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INFO 331
Computer Networking
Technology II
Chapter 6
Wireless Networking
Dr. Jennifer Booker
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Wireless & Mobile Networks
• The number of mobile devices has grown
immensely in the last few years
– 34 million cell phones worldwide as of 1993
– 2 billion [ITU] as of 2005, many Internet-aware
– 5.5 billion by 2011
• Distinguish between wireless connectivity
and the mobility that affords
– Some wireless devices are stationary
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Wireless & Mobile Networks
• Challenges for this context include
– Establishing and maintaining a wireless
connection
– Handing off a wireless client from one part of
the network to another
• Some terminology
– Wireless host is the end user’s device
connected to the network
– Wireless communication links are analogous
to the wired variety
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Terminology
– A base station communicates with the wireless hosts;
e.g. cell towers for cell phones, and access points for
wireless computers
• Base stations connect to the rest of the network,
either through wired or other wireless links
• Infrastructure versus ad hoc mode
– When a wireless host connects in infrastructure
mode, it relies on the network for address resolution,
routing, etc.
– In ad hoc mode, the host performs those functions
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Terminology
• When a host changes from one base
station to another, the change of
attachment is a handoff
• Can categorize wireless networks by the
number of wireless hops (one or more),
and whether it uses infrastructure (e.g. a
base station)
– Single hop, with infrastructure – is typical of a
local wireless connection to a wired network
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Terminology
– Single hop, no infrastructure – like Bluetooth
or ad hoc 802.11 networks
– Multi-hop, with infrastructure – needs a
wireless relay to get to the wired world, like a
wireless mesh network
– Multi-hop, no infrastructure – typically has
mobile nodes as well as hosts; MANETs
(mobile ad hoc networks) and vehicle
versions, VANETs are in this category
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Wireless Links
• If a simple wired Ethernet link is replaced
by a wireless connection
– The hub or switch would be replaced by an
access point
– The host needs a wireless network card
– The Ethernet cable goes in the closet
• So how does this affect service?
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Wireless Links Problems
• Key impacts of changing to wireless are
– Decreasing signal strength with distance from
the access point
– Interference from other sources in the same
frequency range
– Multipath propagation – signals can bounce
around, giving echoes (like talking at edge of
Grand Canyon)
• This results in much higher, and more
variable, bit error rates (BER)
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Wireless Links Problems
• The bit error rates (BER) and signal-tonoise ratio (SNR) are inversely related
– A high SNR means a lower BER
– The BER is expressed as a number, e.g. 10-5
– SNR is given in decibels (dB)
• Can increase SNR somewhat by using
more transmission power
– Higher transmission rates have higher BER
– Can modulate transmission to adapt
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Wireless Links Problems
B
A
C
C’s signal
strength
A’s signal
strength
space
Signal fading:
 B, A hear each other
 B, C hear each other
 A, C can not hear each other
interfering at B
Images from Kurose’s slides
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Some Wireless Protocols
From the author’s PPT slides
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CDMA
• Last term we covered three approaches to
sharing links (multiple access)
– Channel partitioning (TDM and FDM)
– Random access protocols (ALOHA & CSMA)
– Taking turns protocols (polling or token ring)
• Here we need another type of multiple
access protocol – Code Division Multiple
Access (CDMA)
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CDMA
• In CDMA, the original data stream is
multiplied by a code which changes much
faster than the data, the chipping rate
– In the example on page 523, for every bit of
incoming data, the code has eight values
(11101000)
– The data*code product is sent over the link
– The receiver undoes the code, and recovers
the original signal
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CDMA Example
sender
d0 = 1
data
bits
code
Zi,m= di.cm
-1
--1
-1 -1
1 -1
1
--1
1
1 1 1
-1
--1
-1 -1
1 -1
slot 1
-1
-1
slot 1
channel
output
1
--1
1
1 1 1 1 1 1
1
d1 = -1
1 1 1
channel output Zi,m
-1
--1
-1 -1
1 -1
slot 0
1
--1
1
-1
--1
-1 -1
1 -1
slot 0
channel
output
M
Di =  Zi,m.cm
m=1
received
input
code
receiver
1 1 1 1 1 1
1
-1
--1
-1 -1
1 -1
-1
-1
1 1 1
1
--1
1
-1
--1
-1 -1
1 -1
--1
1
1 1 1
-1
--1
-1 -1
1 -1
slot 1
M
1
1
--1
1
-1
--1
-1 -1
1 -1
slot 0
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d0 = 1
d1 = -1
slot 1
channel
output
slot 0
channel
output
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CDMA
• So how does this help??
