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Transcript of Wireless, Mobile Networks6-1 Ch. 6: Wireless and Mobile Networks Background: # wireless (mobile)...
Wireless, Mobile Networks 6-1
Ch. 6: Wireless and Mobile Networks
Background: # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers (5-to-1)! # wireless Internet-connected devices equals
# wireline Internet-connected devices laptops, Internet-enabled phones promise anytime
untethered Internet access two important (but different) challenges
wireless: communication over wireless link mobility: handling the mobile user who changes
point of attachment to network
Wireless, Mobile Networks 6-2
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet Access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Handling mobility in
cellular networks6.8 Mobility and higher-
layer protocols
6.9 Summary
Wireless, Mobile Networks 6-3
Elements of a wireless network
network infrastructure
cell tower in cellular network (e.g. 3G or 4G)
access pointin 802.11 wireless LAN
Wireless, Mobile Networks 6-4
wireless hosts laptop, smartphone run applications may be stationary
(non-mobile) or mobile wireless does not
always mean mobility
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-5
base station typically connected
to wired network responsible for
sending packets between wired network and wireless host(s) in its “area” e.g., cell towers,
802.11 access points
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-6
wireless link typically used to
connect mobile(s) to base station
multiple access protocol coordinates link access
various data rates, transmission distance
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-7
Characteristics of selected wireless links
Indoor10-30m
Outdoor50-200m
Mid-rangeoutdoor
200m – 4 Km
Long-rangeoutdoor
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
Dat
a ra
te (
Mbp
s)
Wireless, Mobile Networks 6-8
hosts “operate in infrastructure mode”
all services via base station (connects mobiles into wired network)
handoff: mobile changes base station providing connection into wired network
Elements of a wireless network
network infrastructure
Wireless, Mobile Networks 6-9
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
Elements of a wireless network
Wireless, Mobile Networks 6-10
Wireless network taxonomy
single hop multiple hops
infrastructure(e.g., APs)
noinfrastructure
host connects to base station (WiFi,
cellular) which connects to
larger Internet
no base station, noconnection to larger
Internet (e.g. Bluetooth)
host may have torelay through several
wireless nodes to connect to larger Internet: mesh net
no base station, noconnection to larger Internet. May have torelay to reach other
wireless nodeMANET (mobile ad hoc netw)
Wireless, Mobile Networks 6-11
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet Access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Handling mobility in
cellular networks6.8 Mobility and higher-
layer protocols
6.9 Summary
Wireless, Mobile Networks 6-12
Wireless Link Characteristics (1)
important differences from wired link ….
decreased signal strength: radio signal strength decreases 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 (e.g. microwave) interfere as well
multipath propagation: radio signals reaching the receiving antenna by two or more paths because radio signal reflects off objects and the ground
…. make communication across (even a point to point) wireless link much more “difficult” => powerful CRC error detection & reliable data transfer protocols
Wireless, Mobile Networks 6-13
Wireless Link Characteristics (2) SNR: signal-to-noise ratio
measures signal strength relative to noise => larger SNR gives easier to extract signal from background noise
three different modulation schemes: amplitude modulation of a signal:
change the amplitude of periodic waveform to encode information (QAM = Quadrature amplitude modulation)
SNR versus Bit-Error-Rate (BER) for given modulation scheme:
increase transmission power -> increase SNR->decrease BER
but: little practical gain beyond certain threshold, stronger signal costs more energy (bad for battery-powered users)
10 20 30 40
QAM256 (8)
QAM16 (4)
BPSK (1)
SNR(decibels)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
Wireless, Mobile Networks 6-14
Wireless Link Characteristics (2) SNR versus Bit-Error-Rate (BER)
for given SNR: choose modulation scheme that meets BER requirement and gives highest throughput
