1© Michel Kadoch

Attributs et Classification des commutateurs ATM

© Michel Kadoch 2

Caractéristiques Architecturales des commutateurs ATM

Les caractéristiques architecturales des commutateurs ATM sont évaluées pour leur pertinence à rencontrer les exigences de performance des protocoles qui supporteront les services à offrir dans B-ISDN.Un commutateur ATM comprend un ensemble de N ports d’entrées, et N ports de sorties, un switch fabric, et un processeur de controle de gestion - management control processor (MCP).

Switch Fabric

MCP

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0

1

N

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0

1

N

Input Controllers Output Controllers

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Contrôleurs I/O

contrôleurs de port d’entrées fournie:

buffering;

cell replication (copying) pour multicast;

traitement des cellules;

translation VPI/VCI;

multiplexage du trafic provenant de plusieurs equippements à basse vitesses;

demandes de connexion de chemin et réservations à travers le switch fabric.

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Switch Fabric

Un “switch fabric” est un mécanisme qui achemine les cellules des ports d’entrée aux ports de sortie. Il doit s’occuper des problèmes suivants :

Établissement d’un chemin entre un port d’entrée et un port de sortie à l’intérieur du commutateur;

La discipline de service pour les ports d’entrées;

Mécanisme de résolution de contention pour les cellules qui compétitionnent pour les liens ou d’autres ressources internes du commutateur (avoir à faire avec le blockage interne);

Support de plusieurs ports d’entrée aux connexions de ports de sortie (parallélisme).

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Management and Control Processor (MCP)

La fonction du MCP est de communiquer avec les controlleurs des ports et de faciliter l’opération de commutation, l’administration et la gestion tel que SNMP ou CMIP (OSI Management).

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Buffering

La prévention de la congestion et du deadlock sont des problèmes majeurs dans la conception de système commutation paquets comme ATM.

La contention d’un lien par une cellule peut bloquer le lien. Buffering ou tamporisation est une solution pour les ports d’entrées et de sorties ou dans le switch fabric ou dans tous ces points ensemble.

Les stratégies “Cell scheduling” sont requises aux ports d’entrées. Une bonne approche est le mécanisme de fenêtre.

Les analyses ont démontré que le “buffering” dans les ports de sorties peut donner un meilleur débit que s’il était apliqué dans les ports d’entrées.

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Classification de Switch Fabrics ATM

ATM Switch Fabrics

Timedivision

Sharedmemory

Sharedmedium

Bus Ring

Spacedivision

Singlepath

Matrix Banyan SortedBanyan

Delta

Multiplepath

AugmentedBanyan

Parallelplanes

Loadsharing

Recirculation

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Commutation Time Division

La connexion physique entre les ports d’entrées et de sorties dans le switch fabric peut être développée en utilisant la méthodologie de « time or space division ».

Dans “time division switching” commutation temporelle, l’utilisation de ressources physiques est multiplexée entre plusieurs connexions d’entrées et de sorties en se basant sur des emplacements en temps discrets. Un bus est un exemple d’un médium physique qui peut faciliter le multiplexage temporel en utilisant de module de mémoire.

M e m o r yM o d u l ecells from input ports

cells to output ports

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Space Division Switching

Dans la méthodologie “space division”, le switch fabric peut supporter plusieurs connexions au même moment. Les connexions sont basées sur la disponibilité de chemins physiques du switch fabric qui ne sont pas en conflit.

2 x 2switchingelement

a0

a1

b0

b1

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Switching Elements

a0

a1

b0

b1

a0

a1

b0

b1

a0

a1

b0

b1

a0

a1

b0

b1

Straight through Lower broadcast

exchange Upper broadcast

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Routing

Routage de cellules à travers un « space-division switch » peut être accomplit par:

self routing label routing

self routing compte sur les interconnexions régulières du switching elements dans le fabric.

label routing, le champ VCI dans l’entête est utilisé par chaque switching element pour prendre les décision de lien de sortie.

