Review of Previous Lecture
• Course Administrative Trivia
• Internet Architecture
• Network Protocols
• Network Edge
• A taxonomy of communication networks
Some slides are in courtesy of J. Kurose and K. Ross
Overview
• Homework 1 out, due 1/18
• Project 1 ready to go on Tlab, should have
found partners
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched networks
• Protocol layers, service models
Access networks and physical media
Q: How to connection 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?
Residential access: point to point access
• Dialup via modem
– up to 56Kbps direct access to
router (often less)
– Can’t surf and phone at same
time: can’t be “always on”
• ADSL: asymmetric digital subscriber line
– up to 1 Mbps upstream (today typically < 256 kbps)
– up to 8 Mbps downstream (today typically < 1 Mbps)
– FDM: 50 kHz - 1 MHz for downstream
4 kHz - 50 kHz for upstream
0 kHz - 4 kHz for ordinary telephone
Residential access: cable modems
• HFC: hybrid fiber coax
– asymmetric: up to 30Mbps downstream, 2 Mbps
upstream
• network of cable and fiber attaches homes to ISP
router
– homes share access to router
• deployment: available via cable TV companies
Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
Cable Network Architecture: Overview
FDM:
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Channels
cable headend
cable distribution
network
home
Company access: local area networks
• company/univ local area
network (LAN) connects
end system to edge router
• Ethernet:
– shared or dedicated link
connects end system
and router
– 10 Mbs, 100Mbps,
Gigabit Ethernet
• deployment: institutions,
home LANs happening now
Wireless access networks
• shared wireless access network
connects end system to router
– via base station aka “access point”
• wireless LANs:
– 802.11b (WiFi): 11 Mbps
– 802.11a, 802.11g 54Mbps
router
base
station
• wider-area wireless access
– provided by telco operator
– 3G ~ 384 kbps
• Will it happen??
– WAP/GPRS in Europe
mobile
hosts
Home networks
Typical home network components:
• ADSL or cable modem
• router/firewall/NAT
• Ethernet
• wireless access point
to/from
cable
headend
cable
modem
router/
firewall
Ethernet
(switched)
wireless
laptops
wireless
access
point
Physical Media
• Bit: propagates between
transmitter/rcvr pairs
• Physical link: what lies
between transmitter &
receiver
• Guided media:
– signals propagate in solid media:
copper, fiber, coax
• Unguided media:
– signals propagate freely, e.g.,
radio
Twisted Pair (TP)
• two insulated copper
wires
– Category 3: traditional
phone wires, 10 Mbps
Ethernet
– Category 5 TP:
100Mbps Ethernet
Physical Media: coax, fiber
Coaxial cable:
Fiber optic cable:
• two concentric copper
• glass fiber carrying light
conductors
pulses, each pulse a bit
• bidirectional
• high-speed operation:
• baseband:
– single channel on cable
– legacy Ethernet
• broadband:
– multiple channels on
cable
– high-speed point-to-point
transmission (e.g., 10’s-100’s
Gps)
• low error rate: repeaters
spaced far apart ; immune to
electromagnetic noise
Overview
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched
networks
• Protocol layers, service models
Internet structure: network of networks
• roughly hierarchical
• at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable
and Wireless), national/international coverage
– treat each other as equals
Tier-1
providers
interconnect
(peer)
privately
Tier 1 ISP
Tier 1 ISP
NAP
Tier 1 ISP
Tier-1 providers
also interconnect
at public network
access points
(NAPs)
Tier-1 ISP: e.g., Sprint
Sprint US backbone network
Seattle
Tacoma
DS3 (45 Mbps)
OC3 (155 Mbps)
OC12 (622 Mbps)
OC48 (2.4 Gbps)
POP: point-of-presence
to/from backbone
Stockton
…
…
Kansas City
.
…
Anaheim
peering
…
…
San Jose
Cheyenne
New York
Pennsauken
Relay
Wash. DC
Chicago
Roachdale
Atlanta
to/from customers
Fort Worth
Orlando
Internet structure: network of networks
• “Tier-2” ISPs: smaller (often regional) ISPs
– Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
– E.g.: UUNet Europe, Singapore telecom
Tier-2 ISP pays
tier-1 ISP for
connectivity to
rest of Internet
tier-2 ISP is
customer of
tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
NAP
Tier 1 ISP
Tier-2 ISP
Tier-2 ISPs
also peer
privately with
each other,
interconnect
at NAP
Tier-2 ISP
Internet structure: network of networks
• “Tier-3” ISPs and local ISPs
– last hop (“access”) network (closest to end systems)
– Tier-3: Turkish Telecom, Minnesota Regional Network
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
local
local
ISP
Tier-2 ISP
ISP
ISP
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
local
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Internet structure: network of networks
• a packet passes through many networks!