SNR may change with mobility: dynamically adapt modulation technique and rate to keep BER low! (e.g. with SNR of 10 dB we have to choose BPSK to avoid high BER)
when sending at high bit rate, differences in amplitude are low => errors while we sample the signal
10 20 30 40
QAM256 (8)
QAM16 (4)
BPSK (1)
SNR(decibels)
BER
10-1
10-2
10-3
10-5
10-6
10-7
10-4
Wireless, Mobile Networks 6-15
Wireless network characteristicsMultiple wireless senders and receivers create
additional problems (beyond multiple access):
AB
C
Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each
other => A, C unaware of their interference at B
A B C
A’s signalstrength
space
C’s signalstrength
Fading problem: B, A hear each other B, C hear each other A, C can not hear each
other as signal strength decreases => interference at B
Wireless, Mobile Networks 6-16
Code Division Multiple Access (CDMA)
uses: 3G mobile networks (UMTS, etc.) channel partitioning protocol unique “code” assigned to each user all users share same frequency, but each
user uses his own code to encode data=> allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
encoded signal = (original data) x (code) decoding: sum of product of encoded signal
and code divide time slot for transmitting a bit into
many “mini slots” view “0” are “-1”
Wireless, Mobile Networks 6-17
CDMA encode/decode
slot 1 slot 0
d1 = -1
1 1 1 1
1- 1- 1- 1-
Zi,m= di.cmd0 = 1
1 1 1 1
1- 1- 1- 1-
1 1 1 1
1- 1- 1- 1-
1 1 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutput
channel output Zi,m
sendercode
databits
slot 1 slot 0
d1 = -1d0 = 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 11
1-1- 1- 1-
slot 0channeloutput
slot 1channeloutputreceiver
code
receivedinput
Di = Zi,m.cmm=1
M
M8x1/8-8x1/8
Wireless, Mobile Networks 6-18
CDMA: two-sender interference
using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!
Sender 1
Sender 2
channel sums together transmissions by sender 1 and 2
(-2-2-2-2)/8 (2+2+2+2)/8
Wireless, Mobile Networks 6-19
CDMA: Code Division MADrawbacks:-Synchronization of all users is required-Multipath propagation: delayed copies of signal may be received which are not orthogonal any longer!-Near-far problem: nearby users will completely swamp far away users (signal of users far away cannot be received correctly) => accurate power control needed -having J more codes, the bandwidth must J times largerMain advantage:- when using FDMA or TDMA for cellular systems, frequencies used in a cell cannot be reused in adjacent cells (to avoid interference); in CDMA one can reuse the frequencies
more details:A.Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison Wesley, 1995.
Wireless, Mobile Networks 6-20
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet Access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Summary
Wireless, Mobile Networks 6-21
IEEE 802.11 Wireless LAN different standards (802.11a/b/g/n) differ in modulation techniques used (=>
different maximal transmission rate) and in frequency range (2.4 or 5 GHz)
all use CSMA/CA (carrier sense multiple access with collision avoidance)
all have base-station and ad-hoc network versions
Wireless, Mobile Networks 6-22
802.11 LAN architecture wireless host
communicates with base station base station = access
point (AP) Basic Service Set (BSS)
contains: wireless hosts access point (AP):
base station ad hoc mode: hosts
only
BSS 1
BSS 2
Internet
hub, switchor router
Wireless, Mobile Networks 6-23
802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11
channels at different (partly overlapping) frequencies admin chooses frequency for AP interference possible: channels can be same or
overlapping host: must associate with an AP
scans channels, listening for beacon frames (beacon = “Blinklicht”) containing AP’s name (service set identifier = SSID) and MAC address
selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP’s
subnet
Wireless, Mobile Networks 6-24
802.11: passive/active scanning
AP 2AP 1
H1
BSS 2BSS 1
1
23
1
passive scanning: (1)beacon frames sent from APs(2)association Request frame
sent: H1 to selected AP (3)association Response frame
sent from selected AP to H1
AP 2AP 1
H1
BSS 2BSS 1
122
3 4
active scanning: (1) Probe Request frame broadcast
from H1(2) Probe Response frames sent
from APs(3) Association Request frame sent:
H1 to selected AP (4) Association Response frame sent
from selected AP to H1
Wireless, Mobile Networks 6-25
IEEE 802.11: multiple access random access protocol as for Ethernet (carrier sense
MAC) sense before transmitting
refrain from transmitting when channel is sensed busy no collision detection during transmission!
