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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?
1-3
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)
ADSL2+ 24Mbps
1-4
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
1-5
Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
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Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend
cable distribution
network (simplified)
home
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Cable Network Architecture: Overview
server(s)
cable headend
cable distribution
network
home
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Cable Network Architecture: Overview
cable headend
cable distribution
network (simplified)
home
1-9
Cable Network Architecture: Overview
FDM:
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Channels
cable headend
cable distribution
network
home
1-10
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
1-11
Wireless access networks
• shared wireless access network
connects end system to router
▫ via base station aka “access point”
• wireless LANs:
▫ 802.11b/g (WiFi): 11 or 54 Mbps
• wider-area wireless access
router
base
station
▫ provided by telco operator
▫ 3G ~ 384 kbps
 Will it happen??
▫ GPRS
▫ WiMAX
mobile
hosts
1-12
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
wireless
laptops
wireless
access
point
1-13
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:
100Mbps Ethernet
1-14
Physical Media: coax, fiber
Coaxial cable:
• two concentric copper
conductors
• bidirectional
• baseband:
▫ single channel on cable
▫ legacy Ethernet
• broadband:
▫ multiple channels on cable
▫ HFC
Fiber optic cable:
 glass fiber carrying light pulses,
each pulse a bit
 high-speed operation:

high-speed point-to-point
transmission (e.g., 10’s100’s Gps)
 low error rate: repeaters spaced
far apart ; immune to
electromagnetic noise
1-15
Physical media: radio
• signal carried in
electromagnetic spectrum
• no physical “wire”
• bidirectional
• propagation environment
effects:
▫ reflection
▫ obstruction by objects
▫ interference
Radio link types:
 terrestrial microwave

e.g. up to 45 Mbps channels
 LAN (e.g., Wifi)

11Mbps, 54 Mbps
 wide-area (e.g., cellular)

