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ATM Layering

ATM Layering. Does not follow 802 layering However, does subdivide the data link layer. OSI. ATM. Data Link. ATM Adaptation Layer (AAL). CS. SAR. ATM (Application-Independent). Physical. Physical. ATM Layering. ATM Layer Manages delivery of 53-octet cells

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ATM Layering

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  1. ATM Layering • Does not follow 802 layering • However, does subdivide the data link layer OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  2. ATM Layering • ATM Layer • Manages delivery of 53-octet cells • Application-independent: Same for all Classes of Service OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  3. ATM Layering • ATM Adaptation Layer • Provides what is needed for different classes of service • Application-dependent OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  4. ATM Layering • ATM Adaptation Layer is Subdivided Further • CS (Convergence Services) • SAR (Segment Assembly/Reassembly) OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  5. ATM Layering • Convergence Services (CS) • Provides whatever services are needed for a particular service class (A, B, C, or D) OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  6. ATM Layering • Segment Assembly/Reassembly (SAR) • Does fragmentation and reassembly if CS frame is longer than the ATM payload field OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical

  7. ATM Layering • Physical Layer • Uses SONET/SDH optical fiber transmission standard (see Module B) OSI ATM DataLink ATM Adaptation Layer (AAL) CS SAR ATM (Application-Independent) Physical Physical: SONET/SDH

  8. ATM Cell • Organization • 5 octets of overhead, 48 of payload (data) 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT res CLP Header Error Control Payload (48 octets)

  9. ATM Cell • Virtual Circuit Number is Hierarchical • Virtual Path Identifier: Identifies a site • Virtual Channel Identifier: Identifies a station at the VPI site 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont)

  10. ATM Cell • Payload Type (2 bits) identifies cell category • 00 = user information (data) • reserved for later definition (1 bit) 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT res CLP Header Error Control

  11. ATM Cell • Cell Loss Priority Bit • 0 if sending within committed information rate • 1 if sending faster: switches will discarded if congestion occurs 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT res CLP Header Error Control

  12. ATM Cell • Header Error Control Field • To check the header (not the payload) for errors • Discard cell if an error is found 8 7 6 5 4 3 2 1 Virtual Path Identifier VPI (cont) Virtual Channel Identifier VCI (cont) VCI (cont) PT res CLP Header Error Control

  13. ATM Overhead • ATM has very high overhead • ATM Layer: 5 octets of overhead for 48 octets of data • CS and SAR layers have their own frames, whose headers take up room in the payload field • Even worse, in streaming mode, for applications that need low latency, cells are sent out without waiting for the SAR frame data fields to be filled to capacity

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