Download Wireless and Mobile Networks II

Special Topics in Computer
Engineering
Wireless and Mobile Networks II
Some of these Slides are Based on Slides by
Kurose and Ross
Prepared for Sections 6.4 – 6.8
of the Book Computer Networking: A Top Down
Approach Featuring the Internet
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CPE 0907532:Wireless and Mobile Networks II
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Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Spring 2006
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
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A wireless network : 802.11
network
infrastructure
Wi-Fi Hotspot
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Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 covers geographical
region
 base station (BS)
analogous to 802.11 AP
 mobile users attach
to network through BS
 air-interface:
physical and link layer
protocol between
mobile and BS
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Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network : Public Switched
Telephone Network (PSTN)
OR the Internet
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Components of cellular network architecture
Coverage area of cell depends on
factors, including :
Transmitting power of base
station
Transmitting power of mobile
station
Obstructing buildings in cell
Height of base station
antennae
Many systems place base
stations at corners where
three cells intersect
A single base station
with directional antennas
can service three cells
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Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
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Cellular networks: the first hop
 Many simultaneous calls take place in a cell
 Calls share the portion of the radio spectrum
allocated to the cellular service provider
 Two techniques for sharing mobile-to-BS radio
spectrum
combined FDMA/TDMA: divide spectrum in frequency
channels, divide each channel into time slots
 CDMA: code division multiple access

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Pure FDM and TDM
 FDMA
 Channel is partitioned into a number of frequency
bands, with each band devoted to a call
 TDMA

Time is partitioned into frames
• Each frame is further partitioned into slots
• Each call being assigned the use of a particular slot in the
frame
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Cellular networks: Combined FDM/TDM
 Channel is partitioned
into a number ( F ) of
frequency sub-bands
 For each sub-band, time
is partitioned into
frames and each frame
into a number of slots , T
 Channel will be able to
time slots
frequency
bands
support F * T
simultaneous calls
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Cellular networks: Code division
multiple access (CDMA)
 Users share same radio frequency
 Each user is allocated a distinct chipping
sequence
 Sender and receiver use the same chipping
sequence
 Receiver recovers sender’s transmission from
among simultaneous transmissions from other
senders
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Cellular standards: brief survey I
2G systems: voice channels
 IS-136 TDMA: combined FDMA/TDMA (north
america)
 GSM (global system for mobile communications):
combined FDMA/TDMA

most widely deployed
 IS-95 CDMA: code division multiple access
GSM
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Don’t drown in a bowl
of alphabet soup: use this
for reference only
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Cellular standards: brief survey II
2.5 G systems: voice and data channels
for those who can’t wait for 3G service: 2G extensions
 general packet radio service (GPRS)


evolved from GSM
data sent on multiple channels (if available)
 enhanced data rates for global evolution (EDGE)
 also evolved from GSM, using enhanced modulation
 Date rates up to 384K
 CDMA-2000 (phase 1)
 data rates up to 144K
 evolved from IS-95
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Cellular standards: brief survey III
3G systems: voice/data
 Universal Mobile Telecommunications Service
(UMTS)
 GSM next step, but using CDMA
 CDMA-2000
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Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Spring 2006
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
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What is mobility?
 spectrum of mobility, from the network perspective:
no mobility
high mobility
mobile wireless user, mobile user,
using same access
connecting/
point
disconnecting
from network
using DHCP.
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mobile user, passing
through multiple
access point while
maintaining ongoing
connections (like cell
phone)
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Mobility: Vocabulary
home network: permanent
“home” of mobile
(e.g., 128.119.40/24)
Permanent address:
address in home
network, can always be
used to reach mobile
e.g., 128.119.40.186
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home agent: entity that will
perform mobility functions on
behalf of mobile, when mobile
is remote
wide area
network
correspondent
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Mobility: more vocabulary
Permanent address: remains
constant (e.g., 128.119.40.186)
visited network: network
in which mobile currently
resides (e.g., 79.129.13/24)
Care-of-address: address
in visited network.
(e.g., 79,129.13.2)
wide area
network
foreign agent: entity
in visited network
that performs
mobility functions on
behalf of mobile.
correspondent: wants
to communicate with
mobile
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How do you contact a mobile friend:
Consider friend frequently changing
addresses, how do you find her?
I wonder where
Alice moved to?
 search all phone
books?
 call her parents?
 expect her to let you
know where he/she is?
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Mobility: approaches
 Let routing handle it: routers advertise permanent
address of mobile-nodes-in-residence via usual
routing table exchange.
 routing tables indicate where each mobile located
 no changes to end-systems
 Let end-systems handle it:
 indirect routing: communication from
correspondent to mobile goes through home
agent, then forwarded to remote
 direct routing: correspondent gets foreign
address of mobile, sends directly to mobile
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Mobility: approaches
 Let routing handle it: routers advertise permanent
not
address of mobile-nodes-in-residence
via usual
scalable
routing table exchange.
to millions of
 routing tables indicate
mobiles where each mobile located
no changes to end-systems
 let end-systems handle it:
 indirect routing: communication from
correspondent to mobile goes through home
agent, then forwarded to remote
 direct routing: correspondent gets foreign
address of mobile, sends directly to mobile

