Transcript Access networks

CS 381
Introduction to computer
networks
Lecture 2
1/29/2015
• Overview:
Introduction
• 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
1-2
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
1-3
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
1-4
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?
1-5
Introduction
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
1-6
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
1-7
Dial-up Modem
central
office
telephone
network
home
PC
 Uses
home
dial-up
modem
Internet
ISP
modem
existing telephone infrastructure
 Computer software makes phone connection
• Handshake: determines link speed, IP address
to ISP
 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
central
office
telephone
network
home
PC
home
dial-up
modem
Internet
ISP
modem
• 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
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
 DSL modem uses telephone wire to communicate with Teleco office
 Advantages over Dial-up:
• Increased upload and download throughput
• Can use computer and telephone at the same time
Digital Subscriber Line (DSL)
Existing phone line:
0-4KHz phone; 4-50KHz upstream data;
50KHz-1MHz downstream data
Internet
home
phone
DSLAM
telephone
network
splitter
DSL
modem
home
PC
central
office
• 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)
Ethernet Internet access
100 Mbps
Institutional
router
Ethernet
switch
To Institution’s
ISP
100 Mbps
1 Gbps
100 Mbps
server
• 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
Ethernet Internet access
100 Mbps
Institutional
router
Ethernet
switch
To Institution’s
ISP
100 Mbps
1 Gbps
100 Mbps
server
• 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)
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 networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction
1-14
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
1-15
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
1-16
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
1-17
Network Core: Circuit Switching
• Two techniques for dividing link bandwidth into “pieces”
• frequency division multiplexing (FDM)
• time division multiplexing (TDM)
Introduction
1-18
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
1-19
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
1-20
Circuit Switching: FDM and TDM
Example: Assume frequency domain divided into 4 circuits
FDM
4 users
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
Introduction
1-21
Circuit Switching: FDM and TDM
Example: TDM
4 users
TDM
frequency
Frame
Frame
time
• 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
1-22
Time Division Multiplexing
frequency
time
Frame
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
1-23
Time Division Multiplexing
frequency
time
Frame
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
1-24
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
Introduction
1-25
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 13000 bit file?
¼ * 6000 bps = 1500 bps
~9 seconds, excluding setup time
Introduction
1-26
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
1-27
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
1-28
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
1-29
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
1-30
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