difficult to receive (sense collisions) when transmitting due to weak received signals that gets overwhelmed by sending signal
can’t sense all collisions in any case: hidden terminal, fading => once transmission is running, it is never aborted (as opposed
to Ethernet when collision is detected)
optional feature: try to avoid collisions when several nodes are transmitting at the same time goal: avoid collisions: CSMA/C(ollision)A(voidance)
Wireless, Mobile Networks 6-26
IEEE 802.11 MAC Protocol: CSMA/CA SIFS= Short Inter-frame Spacing
Time required for a receiving station to sense the end of a frame and start transmitting an ACK (compute CRC etc.)
DIFS= Distributed Inter-frame Spacing sender waits before transmission If the medium is continuously idle
for this duration, only then a node is supposed to transmit a frame.
DIFS = SIFS + (2 * Slot time) where slot time is 20μs in 802.11b
sender receiver
DIFS
data
SIFS
ACK
Wireless, Mobile Networks 6-27
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for time period 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 (binary exponential), repeat step 2
802.11 receiver
- if frame received OK
return ACK after time period SIFS (ACK needed due to hidden terminal problem)
sender receiver
DIFS
data
SIFS
ACK
Wireless, Mobile Networks 6-28
IEEE 802.11 MAC Protocol: CSMA
sender receiver
DIFS
data
SIFS
ACK
• Why not send directly when channel is sensed idle? Assume three senders, one is transmitting and finishes => other senders both start sending => collision
• With random backoff time, one of the senders might send directly while the other waits
• When do collisions occur: same backoff time chosen or senders are hidden from each other (can’t sense whether channel idle)
Wireless, Mobile Networks 6-29
decrement by one slot time
Wireless, Mobile Networks 6-30
IEEE 802.11 MAC Protocol: CSMA
source: IEEE Std 802.11TM-2012 (Revision of IEEE Std 802.11-2007)
Wireless, Mobile Networks 6-31
Optional avoidance mechanism
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 CTS heard by all nodes
sender transmits data frame other stations defer transmissions
avoid data frame collisions completely using small reservation packets!
Wireless, Mobile Networks 6-32
Collision Avoidance: RTS-CTS exchange
APA B
time
RTS(A)RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
Wireless, Mobile Networks 6-33
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 frame: four address fields!
Address 2: MAC addressof wireless host or AP transmitting this frame(sender: wireless host or AP)
Address 1: MAC addressof wireless host or AP to receive this frame (destination within subnet)
Address 3: MAC addressof router interface to which AP is attached (=> send to Internet; see example on next slide)
Address 4: used only in ad hoc mode
bytes
Wireless, Mobile Networks 6-34
InternetrouterH1 R1
H1 MAC addr AP MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
H1 MAC addr R1 MAC addr
dest. address source address
802.3 Ethernet frame
802.11 frame: addressing
from the router’s perspective, the AP is invisible (ethernet subnet!)now, H1 knows MAC
adress of source router
Wireless, Mobile Networks 6-35
InternetrouterH1 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 Ethernet frame
802.11 frame: addressing
address 3 allows theAP to determinecorrect destinationMAC address
framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
expected duration of plannedtransmission (including ACKs)when RTS, CTS or data is sent
frame seq #(since frames may beretransmitted if not ACKed)
frame type(RTS, CTS, ACK, data)
802.11 frame: more
frame sent by access point
detailed specification: http://standards.ieee.org/about/get/802/802.11.html
Wireless, Mobile Networks 6-37
802.11: mobility within same subnet
How does switch know 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 BSS 2BSS 1
H1 detects weakening signal from AP1scans and finds beacon frames of AP2 dissociates with AP1, associates with AP2remains in same IP subnet: keep IP address and all ongoing TCP connections
• better solutions are currently being developed (so that switch is informed when H1 disassociates/associates)
Wireless, Mobile Networks 6-38
802.11: advanced capabilitiesRate adaptation base station, mobile
dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
2 unack’d frames in a row => next lower modul. scheme
10 frames ack’d in a row => next higher modul. scheme
QAM256 QAM16
BPSK
10 20 30 40SNR(dB)
BE
R
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
Wireless, Mobile Networks 6-39
power management node-to-AP: “I am going to sleep until next
beacon frame” (power manag. bit in header) AP knows not to transmit frames to this node
(buffers frames!) node wakes up before next beacon frame (every
100 msec) beacon frame: contains list of mobiles with
buffered AP-to-mobile frames node will stay awake if AP-to-mobile frames to be
sent (=> has to send request to AP) otherwise sleep again until next beacon frame
802.11: advanced capabilities
Wireless, Mobile Networks 6-40
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Summary
Wireless, Mobile Networks 6-41
Mobile Switching
Center
Public telephonenetwork
Mobile Switching
Center
Components of cellular network architecture
connects cells to wired tel. net. manages 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
Wireless, Mobile Networks 6-42
Cellular networks: the first hop
Two techniques for sharing mobile-to-BS radio spectrum
(1) combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots (used in 2G systems)
(Having F different sub-bands and T time slots, how many simultaneous calls can be supported?)
