CS 381Introduction to

computer networksLecture 2

1/29/2015

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching, network structure

1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-2

A closer look at network structure:

• network edge: • applications and hosts

Introduction 1-3

• access networks• Connects end system to

1st router• physical media: wired,

wireless communication links

• network core: • interconnected routers• network of networks

Introduction 1-4

The network edge:

• End systems (hosts): • All Internet applications are implemented at the end systems.

• HTTP, FTP, SSH, SCP, DNS, SMTP

• Reasons for this?

Introduction 1-5

Access networks and physical media

• Access network: • Links connecting an end system to the first router (edge router) on

the path to the Internet core. • Edge router connects end system to Internet.

• Question: • How to connect end systems to edge router?• In other words, how can you connect your smartphone or laptop to

the first router on campus?

Introduction 1-6

Access networks and physical media

• Question: • How to connect end systems to edge router?

• Most common ways:• residential access networks

• Cable modems, DSL, Dial-Up modem• NAT router with Wi-Fi, Ethernet

• institutional access networks (school, company)• mobile access networks

Introduction 1-7

Access networks and physical media

• Two important characteristics of access networks

• bandwidth (bits per second) of access network• Residential (Outgoing): 2Mbps – 50Mbps (and higher)• Residential (Local): 11Mbps – 1.2Gpbs• Institutional (Outgoing): 100s Mbps – multiple Gbps• Institutional (Local): 54Mbps – 10Gpbs• Mobile: Kbps - ~40Mbps

• shared or dedicated

telephonenetwork Internet

homedial-upmodem

ISPmodem

homePC

central office

Uses existing telephone infrastructure Computer software makes phone connection to ISP

• Handshake: determines link speed, IP address

Home modem converts digital output to analog and sends it across phone line.

• Modem: Modulate/Demodulate

The ISP modem converts from analog back to digital and pushes data to edge router.

Dial-up Modem

telephonenetwork Internet

homedial-upmodem

ISPmodem(e.g., AOL)

homePC

central office

• Problems:• Extremely slow with max speed of 56 kbps

• ~42.5 hours to download 1GB worth of data• ~4KHz bandwidth compared to 500MHz using CAT6a cable

• Have to choose: Computer or telephone. • Circuit switched, non-shared access to ISP

Dial-up Modem

telephonenetwork

DSLmodem

homePC

homephone

Internet

DSLAM

Existing phone line:0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data

splitter

centraloffice

Digital Subscriber Line (DSL)

Also uses existing telephone infrastructure DSL modem uses telephone wire to communicate with the Digital Subscriber Line Access

Multiplexer (DSLAM) located in telco central office. Advantages over Dial-up:

• Increased upload and download throughput• Can use computer and telephone at the same time

telephonenetwork

DSLmodem

homePC

homephone

Internet

DSLAM

Existing phone line:0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data

splitter

centraloffice

Digital Subscriber Line (DSL)

• Telephone line carries both digital and telephone signals• Encoded at different frequencies.

• Phone line at 0 - 4KHz• Upstream data at 4 - 50KHz (128 kbps - 1 mbps)• Downstream data at 50KHz - 1MHz (1 - 2 megabits per second)

New technologies emerging for DSL: up to 1Gbps (~2016)

100 Mbps

100 Mbps

100 Mbps1 Gbps

server

Ethernetswitch

Institutionalrouter

To Institution’sISP

Ethernet Internet access

• Typically used in companies, universities, etc• 10 Mbps, 100Mbps, 1Gbps, 10Gbps Ethernet• Multiple switches per building

• Serves rooms with Ethernet ports and Wi-Fi access points

• Fiber connection between switches

100 Mbps

100 Mbps

100 Mbps1 Gbps

server

Ethernetswitch

Institutionalrouter

To Institution’sISP

Ethernet Internet access

• Few routers on campus• Why?

• Campus network can be thought of as a large LAN (Local Area Network)• Similar to your network at home, but with thousands of end systems• Greater complexity, but basic topology is exactly the same• Large number of switches allow local communication (layer 2 routing)• Only communication off campus requires the use of routers (layer 3 routing)

Introduction 1-14

Chapter 1: roadmap

1.1 What is the Internet?1.2 Network edge

end systems, access networks, links

1.3 Network core circuit switching, packet switching, network structure

1.4 Delay, loss and throughput in packet-switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

Introduction 1-15

The Network Core

• Mesh of interconnected routers

• The fundamental question: • how is data transferred through net?

• Compare telephone network and Internet• Telephone network employs “circuit switching” • resources necessary to make call are reserved for duration of

communication

Introduction 1-16

Network Core: Circuit Switching

• End-end resources reserved for “call”• link bandwidth, switch capacity

• Finite capacity, “all circuits are busy”

• dedicated resources: no sharing• circuit-like (guaranteed) performance

• Always true?

• call setup required• Call request time: time to obtain dial tone• Selection time: user dialing numbers, transmitting tones of different frequency• Post selection time: time needed to process dialed numbers until connection to destination device• Some differences in traditional telephone service and cellular telephone service

Introduction 1-17

Network Core: Circuit Switching

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

• Pieces allocated to calls for duration of call• When you are not talking, no one else can utilize your piece of the network

• How can bandwidth of a link be divided into pieces?

