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6: Wireless and Mobile Networks 6-1
Chapter 6Wireless and Mobile Networks
A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:
If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)
If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2004J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2004.
6: Wireless and Mobile Networks 6-2
Chapter 6: Wireless and Mobile Networks
Background:# wireless (mobile) phone subscribers now exceeds # wired phone subscribers!computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet accesstwo important (but different) challenges
communication over wireless linkhandling mobile user who changes point of attachment to network
6: Wireless and Mobile Networks 6-3
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links, characteristics
CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-5
Elements of a wireless networkAd hoc mode
no base stationsnodes can only transmit to other nodes within link coveragenodes organize themselves into a network: route among themselves
Characteristics of selected wireless link standards
384 Kbps
56 Kbps
54 Mbps
5-11 Mbps
1 Mbps802.15
802.11b802.11{a,g}
IS-95 CDMA, GSM
UMTS/WCDMA, CDMA2000
.11 p-to-p link
2G
3G
Indoor
10 – 30m
Outdoor
50 – 200m
Mid rangeoutdoor
200m – 4Km
Long rangeoutdoor
5Km – 20Km
6: Wireless and Mobile Networks 6-6
6: Wireless and Mobile Networks 6-7
Wireless Link CharacteristicsDifferences 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 wellmultipath 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”
6: Wireless and Mobile Networks 6-10
Medium Access Protocols
Channel partitioningRandom accessTaking turnsCode division multiple access
6: Wireless and Mobile Networks 6-11
Code Division Multiple Access (CDMA)
used in several wireless broadcast channels (cellular, satellite, etc) standardsunique “code” assigned to each user; i.e., code set partitioningall users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode dataencoded signal = (original data) X (chipping sequence)decoding: inner-product of encoded signal and chipping sequenceallows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
6: Wireless and Mobile Networks 6-16
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links, characteristics
CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-17
IEEE 802.11 Wireless LAN
802.11b2.4-5 GHz unlicensed radio spectrumup to 11 Mbpsdirect sequence spread spectrum (DSSS) in physical layer
• all hosts use same chipping code
widely deployed, using base stations
802.11a5-6 GHz rangeup to 54 Mbps
802.11g2.4-5 GHz rangeup to 54 Mbps
All use CSMA/CA for multiple accessAll have base-station and ad-hoc network versions
6: Wireless and Mobile Networks 6-19
802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
AP admin chooses frequency for APinterference possible: channel can be same as that chosen by neighboring AP!
host: must associate with an APscans channels, listening for beacon framescontaining AP’s name (SSID) and MAC addressselects AP to associate withmay perform authentication [Chapter 8]will typically run DHCP to get IP address in AP’s subnet
6: Wireless and Mobile Networks 6-22
Avoiding collisions (more)idea: allow sender to “reserve” channel rather than random
access of data frames: avoid collisions of long data framessender 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 RTSRTS heard by all nodes
sender transmits data frameother stations defer transmissions
Avoid data frame collisions completely using small reservation packets!
6: Wireless and Mobile Networks 6-24
802.11 frame: addressing
framecontrol duration address
1address
2address
4address
3 payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
Address 3: used only in ad hoc modeAddress 1: MAC address
of wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 2: MAC addressof wireless host or AP transmitting this frame
6: Wireless and Mobile Networks 6-27
802.11: mobility within same subnet
H1 remains in same IP subnet: IP address can remain sameswitch: 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
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
router
6: Wireless and Mobile Networks 6-28
802.15: personal area network
M radius ofcoverage
S
SS
P
P
P
P
M
S
Master device
Slave device
Parked device (inactive)P
less than 10 m diameterreplacement for cables (mouse, keyboard, headphones)ad hoc: no infrastructuremaster/slaves:
slaves request permission to send (to master)master grants requests
802.15: evolved from Bluetooth specification
2.4-2.5 GHz radio bandup to 721 kbps
6: Wireless and Mobile Networks 6-29
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links, characteristics
CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-30
Components of cellular network architecture
Mobile Switching
Center
Public telephonenetwork, andInternet
Mobile Switching
Center
connects cells to wide area netmanages call setup (more later!)handles mobility (more later!)
