Presentation on theme: "1 ATM - Asynchronous Transfer Mode (ATM). 2 ATM - An Overview of ATM A technology for multiplexing fixed-length cells from a variety of sources to a."— Presentation transcript:
2 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). 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 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 An Overview of ATM
4 ATM - ATM Networking UNI: User-Network Interface NNI: Network-Network Interface ES IS ES IS
5 ATM - ATM Cell Format 酬載 48 Octets GFC VPI VCI PTI CLP HEC 4 8 16 3 1 8 位元 酬載 48 Octets VPI VCI PTI CLP HEC 12 16 3 1 8 位元 (a) UNI Cell format (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 ATM - 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)
10 ATM - 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 - 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 ATM - 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
14 ATM - Physical Link, Virtual Path, and Virtual Channel ATM Layer Phy. Layer virtual channel connection virtual channel linkVC link virtual path connection VP link transmission path Digital Section Regenerator Section Crossing point Endpoint
15 ATM - Virtual Channels v 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) v 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. v A Virtual Channel Identifier (VCI) identifiers a particular VC within a particular VP over a UNI or NNI.
16 ATM - Virtual Paths v 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. v The VP boundaries are delimited by Virtual Path Terminators (VPT). v AT VPTs, both VPI and VCI are processed. v Between VPTs associated with the same VP, only the VPI values are processed (and translated) at ATM network elements. v The VCI values are processed only at VPTs, and are not translated at intermediate ATM network elements.
17 ATM - Why Virtual Paths and Virtual Channel ? A B (1) VCI OP (1) 2 (2) New VCI (127) 2 (35) 0 1 255 0 1 (2) (3) (127) (35) (255) (208) (254) (38) (208) 2 (254) 0 1 255 VCI OP (2) 1 (3) New VCI (35) 1 (255) (254) 1 (38) 0 1 255 Assume the identification field contains 8 bits. All used as VCI. Then the size of the mapping table is 256.
18 ATM - Why Virtual Paths and Virtual Channels ? 0 1 7 0 1 7 0 1 7 (0/1) (7/1) (1/1) (1/31) (0/31) (7/25) (1/25) (7/25) 0 1 31 0 1 0 1 7 VPI OP (0) 2 (7) New VPI (1) 2 (0) 0 1 (7) 2 (1) VPI OP (7) 1 (1) New VPI (0) 1 (0) 0 1 (1) 1 (7) 7 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.
19 ATM - Virtual Channels Examples 1 A D 1 C B (300,10) (100,20) (300,20) (100,10) (300,10) (200,20) (200,10) (100,10) (100,20) 2 3 1 2 2 4 2 VPI/VCI OP (100,10) 2 (200,10) New VPI/VCI (100,20) 2 (200,20) VPI/VCI OP (200,10) 3 (300,20) New VPI/VCI (200,20) 2 (300,10) VPI/VCI OP (300,20) 2 (100,20) New VPI/VCI VPI/VCI OP (200,10) 4 (300,10) New VPI/VCI a b c d Port (a,1) Port (c,2) Port (d,1) Port (b,1) VPI/VCI OP (100,10) 4 (200,10) New VPI/VCI Port (b,2) 4 1 (100,20) VPI/VCI 1 (300,10) VPI/VCI 1 (300,10) VPI/VCI 1 (100,10) VPI/VCI 連線 1 (100,10) VPI/VCI 輸出連線 2 (100,20) (200,10) 輸出連線 輸入連線 連線
24 ATM - AAL Functions FunctionsParameters Error DetectionCRC, length, correlation tags Framing of user data unitsPayload type/segment type Cell sequence indicationCell sequence count field Multiplexing Message ID (MID) Error CorrectionFEC, retransmission Flow Control Credit window Timing RecoveryTime stamp
25 ATM - 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 ATM - CCITT Services Classifications Timing between source and destination Class A Class B Class C Class D Required Not required Bit Rate Constant Variable Connection mode Connection oriented Connectionless Circuit Emulation Packetized voice/video Connection Oriented Data Datagram Attribute 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 ATM - AAL Service Classification Timing between source and destination Class A Class B Class C Class D Required Not required Bit Rate Constant Variable Connection Mode Connection oriented Connectionless Circuit Emulation Packetized voice/video Connection Oriented Data Datagram Attribute AAL1 AAL2 SAAL AAL 5 AAL 4 AAL 3 Signalling (Q.93B)
28 ATM - 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 ATM - 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 Header SN SNP 47 Octets Payload SAR PDU CSI Seq Count EP CRC 1 3 3 1
30 ATM - 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 ATM - AAL 2 (VBR) Protocol Data Unit (PDU) Header SN IT 47 Octets Payload LI CRC SAR PDU ATM PDU SN: Sequence number IT: Information Type:BOM,COM,EOM,SSM Length Indicator
32 ATM - 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 ATM - AAL 3/4 CS and SAR PDU Structures CS-PDU Header 4 Octets CS-PDU User Information <= 65,535 Octets CS-PDU Trailer 4 Octets PAD 0-3 Octets Common Part Indicator BTag BASize Alignment CS User Infor. Length ETag 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 ATM - AAL 3/4 SAR Sublayer SAR-PDU Header 2 Octets Segmentation Unit,SAR-PDU Payload 44 Octets SAR-PDU Trailer 2 Octets SAR Type Length SAR SN MID SAR-PDU CRC 2 4 1 9 6 10 p 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 ATM - 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 User Information <= 65,535 Octets CS-PDU Trailer 8 Octets PAD 0-47 Octets Protocol Control CRC Length 2 2 4
36 ATM - AAL 5 Segmentation and Reassembly CPCS-PDU Payload (CPCS-SDU) <= 65,535 Octets CS-PDU Trailer 8 Octets PAD 0-47 Octets Control CRC-32 Length 2 2 4 User SDU User Data More = T User Data More = T Data T More = F... 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. 48 octets ATM cell
37 ATM - 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 ATM - SAAL Structure Service Specific Coordination Function (SSCF) Service Specific Connection Oriented Peer-to-Peer Protocol (SSCOP) Common Part AAL Peer-to-Peer Protocol (CP-AAL) Service Specific Convergence Sublayer Common Part Peer-to-Peer Message Peer-to-Peer Message Primitives
39 ATM - SAAL v Reside between Q.93B and ATM Layer. v SAAL is used to provide reliable transport of Q.93B messages between peer Q.93B entities. v SAAL CP-AAL uses AAL5 Common Part Protocol. v SSCOP can be used for any reliable service. v SSCF maps primitives from MTP 3 to the required SSCOP signals and vice versa, and v flow control v maintains link status v reports to layer management when a link is released