ATM Technology in Detail - Massey University · PDF fileATM Layering - 3 Convergence ......

ATM Technology in Detail

Professor Richard Harris

Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 2

Objectives

• You should be able to:– Discuss the ATM protocol stack– Identify the different layers and their purpose– Explain the ATM Adaptation Layer– Discuss the key differences between ATM, Frame Relay and

IP technologies


Presentation Outline

• ATM Layering• Adaptation layers in more detail• Comparative reviews of

– ATM versus Frame Relay– ATM versus LAN technologies– ATM versus IP


ATM Layering - 1

• ATM Layering defined by BISDN Reference model

• Distinguish User, Control and Management planes

• Three layers of functionality– Physical– ATM– ATM Adaptation (AAL)


ATM Layering - 2

User PlaneControl Plane Management Plane

Q.2931 TCP/IPFTP, etc

LMI, SNMP,CMIP

AAL AAL

ATM

SDH, SONET,SDH, SONET,DS1, E1, etcDS1, E1, etc

SSCFSSCOPAALCPSA

AL

Examples of Protocol Placement in the BISDN layers

SSCOP - Service Specific Convergence Protocol

SSCF - Service Specific Convergence Function

AALCP - AAL Convergence Protocol


ATM Layering - 3 Convergence

Segmentation and Reassembly

Generic flow controlCell header processingVPI/VCI processingCell muxing & demuxing

Cell rate decouplingHEC header processingCell delineationTransmission frame adaptationTransmission frame generation/recovery

Bit timingPhysical medium

CSSAR

AAL

ATM

TCTransmissionConvergence

PMPhysical Medium

PLPhysical Layer

Laye

r Man

agem

ent

Layer functions


ATM Layering - 4

B-ISDN PDU’s and Service Definitions

Upper LayerProtocols

AAL Layer

ATM Layer

Physical Layer

service definitions

service definitions

service definitions

Upper LayerProtocols

AAL Layer

ATM Layer

Physical Layer

service definitions

service definitions

service definitions

User Layer ProtocolsPDU’s

AAL PDU’s

ATM PDU’s(cells)

Cells - Framing?

= Service Access Point, SAP


ATM - Cell Header - 1, first 5 octets

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

HEC

CPT

VCI

VCI

VCIVPI

GFC VPI

HEC

CPT

VCI

VCI

VCIVPI

VPI

Header at UNI Header at NNI

TransmitOrder

VPI = Virtual Path Identifier VCI = Virtual Channel IdentifierGFC = Generic Flow Control PT = Payload Type C = Cell Loss Priority HEC = Header Error Control

Bit positions


ATM - Cell Header - 2

• Major Header Functions• VPI & VCI information.• Cell Loss Priority Bit - may discard cell if set to 1.• Payload Type - 3 bits.• Generic Flow Control - GFC. For control signalling and OAM

functions at the UNI.• Header Error Control - HEC. An 8 bit field, error checking the

first 4 octets (uses BCH code). Will auto correct single bit errors and identify most multiple bit errors. All resultant uncorrected/errored cells are discarded. HEC may also be used for cell delineation.


ATM - Physical layer - 1

Physical Layer - Examples of Transmission Convergence (TC) functions.

• Cell Rate Decoupling

• Header Error Control - HEC

• Cell Delineation


ATM - Physical Layer - 2

Transmission Convergence - Cell Rate Decoupling

• If no information is passed down from the ATM layer, the TC sub-layer in the transmitter generates Idle Cells to maintain the specified data rate.

• Some systems are Cell-based with no framing (eg. LAN’s), others are Frame-based, eg. STM-1 frames in SDH. In the former case the user rate is 149.760 Mbps. In the latter case the rate is the well known 155.520 Mbps. ie. 149.760 = 155.520 * (26 : 27).

• In a practical case this means inserting a PLOAM cell after each26 cells. See ATM LAN networks.


