1. D Channel Data Link Protocol Link access procedure on the D channel - LAPD

2. LAPD 1 Goals of LAPD Framing – signal beginning and end of transmission delimit user data within frame Addressing – indicate which device is the transmitter and receiver Sequencing – maintain sequence numbering of transmitted data

3. LAPD 2 Acknowledgment – acknowledge receipt of frames Time outs – handle situations where an appropriate reply does not arrive within specified time Error-control – detect error bits, out-of- sequence frames and correct these errors

4. LAPD 3 Flow control – provide mechanism os that a receiver is not flooded with frames from a fast transmitter Standards described in various CCITT documents Also referred to as Digital Subscriber Signaling System No.1 DSS1 Data Link

5. LAPD Frame 1 Unit of transmission in LAPD is a frame Frame fields Flag – 01111110 – hex 7E – indicates start and end of frame Address – indicates user device, service and receiver – 2 octets (bytes) Control – identifies type of frame may include sequence ACK numbers

6. LAPD Frame 2 Information – Contains DSS1 netowrk layer – user data and or LAPD management information, variable length but octet aligned Frame clock sequence – 16 bit number from CRC to detect errors

7. Zero bit insertion After every fifth bit insert a zero Tx wants to send 011111 11111 0111110 – no gaps 01111101111100111110 Six bits in a row is a flag Seven bits in a row is an abort signal Diagram LAPD frame format p105