Asynchronous Transfer Mode - zcu.czledvina/DHT/tugraz/ATM_Revisited.pdfAsynchronous Transfer Mode...

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ATMAsynchronous Transfer Mode

…revisited

ACN 2007

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ATM GOAL

To establish connections between an arbitrarynumber of hosts ...

... over channels that fulfills a certain QoS –level.

-> ATM networks make it possible !

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Basics

ATM characteristics

Addressing, Routing, Signaling

ATM header structure

ATM Layer

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ATM characteristics – Addressing, Routing,Signaling – ATM Header structure – ATM Adaptation Layer

How does an ATM-network look like ?

ATM

Switch

ATM

Switch

Host

A

Host

B

Host

D

Host

C

UNI

NNI

UNI

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ATM characteristics – Addressing, Routing, Signaling –ATM Header structure – ATM Adaptation Layer

What kind of service is used ?

connection oriented• Virtual Channels/Circuits (VC)• VC establishing before transmission (connection setup ->

PVC, SVC)• (VPI + VCI) to identify such a Virtual Channel

Host

A

Host

BVC

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packet switched• 53-byte packets -> cells

(5 byte header, 48 byte payload)• fixed cell size

advantages: hardware implementation,parallelization, improve queue-control, find cellboundaries ...

• transferred asynchronous???

5 bytes 48 bytes

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How work the switches ?

statistical multiplexing of the cells• many VCs on one link (port)• no disordering of cells !

De-multiplexing

input (VCIin, port)

switchingfabric

table

output

multiplexing (QoS)

bufferdiscard

(VCIout, port)

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6 categories of services:(ATM Forum)• CBR – constant bit rate• VBR-RT – variable bit

rate-real time• VBR-NRT – variable bit

rate-non real time• ABR – available bit rate• UBR – unspecified bit

rate• GBR – guaranteed bit

rate (extension to UBR)

B Bmax

ABR, UBR

CBR

VBR

t

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• ABR (Available bit rate):• Source follows network feedback.• Max throughput with minimum loss.

• UBR (Unspecified bit rate):• User sends whenever it wants. No feedback. No guarantee. Cells

may be dropped during congestion.• CBR (Constant bit rate): User declares required rate.

Throughput, delay and delay variation guaranteed.• VBR (Variable bit rate): Declare avg and max rate.

• rt-VBR (Real-time): Conferencing. Max delay guaranteed.• nrt-VBR (non-real time): Stored video.

B Bmax

ABR, UBR

CBR

VBR

t

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each of that service defines several of thetraffic parameters:• PCR – peak cell rate• BT – burst tolerance• MCR – minimum cell rate• SCR sustainable cell rate• MBL maximum burst lengthQoS parameters:• CLR – cell loss ratio• CDV – cell delay variation• CTD cell transfer delay• Mean CTD – mean cell transfer delay ...

Drop cell,or set CLP

Cell ra

te

SCR

Volume:

PCR*BL

Drop cell,or set CLP

Cell ra

te

Volume:

PCR*MBL

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How do we attain a certain QoS – level ?signaling process:• negotiate traffic parameters (specify service category)

and QoS parameters= CAC (call admission control) -> reject/accept VC

• allocate resources: bandwidth, buffer ... on the VC

transfer process: (traffic shaping -> end-stations)• flow control• congestion control

fields in the ATM header

Compare to Traffic Policing (GCRA) Generic Cell Rate Algorithm

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QoS Terms II Generic Cell Rate Algorithm GCRA(I,L)

• I = Increment = Inter-cell Time = Cell size/PCR• L = Limit → Leaky bucket of size I + L and rate 1

LCT = Last ComplianceTimeX = Bucket contents at LCTF = Bucket contents now

F = X – (t-LCT)

F < 0?

F > L?

X = F + I; LCT = tConforming Cell

Non-ConformingCell

No F = 0

No

Yes

Yes

Leaky Bucket AlgorithmL

L + I

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How do we attain a certain QoS – level ?

ATM ABR Congestion Control: EFCI:• EFCI: explicit forward congestion indication:

• Based on negative feedback ("bad things are happening") tosender

• Congested node (queue length > threshold) marks EFCI bitin sender-to-receiver cell

• Receiver sees EFCI set and notifies sender• Sender decreases cell rate:

• ACR: allowed cell rate• ACR = max(ACR *multiplicative decrease, MCR)• Sender increases cell rate if no negative feedback in an

update interval:• ACR = min(ACR+ additive_increase, PCR)

PCR

MCR

EFCI bit: second bit of the three-bit PTI field

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How do we attain a certain QoS – level ?