– Interfering signals add onto the signal you
want to receive
– If the code is “chosen properly,” the desired
signal can be picked out of the sum of your
signal plus garbage
• It’s kind of like being able to follow one
conversation in a crowded room
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802.11 LAN Protocols
• The WiFi or 802.11 protocols are used for
local wireless networks
• 802.11g and 802.11n are most common
– Both provide service at up to 54 Mbps
– 802.11a operates at 5.8 GHz
– 802.11g operates at 2.4 GHz
– 802.11n uses multiple antennae at 2.4 GHz
• All use CSMA/CA as medium access
protocol, have the same frame structure
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802.11 LAN Protocols
• All 802.11 protocols can slow themselves
down for longer distances, or to deal with
interference
• All can use infrastructure or ad hoc mode
• They differ at the physical layer
– Notice each band is a range of frequencies
(2.4 – 2.485 or 5.1 – 5.8 GHz); and they
typically have 11 channels in that range
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802.11 LAN Protocols
• Both 2.4 (for .11b, g, and n) and 5.8 GHz
(for .11a) frequency ranges have
disadvantages
– 2.4 GHz has more interference from cell
phones and microwave ovens
– 5.8 GHz needs more power for a given
distance, and suffers more from multipath
propagation
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802.11 LAN Protocols
• What wavelength are the 802.11 bands?
ln = c = 3E10 cm/s
l = c/n
• For 2.4 GHz, l = 3E10 cm/s / 2.4E9 s-1 = 12.5 cm
or about 5”
• For 5.8 GHz, l = 3E10 cm/s / 5.8E9 s-1 = 5.2 cm
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802.11 Architecture
• A basic service set (BSS) is an access point
(base station) and one or more wireless hosts
• The access points for various BSSs are
connected to each other via hubs, switches,
or routers
• Every wireless adapter has a 6 byte MAC
address, and the access point has a MAC
address
– Again, MAC addresses are managed by IEEE
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802.11 LAN Architecture
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802.11 Architecture
• In infrastructure mode, the access points
are essential elements
• In ad hoc mode, there are no access
points, and wireless devices communicate
independently
– This could be used to network with another
laptop directly, for example
– The outside world isn’t visible in ad hoc mode
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Channels & Association
• In infrastructure mode, need to associate
with an access point before data can be
sent or received
• Each access point is given a Service Set
Identifier (SSID), and channel
– The SSID is a readable name, like ‘sixflags’
– Channels 1-11 are available, but only
channels 1, 6, and 11 are non-overlapping
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It’s a jungle out there!
• A Wi-Fi jungle is when you can choose
from multiple access points (APs),
possibly using the same channels
– Could occur downtown, where many cafés
and local networks could intersect
• How tell the networks (APs) apart?
– Each AP sends beacon frames periodically,
with the AP’s SSID and MAC address
– You choose which AP with which to associate
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Passive vs Active scanning
• When access points broadcast their
presence, and you merely look for their
beacon frames, this is passive scanning
• A wireless host can also broadcast a
signal to look for APs, this is active
scanning
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After association
• Once an AP has been selected for
association, generally DHCP is used to get
an IP address, find DNS servers, etc.