e.g. 3G: hundreds of kbps
 satellite



up to 45Mbps channel (or
multiple smaller channels)
270 msec end-end delay
geosynchronous versus low
altitude
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Teleports
• A telecommunications port—or, more commonly, teleport—is
a satellite ground station with multiple parabolic antennas
(i.e., an antenna farm) that functions as a hub connecting a
satellite or geocentric orbital network with a terrestrial
telecommunications network.
• Teleports may provide various broadcasting services among
other telecommunications functions, such as uploading
computer programs or issuing commands over an uplink to a
satellite.
17
Teleports
Balambu Earth Station: Nepal Telecom=>Teleport
18
Satellites
• Geostationary Satellites
• Medium-Earth Orbit Satellites
• Low-Earth Orbit Satellites
19
Geostationary Satellites
• At altitude approx. 36000Km above equatorial plane, satellite
rotation period is 24hrs.
• Satellite is stationary with respect to Earth.
• With current technology, it is unwise to have geostationary
satellites spaced much closer than 2 degrees in the 360degree equatorial plane, to avoid interference.
• With a spacing of 2 degrees, there can only be 360/2 = 180 of
these satellites in the sky at once.
• However, each transponder can use multiple frequencies and
polarizations to increase the available bandwidth.
20
Medium-Earth Orbit Satellites
• At much lower altitudes, we find the MEO (Medium-Earth
Orbit) satellites.
• As viewed from the earth, these drift slowly in longitude,
taking something like 6 hours to circle the earth. Accordingly,
they must be tracked as they move through the sky. Because
they are lower than the GEOs.
• GPS (Global Positioning System) satellites orbiting at about
18,000 km are examples of MEO satellites.
21
Low-Earth Orbit Satellites
• Moving down in altitude, we come to the LEO (Low-Earth
Orbit) satellites.
• Due to their rapid motion, large numbers of them are needed
for a complete system.
• On the other hand, because the satellites are so close to the
earth, the ground stations do not need much power, and the
round-trip delay is only a few milliseconds.
22
Satellite
23
Terrestrial Radio Links
• Radio channels carry signals in the electromagnetic
spectrum.
• They are an attractive media because require no
physical "wire“ to be installed, can penetrate walls,
provide connectivity to a mobile user, and can
potentially carry a signal for long distances.
• The characteristics of a radio channel depend
significantly on the propagation environment and
the distance over which a signal is to be carried.
24
Terrestrial Radio Links
• those that operate as local area networks (typically spanning 10's to a few
hundred meters) e.g. WLAN and
• wide-area radio channels that are used for mobile data services (typically
operating within a metropolitan region) e.g. WAP, 3G etc
25
Satellite frequency band
•
•
•
•
L-bank (1-2GHz)
C-Bank (4-8GHz)
KU-Band (12-18GHz)
KA-Band (26.5-40GHz)
26
Satellite frequency band
• L-bank (1-2GHz)
▫ Being a relatively low frequency, L-band is easier to process,
requiring less sophisticated and less expensive RF equipment.
▫ L-Band is also used for low earth orbit satellites, military satellites,
and terrestrial wireless connections like GSM mobile phones. It is
also used as an intermediate frequency for satellite TV where the
Ku or Ka band signals are down-converted to L-Band at the antenna
• C-Bank (4-8GHz)
▫ Satellite C-band usually transmits around 6 GHz and receives
around 4 GHz. It uses large (2.4- 3.7 meter) antennas
▫ C-band is typically used by large ships that traverse the oceans on a
regular basis and require uninterrupted, dedicated, always on
connectivity as they move from region to region.
▫ C-band is also used for terrestrial microwave links
27
Satellite frequency band
• KU-Band (12-18GHz)
▫ Ku-Band is most commonly used for satellite TV and is used for
most VSAT systems.
▫ There is much more bandwidth available in Ku -Band and it is therefore
less expensive that C or L-band.
▫ Ku band coverage is generally by regional spot beams, covering major
land areas with TV reception
▫ VSAT Antenna sizes typically range from the standard 1 meter, to 1.5
meters & as low as 60cm for spread spectrum operation
• KA-Band (26.5-40GHz)
▫ is an extremely high frequency requiring great pointing accuracy and
sophisticated RF equipment.
▫ It is commonly used for high definition satellite TV. It is also used today
for terrestrial VSAT services
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Internet service provider
• An Internet service provider (ISP) is an organization that provides access
to the Internet. Access ISPs directly connect clients to the Internet using
copper wires, wireless or fiber-optic connections.
• Hosting ISPs are a kind of colocation center that leases server space to
smaller businesses and other people. Transit ISPs provide large amounts
of bandwidth for connecting hosting ISPs to access ISPs
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ISPs
• Tier1
• Tier2 and
• Tier3 ISPS
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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)
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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
San Jose
Cheyenne
peering
…
….
Kansas City
New York
Pennsauken
Relay
Wash. DC
Chicago
Roachdale
…
…
…
Anaheim
Atlanta
to/from customers
Fort Worth
Orlando
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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
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
1-34
Internet structure: network of networks
• “Tier-3” ISPs and local ISPs
▫ last hop (“access”) network (closest to end systems)
local
ISP
Local and tier3 ISPs are
customers of
higher tier
ISPs
connecting
them to rest
of Internet
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
1-35
Internet structure: network of networks
• a packet passes through many networks!
local
ISP
Tier 3
ISP
Tier-2 ISP
local
ISP
local
ISP
local
ISP
Tier-2 ISP
Tier 1 ISP
Tier 1 ISP
Tier-2 ISP
local
local
ISP
ISP
NAP
Tier 1 ISP
Tier-2 ISP
local
ISP
Tier-2 ISP
local
ISP
36
Delays
• As a packet travels from one node to the subsequent node
along the path, the packet suffers from several different
types of delays at each node along the path.
• The most important of these delays are the
▫ Nodal processing delay /Processing Delay
▫ Queuing delay
▫ Transmission delay and
▫ Propagation delay
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Delays
• Nodal processing delay /Processing Delay
▫ The time required to examine the packet's header and determine where
to direct the packet is part of the processing delay.
▫ The processing delay can also include other factors, such as the time
needed to check for bit-level errors in the packet that occurred in
transmitting the packet's bits from the upstream router to router
• Queuing delay
▫ After this nodal processing, the router directs the packet to the queue that
precedes the link to router B.
▫ At the queue, the packet experiences a queuing delay as it waits to be
transmitted onto the link.
▫ The queuing delay of a specific packet will depend on the number of other,
earlier-arriving packets that are queued and waiting for transmission
across the link; the delay of a given packet can vary significantly from
packet to packet.
▫ If the queue is empty and no other packet is currently being transmitted,
then our packet's queuing delay is zero
38
Delays
• Transmission Delay
▫ The amount of time required to transmit all of the packet's bits into the
link.
• Propagation Delay
▫ The time required to send packet from one router to another router. The
bit propagates at the propagation speed of the link.
▫ The propagation speed depends on the physical medium of the link and
the distance from source to destination.
• Total Delay
▫ Total Delay dtotal = dproc + dqueue + dtrans + dprop
• If there is N node and all the delays are same then the total delay is
▫ Total Delay dtotal = N (dproc + dqueue + dtrans + dprop)