ATM ABR Congestion Control: explicit rates• Sender declares every Nth cell as "RM" cell:

• RM: resource management• Records its PCR, allowed_cell_rate in RM cell• ER (explicit rate) field in RM cell: used by switches to set source rate

• Switch on sender-to-receiver path: if congested• Determine new rate for that source (consider PCR, ACR)• set ER field to indicate new rate only if new rate less than current ER

value

cell

cell PCR, ACR, ER

PCR, ALC,__

cell PCR, ACR, ER

Sender

Receiver

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Connection-Oriented, Packet-Switched ServiceFixed Cell SizeStatistical Multiplexing at the switchesnegotiating QoS-level for each connectionControl mechanisms based on ATM header info

SUMMARY

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ATM characteristics – Addressing, Routing, Signaling – ATMHeader structure – ATM Adaptation Layer

ATM header UNI/NNI (ATM Layer)

GFC ... generic flow control (only UNI, NNI: GFC is part of VPI)VPI ... virtual path identifierVCI ... virtual channel identifierPTI ... payload type identifier (EFCI-bit)CLP ... cell loss priorityHEC ... header error checksum (CRC-8)

GFC PTIVPI CLPVCI HEC 4 8 16 3 1 8

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Physical Layer

The ATM Reference Model

ATM Layer

AAL

Control Plane User PlaneManagement Plane higher-layer applications

(e.g. HTTP->TCP->IP)

Data Stream

Bit Stream

5 bytes 48 bytesATM Header Payload

53 bytesATM Cell

ATM characteristics – Addressing, Routing, Signaling – ATM Header

structure – ATM Layer

signalling

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ATM Layer• Common Flow Control (generic flow control GFC)• Cell Switching (based on VPI/VCI)• Cell Multiplexing, De-multiplexing

Physical layer• TC: Cell rate de-coupling, HEC generate/verify

transmission frame adaptation, cell delineation,• PMD: Bit-timing, Physical Medium



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• TC Transmission convergence• Cell rate de-coupling by adding idle cells: Sender and

receiver have to operate independent in terms of cell-clock ( „asynchronous“ in terms of ATM)

• HEC: generate and verify the header error checksum HEC

• cell delineation• Via cell delimiter

• Via HEC-correlation,PRESYNC

Bit-weise

Zellen-

weise

HEC

OKHEC

falsch

DELTA

HEC’s OK

ALPHA

HEC’s

falsch

HUNT

SYNC

Bit-wiseHOCOK

AlphaHEC’Swrong

DeltaHEC’SOK

Cell-wise

HOCwrong



5 bytes 48 bytes

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• Physical media dependent (examples)• n*56/n*64 kbps• 1.5 / 2 Mbps (T1/E1)• 6 / 8 Mbps (T2/E2)• 25 Mbps• 45/34 Mbps (E3/T3)• 155 Mbps (OC3c)• 625 Mbps (OC12)• ...



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structure – ATM Adaptation Layer

ATM Adaptation Layer (AAL)Data Stream

48 byte payload

Convergence Sublayer CS

Segmentation /

Reassembly SAR

- segment data stream into PDUs (protocol dataunit; likely to be application dependent)

- encapsulate a PDU (header and/or trailer)-> CS-PDU

higher-layer applications

- segment the CS-PDU into SDUs (service dataunit)

- encapsulate a SDU (header and/or trailer)-> 48 byte payload

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structure – ATM Adaptation Layer

How a PDU, SDU and the corresponding header/trailerlooks like is defined in 4 AAL-types:

AAL 1: CBR; voice, audio, videoAAL 2: multiplexing of more streams; voice,

audio, video (low bit-rate)AAL 3/4: ABR; dataAAL 5: ABR; data (evolved from AAL 3/4,

eliminates much overhead)

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Switching PacketsDatagram or connectionless approach

Every packet contains the complete destination addressForwarding table contains information for a switch how to forward a

packetData can take different path and can arrive out of orderHard to create the forwarding table in large networks

Virtual circuit or connection orientedRequires the setup of a virtual connection of two hosts before any data is

sentVirtual circuit identifier (VCI) identifies the connection in combination with

a virtual circuit (VC) tableData takes the same path and arrives in sequences2 connection types: Permanent vs. switched


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Switching PacketsPVC – Permanent virtual circuit

Network administrator establishes and deletes virtual circuitsSVC – Switched virtual circuit

A host sets up and deleted virtual circuit without the involvement of anetwork administrator -> Signaling Advantages:

Universal connectivityMore efficient resource utilization

ATM –Signaling:The exchange of information specifically concerned with theestablishment and control of connections in an ATM network is calledsignaling.


