代写代考 COMP 3331/COMP 9331

Computer Networks and Applications COMP 3331/COMP 9331
Introduction to Computer Networks
Reading Guide: Chapter 1, Sections 1.1 – 1.4

Acknowledgment
v Majority of lecture slides are from the author’s lecture slide set § Enhancements + additional material

Introduction
vGet “feel,” “big picture,” introduction to terminology
§ more depth, detail later in course
vApproach:
§use Internet as example
Overview/roadmap:
v What is the Internet?
v What is a protocol?
v Network edge: hosts, access network, physical media
v Network core: packet/circuit switching, internet structure
v Performance: loss, delay, throughput v Protocol layers, service models
v Security (self study, not on exam)
v History (self study, not on exam)
Hobbe’s Internet Timeline – http://www.zakon.org/robert/internet/timeline/

Quiz: What is the Internet?
A. One single homogenous network
B. An interconnection of different computer networks
C. An infrastructure that provides services to networked applications D. Something else (answer in comments on Zeeting)
Answer: B and C as explained on the next few slides Answers: B and C are valid answers
Open a browser and type: pollev.com/salil

The Internet: a “nuts and bolts” view
Billions of connected computing devices:
§ hosts = end systems
§ running network apps at Internet’s “edge”
Packet switches: forward packets (chunks of data)
§ routers, switches Communication links
§ fiber, copper, radio, satellite § transmission rate: bandwidth
§ collection of devices, routers, links: managed by an organization
mobile network
national or global ISP
local or regional
home network
enterprise network
content provider network
datacenter network

“Fun” Internet appliances Web-enabled toaster +
Security Camera
weather forecaster
Picture frame
Tweet-a-watt: monitor energy use
Amazon Echo
Networked TV Set top Boxes
sensorized, bed mattress
Internet refrigerator
Smart Lightbulbs
AR devices

The Internet: a “nuts and bolts” view
v Internet: “network of networks” § Interconnected ISPs
§ protocols are everywhere
• controlsending,receivingof
• e.g., HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4G, Ethernet
§Internet standards
• RFC: Request for Comments
• IETF: Internet Engineering Task Force
mobile network 4G
national or global ISP
local or regional ISP
Streaming video
datacenter network
home network
content provider network
enterprise network

The Internet: a “service” view
v Infrastructure that provides services to applications:
§ Web, streaming video, multimedia teleconferencing, email, games, e- commerce, social media, inter- connected appliances, …
§ provides programming interface to distributed applications:
• “hooks” allowing sending/receiving apps to “connect” to, use Internet transport service
• provides service options, analogous to postal service
mobile network
national or global ISP
Streaming video
datacenter network
local or regional ISP
home network
enterprise network
content provider network

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What’s a protocol?
Human protocols:
§ “what’s the time?” § “I have a question” § introductions
… specific messages sent
… specific actions taken when message received, or other events
Network protocols:
§ computers (devices) rather than humans
§ all communication activity in Internet governed by protocols
Protocols define the format, order of messages sent and received among
network entities, and actions taken on msg transmission, receipt

What’s a protocol?
A human protocol and a computer network protocol:
Got the time?
time Q: other human protocols?
TCP connection request
TCP connection response
GET http://gaia.cs.umass.edu/kurose_ross

Quiz: Internet of Things
How many Internet-connected devices do you have in your home (include your computers, phones, tablets)?
A. Less than 10
B. Between 10 to 20 C. Between 20 to 50 D. Between 50 to 100 E. More than 100
Open a browser and type: pollev.com/salil

Introduction: roadmap
vWhat is the Internet? vWhat is a protocol?
vNetwork edge: hosts, access network, physical media vNetwork core: packet/circuit switching, internet
vPerformance: loss, delay, throughput v Security
vProtocol layers, service models

A closer look at Internet structure
Network edge:
vhosts: clients and servers vservers often in data centers
national or global ISP
mobile network
local or regional ISP
home network
enterprise network
content provider network
datacenter network

A closer look at Internet structure
Network edge:
vhosts: clients and servers vservers often in data centers
Access networks, physical media:
vwired, wireless communication links
national or global ISP
mobile network
local or regional ISP
home network
enterprise network
content provider network
datacenter network

A closer look at Internet structure
mobile network
Network edge:
vhosts: clients and servers vservers often in data centers
Access networks, physical media:
vwired, wireless communication links
Network core:
§interconnected routers §network of networks
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network

Access networks and physical media
Q: How to connect end systems to edge router?
v residential access nets
v institutional access networks (school,
v mobile access networks (WiFi, 4G/5G)
What to look for:
§ transmission rate (bits per second) of access network?
§ shared or dedicated access among users?
mobile network
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network

Access networks: digital subscriber line (DSL)
central office
DSL access multiplexer
telephone network
DSL splitte modem
voice, data transmitted at different frequencies over dedicated line to central office
§ use existing telephone line to central office DSLAM • data over DSL phone line goes to Internet
• voice over DSL phone line goes to telephone net

