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Contents

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5.5    Advantages of the OSI Reference Model    42

5.6    The Seven Layers of the OSI Reference Model    43

5.6.1    Layer    1: The Physical Layer    43

5.6.2    Layer    2: The Data Link Layer    43

5.6.3    Layer    3: The Network Layer    44

5.6.4    Layer    4: The Transport Layer    44

5.6.5    Layer    5: The Session Layer    45

5.6.6    Layer    6: The Presentation Layer    45

5.6.7    Layer    7: The Application Layer    46

5.7    Comprehension Issues    46

5.7.1    An Analogy: The Move to Europę    47

6    The Abis-lnterface_51

6.1    Channel Configurations    51

6.2    Alternatives for Connecting the BTS to the BSC 52

6.2.1    BTS Connection in a Serial Configuration    54

6.2.2    Connection of BTSs in Star Configuration    55

6.3    Signaling on the Abis-lnterface    55

6.3.1    OSI Protocol Stack on the Abis-lnterface    55

6.3.2    Layer    2    56

6.3.3    Layer    3    71

6.4    Bringing an Abis-lnterface Into Service    87

6.4.1    Layer    1    87

6.4.2    Layer    2    87

7    The Air-lnterface of GSM_89

7.1    The Structure of the Air-lnterface in GSM    89

7.1.1    The FDMA/TDMA Scheme    89

7.1.2    Frame Hierarchy and Frame Numbers    90

7.1.3    Synchronization Between Uplink and Downlink 93

7.2    Physical Versus Logical Channels    94

7.3    Logical-Channel Configuration    94