MSC
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
cell
wired network
6: Wireless and Mobile Networks 6-31
Cellular networks: the first hopTwo techniques for sharing
mobile-to-BS radio spectrumcombined FDMA/TDMA:divide spectrum in frequency channels, divide each channel into time slotsCDMA: code division multiple access
frequencybands
time slots
6: Wireless and Mobile Networks 6-32
Cellular standards: brief survey
1G systems: only for voice, analog FDMA, extinct
2G Systems: digitalIS-136 TDMA
widely deployed in North Americaglobal system for mobile communications (GSM)
deployed in Europe in early 1990s and most widely deployedcombined FDM/TDM
IS-95 CDMAdeployed in North America and Korea
6: Wireless and Mobile Networks 6-33
Cellular standards: brief survey
2.5 G systems: voice and data channelsfor those who can’t wait for 3G service: 2G extensionsgeneral 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 144Kevolved from IS-95
6: Wireless and Mobile Networks 6-34
Cellular standards: brief survey
3G systems: voice/dataUniversal Mobile Telecommunications Service (UMTS)
GSM next step, but using CDMACDMA-2000
….. more (and more interesting) cellular topics due to mobility (stay tuned for details)
6: Wireless and Mobile Networks 6-35
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links, characteristics
CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-36
What is mobility?
spectrum of mobility, from the network perspective:
no mobility high mobility
mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
mobile user, connecting/ disconnecting from network using DHCP.
mobile wireless user, using same access point
How do you contact a mobile friend:
search all phone books?call her parents?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?
6: Wireless and Mobile Networks 6-37
6: Wireless and Mobile Networks 6-38
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 locatedno changes to end-systems
Let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing: correspondent gets foreign address of mobile, sends directly to mobile
6: Wireless and Mobile Networks 6-39
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 locatedno changes to end-systems
let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing: correspondent gets foreign address of mobile, sends directly to mobile
not scalable
to millions ofmobiles
6: Wireless and Mobile Networks 6-40
Indirect Routing: moving between networks
suppose mobile user moves to another network
registers with new foreign agentnew foreign agent registers with home agenthome agent update care-of-address for mobilepackets continue to be forwarded to mobile (but with new care-of-address)
mobility, changing foreign networks transparent: on going connections can be maintained!
6: Wireless and Mobile Networks 6-44
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 locatedno changes to end-systems
let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remotedirect routing: correspondent gets foreign address of mobile, sends directly to mobile
not scalable
to millions ofmobiles
6: Wireless and Mobile Networks 6-47
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links, characteristics
CDMA6.3 IEEE 802.11 wireless LANs (“wi-fi”)6.4 Cellular Internet Access
architecturestandards (e.g., GSM)
Mobility6.5 Principles: addressing and routing to mobile users6.6 Mobile IP6.7 Handling mobility in cellular networks6.8 Mobility and higher-layer protocols
6.9 Summary
6: Wireless and Mobile Networks 6-48
Mobile IP
RFC 3220has 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 datagramsagent discoveryregistration with home agent
6: Wireless and Mobile Networks 6-49
Mobile IP: agent discoveryagent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)
RBHFMGV bits reserved
type = 16
type = 9 code = 0 checksum
router address standard
ICMP fields
mobility agent advertisement
extension
length sequence #
registration lifetime
0 or more care-of-addresses
0 8 16 24
H,F bits: home and/or foreign agent
R bit: registration required
6: Wireless and Mobile Networks 6-50
Mobile IP: registration example
visited network: 79.129.13/24 home agent
HA: 128.119.40.7 foreign agent
COA: 79.129.13.2 COA: 79.129.13.2
….
ICMP agent adv. 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 req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format….
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
6: Wireless and Mobile Networks 6-51
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 networkcould be home network
6: Wireless and Mobile Networks 6-56
Mobility: GSM versus Mobile IPGSM 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-of-address
6: Wireless and Mobile Networks 6-57
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 handoffTCP interprets loss as congestion, will decrease congestion window un-necessarilydelay impairments for real-time trafficlimited bandwidth of wireless links
6: Wireless and Mobile Networks 6-58
Chapter 6 Summary
Wirelesswireless links:
capacity, distancechannel impairmentsCDMA
IEEE 802.11 (“wi-fi”)CSMA/CA reflects wireless channel characteristics
cellular accessarchitecturestandards (e.g., GSM, CDMA-2000, UMTS)
Mobilityprinciples: addressing, routing to mobile users
home, visited networksdirect, indirect routingcare-of-addresses
case studiesmobile IPmobility in GSM
impact on higher-layer protocols