Asynchronous Transfer Mode (ATM) Networks! All source media is first broken down into a stream of fixed sized units known as cells.! Cell streams relating to different media types are multiplexed together on a statistical basis for transmission and switching.! The resulting networks are known as Cell Relay Networks or Asynchronous Transfer Mode (ATM) Networks.! Transfer Mode: - Packet oriented transfer mode using ATDM - No link-by-link error control - No link-by-link flow control - End-to-End error control if needed - Use of internal virtual circuits - Switching based on table look-up - Fixed size blocks (cells) ATM-1
ATM Protocol Architecture The protocol reference model involves three separate planes: User plane: Provides for user information transfer, along with associated controls (e.g., flow control, error control) Control plane: Performs call control and connection control functions Management plane: Includes plane management, which performs management functions related to a system as a whole and provides coordination between all the planes, and layer management, which performs management functions relating to resources and parameters residing in its protocol entities ATM-2
ATM Protocol Architecture: ATM Functions! ATM Layer: - Common to all services and provides cell transfer capabilities. - Provides logical connections upon the physical layer:! Virtual Channels! Virtual Paths! ATM Adaptation Layer (AAL): - Provides a range of alternative service types known as service classes. - Converts the source information into streams of 48-octet segments. ATM-3
ATM Transport Hierarchy! Transmission Path Level: Network elements that assemble and disassemble the payload of a transmission system. Cell delineation and header error control are required at the endpoints of each transmission path.! Digital Section Level: Network elements that assemble and disassemble a continuous bit or byte stream. Refers to the exchanges or signal transfer points involved in switching data streams.! Regenerator Section Level: A portion of a digital section (e.g.; a repeater). ATM-4
Virtual Path / Virtual Channel Terminology ATM-5
ATM Logical Connections! Logical connections in ATM are referrred to as Virtual Channel Connections (VCCs).! VCCs are used for: - user-to-user exchange of variable rate, full duplex, fixedsize cells carrying user data. - user-to-network exchange of control signaling information. - network-to-network exchange of management and routing information.! A Virtual Path Connection (VPC) is a bundle of VCCs that have the same endpoints.! All the cells flowing over all VCCs in a single VPC are switched together.! To set up a VCC, there must first be a VPC to the required destination node with sufficient available capacity to support the VCC with the appropriate QOS.! The virtual path control mechanisms include: - calculating routes - allocating capacity - storing connection state information ATM-6
ATM Connection Relationships! Logical connections in ATM are referrred to as Virtual Channel Connections (VCCs).! VCCs are a concatenation of Virtual Channels (VCs).! A Virtual Path is a group of VCs going the same way. ATM-7
Call Establishment Using Virtual Paths ATM-8
Hierarchial Layer-to-Layer Relationship ATM-9
VC Switching Principles! Each VC has a Protocol Connection Identifier (PCI) that is assigned to the VC on each link of the network. The PCI has only local significance.! Routing Scheme Principle: Cell Switching Routing Principle ATM-10
VPIs and VCIs! The PCI consists of two subfields: - Virtual Path Identifier (VPI) - Virtual Channel Identifier (VCI) A Virtual Path is a bundle of Virtual Channels to be switched as a single unit.! Routing can be performed using either subfield or a combination of the two.! Example VP and VC Routing: Example of VP and VC Routing ATM-11
Representation of VP and VC Switching Hierarchy ATM-12
