1. CS 381 Introduction to computer networks
Lecture 2
1/29/2015

2. Introduction Overview: What is the Internet? What is a protocol?
Network edge –
Hosts
Access networks
Physical media
Network core –
Packet/circuit switching
Internet structure
Performance –
Loss
Delay
Throughput
Protocol layers
Service models
History
Introduction

3. What’s the Internet: “nuts and bolts” view
smartphone PC
server
wireless
laptop
millions of connected computing devices:
hosts = end systems
running network apps
mobile network
global ISP
regional ISP
home
network
institutional
communication links
fiber, copper, radio, satellite
transmission rate: bandwidth
wired
links
wireless
Packet switches: forward packets (chunks of data)
routers and switches
router
Introduction 3

4. Internet Appliances
Introduction

5. What’s a protocol? a human protocol and a computer network protocol:Hi
TCP connection
request Hi
TCP connection
response
Got the
time?
Get
2:00
<file>
time
Q: other human protocols?
Introduction

6. A closer look at network structure:
Network edge:
applications and hosts
Access networks
Connects end system to 1st router
physical media: wired, wireless communication links
Network core:
interconnected routers
network of networks
Introduction

7. The network edge: End systems (hosts): Reasons for this?
All Internet applications are implemented at the end systems.
HTTP, FTP, SSH, SCP, DNS, SMTP
Reasons for this?
Introduction

8. Access networks and physical media
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

9. 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
1-9 9

10. 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 – 1.2Gbps
Institutional (Outgoing): 100s Mbps – multiple Gbps
Institutional (Local): 10Mbps – 10Gbps
Mobile: Kbps - ~40Mbps
shared or dedicated
Introduction
1-10 10

11. Dial-up Modem Uses existing telephone infrastructure
network
Internet
home dial-up
modem
ISP modem
home PC
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.

12. Dial-up Modem Problems: Extremely slow with max speed of 56 kbps
telephone
network
Internet
home dial-up
modem
ISP modem
home PC
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

13. Digital Subscriber Line (DSL)
Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data
Internet
home
phone
telephone
network
splitter
home PC
DSL
modem
central
office
Also uses existing telephone infrastructure
Advantages over Dial-up:
Increased upload and download throughput
Can use computer and telephone at the same time

14. Digital Subscriber Line (DSL)
Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data
Internet
home
phone
telephone
network
splitter
home PC
DSL
modem
central
office
Telephone line carries both digital and telephone signals
Encoded at different frequencies.
Phone line at 0 - 4KHz
Upstream data at KHz (128 kbps - 1 mbps)
Downstream data at 50KHz - 1MHz (1 - 2 megabits per second)
New technologies emerging for DSL: up to 1Gbps (~2016)

15. to/from central office
Home Network
wireless
devices
to/from central office
often combined
in single box
wireless access
point (54 Mbps – 1.2 Gbps)
router, firewall, NAT
cable or DSL modem
wired Ethernet (100 Mbps – 1 Gbps)
Introduction

16. 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
1 Gbps
server
Ethernet
switch
Institutional
router
To Institution’s ISP

17. 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)
100 Mbps
1 Gbps
server
Ethernet
switch
Institutional
router
To Institution’s ISP

18. Wireless access networks
shared wireless access network connects end system to router
via base station aka “access point”
wide-area wireless access
provided by telco (cellular)
10’s km
between 1 and 10 Mbps
3G, 4G: LTE
wireless LANs:
within building (~100 ft)
802.11b/g/n/ac (WiFi)
to Internet
to Internet
Introduction

19. Host: sends packets of data
host sending function:
takes application message
breaks into smaller chunks, known as packets, of length L bits
transmits packet into access network at transmission rate R
link transmission rate
link capacity
link bandwidth
two packets,
L bits each 2 1
R: link transmission rate
host
L (bits)
R (bits/sec)
packet
transmission
delay
time needed to
transmit L-bit
packet into link = =

20. Physical media bit: propagates between transmitter/receiver pairs
physical link: what lies between transmitter & receiver
guided media: signals propagate in solid media: copper, fiber, coax
twisted pair (TP)
two insulated copper wires
Category 5: 100 Mbps, 1Gpbs Ethernet
Category 6: 10Gbps
Introduction

21. Physical media: coax, fiber
Coaxial cable:
Center copper conductor surrounded by insulation
bidirectional
broadband:
multiple channels on cable
Fiber optic cable:
glass fiber carrying light pulses, each pulse a bit
high-speed operation:
high-speed point-to-point transmission (e.g., 10s - 100s Gpbs transmission rate)
low error rate:
repeaters spaced far apart
immune to electromagnetic noise
Introduction

22. Physical media: radio Unguided media: Propagation environment effects:
signals propagate freely, e.g., radio
signal carried in electromagnetic spectrum
no physical “wire”
Bidirectional
Propagation environment effects:
reflection
obstruction by objects
Interference
radio link types:
Terrestrial microwave
Up to 45 Mbps channels
LAN (e.g., WiFi)
11Mbps – 1.3 Gbps
Wide-area (e.g., cellular)
3G/4G cellular: ~ few Mbps
Satellite
Kbps to 45Mbps channel (or multiple smaller channels)
270 msec end-end delay
geosynchronous versus low altitude
Introduction

23. Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge
end systems, access networks
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction

24. The Network Core Mesh of interconnected routers
The fundamental question:
how is data transferred through network?
Compare telephone network and Internet
Telephone network employs “circuit switching”
resources necessary to make call are reserved for duration of communication
Introduction

25. 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

26. 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

27. Network Core: Circuit Switching
Two techniques for dividing link bandwidth into “pieces”
frequency division multiplexing (FDM)
time division multiplexing (TDM)
Introduction
1-27 27

28. 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
1-28 28

29. 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
1-29 29

30. Circuit Switching: FDM and TDM
4 users
Example: Assume frequency domain divided into 4 circuits
FDM
frequency
time
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
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-30 30

31. Circuit Switching: FDM and TDM
4 users
Example: TDM
TDM
frequency
time
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
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction

32. Time Division Multiplexing
frequency
Frame
Frame
time
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)
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-32 32

33. Time Division Multiplexing
frequency
Frame
Frame
time
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 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
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-33 33

34. Frequency Division Multiplexing
4 users
frequency
time
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
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-34 34

35. Frequency Division Multiplexing
4 users
frequency
time
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 bit file?
¼ * 6000 bps = 1500 bps
~9 seconds, excluding setup time
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction
1-35 35

36. 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 Mbps
Frame rate is 1 second
Each link uses TDM with 24 slots/sec
500 msec to establish end-to-end circuit
Introduction

37. 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 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 msec to establish end-to-end connection = 10.5 seconds.
Introduction
1-37 37

38. 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

39. 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).
Introduction

40. Packet Switched Networks
Distributed applications communicate by sending messages to each other.
Can contain any kind of data: video, audio, jpeg, mp3, , …
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