Packet over SONET/SDH

Packet Over SONET/SDH :
Many of us use the POS interfaces in the routers .
So here we will have one small technical summary about POS(packet over SONET/SDH)
POS also known as PPP over SONET/SDH . This is scheme which uses PPP encapsulation to map IP datagrams into the SONET/SDH payload .

Why SONET/SDH payload and PPP are used for enacapsulation :
Since SONET/SDH is point to point circuit , PPP is well suited here .

POS layers :
There are three pos layers. They are
1 . Bottom layer : mapping into SONET/SDH
2 . Mid layer - Framing of PPP with HDLC .
3 . Top layer – IP encapsulation into PPP
Here I rememember Sridhar G , my mentor in HCL . I created the stream for POS interface and traffic was not success . At that time , he told me to select the HDLC in Agilent RT.
For Ethernet , We use some other encapsulation in RT. Please share that info if u know.

Operation of POS is :

When transmitting : IP -> PPP->FCS generation->Byte stuffing->scrambling-SONET/SDH framing
When receiving : SONET/SDH deframing->descrambling->de stuffing->FCS detection->PPP->IP


Bit stuffing :

In PPP , packet begins and ends with 01111110 .when ever sender sees five consecutives 1s , it automatically inserts the 0 before transmitting . This is stuffing
When ever receiver five consecutive 1s followed by 0 , it destuffs the 0

The main components :

Flag – delimiter of the Frame
Address – Address in PPP packet
Protocol – identifies the network layer protocol
FCS – frame check sequence
Control – Control information in PPP