• To be allowed to associate, might have to
authenticate the host
– Can specify which MAC addresses are
allowed to associate
– May require logging into the network, verify
identity with a Radius or Diameter server
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802.11 Multiple Access Control
• Ethernet has been very successful
– Recall it used CSMA/CD – carrier sense
multiple access with collision detection
– Wait for a pause in traffic before transmitting,
and sense when a collision occurs
• 802.11 uses a variation of this, CSMA/CA
– Collision avoidance instead of detection
– Also adds link-layer acknowledgement &
retransmission (ARQ)
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802.11 Collision Avoidance
• Why no collision detection?
– It requires ability to send and receive at the
same time - here the received signal is weak
compared to the sent signal, so it’s expensive
to make hardware to do this
– The hidden terminal problem and fading make
it impossible to detect all collisions
• So 802.11 always transmits a full frame
– Unlike Ethernet, it won’t stop midtransmission
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802.11 ARQ
• To transmit data from sender to receiver:
– Sender waits a short time period DIFS
(distributed inter-frame spacing)
– Sender transmits the data using CSMA/CA
– Data gets to receiver
– Receiver validates integrity of data with CRC
– Waits a time SIFS (short inter-frame spacing)
– The receiver sends an ACK
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802.11 ARQ
sender
receiver
DIFS
data
SIFS
ACK
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802.11 ARQ
• 802.11 uses CRC to check for bit errors
– You recall the cyclic redundancy check, right?
• If channel is busy when a transmission is
ready
– Wait a random time of idle channel, and
transmit when the channel is idle; don’t count
down when the channel is busy
– Why? This avoids collisions when multiple
hosts are waiting for a clear channel
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802.11 ARQ
• So in wireless communication, it’s all
about AVOIDING COLLISIONS!
• If the source doesn’t get an ACK within
some time, it retransmits
• If some number of retransmissions aren’t
ACKed, discard the frame
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802.11 Reservation Scheme
• There is an optional scheme to avoid collision
even when there are hidden hosts
• It’s very polite – each host asks for permission to
transmit
– Sort of like the polling protocols
• Sender sends a Request To Send (RTS) frame
to the AP
• AP broadcasts a Clear To Send (CTS) frame to
reserve use of channel by that sender
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802.11 Reservation Scheme
• Sender then transmits exclusively during
that time period – other hosts know from
getting the CTS to be quiet
• This is very effective at avoiding collisions,
but has time overhead to exchange RTS
and CTS messages
– Often used for sending large data files
– May establish threshold, so only files larger
than threshold are allowed to use RTS/CTS
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802.11 point-to-point
• Using directional antennae, the 802.11
protocols can be used up to 80 kilometers
of distance
– This was done in India in 2004, for example
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802.11 Frames
• A frame in 802.11 consists of 34 bytes of
header and trailer, plus 0 to 2312 bytes of
data (payload)
– Data generally limited to 1500 bytes due to
Ethernet limit
– Data is usually an IP datagram or ARP packet
– Hence the 802.11 protocols are both link and
physical layer protocols
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802.11 Frame Fields
–
–
–
–
–
–
–
–
–
Frame control (2 B, expanded on next slide)
Duration (2 B) for timeout or CTS period
Address 1 (6 B) MAC of destination node
Address 2 (6 B) MAC of transmitting node
Address 3 (6 B) MAC of router leaving this BSS
Sequence control (2 B) just like in TCP
Address 4 (6 B) used only for ad hoc networks
Payload (data) (0-2132 B)
CRC code (4 B) [size verified here]
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802.11 Frames
frame seq #
(for reliable ARQ)
duration of reserved
transmission time (RTS/CTS)
2
2
6
6
6
frame
address address address
duration
control
1
2
3
2
Protocol
version
2
4
1
Type
Subtype
To
AP
6
2
1
seq address
4
control
1
From More
AP
frag
1
Retry
1
0 - 2312
4
payload
CRC
1
Power More
mgt
data
bytes
1
1
WEP
Rsvd
bits
frame type
(RTS, CTS, ACK, data)
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802.11 Frame Fields
• The sizes for frame control parts are in bits
(total 16 bits = 2 bytes)
– The Type field also distinguishes association
frames from normal data frames
– WEP is an encryption mode
• The duration field can be the timeout
interval, or time for a clear to send (CTS)
• Address 3 is critical for communicating
across wireless networks
