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CS 4594 Broadband Communications

CS 4594 Broadband Communications. ATM. ATM. ATM = Asynchronous Transfer Mode Asynchronous - in contrast to SONET/SDH in which large frames of data are sent on a completely regular basis, completely synchronized.

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CS 4594 Broadband Communications

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  1. CS 4594 Broadband Communications ATM

  2. ATM • ATM = Asynchronous Transfer Mode Asynchronous - in contrast to SONET/SDH in which large frames of data are sent on a completely regular basis, completely synchronized. • Purpose: Convey voice, video, and data service over the same channel. Each service has different timing requirements. • ATM was developed to meet the new telephone network model called Broadband ISDN = Integrated Services Digital Network

  3. ATM Layers • ATM has three major layers: AAL, ATM, Physical. AAL • AAL provides network services. Converts user data into fixed length cells (53 bytes) • ATM handles switching, traffic management, etc. • Physical connects to a variety of different media ATM Physical

  4. ATM Cells • ATM transports data in small packets called cells. • Each cell is 53 bytes long with a 5 byte header and a 48 byte payload.

  5. ATM Cell Header • The ATM cell header has two different forms: • UNI = User-Network Interface - as the cell enters the network • NNI = Network-Network Interface - as it moves through the network.

  6. ATM Cell Header – Fields • Labeling – Cell ID • GFC = Generic Flow Control – 4 bits for UNI, not present in NNI, misnamed, now used for multiplexing user channels • VPI = Virtual Path Identifier – 8 bits for UNI, 12 bits for NNI • VCI = Virtual Channel Identifier – 16 bits • Cell Management • PTI = Payload Type – 3 bits • CLP = Cell Loss Priority • Error Control and Framing • HEC = Header Error Control – 8 bits, CRC check on header

  7. ATM – Virtual Circuits • VPI/VCI form a label for the cell that determines how the cell will be handled in the next switch. • Each switch has a number of inputs and outputs, each with a identifying number. • When a cell enters a switch, its VPI/VCI is input into a table look-up that returns the output identifier and a new VPI/VCI.

  8. ATM – Virtual Circuit Types • PVC = Permanent Virtual Circuit – set up manually by programming each switch along the route • SVC = Switched Virtual Circuit – set up dynamically by signaling procedures.

  9. AAL = ATM Adaptation Layer • ALL is the top layer among ATM layers AAL ATM Physical

  10. Role of AAL • Provides interface between application layers and ATM network itself • Implemented in endpoint devices (at edge of network) • Supports a number of different service classes • Shields ATM layer from knowing anything about user data

  11. Classical AAL Classes

  12. Modern Service Categories • CBR = Constant Bit Rate • rt-VBR = Real Time Variable Bit Rate • nrt-VBR = Non Real Time Variable Bit Rate • ABR = Available Bit Rate • UBR = Unspecified Bit Rate • GFR = Guaranteed Frame Rate

  13. Traffic Contract • Agreement between user and network • Contains • Traffic descriptors – specifying ATM cell flow • QoS parameters – specifing timing and error qualities • Conformance checking rules

  14. Traffic Parameters • PCR = Peak Cell Rate • SCR = Sustainable Cell Rate • MBS = Maximum Burst Size • MCR = Minimum Cell Rate

  15. QoS Parameters • Timing • Cell Transfer Delay (CTD) • Cell Delay Variation (CDV) • Errors • Nonegotiable • Cell Error Ratio (CER) • Cell Misinsertion Rate (CMR) • Severely-Errored Cell Block Ratio (SECBR) • Negotiable • Cell Loss Ratio (CLR) • Cell Misinsertion Rate (CMR)

  16. AAL Sublayers Service specific convergence sublayer (SSCS) Common part convergence sublayer (SSCS) Segmentation and reassembly sublayer (SAR)

  17. Sublayers of AAL • CS = Convergence Sublayer (CS) • Performs functions such as end-to-end synchronization, traffic control and monitoring, cell loss control • Sublayers • Service Specific (SSCP) – application dependent • Common Part (CPCS) – common to all users of a service • Segmentation and Reassembly (SAR) • Performs repackaging between convergence layer packets and ATM cells

  18. AAL Types • Designed to handle the different service types • Specify architectural details, traffic parameters, packet formats and procedures

  19. AAL-0 • Unofficial • Designed to send ATM cells directly from ATM-cell based application to ATM cell • Has empty CS and SAR

  20. AAL-0 Operations Higher Layers ATM Cell ATM Cell ATM Cell ATM Cell AAL Layers ATM Layer ATM Cell ATM Cell ATM Cell ATM Cell

