Computer Systems Introduction to Networks
Dr. Mian M. Hamayun
m.m.hamayun@bham.ac.uk
Lecture Objective
The objective of this lecture is to develop a basic understanding of Computer Networks (in general) and the Internet.
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Lecture Outline
What is the Internet?
How does the Internet look like?
What is a Distributed System?
Internet vs. Intranet (& Firewall)
Network Principles
Switching Schemes
Internet Structure: Network of Networks Summary
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What is the Internet? (Nuts and Bolts)
PC server
wireless laptop
Millions (billions?) of connected computing devices:
mobile network
smartphone
hosts = end systems running network apps
home network
institutional network
global ISP
regional ISP
router
links
wireless Communication links
wired links
fiber, copper, radio, satellite transmission rate: bandwidth
Packet switches: forward packets (chunks of data)
routers and switches
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“Fun” Internet Appliances
IP picture frame http://www.ceiva.com/
Web-enabled toaster + weather forecaster
Tweet-a-watt: monitor energy use
Internet refrigerator
Internet phones
Slingbox: watch,
control cable TV remotely
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What is the Internet? (Nuts and Bolts)
Internet: “network of networks” Interconnected ISPs
Protocols control sending, receiving of msgs
e.g., TCP, IP, HTTP, Skype, 802.11
Internet standards
RFC: Request for comments
IETF: Internet Engineering Task Force
mobile network
home network
institutional network
global ISP
regional ISP
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What is the Internet? (A Service View)
Infrastructure that provides
services to applications:
Web, VoIP, email, games, e- commerce, social nets, …
Provides programming
interface to apps
hooks that allow sending and receiving app programs to “connect” to Internet
provides service options, analogous to postal service
mobile network
home network
institutional network
global ISP
regional ISP
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What is the Internet? (Access Network & Physical Media)
Q: How to connect end systems
to edge router?
residential access nets
institutional access networks (school, company)
mobile access networks keep in mind:
bandwidth (bits per second) of access network?
shared or dedicated?
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What is the Internet? (A Closer Look)
network edge:
hosts: clients and servers
servers, often in data centers
access networks, physical media: wired, wireless
communication links
network core:
interconnected routers network of networks
mobile network
home network
institutional network
global ISP
regional ISP
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Access Net: Home Network
wireless devices
often combined in single box
to/from headend or central office
cable or DSL modem
router, firewall, NAT wired Ethernet (100 Mbps)
wireless access point (54 Mbps)
2015
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Access Net: Enterprise (Ethernet)
Ethernet switch
institutional link to ISP (Internet)
institutional router
institutional mail, web servers
typically used in companies, universities, etc
10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates today, end systems typically connect into Ethernet switch
2015
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Access Net: Wireless Networks
shared wireless access network connects end system to router via base station aka “access point”
Wireless LANs:
within building (100 ft)
Wide-area wireless access
provided by telco (cellular) operator, 10’s km
between 1 and 100 Mbps
802.11b/g (WiFi): 11, 54 Mbps transmission rate
3G, 4G, 5G (Comparison)
to Internet
to Internet
2015
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Host: Sends Packets of Data
Host sending function:
takes application message
breaks into smaller chunks, known as packets, of length L bits
transmitspacketintoaccess network at transmission rate R
link transmission rate, aka link capacity, aka link bandwidth
two packets, L bits each
2 1 host
R: link transmission rate
packet transmission
2015 delay
= time needed to = transmit L-bit
packet into link
L (bits)
R (bits/sec)
1-13
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The Network Core
mesh of interconnected routers
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
2015
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The Network Core: Key Functions
routing: determines source-destination route taken by packets
routing algorithms routing algorithm
local forwarding table header value output link 0100 3
0101 2 0111 2 1001 1
forwarding: move packets from router’s input to appropriate router output
3
dest address in arriving packet’s header
1 2
0111
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Barrett Lyon / The Opte Project Visualization of the Routing Paths of the Internet.
What the Internet Actually Looks Like?
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2003
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2009
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2015
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Korea Telecom (Zoomed)
More at: http://time.com/3952373/internet-opte-project/
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Distributed Systems
“A distributed system is one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages”
–Coulouris et al. (2005)
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What does this allow for?
On a network of computers concurrency is the norm.
There is no single global notion of the correct time.
Each component in a network may fail independently.
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A Typical Portion of the Internet
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The Internet – A Distributed Systems
The Internet is a huge distributed system that enables users all over the world to make use of its services.
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A Typical Intranet
Instructor’s Guide for Coulouris, Dollimore and Kindberg
Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005
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The Intranet
An intranet is a part of the Internet that is separately administered and uses a firewall to enforce its own local security policies. Users in an intranet share data by means of file services.
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What is a Firewall?
A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.
A firewall typically establishes a barrier between a trusted internal network and untrusted external network, such as the Internet
Firewalls are generally categorized as network-based or host-based
Types include Network layer or packet filters and Application- layer firewalls
More details:
https://www.cisco.com/c/en/us/products/security/firewalls/w hat-is-a-firewall.html
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What is a Firewall?
