Computer Science, FSU 1

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

Computer Science, FSU 3

Elements of a Network• hosts, end-systems

– pc’s, workstations, servers

– PDA’s, 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

local ISP

companynetwork

regional ISP

router workstationserver

mobile

Computer Science, FSU 4

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

LANs

International lines

ISP ISPcompany university

national network

regionalnetwork

NAP

on-line services

companyaccess via

modem

Computer Science, FSU 6

NAPs, NSPs, ISPs• NSP: National Service Provider (Tier 1 Backbones)

– Example: Verizon, At&T

• NAP: National Access Point

NAPNAPNAPNAP

NAPNAP

NAPNAP

National ProviderNational Provider

National ProviderNational Provider Regional Regional ProviderProvider

customerscustomers

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NAP and Private Peering

Sprint network

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Another Interesting figure about Internet found from the

Internet

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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

Computer Science, FSU 14

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

Computer Science, FSU 15

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!

Computer Science, FSU 16

Circuit Switching vs Packet SwitchingItem 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

Computer Science, FSU 17

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