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Asynchronous Transfer Mode (ATM)

Asynchronous Transfer Mode (ATM). An Overview of ATM. A technology for multiplexing fixed-length cells from a variety of sources to a variety of remote locations. Capable of moving data at a wide range of speeds, but aimed at very high speed (100-1000 Mb/s).

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Asynchronous Transfer Mode (ATM)

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  1. Asynchronous Transfer Mode (ATM)

  2. An Overview of ATM • A technology for multiplexing fixed-length cells from a variety of sources to a variety of remote locations. • Capable of moving data at a wide range of speeds, but aimed at very high speed (100-1000 Mb/s). • Capable of handling data from a variety of media (e.g. voice, video, and data) using a single interface. • ATM is a connection-oriented protocol. • Connections can be switched or permanent. • Signalling procedures are used to set up switched calls. • Certain quality of service (QoS) is guaranteed for each connection. QoS parameters may include as cell loss rate, max./avg. cell delay, delay jitter, cell error rate, cell misinsertion rate, etc. • QOS is negotiated at connection setup.

  3. An Overview of ATM • ATM operates on a best effort basis; cells with errors, or that encounter congestion, are silently dropped. • Two types of connections: point-to-point and multipoint. • Service Carried in Fixed Length Cells (53 octets). • 5 Octets Header • 48 Octets Payload

  4. ATM Networking IS IS IS ES ES UNI: User-Network Interface NNI: Network-Network Interface

  5. ATMCell Format 酬載 48 Octets GFC VPI VCI PTI CLP HEC 4 8 16 3 1 8 位元 (a) UNI Cell format 酬載 48 Octets VPI VCI PTI CLP HEC 12 16 3 1 8 位元 (b) NNI Cell format • GFC: Generic Flow Control (4 bits). Used by the flow control mechanism at the UNI. • VPI: Virtual Path Identifier (8 bits). Used for directing cells within the ATM network. • VCI: Virtual Channel Identifier (16 bits). • PTI: Payload Type Identifier (3 bits). Identifies the type of data being carried by the cell. • CLP: Cell Loss Priority (1 bit). 1 = low priority. • HEC: Header Error Correction (8 bits). Generated and inserted by the physical layer. For first 4 octets. Correct single-bit errors and detect some multiple-bit errors. bit 1: AAL indication bit 2: EFCI (upstream congestion) bit 3: data or OAM cells

  6. Pre-assigned VPI/VCI Values • Unassigned Cell Indication (VPI = 0, VCI = 0) • Meta signalling (VCI=1) • Meta signalling is the bootstrap procedure used to establish and release a signalling VC. Not used for PVC setup. • General broadcasting signalling (VCI=2) • OAM F4 flow indication -- segment and end-to-end (VCI=3 and VCI=4) • F4: VP level OAM • F5: VC level OAM, segment or end-to-end (PT=100 or 101) • Point-to-Point Signalling (VCI=5) • Carriage of Interim Local Management Interface (ILMI) messages (VPI=0, VCI=16)

  7. Preassigned, Reserved VPI/VCI Values

  8. VPI/VCI OP New VPI/VCI 100 1 300 200 2 400 300 300 400 300 300 400 100 400 200 100 200 200 100 200 100 VPI/VCI OP New VPI/VCI 100 0 100 150 2 200 Cell Multiplexing and Cell Switching Examples 200 200 100 100 100 100 100 0 0 UTP UTP 300 300 1 1 SMF SMF 400 200 200 2 2 MMF MMF 150 150 150 3 3 UTP UTP ATM Switch

  9. ATM Protocol Stack Management plane Control plane User plane Higher Layers Higher Layers ATM Adaptation Layer Virtual Channel Functions Plane management ATM Layer Virtual Path Functions Layer management Physical Layer (PMD)

  10. ATM Layer Service • Transparent transfer of 48-octet data unit • Deliver data in sequence on a connection • Two levels of multiplexing (VC, VP) • Three types of connections • Point-to-point • Point-to-Multipoint • Multipoint-to-Multipoint (??) • Transport is best-effort • Network QoS negotiation • Traffic control and congestion control

  11. ATM Layer Functions • Cell multiplexing and switching • Cell rate decoupling • Cell discrimination based on pre-defined VPI/VCI • Quality of Service (QoS) • Payload type characterization • Generic flow control • Loss priority indication and Selective cell discarding • Traffic shaping

