cs 372 – introduction to computer networks* lecture 3: wednesday june 23

Chapter 1, slide: 1

CS 372 – introduction to computer networks*Lecture 3: Wednesday June 23

Announcements:

Assignment 1 is posted online and is due next Tuesday

Quiz on next Tuesday Lab 1 is posted and is due next Monday No late lab and assignment will be

accepted!

Acknowledgement: slides drawn heavily from Kurose & Ross

* Based in part on slides by Bechir Hamdaoui, Paul D. Paulson, and Dina Katabi.

The network core: Packet switching

Data transmitted in small, independent pieces Source divides outgoing messages into

packets Destination recovers original data

Each packet travels independently Includes enough information for delivery May follow different paths Can be retransmitted if lost

Chapter 1, slide: 2

The network core:Functions of packet-switching networks

Packet construction encode/package data at source

Packet transmission send packet from source to destination

Packet interpretation unpack/decode data from packet at destination acknowledge receipt

Chapter 1, slide: 3

The network core: other functions

Route discovery Traffic/congestion control Retransmitting lost packets Determining type of data

messages service requests/responses files audio/video etc.

etc.

Chapter 1, slide: 4

Packet switching: Reordering and different path

Chapter 1, slide: 5

Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Chapter 1, slide: 6

Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Network access and physical media5 Internet structure and ISPs 6 Protocol layers, service models7 Delay & loss in packet-switched

networks

Chapter 1, slide: 7

Access networks and physical media

Q: How to connect end systems to edge router?

residential access nets

institutional access networks (school, company)

mobile/wireless access networks

Physical Media

why is it needed? to propagate bits between sender/receiver pairs

what is it? a physical link that lies between sender &

receiver

two types of media: guided media: signals propagate in solid media

unguided media: signals propagate freely, e.g., wireless radio

Chapter 1, slide: 8

Chapter 1, slide: 9

Residential access: point to point access Dialup via modem

regular twisted-pair copper phone lines

up to 56Kbps direct access to router (often less)

rate depends on thickness and distance

may pick up interference (“noise”)

can’t surf and phone at same time: can’t be “always on”

Chapter 1, slide: 10

Residential access: point to point access ADSL: asymmetric digital subscriber

line regular phone lines transmission rates depend on

length point-to-point medium (dedicated) up to 1 Mbps upstream

(today typically < 256 kbps)

up to 8 Mbps downstream (today typically < 1 Mbps)

FDM: 50 kHz - 1 MHz for downstream

4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary

telephone

Guided Media: coaxial cable

two concentric copper conductors baseband:

single channel on cable legacy Ethernet

broadband: multiple channels on cable hybrid fiber-coax cable (HFC)

Cable TV rate depends on thickness and distance less interference than twisted pair

Chapter 1, slide: 11

Chapter 1, slide: 12

Residential access: cable modems

HFC: hybrid fiber coax asymmetric up to 30Mbps downstream up to 2 Mbps upstream

network of cable and fiber attaches homes to ISP router

Shared medium deployment: available via cable TV

companies

Chapter 1, slide: 13

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5,000 homes

Chapter 1, slide: 14

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

server(s)

Chapter 1, slide: 15

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Chapter 1, slide: 16

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

Channels

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

DATA

DATA

CONTROL

1 2 3 4 5 6 7 8 9

FDM:

Chapter 1, slide: 17

Company access: local area networks

local area networks (LAN), more in chapter 5 connect end system to edge router E.g., universities, companies

Example: Ethernet:

shared or dedicated link connects end system to router

10 Mbs, 100Mbps, Gigabit Ethernet

Chapter 1, slide: 18

Wireless access networks

wireless access network connects end system to router via base station or “access point”

Examples: wireless LANs:

802.11b/g (WiFi): 11 or 54 Mbps

wider-area wireless access provided by telcomm operator 3G ~ 384 kbps GPRS in Europe/US

basestation

mobilehosts

router

Chapter 1, slide: 19

Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Network access and physical media5 Internet structure and ISPs6 Protocol layers, service models

7 Delay & loss in packet-switched networks

Chapter 1, slide: 20

Internet structure: network of networks

roughly hierarchical: tier 1, tier 2, and tier 3 at center: “tier-1” ISPs

e.g., MCI, Sprint, AT&T, Cable and Wireless, national/international coverage

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)

Chapter 1, slide: 21

Tier-1 ISP: e.g., Sprint

Sprint US backbone network

Seattle

Atlanta

Chicago

Roachdale

Stockton

San Jose

Anaheim

Fort Worth

Orlando

Kansas City

CheyenneNew York

PennsaukenRelay

Wash. DC

Tacoma

DS3 (45 Mbps)OC3 (155 Mbps)OC12 (622 Mbps)OC48 (2.4 Gbps)

Chapter 1, slide: 22

Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

Chapter 1, slide: 23

Internet structure: network of networks

“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Chapter 1, slide: 24

Internet structure: network of networks

a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Chapter 1, slide: 25

Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Network access and physical media5 Internet structure and ISPs6 Protocol layers, service models7 Delay & loss in packet-switched

networks

Chapter 1, slide: 26

Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of organizing structure of

network?

Or at least our discussion of networks?

Chapter 1, slide: 27

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

Chapter 1, slide: 28

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below

Chapter 1, slide: 29

Why layering?