Physical Layer - 3

Header Error Control (HEC) function• The HEC protects the first 4 octets of Header only - not the cell payload!• Designed to correct isolated single bit errors and detect a very wide range

of multiple bit errors.• The algorithm is conservative.

Single -bit error detected(Correction)

Error detected(Cell discarded)

DetectionMode

CorrectionMode

Multi-bit error detected(Cell discarded)

No error detected(No action)

No errorDetected

(No action)


Physical Layer - 4

• Cell Delineation - Assuming no errors, HEC’s in successive cells will only be correct if the cell boundaries have been maintained on reception.

• Assume the TC sublayer examines 40 bits in a shift register. If this is a valid header the last 8 bits will be the correct HEC for the first 32 bits, ie. the cell boundary has been maintained. If the HEC is incorrect, a one bit shift is made and the HEC recalculated. This bit by bit checking is defined as the ‘hunt’ state.

presynch

hunt

synch

Correct HEC

Cell-by-cellcheck

Incorrect HEC

Alpha consecutive Incorrect HEC

Delta consec.correct HEC

Cell-by-Cell check

Bit-by-Bit Check


Adaptation Layer - AAL

• AAL is the layer which ‘adapts’ the cell-based ATM layer to packet, datagram, or bit stream oriented higher layers.

• Four different types of AAL.

• AAL is considered to consist of 2 internal sublayers:* Convergence Sublayer (CS)*Segmentation and Reassembly Sublayer (SAR)


Adaptation Layer - 2

Service Classification / AAL Types

ServiceParametersTimingCompensation

Bit Rate

ConnectionMode

Example

AAL Type

Class A Class B Class C Class D

Required Not Required

Constant Variable

Connection-Oriented (CO)

Connection-Less (CL)

circuitemulation

variablebit ratevideo

CO datatransfer

CL datatransfer

Type 1 Type 2 Type3Type 5

Type 4


AAL - 3

A Virtual Connection (VC) endpoint consists of a physical mediuminterface, an ATM layer, and the Adaptation layer as shown.

The figure shows multiple user services being simultaneously supported by the AAL, each providing for a different type of transfer: eg.simultaneous video and file transfers.

RXVCI/VPI

RXVCI/VPI

TXVCI/VPI

TXVCI/VPI

RXVCI/VPI

TXVCI/VPI

Convergence SublayerSegmentation and Reassembly Sublayer

Service RService QService P

ATM cell transport media

AAL

ATMLayer


AAL - 4

AAL Types• AAL Type 1 (AAL1) - is specified for Constant Bit Rate (CBR)

services for which clock signals at the receiver may need to be provided by the AAL from the incoming bit stream, eg. circuit emulation.

• AAL Type 2 (AAL2) - is specified for Variable Bit Rate (VBR) services, eg. certain video services.

• AAL Type 3/4 (AAL3/4) - is specified for packet transport services. This AAL is a merging of the earlier specified types 3and 4

• AAL Type 5 (AAL5) - Simpler than AAL3/4; provides best working with current LAN technologies.


AAL - 5

For AAL3/4 and AAL5, the I.363 Rec. further divides the CS into an upper and lower half; ie.

• Service Specific Convergence Sublayer (SSCS)• Common Part Convergence Sublayer (CPCS)

SSCSSSCSCPCSCPCSCS CS

SAR SAR SAR SAR

ATM

Type 1 Type 2 Type 3/4 Type 5


AAL - 6

AAL Convergence and SAR - a generic view, transmit side

HHH

Higher Layer PDU

AAL_SDU

header trailerCS_PDU payload

SAR_PDU payload44,47 or 48

48

53

H T

Cells Transmitted

CS and SAR


The Type 1 AAL - 1

• Transfer of Constant Bit Rate Services• Transfer of Timing Information• Indication of Lost/Errored Information• Functions:

– Segmentation and reassembly of user information– Handling of cell delay variation– Handling of lost & miss-inserted cells– Source clock recovery at receiver– Monitoring and handling of certain bit errors


