D Channel Data Link Protocol

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