Access net: digital subscriber line (DSL)
DSL splitter modem
Different data rates for upload and download (ADSL)
§24-52 Mbps dedicated downstream transmission rate §3.5-16 Mbps dedicated upstream transmission rate
ADSL over POTS
voice, data transmitted
at different frequencies over dedicated line to central office
High-pass filter for data
Low-pass filter for voice

Access net: digital subscriber line (DSL)

Access networks: cable-based access
cable splitter modem
cable headend
O VVVVVV N IIIIIIDDT DDDDDDAAR EEEEEETTO OOOOOOAAL
123456789 Channels
frequency division multiplexing (FDM): different channels transmitted in different frequency bands

Access networks: cable-based access
cable headend
cable splitter modem
data, TV transmitted at different frequencies over shared cable
distribution network
cable modem termination system
§ HFC: hybrid fiber coax
• asymmetric: up to 40 Mbps – 1.2 Gbs downstream transmission rate, 30-100 Mbps
upstream transmission rate
§ network of cable, fiber attaches homes to ISP router • homes share access network to cable headend
• Unlike DSL, which has dedicated access to central office

Fiber to the home/premise/curb
v Fully optical fiber path all the way to the home (or premise or curb)
§ e.g., NBN, Google, Verizon FIOS § ~30 Mbps to 1Gbps

Access networks: home networks
wireless devices
often combined in single box
WiFi wireless access point (54, 450 Mbps)
to/from headend or central office
cable or DSL modem
router, firewall, NAT wired Ethernet (1 Gbps)

Wireless access networks
Shared wireless access network connects end system to router § via base station aka “access point”
Wireless local area networks (WLANs)
§ typically within or around building (~100 ft)
§ 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate
to Internet
Wide-area cellular access networks
§ provided by mobile, cellular network operator (10’s km)
§ 10’s Mbps
§ 4G cellular networks (5G coming)
to Internet

Access networks: enterprise networks
Ethernet switch
Enterprise link to
ISP (Internet) institutional router
institutional mail, web servers
§ companies, universities, etc.
§ mix of wired, wireless link technologies, connecting a mix of switches
and routers (we’ll cover differences shortly)
§ Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps § WiFi: wireless access points at 11, 54, 450 Mbps

Sample results
Can you explain the differences?
FTTC + Cable + WiFi @ my home
Wired Network @ CSE
4G Network

Quiz: Your access network
Your residential ISP provides connectivity using the following technology: A. DSL
C. Fiber to the home/premise/curb
D. Mobile (3G/4G/5G)
E. Satellite
F. Something Else (type in Zeetings comment)
Open a browser and type: pollev.com/salil

Links: physical media
§ bit: propagates between transmitter/receiver pairs
§ physical link: what lies between transmitter & receiver
§ guided media:
• signals propagate in solid
media: copper, fiber, coax
§ unguided media:
• signals propagate freely, e.g., radio
Twisted pair (TP)
§ two insulated copper wires
• Category 5: 100 Mbps, 1 Gbps Ethernet • Category 6: 10Gbps Ethernet
SELF STUDY NOT ON EXAM

Links: physical media
Coaxial cable:
§ two concentric copper conductors
§ bidirectional
§ broadband:
• multiple frequency channels on cable • 100’s Mbps per channel
Fiber optic cable:
§ glass fiber carrying light pulses, each pulse a bit
§ high-speed operation:
• high-speed point-to-point
transmission (10’s-100’s Gbps) § low error rate:
• repeaters spaced far apart
• immune to electromagnetic noise
SELF STUDY NOT ON EXAM

Links: physical media
Wireless radio
§signal carried in electromagnetic spectrum
§no physical “wire” §broadcast and “half-duplex”
(sender to receiver)
§propagation environment effects:
• reflection
• obstruction by objects • interference
Radio link types:
§terrestrial microwave • upto45Mbpschannels
§ Wireless LAN (WiFi) • Up to 100’s Mbps
§ wide-area (e.g., cellular) • 4G cellular: ~ 10’s Mbps
§ satellite
• up to 45 Mbps per channel • 270 msec end-end delay
• geosynchronous versus low- earth-orbit
SELF STUDY NOT ON EXAM

The network core
v mesh of interconnected routers
v packet-switching: hosts break application-layer messages into packets
§ forward packets from one router to the next, across links on path from source to destination
§ each packet transmitted at full link capacity
§ Is used in the Internet
v circuit-switching: an alternative used in legacy telephone networks which was considered during the design of the Internet
mobile network
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
datacenter network

Alternative to packet switching: circuit switching
end-end resources allocated to, reserved for “call” between source and destination
v in diagram, each link has four circuits.
§ call gets 2nd circuit in top link and 1st circuit in right link.
v dedicated resources: no sharing §circuit-like (guaranteed) performance
v circuit segment idle if not used by call (no sharing)
v commonly used in traditional telephone networks

