Networking Circuit Switching Packet Switching Aaron Stevens - - PDF document

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Networking Circuit Switching Packet Switching Aaron Stevens - - PDF document

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9/27/2012 CS101 Lecture 07: Networking Circuit Switching Packet Switching Aaron Stevens (azs@bu.edu) 27 September 2012 Computer Science What You ll Learn Today Computer Science What is a communications network? What are the

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Computer Science

CS101 Lecture 07: Networking

Circuit Switching Packet Switching

Aaron Stevens (azs@bu.edu)

27 September 2012

Computer Science

What You’ll Learn Today

  • What is a communications network?
  • What are the implications of the network topology

and transmission strategy?

  • How do computers connect to each other?
  • What are the building blocks of the Internet?
  • How is data transferred across a network?
  • How does my computer connect to the Internet?
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Computer Science

Communications Networks

  • What is a network?
  • What are its defining characteristics?

Computer Science

The Telephone Network

POTS (the plain old telephone system), a.k.a PSTN (Public Switched Telephone Network) To connect a phone call, the caller’s phone must be physically connected to connect to the receiver’s phone. Connecting these circuits (called switching) takes place at dedicated facilities called central offices.

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Computer Science

POTS Circuit Switching

Telephone wires leave your house, and connect to the central office. At the central office, connections are made to other telephone lines…

Image from www.exegesis.uklinux. net.

Computer Science

POTS Circuit Switching

Telephone operators used to actually switch wires to connect the calls. In the PSTN, this connection (called switching) is done electronically.

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POTS/PSTN Today

Modern telephone systems are hybrid. The audio is digitized at the exchange, then converted back to analog at the receiving exchange. A Nortel switch, which servers tens of thousands of customers.

(Image from Wikimedia Commons)

Computer Science

Modem

Early computer networking used the phone network: dial up. A modem converts computer signals into sounds.

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Computer Networking

Computer Network

A collection of computing devices connected in order to communicate and share resources. Connections between computing devices can be physical using wires or cables, or wireless using radio waves or infrared signals. Why connect computers together?

Computer Science

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Clients and Servers

Computer networks often operate in a client/server

  • model. Examples:
  • Browser / web server
  • File transfer client / FTP server
  • What about Peer-to-Peer?
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Networking Terms

Client A computer which uses (consumes) resources from the network. Server A computer that stores and manages files or applications for multiple users on a network. Example: Web Server A computer dedicated to responding to requests (from the browser client) for web pages.

Computer Science

12

Network Nodes

A network node is any device on a network:

  • Computer (desktop, laptop, PDA, etc.)
  • Server (web server, mail server, etc. )
  • Router (device which directs traffic)
  • Firewall (access control device)
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Network Topologies

Local-area Network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area. LAN Network Topology Describes the physical wiring plan for connections between nodes on a network. Also describes how messages are sent between nodes.

Computer Science

Ring Network Topology

A ring network connects all nodes in a closed loop on which messages travel in one direction.

  • Each node has two neighbors

How many hops? Complexity?

Image source: http://www.edrawsoft.com/Network-Topologies.php

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Computer Science

Bus Network Topology

On a bus network nodes are connected to a single communication line that carries messages in both directions. How many hops? Complexity?

Image source: http://www.edrawsoft.com/Network-Topologies.php

Computer Science

Star Network Topology

A star network centers around one node to which all others are connected and through which all messages are sent. How many hops? Complexity?

Image source: http://www.edrawsoft.com/Network-Topologies.php

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Ethernet

Where does the name come from?

CSMA/CD – Carrier Sense Multiple Access/Collision Detection

  • How do you have a conversation in a large group?

Originally used Bus topology (10Base-2, Coax)

  • Advantages/Disadvantages?

Moved to Star topology (10Base-T, 100Base-TX; UTP)

Computer Science

Network Interface Controller

Each node has a network interface controller (NIC) connected to its circuit board.

The CPU treats the NIC as an input/output device. It communicates by reading or writing bytes of data to the NIC. Each NIC has a unique Media Access Control (MAC) address, which distinguishes it from all other NICs.

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Computer Science

Types of Networks

Local-area Network (LAN) A network that connects a relatively small number of machines in a relatively close geographical area.

Wide-area network (WAN) A network that connects local-area networks over a potentially large geographic distance. Gateway/Router A particular computer on a LAN which directs all communication going between that LAN and other networks.

Computer Science

LAN/WAN Example

LANs separated by a great distance are connected by High speed communication links to create a WAN.

