INF4208_Part1_Wireless Networks.testopdf

Introduction 1-1
INF4208
-- Mobile and Wireless
networks
The content of this course is adapted from the
Book:
"Computer Networking: A Top Down Approach,
Chapter 6"
DOMGA KOMGUEM Rodrigue --
rodrigue.domga-komguem@facsciences-uy1.cm

Wireless, Mobile Networks 6-2
Background:
v # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers (5-to-1)!
v # wireless Internet-connected devices equals #
wireline Internet-connected devices
§ laptops, Internet-enabled phones promise anytime
untethered Internet access
v two important (but different) challenges
§ wireless: communication over wireless link
§ mobility: handling the mobile user who changes point
of attachment to network

Elements of a wireless
network
network
infrastructure

wireless hosts
v laptop, smartphone
v run applications
v may be stationary
(non-mobile) or mobile
§ wireless does not
always mean mobility
network
network
infrastructure

base station
v typically connected to
wired network
v relay - responsible for
sending packets
between wired
network and wireless
host(s) in its “area”
§ e.g., cell towers,
802.11 access
points
network
network
infrastructure

wireless link
v typically used to
connect mobile(s) to
base station
v also used as backbone
link
v multiple access
protocol coordinates
link access
v various data rates,
transmission distance
network
network
infrastructure

Characteristics of selected wireless
links
Indoor
10-30m
Outdoor
50-200m
Mid-range
outdoor
200m – 4 Km
Long-range
outdoor
5Km – 20 Km
.056
.384
1
4
5-11
54
2G: IS-95, CDMA, GSM
2.5G: UMTS/WCDMA, CDMA2000
802.15
802.11b
802.11a,g
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
4G: LTWE WIMAX
802.11a,g point-to-point
200 802.11n
Data
rate
(Mbps)

infrastructure mode
v base station connects
mobiles into wired
network
v handoff: mobile
changes base station
providing connection
into wired network
network
network
infrastructure

ad hoc mode
v no base stations
v nodes can only
transmit to other
nodes within link
coverage
v nodes organize
themselves into a
network: route
among themselves
network

Wireless network taxonomy
single hop multiple hops
infrastructure
(e.g., APs)
no
infrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET

Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,
characteristics
6.3 Cellular Internet
Access
§architecture
§standards (e.g.,
GSM)
Mobility
6.4 Principles:
addressing and
routing to mobile
users
6.5 Mobile IP
6.6 Handling mobility
in cellular networks
6.7 Mobility and
higher-layer
protocols

Wireless Link Characteristics (1)
important differences from wired link ….
§decreased signal strength: radio signal
attenuates as it propagates through matter
(path loss)
§interference from other sources: standardized
wireless network frequencies (e.g., 2.4 GHz)
shared by other devices (e.g., phone); devices
(motors) interfere as well
§multipath propagation: radio signal reflects off
objects ground, arriving ad destination at
slightly different times
…. make communication across (even a point to
point) wireless link much more “difficult”

Wireless Link Characteristics (2)
v SNR: signal-to-noise ratio
§ larger SNR – easier to
extract signal from noise
(a “good thing”)
v SNR versus BER tradeoffs
§ given physical layer:
increase power -> increase
SNR->decrease BER
§ given SNR: choose physical
layer that meets BER
requirement, giving highest
thruput
•SNR may change with
mobility: dynamically
adapt physical layer
(modulation technique,
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4

Wireless network characteristics
Multiple wireless senders and receivers create
additional problems (beyond multiple access):
A
B
C
Hidden terminal problem
v B, A hear each other
v B, C hear each other
v A, C can not hear each
other means A, C unaware
of their interference at B
A B C
A’s signal
strength
space
C’s signal
strength
Signal attenuation:
v B, A hear each other
v B, C hear each other
v A, C can not hear each
other interfering at B

Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,
characteristics
6.3 IEEE 802.11
wireless LANs
(“Wi-Fi”)
Access
§architecture
§standards (e.g.,
Mobility
6.5 Principles:
addressing and
routing to mobile
users
6.6 Mobile IP
6.8 Mobility and
higher-layer
protocols

IEEE 802.11 Wireless LAN
802.11b
v 2.4-5 GHz unlicensed
spectrum
v up to 11 Mbps
v direct sequence spread
spectrum (DSSS) in
physical layer
§all hosts use same
chipping code
802.11a
§5-6 GHz range
§up to 54 Mbps
802.11g
§2.4-5 GHz range
§up to 54 Mbps
802.11n: multiple antennae
§2.4-5 GHz range
§up to 200 Mbps
v all use CSMA/CA for multiple access
v all have base-station and ad-hoc network
versions

