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An Introduction to Computer Networks. Lecture 7: Switch fabric design. University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani. Outline. Introduction What are switches? Basic concepts Design factors Performance factors Functional requirements

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an introduction to computer networks

An IntroductiontoComputer Networks

Lecture 7: Switch fabric design

University of Tehran

Dept. of EE and Computer Engineering

By:

Dr. Nasser Yazdani

Introduction to Computer Network

outline
Outline
  • Introduction
    • What are switches?
    • Basic concepts
  • Design factors
  • Performance factors
  • Functional requirements
  • Design architectures

Introduction to Computer Network

switches

Node

incoming links

outgoing links

Memory

Switches
  • A means to create connectivity
  • Why switching? Efficient utilization of resources
  • Store and forward scheme

Introduction to Computer Network

basic concepts
Basic concepts
  • An ideal switch: route all incoming packet to their requested outputs with the following conditions:
    • No loss
    • Minimum delay
    • Preserving order
  • Output contention: At least two input goes to one output. we need buffer and queuing (how much?)
  • Congestion: no buffer space left. Drop packet

Introduction to Computer Network

design factors
Design Factors
  • Throughput
    • Max throughput- N x line speed where N is the # of input line
    • Ave throughput – At random is %60.
    • Packet per second (PPS)- # of packets switched per second
    • Throughput depends on the traffic.
  • Quality of service:
    • Ave delay of cells
    • Jitter
    • Reliability in switching cells.

Introduction to Computer Network

design factors1
Design Factors
  • Scalability: how the architecture scale, linear? Square?
    • Regarding the speed of input line
    • Regarding the # of input port.
  • Cost:
    • # of logic gates
    • Memory
    • Bandwidth or # of pines

Introduction to Computer Network

performance factors
Performance Factors
  • Traffic pattern:
    • Packet arrival rate.
    • Destination distribution
  • Addressing and operation:
    • Unicast or multicast operation
  • Priority- Differentiation among packets

Introduction to Computer Network

functional requirement
Functional requirement
  • User plane:
    • Data is carried transparently
    • Header are processed and VPI/VCI is used for routing.

0

0

LC

SFC

IPP

LC

OPP

Central unit

Introduction to Computer Network

functional requirement1
Functional requirement
  • Cntrl plane: Signaling
    • Identified by VPI/VCI, 5/0 is for call admission.
    • May use SFC for transporting ctrl packets.

IPP+ SFC+ OPP Store and forward system.

CAC

LC

SFC

IPP

LC

OPP

CAC- Call

Admission Control

Introduction to Computer Network

functional requirement2
Functional requirement
  • Management plane: System Management (SM) must handle management cell.
    • Should support Network wide operations.
    • May use SFC for transporting management cells.

SM

LC

SFC

IPP

LC

OPP

Introduction to Computer Network

workstation based switch

I/O bus

CPU

Interface 1

Interface 2

Interface 3

Main memory

Workstation-Based switch
  • Aggregate bandwidth
    • 1/2 of the I/O bus bandwidth
    • capacity shared among all hosts connected to switch
    • example: 800Mbps bus can support 8 T3 ports
  • Packets-per-second
    • must be able to switch small packets
    • 100,000 packets-per-second is achievable
    • e.g., 64-byte packets implies 51.2Mbps

Introduction to Computer Network

switch fabric design
Switch fabric design
  • Shared Media
    • Bus Architecture
    • Shared buffer
  • Space division
    • Cross bar architecture
  • Combination of above

Introduction to Computer Network

bus architecture
Bus Architecture
  • IPP puts cells on bus
  • OPP buffer cells
  • Control Processor (CP)
    • exchanges control messages
    • configures connections
  • Bus interconnects various components.

» Innonblocking systems bandwidth is equal to sum of external link bandwidths;

» bus width must increase with number of links

» capacitive loading reduces clock rate as number of links grows

Introduction to Computer Network

divided bus with knockout concentrators

w/n

IPP

IPP

IPP

IPP

w

OPPs

Divided Bus with Knockout Concentrators
  • Split bus into n “minibuses” with w/n wires each
  • Each minibus driven by justone IPP.
    • cuts capacitive loading in half
    • adding fanout componentsallows higher clock frequencies
  • OPPs concentrate n minibusesonto L<n outputs (optional)
  • OPPs must each be able tobuffer up to L cells in parallel
  • Parallel reception complicates control somewhat
  • Concentration reduces required OPP memory bandwidth

Introduction to Computer Network

knockout switch
Knockout Switch
  • Concentrator
    • select l of

npackets

  • Complexity: n2

Inputs

D

D

D

D

D

D

D

D

D

D

D

D

D

D

1

2

3

4

Outputs

Introduction to Computer Network

shared buffer switches

CP

OPP

IPP

. . .

. . .

. . .

Shared Buffer Switches
  • For switches with 10 or more links, can reduce required memory by up to an order of magnitude.
  • Queues are rarely full, then, memory for queues is unused mostly
  • With shared memory, we can achieve same performance level with less memory.
  • Requires a central memory with bandwidth equal to twice the external link bandwidth.
  • Per output or per flow queues typically implemented as linked lists.

Introduction to Computer Network

crossbar architecture
crossbar Architecture
  • crossbar allows multiple cells to pass in parallel to distinct outputs

» use of point-to-point transmission eliminates capacitive loading at circuit board level

» parallelism reduces data path width at IPPs, OPPs

  • control circuit arbitrates access to outputs
  • retains quadratic complexity, but concentrates it within chip, reducing system cost

IPP

OPP

Introduction to Computer Network