Circuit switching: FDM and TDM
Frequency Division Multiplexing (FDM)
v optical, electromagnetic frequencies divided into (narrow) frequency bands
v each call allocated its own band, can transmit at max rate of that narrow band
Time Division Multiplexing (TDM)
§time divided into slots
§ each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band, but only during its time slot(s)
frequency frequency

Timing in Circuit Switching
Circuit Establish ment
Circuit Tear- down
Information

Why circuit switching is not feasible?
Ø Inefficient
• Computer communications tends to be very bursty. For
example, viewing a sequence of web pages
• Dedicated circuit cannot be used or shared in periods of silence
• Cannot adapt to network dynamics Ø Fixed data rate
• Computers communicate at very diverse rates. For example, viewing a video vs using telnet or web browsing
• Fixed data rate is not useful
Ø Connection state maintenance
• Requires per communication state to be maintained that is a considerable overhead
• Not scalable

Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”
1. Internet Address
2. Age (TTL)
3. Checksum to protect header
01000111100010101001110100011001

Packet Switching
v Data is sent as chunks of formatted bits (Packets)
v Packets consist of a “header” and “payload”
§ payload is the data being carried
§ header holds instructions to the network for how to handle packet (think of the header as an API)

Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”
v Switches “forward” packets based on their headers

Peek ahead: Two key network-core functions
llocall fforwardiing ttable
header value
0100 0101 0111 1001
output link
routing algorithm
§ global action: determine source- destination paths taken by packets
§ routing algorithms
Forwarding:
v local action: move arriving packets from router’s input link to appropriate router output link
destination address in arriving packet’s header

Timing in Packet Switching
paylo h d ad r
time What about the time to process the packet at the switch? • We’ll assume it’s relatively negligible (mostly true)

Timing in Packet Switching
paylo h d ad r
Could the switch start transmitting as soon as it has processed the header?

Timing in Packet Switching
paylo h d ad r
Yes! This would be called a “cut through” switch
Could the switch start transmit as soon as it has processed the header?

Timing in Packet Switching
paylo h d ad r
time We will always assume a switch processes/forwards a packet after it has received it entirely.
This is called “store and forward” switching

Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”
v Switches “forward” packets based on their headers

Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”
v Switches “forward” packets based on their headers
v Each packet travels independently
§ no notion of packets belonging to a “circuit”

Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”
v Switches “forward” packets based on their headers v Each packet travels independently
v No link resources are reserved. Instead, packet switching leverages statistical multiplexing

Three Flows with Bursty Traffic
Data Rate 1
Data Rate 2
Data Rate 3

When Each Flow Gets 1/3rd of Capacity
Data Rate 1
Data Rate 2
Data Rate 3
like circuit switching
Overloaded

When Flows Share Total Capacity
packet switching
No Overloading
Statistical multiplexing relies on the assumption that not all flows burst at the same time
Very similar to insurance, and has same failure case

Three Flows with Bursty Traffic
Data Rate 1
Data Rate 2
Data Rate 3

Three Flows with Bursty Traffic
Data Rate 1+2+3 >> Capacity
What do we do under overload?

Statistical multiplexing: pipe view
pkt tx time

Statistical multiplexing: pipe view

Statistical multiplexing: pipe view
No Overload

Statistical multiplexing: pipe view
Queue overload
into Buffer
Transient Overload
Not such a rare event

Statistical multiplexing: pipe view
Queue overload
into Buffer
Transient Overload
Buffer absorbs transient bursts Not a rare event!

Statistical multiplexing: pipe view
Queue overload
into Buffer
What about persistent overload?
Will eventually drop packets

Packet switching versus circuit switching
packet switching allows more users to use network!
§ 1 Mb/s link § each user:
• 100 kb/s when “active” • active 10% of time
v circuit-switching: § 10 users
vpacket switching:
§ with 35 users, probability > 10 active at same time is less than .0004
Q: how did we get value 0.0004? Q: what happens if > 35 users
1 Mbps link
Hint: Bernoulli Trials and Binomial Distribution

Binomial Probability Distribution
v A fixed number of observations (trials), n § E.g., 5 tosses of a coin
v Binary random variable
§ E.g., head or tail in a coin toss
§ Often called as success or failure
§ Probability of success is p and failure is (1-p)
v Constant probability for each observation

Binomial Distribution: Example
v Q: What is the probability of observing exactly 3 heads in a sequence of 5 coin tosses
§ One way to get exactly 3 heads is: HHHTT
§ Probability of this sequence occurring = (1/2) x (1/2) x (1/2) x (1-1/2) x (1-1/2) = (1/2)5
§ Another way to get exactly 3 heads is: THHHT
§ Probability of this sequence occurring = (1-1/2) x (1/2) x (1/2) x (1/2) x (1-1/2)
§ How many such unique combinations exist?

Binomial Distribution: Example
P (3 heads and 2 tails) = 10 x (1/2)5 = 0.3125

Binomial Distribution

Packet switching versus circuit switching
v Let’s revisit the earlier problem
v N = 35 users
v Prob (# a

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