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802.11 Frame Fields
• Sequence numbers are also used to tell
multipath echoes apart, in addition to
detecting retransmissions
• Address 4 is only used for ad hoc
networks
• The CRC field (4 B) is particularly
important, since there is a large chance of
bit errors
•
We’ll ignore the other fields for now
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Mobility within IP subnet
• If a host moves between BSS’ within the
same subnet (i.e. they are not connected
by a router), it’s relatively easy for the
handoff from one AP to another to occur
• If the BSS’ are connected by a hub,
there’s no problem – the host
disassociates from one AP and associates
with another
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Mobility within subnet
• If the BSS’ are connected by a switch, the
self-learning features of switches is too
slow to keep up well
– The new AP has to send a broadcast Ethernet
message to update the switch with the new
association
• An 802.11f standards group was working
on this issue – standard was withdrawn
2/06
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Advanced 802.11 Features
• 802.11 hints at supporting added features
– Adapt transmission rate, depending on the
SNR (signal to noise ratio) and other channel
characteristics (e.g. lost frames)
– Power management, by limiting the time
various functions are on; done by putting itself
to sleep
• It can tell its access point it’s asleep, so frames
aren’t sent to it until it wakes up!
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802.15 WPAN
• The 802.11 standards are designed for wireless
communication up to 100 meters
• The 802.15.1 wireless personal area network
(WPAN) is for ad hoc wireless networking with a
range of about 10 meters
• Based on Bluetooth, it’s designed to handle up
to eight ‘active’ local devices near a host in a
piconet, controlled by a master node
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802.15 WPAN
• The master node decides which devices
are active or parked
– Can have up to 255 parked devices
• Operates at 2.4 GHz using TDM with slot
of 625 ms, and 79 channels
– Hops randomly across channels (frequencyhopping spread spectrum, or FHSS)
– Data rates up to 4 Mbps
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Zigbee
• The 802.15.4 standard defines Zigbee,
which is targeted at low power, low data
rate, infrequent short range
communications
– Home temperature and security sensors, for
examples – from the Internet of Things
– Channel data rates from 20-250 kbps
– There are ‘full function’ devices which can act
as a master controller for ‘reduced function’
devices
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Cellular Internet Access
• Since Wi-Fi is limited to about 100 meters,
how do we connect to the Internet when
far from an access point?
– Use your cell phone!
• Key concerns are:
– Is it fast?
– Is it reliable?
– Is it going to be better than a long distance
wireless LAN?
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GSM Cellular Architecture
• Cellular architecture is broken into … cells
• Each cell is a geographic area served by a
cell tower, which routes through a mobile
switching center (MSC)
– Acts like a switching center or central office
• The center is connected to the Internet
directly, and/or the phone system (Public
Switched Telephone Network)
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2G Cellular Architecture
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Sharing Frequencies
• Each cell tower handles many calls
simultaneously, so multiple access
protocols are needed
– Combined FDM and TDM
– CDMA (code division, not carrier sense)
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Cell Technology Generations
• The standards used for communication
between cell phones and cell towers are
grouped by the generation of technology
involved
• First Generation (G1) was the analog
FDMA phone, now obsolete in the US
• Second Generation (G2) was the start of
digital phone service (no data)
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Second Generation
• Second generation cell phones used
– IS-136, a combined FDM/TDM derived from
FDMA
– GSM, a European-initiated FDM/TDM, now
widely used in North America
– IS-95, a CDMA-based approach from
Qualcomm
• To bridge the gap to third generation,
generation 2.5 was developed
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Generation 2.5
• Generation 2.5 includes
– GPRS, an upgrade from GSM which uses
circuit switching (slow and inefficient for
Internet); max data rate only 9.6 kbps
– EDGE, was to replace GSM/GPRS and crank
data rate up to 384 kbps
– CDMA2000, an upgrade of IS-95 to get up to
144.4 kbps, also called 1xRTT
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3G
• UMTS is a typical third generation cellular
technology
– It’s built on top of GSM technology for
backward compatibility