Virtual Path / Virtual Channel Characteristics! VCC Characteristics: - QOS: - cell loss ratio, - cell delay variation, - etc. - Switched and Semipermanent VCCs: - a switched VCC is an on-demand VCC. - a semipermanent VCC is set up by configuration or network management action. - Cell Sequence Integrity: - sequence of transmitted cells within a VCC is preserved. - Traffic Parameter Negotiation and Monitoring: - average rate, - peak rate, - burstiness, - peak duration, - etc.! VPC Characteristics: - QOS: - Switched and Semipermanent VCCs: - Cell Sequence Integrity: - Traffic Parameter Negotiation and Monitoring: - Virtual Channel Identifier Restriction within a VPC: - one or more VCIs may not be available to the user of the VPC (e.g.; VCIs used for network management.) ATM-13
ATM Cell Format! Cell Structure: Header (5 Octets) Information Field (48 Octets)! User-Network Interface (UNI) Header Format: 8 7 6 5 4 3 2 1 Generic Flow Control VPI cont. Virtual Channel Identifier Virtual Path Identifier Payload Type Header Error Control (HEC) CLP Notes: GFC - Enables a local switch to regulate (flow control) the entry of cells by a user into the network. CLP - Cell Loss Priority - provides guidance to the network in the event of congestion: - 0 indicates cell of relatively high priority. - 1 indicates cell is subject to discard. PT: 000 - user data, no congestion, SDU type 0 001 - user data, no congestion, SDU type 1 010 - user data, congestion, SDU type 0 011 - user data, congestion, SDU type 1 100 - OAM segment associated cell 101 - OAM end-to-end associated cell 110 - Resource management cell 111 - Reserved for future function ATM-14
ATM Cell Format cont.! Network-Network Interface (NNI) Header Format: 8 7 6 5 4 3 2 1 Virtual Path Identifier Virtual Channel Identifier Payload Type Header Error Control (HEC) CLP ATM-15
Generic Flow Control (GFC) ATM-16
GFC - Single Group of Connections: 1-Q Model! The controlled equipment (terminal equipment - TE), initializes two variables: TRANSMIT is a flag initialized to SET (1), and GO_CNTR, which is a credit counter, is initialized to 0.! A third variable, GO_VALUE, is either initialized to 1 or set to some larger value at configuration time. The rules for transmission are as follows: 1. If TRANSMIT = 1, cells on uncontrolled connections may be sent at any time. If TRANSMIT = 0, no cells may be sent on either controlled or uncontrolled connections. 2. If a HALT signal is received from the controlling equipment, TRANSMIT is set to 0 and remains at zero until a NO_HALT signal is received, at which time TRANSMIT is set to 1. 3. If TRANSMIT = 1 and there is no cell to transmit on any uncontrolled connections, then If GO_CNTR > 0, then the TE may send a cell on a controlled connection. The TE marks that cell as a cell on a controlled connection and decrements GO_CNTR. If GO_CNTR = 0, then the TE may not send a cell on a controlled connection. 4. The TE sets GO_CNTR to GO_VALUE upon receiving a SET signal; a null signal has no effect on GO_CNTR. The HALT signal is used logically to limit the effective ATM data rate and should be cyclic. For example, to reduce the data rate over a link by half, the HALT command is issued by the controlling equipment so as to be in effect 50% of the time. This is done in a predictable, pattern over the lifetime of the physical connection. ATM-17
GFC - Two Groups of Connections: 2-Q Model! For the two-queue model, there are two counters, each with a current counter value and an initialization value: GO_CNTR_A, GO_VALUE_A, GO_CNTR_B, and GO_VALUE_B. This enables the network to control two separate groups of connections. ATM-18
Header Error Control (HEC) at Receiver! At initialization, the receiver's error correction algorithm is in the default mode for single-bit error correction.! As each cell is received, the HEC calculation and comparison is performed. As long as no errors are detected, the receiver remains in error correction mode.! When an error is detected, the receiver will correct the error if it is a single-bit error or will detect that a multibit error has occurred.! In either case, the receiver now moves to detection mode.! The receiver remains in detection mode as long as errored cells are received. When a header is examined and found not to be in error, the receiver switches back to correction mode. ATM-19
