ATM Layering Does not follow 802 layering –However, does subdivide the data link layer ATM ATM (Application-Independent) Physical OSI Data Link Physical ATM Adaptation Layer (AAL) CS SAR

ATM Layering ATM Layer –Manages delivery of 53-octet cells –Application-independent: Same for all Classes of Service ATM ATM (Application-Independent) Physical OSI Data Link Physical ATM Adaptation Layer (AAL) CS SAR

ATM Layering ATM Adaptation Layer –Provides what is needed for different classes of service Application-dependent ATM ATM (Application-Independent) Physical OSI Data Link Physical ATM Adaptation Layer (AAL) CS SAR

ATM Layering ATM Adaptation Layer is Subdivided Further –CS (Convergence Services) –SAR (Segment Assembly/Reassembly) ATM ATM (Application-Independent) Physical OSI Data Link Physical ATM Adaptation Layer (AAL) CS SAR

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

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

ATM Layering Physical Layer –Uses SONET/SDH optical fiber transmission standard (see Module B) ATM ATM (Application-Independent) Physical: SONET/SDH OSI Data Link Physical ATM Adaptation Layer (AAL) CS SAR

ATM Cell Organization –5 octets of overhead, 48 of payload (data) resCLP Virtual Path Identifier VPI (cont)Virtual Channel Identifier VCI (cont) PT Header Error Control Payload (48 octets)

ATM Cell Virtual Circuit Number is Hierarchical –Virtual Path Identifier: Identifies a site –Virtual Channel Identifier: Identifies a station at the VPI site Virtual Path Identifier VPI (cont)Virtual Channel Identifier VCI (cont)

ATM Cell Payload Type (2 bits) identifies cell category –00 = user information (data) reserved for later definition (1 bit) resCLP Virtual Path Identifier VPI (cont)Virtual Channel Identifier VCI (cont) PT Header Error Control

ATM Cell Cell Loss Priority Bit 0 if sending within committed information rate 1 if sending faster: switches will discarded if congestion occurs resCLP Virtual Path Identifier VPI (cont)Virtual Channel Identifier VCI (cont) PT Header Error Control

ATM Cell Header Error Control Field –To check the header (not the payload) for errors –Discard cell if an error is found resCLP Virtual Path Identifier VPI (cont)Virtual Channel Identifier VCI (cont) PT Header Error Control

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