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Computer Networks & The InternetComputer Networks & The Internet
Lecture 2Lecture 2
Imran AhmedImran AhmedUniversity of Management & TechnologyUniversity of Management & Technology
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Agenda
• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models
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A closer look at network structure:
• network edge: applications and hosts
• network core: – routers– network of networks
• access networks, physical media: communication links
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The network edge:
• end systems (hosts):– run application programs
– e.g. Web, email
– at “edge of network”
• client/server model– client host requests, receives service
from always-on server
– e.g. Web browser/server; email client/server
• peer-peer model:– minimal (or no) use of dedicated
servers
– e.g. Gnutella, KaZaA
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Network edge: connection-oriented service
• Goal: data transfer between end systems
• Handshaking: setup (prepare for) data transfer ahead of time– Hello, hello back human
protocol
– Set up “state” in two communicating hosts
• TCP – Transmission Control Protocol– Internet’s connection-
oriented service
• TCP service [RFC 793]
• Reliable, in-order byte-stream data transfer– Loss: acknowledgements
and retransmission
• Flow control:– Sender won’t overwhelm
receiver
• Congestion control:– Senders “slow down
sending rate”, when network congested
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Network edge: connectionless service
• Goal: data transfer between end systems– Same as before!
• UDP – user Datagram Protocol [RFC768]:– Connectionless
– Unreliable data transfer
– No flow control
– No congestion control
• App’s using TCP:– HTTP (Web), FTP (file
transfer), Telnet (remote login), SMTP (email)
• App’s using UDP:– Streaming media,
teleconferencing, DNS, Internet telephony
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Agenda
• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models
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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”
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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
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Circuit Switching
A B
Source Destination
It’s the method used by the telephone network. A call has three phases:
1. Establish circuit from end-to-end (“dialing”),2. Communicate,3. Close circuit (“tear down”).
Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire: each
call has its own private, guaranteed data rate from end-to-end.
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Circuit Switching Telephone Network
Source“Caller”
Central Office“C.O.”
Destination“Callee”
Central Office“C.O.”
TrunkExchange
Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000
calls.
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Packet Switching
A
R1
R2
R4
R3
B
Source Destination
It’s the method used by the Internet. Each packet is individually routed packet-by-packet,
using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at
the same time, therefore a packet switch has buffers.
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Packet SwitchingSimple router model
R1Link 1
Link 2
Link 3
Link 4
Link 1, ingress Link 1, egress
Link 2, ingress Link 2, egress
Link 3, ingress Link 3, egress
Link 4, ingress Link 4, egress
ChooseEgress
ChooseEgress
ChooseEgress
ChooseEgress
“4”
“4”
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Why does the Internet usepacket switching?
1. Efficient use of expensive links:– The links are assumed to be expensive and scarce. – Packet switching allows many, bursty flows to share the
same link efficiently.– “Circuit switching is rarely used for data networks, ...
because of very inefficient use of the links” - Gallager
2. Resilience to failure of links & routers:– ”For high reliability, ... [the Internet] was to be a datagram
subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum
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Some Definitions• Packet length, P, is the length of a packet in bits.• Link length, L, is the length of a link in meters.• Data rate, R, is the rate at which bits can be sent, in bits/second,
or b/s.1
• Propagation delay, PROP, is the time for one bit to travel along a link of length, L.
PROP = L/c.• Transmission time, TRANSP, is the time to transmit a packet of
length P. TRANSP = P/R.
• Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link:
Latency = PROP + TRANSP.
1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second.
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Packet Switching
Host A
Host B
R1
R2
R3
A
R1
R2
R4
R3
B
TRANSP1
TRANSP2
TRANSP3
TRANSP4
PROP1
PROP2
PROP3
PROP4
Source Destination
“Store-and-Forward” at each Router
( )i ii
TRANSP PROP Minimum end to end latency
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Agenda
• Network & its types• What’s the Internet?• What’s a protocol?• History• Network edge• Network core• Access net, physical media• Internet/ISP structure• Performance: loss, delay• Protocol layers, service models
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Access networks and physical media
Q: How to connect end systems to edge router?
• residential access nets• institutional access
networks (school, company)
• mobile access networks
Keep in mind: • bandwidth (bits per second) of
access network?
• shared or dedicated?
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Internet access technologies
• Previously, most people use 56K dial-up lines to access the Internet, but a number of new access technologies are now being offered.
• The main new access technologies are:– Digital Subscriber Line (DSl,ADSL)– Cable Modems– Local Area Networks– Wireless Networks
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Access networks: DSL & ADSL
• Digital Subscriber Line (DSL) is one of the most used technologies now being implemented to significantly increase the data rates over traditional telephone lines.
• Historically, voice telephone circuits have had only a limited capacity for data communications because they were constrained by the 4 kHz bandwidth voice channel.
• Most local loop telephone lines actually have a much higher bandwidth and can therefore carry data at much higher rates.
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Access networks: DSL & ADSL
• DSL services are relatively new and not all common carriers offer them.
• Two general categories of DSL services have emerged in the marketplace. – Symmetric DSL (SDSL) provides the same
transmission rates (up to 128 Kbps) in both directions on the circuits.
– Asymmetric DSL (ADSL) provides different data rates to (up to 640 Kbps) and from (up to 6.144 Mbps) the carrier’s end office. It also includes an analog channel for voice transmissions.
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DSL ArchitectureLocal Carrier End Office
Line Splitter
Customer Premises
Telephone
DSL Modem
Hub
Computer Computer
Local Loop
MainDistribution
Frame
CustomerPremises
CustomerPremises
VoiceTelephoneNetwork
DSL AccessMultiplexer
ATM Switch
ISP POP
ISP POP
ISP POP
ISP POP
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Access networks: Cable modems
• One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of 1.5-10 Mbps and a downstream rate of 2-30 Mbps.
• A few cable companies offer downstream services only, with upstream communications using regular telephone lines.
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Cable Modem Architecture
Cable Company Distribution Hub
Cable Splitter
Customer Premises
TV
Cable Modem
Hub
Computer Computer
SharedCoaxCable
System
Combiner
CustomerPremises
CustomerPremises
TV VideoNetwork
Cable ModemTermination
System
ISP POP
Cable CompanyFiber Node
Optical/ElectricalConverter
Downstream
Upstream
Router
Cable Company
Fiber Node
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Access networks: Local area networks
• Company/univ. local area network (LAN) connects end system to edge router
• Ethernet:– Shared or dedicated link, connects end systems
and router– 10 Mbs, 100mbs, Gigabit Ethernet etc.
• Details will be available in future
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Access networks: Wireless networks
• Shared wireless access network connects end system to router via base stations aka “access point”
• Wireless Lans:– 802.11b (WiFi): 11 Mbps
• Wide-area wireless access:– Provided by telecommunication companies
– WAP/GPRS etc.
• Satellite:– Up to 50 Mbps channels or multiple smaller channles
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Physical Media
• Bit: propagates between transmitter/rcvr pairs
• Physical link: What lies between transmitter & receiver
• Guided media:– Signals propagates in solid
media; copper, fiber, coax.
• Unguided media:– Signals propagates freely,
e.g., radio
Twisted Pair (TP)
• Two insulated copper wires:– Category 3: traditional
phone wires, 10 Mbps (Ethernet)
– Category 5: 100 Mbps (Ethernet)
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Physical Media
Coaxial cable:• Two concentric copper
conductors
• Bidirectional
• Baseband:– Single channel on cable
• Broadband:– Multiple channel 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., 5 Gps)
• Low error rate: repeaters spaced far apart; immune to electromagnetic noise