The Type 1 AAL - 2

AAL Type - SAR_PDU Format

4 bits 4 bits

47 Octets

SN SNP SAR_PDU Payload

SAR_PDUHeader

SAR_PDU

SN = Cell Sequence NumberSNP = Sequence Number Protection


The Type 1 AAL - 3

Sequence Number (SN) field• Three Bit Sequence Count field• CSI Bit - carries the CS Indication - from CS layer

CSI Bit

Sequence Count Field - 3 bits

Sequence No. Field - 4 bits


The Type 1 AAL - 4

Sequence Number Protection (SNP) field• Provides error protection and some auto correction• The SN field is protected by a 3 bit CRC code which provides

single bit correction and multiple bit detection.• The resulting 7 bit word is protected by an even parity check bit

(simple error detection).

Even ParityBit

CRC Field - 3 bitsSNP Field Format


The Type 1 AAL - 5

The CS Protocol for AAL Type 1A number of functions may be handled -eg. Source Clock recovery at the receiver, may use Adaptive Clock

or Time Stamp.• Adaptive Clock

– The local CS reads the buffer of the incoming cells. The buffer fill level provides a mechanism to control the local clock.

• Synchronous Residual Time Stamp (SRTS)– Uses a Residual Time Stamp (RTS) to measure and convey

information to the receiver about the difference between a common reference clock derived from the network and the service clock at the sender. The RTS is transmitted in the CSI bits of successive SAR_PDU’s.


AAL Type 3/4 - Overview

CPCS_PDU payload

SARhead

SARtrail

Higher Layer PDU

pad

CPCS-H CPCS-T

SARhead

SARtrail

SAR_PDUpayload

SAR_PDUpayload

SAR_PDUpayload

SAR_PDUpayload

SAR_PDU

SAR_PDU

SAR_PDU

SAR_PDU

EOM

ATM cell

BOM

COM

COMBOM = Beginning of MessageCOM = Continuation of Message

EOM = End of Message


AAL Type 5 - Overview

SAR_PDUpayload

SAR_PDUpayload

Higher Layer PDU

CPCS_PDU payload

SAR_PDUpayload

SAR_PDUpayload

CPCStrailerpa

d

ATM Cell

AAU 0

AAU 0

AAU 0

AAU 1AAU = ATM user-to-userIndication. It is inserted into the Payload type field of the ATMCell header.


Comparing AAL3/4 and AAL5

CPCS-PDU Comparison

CPI = Common Part Indicator Btag = Beginning TagBA Size = Buffer Allocation Size AL = 32 bit AlignmentEtag = Ending Tag Length = CPCS_PDU Length

AAL3/4 Octets

AAL5

CPCS_PDU payload CPCS-UU CPI Length CRCpad

1 - 65,535 0-47 1 1 2 4Octets

CPCS_PDU trailer

EtagCPI Btag BA Size CPCS_PDU payload AL Lengthpad1 1 2 0-3 1 1 21 - 65,535

CPCS_PDU header CPCS_PDU trailer

CPCS-UU = CPCS user-to-user indicator


Comparing AAL3/4 and AAL5

A comparison at the SAR sublayer is made. (The AAL type1 SAR_PDU is also drawn for reference)

SNPSN SAR_PDU payload

SAR_PDU payload

47 octets1 octet

SAR_PDU payloadSNST MID LI CRC

2 octets 44 octets 2 octets

48 octets

Type 1

Type 3/4

Type 5

SN = Sequence No. ST = Segment Type MID = Multiplexing Ident.LI = Length Indicate CRC = Cyclic Redundancy Check


AAL5 - A final note

A single bit in the PTI field of the cell header is termed the ATM-layer-user-to-ATM-layer-user parameter - AAU.

• AAU = 1 to mark the last SAR_PDU of a CPCS_PDU(or the only SAR_PDU of a small CPCS_PDU).