  12. Cell Rate Decoupling and Cell Discrimination • Cell Rate Decoupling • ATM sending entity adds unassigned cells to the assigned cell stream in order to adjust to the cell rate acquired by the payload capacity of the physical layer (R). • The receiving ATM entity shall extract and discard the unassigned and invalid cells from the flow of cells coming from the physical layer (R). • Cell Discrimination • Meta signalling • General broadcast signalling • Point-ot-point Signalling • Segment OAM F4 flow cell • End-to-end OAM F4 flow cell • ILMI message • User data

  13. Virtual Channels, Virtual Paths, and the Physical Channel 虛擬路徑 虛擬路徑 虛擬通道 實體線路 虛擬通道 100 100 100 100 200 200 細胞 100 100 200 200 200 200/300 300/10 300/40 100/100 200 300 300 10 10 300 20 300 20 30 30 40 40 連線(VPI/VCI) = (100,100),(100,200),(200,100), (200,200),(200,300),(300,10), (300,20),(300,30),(300,40)

  14. Physical Link, Virtual Path, and Virtual Channel virtual channel connection VC link virtual channel link ATM Layer virtual path connection VP link VP link transmission path Phy. Layer Digital Section Regenerator Section Crossing point Endpoint

  15. Virtual Channels • The virtual Channel (VC) is the fundamental unit of transport in a B-ISDN. Each ATM cell contains an explicit label in its header to identify the virtual channel. • a Virtual Channel Identifier (VCI) • a Virtual Path Identifier (VPI) • A virtual channel (VC) is a communication channel that provides for the transport of ATM cells between two or more endpoints for the purpose of user-user, user-network, network-network information transfer. The points at which the ATM cell information payload is passed to a higher layer signify the endpoints of a VC. • A Virtual Channel Identifier (VCI) identifiers a particular VC within a particular VP over a UNI or NNI.

  16. Virtual Paths • A Virtual Path (VP) is a group of Virtual Channels that are carried on the same physical facility and at a given reference point in the VP share the same Virtual Path Identifier (VPI) value. • The VP boundaries are delimited by Virtual Path Terminators (VPT). • AT VPTs, both VPI and VCI are processed. • Between VPTs associated with the same VP, only the VPI values are processed (and translated) at ATM network elements. • The VCI values are processed only at VPTs, and are not translated at intermediate ATM network elements.

  17. Why Virtual Paths and Virtual Channel ? Assume the identification field contains 8 bits. All used as VCI. Then the size of the mapping table is 256. New New VCI OP VCI OP VCI VCI 0 0 (2) 1 (3) (1) 2 (2) (35) 1 (255) 1 (127) 2 (35) 1 (254) 1 (38) (208) 2 (254) 255 255 (1) (3) (2) (127) (255) (35) (208) (38) 0 (254) 0 A B 1 1 255 255

  18. 0 1 31 New VPI OP VPI New 0 VPI OP (7) 1 (1) VPI (0) 1 (0) 1 0 (0) 2 (7) (1) 1 (7) (1) 2 (0) 1 7 (7) 2 (1) 7 Why Virtual Paths and Virtual Channels ? Assume the identification field contains 8 bits. VPI takes 3 bits and VCI takes 5 bits. Then the size of the mapping table is 8. (1/1) (0/1) (0/31) (1/31) (7/25) (7/25) 0 0 (7/1) 1 0 (0/31) 1 1 31 (1/25) 7 0 7 1 31 0 1 7

  19. New VPI/VCI OP VPI/VCI (100,10) 2 (200,10) (100,20) 2 (200,20) Virtual Channels Examples Port (a,1) Port (c,2) New VPI/VCI OP VPI/VCI 輸出連線 (200,10) 4 (300,10) 輸入連線 連線 VPI/VCI (100,10) 連線 VPI/VCI 1 (100,10) D A a 1 4 c 2 (100,20) 1 (300,10) 2 2 (100,20) (300,10) 輸入連線 (200,10) (200,10) (200,20) 連線 VPI/VCI 輸入連線 (300,10) (100,20) 1 (300,10) (300,20) 連線 4 VPI/VCI 1 B C b 2 d 2 1 3 1 (100,20) 輸出連線 (100,10) 連線 VPI/VCI Port (b,1) Port (d,1) New New 1 (100,10) VPI/VCI OP VPI/VCI OP VPI/VCI VPI/VCI Port (b,2) (200,10) 3 (300,20) (300,20) 2 (100,20) New (200,20) 2 (300,10) VPI/VCI OP VPI/VCI (100,10) 4 (200,10)