802.11 LAN architecture
v wireless host communicates with
base station
§ base station = access
point (AP)
v Basic Service Set (BSS) (aka
“cell”) in infrastructure mode
contains:
§ wireless hosts
§ access point (AP): base
station
§ ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switch
or router

802.11: Channels, association
v 802.11b: 2.4GHz-2.485GHz spectrum divided into
11 channels at different frequencies
§AP admin chooses frequency for AP
§interference possible: channel can be same as
that chosen by neighboring AP!
v host: must associate with an AP
§scans channels, listening for beacon frames
containing AP’s name (SSID) and MAC
address
§selects AP to associate with
§may perform authentication [Chapter 8]
§will typically run DHCP to get IP address in
AP’s subnet

802.11: passive/active
scanning
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
3
1
passive scanning:
(⑴ )beacon frames sent from APs
(⑵ )association Request frame sent:
H1 to selected AP
(⑶ )association Response frame sent
from selected AP to H1
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
2
3
4
active scanning:
(⑴ )Probe Request frame
broadcast from H1
(⑵ )Probe Response frames sent
from APs
(⑶ )Association Request frame
sent: H1 to selected AP
(⑷ )Association Response frame
sent from selected AP to H1

Wireless, Mobile
Networks
6-20
IEEE 802.11: multiple access
v avoid collisions: 2+ nodes transmitting at same time
v 802.11: CSMA - sense before transmitting
§ don’t collide with ongoing transmission by other node
v 802.11: no collision detection!
§ difficult to receive (sense collisions) when transmitting due to
weak received signals (fading)
§ can’t sense all collisions in any case: hidden terminal, fading
§ goal: avoid collisions: CSMA/C(ollision)A(voidance)
space
A
B
C
A B C
A’s signal
strength
C’s signal
strength

IEEE 802.11 MAC Protocol:
CSMA/CA
802.11 sender
1 if sense channel idle for DIFS then
transmit entire frame (no CD)
2 if sense channel busy then
start random backoff time
timer counts down while channel idle
transmit when timer expires
if no ACK, increase random backoff
interval, repeat 2
802.11 receiver
- if frame received OK
return ACK after SIFS (ACK needed due
to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK

Avoiding collisions (more)
idea: allow sender to “reserve” channel rather than
random access of data frames: avoid collisions of
long data frames
v sender first transmits small request-to-send (RTS)
packets to BS using CSMA
§ RTSs may still collide with each other (but they’re
short)
v BS broadcasts clear-to-send CTS in response to
RTS
v CTS heard by all nodes
§ sender transmits data frame
§ other stations defer transmissions
avoid data frame collisions completely
using small reservation packets!

Collision Avoidance: RTS-CTS
exchange
AP
A B
time
RTS(A)
RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer

frame
control
duration
address
1
address
2
address
4
address
3
payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seq
control
802.11 frame:
addressing
Address 2: MAC address
of wireless host or AP
transmitting this frame
Address 1: MAC address
of wireless host or AP
to receive this frame
Address 3: MAC
address
of router interface to
which AP is attached
Address 4: used only
in ad hoc mode

Internet
router
H1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr
dest. address source address
802.3 frame
802.11 frame:
addressing

frame
control
duration
address
1
address
2
address
4
address
3
payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seq
control
Type
From
AP
Subtype
To
AP
More
frag
WEP
More
data
Power
mgt
Retry Rsvd
Protocol
version
2 2 4 1 1 1 1 1 1
1 1
duration of reserved
transmission time (RTS/CTS)
frame seq #
(for RDT)
frame type
(RTS, CTS, ACK, data)
802.11 frame: more

802.11: mobility within same
subnet
v H1 remains in
same IP subnet: IP
address can
remain same
v switch: which AP
is associated with
H1?
§self-learning (Ch.
5): switch will see
frame from H1 and
“remember”
which switch port
can be used to
reach H1
H1 BBS 2
BBS 1

802.11: advanced capabilities
Rate adaptation
v base station, mobile
dynamically change
transmission rate
(physical layer
modulation technique)
as mobile moves, SNR
varies
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
10 20 30 40
SNR(dB)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
operating point
1. SNR decreases, BER
increase as node moves
away from base station
2. When BER becomes
too high, switch to lower
transmission rate but with
lower BER

power management
v node-to-AP: “I am going to sleep until
next beacon frame”
§ AP knows not to transmit frames to this node
§ node wakes up before next beacon frame
v beacon frame: contains list of mobiles
with AP-to-mobile frames waiting to be
sent
§ node will stay awake if AP-to-mobile frames to be sent;
otherwise sleep again until next beacon frame
802.11: advanced capabilities

