Transcript Introduction to Networking - FSU Computer Science Department

Introduction to Networking
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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
– pc’s, workstations, servers
– PDA’s, phones, toasters
running network apps
• communication links
router
server
local ISP
– Point-to-point, multiaccess
– fiber, copper, radio, satellite
workstation
mobile
regional ISP
• routers: forward packets
(chunks) of data thru network
• internet: network of networks
• Internet is a specific internet
company
network
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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
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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
NAP
International
lines
regional
network
national
network
ISP
company
on-line
services
ISP
university
company
access via
modem
LANs
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NAPs, NSPs, ISPs
• NSP: National Service Provider (Tier 1 Backbones)
– Example: Verizon, At&T
• NAP: National Access Point
National Provider
NAP
NAP
NAP
NAP
National Provider
Regional
Provider
customers
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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
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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 > 10 active is < 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
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Requests “connection” to receiver
Waits for network to form connection
Leaves connection in place while sending data
Terminates connection when no longer needed
• Network
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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
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