Image source: http://www.air-stream.org.au/files/wide_area_network.gif

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Computer Science

Circuit Switched Network

Computers can be connected over a circuit switched network (e.g. phone lines), creating a circuit between the source and destination.

A Switched Circuit connects devices A and B.

Image from www.tcpipguide.com.

Computer Science

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

 pieces allocated to calls  resource piece idle if not

used by owning call (no sharing)

 dividing link bandwidth

into “pieces”

  • frequency division
  • time division

Introduction 1-22

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Computer Science

Circuit Switching: FDM and TDM

FDM frequency time TDM frequency time 4 users Example:

Introduction 1-23 Computer Science

Circuit Switching: Details

  • How many circuits do you need to connect

1 pair of computers? 2 pairs? N pairs?

  • What happens when a backhoe cuts the

line?

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Computer Science

Bursty Data Transmissions

Computers send data in irregular bursts. Example: email. Using a switched network connection is wasteful – the circuits must be connected even if no data is being sent!

Computer Science

Packet Switching

Packet A unit of data sent across a network. Packet switching Messages are divided into fixed-sized, numbered packets. Packets are individually routed to their destination. Reassembled into messages. Router A network device that directs a packet between networks toward its final destination.

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Computer Science

Nodes send packets of data along routes to a destination, without a dedicated circuit. Packets (even in the same transmission) can take different routes.

Image from http://www.teach-ict.com/technology_explained/packet_switching/packet.switching.gif

Packet Switching Network

Image from www.tcpipguide.com.

Computer Science

Packet Switching: Details

  • What happens to packets when they

arrive?

  • Are they ready for consumption?
  • What happens if some packet(s) get lost?
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Computer Science

Network Core: Packet Switching

each end-end data stream divided into packets

 user A, B packets share

network resources

 each packet uses full link

bandwidth

 resources used as needed

resource contention:

 aggregate resource

demand can exceed amount available

 congestion: packets

queue, wait for link use

 store and forward:

packets move one hop at a time

  • node receives complete

packet before forwarding Bandwidth division into “pieces” Dedicated allocation Resource reservation

Introduction 1-29 Computer Science

Packet Switching: Statistical Multiplexing

 sequence of A & B packets has no fixed timing pattern

  • bandwidth shared on demand: statistical multiplexing.

 TDM: each host gets same slot in revolving TDM frame.

A B C

100 Mb/s Ethernet 1.5 Mb/s

D E

statistical multiplexing

queue of packets waiting for output link

Introduction 1-30

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Computer Science

Packet switching versus circuit switching

Example:

  • 1 Mb/s link
  • each user:
  • 100 kb/s when “active”
  • active 10% of time

circuit-switching:

  • 10 users

packet switching:

  • with 35 users, probability >

10 active at same time is less than .0004

Packet switching allows more users to use network!

N users 1 Mbps link

Introduction 1-31

Don’t mix up… Mb/s vs. MB/s Bits vs. Bytes

Computer Science

Packet switching versus circuit switching

 great for bursty data

  • resource sharing
  • simpler, no call setup

 excessive congestion: packet delay and loss

  • protocols needed for reliable data transfer,

congestion control

 Q: How to provide circuit-like behavior?

  • bandwidth guarantees needed for audio/video apps
  • still an unsolved problem (can’t do it!)

Is packet switching a “slam dunk winner?”

Introduction 1-32

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Circuit or Packet Switching?

  • For which kind of information is circuit switching

preferred?

  • For which kinds of information is packet switching

preferred?

  • Are all packets treated the same by the network?

Should some be prioritized?

  • What current events topic relates to this?

Computer Science

Packet Switching

What’s so great about packet switching?

  • Efficient use of wires/circuits
  • Multiple paths between source and destination
  • Slow growth of network infrastructure as

number of users increases

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Computer Science

What’s Not To Love?

  • Time required to reassembly

messages, resend missing packets.

  • Why does this matter?
  • Speed of delivery: latency is not

guaranteed.

  • Inefficient for small packets

Computer Science

Circuit or Packet Switching?

  • For which kind of information is circuit switching

preferred?

  • For which kinds of information is packet switching

preferred?

  • Are all packets treated the same by the network?

Should some be prioritized?

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Computer Science

What You Learned Today

  • Circuit Switching
  • Packet Switching
  • Network Topology
  • How Your Computer Connects to the

Internet

Computer Science

Announcements and To-Do List

  • Readings this week:
  • Reed ch 3, pp 44-50 (today)
  • Watch: history of Internet video
  • linked to class schedule page
  • Start at time 19:45, to end (about 40 minutes)
  • Reed ch 3, pp. 51-53 (Tuesday))
  • HW04 due Wednesday 10/3