Transcript Wireless and Mobile Networks

Chapter 6: Wireless and Mobile
Networks
Objectives
 Introduction
 Wireless links,
characteristics

CDMA
 IEEE 802.11 wireless
LANs (“wi-fi”)
 Cellular Internet
Access


 Principles: addressing
and routing to mobile
users
 Mobile IP


Handling mobility in
cellular networks
Mobility and higher-layer
protocols
architecture
standards (e.g., GSM)
1
1. Wireless Internet:
Introduction
2
What Is Wireless Networking?
 The use of infra-red (IR) or radio frequency
(RF) signals to share information and
resources between devices
 A hot computer industry buzzword:
Lots of advertising by companies and media
 Wireless Broadband, 3G wireless, 4G, WAP,
iMode, Bluetooth, WiFi

 Mobile Internet, Pervasive Computing,
Nomadic Computing, M-commerce

Ubiquitous; Global; Revolutionary
3
Two Popular 2.4 GHz Standards:
 IEEE 802.11
Fast (11b)
 High Power
 Long range
 Single-purpose
 Ethernet replacement
 Easily Available

 Bluetooth
Slow
 Low Power
 Short range
 Flexible
 Cable replacement

• Apple Airport, iBook, G4
• Cisco Aironet 350
4
Pros and Cons of 802.11:
 Pro:
High bandwidth (up to 11 Mbps)
 Two modes of operation: infrastructure vs. ad hoc

 Con:
 Incompatibility between old and new cards
 Signal blocked by reinforced concrete or tinted glass
 No standard for hand-off between base stations
 Some channel numbers overlap spectrum
 High power consumption in laptops
5
Bluetooth
 Think USB, not Ethernet
 Cable replacement technology
 Created by Ericsson
 PAN - Personal Area Network
 1-2 Mbps connections
 1600 hops per second FHSS
 Includes synchronous, asynchronous, voice connections
 Small, low-power, short-range, cheap, versatile
radios
 Used as Internet connection, phone, or headset
 Master/slave configuration and scheduling
6
Future of Wireless
 Higher data rates
 Better security
 Wider selection of products
 Lower prices
 Zero configuration networking
 More end-user focus
 Better software
 Less visible
 More popular
7
2. Background:
Networking Terminology
8
Internet Protocol Stack
 Application: supporting network
applications and end-user services

FTP, SMTP, HTTP, DNS, NTP
 Transport: end to end data transfer
 TCP, UDP
 Network: routing of datagrams
from source to destination

IPv4, IPv6, BGP, RIP, routing protocols
 Data Link: hop by hop frames,
channel access, flow/error control

PPP, Ethernet, IEEE 802.11b
Application
Transport
Network
Data Link
Physical
001101011...
 Physical: raw transmission of bits
9
Wireless Internet Technologies
 Mobile devices (e.g., notebooks, laptops,
PDAs, cell phones, wearable computers)
 Wireless network access
Bluetooth (1 Mbps, up to 3 meters)
 IEEE 802.11b (11 Mbps, up to 100 meters)
 IEEE 802.11a (55 Mbps, up to 20 meters)

 Operating modes:
 Infrastructure mode (access point)
 Ad hoc mode
 Wireless Web, WiFi “hot spots”
10
Elements of a wireless network
Ad hoc mode
 no base stations
 nodes can only
transmit to other
nodes within link
coverage
 nodes organize
themselves into a
network: route among
themselves
11
Wireless Link Characteristics
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”
12
Code Division Multiple Access (CDMA)
 used in several wireless broadcast channels





(cellular, satellite, etc) standards
unique “code” assigned to each user; i.e., code set
partitioning
all users share same frequency, but each user has
own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping
sequence)
decoding: inner-product of encoded signal and
chipping sequence
allows multiple users to “coexist” and transmit
simultaneously with minimal interference (if codes
are “orthogonal”)
13
CDMA: two-sender interference
14
Chapter 6: Wireless and Mobile
Networks
 Introduction
 Wireless links,
characteristics

CDMA
 IEEE 802.11 wireless
LANs (“wi-fi”)
 Cellular Internet
Access


architecture
standards (e.g., GSM)
 Principles: addressing
and routing to mobile
users
 Mobile IP


Handling mobility in
cellular networks
Mobility and higher-layer
protocols
15
IEEE 802.11 Wireless LAN
 802.11b
 2.4-5 GHz unlicensed
radio spectrum
 up to 11 Mbps
 direct sequence spread
spectrum (DSSS) in
physical layer
• all hosts use same
chipping code
 widely deployed, using
base stations
 802.11a
 5-6 GHz range
 up to 54 Mbps
 802.11g
 2.4-5 GHz range
 up to 54 Mbps
 All use CSMA/CA for
multiple access
 All have base-station
and ad-hoc network
versions
16
Where does 802.11 live in the OSI?
Telnet, FTP, Email, Web, etc.
Application
Presentation
Session
TCP, UDP
IP, ICMP, IPX
Transport
Network
Logical Link Control - 802.2
(Interface to upper layer protocols)
MAC
Data Link
Wireless lives at
802.3, 802.5, 802.11
Layers 1 & 2
only!
Phy. Layer Convergence Protocol
LAN:10BaseT,10Base2,10BaseFL
WLAN: FHSS, DSSS, IR
Physical
17
802.11: Channels, association
 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!
 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
 will typically run DHCP to get IP address in AP’s
subnet
18
Avoiding collisions
idea: allow sender to “reserve” channel rather than random
access of data frames: avoid collisions of long data frames
 sender first transmits small request-to-send (RTS) packets
to BS using CSMA
 RTSs may still collide with each other (but they’re short)
 BS broadcasts clear-to-send CTS in response to RTS
 RTS heard by all nodes
 sender transmits data frame
 other stations defer transmissions
Avoid data frame collisions completely
using small reservation packets!
19
802.11 frame: addressing
2
2
6
6
6
frame
address address address
duration
control
1
2
3
Address 1: MAC address
of wireless host or AP
to receive this frame
2
6
seq address
4
control
0 - 2312
4
payload
CRC
Address 4: used only
in ad hoc mode
Address 3: MAC address
of router interface to
which AP is attached
Address 2: MAC address
of wireless host or AP
transmitting this frame
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802.11: mobility within same subnet
 H1 remains in same IP
subnet: IP address
can remain same
 switch: which AP is
associated with H1?