Effect of Error in Cell Header ATM-20
Transmission of ATM Cells! I.432 specifies that ATM cells may be transmitted at the following data rates: - 25.6 Mbps - 51.84 Mbps - 155.52 Mbps - 622.08 Mbps! Two approaches are defined: - a cell based physical layer - an SDH based physical layer ATM-21
Cell Based Physical Layer! The interface structure consists of a continuous stream of 53- octet cells.! No framing is imposed.! Because there is no frame structure, some form of synchronization is needed.! Synchronization is achieved on the basis of the Header Error Control (HEC) field in the cell header. ATM-22
Cell Delineation! HUNT state: - bit-by-bit hunt for a correct HEC.! PRESYNCH state: - cell-by-cell hunt to insure that the HEC is correct consecutive times.! SYNC state: - HEC is used for error detection and correction. - cell delineation is lost if HEC is incorrect consecutive times. ATM-23
SONET/SDH Signal Hierarchy! SONET (Synchronous Optical Network): ANSI standard (originally proposed by BellCore)! SDH (Synchronous Digital Hierarchy): A compatible ITU-T recommendation G.707-G.709 SONET Designation SDH Designation Data Rate Mbps Payload Rate STS-1 51.84 50.112 STS-3 STM-1 155.52 150.336 STS-9 466.56 451.008 STS-12 STM-4 622.08 601.344 STS-18 933.12 902.016 STS-24 1244.16 1202.688 STS-36 1866.24 1804.032 STS-48 STM-16 2488.32 2405.376 STS-96 4876.64 4810.752 STS-192 STM-64 9953.28 9621.504 STS-768 STM-256 39813.12 38486.016 STS-3072 159252.48 153944.064 ATM-24
SONET/SDH System Hierarchy! Photonic: the physical layer! Section: creates the basic SONET frames! Line: responsible for synchronization, multiplexing of data onto SONET frames, and switching.! Path: responsible for end-to-end transport of data. ATM-25
SONET/SDH Frame Formats! The basic SONET building block is the STS-1 frame, which consists of 810 octets and is transmitted 8000 times per second for an overall data rate of 51.84Mbps. ATM-26
SONET STS-1 Overhead Octets ATM-27
SONET STS-1 Overhead Octets cont. ATM-28
Location of the Synchronous Payload Environment (SPE)! The SPE of an STS-1 frame can float w.r.t. to the frame.! The actual payload (87 columns x 9 rows) can straddle two frames.! The H1 and H2 octets in the line overhead indicate the start of the payload.! Because of timing variations between nodes, each node recalculates the pointer to alert the next receiving node of the exact location of the start of the payload.! The pointer value can increase or decrease by one byte position at each node.! When the pointer decrements, H3 holds the extra octet for that frame. ATM-29
STS-1 Pointer Adjustment ATM-30
SDH Based Physical Layer! A framing structure is imposed on the ATM cell stream using the STM-1 (STS-3) frame.! Since the payload capacity (2,340 octets) is not an integer multiple of the cell legth (53 octets), a cell may cross a payload boundary.! The H4 octet is set at the sending side to indicate the next occurrence of a cell boundary (value of 0 to 52) from H4. ATM-31
ATM Service Categories! Real-Time Service: - Constant Bit Rate (CBR) - Real-Time Variable Bit Rate (rt-vbr)! Non-Real-Time Service: - Non-Real-Time Variable Bit Rate (nrt-vbr) - Available Bit Rate (ABR) Application specifies a peak cell rate (PCR) that it will use and a minimum cell rate (MCR) that it requires. - Unspecified Bit Rate (UBR) This is a best-efforts service using capacity not allocated to any other service. - Guaranteed Frame Rate (GFR) Designed specifically to support IP backbone subnetworks. GFR provides better service than UBR for frame-based traffic, including IP and Ethernet. GFR allows a user to reserve capacity for each GFR VC. The user is guaranteed this minimum capacity. Additional frames may be transmitted if the network is not congested. ATM-32
AAL Service Classes! AAL services provided between source and destination: ATM-33
AAL Sublayers! Convergence Sublayer (CS): Performs a convergence function between the service offered at the layer interface and that provided by the underlying ATM layer.! Segmentation and Reassembly (SAR) Sublayer: Provides cell segmentation and reassembly functions. AAL Sublayers ATM-34