• AAU = 0 for cells carrying the beginning and continuation SAR_PDU’s ( for CPCS_PDU’s spanning more than one cell. See Slide titled

• “AAL Type 5 - Overview”

Cell Header SAR_PDU 48 octets

One Bit AAU in PTI


Comparative Reviews

• Now that we have provided an overview of ATM and other forms of packet switching, we shall now revisit these earlier technologies and provide some comparative reviews of– ATM versus Frame Relay– ATM versus LAN technologies– ATM versus IP


ATM Versus Frame Relay - 1

• The following diagrams compare the headers for Frame Relay and ATM respectively. Notice that they are actually very similar:

Frame Relay ATM

8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1

DLCI C/R EB D

I (variable)FCS

Flag

DLCI F E

Flag

FCS

GFC VPIVPI VCI

VCIVCI PT C

HECPayload (fixed @ 48 bytes)



• Key to acronyms:– B = Backward explicit congestion notification bit (BECN)– C = Cell loss priority bit (CLP)– C/R = Command/response bit– D = Discard eligibility bit (DE)– DLCI = data link connection identifier– E = Address extension bit– F = Forward explicit congestion notification bit (FECN)– FCS = Frame check sequence field– GFC = Generic Flow Control field– HEC = Header Error Control field– I = Information field (user traffic)– PT = Payload Type identifier– VCI = Virtual Channel Identifier– VPI = Virtual Path Identifier



• Comments:– Each contains a virtual circuit ID– Both contain bits to allow traffic to be tagged if there are any

difficulties present in the network. (DE in Frame Relay and CLP in ATM)

– Both allow for congestion notification. For FR this facility is provided in the FECN and BECN bits. For ATM this is done in the bits residing in the payload type identifier PTI.

• Note: ATM provides no mechanism for identifying forward or backward congestion notification bits.

– The Frame Relay header is embedded in another PDU which is considered part of the overall FR header and trailer. By contrast, ATM doesn’t have flag type fields and error checking is done in the fifth byte of the header.



Attribute Frame Relay ATM

Application support Asynchronous data –not designed for voice.

Asynchronous,synchronous voice, video,data.

Connection mode Connection-oriented Connection-orientedCongestionmanagement

Yes, congestionnotification, traffictagging and possibletraffic discard.

Yes, congestionnotification, traffictagging and possibletraffic discard.

PVC Yes YesSVC (connection ondemand)

Yes Yes

Congestion notificationtechnique

The FECN and BECNbits

The CN bits in the PTIfield

LAN or WANtechnology

WAN based Either

PDU size Variable (PDU iscalled a frame)

Fixed at 53 bytes. PDU iscalled a “cell”.

ACK/NAKretransmissions

No Only for signalling trafficon SVCs


ATM Versus LAN Technologies - 1

Attribute Ethernet IEEE 802.3 ATM

Application support Asynchronous data –with some voice, butnot designed for voice.

Asynchronous data –with some voice, but notdesigned for voice.


Connection mode Connectionless Connectionless Connection-orientedCongestionmanagement

Collision detection Collision detection Yes, congestionnotification, traffictagging and possibletraffic discard.

Method of identifyingtraffic

48-bit MAC address 48-bit MAC address Virtual circuit ID: TheVPI/VCI and an ATMaddress


None None The CN bits in the PTIfield

PDU size Variable Variable Fixed at 53 bytes. PDU iscalled a “cell”.


No No Only for signalling trafficon SVCs


ATM Versus IP - 1

• These two technologies use different identifiers and different encapsulation headers.

• ATM is connection oriented• IP is connectionless.


ATM Versus IP - 2Attribute IP ATM

Application support Asynchronous data –not designed for voice.


Connection mode Connectionless Connection-orientedCongestionmanagement

None Yes, congestionnotification, traffictagging and possibletraffic discard.

Method of identifyingtraffic

32-bit IP address Virtual circuit ID: TheVPI/VCI and an ATMaddress


None The CN bits in the PTIfield

PDU size Variable Fixed at 53 bytes. PDU iscalled a “cell”.


No Only for signalling trafficon SVCs

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