  20. Virtual Path/Virtual Channel (VP/VC) Switches VC1 VP3 VP1 A C VP Switch VC2 SW1 SW4 VP/VC Switch (VP Terminator, VPT) SW3 VP4 VP2 VC4 SW5 SW2 B D VC3

  21. ATM Adaptation Layers (AAL) • AAL Reference Structure • AAL Type 1 • AAL Type 2 • AAL Type 3/4 • AAL Type 5 • SAAL Management plane Control plane User plane Higher Layers Higher Layers ATM Adaptation Layer Plane management Virtual Channel Functions ATM Layer Virtual Path Functions Layer management Physical Layer (PMD)

  22. AAL Reference Structure SAP Service Specific CS (SCCS) (may be Null) Convergence Sublayer (CS) Primitives AAL Common Part CS (CPCS) Primitives SAR SAR(Common) SAP

  23. AAL Layer MGMT Service Specific Layers message AAL 48 byte payloads ... ATM add 5 byte header ... Transmission Convergence Sublayer PMD

  24. AAL Functions Functions Parameters Error Detection CRC, length, correlation tags Framing of user data units Payload type/segment type Cell sequence indication Cell sequence count field Multiplexing Message ID (MID) Error Correction FEC, retransmission Flow Control Credit window Timing Recovery Time stamp

  25. ATM Adaptation Layers (AAL) • In order to carry data units longer than 48 octets in ATM cells, an adaptation layer is needed. • The ATM adaptation layer (AAL) provides for segmentation and reassembly of higher-layer data units and for detection of errors in transmission. • Since the ATM layer simply carries cells without concern for their contents, a number of different AALs can be used across a single ATM interface. • The AAL maps the user, control, or management protocol data units into the information field of the ATM cell and vice versa. • To reflect the spectrum of applications, four service classes have been defined by CCITT.

  26. CCITT Services Classifications Class A Class B Class C Class D Attribute Connection Packetized Circuit Datagram Oriented Emulation voice/video Data Timing between source and destination Required Not required Bit Rate Constant Variable Connection mode Connection oriented Connectionless • Examples: • Class A (CBR): 64kbps digital voice • Class B (rt-VBR, nrt-VBR): Variable bit rate encoded video • Class C (UBR,ABR): Frame Relay over ATM, File Transfer (Telnet, FTP, TCP),... • Class D (ABR): SMDS over ATM, IP over ATM,... • Class X: Raw Cell Service (e.g., proprietary AAL)

  27. AAL Service Classification Class A Class B Class C Class D Signalling Connection Packetized Circuit Datagram (Q.93B) Oriented Emulation voice/video Data Attribute AAL 4 AAL 3 AAL1 AAL2 SAAL AAL 5 Timing between source and destination Required Not required Bit Rate Constant Variable Connection Mode Connection oriented Connectionless

  28. AAL Types • Five AALs have been accepted for consideration by the CCITT. • AAL 1 is meant for constant-bit-rate services (voice). • AAL 2 is meant for variable-bit-rate services with a required timing relationship between source and destination (audio and video). • AAL 3 was originally meant for connection-oriented variable -bit-rate services without a required timing relationship; it has now been merged with AAL 4. • AAL 3/4 and 5 are meant for connectionless services (e.g. connectionless data). • Only AALs 3/4 and 5 are of interest for IP networking.

  29. AAL 1 (Constant Bit Rate -CBR) Functions • Emulation of DS1 and DS3 Circuits • Distribution with forward error correction • Handle cell delay for constant bit rate • Transfer timing information between source and destination • Transfer structure information (structure pointer) • Provide indication of unrecoverable lost or errored information SAR PDU Header SN SNP 47 Octets Payload Seq CRC EP CSI Count 1 3 3 1

  30. AAL1 Supports Circuit Emulation • Synchronous Residual Time Stamp (SRTS) • DS1, DS3 • Require accurate frequency clock • 4-bit Residual Time Stamp (RTS) for clock aligning • RTS is generated once every 8 cell times, carried in CSI bit of odd cells • Structured Data Transfer (SDT) • nxDS0 (64kbps) • 1-octect pointer carried in payload once every two cells (even cells) indicates the offset into the current payload of the first octect of an nxDS0 payload • The octect contains 1 reserved bit and 7-bit offset field which points to start of up to 93 octect structure (47+46=93)

