Introduction 1-1

Chapter 1Part 2Network Core

These slides derived from Computer Networking: A Top Down Approach ,6th edition. Jim Kurose, Keith RossAddison-Wesley, March 2012.

Comp 365Computer Networks

Fall 2014

Introduction 1-2

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching, network structure

1.4 Delay, loss and throughput in packet-switched networks

1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-3

The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

Resources reserved

Resources reserved

Resources allocated on demand.

Resources allocated on demand.

Core Resources: buffers, link transmission rateCore Resources: buffers, link transmission rate

Introduction 1-4

The Network Core

The internet is packet switched

Telephone network is circuit switched

We need to understand circuit switching to know why it’s not used for the internet

Introduction 1-5

Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required State maintained Data transferred at a

guaranteed rate

Introduction 1-6

Network Core: Circuit Switching

End-end resources reserved for “call”

Links between circuit switches

Each link can support n circuits

There can be n simultaneous connections.

Each circuit thus gets 1/n of the link’s bandwidth.

Introduction 1-7

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls resource piece idle if

not used by owning call (no sharing)

When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts.

dividing link bandwidth into “pieces” frequency division time division

Introduction

Alternative core: circuit switchingend-end resources allocated

to, reserved for “call” between source & dest:

In diagram, each link has four circuits. call gets 2nd circuit in top link

and 1st circuit in right link. dedicated resources: no sharing

circuit-like (guaranteed) performance

circuit segment idle if not used by call (no sharing)

Commonly used in traditional telephone networks

1-8

Introduction 1-9

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

FDM: frequency-division multiplexingFDM: frequency-division multiplexing

TDM: time-division multiplexingTDM: time-division multiplexing

Each connection gets one band of frequencies

Each connection gets one band of frequencies

Each connection gets one time slot

Each connection gets one time slot

Introduction 1-10

Circuit Switching: FDM and TDM

FDM

frequency

time

4 users

Example: telephone networks

FDM: frequency-division multiplexingFDM: frequency-division multiplexing

4kHz4kHz

4kHz4kHz

4kHz4kHz4kHz4kHz

4kHz = 4,000 hertz = 4,000 cycles per second4kHz = 4,000 hertz = 4,000 cycles per second

Radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz

Introduction 1-11

Circuit Switching: FDM and TDM

TDM

frequency

time

4 users

Example:

TDM: time-division multiplexingTDM: time-division multiplexing

One time slotOne time slot

One frameOne frame

Circuit gets same time slot in every frameCircuit gets same time slot in every frame

Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot.

Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate?

Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot.

Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate? 64 kbps

Introduction 1-12

Numerical example

How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links have transmission rate of 1.536

Mbps Each link uses TDM with 24 slots/sec, 24

slots/frame. 500 msec to establish end-to-end circuit

Let’s work it out!

Introduction 1-13

Numerical example

How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links have transmission rate of 1.536 Mbps Each link uses TDM with 24 slots/sec, 24 slots/frame. 500 msec to establish end-to-end circuit

Soln: determine how many slots we need. To do this must figure

out how many bits/slot:

(1.536 x 106 bits/sec) / (24 slots/sec) = 64,000 bits / slot Now can determine how many slots we need:

640,000 bits / (64,000 bits/slot) = 10 slots. So need 10 seconds + 500 msec = 10.5 seconds. This does not account for propagation delays.

Circuit switching: analysis

Disadvantages: Network resources are wasted during “silent”

times (when no one is talking but still connected)

Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software.

Advantage: Guaranteed bandwith and transmission time Necessary for some applications (streamed

music/video)

Introduction 1-14

Introduction 1-15

Network Core: Packet Switching

transmission: Each end-end data stream divided into packets Each packet travels through communication links Links connected by packet switches

Routers Or link-level switches.

