Transport of AAL1 Frames over PSN

1
Transport of AAL1 Frames over PSN

• draft-davari-pwe3-aal12-over-psn-01.txt
• Shahram Davari
• Senior Product Research Engineer
• PMC-Sierra
• March 2003

2
Changes Made to PDU mode in ATM Encap draft

• Removed the requirement to reassemble fragments
  at the egress PE
• Replaced FRAG bits with UU-bit
• This change improves the PDU mode
• By not re-ordering the OAM cells
• By not introducing excess delay
• By not requiring reassembly buffer
• This change actually made PDU mode applicable to
  other AAL types

3
Why is the Draft Informational?

• It reuses the ATM transport modes of ATM Encap
  draft (Martini)
• N1 cell mode (mandatory), 11 cell mode and PDU
  mode (optional)
• It does not introduce any new protocol
• Except LR bits for (PSN) failure indication, and
• Packet loss handling due to congestion
• Which should actually be added to ATM Encap draft

4
Raised Issues Proposed Solutions

• How can we ensure correct AAL1 operation after
  packet loss?
• Concatenate N cells per packet
• where Nlt8
• Can tolerate random packet loss
• Replace the lost packets with pre-configured
  dummy packets
• Optionally use interpolation to conceal packet
  loss
• Requires detailed knowledge of customer traffic
• ATM AIS generation at egress PE does not help.

5
Raised Issues Proposed Solutions

• Wont the ATM OAM transparency result in variable
  packet sizes, which is undesirable for AAL1 clock
  recovery?
• Not for N1 and 11 cell modes
• Two choices for PDU mode
• Dont send OAM cells
• Delay OAM transmission until after transmission
  of a pre-configured fixed-size packet
• May cause OAM cell mis-ordering

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Suggestion for Moving Forward

• Add the proposed solutions to raised issues to
  the draft
• Add PSN failure and packet loss handling to ATM
  encap draft
• Accept this draft as an informational WG draft