Introduction 1-18

Network Core: Circuit Switching

• Two techniques for dividing link bandwidth into “pieces”• frequency division multiplexing (FDM)• time division multiplexing (TDM)

Introduction 1-19

Network Core: Circuit Switching

• Frequency division multiplexing the frequency spectrum is divided among the connections across the link

• Recall that with DSL telephone link is divided into three frequency “bands”

• Telephone use

• Data upload

• Data download

• Link dedicates a frequency band for each connection for duration of communication

• The width of the frequency band allocated to a particular connection is called ?????

• Bandwidth!

Introduction 1-20

Network Core: Circuit Switching

• Time division multiplexing

• Time is divided into frames of fixed duration• Example: 4 users, each user has access to the link for ¼ time per frame

• Each frame is divided into slots of fixed duration• User has full bandwidth access to the link when active

• Each connection gets one time slot per frame• User is idle for N-1/N time, where• N = number of connections per frame

Introduction 1-21

Circuit Switching: FDM and TDM

FDM

frequency

time

4 users

Example: Assume frequency domain divided into 4 circuits

• Example: Total bandwidth is 40Mhz• Each user is allocated ¼ of the total bandwidth, 10Mhz each• Resources are dedicated for the duration of the connection

• DSL works this way• Instead of multiple users:

• 3 channels – telephone, data upload, data download

Introduction 1-22

Circuit Switching: FDM and TDM

TDM

frequency

time

4 users

Example: TDM

Frame Frame

• Example: Total bandwidth is 40Mhz• Each user is allocated all of the total bandwidth, 40Mhz for ¼ of the time of a frame• Resources are dedicated for the duration of the connection

• Bluetooth works this way• Instead of multiple users:

• 40 channels• Data divided into packets, each packet transmitted on one of the 40 channels

Introduction 1-23

Time Division Multiplexing

frequency

timeFrame Frame

• Assume transmission rate of link is 4000 bits per second

Frame = 1 second, link is divided among four communications (I.e., link is supporting 4 circuits: 0, 1, 2, 3).

Each circuit gets a 1/4 second timeslot per second.

During timeslot gets full transmission rate:

1/4 second * 4000 bps = 1000 bps.

Transmission rate for each circuit is 1000 bps

How long to transmit a 5000 bit file?

5 seconds (Note: The example does not consider setup time)

Introduction 1-24

Time Division Multiplexing

frequency

timeFrame Frame

• Assume transmission rate of link is 6000 bits per second

• Another Example:

Frame = 2 seconds, 4 circuits

Each circuit gets ½ second timeslot per frame

How long does it take to transmit a 13000 bit file?

rate link: 6kbps, 12kbps throughput per frame

½ second * 6000 bps = 3kbps

3kb transmitted per frame

5 frames needed to transmit 13kb.

total time: ~9 seconds, excluding setup time

Introduction 1-25

Frequency Division Multiplexing

frequency

time

4 users

• Assume bandwidth of the link is 4000 bps and each communication (circuit) receives equal bandwidth.

• Each circuit gets ¼ of the 4000 bps throughput for the duration of the communication.

• How long for a given circuit to transmit a 5000 bit file?

¼ * 4000 bps = 1000 bps

5 seconds, excluding setup time

Introduction 1-26

Frequency Division Multiplexing

frequency

time

4 users

• Assume bandwidth of the link is 6000 bps and each communication (circuit) receives equal bandwidth.

• Another Example:

• Each circuit gets ¼ of the 6000 bps throughput for the duration of the communication.

• How long for a given circuit to transmit a 13000 bit file?

¼ * 6000 bps = 1500 bps

~9 seconds, excluding setup time

Introduction 1-27

One more example

• How long does it take one connection to send a file of 640,000 bits from host A to host B over a circuit-switched network?

Assuming:

• All links are 1.536 Mbps• Frame rate is 1 second• Each link uses TDM with 24 slots/sec• 500 msec to establish end-to-end circuit

Introduction 1-28

Numerical example

• How long does it take one connection to send a file of 640,000 bits from host A to host B over a circuit-switched network?

• Capacity of link is 1.536 Mbps

• With 24 slots, each connection gets bandwidth of 1.536/24 = 64Kbps

• So each connection has bandwidth of 64Kbps

• (640,000)/64 = 10 seconds to transmit file + 500 msec to establish end-to-end connection = 10.5 seconds.

Introduction 1-29

Network Core: Packet Switching

• Internet is a packet switching rather than circuit switching network.

• Reservations not accepted • No reserving of communication links, • no guarantee of given bandwidth • In fact, No guarantees at all!

• How can we demonstrate this?• Ping command

Introduction 1-30

Network Core: Packet Switching

• Internet is a best-effort network: • It will allocate whatever resources are available at the time they are requested.• Hopefully all data will make it from sender to receiver:

• Might take a very long time• Might not arrive in the same order it was sent• Might not arrive at all• The application is not informed if any of these problems happen (or don’t).

Packet Switched Networks

• Distributed applications communicate by sending messages to each other.• Can contain any kind of data: video, audio, jpeg, mp3, email, …

• Sender divides long messages into smaller chunks called packets.• Each layer of the OSI model will attach a header with information to the packet• Packet generation happens on client devices. Network core components do little to change packet header

information.

• Packets get shuttled between packet switches (routers, link-layer switches). • Network Layer protocols: communication between source and destination client devices• Link Layer Protocols: single hop communication between clients, switches, and routers

• Packet switches have input links and output links• Routing vs. forwarding• Store-and-forward