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Representative Architecture

Matrix and Fully-Connected Switches Also referred to as crossbar switches. Provide N2 paths between N input and N output ports. A single contact pair for each input-ouput combination.

inputs outputs

Complexity grows with the number of links, i.e., O(N2)

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Knockout Switch

A knockout switch is a matrix architecture. Any input can transmit to any output. The switch is self routing.

input

0

1

N-1

output

Knockoutconcentrator

filter

R R

0 N-1

Filtering is based on the packet destination address

The concentrator with R buffers (R<N)implements a selection algorithm for selecting R cells outof the maximum N possible cells arriving at that port. Worse case:N-R cells lost

R out of N selection algorithm is used by the concentrator

bus

bus drivers

Separate outputbuffers can be maintained and a shifter can beused to allocatecells to thebuffers in order to ensure propersequencing of transmitted cells on each virtual connection in anATM Network.

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Multistage Switches

Two of the basic switching fabrics used for multistage switching are the Banyan and the Delta networks.

Delta Networks

N input port switch composed of b x b simpler switching elements. There are K stages, where N = bK (K=logbN) and N/b switches in each stage (column).

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Example of Delta network

8 x 8 Delta 2 network, where b = 2, N = 8, K = log28 = 3 columns (switches in a column)

K

01

01

01

01

01

01

01

01

01

01

01

01

000001

010011

100101

110111

N

01

234

56

7

one half of

the deck

theother half of

the deck

a perfect shuffle a perfect shuffle

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Multiple connectionsThe bits of the destination address provide the required routing tags. The digits in the destination address are used to set the state of the stages.

01

01

01

01

01

01

01

01

01

01

01

01

001010011

100101

110111

0123

4567

000

Perfect shuffle Perfect shuffleStage 1 Stage 2 Stage 3

011

101

011

101

011

101

011

101

0

10

1 1

1

destination port

address

white bitcontrolsswitchsetting

in each stage

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Internal blocking Internal link blocking as well as output blocking can happen in a Delta

network. The following example illustrates an internal blocking for connections of input 0 to output 3 and input 4 to output 2.

01

01

01

01

01

01

01

01

01

01

01

01

001010011

100101

110111

01

23456

7

000

Perfect shuffle Perfect shuffleStage 1 Stage 2 Stage 3

blocking link011

010

011

010

??? ???

???

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Output Blocking The following example illustrates output blocking for the connections between input 1 and output 6, and input 3 and output 6.

01

01

01

01

01

01

01

01

01

01

01

01

001010011

100101

110111

01

23456

7

000

Perfect shuffle Perfect shuffleStage 1 Stage 2 Stage 3

110

110

110

110

110

110

output blocking

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Blocking Reduction

Means to reduce the probability of blocking or increase throughput: installation of a distribution (randomizing) network in front of the

switch to reduce or eliminate the possibility of internal blocking. Recirculate packets that cause blocking to the input ports,

including them in the next cycle. Provide a contention resolution phase among the input ports. Subject the traffic between the stages of the network to a

handshake protocol that synchronizes transmission and reception across the stages.

Add extra stages to the multistage network and produce extra paths.

Provide a number of buffers on each link or in the switching elements.

Increase the bandwidth of the internal links relative to the input links.

Increase network throughput by using several networks in parallel.

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The Banyan Network

The banyan network is another self-routing switching fabric, similar in topology to the delta network.

