! The resulting networks are known as Ce l Re ay Networks ...ziegler/CIS49.2/Notes_pdf/10_ATM.pdf · Copyright © November 11, 2009 by Chaim Ziegler, Ph.D. ATM-17 GFC - Single Group

ATM-1Copyright © November 11, 2009 by Chaim Ziegler, Ph.D.

Asynchronous Transfer Mode (ATM) Networks

! All source media is first broken down into a stream of fixedsized units known as cells.

! Cell streams relating to different media types are multiplexedtogether on a statistical basis for transmission and switching.

! The resulting networks are known as Cell Relay Networks orAsynchronous 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 Protocol Architecture

The protocol reference model involves three separate planes:

• User plane: Provides for user information transfer, along with associatedcontrols (e.g., flow control, error control)

• Control plane: Performs call control and connection control functions

• Management plane: Includes plane management, which performs managementfunctions related to a system as a whole and providescoordination between all the planes, and layer management,which performs management functions relating to resourcesand parameters residing in its protocol entities


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 Transport Hierarchy

! Transmission Path Level:Network elements that assemble and disassemble the payloadof a transmission system. Cell delineation and header errorcontrol are required at the endpoints of each transmissionpath.

! Digital Section Level:Network elements that assemble and disassemble acontinuous bit or byte stream. Refers to the exchanges orsignal transfer points involved in switching data streams.

! Regenerator Section Level:A portion of a digital section (e.g.; a repeater).


Virtual Path / Virtual Channel Terminology


ATM Logical Connections

! Logical connections in ATM are referrred to as VirtualChannel Connections (VCCs).

! VCCs are used for:- user-to-user exchange of variable rate, full duplex, fixed-

size 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 thathave the same endpoints.

! All the cells flowing over all VCCs in a single VPC areswitched together.

! To set up a VCC, there must first be a VPC to the requireddestination node with sufficient available capacity to supportthe VCC with the appropriate QOS.

! The virtual path control mechanisms include:- calculating routes- allocating capacity- storing connection state information


ATM Connection Relationships

! Logical connections in ATM are referrred to as VirtualChannel Connections (VCCs).

! VCCs are a concatenation of Virtual Channels (VCs).

! A Virtual Path is a group of VCs going the same way.


Call Establishment Using Virtual Paths


Hierarchial Layer-to-Layer Relationship


Cell Switching Routing Principle

VC Switching Principles

! Each VC has a Protocol Connection Identifier (PCI) that isassigned to the VC on each link of the network. The PCI hasonly local significance.

! Routing Scheme Principle:


Example of VP and VC Routing

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 beswitched as a single unit.

! Routing can be performed using either subfield or acombination of the two.

! Example VP and VC Routing:


Representation of VP and VC Switching Hierarchy


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 ispreserved.

- 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 ofthe VPC (e.g.; VCIs used for networkmanagement.)


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 Virtual Path Identifier

VPI cont.

Virtual Channel Identifier

Payload Type CLP

Header Error Control (HEC)

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 0001 - user data, no congestion, SDU type 1010 - user data, congestion, SDU type 0011 - user data, congestion, SDU type 1100 - OAM segment associated cell101 - OAM end-to-end associated cell110 - Resource management cell111 - Reserved for future function


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 CLP

Header Error Control (HEC)


Generic Flow Control (GFC)


GFC - Single Group of Connections: 1-Q Model

! The controlled equipment (terminal equipment - TE), initializestwo 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 setto some larger value at configuration time. The rules fortransmission are as follows:

1. If TRANSMIT = 1, cells on uncontrolled connections maybe sent at any time. If TRANSMIT = 0, no cells may besent on either controlled or uncontrolled connections.

2. If a HALT signal is received from the controllingequipment, TRANSMIT is set to 0 and remains at zerountil a NO_HALT signal is received, at which timeTRANSMIT is set to 1.

3. If TRANSMIT = 1 and there is no cell to transmit on anyuncontrolled connections, then—If GO_CNTR > 0, then the TE may send a cell on a

controlled connection. The TE marks that cell as acell on a controlled connection and decrementsGO_CNTR.

—If GO_CNTR = 0, then the TE may not send a cell ona controlled connection.

4. The TE sets GO_CNTR to GO_VALUE upon receiving aSET signal; a null signal has no effect on GO_CNTR.

The HALT signal is used logically to limit the effective ATM datarate and should be cyclic. For example, to reduce the data rate overa link by half, the HALT command is issued by the controllingequipment so as to be in effect 50% of the time. This is done in apredictable, pattern over the lifetime of the physical connection.


GFC - Two Groups of Connections: 2-Q Model

! For the two-queue model, there are two counters, each with acurrent counter value and an initialization value:GO_CNTR_A, GO_VALUE_A, GO_CNTR_B, andGO_VALUE_B. This enables the network to control twoseparate groups of connections.


Header Error Control (HEC) at Receiver

! At initialization, the receiver's error correction algorithm is inthe default mode for single-bit error correction.