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Mobility: registration
visited network
home network
2
1
wide area
network
foreign agent contacts home
agent home: “this mobile is
resident in my network”
mobile contacts
foreign agent on
entering visited
network
End result:
 Foreign agent knows about mobile
 Home agent knows location of mobile
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Mobility via Indirect Routing
foreign agent
receives packets,
forwards to mobile
home agent intercepts
packets, forwards to
foreign agent
home
network
visited
network
3
wide area
network
correspondent
addresses packets
using home address
of mobile
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1
2
4
mobile replies
directly to
correspondent
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Indirect Routing: comments
 Mobile uses two addresses:
permanent address: used by correspondent (hence
mobile location is transparent to correspondent)
 care-of-address: used by home agent to forward
datagrams to mobile
 foreign agent functions may be done by mobile itself
 triangle routing: correspondent-home-networkmobile
 inefficient when
correspondent, mobile
are in same network

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Indirect Routing: moving between networks
 suppose mobile user moves to another
network
registers with new foreign agent
 new foreign agent registers with home agent
 home agent update care-of-address for mobile
 packets continue to be forwarded to mobile (but
with new care-of-address)

 mobility, changing foreign networks
transparent: on going connections can be
maintained!
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Mobility via Direct Routing
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
home
network
4
wide area
network
2
correspondent
requests, receives
foreign address of
mobile
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visited
network
1
3
4
mobile replies
directly to
correspondent
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Mobility via Direct Routing: comments
 overcome triangle routing problem
 non-transparent to correspondent:
correspondent must get care-of-address
from home agent

what if mobile changes visited network?
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Accommodating mobility with direct routing
 anchor foreign agent: FA in first visited network
 data always routed first to anchor FA
 when mobile moves: new FA arranges to have data
forwarded from old FA (chaining)
foreign net visited
at session start
wide area
network
anchor
foreign
agent
1
2
4
5
correspondent
agent
correspondent
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3
new foreign
agent
new
foreign
network
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mobility with direct routing Details
Step I: data forwarded to mobile in foreign network where mobile was located when
session first started
Step 2: mobile moves to a new foreign network
Step 3: mobile registers with new foreign agent
Step 4: new foreign agent provides anchor foreign agent with mobile new COA
Step 5: anchor foreign agent receives an encapsulated datagram for a departed
mobile it re-encapsulate datagram and forward it to mobile using new COA
If mobile node moves again to a new foreign network, foreign agent in new network
contacts anchor foreign agent to set up forwarding to new foreign network
foreign net visited
at session start
wide area
network
anchor
foreign
agent
1
2
4
5
correspondent
agent
correspondent
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3
new foreign
agent
new
foreign
network
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Chapter 6 outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics

CDMA
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 6.4 Cellular Internet
Access


architecture
standards (e.g., GSM)
Spring 2006
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Handling mobility in
cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
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Mobile IP
 Internet architecture and protocols for
supporting mobility collectively known as
Mobile IP
 Defined in RFC 3220
 has many features we’ve seen:

home agents, foreign agents, foreign-agent
registration, care-of-addresses, encapsulation
(packet-within-a-packet)
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Mobile IP Three Components
 Indirect routing of datagrams
 The manner in which datagrams are forwarded to
mobile nodes by a home agent including
• Rules for forwarding datagrams
• Rules for handling error conditions
• Several forms of encapsulation [RFC 2003, RFC 2004]
 Agent discovery
 Protocols used by agents to advertise services
 Protocols for mobile nodes to solicit services
 Registration with home agent
 Protocols used by mobile node and/or foreign agent
to register and deregister COAs with home agent
 Security: discussed in chapter 8
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Mobile IP: indirect routing
foreign-agent-to-mobile packet
packet sent by home agent to foreign
agent: a packet within a packet
dest: 79.129.13.2
dest: 128.119.40.186
dest: 128.119.40.186
Permanent address:
128.119.40.186
dest: 128.119.40.186
Care-of address:
79.129.13.2
packet sent by
correspondent
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Mobile IP: agent discovery
 agent advertisement: foreign/home agents advertise
service by broadcasting ICMP messages (typefield = 9)
IP of Agent
0
16
8
type = 9
24
checksum
=9
code = 0
=9
H : a home agent
F : a foreign agent
standard
ICMP fields
router address
R: registration required
•A mobile user in this network must type = 16
register with a foreign agent
length
registration lifetime
•Cannot obtain a care-of address in
foreign network and assume
functionality of foreign agent
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sequence #
RBHFMGV
bits
reserved
0 or more care-ofaddresses
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mobility agent
advertisement
extension
32
Mobile IP: registration example
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
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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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Components of cellular network architecture
recall:
correspondent
wired public
telephone
network
MSC
MSC
MSC
MSC
MSC
different cellular networks,
operated by different providers
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Handling mobility in cellular networks
 home network: network of cellular provider you
subscribe to (e.g., Fastlink, Jo Mobilecom)
 home location register (HLR): database in home
network containing permanent cell phone #,
profile information (services, preferences,
billing), information about current location
(could be in another network)
 visited network: network in which mobile currently
resides
 visitor location register (VLR): database with
entry for each user currently in network
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GSM: indirect routing to mobile
home
network
HLR
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
correspondent
home
Mobile
Switching
Center
1
3
VLR
4
mobile
user
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Mobile
Switching
Center
Public
switched
telephone
network
call routed
to home network
home MSC sets up 2nd leg of call
to MSC in visited network
MSC in visited network completes
call through base station to mobile
visited
network
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GSM: handoff with common MSC
 Handoff goal: route call via
new base station (without
interruption)
 reasons for handoff:

VLR Mobile

Switching
Center
old
routing
old BSS

new
routing
new BSS
stronger signal to/from new
BSS (continuing connectivity,
less battery drain)
load balance: free up channel
in current BSS (cell
overloaded with a large # of
calls)
GSM doesn’t mandate why to
perform handoff (algorithms
used to determine whether or
not to perform handoff)
• only how (mechanism)
 handoff initiated by old BSS
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GSM: handoff with common MSC
VLR Mobile
Switching
Center 2
4
1
8
old BSS
5
Spring 2006
7
3
6
new BSS
1. old BSS informs MSC of impending
handoff, provides list of 1+ new BSSs
2. MSC sets up path (allocates resources)
to new BSS
3. new BSS allocates radio channel for
use by mobile
4. new BSS signals MSC, old BSS: ready
5. old BSS tells mobile: perform handoff to
new BSS
6. mobile, new BSS signal to activate new
channel
7. mobile signals via new BSS to MSC:
handoff complete. MSC reroutes call
8 MSC-old-BSS resources released
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GSM: handoff between MSCs
 anchor MSC: first MSC
visited during call

home network
correspondent
Home
MSC
anchor MSC
PSTN
call remains routed through
anchor MSC
 Two alternatives
 Maintaining a single MSC-hop
from anchor MSC to current
MSC (illustrated in figure)
MSC
MSC
MSC
(a) before handoff
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GSM: handoff between MSCs
 anchor MSC: first MSC
visited during call

home network
correspondent
Home
MSC
anchor MSC
PSTN
MSC
MSC
MSC
(b) after handoff
Spring 2006
call remains routed through
anchor MSC
 Two alternatives
 Maintaining a single MSC-hop
from anchor MSC to current
MSC (illustrated in figure)
 Chain the MSCs visited by
mobile
• An old MSC forward ongoing call
to new MSC each time mobile
moves to new MSC
• IS-41 allows optional path
minimization step to shorten
multi-MSC chain
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Mobility: GSM versus Mobile IP
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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Wireless, mobility: impact on transport protocols
 logically, impact should be minimal …
 best
effort service model remains unchanged
 TCP and UDP can (and do) run over wireless, mobile
 … but performance-wise:
 packet loss/delay due to bit-errors (discarded
packets, delays for link-layer retransmissions), and
handoff
 TCP interprets loss as congestion, will decrease
congestion window un-necessarily
 TCP’s congestion control could be problematic in
wireless networks
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Wireless, mobility: impact on application protocols
limited bandwidth of wireless links
 Applications must treat bandwidth as a scare
commodity
 Serving content to a browser on 3G phone
 same image-rich content is not possible like serving a browser
operating over a wired connection
 mobility makes possible a rich set of location-aware
and context-aware applications
 we’ve only seen tip of iceberg
 impact of wireless and mobile networks on networked
applications and their protocols!
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