dB =10log P1 P2

! HDLC operation consists of exchange of Iframes,
and some U-frames.
! Operation of HDLC involves three phases.
! First one side or another initializes the Data Link so that
frames may be exchanged in an orderly fashion.
! During this phase, the options that are to be used are agreed
upon.
! Then the two sides exchange user data and the control
information to exercise flow and error control.
! Finally, one of the two sides will signal termination of
operation.
! Data Transfer:
! Once initialization has taken place a logical connection is
established.
! Both sides may begin to send user data in I-frames
! SNs will be either modulo 8 or modulo 128 depending on
whether 3 or 7 bit sequences are used.
! RR (received ready) frame is used when there is no reverse
user data to carry acknowledgement.
! RNR (Receiver Not Ready) acknowledges I-frames asking
the peer entity to suspend transmission of I-frames.
! REJ (Reject) indicates the Go Back n ARQ.
! SREJ (Selective Reject) is used to request retransmission of a
single frame.
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! Initialization:
! This may be requested by either side.
! Initialization procedure signals the other side that
initialization is requested and specifies which of the
three mode (NRM, ARM, ABM) is requested and whether
3 or 7 bit sequence numbers are to be used.
! If the other side accepts then the HDLC Module at that
end will send an UA (unnumbered acknowledgement)
frame back to initiating side.
! If request is rejected then DM (Disconnected Mode)
frame is sent.
! Disconnect:
! Either HDLC module can initiate a disconnect.
! HDLC issues a disconnect by replying with a DISC
(disconnect) frame.
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