  31. AAL 2 (VBR) Protocol Data Unit (PDU) ATM PDU SAR PDU Header SN IT 47 Octets Payload LI CRC SN: Sequence number IT: Information Type:BOM,COM,EOM,SSM Length Indicator

  32. AAL 3/4 • The variable bit rate (VBR) adaptation layer, defined in CCITT recommendation I.363, is defined for services (e.g. data) that require bursty bandwidth. • Comprises two sublayers: • the convergence sublayer (CS) • the segmentation and reassembly sublayer (SAR)

  33. AAL 3/4 CS and SAR PDU Structures CS-PDU Trailer 4 Octets CS-PDU Header 4 Octets PAD 0-3 Octets CS-PDU User Information <= 65,535 Octets Common Part Indicator CS User Infor. Length Alignment ETag BTag BASize 1 1 2 1 1 2 • CPI:00000000 • Btag/Etag: Beginning/Ending Tag -- 256 increment counters • BAsize: receiving side maximum buffering requirement (>= CPCS-PDU) • Pad: make CPCS-PDU on 32-bit boundary • AL(Alignment): make trailer 32-bit aligned • Length: CPCS-PDU size

  34. AAL 3/4 SAR Sublayer SAR-PDU Header 2 Octets SAR-PDU Trailer 2 Octets Segmentation Unit ,SAR-PDU Payload 44 Octets p SAR SN SAR Type SAR-PDU CRC Length MID 2 4 1 9 6 10 • ST: COM(00),BOM(10),EOM(01),SSM(11) • SN: Modulo 16 sequence counter • P(Priority): 1- Priority CS-PDU, 0- Normal CS-PDU • MID (Multiplexing ID) -- Multiplexing multiple CPCS connections on a single ATM connection • LI: Length <=44 • CRC on Cell Header, SAR-PDU payload and LI

  35. AAL 5 PDU Structure • The Simple and Efficient Adaptation Layer (SEAL), attempts to reduce the complexity and overhead of AAL 3/4. • It eliminates most of the protocol overhead of AAL 3/4. • AAL 5 comprises a convergence sublayer and a SAR sublayer, although the SAR is essentially null. CS-PDU Trailer 8 Octets PAD 0-47 Octets CS-PDU User Information <= 65,535 Octets Protocol Control CRC Length 2 2 4

  36. AAL 5 Segmentation and Reassembly User SDU CS-PDU Trailer 8 Octets PAD 0-47 Octets CPCS-PDU Payload (CPCS-SDU) <= 65,535 Octets 48 octets 48 octets ... More = F More = T More = T Data T User Data User Data ATM cell ATM cell ATM cell • Control • CPCS-UU -- CPCS User-to-User Indication (1 octet). Transparently transfer CPCS user to user information • CPI -- Common Part Indicator (1 octet). Align trailer to 64 bits. Possible identification of layer management message. CRC-32 Control Length 2 2 4

  37. AAL 5 • When a network node has a user datagram to transmit, it first converts it to a CS-PDU by adding the pad (if necessary) and trailer. • Then it breaks the CS-PDU into 48-octet SAR-PDUs and transmits each in an ATM cell on the same virtual channel. • Since there is no AAL 5 SAR header, an end-of-frame indication in the ATM cell header is required: SDU type of 1 (binary value 0X1) in the PTI field. • The receiver simply concatenates cells as they are received, watching for the end-of-frame indication. • The higher layer is responsible for ignoring PDUs with CRC errors. • Some applications may discard PDUs with errors; others may choose to use them.

  38. SAAL Structure Primitives Service Specific Coordination Function (SSCF) Service Specific Convergence Sublayer Peer-to-Peer Message Service Specific Connection Oriented Peer-to-Peer Protocol (SSCOP) Peer-to-Peer Message Common Part AAL Peer-to-Peer Protocol (CP-AAL) Common Part Primitives

  39. SAAL • Reside between Q.93B and ATM Layer. • SAAL is used to provide reliable transport of Q.93B messages between peer Q.93B entities. • SAAL CP-AAL uses AAL5 Common Part Protocol. • SSCOP can be used for any reliable service. • SSCF maps primitives from MTP 3 to the required SSCOP signals and vice versa, and • flow control • maintains link status • reports to layer management when a link is released

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