Dealing with complex systems:

Easing assignment of tasks identify relationship among pieces of

complex systems

Easing maintenance, updating of system change of implementation of layer’s service

transparent to rest of system e.g., change in gate procedure doesn’t

affect rest of system

Chapter 1, slide: 30

Internet protocol stack application: supporting network

applications FTP, SMTP, HTTP

transport: process-process data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

Chapter 1, slide: 31

sourceapplicatio

ntransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

application

transportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

Chapter 1, slide: 32

ISO/OSI Model: late 70’s

application

transport

network

link

physical

presentation

application

session

transport

network

data link

physical

7-layer ISO/OSI model(OSI: open system interconnections)

5-layer Internet Protocol Stack

Chapter 1, slide: 33

Chapter 1: roadmap

1 What is the Internet?2 Network edge3 Network core4 Network access and physical media5 Internet structure and ISPs 6 Protocol layers, service models7 Delay & loss in packet-switched

networks

End-to-end delay (nodal delay) : Total time from initiating “send” (from

source) to completed “receive” (at destination)

Throughput : Rate (bits/sec) at which bits are actually

being transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time

Network performance metrics

Chapter 1, slide: 34

Chapter 1, slide: 35

Sources of packet delay

1. nodal processing: check bit errors determine output link

2. queueing time waiting at output

link for transmission depends on

congestion level of router

A

Bnodal

processing queueing

Chapter 1, slide: 36

3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) trans. delay = L/R

4. Propagation delay: d = length of physical link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

Note: s and R are very different quantities!

A

B

propagation

transmission

nodalprocessing queueing

Sources of packet delay

How do loss and delay occur?

A

B

packet being transmitted (delay)

packets queueing (delay)

packets get dropped (loss)if no free buffers

Chapter 1, slide: 37

Packet loss

queue (buffer) preceding link in buffer has finite capacity

packet arriving at a full queue is dropped (lost)

lost packet may be retransmitted by previous node, by source, or not at all

A

B

packet being transmitted

packet arriving tofull buffer is lost

buffer (waiting area)

Chapter 1, slide: 38

Chapter 1, slide: 39

Caravan analogy

Cars run at 100 km/hr (speed of propagation)

Booth takes 12 sec to service a car (transmission time)

car~bit; caravan ~ packet

Q: How long until caravan is lined up before 2nd toll booth?

Time to “push” entire caravan through toll booth = 12*10 = 120 sec = 2 mns

Time for last car to propagate from 1st to 2nd toll both: =100km/(100km/hr)= 1 hr

A: 1 hr 2 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km

Chapter 1, slide: 40

Caravan analogy (more)

Cars now “propagate” at 1000 km/hr

Toll booth now takes 1 min to service a car

Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.

1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!

toll booth

toll booth

ten-car caravan

100 km

100 km

Chapter 1, slide: 41

Example

Host A Host Btrans. rate R = 1 Mbps

distance = 1 km, speed = 2x108m/s

Packet length = L bits

Question: Which bit is being transmitted at the time the first bit

arrives at Host B for

Answer:First bit arrives after 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 =

6 µsec

After 6 µsec6 bits are already transmitted; so 7th bit is being transmitted

Chapter 1, slide: 42

Nodal delay

dproc = processing delay typically a few microsecs or less

dqueue = queuing delay depends on congestion

dtrans = transmission delay = L/R, significant for low-speed links

dprop = propagation delay a few microsecs to hundreds of msecs

proptransqueueprocnodal ddddd

Chapter 1, slide: 43

Queueing delay (revisited)

Every second: aL bits arrive to queue Every second: R bits leave the router Question: what happens if aL > R ? Answer: queue will fill up, and packets will get

dropped!!

aL/R is called traffic intensity

queuePacket arrival rate= a packets/sec

Link bandwidth = R bits/sec

Packet length = L bits

Chapter 1, slide: 44

Queueing delay (revisited)

La/R ~ 0: avg. queueing delay small

La/R -> 1: delays become large La/R > 1: more “work” than can

be serviced, average delay infinite!

queuePacket arrival rate= a packets/sec

Link bandwidth = R bits/sec

Packet length = L bits

Chapter 1, slide: 45

Exercise 1Transmission vs. propagation

Host A Host Btrans. rate R = ?

distance = 2 km, speed = 2x108m/s

L=100Bytes

Question: At what rate (bandwidth) R would the propagation delay

equal the transmission delay?

Answer: Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec Transmission delay = 100x8 (bits)/R Prop. Delay = trans. Delay => R=105x100x8 = 80

Mbps

Chapter 1, slide: 46

Exercise 2Voice over IP

Host A Host Btrans. rate R = 1Mbps

delay_prop = 2mseca=64Kbps

L=48 Bytes

Host A converts analog to digital at a=64Kbps groups bits into L=48Byte packets sends packet to Host B as soon it gathers a packet

Host B As soon as it receives the whole pckt, it converts it to analog

Question: How much time elapses from the 1st bit is created until the

last bit arrives at Host B?

Chapter 1, slide: 47

Exercise 2Voice over IP

Host A Host Btrans. rate R = 1Mbps

delay_prop = 2msec

Answer: Time to gather 1st pkt: 48x8 (bits)/64x1000 (b/s) = 6 msec

Time to push 1st pkt to link: 48x8 (bits)/1x106 (b/s) = 0.384 msec

Time to propagate: 2 msec

Total delay = 6 + 0.384 + 2 = 8.384 msec

a=64Kbps

L=48 Bytes

Chapter 1, slide: 48

Introduction: Summary

Covered a “ton” of material!

Internet overview what’s a protocol? network edge, core,

access network packet-switching

versus circuit-switching

Internet/ISP structure performance: loss, delay layering and service

models

You now have: context, overview,

“feel” of networking more depth, detail

to follow!