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ATM Layered Model

ATM Layered Model

Physical Layer

• Physical layer consists of two sublayers
  • Physical Medium (PM) sublayer
    • correct transmission and reception of bits

    • medium dependent (optical, electrical)

  • Transmission Convergence (TC) sublayer
    • Maps recovered bitstream into the ATM cells

    • Maps the cells into the transmission mode e.g. SDH, PDH, cell-based

• Two options for cell transmission
  • At the NNI and within the network SDH is preferred transport mechanism (PDH in early versions)

  • At the UNI a cell-based transport is preferred

  • Data rates for both options - 155Mbps 622Mbps

ATM Cell Transmission

• Transmission of ATM cells in an SDH signal

  
  
  

ATM Layer

• ATM layer is fully independent of physical medium

• Four main Functions
  • Multiplexing and demultiplexing from different connections (using VCI) into a single cell stream

  • Cell header extraction/insertion for communication with Adaptation Layer

  • Translation of VCI at ATM switching nodes

  • Implementation of flow control mechanism upon the UNI

Cell Multiplexing

ATM Adaptation Layer

AAL

• AAL links specific services of B-ISDN to generic ATM cells

• AAL supports four classes of service

• AAL has two sublayers
  • Segmentation and Reassembly (SAR) of the Protocol Data Units (PDU) from the higher layer

  • Convergence sublayer (CS) - service specific functions

AAL Service Classes

• Classification of Services for AAL

  
  
  

• Examples
  • Voice: Class A Service

  • Variable Bit Rate Video: Class B Service

  • X.25 Packet/Signalling Services: Class C Service

  • LAN Interconnect: Class D Service

Class A

• Sometimes called "Circuit Emulation"

• Timing relationship between source and destination

• Constant bit rate

• Connection-oriented

• Examples include 64kbps, transparent carriage of T1 or E1 signals

Class B

• Timing relationship between source and destination

• Connection-oriented

• Variable bit rate

• Examples include variable video and audio bit rates

Class C

• No timing relationship between source and destination

• Variable bit rate

• Connection oriented

• Examples are connection-oriented data transfer in the user plane and signalling in the control plane

Class D

• Similar to C but connectionless

• Example: connectionless data transport such as LAN interconnection traffic

AAL types

• To support the four service classes, five AAL types are used, initial definitions
  • Type 1 supports class A

  • Type 2 supports class B

  • Type 3 supports class C

  • Type 4 supports class D

• Types 3 and 4 merged to unified 3/4 type which can be applied in a connection of a connectionless manner

• Type 5 defined
  • Simple and Efficient Adaptation Layer (SEAL)

  • suitable for Class C services for which type 3/4 is too complex

ATM Higher Layers

• In some cases the AAL will also have to support higher network layers
  • For example in the case of signalling the AAL must be able to support the necessary Layer3 signalling protocols, while for the support of Class D (connectionless) services, the AAL should support a higher layer that contains addressing and routing functions (e.g. TCP/IP)

• The exact alignment between the layers of the OSI and B-ISDN PRMs is still not clear, and requires further definition by CCITT