– SGSN nodes act like switches to
communicate with mobile nodes
– GGSN nodes act like gateway routers to
connect SGSNs to each other and the Internet
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3G Architecture
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3G Edge Technology
• Third generation cellular technology uses a
Radio Network Controller (RNC) to connect to
mobile users
– An RNC may be connected to several cell
transceivers but then has to connect to both the 2G
phone technology (an MSC) as well as 3G Internet
technology (an SGSN)
• UMTS uses multiple frequencies at once
via Direct Sequence Wideband CDMA
(DS-WCDMA), also known as HSPA
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4G LTE
• 4G Long Term Evolution (LTE) came from the
3rd Generation Partnership (3GPP)
– Features Evolved Packet Core (EPC), an all-IP
approach with good resource management to ensure
high quality of service (QoS)
– Uses both FDM and TDM to create orthogonal
frequency division multiplexing (OFDM) and
reallocating slots as often as every millisecond
– Uses multiple input multiple output (MIMO) antennae
(like 802.11n) to get up to 100 Mbps downstream and
50 Mbps upstream
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WiMAX
• WiMAX was the World Interoperability for
Microwave Access, IEEE 802.16
• Created by Sprint in 2006, it was the first
4G cell technology
• Based more on wireless networking
technology more than cellular
• It is being replaced by LTE, which came
out in 2010
http://www.pcmag.com/article2/0,2817,2403490,00.asp
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Mobility Management
• That concludes addressing the wireless
aspect of networking
• Now, how do we handle a host moving
from one part of the network to another?
– From the network layer, a laptop that moves
around in one subnet isn’t mobile
– From the link layer, if they stay keep using
one access point, they aren’t mobile
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What is mobile?
• Does a user connect separately at
different parts of the network, or need to
maintain a connection while moving?
• Does their IP address need to be the
same?
• What wired infrastructure is available?
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Mobility Terms
• Your home network is the network you
started in
– Your first hop router is a home agent
• While moving, you are in a foreign or
visited network
– Your first hop router is a foreign agent
• You want to communicate with a
correspondent
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Mobility Terms
Home agent in
home network
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Addressing
• As hinted in the previous slide, addressing
is a key concern
• How does the visited network indicate the
home host is there?
– Could update routing tables to indicate that
particular address is in the visited network
– But what about when 1000’s of users are
mobile? Routing tables would get huge &
hard to maintain
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Addressing
• Instead, push mobility concerns to the edge of
the network – the edge routers
– Let the home agent keep track of the permanent
(home) address, and the foreign address
– A care-of-address (COA) is the address of the foreign
agent of the host
– The COA is used to re-route datagrams to the foreign
agent, who then passes them to the host
• Use this via indirect or direct routing
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Indirect Routing
• We could blindly forward datagrams to the home
agent
– Let it change the address to the COA/foreign agent
– The foreign agent sends them to the host
• It works, but it’ll take a while
• The home agent needs to encapsulate the
datagram to get to the COA, who unwraps it
– This is like tunneling for IPv6
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Indirect Routing
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Indirect Routing
• So for indirect routing, we need
–
–
–
–
A mobile node to foreign agent protocol
A foreign agent to home agent protocol
A home agent encapsulation protocol
A foreign agent de-encapsulation protocol
• Every time the node moves to a new foreign
agent, it has to register its presence
(association) and update its home agent
• Is used in the mobile IP standard (RFC 5944)
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Direct Routing
• Direct routing avoids the inefficiency
inherent in indirect routing
– The correspondent goes through a
corresponding agent (router), who learns the
COA of the node
– Then the corresponding agent sends data
directly to the COA
• Need a mobile-user location protocol, to
get the COA from the home agent
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Direct Routing
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Direct Routing
• But how update the corresponding agent if
the node’s COA changes during a
session?