AAL Sublayers! Convergence Sublayer (CS): Performs a convergence function between the service offered at the layer interface and that provided by the underlying ATM layer.! Segmentation and Reassembly (SAR) Sublayer: Provides cell segmentation and reassembly functions. ATM-35
AAL 1! The CS protocol endeavors to maintain a constant bit rate stream between the source and destination SAPs.! The agreed rate must be maintained even when occasional cell losses or cell transfer time variations occur.! Cell losses are overcome in an agreed way (e.g., by inserting dummy bits/bytes into the delivered stream.! Cell transfer delay variations are compensated by buffering segments at the destination. AAL 2! There is a timing relationship between the source and destination SAPs determined by the frame rate.! The amount of information associated with a frame may vary from one frame to the next.! The CS at the source receives bursts of information at the frame rate with each burst containing a variable amount of information.! The CS protocol at the destination must endeavor to output the received information in this same way even when occasional cell losses or cell transfer time variations occur.! Time variations are overcome using similar techniques as described for AAL 1. ATM-36
AAL 1 and AAL 2 SAR PDU Formats SAR PDU Formats: (a) AAL 1 (b) AAL 2 Notes: IT: LI: indicates either the position of the segment in relation to a submitted message unit (BOM, COM, or EOM), or whether the segment contains timing and other information. the last (EOM) segment may not be full and LI indicates the number of useful bytes. ATM-37
AAL 3/4! Provides a connectionless or a connection oriented data transfer service. - For connectionless service, each block of data presented to the SAR (SAR SDU) is treated independently. - For connection oriented service, it is possible to define multiple SAR logical connections over a single ATM connection.! Service may be message mode or streaming mode. - Message mode service transfers framed data. A single block of data from the layer above AAL is transferred in one or more cells. - Streaming mode service supports the transfer of lowspeed continuous data with low delay requirements. The data are presented to the AAL in fixed-size blocks that may be as small as an octet. One block is transferred per cell. ATM-38
AAL 3/4 CS PDU and SAR PDU Formats CS & SAR PDU Format for AAL 3/4 Notes: Type (Common Part Indicator): Currenty only 0 is defined BETag: Begin-End Tag (a mod 256 sequence number) Sender changes value for each successive CS PDU BA: Buffer allocation (helps destination CS allocate memory) Indicates maximum buffer needed to reassemble CS SDU For message mode, equals the CS PDU length For streaming mode, CS PDU length Pad: (to make CS PDU a multiple of 4 octets) AL: Alignment (dummy octet to make trailer 4 octets) Length: length of the CS PDU payload field ST: Segment type (BOM, COM, EOM) SN: Sequence number MID: Message identifier (or Multiplexing ID) LI: Length indicator (number of useful octets) ATM-39
Example of AAL 3/4 Transmission ATM-40
AAL 3/4 and AAL 5 Common Part CS (CPCS) PDUs ATM-41
AAL 5! Provides a similar service to that of AAL 3/4 with a reduced number of control fields in both the CS and SAR PDUs.! Known as the Simple and Efficient Adaptation Layer (SEAL).! AAL 5 is also used in the C plane (Signaling AAL). CS & SAR PDU Formats for AAL 5 - There is no header in the CS PDU. - The PDU is padded to an integral multiple of 48 octets. - The UU fields enables the two correspondent user layers to relate the AAL SDU to a particular SAP. - The use of the CPI field is yet to be defined. - Length field indicates the number of octets in data field. - There is no header or trailer in the SAR PDU. - The SAR protocol is said to be null. - Segments relating to the same CS PDU are identified using the field in the header of the ATM cell that is used to transport the segment. - The SDU Type Bit in user data cells, is used to indicate BOM, COM (type 0) or EOM (type 1) ATM-42
Example of AAL 5 Transmission ATM-43