Redes de Computadores - James F. Kurose, Keith w. Ross (2da Edición)
Introduction 1-1 Chapter 1 Part 2 Network Core These slides derived from Computer Networking: A Top...
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Transcript of Introduction 1-1 Chapter 1 Part 2 Network Core These slides derived from Computer Networking: A Top...
Introduction 1-1
Chapter 1Part 2Network Core
These slides derived from Computer Networking: A Top Down Approach ,6th edition. Jim Kurose, Keith RossAddison-Wesley, March 2012.
Comp 365Computer Networks
Fall 2014
Introduction 1-2
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 models1.6 Networks under attack: security1.7 History
Introduction 1-3
The Network Core
mesh of interconnected routers
the fundamental question: how is data transferred through net? circuit switching:
dedicated circuit per call: telephone net
packet-switching: data sent thru net in discrete “chunks”
Resources reserved
Resources reserved
Resources allocated on demand.
Resources allocated on demand.
Core Resources: buffers, link transmission rateCore Resources: buffers, link transmission rate
Introduction 1-4
The Network Core
The internet is packet switched
Telephone network is circuit switched
We need to understand circuit switching to know why it’s not used for the internet
Introduction 1-5
Network Core: Circuit Switching
End-end resources reserved for “call”
link bandwidth, switch capacity
dedicated resources: no sharing
circuit-like (guaranteed) performance
call setup required State maintained Data transferred at a
guaranteed rate
Introduction 1-6
Network Core: Circuit Switching
End-end resources reserved for “call”
Links between circuit switches
Each link can support n circuits
There can be n simultaneous connections.
Each circuit thus gets 1/n of the link’s bandwidth.
Introduction 1-7
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)
When two hosts want to communicate network establishes a dedicated end-to-end connection between the hosts.
dividing link bandwidth into “pieces” frequency division time division
Introduction
Alternative core: circuit switchingend-end resources allocated
to, reserved for “call” between source & dest:
In diagram, each link has four circuits. call gets 2nd circuit in top link
and 1st circuit in right link. dedicated resources: no sharing
circuit-like (guaranteed) performance
circuit segment idle if not used by call (no sharing)
Commonly used in traditional telephone networks
1-8
Introduction 1-9
Circuit Switching: FDM and TDM
FDM
frequency
time
TDM
frequency
time
4 users
Example:
FDM: frequency-division multiplexingFDM: frequency-division multiplexing
TDM: time-division multiplexingTDM: time-division multiplexing
Each connection gets one band of frequencies
Each connection gets one band of frequencies
Each connection gets one time slot
Each connection gets one time slot
Introduction 1-10
Circuit Switching: FDM and TDM
FDM
frequency
time
4 users
Example: telephone networks
FDM: frequency-division multiplexingFDM: frequency-division multiplexing
4kHz4kHz
4kHz4kHz
4kHz4kHz4kHz4kHz
4kHz = 4,000 hertz = 4,000 cycles per second4kHz = 4,000 hertz = 4,000 cycles per second
Radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz
Introduction 1-11
Circuit Switching: FDM and TDM
TDM
frequency
time
4 users
Example:
TDM: time-division multiplexingTDM: time-division multiplexing
One time slotOne time slot
One frameOne frame
Circuit gets same time slot in every frameCircuit gets same time slot in every frame
Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot.
Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate?
Transmission rate of a circuit: frame rate multiplied by the number of bits in a slot.
Example: 8,000 frames per sec, slot has 8 bits, then what is the transmission rate? 64 kbps
Introduction 1-12
Numerical example
How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links have transmission rate of 1.536
Mbps Each link uses TDM with 24 slots/sec, 24
slots/frame. 500 msec to establish end-to-end circuit
Let’s work it out!
Introduction 1-13
Numerical example
How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links have transmission rate of 1.536 Mbps Each link uses TDM with 24 slots/sec, 24 slots/frame. 500 msec to establish end-to-end circuit
Soln: determine how many slots we need. To do this must figure
out how many bits/slot:
(1.536 x 106 bits/sec) / (24 slots/sec) = 64,000 bits / slot Now can determine how many slots we need:
640,000 bits / (64,000 bits/slot) = 10 slots. So need 10 seconds + 500 msec = 10.5 seconds. This does not account for propagation delays.
Circuit switching: analysis
Disadvantages: Network resources are wasted during “silent”
times (when no one is talking but still connected)
Establishing end-to-end circuits and reserving end-to-end bandwidth is complicated and requires complex signaling software.
