202401111021
Status: #idea
Tags: CN

Introduction to Networks

• Information - Data that is useful
• Media can't transfer all information at once. So, a computer encodes it in binary
• On-off keying: You turn on/off a certain property of the channel, in order to encode information
• Protocol dictates how to behave, based on information received.
• Packets
  • Collection of bits/bytes
  • Chunks of information
• Headers
  • Information added by a protocol onto a packet
  • Where to go, timestamp, etc.
• Network core
  • Managed by ISPs
  • Part of the network in between the network edge
• Interface
  • Physical port through which information can be sent/received

What is the Internet?

Engineering view

• Network of smaller networks
  • Interconnected ISPs

Components

1. Lots of connected devices
   • Hosts/end systems
   • Able to communicate with other devices
   • Running network applications
2. Communication links
   • Fiber, copper, radio, satellite
   • Their transmission rate: bandwidth
3. Packet switches
   • Forward packets
   • Switches, routers, etc.
4. Protocols
   1. Control sending an receiving of messages
   2. Eg: TCP, IP, HTTP, 802.11

Service view

• Infrastructure that provides services to applications
  • Eg: Web, VoIP, Email, Games

Protocol

• A set of rules we always follow
• They define the format, order of messages Tx/Rx among network entities and actions taken on message Tx/Rx

Cerf and Kahn's inter-networking principles

• Minimalism and autonomy
  • No internal changes required to interconnect networks
• Best effort service model
• Stateless routers
• Decentralised control

A Closer Look at Network Edge

1. Network edge
   • Hosts: Clients and servers
   • Servers often in data centres
2. Access networks, physical media
   • Wired/wireless communication links
3. Network core
   • Interconnected routers
   • Network of networks

Access Networks

• Residential access nets
• Institutional access networks (school, company)
• Mobile access networks
  • Wireless media
  • Usually, we use last hop to decide (but the area is grey)

Digital Subscriber Line (DSL)

• TODO: ADD IMAGE FROM 1-30
• Uses existing telephone lines
• Goes to central office DSL access multiplexer (DSL-AM)
  • Data over DSL phone line goes to internet
  • Voice over DSL goes to telephone net
• Bandwidth
  • $<2.5\text{Mbps}$ (typically $<1\text{ Mbps}$) upstream
  • $<24\text{Mbps}$ (typically $<10\text{ Mbps}$) downstream

Cable Network

• Internet over TV's coaxial line
• Everyone can hear the data on the line
• Uses frequency division multiplexing (FDM) similar to radio
• Hybrid fiber coax (HFC)
  • Asymmetric: 30Mbps down, 2Mbps up
• Homes share access to cable headend

Physical Media

Guided media

• Signals propagate in solid media
• Twisted pair
  • Needs 2 wires to send/recieve data
  • Twisted to redudce interference
• Coaxial cable
  • 2 concentric copper conductors
  • Birdirectionall
  • Broadband: Multiple channels on cable
• Fiber Optic Cable
  • Each pulse is a bit
  • 10-100Gbps
  • Low error rate

Unguided media

• Signals propagate freely
• Radio
  • Signal carried in EM spectrum
  • Bidirectional (but only 1 person can talk at a time)
  • Propagation environment effects:
    • Reflection-> Same wave arrives at different times, i.e. multipath
    • Obstruction by objects
    • Interference-> More packets get dropped
  • Types:
    • Terrestrial microwave
    • LAN (Eg. WiFi)
    • Wide area (Eg. cellular)
    • Satellite
      • ~270ms delay
      • Geosynchronous vs low altitude

Network Core

• Mesh of interconnected routers
• Does not run applications

Packet Switching vs Circuit Switching

• Packet switching allows more users to use the network
• Packet switching can lead to excessive congestion

Structure of the Internet

• ISPs connect to each other via peering links
• Sometimes, ISPs connect to an IXP
• TODO: Image from 1-56

Loss & Delay in Networks

• Packets queue in router buffers
  • Packet arrival rate $>$ output link capacity
  • Beyond the buffer's capacity, packets are dropped by the router
• $$d_{nodal}=d_{proc}+d_{queue}+d_{trans}+d_{prop}$$
  • $d_{proc}$
    • Nodal processing
    • Check bit errors
    • Determine the output link
    • Typically $<1ms$
  • $d_{queue}$
    • Time waiting at output link
    • Depends on congestion level at router
  • $d_{trans}=L/R$
    • Transmission delay
    • $L$ -> Packet length; $R$ -> Link bandwidth
  • $d_{prop}=d/s$
    • $d$ ->Length of physical link
    • $s$ -> Propagation speed in medium

Throughput

• Rate at which bits are transferred between sender and receiver
• Usually, the throughput of the thinnest pipe is used for the overall throughput

Layering

• Helps to deal with complex systems.
• Different protocols can communicate using layering
• Helps in modularisation
• Each layer adds a level to the message/payload
• TODO: Slidee 1-74 (OSI model layers)

Netowrk Securitty

• Internet was not designed with (much) security in mind
• Denial of Service (DoS)
  • Make resources unavailable to legitimate users by spamming the resource with bogus traffic
• Packet Sniffing
  • Promiscuous network interface records all packets
• IP spoofing
  • Send a packet with a false source address

References

1. Chapter 2 in Tanenbaum for Physical layer