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== Introduction ==


Point-to-Point Networks are used around the world to provide fast, secure, and reliable communication.



Point-to-Point Protocol (PPP)

PPP is the protocol used for communication over a point-to-point link. It does this by transporting datagrams from a predetermined network protocol across the link. The connection passes through four stages starting with a link establishment stage where the two parties use LCP (link control protocol) datagrams to ensure a connection is up and running. After which they can enter the configuration stage to negotiate the parameters of the PPP frames that will be sent back and forth over the connection. Once the initial parameters are set the connection enters the monitoring stage, which regularly tests the connection to make sure it is running smoothly. A typical point-to-point connection is always on, but a user may choose to terminate the link at any time. This is done using LCP.

PPP Frame

The layout of a standard PPP frame:

|---|---|---|---|---|---|
| i | ii|iii| iv| v | vi|
|---|---|---|---|---|---|

i.Flag (1 byte)
The flag byte is used to indicate the beginning or end of a frame.

ii. Address(1 byte)
Because there is only one destination in a point-to-point network this byte is usually set to the value 11111111 known as the broadcast address. This value is ignored by PPP, however it may be assigned a value for clarity.

iii. Control (1 byte)
Tells the destination what to do with the packet once the frame has been received.

iv. Protocol (2 bytes)
These bytes are used to identify the transmission protocol(TCP for instance) used in the data field.

v. Data 
The data field can have a variable length, and contains a datagram of the specified protocol in the protocol field.

vi. Frame Check Sequence
Reserves the last 2 bytes of the frame to make it easier to locate it's end. It can also be 4 bytes if necessary to help improve error checking.