– Use an anchor foreign agent (first foreign
agent used) to keep track of the current COA
– Then if the node is out of the anchor’s
network, encapsulate it and forward to the
current foreign agent
• A little tedious, but probably more efficient
than indirect routing 
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Mobile IP
• How mobile IP addresses can be handled
is a huge topic
• RFC 5944, hinted earlier, defines many
allowable approaches
– With or without foreign agents
– How agents and nodes can discover each
other
– Single or multiple COAs
– Many forms of encapsulation
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Mobile IP
• The three key functions of mobile IP are
– Discovery - how agents and nodes advertise
their presence to each other
– Registration – how nodes and agents
register and deregister COAs with one’s home
agent
– Indirect routing – how home agents can
reroute datagrams, with forwarding rules,
error handling, and different forms of
encapsulation
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Agent Discovery
• A node arriving at a new network needs to
identify the network
– This is called agent discovery
• Two ways to do this are agent
advertisement or agent solicitation
• Agent advertisement is when the agent
broadcasts its services over ICMP
(type 9, router discovery)
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Agent Advertisement
• The broadcast gives the IP address of the
router (agent) and:
– Whether the agent is willing to act as a home
and/or foreign agent (H or F bits)
– If registration is needed before you can get a
COA in a foreign network (R bit)
– If other forms of encapsulation is needed
(M or G bits)
– COA data (one or more COA addresses)
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ICMP Agent Advertisement
0
16
8
type = 9
24
checksum
=9
code = 0
=9
standard
ICMP fields
router address
type = 16
length
registration lifetime
sequence #
RBHFMGV
bits
reserved
0 or more care-ofaddresses
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Agent Solicitation
• Agent solicitation is used when a node
wants to find agents without waiting for
advertisements
– Solicitations are ICMP messages with
type = 10
• When an agent gets a solicitation, it
responds directly to the node, and
registration proceeds normally from there
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Registration with home agent
• When a mobile node gets a COA, that
address must be registered with its home
agent (router)
• This could be done by the foreign agent,
or by the node
• In the former case, there are four steps
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Registration with home agent
1. Node sends registration message to foreign
agent (over UDP, port 434)
2. Foreign agent gets message, records node’s
permanent IP address, and sends
registration message (UDP:434) to home
agent
3. Home agent verifies the message, and
connects node’s permanent IP to the COA
4. Foreign agent gets registration reply, and
forwards it to the mobile node
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Registration with home agent
home agent
HA: 128.119.40.7
foreign agent
COA: 79.129.13.2
visited network: 79.129.13/24
ICMP agent adv.
COA: 79.129.13.2
….
registration req.
COA: 79.129.13.2
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 9999
identification: 714
encapsulation format
….
Mobile agent
MA: 128.119.40.186
registration req.
COA: 79.129.13.2
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 9999
identification:714
….
registration reply
time
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
encapsulation format
….
registration reply
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
….