Intranet
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Example Applications – Cloud Computing
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Network Principles
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Networking is all about sending messages
over a carrier
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Key Terms
Latency = any kind of delay that happens in data communication over a network
Bandwidth = transmission capacity of a computer network or telecommunication system
Speed = rate at which something is able to move http://www.speedtest.net
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Packet Transmission
Logical unit of information transmitted via network Otherwise known as a message
Message = data item of arbitrary length
Message subdivided into packets before transmission
Sequence of 01100010 01101001 01101110 01100001 01110010 01111001 00100000 01100100 01100001 01110100 01100001 of restricted length
Plus addressing information
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Switching Schemes
Network consists of a set of nodes, connected by circuits
To transmit information between two nodes, a switching scheme is required
Broadcast – no switching, everything transmitted to everyone Circuit Switching – plain old telephone system. Connected by
switch at the exchange
Packet Switching – store-and-forward network. Computer at each switching node. Like postal system
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Circuit Switching
End-to-End resources allocated to, reserved for “call” between source & destination:
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
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Packet Switching
packet switching allows more users to use network!
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Packet Switching: Store and Forward
L bits
per packet
321
source
destination
takes L/R seconds to transmit (push out) L-bit packet into link at R bps
store and forward: entire packet must arrive at router before it can be transmitted on next link
end-end delay = 2L/R (assuming zero propagation delay)
In general: dend-to-end = NL/R (with N links and N-1 routers)
R = 1.5 Mbps
one-hop transmission delay = 5 sec
R bps
R bps
one-hop numerical
example:
L = 7.5 Mbits
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Packet Switching: Queueing Delay / Loss
A
B
R = 100 Mb/s
C
R = 1.5 Mb/s queue of packets
waiting for output link
D
E
Queuing and packet loss:
If arrival rate (in bits) to link exceeds transmission rate of link for a period of time:
packets will queue, wait to be transmitted on link
packets can be dropped (lost) if memory (buffer) fills up
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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 step-wise approach to describe current Internet structure
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Internet Structure: Network of Networks
Question: given millions of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
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…
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Internet Structure: Network of Networks
Question: given millions of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
connecting each access ISP to each other directly doesn’t scale: O(N2) connections.
access net
access net
access net
access net
access net
access net
access net
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Internet Structure: Network of Networks
Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.
…
access net
access net
access net
…
access net
access net
access net
access net
access net
access net
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global ISP
access net
access net
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access net
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access net
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Internet Structure: Network of Networks
But if one global ISP is viable business, there will be competitors …
…
…
access net
access net
access net
access net
access net
access net
access net
ISP A
access net
access net
…
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ISP C
access net
access access net
access net
…
net
ISP B
access net
access net
access net
…
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Internet Structure: Network of Networks
But if one global ISP is viable business, there will be competitors … which must be connected
…
…
access net
access net
access net
access net
access net
access net
ISP A
access net
IXP
access net
…
…
ISP C
access net
access net
access net
access net
Internet exchange point
IXP
ISP B
peering link
access access net
access net
access net
…
net
…
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Internet Structure: Network of Networks
… and regional networks may arise to connect access nets to ISPs
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…
access net
access net
access net
access net
access net
access net
IXP
access net
access net
ISP A
access net
access net
…
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ISP C
ISP B
regional net
access access net
access net
access net
IXP
access net
access net
…
net
…
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Internet Structure: Network of Networks
… and content provider networks (e.g., Google, Microsoft) may run their own network, to bring services, content
close to end users
access net
ISP A
access net
access net
access net
access net
IXP
access net
access net
access net
access net
ISP B
access net
Content provider network
IXP
ISP B
regional net
access
access net net
access net
access net
access net
access net
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…
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Internet Structure: Network of Networks
Tier 1 ISP Tier 1 ISP
Google
IXP IXP IXP
Regional ISP Regional ISP
access access access access access access access access ISP ISP ISP ISP ISP ISP ISP ISP
At center: small # of well-connected large networks
“tier-1” commercial ISPs (e.g., Sprint, AT&T, NTT), national & international coverage
content provider network (e.g, Google): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs
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Summary
In this lecture:
What is the Internet and how can we visualize it.
What is a Distributed System and how the Internet is also considered a distributed system
We have discussed the difference between Internet and Intranets; We have also looked at the role of Firewalls.
We have discussed the Packet and Circuit Switching schemes and pros / cons of each.
We concluded this lecture by having a look at the Internet Structure, which is a very large Network of Networks.
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References / Links
Chapter #1: Computer Networks and the Internet, Computer Networking: A Top-Down Approach (7th
edition) by Kurose & Ross What is a Firewall:
https://www.cisco.com/c/en/us/products/security/firewalls/w hat-is-a-firewall.html
4G vs. 5G Compared: https://www.tomsguide.com/features/5g-vs-4g
What is an IXP
https://www.cloudflare.com/learning/cdn/glossary/internet-exch ange-point-ixp/
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