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CS101 Lecture 6: Networking: Topology, Packet Switching Aaron Stevens 28 January 2009 1 Overview/Questions What is a communications network? For example, the phone network Why connect computers together? How do computers

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Aaron Stevens

28 January 2009

CS101 Lecture 6: Networking: Topology, Packet Switching

Overview/Questions

– What is a communications network?

For example, the phone network

– Why connect computers together? – How do computers connect to each other? – What are the typical models for creating computer networks? – How is data transferred across a network?

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The Telephone Network

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

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.

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POTS Circuit Switching

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

Computer Networking

Computer Network

A collection of computing devices connected in

rder 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?

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Clients and Servers

Computer networks operate in a client/server

model. Examples:

– Browser / web server – File transfer client / FTP server

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.

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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)

Modem

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

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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.

Network Topologies

Local-area Network (LAN) A network that connects a relatively small number

f machines in a relatively close geographical area.

Network Topology Describes the physical wiring plan for connections between nodes on a network. Also describes how messages are sent between nodes.

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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

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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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

Ethernet The industry standard bus technology for local-area networks.

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.

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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

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.

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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?

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!

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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, then reassembled into messages. Router A network device that directs a packet between networks toward its final destination.

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.

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Packet Switching: Details

– What happens to packets when they arrive? Are they ready for consumption? – What happens if some packet(s) get lost?

Advantages of Packet Switching

– Efficient use of wires/circuits – Multiple paths between source and destination – Slow growth of network infrastructure as number of customers increases

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Disadvantages of Packet Switching

– Time required to reassembly messages, resend missing packets.

Why does this matter?

– Speed of delivery: latency is not guaranteed. – Inefficient for small packets.

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?

SLIDE 14

1

Aaron Stevens

28 January 2009

CS101 Lecture 6: Networking: Topology, Packet Switching

Overview/Questions

– What is a communications network?

– Why connect computers together? – How do computers connect to each other? – What are the typical models for creating computer networks? – How is data transferred across a network?

2

The Telephone Network

POTS Circuit Switching

3

POTS Circuit Switching

Computer Networking

Computer Network

A collection of computing devices connected in

Connections between computing devices can be physical using wires or cables, or wireless using radio waves or infrared signals. Why connect computers together?

4

Clients and Servers

Computer networks operate in a client/server

– Browser / web server – File transfer client / FTP server

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.

5

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)

Modem

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

6

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.

Network Topologies

Local-area Network (LAN) A network that connects a relatively small number

Network Topology Describes the physical wiring plan for connections between nodes on a network. Also describes how messages are sent between nodes.

7

Ring Network Topology

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

How many hops? Complexity?

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?

8

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?

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.

9

WAN Example

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

Circuit Switched Network

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

10

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?

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!

11

Packet Switching

Packet A unit of data sent across a network. Packet switching Messages are divided into fixed-sized, numbered

destination, then reassembled into messages. Router A network device that directs a packet between networks toward its final destination.

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

Packet Switching Network

12

Packet Switching: Details

– What happens to packets when they arrive? Are they ready for consumption? – What happens if some packet(s) get lost?

Advantages of Packet Switching

– Efficient use of wires/circuits – Multiple paths between source and destination – Slow growth of network infrastructure as number of customers increases

13

Disadvantages of Packet Switching

– Time required to reassembly messages, resend missing packets.

– Speed of delivery: latency is not guaranteed. – Inefficient for small packets.

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?

14

Take-Away Points

– Client-server model – LAN/WAN Topologies – Circuit Switching – Packet switching

Student To Dos

– Readings this week:

– Lab 03 on Thursday 1/29 is on HTML – Quiz 1 is on Friday 1/30