local
ISP
Tier 3
local
local
ISP
Tier-2 ISP
ISP
ISP
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
local
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
Overview
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched
networks
• Protocol layers, service models
How do loss and delay occur?
packets queue in router buffers
• packet arrival rate to link exceeds output link capacity
• packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Four sources of packet delay
• 1. processing:
• 2. queueing
– check bit errors
– time waiting at output
link for transmission
– determine output link
– depends on congestion
level of router
transmission
A
propagation
B
processing
queueing
Delay in packet-switched networks
3. Transmission delay:
4. Propagation delay:
• R=link bandwidth (bps)
• d = length of physical link
• L=packet length (bits)
• s = propagation speed in
medium (~2x108 m/sec)
• time to send bits into
link = L/R
transmission
A
• propagation delay = d/s
Note: s and R are very
different quantities!
propagation
B
processing
queueing
Caravan analogy
100 km
ten-car
caravan
toll
booth
• Cars “propagate” at
100 km/hr
• Toll booth takes 12 sec to
service a car
(transmission time)
• car~bit; caravan ~ packet
• Q: How long until caravan
is lined up before 2nd toll
booth?
100 km
toll
booth
• Time to “push” entire
caravan through toll
booth onto highway =
12*10 = 120 sec
• Time for last car to
propagate from 1st to
2nd toll both:
100km/(100km/hr)= 1 hr
• A: 62 minutes
Caravan analogy (more)
100 km
ten-car
caravan
toll
booth
• Cars now “propagate” at
1000 km/hr
• Toll booth now takes 1
min to service a car
• Q: Will cars arrive to
2nd booth before all
cars serviced at 1st
booth?
100 km
toll
booth
• Yes! After 7 min, 1st car
at 2nd booth and 3 cars
still at 1st booth.
• 1st bit of packet can
arrive at 2nd router
before packet is fully
transmitted at 1st router!
– See Ethernet applet at AWL
Web site
Nodal delay:
Total delay at each node along the path
d nodal  d proc  d queue  d trans  d prop
• dproc = processing delay
– typically a few microsecs or less
• dqueue = queuing delay
– depends on congestion
• dtrans = transmission delay
– = L/R, significant for low-speed links
• dprop = propagation delay
– a few microsecs to hundreds of msecs
Queueing delay (revisited)
• R=link bandwidth (bps)
• L=packet length (bits)
• a=average packet arrival
rate
traffic intensity = La/R
• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large
• La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
“Real” Internet delays and routes
• What do “real” Internet delay & loss look like?
• Traceroute program: provides delay measurement
from source to router along end-end Internet path
towards destination. For all i:
– sends three packets that will reach router i on path towards
destination
– router i will return packets to sender
– sender times interval between transmission and reply.
3 probes
3 probes
3 probes
“Real” Internet delays and routes
traceroute: zappa.cs.nwu.edu to www.zju.edu.cn
Three delay measements from
Zappa.cs.cs.nwu.edu to 1890mpl-idf-vln-122.northwestern.edu
1 1890mpl-idf-vln-122.northwestern.edu (129.105.100.1) 0.287 ms 0.211 ms 0.193 ms
2 lev-mdf-6-vln-54.northwestern.edu (129.105.253.53) 0.431 ms 0.315 ms 0.321 ms
3 abbt-mdf-1-vln-902.northwestern.edu (129.105.253.222) 0.991 ms 0.950 ms 1.151 ms
4 abbt-mdf-4-ge-0-1-0.northwestern.edu (129.105.253.22) 1.659 ms 1.255 ms 1.520 ms
5 starlight-lsd6509.northwestern.edu (199.249.169.6) 1.713 ms 1.368 ms 1.278 ms
6 206.220.240.154 (206.220.240.154) 1.284 ms 1.204 ms 1.279 ms
trans-oceanic
7 206.220.240.105 (206.220.240.105) 2.892 ms 2.003 ms 2.808 ms
link
8 202.112.61.5 (202.112.61.5) 116.475 ms 196.663 ms 241.792 ms
9 sl-gw25-stk-1-2.sprintlink.net (144.223.71.221) 145.502 ms 150.033 ms 151.715 ms
10 sl-bb21-stk-8-1.sprintlink.net (144.232.4.225) 166.762 ms 177.180 ms 166.235 ms
11 sl-bb21-hk-2-0.sprintlink.net (144.232.20.28) 331.858 ms 340.613 ms 346.332 ms
12 sl-gw10-hk-14-0.sprintlink.net (203.222.38.38) 346.842 ms 356.915 ms 366.916 ms
13 sla-cent-3-0.sprintlink.net (203.222.39.158) 482.426 ms 495.908 ms 509.712 ms
14 202.112.61.193 (202.112.61.193) 515.548 ms 501.186 ms 509.868 ms
15 202.112.36.226 (202.112.36.226) 537.994 ms 561.658 ms 541.695 ms
16 shnj4.cernet.net (202.112.46.78) 451.750 ms 263.390 ms 342.306 ms
17 hzsh3.cernet.net (202.112.46.134) 349.855 ms 366.082 ms 380.849 ms
18 zjufw.zju.edu.cn (210.32.156.130) 350.693 ms 394.553 ms 366.636 ms
19 * * *
* means no reponse (probe lost, router not replying)
20
***
21 www.zju.edu.cn (210.32.0.9) 353.623 ms 397.532 ms 396.326 ms
Packet loss
• Queue (aka buffer) preceding link in buffer
has finite capacity
• When packet arrives to full queue, packet is
dropped (aka lost)
• Lost packet may be retransmitted by previous
node, by source end system, or not
retransmitted at all
Overview
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched
networks
• Protocol layers, service models
Protocol “Layers”
Networks are complex!
