An Introduction to Computer Networks

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

Introduction to Computer Network

Node

Memory

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

Introduction to Computer Network

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
• 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 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
• Traffic pattern:
• Packet arrival rate.
• Destination distribution
• Unicast or multicast operation
• Priority- Differentiation among packets

Introduction to Computer Network

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 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

Introduction to Computer Network

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

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
• Shared Media
• Bus Architecture
• Shared buffer
• Space division
• Cross bar architecture
• Combination of above

Introduction to Computer Network

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

Introduction to Computer Network

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.
• 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
• 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

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 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