Network-on-Chip

Network Adapter and Network Issues

System-on-Chip Group, CSE-IMM, DTU

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NoC Overview Slide

Network AdapterRouting Node

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Overview

•Network Adapter Session / Transport Layer Plug and play interface Traffic encapsulation

•Network Topology Protocol

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Network Adapter

• Functions Encapsulation Service Management Interface multiple IPs to a single NoC

port

• Sockets OCP VCI

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Encapsulation

• Convert messages into packets Header to payload ratio Header is overhead

Routing information Control information (such as services, flit

number, etc) Possible error-correction

• Broadcast, narrowcast services

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Service Management

• Definition: Acquire, retain, use and relinquishany service in a predictable way!!

• Types of Services: BE: not guarantee, only correctness and

completion of transmission is guaranteed GS: provides bounded guarantees

Latency Bandwidth Power etc…

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Sockets

• Point-to-point connection (abstraction)• Abstract away the network details for

the IP cores• Examples: OCP, VCI, etc• Limitations (a bus-based view):

Broadcast, narrowcast, services are not supported

Easy means for GS service request, retention and teardown not supported

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Network

• Two characteristics Topology Protocol

• Flow-Control• Quality-of-Service

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Topology

• Form: relates to geometry Scalable with area and power Easy to lay out in 2D chip plane

• Nature of link: relates to unidirectional or bidirectional links

• Presence of IP core

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Forms of Topology

• preferred for easy of layout• better utilization of available bandwidth

K-ary 2-cube K-ary tree

• better hardware utilization for same bandwidth• good to exploit locality of traffic

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Nature of Topological Link

• Common variations: torus and mesh

Uni-directional Bi-directional

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IP Core of Topology

Direct network

Indirect network

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Irregular Topologies

• Hybrid, asymmetric and hierarchical

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Router Architecture

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Buffering Schemes

• Input Head-of-line blocking

• Output Expensive in terms of hardware

• Virtual-output Moderately buffer cost at very high

improvement in performance

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Crossbar and Arbitration Unit

• Crossbar connects input port to output port

• Arbitration is used to prioritize, setup and manage crossbar connections Possibly programmable for best-effort

and guaranteed service connections

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Protocol

• Many dimensions Circuit vs packet switched Connectionless or connection-

oriented Adaptive or deterministic Minimal or non-minimal Delay or loss Centralized or decentralized control

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Common Routing Mechanisms

• Store-and-forward• Virtual-cut through• Wormhole

Protocol

RouterStallingLatency Storage

Store-and-forward

Packet Packet At two nodes and link between them

Virtual-cut through

Header Header At all nodes and links spanned by the packet

Wormhole Header Packet At the local node

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Flow-Control

• Network-level: NA-to-NA In-order delivery Packet Acknowledgment Credit based injection schemes

• Link-Level: Node-to-Node Congestion look-ahead or stalling Virtual channel selection

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Quality-of-Service

• End-to-end Reserving virtual circuits from source to

destination One-way, round-trip or just reverse-way Connection management overhead!!

• Node-to-Node Logically independent resource allocation

(avoid contention) Division of link bandwidth!!

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Conclusion

• Each NoC level offers many parameters such as topology, packet size, buffereing, that to optimize the implementation

• Sockets are enable plug’n’play of IP cores, thus flexibility in placement anywhere within the network geometry

• Topology is influenced by placement of IP cores• Many protocol choices available, with wormhole

costing the least in terms of buffering• Buffers are most area consuming component

within the routers

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References

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