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UNI vs. NNIUNI: User-network interface;P-NNI: Private network-network interface

ATM switch

UNI

User-to-network interaction Network-to-network interaction

UNIUNI

NNI

ATM switch

User-to-user interaction


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Signaling:

Ingress

Switch

Egress

Switch

Host

A

Host

B

source address

destination addresstopology state

traffic and QoS parameter


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UNI vs. NNI GFC (4 bit)

8 bit

Cell header (5 Byte)

Cell payload (48 Byte)

Cell format (UNI)

16 bitHEC

(8 bit)

PTI (3 bit)

VPI VCI

CLP(1 bit)

12 bit

Cell format (NNI)

16 bitHEC

(8 bit)

PTI (3 bit)

VPI VCI

CLP(1 bit)

ATM characteristics – Addressing, Routing, Signaling – ATM Headerstructure – ATM Adaptation Layer

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Virtual Path and Virtual Channel


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How do we establish a VC ?

Addressing:• unique for each host• 20 bytes (6 byte MAC included)• 3 types (DCC, ICD, NSAP)• …


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3 different ATM- addressing schemes are defined• Data country code (DCC) Format (20 Byte)• International code designator (ICD) Format (20 Byte)• Network service access point (NSAP) by E.164 (20 Byte)

AFI

AFI

AFI

DCC Data country codeDCC

ICD

HO-DSP

HO-DSP

HO-DSPE.164

ESI

ESI

ESI

ESI End-system Identifier

SEL

SEL

SEL

SEL Selector

IDI Initial domain Id.

DSP Domian specific part

ICD Internat. code designator

AFI Address formatDCC FormatIDI

IDI

IDI

ICD Format

NSAP Format E.164

ISO

BSI

ITU-T


E.164 is an ITU-T recommendation which defines the international public telecommunication numbering plan used in the PSTN

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NSAP network service access pointAFI authority and service identifierIDI international domain identifierICD international code designator (BSI)DCC data country code (ISO)IDP initial domain part (AFI + IDI)DSP domain specific partESI end system identifierSEL selector (like link type field)


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Signaling:• to define the route from the source host to the destination

host -> VC• calculate a desired VC according to:

• source address and destination address• topology state (nodes, links)• traffic and QoS parameters ...

-> DTL (Desired Transit List)• source routing• generalized Dijkstra algorithm (OSPF)


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1. host A sends a setup message(destination address, traffic and QoS parameter, high-level protocolbindings, ...)

2. ingress switch calculates a DTL and adds it to themessage (routing)

3. the setup message follows the DTL to every switchspecified, setting up the table in the switch fabric

4. host B sends the message back (also table setup)

Connection Setup


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0

13

2

0

1 3

2

0

13

2

511

4

7

Switch 3

Host B

Switch 2

Host A

Switch 1Switch 1

VCI-in port VCI-out5 1 1111 0 7

......(VCI) with local validity


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every switch along the path specified by the DTLand the destination host can reject the VC-> maybe another try by the ingress switch = a socalled crank-backwhen there is a node or link failure -> VC isterminated an needs to be re-establishedVC teardown with a teardown message


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Point-to-Point (PTP) Connection


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Point-to-Multipoint (PTM) Connection


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Inititate a PTP ConnectionUNI 4.0 specifies the signalling protocols usedacross UNI http://www.atmforum.org/UNI 4.0 specification is based on the Q.2931public network signalling protocol developed bythe ITU-T


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Inititate a PTP Connection


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Accept a Connection


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Reject a Connection …


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User clears a connection …


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Network clears a connection …


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Overview Multipoint Connection …Initiated by a setup message from the root nodeto a single leaf nodeRoot can send Add Party and Drop Partymessages to add/delete leafs in an existingconnectionLeaf-Initiated Join Capability: Leaf node can joinexisting connection with or without theintervention of the root node


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UNI Specifications …UNI 4.0: The functions are a superset of UNI3.1and include both - a mandatory core of functionsand many optional features:

Anycast ServicesExplicit signalling of QoS across the UNIMPC (MPOA Client): Leaf initiated joins are supported


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ILMIIntegrated Local Management Interface providesdiagnostic, monitoring and configuration servicesacross the UNIILMI uses the Simple Network ManagementProtocol (SNMP) and a Management InformationBase (MIB)It allows neighbouring hosts to determine variouscharacteristics of each other e.g. the size of eachother’s connection space, …


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ILMI Address Registration


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ISDN Signalling (vs. ATM)

Telemetry

NetworkDatalink

Physical

OSI layer

Signal Packet

Control plane, D-channel

Layer 1 (I.430, I.431)

LAP-D (Q.921, HDLC)

Q.931 X.25Packet level

X.25LAP-B

X.25Packet level

Circuitswitching

User plane, B-channel

Packetswitching

Leasedcircuit

Separate user plane and control planeBRA: 2 B-ch. á 64 kbps, D- ch. 16 kbpsPRA: 30 B-ch. á 64 kbps, D- ch. 64 kbps

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ISDN Signalling (vs. ATM)

exch. exch.SetupSetup ack

SetupCall proc.Call proc.Alerting

AlertingConnect

Conn. ack

SS 7

B-channel

ConnectConn. ack

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