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John Magee 8 July 2013

CS101 Lecture 5:

Networking:

Topology, Packet Switching

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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 (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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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.

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POTS 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)

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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?

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Clients and Servers

Computer networks 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.

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OSI Model

OSI Layers Examples

Open Systems Interconnection

Layered network protocol design

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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)

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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.

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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.

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.

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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)

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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

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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.

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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Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

Introduction 1-23

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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?

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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!

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.

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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.

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 1-28

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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 1-29

Packet-switching: store-and-forward

takes L/R seconds to transmit (push out) packet of L bits on to link at R bps

store and forward: entire packet must arrive at router before it can be transmitted on next link

delay = 3L/R (assuming zero propagation delay)

Example:

L = 7.5 Mbits

R = 1.5 Mbps

transmission delay = 15 sec

R R R

L

more on delay shortly …

Introduction 1-30

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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

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

Is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

Introduction 1-32

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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?

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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.

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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?

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Take-Away Points

– Client-server model

– LAN/WAN Topologies

– Circuit Switching

– Packet switching