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Registration with home agent
• When registration is complete, the node
can get data sent to its permanent address
via the new COA
– The actual registration lifetime granted (in
seconds) is less than that requested
– The identification number acts like a
sequence number, to match reply with its
request
• Deregistering a COA isn’t needed, since it
will be overwritten by a new COA
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Managing Cellular Mobility
• For contrast to IP networks, let’s peek at
how cellular networks manage handing off
a connection
• Look at the GSM architecture, since it’s a
mature example
– It follows an indirect approach
– The home network is officially called the home
public land mobile network (PLMN)
– The foreign network is here a visited network
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Managing Cellular Mobility
• The home network maintains a home
location register (HLR) with your cell
phone number subscriber information, and
current location information
• A switch in the home network, the gateway
mobile services switching center (GMSC),
is contacted when an outside call is placed
to the cell phone
– Here call this switch the home MSC
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Managing Cellular Mobility
• The visited network maintains the visitor
location register (VLR), with an entry for
each mobile user currently in the network
– The VLR and the MSC are generally
colocated
• So a given cellular network is the home
network for its subscribers, and a visited
network for phones from other providers
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Routing Calls to Cellular User
• For a call to get to a cellular user:
– A correspondent places the call
– The call is routed to the MSC in the home
network
– The home MSC checks the HLR to see where
the user is located
• It might return the mobile station roaming number
(MSRN, here just roaming number), a fake phone
number which points to the user when in the
network
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Routing Calls to Cellular User
• Or it will return the VLR of the visited network; the
MSC will ask the VLR for the roaming number
– Given the roaming number, the MSC can now
route the call to the VLR and get to the user
• For this to work, the user must exchange
signaling messages with the VLR, who
then passes that information to the HLR
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Routing Calls to Cellular User
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Handoffs in GSM
• Handoff is when a user changes
association during a call
– Here from the old base station to the new
base station
• If both base stations share the same MSC,
life is easier
– Might need to handoff due to weak signal, or
high traffic load on the old base station
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Handoffs in GSM
• The handoff process includes
– Old base station (BS) informs MSC that
handoff is needed
– MSC sets up path for new BS and opens
channel
– New BS allocates resources and new channel
– New BS tells MSC and old BS that user
should be told what’s going on
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Handoffs in GSM
– Mobile user is told it should handoff
– Mobile and new BS exchange messages to
activate new channel
– Mobile user sends handoff complete message
to new BS
– Old BS de-allocates resources
• So how does this process change when a
different MSC is involved?
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Handoffs in GSM
• For handoff between MSCs, the first one is the
anchor MSC
• The anchor MSC stays the same regardless of
where the user goes
• The current user location is the visited MSC
• Hence the home MSC, anchor MSC, and visited
MSC are tracked throughout the call
– IS-41 networks maintain chains of MSCs
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GSM versus IP networks
GSM element
Comment on GSM element
Mobile IP element
Home system
Network to which the mobile user’s permanent
phone number belongs
Home network
Gateway Mobile
Switching Center, or
“home MSC”. Home
Location Register
(HLR)
Home MSC: point of contact to obtain routable
address of mobile user. HLR: database in
home system containing permanent phone
number, profile information, current location of
mobile user, subscription information
Home agent
Visited System
Network other than home system where
mobile user is currently residing
Visited network
Visited Mobile
services Switching
Center.
Visitor Location
Record (VLR)
Visited MSC: responsible for setting up calls
to/from mobile nodes in cells associated with
MSC. VLR: temporary database entry in
visited system, containing subscription
information for each visiting mobile user
Foreign agent
Mobile Station
Roaming Number
(MSRN), or “roaming
number”
Routable address for telephone call segment
between home MSC and visited MSC, visible
to neither the mobile nor the correspondent.
Care-ofaddress
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Mobile effect on higher layers
• Mobile protocols clearly affect the physical, link,
and often the network layers
• Are transport and application layers affected?
– Mostly performance is affected
– Since TCP retransmits lost segments, much worse
performance can be seen under wireless
• TCP can’t tell if packet was lost, or had bit errors, or during
handoff
• The congestion window size (CongWin) is reduced
frequently, reducing efficiency, even though there may be
little actual congestion
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Mobile effect on higher layers
• Ways around this have been proposed
– Use ARQ and/or FEC to detect and repair bit errors
– Split TCP into two segments; one wired and one
wireless
– Use TCP-aware link protocols
– Change TCP so it handles wireless losses differently
than wired losses
• Applications need to consider low bandwidth,
e.g. from 3G phone, and small image sizes
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Summary
• Wireless and mobile computing has
revolutionized telephony
– Computers and phones used to be completely
separate devices!
– Huge contribution to ubiquitous computing
• We examined traits of wireless connections, the
802.11 family of protocols, two 802.15 protocols,
and how 3G and 4G cellular networks provide
Internet access
– Concluded looking at wireless mobility issues
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