M
radius of
coverage
S
S
S
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)
P
802.15: personal area network
v less than 10 m diameter
v replacement for cables
(mouse, keyboard,
headphones)
v ad hoc: no infrastructure
v master/slaves:
§ slaves request permission to
send (to master)
§ master grants requests
v 802.15: evolved from
Bluetooth specification
§ 2.4-2.5 GHz radio band
§ up to 721 kbps

Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,
characteristics
6.3 IEEE 802.11
wireless LANs
(“Wi-Fi”)
access
§architecture
§standards (e.g.,
Mobility
6.5 Principles:
addressing and
routing to mobile
users
6.6 Mobile IP
6.8 Mobility and
higher-layer
protocols

Mobile
Switchin
g
Center
Public telephone
network
Mobile
Switchin
g
Center
Components of cellular network architecture
v connects cells to wired tel. net.
v manages call setup (more later!)
v handles mobility (more later!)
MSC
v covers
geographical region
v base station (BS)
analogous to 802.11
AP
v mobile users
attach to network
through BS
v air-interface:
physical and link
layer protocol
between mobile and
BS
cell
wired network

Cellular networks: the first
hop
Two techniques for sharing
mobile-to-BS radio
spectrum
v combined FDMA/TDMA:
divide spectrum in
frequency channels, divide
each channel into time slots
v CDMA: code division
multiple access
frequency
bands
time slots

BSC
BTS
Base transceiver station (BTS
Base station controller (BSC)
Mobile Switching Center (MSC
Mobile subscribers
Base station system (BSS)
Legend
2G (voice) network architecture
MSC
Public
telephone
network
Gateway
MSC
G

3G (voice+data) network architecture
radio
network
controller
MSC
SGSN
Public
telephone
network
Gateway
MSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public
Internet
GGSN
G
Key insight: new cellular data
network operates in parallel
(except at edge) with existing
cellular voice network
v voice network unchanged in core
v data network operates in parallel

radio
network
controller
MSC
SGSN
Public
telephone
network
Gateway
MSC
G
Public
Internet
GGSN
G
radio access network
Universal Terrestrial Radio
Access Network (UTRAN)
core network
General Packet Radio Service
(GPRS) Core Network
public
Internet
radio interface
(WCDMA, HSPA)
3G (voice+data) network architecture

Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,
characteristics
6.3 IEEE 802.11
wireless LANs
(“Wi-Fi”)
Access
§architecture
§standards (e.g.,
Mobility
6.5 Principles:
addressing and
routing to mobile
users
6.6 Mobile IP
6.8 Mobility and
higher-layer
protocols

What is mobility?
v spectrum of mobility, from the network perspective:
no mobility high mobility
mobile wireless user,
using same access
point
mobile user,
passing through
multiple access
point while
maintaining
ongoing
connections (like
cell phone)
mobile user,
connecting/
disconnecting
from network
using DHCP.

wide area
network
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
home agent: entity that will
perform mobility functions
on behalf of mobile, when
mobile is remote

Mobility: more vocabulary
wide area
network
care-of-address: address
in visited network.
(e.g., 79,129.13.2)
visited network: network
in which mobile
currently resides (e.g.,
79.129.13/24)
permanent address:
remains constant (e.g.,
128.119.40.186)
foreign agent:
entity in visited
network that
performs mobility
functions on behalf
of mobile.
correspondent:
wants to
communicate with
mobile

How do you contact a mobile
friend:
v search all phone
books?
v call her parents?
v expect her to let you
know where he/she is?
I wonder where
Alice moved to?
Consider friend frequently
changing addresses, how do
you find her?

Mobility: approaches
v 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
v 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

v 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
v 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
not
scalable
to millions of
mobiles
Mobility: approaches

wide area
network
Mobility: registration
end result:
v foreign agent knows about mobile
v home agent knows location of mobile
home
network
visited network
1
mobile
contacts
foreign agent
on entering
visited network
2
foreign agent contacts home
agent home: “this mobile is
resident in my network”

Mobility via indirect routing
wide area
network
home
network
visited
network
3
2
4
1
correspondent
addresses packets
using home address
of mobile
home agent intercepts
packets, forwards to
foreign agent
foreign agent
receives packets,
forwards to mobile
mobile replies
directly to
correspondent

Indirect Routing: comments
v 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
v foreign agent functions may be done by
mobile itself
v triangle routing: correspondent-home-
network-mobile
§inefficient when
correspondent, mobile
are in same network

Indirect routing: moving between
networks
v 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)
v mobility, changing foreign networks
transparent: on going connections can be
maintained!

