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ATM Transmission

ATM Transmission

Physical Layers

• ATM does not specify a physical layer -alternatives include:
  • T1, T3 telephone lines

  • Fibre channel

  • TAXI - FDDI fibre optic physical layer minus tokens and Station ManagemenT

  • Token ring or STP

  • UTP - key to ATM expanding into networking

  • ATM over SONET/SDH

SONET/SDH

• ATM specified for STM-1(STS-3) and STM-4 (STS-12) giving 155 Mbps and 622 Mbps

• ATM cells are packed into STM frames

• STM-1 possible payload

		270 x 9 = 2430
		2430 - (9x9 + 9 x1) = 2340 octets

• 2340 octets not an integer multiple of 53

• ATM cells are split across frames and a pointer in the path header indicates next cell boundary

• SDH can combine several ATM streams

Throughput

• STM-1
  • nominally 155 Mbps

  • less STM-1 path overhead 150 Mbps

  • less ATM header 136 Mbps