• many “pieces”:
– hosts
– routers
– links of various
media
– applications
– protocols
– hardware,
software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Why layering?
Dealing with complex systems:
• Explicit structure allows identification, relationship
of complex system’s pieces
– layered reference model for discussion
• Modularization eases maintenance, updating of
system
– change of implementation of layer’s service
transparent to rest of system
– e.g., change in gate procedure doesn’t affect rest
of system
• Layering considered harmful?
Internet protocol stack
• application: supporting network applications
– FTP, SMTP, HTTP
• transport: host-host data transfer
– TCP, UDP
• network: routing of datagrams from source
to destination
– IP, routing protocols
• link: data transfer between neighboring
network elements
– PPP, Ethernet
• physical: bits “on the wire”
application
transport
network
link
physical
Layering: logical communication
Each layer:
• distributed
• “entities”
implement
layer functions
at each node
• entities
perform
actions,
exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
Layering: logical communication
E.g.: transport
• take data from app
• add addressing,
reliability check
info to form
“datagram”
• send datagram to
peer
• wait for peer to
ack receipt
• analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
source
message
segment Ht
datagram Hn Ht
frame
Hl Hn Ht
M
M
M
M
Encapsulation
application
transport
network
link
physical
Hl Hn Ht
M
link
physical
Hl Hn Ht
M
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
Hl Hn Ht
M
M
router
Internet History
1961-1972: Early packet-switching principles
• 1961: Kleinrock - queueing
theory shows effectiveness
of packet-switching
• 1972:
– ARPAnet public demonstration
• 1964: Baran - packetswitching in military nets
– NCP (Network Control Protocol)
first host-host protocol
• 1967: ARPAnet conceived by
Advanced Research
Projects Agency
– first e-mail program
• 1969: first ARPAnet node
operational
– ARPAnet has 15 nodes
Internet History
1972-1980: Internetworking, new and proprietary nets
• 1970: ALOHAnet satellite
network in Hawaii
Cerf and Kahn’s internetworking
principles:
• 1974: Cerf and Kahn architecture for
interconnecting networks
– minimalism, autonomy - no
internal changes required to
interconnect networks
• 1976: Ethernet at Xerox
PARC
– best effort service model
• late70’s: proprietary
architectures: DECnet, SNA,
XNA
– decentralized control
• late 70’s: switching fixed
length packets (ATM
precursor)
• 1979: ARPAnet has 200 nodes
– stateless routers
define today’s Internet
architecture
Internet History
1980-1990: new protocols, a proliferation of networks
• 1983: deployment of
TCP/IP
• 1982: smtp e-mail
protocol defined
• 1983: DNS defined for
name-to-IP-address
translation
• 1985: ftp protocol
defined
• 1988: TCP congestion
control
• new national networks:
Csnet, BITnet, NSFnet,
Minitel
• 100,000 hosts
connected to
confederation of
networks
Internet History
1990, 2000’s: commercialization, the Web, new apps
• Early 1990’s: ARPAnet
decommissioned
• 1991: NSF lifts restrictions on
commercial use of NSFnet
(decommissioned, 1995)
• early 1990s: Web
Late 1990’s – 2000’s:
• more killer apps: instant
messaging, P2P file sharing
• network security to
forefront
• est. 50 million host, 100
– hypertext [Bush 1945, Nelson
million+ users
1960’s]
• backbone links running at
– HTML, HTTP: Berners-Lee
Gbps
– 1994: Mosaic, later Netscape
– late 1990’s:
commercialization of the Web
Summary
• Network access and physical media
• Internet structure and ISPs
• Delay & loss in packet-switched networks
• Protocol layers, service models
• More depth, detail to follow!
• Homework 1 out, due 1/18.
• Project 1 ready to go on Tlab, should have found
partners.
• Email your team info to the TA