Introduction to Networking

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Introduction to Networking

• What is a computer network?
• Internet Architecture
• Statistical multiplexing and packet switching
• Connection-oriented vs Connectionless
• Fundamental issues in computer networking
• Readings
• Sections 1.1-1.5, 2.5.4, 2.5.5

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What is a Computer Network?

• Network provides connectivity
• A set of computers/switches connected by
  communication links
• Many topologies possible

• Many different physical media
• Coaxial cable, twisted pair, fiber optic, radio,
  satellite
• Local area networks vs Wide area networks

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Elements of a Network

• hosts, end-systems
• pcs, workstations, servers
• PDAs, phones, toasters
• running network apps
• communication links
• Point-to-point, multiaccess
• fiber, copper, radio, satellite
• routers forward packets (chunks) of data thru
  network
• internet network of networks
• Internet is a specific internet

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What is The Internet?

• The Internet
• collection of networks and routers that span the
  world and use the TCP/IP protocols to form a
  single, cooperative virtual network
• intranet
• connection of different LANs within an
  organization
• private
• may use leased lines
• usually small, but possibly hundreds of routers
• may be connected to the Internet (or not), often
  by firewall

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Internet Architecture
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NAPs, NSPs, ISPs

• NSP National Service Provider (Tier 1 Backbones)
• Example Verizon, AtT
• NAP National Access Point

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NAP and Private Peering
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Sprint network
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Another Interesting figure about Internet found
from the Internet
http//www.cs.fsu.edu/zzhang/Internet_map.pdf
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Fundamental Issues in Networking

• Naming/Addressing
• How to find name/address of the party (or
  parties) you would like to communicate with
• Address byte-string that identifies a node
• Routing/Forwarding process of determining how to
  send packets towards the destination based on its
  address
• Finding out neighbors, building routing tables
• Resource sharing
• Fundamentally, all nodes use a shared
  infrastructure to send/receive information. If
  all nodes becomes aggressive, everybody will be
  hurt.

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Multiplexing Strategies

• Sharing of network resources among multiple users

• Common multiplexing strategies
• Time Division Multiplexing (TDM)
• Frequency Division Multiplexing (FDM)
• These two strategies are circuit switching
  technology

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Circuit Switched Networks

• All resources (e.g. communication links) needed
  by a call dedicated to that call for its duration
• Example telephone network

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Statistical Multiplexing

• Time division, but on demand rather than fixed
• Reschedule link on a per-packet basis
• Packets from different sources interleaved on the
  link
• Buffer packets that are contending for the link
• Buffer buildup is called congestion
• This is packet switching, used in computer
  networks

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Packet Switched Networks

• Data entering network is divided into chunks
  called packets
• Store-and-forward approach packets buffered
  before transmission
• Packets traversing network share resources with
  other packets
• On demand resource use statistical resource
  sharing
• Fewer resources queuing delay, packet loss

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Why Statistically Share Resources

• Efficient utilization of the network
• Example scenario
• Link bandwidth 1 Mbps
• Each call requires 100 Kbps when transmitting
• Each call has data to send only 10 of time
• Circuit switching
• Each call gets 100 Kbps supports 10 simultaneous
  calls
• Packet switching
• Supports many more calls with small probability
  of contention
• 35 ongoing calls probability that gt 10 active is
  lt 0.00044!

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Circuit Switching vs Packet Switching
Item Circuit-switched Packet-switched
Dedicated copper path Yes No
Bandwidth available Fixed Dynamic
Potentially wasted bandwidth Yes No
Store-and-forward transmission No Yes
Each packet follows the same route Yes No
Call setup Required Not Needed
When can congestion occur At setup time On every packet
Effect of congestion Call blocking Queuing delay
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Connection-Oriented Service

• Sender
• Requests connection to receiver
• Waits for network to form connection
• Leaves connection in place while sending data
• Terminates connection when no longer needed
• Network
• Receives connection request
• Establishes connection and informs sender
• Transfers data across connection
• Removes connection when sender requests

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Connectionless Service

• Sender
• Forms packet to be sent
• Places address of intended recipient in packet
• Transfers packet to network for delivery
• Network
• Uses destination address to forward packet
• Delivers the packet to destination

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Connection-Oriented vs Connectionless

• Connection-Oriented
• Telephone System, Virtual Circuit Model
• Path is setup before data is sent
• Data identifies the connection
• All data follows the same path
• Connectionless
• Postal System, Datagram Model
• No path setup before transmitting data
• Packet contains identification of destination
• Each packet handled independently

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Connection-Oriented vs Connectionles

• Connection-Oriented
• Connection setup overhead
• State in packet switches
• Can reserve bandwidth
• Connectionless
• Stateless and less overhead
• Resource reservation not possible
• Allows broadcast/multicast

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Fundamental Problems in Networking

• What can go wrong?
• Bit-level errors due to electrical interferences
• Packet-level errors packet loss due to buffer
  overflow/congestion
• Out of order delivery packets may takes
  different paths
• Link/node failures cable is cut or system crash
• What can be done?
• Add redundancy to detect and correct erroneous
  packets
• Acknowledge received packets and retransmit lost
  packets
• Assign sequence numbers and reorder packets at
  the receiver
• Sense link/node failures and route around failed
  links/nodes
• Goal to fill the gap between what applications
  expect and what underlying technology provides

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Summary

• The pieces of a network
• Internet architecture
• Packet switching vs circuit switching
• Statistical multiplexing
• Connection-oriented vs connectionless
• Fundamental issues in networking
• Addressing/Naming and routing/forwarding
• Error/Flow/Congestion control