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ATM over SONET/SDH

ATM over SONET/SDH. Fiksman Evgeny Gurovich Alexander DigLab Seminar. Problems to solve. Each pair of multiplexors (A - A’) in the system has to be manufactured by the same supplier. Access to a single tributary circuit, demands demultiplexing the whole structure and then remultiplexing.

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ATM over SONET/SDH

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  1. ATM over SONET/SDH Fiksman Evgeny Gurovich Alexander DigLab Seminar

  2. Problems to solve • Each pair of multiplexors (A - A’) in the system has to be manufactured by the same supplier. • Access to a single tributary circuit, demands demultiplexing the whole structure and then remultiplexing. • The multiplexing structure different in some countries. • There is an enormous cost benefit in removing the multiplexors entirely.

  3. Synchronous Optical Network • The basic structure in Sonet is a frame of 810 bytes (STS-1). • Sent every 125msec - minimum speed 51.84 Mbps • The first three columns of every row are used for administration and control of the multiplexing system.

  4. Sonet Synchronous Payload Envelope • SPE is pointed by H1, H2. • Small differences in the clock are accommodated by H3 pointer. Multiple STS-1 frames can be • byte-multiplexed together. Four STS-1 to STS-4 • phase-aligned with their payloads. Four STS-1 to STS-4c.

  5. Synchronous Digital Hierarchy • STM-1 is STS-3c • Minimum speed 155.52 Mbps • Accommodates both US and the European line speeds. • Faster line speeds are obtained in the same way as in Sonet - by byte interleaving of multiple STM-1 frames. Four STM-1 frames may be multiplexed to form an STM-4 or STM-4c at 622.08 Mbps.

  6. Asynchronous Transfer Mode • Data transferred in small packets called “cells” • Information flow is along paths called “virtual channels”. • ATM is designed to make hardware switching simple. • At the edges of the network, user data frames are broken up into cells. • The ATM cell switching network only checks cell headers for errors (HEC) and simply discards errored cells.

  7. ATM over SDH physical interface • Cells are transmitted row by row without regard to row or cell boundaries. • There are a number of end-to-end TDM channels across the link, independent of the main channel - primary function of SOH & LOH. • Three standards: • Sonet STS-3c • STM-1 • Sonet “Lite” • In the early specifications H4 points to the cell boundary. • Now cell delineation done only by the HEC method.

  8. HEC method for cell delineation • HCS is a CRC-8 calculation over the first 4 octets of the ATM cell header. • While searching for the cell boundary location, the cell delineation circuit is in the HUNT state. • On correct HCS the cell delineation state machine locks on the particular cell boundary and enters the PRESYNC state. • If no HCS errors are detected in this PRESYNC period then the SYNC state is entered.

  9. Bibliography • Asynchronous Transfer Mode (ATM) Technical Overview - IBM, International Technical Support Organization, SG24-4625-00. • PMC - PM5346 Datasheet.

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