! As each cell is received, the HEC calculation and comparisonis performed. As long as no errors are detected, the receiverremains in error correction mode.

! When an error is detected, the receiver will correct the error ifit is a single-bit error or will detect that a multibit error hasoccurred.

! In either case, the receiver now moves to detection mode.

! The receiver remains in detection mode as long as errored cellsare received. When a header is examined and found not tobe in error, the receiver switches back to correction mode.


Effect of Error in Cell Header


Transmission of ATM Cells

! I.432 specifies that ATM cells may be transmitted at thefollowing 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


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 ofsynchronization is needed.

! Synchronization is achieved on the basis of the Header ErrorControl (HEC) field in the cell header.


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.


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

SONETDesignation

SDHDesignation

Data RateMbps

PayloadRate

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


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.


SONET/SDH Frame Formats

! The basic SONET building block is the STS-1 frame, whichconsists of 810 octets and is transmitted 8000 times persecond for an overall data rate of 51.84Mbps.


SONET STS-1 Overhead Octets


SONET STS-1 Overhead Octets cont.


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 theexact location of the start of the payload.

! The pointer value can increase or decrease by one byte positionat each node.

! When the pointer decrements, H3 holds the extra octet for thatframe.


STS-1 Pointer Adjustment


SDH Based Physical Layer

! A framing structure is imposed on the ATM cell stream usingthe STM-1 (STS-3) frame.

! Since the payload capacity (2,340 octets) is not an integermultiple of the cell legth (53 octets), a cell may cross apayload boundary.

! The H4 octet is set at the sending side to indicate the nextoccurrence of a cell boundary (value of 0 to 52) from H4.


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 willuse and a minimum cell rate (MCR) that it requires.

- Unspecified Bit Rate (UBR)This is a best-efforts service using capacity notallocated to any other service.

- Guaranteed Frame Rate (GFR)Designed specifically to support IP backbonesubnetworks. GFR provides better service than UBRfor frame-based traffic, including IP and Ethernet.GFR allows a user to reserve capacity for each GFRVC. The user is guaranteed this minimum capacity.Additional frames may be transmitted if the networkis not congested.


AAL Service Classes

! AAL services provided between source and destination:


AAL Sublayers

AAL Sublayers

! Convergence Sublayer (CS):Performs a convergence function between the serviceoffered at the layer interface and that provided by theunderlying ATM layer.

! Segmentation and Reassembly (SAR) Sublayer:Provides cell segmentation and reassembly functions.


AAL Sublayers

! Convergence Sublayer (CS):Performs a convergence function between the serviceoffered at the layer interface and that provided by theunderlying ATM layer.

! Segmentation and Reassembly (SAR) Sublayer:Provides cell segmentation and reassembly functions.


AAL 1

! The CS protocol endeavors to maintain a constant bit ratestream between the source and destination SAPs.

! The agreed rate must be maintained even when occasional celllosses or cell transfer time variations occur.

! Cell losses are overcome in an agreed way (e.g., by insertingdummy bits/bytes into the delivered stream.

! Cell transfer delay variations are compensated by bufferingsegments at the destination.

AAL 2

! There is a timing relationship between the source anddestination SAPs determined by the frame rate.

! The amount of information associated with a frame may varyfrom one frame to the next.

! The CS at the source receives bursts of information at theframe rate with each burst containing a variable amount ofinformation.

! The CS protocol at the destination must endeavor to outputthe received information in this same way even whenoccasional cell losses or cell transfer time variations occur.

! Time variations are overcome using similar techniques asdescribed for AAL 1.


SAR PDU Formats: (a) AAL 1 (b) AAL 2

AAL 1 and AAL 2 SAR PDU Formats

Notes: IT: 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 otherinformation.

LI: the last (EOM) segment may not be full and LIindicates the number of useful bytes.


AAL 3/4

! Provides a connectionless or a connection oriented datatransfer 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 ATMconnection.

! 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 transferredin one or more cells.

- Streaming mode service supports the transfer of low-speed continuous data with low delay requirements.The data are presented to the AAL in fixed-size blocksthat may be as small as an octet. One block istransferred per cell.


CS & SAR PDU Format for AAL 3/4

AAL 3/4 CS PDU and SAR PDU Formats

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 SDUFor message mode, equals the CS PDU lengthFor 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)


Example of AAL 3/4 Transmission


AAL 3/4 and AAL 5 Common Part CS (CPCS) PDUs


CS & SAR PDU Formats for AAL 5

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).

- 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 isused to transport the segment.

- The SDU Type Bit in user data cells, is used to indicateBOM, COM (type 0) or EOM (type 1)


Example of AAL 5 Transmission

Documents

! The resulting networks are known as Ce l Re ay Networks ...ziegler/CIS49.2/Notes_pdf/10_ATM.pdf · Copyright © November 11, 2009 by Chaim Ziegler, Ph.D. ATM-17 GFC - Single Group