Networks - Chapter 0 - Motivation 1spp

8/6/2019 Networks - Chapter 0 - Motivation 1spp

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Computer Networking and the Internet:

Design Issues


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MotivationMotivationWhat is the importance of the computer

networking and the Internet? Applications: does it really make our lives easier?

Potential uses: where is it heading to?

As computer networking engineers:

What do we need to know about networks issues?

What do we do with these issues?


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Applications: Efficient WorkflowApplications: Efficient Workflow

Access to multiple sources Minimize travel No 2nd party involvement

Paperless information entry Status update Auditing Alerts

Engineering

Server

Machine

Automation

ManufacturerInternet

Process

Server

Office Plant

IT

Engineering

Server

Machine

Automation


Process

ServerEngineering

Server

MachineMachine

Automation


Process

Server

Office Plant

IT

Security Server

Machine

Controller


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Computer Networking ComponentsComputer Networking Components

and Issuesand Issues

(A Network Design Perspective)(A Network Design Perspective)


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Information, Computers, NetworksInformation, Computers, Networks

Information:

anything that can be represented in bits

Properties:

Information can be infinitely replicable

Computers can manipulate information

Networks create access to information


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NetworksNetworks

Potential of networking:

Move bits everywhere, cheaply, and withdesiredperformance characteristics

Break the space barrierfor information

Networks provide connectivity


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ConnectivityConnectivity

Building Blocks

Links: coax cable, optical fiber, wireless, etc.

Nodes: general-purpose workstations or devices

Direct Connectivity: Point-to-point

Multiple access

Point-to-Point

Bus

Node 1 Node 2 Node n


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Indirect Connectivity

Switched networksSwitches create temporary

physical connections E.g. Circuit-switched

Telephone Network

Inter-networks

Routers create temporarylogical connections E.g. Packet-switched

Internet


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The Internet:

Best-effort (i.e.; noperformance guarantees)

Packet-by-packet delivery

A point-to-point physical

link: Always-connected

Fixed bandwidth

Fixed delay

Zero-jitterPoint-to-Point


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PointPoint--toto--Point ConnectivityPoint Connectivity

Physical layer is, at least, needed for coding,modulation, etc.

Link layer is needed only if the link is: Shared: needs framing, medium access control,

multiplexing, etc.

Unreliable: needs reliability techniques

used sporadically and traffic can flood receivers:needs flow control mechanisms

No need for protocol concepts like addressing,names, routers, hubs, forwarding, or filtering in

point-to-point connections


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Connecting N usersConnecting N usersDirectlyDirectly

Bus:broadcast, collisions, media access control

Full mesh: Cost vs simplicity

. . .

Full meshBus

Address concept is needed if we want a specificreceiver (or receivers) to consume the packet!


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List of Design Problems (so far)List of Design Problems (so far)

Topologies

Framing

Error control

Flow control

Multiple access

How to share a link?


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How to build Scalable Networks?How to build Scalable Networks?

Scaling: the system allows for an increase of a

key parameter (e.g., the number of nodes),

without severely compromising the resources. Inefficiency limits scaling

Direct connectivity is inefficient & hence does

not scale

Mesh: inefficient in terms of the number of links Bus: inefficient in terms of the bandwidth use


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Filtering and ForwardingFiltering and Forwarding

Filtering: choosing a subset of elements from a set

Filtering is the key to efficiency & scaling

Forwarding: sending packets to a filtered subset of thelinks or nodes

Packet sent to one link or node improves efficiency

Solution: Build nodes that filter/forward and connectindirectly such as switches & routers


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ConnectingConnectingNNNodesNodesIndirectlyIndirectly

Star: One-hop path from a node to any other node

Switch S can filter and forward!

Reliability problem (single point of failure) Bandwidth problem (bottleneck at the switch)

It may forward multiple packets in parallel for

additional efficiency!

Star

S


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ConnectingConnectingNNNodesNodesIndirectlyIndirectly

Ring: each node is connected to two neighboringnodes forming a ring.

All nodes do forwarding and filtering

Near-minimal number of links

Reliability to link failure

Ring


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Connecting Nodes Indirectly:Connecting Nodes Indirectly:InterInter--NetworksNetworks

= Internet

The goal is to design the black box on the right

Inter-Network is a network of networks

E.g., the Internet


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InterInter--Networks: Networks of NetworksNetworks: Networks of Networks

Internetworking involves two fundamental

design problems: heterogeneity andscaleConcepts: To handle heterogeneity: use translation, address & name

resolution, fragmentation, etc.

To handle scaling: use hierarchical addressing, routing,naming, address allocation, congestion control, etc.

Covered in more detail in Internetworking course


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Additions to the Design Problem ListAdditions to the Design Problem List

Switching, bridging,

routingReliability

Fragmentation

Naming, addressing

Congestion control,

traffic management


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How to do system design? (Design Ideas)How to do system design? (Design Ideas)

Example Goal: Design an Inter-network

Resources:

Computation: planning, analysis, etc. Time: project management, due dates, etc.

Labor: team sizing, organizing, managing, etc.

Space: location, size, dimensions, etc. Money: cost, budget analysis, investment, etc.

Design Rule: tradeoff cheaper resourcesagainst expensive ones to meet design goals Life is all about compromising and optimizing


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Statistical MultiplexingStatistical Multiplexing

Reduce resource requirements by exploitingstatistical knowledge of the system.

E.g., average rate service rate peak rate

If the service rate < the average rate, then thesystem becomes unstable!!

First, design to ensure system stability!!

Then, for a stable multiplexed system:

Gain = peak rate/service rate.

Cost: buffering, queuing delays, and data loss.


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Stability of a Multiplexed SystemStability of a Multiplexed System

Average Input Rate >Average Output Rate

System will become unstable!

How to ensure stability ?

1. Reserve enough capacity so that the average

demand is less than the reserved capacity2. Dynamically detect overload and adapt either the

demand or the capacity to resolve the overload


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WhatWhats a performances a performancetradeofftradeoff??

R = link bandwidth (bps)

L = packet length (bits)a = averagepacket arrival rate

Traffic Intensity (or Link Utilization) = La/R

(Average traffic divided by the link capacity)

A situation where you cannot get something for nothing!

Also known as a zero-sum game.


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WhatWhats a performances a performancetradeofftradeoff??

La/R ~ 0: average queuingdelay small (Wasteful Design)

La/R 1: delay become large

La/R > 1: infinite average delay(service degrades unboundedly instability)!

Statistics suggest that the besttradeoffis when La/R ~ 0.8


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Example Design:Example Design: CircuitCircuit--SwitchingSwitching

Divide link bandwidth intopieces

Reserve pieces onsuccessive links and tiethem together to form acircuit

Map traffic into thereserved circuits

Resources are wasted ifunused: expensive design.

Circuit-switching: a form of multiplexing

Mapping can be done without headers.

Everything inferred from timing (i.e. connection is

immediately available).


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Example Design:Example Design:PacketPacket--SwitchingSwitching

Chop up data (not links!)

into packetsPackets: data + meta-data

(e.g., header)

Switch packets atintermediate nodes

Store-and-forwardifbandwidth is not

immediately available.

Bandwidth division intopieces

Dedicated allocation

Resource reservation

Packet-switching: another form of multiplexing:

Cost: self-descriptive header per-packet (i.e.; overhead),

buffering, and delays for applications.


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Summary of System Design IdeasSummary of System Design Ideas

Multiplexing

Statistical Multiplexing

Stability and

performance tradeoffsCircuit switching

Packet switching


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Networks - Chapter 0 - Motivation 1spp

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