  21. AAL-1 • Designed for Class A= CBR • Transports a constant stream of bits with timing • Applications: circuit emulation, uncompressed voice and video. • May handle: • segmentation and reassembly (SAR) • handling cell delay variation (buffering) • handling cell delay • handling lost and mininserted cells • clock management • recovery of source data framing • monitoring and correction of error in user data

  22. AAL-1 PDU • SAR header • SN (4 bits): sequence number • CSI (1 bit): Convergence layer indicator (can be used for clock) • Sequence number (3 bits) • SNP (4 bits): sequence number protection • 3-bit CRC • 1 bit parity

  23. H H H H 1 1 1 1 47 47 47 47 H H H H ATM Cell ATM Cell ATM Cell ATM Cell 5 5 5 5 48 48 48 48 AAL-1 Operations Higher Layers Bit Stream 47 47 47 47 (bytes) CS AAL Layers SAR ATM Layer

  24. AAL-2 • Designed to handle Class B, rt-VBR • Transfers multiplexed streams of bits and timing • Handles errors or indicates when errors have occurred • Two layers: packet and PDU (cell)

  25. 3 3 3 3 H STF STF H H H STF STF 47 1 1 1 1 47 47 47 H H H H ATM Cell ATM Cell ATM Cell ATM Cell 5 5 5 5 48 48 48 48 AAL-2 Operations Higher Layers User 1 User 2 User 3 User 1 packet AAL 2 Layers PDU ATM Layer

  26. AAL-2 CPS Packet • CPS-PH (3 octets) common part sublayer packet header • CID (8 bits): channel id for multiplexing • LI (6 bits): length indicator for number of octets in variable payload structures • UUI (5 bits): user-to-user information • HEC (5 bits): error control for packet header

  27. AAL-2 CS-PDU • STF (8 bits): start field • OSF (6 bits): offset • SN/P (2 bits): error correction and detection for start field • Payload data (47 bytes) • PAD (padding as needed for real-time)

  28. AAL-3/4 • Designed for Class C and Class D (nrt-VBR) connection and connectionless • Can do multicasting (network-based distribution) • Can do multiplexing (multiple streams through same service) • Designed to be highly reliable • CRC check on cells • SAR support • Sequencing • Mux id • Beginning and end tags • HEC • Multiple modes of operation (assured and non assured delivery)

  29. 4 4 4 75 75 75 5 5 5 4 4 4 H H H P P P T T T 2 2 2 2 44 44 44 44 2 2 2 2 H H H H T T T T 48 48 48 48 H H H H ATM Cell ATM Cell ATM Cell ATM Cell 5 5 5 5 48 48 48 48 AAL-3/4 Operations Higher Layers packet packet packet 75 75 75 (bytes) SSCS CPCS AAL Layers SAR ATM Layer

  30. AAL-3 structures CPI BT BA size User Data PAD AL ET Length Header (1+1+2) Trailer (1+1+2)

  31. AAL-3/4 CPCS-PDU • Header (4 octets) • CPI (1 octets): Common Part Indicator – defines how the other parts are encoded • BT (1 octet): Beginning Tag • BA size (1 octet): Buffer Allocation size • User Data (1-65,535 octets) • PAD: padding to keep PDU length a multiple of 4 octets • Trailer (4 octets) • AL (1 octet): Alignment to make CPS trailer 4 octets • ET (1 octet): Ending Tag • Length (2 octets) of entire PDU

  32. AAL-3/4 SAR-PDU • Header (2 octets) • ST (2 bits): Segment Type: BOM 10, COM 00, EOM 01 • SN (4 bits): Sequence Number • MUX-ID (10 bits): multiplexing id • SAR-PDU Payload (44 octets) segment of CPCS-PDU • Trailer (2 octets) • LI (6 bits): length indicator • CRC (10 bits): CRC check on segment

  33. AAL-5 • Designed for Class C and Class D (nrt-VBR) connection and connectionless • Simplified (lightweight) version of AAL-3/4 • No overhead in cells

  34. 75 75 75 13 13 13 8 8 8 P P P T T T 48 48 48 48 48 48 48 48 H H H H ATM Cell ATM Cell ATM Cell ATM Cell 5 5 5 5 48 48 48 48 AAL-5 Operations Higher Layers packet packet packet 75 75 75 (bytes) SSCS CPCS AAL Layers SAR ATM Layer

  35. AAL-5 SDUs • CPCS-PDU • User Data: 1-65,535 octets • PAD: 0-47 octets • CPCS-PDU trailer • CPCS UU: 1 octet • CPI: 1 octet • CPCS-PDU length: 2 octets • CRC: 4 octets

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