Switches use store-and-forward transmission Switch receives entire packet before it transmits any of it again

Introduction 1-16

Network Core: Packet Switching

transmission:

Introduction 1-17

Network Core: Packet Switching

each end-end data stream divided into packets

user A, B packets share network resources

each packet uses full link bandwidth

resources used as needed

Bandwidth division into “pieces”

Dedicated allocationResource reservation

Introduction 1-18

Network Core: Packet Switching

Switching delays Assume packet has L bits Assume there are Q switches Assume a switch transmits at rate of R Takes each switch how long to transmit the

packet? L/R

Total delay is: QL/R

Introduction 1-19

Packet-switching: store-and-forward

This Figure: takes L/R seconds to

transmit (push out) packet of L bits on to link at R bps

store and forward: entire packet must arrive at router before it can be transmitted on next link

delay = 3L/R (assuming zero propagation delay)

Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay =

?? 15 sec

R R RL

more on delay shortly …

Introduction 1-20

Network Core: Packet Switching

Switching delays: Each switch has multiple links Each link has buffer If packet arrives and another packet is already being

transmitted on that link, must wait in queue Called queuing delay. Varies depending on network congestion Packet loss: queue is full when packet arrives.

Introduction 1-21

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.

TDM: each host gets same slot in revolving TDM frame.

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

On-demand sharing of resources is called statistical multiplexingOn-demand sharing of resources is called statistical multiplexing

Network Layer 4-22

Two key network-core functions

forwarding: move packets from router’s input to appropriate router output

routing: determines source-destination route taken by packets

routing algorithms

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

1

23

0111

dest address in arrivingpacket’s header

Introduction 1-23

Packet switching versus circuit switching

1 Mb/s link For each user

assume: 100 kb/s when “active” active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active at same time is less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

Q: how did we get value 0.0004?

A: see this pdf:https://147.129.181.14/~barr/classes/comp365/lectures/Prob10orMoreUsers.pdf

Introduction 1-24

Packet switching versus circuit switching

packet switching: with 35 users, probability > 10 active at same time is

less than .0004 With 10 or fewer simultaneously active users, aggregate

arrival rate of data is less than or equal to 1 Mbps. No loss of time Allows essentially same performance as circuit switching But allows > 3 times number of users

Packet switching allows more users to use network!

Introduction 1-25

Packet switching versus circuit switching

great for bursty data resource sharing simpler, no call setup

excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem (chapter 7)

Is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

Most networks use packet-switching; even telephone networks are going to this!Most networks use packet-switching; even telephone networks are going to this!

Routing

How do packets make their way through packet-switched Networks?

Introduction 1-26

Internet structure: network of networks

End systems connect to Internet via access ISPs (Internet Service Providers) Residential, company and university ISPs

Access ISPs in turn must be interconnected. So that any two hosts can send packets to

each other Resulting network of networks is very complex

Evolution was driven by economics and national policies

Let’s take a stepwise approach to describe current Internet structure

Internet structure: network of networks

Question: given millions of access ISPs, how to connect them together?

accessnet

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Internet structure: network of networks

Option: connect each access ISP to every other access ISP?

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connecting each access ISP to each other directly doesn’t

scale: O(N2) connections.

Internet structure: network of networks

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Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.

globalISP

Internet structure: network of networks

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But if one global ISP is viable business, there will be competitors ….

ISP B

ISP A

ISP C

Internet structure: network of networks

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But if one global ISP is viable business, there will be competitors …. which must be interconnected

ISP B

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IXP

IXP

peering link

Internet exchange point

Internet structure: network of networks

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… and regional networks may arise to connect access nets to ISPS

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IXP

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regional net

Internet structure: network of networks

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… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users

ISP B

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ISP B

IXP

IXP

regional net

Content provider network

Introduction

Internet structure: network of networks

at center: small # of well-connected large networks “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national &

international coverage content provider network (e.g, Google): private network that connects

it data centers to Internet, often bypassing tier-1, regional ISPs 1-35

accessISP

accessISP

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accessISP

Regional ISP Regional ISP

IXP IXP

Tier 1 ISP Tier 1 ISP Google

IXP

Introduction

Tier-1 ISP: e.g., Sprint

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POP: point-of-presence

1-36