01

01

01

01

01

01

01

01

01

01

01

01

001010011

100101110111

0123

4567

000

Stage 1 Stage 2 Stage 3

011

101

011

101

011

101 0

10

1

1

1011

101

011

101

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Adding Load Distribution Because of internal blocking, input traffic is sorted on the

destination ports. The network is known as Batcher sorter

01

01

01

01

01

01

01

01

01

01

01

01

001

010011

100101

110111

000

Perfect shuffle Perfect shuffleStage 1 Stage 2 Stage 3

B

A

01

01

01

01

A

B

A = Connection to output port 3 (011)B = Connection to output port 2 (010)

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Output blocking

To eliminate output contention if there are multiple packets with the same destination:

Remove the duplicate-destination packets from the group and include them in the next batch for sorting and routing;

submit only requests (into the group to be sorted) that have no destination in common.

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Adding Recirculation

Recirculating networks can redistribute, to the input ports, packets that were not successfully delivered in a given cycle.

Switchfabric

recirculation buffer

This technique deals with output or internal blocking.

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Starlight Switch Structure

The Trap network examines the output of the sort network and removes all packets with the same destination address. The duplicates are routed back to the sort network for the next cycle with a higher priority.

Concentrator

Sortnetwork

Trapbuffer Routing

Banyan Network

Input Output

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Adding a contention resolution phase

Three Phase AlgorithmPhase 1

The input ports that have traffic, submit a transmission request to the sorting network. The sorter sorts the requests based on the destination. One of the requests that contests the access to the destination is selected; the others with the same destination address are purged.

Phase 2The sorter sends an acknowledgement to the input ports from which requests have been received and selected.

Phase 3The ports selected transmit their packets.

This method can have some blocking problems, and phase 1 and 2 are overhead.

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Adding Extra Stages

One way to augment a Banyan network is to add extra stages to increase the number of paths from input ports to output ports.

Each additional stage doubles the number of paths between any input-output pair.

A better load distribution can increase throughput, alleviate blocking, and make the fabric less sensitive to the load on input ports.

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Adding Internal Buffers

Adding buffers also augments a Banyan network. The resulting configurations are known as buffered Banyans.

The Integrated Service Packet Network (ISPN) described by Turner1.

The switch is constructed from 2 x 2 switches in which each link buffers one packet. The fabric connects 1024 ports using 5120 switching elements arranged in 10 stages. The internal links operate at eight times the speed of the external ports. So, for 100% load, an input port results in only 12.5% load on the internal links. A back pressure flow mechanism prevents contention at the internal links. When a packet arrives at a free link, the packet is not buffered.

1. J. Turner, « Design of an Integrated Service Packet Network », Proceedings Ninth ACM Data Comm. Symposium, ACM, New York, 1985,pp. 125-133.

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Adding Parallel Switching Planes

One way to increase throughput is to connect the input and output ports to multiple planes of the switch fabric.

Each input port can distribute its traffic to the multiple homogeneous fabric planes. Similarly, each output port can be fed from multiple fabric planes.

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Putting it all together The Sunshine switch is an example that incorporates: Batcher Banyan; Internal and output buffering; parallel fabrics; recirculation with dedicated input ports; self routing; level of service priority; trunk groups between internal nodes.

Batchersort

1

N

TrapNetwork

Concentrator

Banyan 1

Banyan K

1

N

Delay T T

Select at mostK cells per destination.

Separate selectedcells from the rest.

Selector

Input port Output port

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Commercial Systems

ATM strategy for major vendor classes and some commercially available ATM switches.

Hub and Router VendorsUses two stage strategy for introducing ATM.Phase 1

Add interface board to connect hubs or routers to an ATM switch over SONET (OC-3) or T3.Phase 2

The next step will be to incorporate an ATM switching module (switch fabric) into

the hub or router. Several vendors are considering attaching to an ATM adaptor box.

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Multiplexer and WAN Vendors

They are working on or have already released ATM switches or adapters for the LAN. Examples: ATMX from Adaptive, Newbridge’s 361X ATM switch, the Synchrony product from Ascom Timplex, Stratacom’s BPX, GTE Government System’s ATM switch.

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Computer Systems Vendors

IBM, NEC, DEC, Sun Microsystems, HP, and AT&T are developing intelligent hubs that offer ATM support.