self-learning: switch
will see frame from H1
and “remember” which
switch port can be
used to reach H1
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
21
802.15: personal area network
 less than 10 m diameter
 replacement for cables
(mouse, keyboard,
headphones)
 ad hoc: no infrastructure
 master/slaves:


slaves request permission to
send (to master)
master grants requests
 802.15: evolved from
Bluetooth specification


2.4-2.5 GHz radio band
up to 721 kbps
P
S
P
radius of
coverage
M
S
P
S
P
M Master device
S Slave device
P Parked device (inactive)
22
Chapter 6: Wireless and Mobile
Networks
 Introduction
 Wireless links,
characteristics

CDMA
 IEEE 802.11 wireless
LANs (“wi-fi”)
 Cellular Internet
Access


architecture
standards (e.g., GSM)
 Principles: addressing
and routing to mobile
users
 Mobile IP


Handling mobility in
cellular networks
Mobility and higher-layer
protocols
23
Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup
 handles mobility
 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
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
24
Cellular networks: the first hop
Two techniques for sharing
mobile-to-BS radio
spectrum
 combined FDMA/TDMA:
divide spectrum in
frequency channels, divide
each channel into time
slots
frequency
bands
 CDMA: code division
multiple access
time slots
25
Cellular standards: brief survey
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 global system for mobile communication (GSM)
Uses FDM/TDM for the air interface
data sent on multiple channels
 enhanced data rates for global evolution (EDGE)


also evolved from GSM, using enhanced modulation
Date rates up to 384Kbps
 CDMA-2000 (phase 1)


data rates up to 144Kbps
evolved from IS-95, which uses CDMA
3G systems: ( CDMA phase II) voice/data
 Universal Mobile Telecommunications Service (UMTS)
GSM next step, but using CDMA
 CDMA-2000

26
Chapter 6: Wireless and Mobile
Networks
 Introduction
 Wireless links,
characteristics

CDMA
 IEEE 802.11 wireless
LANs (“wi-fi”)
 Cellular Internet
Access


architecture
standards (e.g., GSM)
 Principles: addressing
and routing to mobile
users
 Mobile IP


Handling mobility in
cellular networks
Mobility and higher-layer
protocols
27
What is mobility?
 spectrum of mobility, from the network perspective:
no mobility
mobile wireless user, mobile user,
using same access
connecting/
point
disconnecting
from network
using DHCP.
high mobility
mobile user, passing
through multiple
access point while
maintaining ongoing
connections (like cell
phone)
28
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
29
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

30
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!
31
Chapter 6: Wireless and Mobile
Networks
 Introduction
 Wireless links,
characteristics

CDMA
 IEEE 802.11 wireless
LANs (“wi-fi”)
 Cellular Internet
Access


architecture
standards (e.g., GSM)
 Principles: addressing
and routing to mobile
users
 Mobile IP


Handling mobility in
cellular networks
Mobility and higher-layer
protocols
32
Mobile IP
 RFC 3220
 has many features we’ve seen:
 home agents, foreign agents, foreign-agent
registration, care-of-addresses, encapsulation
(packet-within-a-packet)
 three components to standard:
 indirect routing of datagrams
 agent discovery
 registration with home agent
33
Mobile IP: agent discovery
 agent advertisement: foreign/home agents advertise
service by broadcasting ICMP messages (typefield = 9)
0
type = 9
24
checksum
=9
code = 0
=9
H,F bits: home
and/or foreign agent
R bit: registration
required
16
8
standard
ICMP fields
router address
type = 16
length
registration lifetime
sequence #
RBHFMGV
bits
reserved
0 or more care-ofaddresses
mobility agent
advertisement
extension
34
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
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
….
35
Handling mobility in cellular networks
 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)
 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
36
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
37
Wireless, mobility: impact on higher layer 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
 delay impairments for real-time traffic
 limited bandwidth of wireless links

38
Chapter 6 Summary
Wireless
 wireless links:



capacity, distance
channel impairments
CDMA
 IEEE 802.11 (“wi-fi”)
 CSMA/CA reflects
wireless channel
characteristics
 cellular access
 architecture
 standards (e.g., GSM,
CDMA-2000, UMTS)
Mobility
 principles: addressing,
routing to mobile users



home, visited networks
direct, indirect routing
care-of-addresses
 case studies
 mobile IP
 mobility in GSM
39