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Sasha Vainshtein

IETF-56 PWE3 Workgroup 19-Mar-03, San Francisco Edge-to-edge Emulation of Structured TDM Services over PSN: Open Issues. Sasha Vainshtein. What This Is About. Consensus on emulation of unstructured TDM (E1, T1, E3, T3) looks very close :

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Sasha Vainshtein

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  1. IETF-56PWE3 Workgroup19-Mar-03, San FranciscoEdge-to-edge Emulation of Structured TDM Services over PSN:Open Issues Sasha Vainshtein

  2. What This Is About • Consensus on emulation of unstructured TDM (E1, T1, E3, T3) looks very close: • Raw TDM payload encapsulated "as is" in packets of fixed size • No alignment presumed or preserved • The packet payload size is a matter of convenience • Different sizes should be supported in order to allow trade-off between BW efficiency and packetization latency • Mandatory defaults can be agreed upon • Why is the situation with structured TDM so different? • Different people mean different things when they talk about "structured TDM emulation" • A clear WG position on scope, requirements and architecture seems to be lacking • The WG input is solicited on a number of issues

  3. What is Structured TDM • A Frame Alignment Signal structures the trunk bit-stream at two levels: • TDM frames: • Always present • Always octet-aligned • Number of octets per frame is trunk-specific • DS0 channels use a single octet per frame • NxDS0 services use a subset of N octets per frame • Some applications depend on NxDS0 frame integrity • Always 8000 frames per second • An NxDS0 service carries Nx64 kbit/sec, 1 <= N <=31 • Multiframes/superframes • May be absent on some trunks • Not synchronized between different trunks • The number of frames per multiframe is trunk-dependent • Bit-error detection and OAM channels • DS0 channel-associated signaling (CAS)

  4. Why Structured TDM Emulation? • Suggested rationales vary between: • Save the PSN BW by skipping unused channels • This reasoning has been used for AAL5 SDU in ATM • Fidelity of BW-saving emulation may be limited • Split a single trunk attachment circuit between several applications • Different groups of timeslots are used by different applications and are carried by different PWs • Assumes a cross-connect NSP in the PE • Improve resilience of the attachment circuit to packet loss • The "interface down" condition in the CE never occurs • Can be achieved in unstructured emulation under reasonable assumptions • Improve resilience of specific applications to packet loss • Interpolate between lost packets carrying Voice samples • Application-specific BW management • E.g., graceful congestion handling

  5. Splitting the Access Circuit Trunk A DS0 Grooming Application

  6. Which Structures to Preserve? • Answers vary between: • NxDS0 frames • Supports all application requirements for data integrity • Supports interworking of services originating in different trunks • Allows extension of N beyond a single trunk • Treats CAS as encoding of CE application signaling • Signaling phase relative to data is not perfect • "Form" of signals can be preserved • Signaling may be carried in separate packets or travel appended to the NxDS0 payload • NxDS0 frames without CAS or trunk multiframes with CAS: • CAS remains interleaved with the TDM payload • Additional timeslot required for E1 trunks • Problematic with existing NSP blocks • Problematic handling of packet loss • "Replacement logic" for data and signaling is different

  7. Which Structures to Preserve (2)? • ATM-CES-like: • NxDS0 frames for NxDS0 without CAS • "Superframe structures with signaling" for NxDS0 with CAS • Requires extraction/insertion of CAS as a set of bit-streams • Makes services with CAS trunk-specific • Multiframe alignment may be preserved at the expense of additional delay in the NSP • Requires double capacity of the IWF/NSP interface • Handling of lost packets is problematic • Always NxDS0 multiframes/superframes • Problematic if multiframe structures do not exist in the trunk • End services originating in different trunks will not interwork • Loop Emulation-like: • Frame integrity is not preserved, only DS0 integrity • Suits only telephony applications • Allows additional BW saving

  8. Granularity of Packet Latency? • Important for low-rate services: • Trade-off between packet latency and BW efficiency becomes essential • The rate varies from 64 kbit/s to 1920 kbit/s • Latency of NxDS0 PWs between the given pair of PEs should not depend on N • The answers vary between: • 125 microsecond granularity is mandatory • 125 microsecond granularity is recommended • The granularity is not important at all

  9. Piece of Wire or Distributed DXC? • Trade-off between the IWF and NSP Complexity • Answers vary between: • A "piece of wire" approach (which wire?) • Absolute timeslot positions in the trunk must be preserved • CAS alignment with data is fully preserved (NSP may impair it later) • Handling of lost packets may be non-trivial • A DXC approach • Absolute timeslot positions are irrelevant • TDM data and signaling may follow different paths • Synchronization between data and signals may be impaired • Reliable signaling delivery may be an issue • A Mix of both • Absolute timeslot positions are irrelevant • CAS alignment with data is preserved

  10. Questions?

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