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# Computer Network
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# Computer Network
### Definition:
A computer network is a group of devices that are connected to one another in order to exchange information or share resources.
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## The Internet: "nuts and bolts" view
- Billions of connected computing devices:
</img>
- Hosts = end systems
- Running network apps
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## The Internet: "nuts and bolts" view
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## The Internet: "nuts and bolts" view
- Internet: “network of networks”
- Interconnected ISPs
- Protocols control sending, receiving of messages
- e.g., TCP, IP, HTTP, Skype, 802.11
- Internet standards
- RFC: Request For Comments
- IETF: Internet Engineering Task Force
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# What’s a protocol?
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# What’s a protocol?
Protocols define the format and order of messages sent and received among network entities, and actions taken on message transmission and receipt.
- specific messages sent
- specific actions taken when messages received, or upon other events
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# What’s a protocol?
### Human protocols:
- “what’s the time?”
- “I have a question”
- Introductions...
### Network protocols:
- Machines rather than humans
- All communication activity in Internet governed by protocols
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# Protocol Examples
- a human protocol and a computer network protocol:
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# Access network:
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# Access network:
## digital subscriber line (DSL)
</img>
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# Access network:
## digital subscriber line (DSL)
- 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
- Asymmetric access: downstream and upstream rates are different
- < 2.5 Mbps upstream transmission rate (typically < 1 Mbps)
- < 24 Mbps downstream transmission rate (typically < 10 Mbps)
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# Access network:
## cable network
</img>
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# Access network:
## cable network
- HFC: hybrid fiber coax
- asymmetric: up to 42.8 Mbps downstream transmission rate, 30.7 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
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# Enterprise access networks:
## Ethernet
</img>
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# Enterprise access networks:
## Ethernet
- Widely used in companies, universities, etc.
- 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates
- Today, end systems typically connected by Ethernet switches
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# Access network:
## home network
</img>
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# Wireless access networks
- Shared wireless access network connects end system to router
- via base station, aka “access point”
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# Wireless access networks
</img>
- Wireless LANs:
- Within building (~100 ft.)
- 802.11 (WiFi)
- 54~1300 Mbps transmission rate
</img>
- Wide-area wireless access
- Provided by telco (mobile) operator, 10’s of km
- Between 1 and 10 Mbps
- 3G, 4G, LTE (“Long Term Evolution”), 5G
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# Internet of Things (IoT)
</img>
Link
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# Physical Media
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# 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
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# Physical Media
### Types of physical media:
- Twisted pair (TP)
- Copper wire
- Coaxial cable
- Fibre optic
- Terrestrial radio spectrum
- Satellite radio spectrum
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# Physical Media
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## Twisted pair:
- Two insulated copper wires
- Category 5: 10 Mbps, 1 Gbps Ethernet
- Category 6: 10 Gbps
</img>
## Coaxial cable:
- Two concentric copper conductors
- Can achieve high data transmission rates
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# Physical Media
</src>
### Fiber optic cable:
- Glass fibre carrying light pulses representing bits
- High-speed operation:
- High-speed point-to-point transmission (e.g., 10’s-100’s Gbps transmission rate)
- Low error rate:
- Repeaters spaced far apart
- Immune to electromagnetic noise
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# Physical Media
### Radio
- Signal carried in electromagnetic spectrum
- No physical “wire”
- Carry a signal for long distances
- Propagation environment effects:
- Reflection
- Obstruction by objects
- Interference
Classified into 3 groups:
- Very short distance (e.g. Bluetooth)
- 5~10 metres
- LAN (e.g., WiFi)
- 10 to a few hundred meters
- Wide-area (e.g., cellular/mobile)
- Tens of miles
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# Physical Media
### Satellite Radio Channels
</img>
- Two types of satellites for communications:
- geostationary (~36000 km above earth, stationary)
- used where DSL or cable-based access is unavailable
- low-earth orbiting (closer to earth, move over the surface)
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# Network security
### Network security
- How bad actors can attack computer networks
- How to defend networks against attacks
- How to design architectures resistant to attacks
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# Network security
### Internet originally designed with little security
- Original vision: “a group of mutually trusting users attached to a transparent network”
- Internet protocol designers playing “catch-up”
- Security considerations in all layers!
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# Network security
# Packet sniffing:
- Broadcast media (shared Ethernet, wireless)
- “Promiscuous” network interface reads/records all packets (e.g., including passwords!) passing by
</src>
- Wireshark software used in the labs is an open source packet-sniffer
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# Summary
- An overview of Networks and network components
- An overview of protocols
- Different types of physical media for data transmission in networks
- Network security
## Reference:
1. Computer Networking: A Top-Down Approach [Chapter 1]