Advantage: Guaranteed bandwith and transmission time Necessary for some applications (streamed
music/video)
Introduction 1-14
Introduction 1-15
Network Core: Packet Switching
transmission: Each end-end data stream divided into packets Each packet travels through communication links Links connected by packet switches
Routers Or link-level switches.
Switches use store-and-forward transmission Switch receives entire packet before it transmits any of it again
Introduction 1-16
Network Core: Packet Switching
transmission:
Introduction 1-17
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
Bandwidth division into “pieces”
Dedicated allocationResource reservation
Introduction 1-18
Network Core: Packet Switching
Switching delays Assume packet has L bits Assume there are Q switches Assume a switch transmits at rate of R Takes each switch how long to transmit the
packet? L/R
Total delay is: QL/R
Introduction 1-19
Packet-switching: store-and-forward
This Figure: 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 RL
more on delay shortly …
Introduction 1-20
Network Core: Packet Switching
Switching delays: Each switch has multiple links Each link has buffer If packet arrives and another packet is already being
transmitted on that link, must wait in queue Called queuing delay. Varies depending on network congestion Packet loss: queue is full when packet arrives.
Introduction 1-21
Packet Switching: Statistical Multiplexing
Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing.
TDM: each host gets same slot in revolving TDM frame.
A
B
C100 Mb/sEthernet
1.5 Mb/s
D E
statistical multiplexing
queue of packetswaiting for output
link
On-demand sharing of resources is called statistical multiplexingOn-demand sharing of resources is called statistical multiplexing
Network Layer 4-22
Two key network-core functions
forwarding: move packets from router’s input to appropriate router output
routing: determines source-destination route taken by packets
routing algorithms
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
1
23
0111
dest address in arrivingpacket’s header
Introduction 1-23
Packet switching versus circuit switching
1 Mb/s link For each user
assume: 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
Q: how did we get value 0.0004?
A: see this pdf:https://147.129.181.14/~barr/classes/comp365/lectures/Prob10orMoreUsers.pdf
Introduction 1-24
Packet switching versus circuit switching
packet switching: with 35 users, probability > 10 active at same time is
less than .0004 With 10 or fewer simultaneously active users, aggregate
arrival rate of data is less than or equal to 1 Mbps. No loss of time Allows essentially same performance as circuit switching But allows > 3 times number of users
Packet switching allows more users to use network!
Introduction 1-25
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 (chapter 7)
Is packet switching a “slam dunk winner?”
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Most networks use packet-switching; even telephone networks are going to this!Most networks use packet-switching; even telephone networks are going to this!
Routing
How do packets make their way through packet-switched Networks?
Introduction 1-26
Internet structure: network of networks
End systems connect to Internet via access ISPs (Internet Service Providers) Residential, company and university ISPs
Access ISPs in turn must be interconnected. So that any two hosts can send packets to
each other Resulting network of networks is very complex
Evolution was driven by economics and national policies
Let’s take a stepwise approach to describe current Internet structure
Internet structure: network of networks
Question: given millions of access ISPs, how to connect them together?
accessnet
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Internet structure: network of networks
Option: connect each access ISP to every other access ISP?
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connecting each access ISP to each other directly doesn’t
scale: O(N2) connections.
Internet structure: network of networks
accessnet
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Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.
globalISP
Internet structure: network of networks
accessnet
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But if one global ISP is viable business, there will be competitors ….
ISP B
ISP A
ISP C
Internet structure: network of networks
accessnet
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But if one global ISP is viable business, there will be competitors …. which must be interconnected
ISP B
ISP A
ISP C
IXP
IXP
peering link
Internet exchange point
Internet structure: network of networks
accessnet
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… and regional networks may arise to connect access nets to ISPS
ISP B
ISP A
ISP C
IXP
IXP
regional net
Internet structure: network of networks
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… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users
ISP B
ISP A
ISP B
IXP
IXP
regional net
Content provider network
Introduction
Internet structure: network of networks
at center: small # of well-connected large networks “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national &
international coverage content provider network (e.g, Google): private network that connects
it data centers to Internet, often bypassing tier-1, regional ISPs 1-35
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
accessISP
Regional ISP Regional ISP
IXP IXP
Tier 1 ISP Tier 1 ISP Google
IXP
Introduction
Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
…
………
POP: point-of-presence
1-36