will be processing pointers in a different way when we are using concatenated structures, which we will be discussing later.