dB =10log P1 P2

! LAPD:
! Link Access Procedure D channel is another simplified subset of
HDLC issued by ITU-T as part of its recommendation on ISDN.
! LAPD provides DLC over channel D.
! There are several key differences between LABD and HDLC.
! LAPD is restricted to ABM and always use 7-bit SN.
! The FCS of LAPD is always 16 bit CRC.
! The address field of LAPD is a 16 bit field that actually contains
two sub addresses.
! LAPM:
! Link Access Procedure for Modems is designed to do
asynchronous–synchronous conversion, error detection, and
retransmission.
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! LLC:
! Logical Link Control is a part of IEEE 802 family of
standards for controlling operation over a LAN.
! LLC is lacking some features found in HDLC and
also has some features not found in HDLC.
! The most obvious difference is in the frame format.
! Link control functions in the case of LLC are divided
between two layers:
! A MAC and the LLC layer.
! The previous shows the structure of the combined MAC/LLC frame.
! Shaded portion corresponds to fields produced by LLC.
! The rest are added by the MAC.
! Two addresses are required since there is no concept of primary and
secondary.
! Error detection is done at the MAC layer by 32-bit CRC.
! At the LLC layer there are the destination and source services Access
Points (DSAP and SSAP) identifying the logical user of LLC at the
source and destination systems.
! LLC control field has the same format as the HDLC limited to 7 bit
SNs.
! Operationally LLC offered three forms of services.
! The connection-oriented mode service is the same as ABM of HDLC.
! The other two services, unacknowledged connectionless and
acknowledged connectionless.
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