1 2
3
4
Mobility via direct routing
home
network
visited
network
correspondent
requests, receives
foreign address of
mobile
correspondent
forwards to foreign
agent
foreign agent
receives packets,
forwards to mobile
mobile replies
directly to
correspondent

Mobility via direct routing:
comments
v overcome triangle routing problem
v non-transparent to correspondent:
correspondent must get care-of-address from
home agent
§what if mobile changes visited network?
1 2
3
4

wide area
network
1
foreign net
visited
at session start
anchor
foreign
agent
2
4
new foreign
agent
3
corresponden
t
agent
corresponden
t
new
foreign
network
Accommodating mobility with direct
routing
v anchor foreign agent: FA in first visited
network
v data always routed first to anchor FA
v when mobile moves: new FA arranges to
have data forwarded from old FA
(chaining)
5

Chapter 6 outline
6.1 Introduction
Wireless
6.2 Wireless links,
characteristics
6.3 IEEE 802.11
wireless LANs
(“Wi-Fi”)
Access
§architecture
§standards (e.g.,
Mobility
6.5 Principles:
addressing and
routing to mobile
users
6.6 Mobile IP
6.8 Mobility and
higher-layer
protocols

Mobile IP
v RFC 3344
v has many features we’ve seen:
§home agents, foreign agents, foreign-agent
registration, care-of-addresses, encapsulation
(packet-within-a-packet)
v three components to standard:
§indirect routing of datagrams
§agent discovery
§registration with home agent

Mobile IP: indirect routing
Permanent
address:
128.119.40.186
Care-of address:
79.129.13.2
dest: 128.119.40.186
packet sent by
correspondent
dest: 79.129.13.2 dest: 128.119.40.186
packet sent by home agent to foreign
agent: a packet within a packet
dest: 128.119.40.186
foreign-agent-to-mobile
packet

Mobile IP: agent discovery
v agent advertisement: foreign/home agents
advertise service by broadcasting ICMP
messages (typefield § 9)
RBHFMGV
bits
reserved
type = 16
type = 9 code = 0
= 9
checksum
= 9
router address
standard
ICMP fields
mobility agent
advertisement
extension
length sequence #
registration lifetime
0 or more care-of-
addresses
0 8 16 24
R bit: registration
required
H,F bits: home
and/or foreign
agent

Mobile IP: registration
example visited network: 79.129.13/24
home agent
HA: 128.119.40.7
foreign agent
COA: 79.129.13.2
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
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
….
time
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
….

Components of cellular network architectu
correspondent
MSC
MSC
MSC
MSC
MSC
wired
public
telephone
network
different cellular networks,
operated by different providers
recall:

Handling mobility in cellular
networks
v home network: network of cellular provider
you subscribe to (e.g., Sprint PCS, Verizon)
§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)
v visited network: network in which mobile
currently resides
§visitor location register (VLR): database with
entry for each user currently in network
§could be home network

Public
switched
telephone
network
mobile
user
home
Mobile
Switching
Center
HLR
home
network
visited
network
correspondent
Mobile
Switchin
g
Center
VLR
GSM: indirect routing to
mobile
1 call routed
to home network
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
3
home MSC sets up 2nd leg of call
to MSC in visited network
4
MSC in visited network completes
call through base station to mobile

Mobile
Switchin
g
Center
VLR
old BSS
new BSS
old
routing
new
routing
GSM: handoff with common
MSC v handoff goal: route
call via new base
station (without
interruption)
v reasons for handoff:
§ stronger signal to/from new
BSS (continuing connectivity,
less battery drain)
§ load balance: free up channel
in current BSS
§ GSM doesnt mandate why to
perform handoff (policy), only
how (mechanism)
v handoff initiated by
old BSS

Mobile
Switchin
g
Center
VLR
old BSS
1
3
2
4
5 6
7
8
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
GSM: handoff with common
MSC

home
network
Home
MSC
PSTN
correspondent
MSC
anchor MSC
MSC
MSC
(a) before handoff
GSM: handoff between MSCs
v anchor MSC: first MSC
visited during call
§call remains routed
through anchor MSC
v new MSCs add on to end
of MSC chain as mobile
moves to new MSC
v optional path
minimization step to
shorten multi-MSC chain

home
network
Home
MSC
PSTN
correspondent
MSC
anchor MSC
MSC
MSC
(b) after handoff
v anchor MSC: first MSC
visited during call
§call remains routed
through anchor MSC
v new MSCs add on to end
of MSC chain as mobile
moves to new MSC
v optional path
minimization step to
shorten multi-MSC chain
GSM: handoff between MSCs

Mobility: GSM versus Mobile
IPGSM element Comment on GSM element Mobile IP
element
Home system Network to which 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-of-
address

Wireless, mobility: impact on higher layer
protocols
v logically, impact should be minimal …
§best effort service model remains unchanged
§TCP and UDP can (and do) run over wireless,
mobile
v … 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
§delay impairments for real-time traffic
§limited bandwidth of wireless links

INF4208_Part1_Wireless Networks.testopdf

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