(2) CDMA: code division multiple access (used in 3G UMTS)
frequencybands
time slots
Wireless, Mobile Networks 6-43
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet Access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Summary
Wireless, Mobile Networks 6-44
Various degrees of mobility? 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 (e.g. cellular access in an ICE)
mobile user, connecting/ disconnecting from network using DHCP (e.g. moving to different room).
wide area network
Wireless, Mobile Networks 6-45
Mobility: vocabularyhome network: permanent “home” of mobile(e.g., 128.119.40/24)
permanent address: address in home network, can always be used to reach mobilee.g., 128.119.40.186
home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote
Wireless, Mobile Networks 6-46
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
Wireless, Mobile Networks 6-47
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?
Wireless, Mobile Networks 6-48
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
search all phone books
call her parents
expect her to let you know
Wireless, Mobile Networks 6-49
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
not scalable
to millions of mobiles
Mobility: approaches
wide area network
Wireless, Mobile Networks 6-50
Mobility: registration
end result: foreign agent knows about mobile home agent knows location of mobile
home networkvisited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent: “this mobile is resident in my network”
Wireless, Mobile Networks 6-51
Mobility via indirect routing
wide area network
homenetwork
visitednetwork
3
2
41
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
correspondentsource address is
permanent address
=> usage: see Mobile IP (later)
Wireless, Mobile Networks 6-52
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!
Wireless, Mobile Networks 6-53
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
inefficient when correspondent and mobile are in same network
1 23
4
Wireless, Mobile Networks 6-54
Mobility via direct routing
homenetwork
visitednetwork
correspondent requests, receives foreign address of mobile
correspondent forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent
used for routing telephone calls to mobile users in several mobile telephone network standards, including GSM
Wireless, Mobile Networks 6-55
Mobility via direct routing: comments
more efficient if correspondent is in same or ”close” network
non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited network? how to inform correspondent?
1 23
4
Wireless, Mobile Networks 6-56
wide area network
1
foreign net visited at session start
anchorforeignagent
2
4
new foreignagent
3
correspondentagent
correspondent
new foreignnetwork
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) requires additional coordination among the foreign
agents
5
Wireless, Mobile Networks 6-57
Chapter 6 outline
6.1 Introduction
Wireless6.2 Wireless links,
characteristics CDMA
6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)
6.4 Cellular Internet Access
Mobility6.5 Principles:
addressing and routing to mobile users
6.6 Mobile IP6.7 Summary
Wireless, Mobile Networks 6-58
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 packetRFC 5944
Wireless, Mobile Networks 6-59
Mobile IP: agent discovery
agent advertisement: foreign/home agents advertise service for mobile nodes by broadcasting ICMP messages (type field = 9)=> mobile node gets care-of-address
Wireless, Mobile Networks 6-60
Mobile IP: registration example
visited network: 79.129.13/24home agent
HA: 128.119.40.7foreign agentCOA: 79.129.13.2
mobile agentMA: 128.119.40.186
registration req. COA: 79.129.13.2
HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification:714….
registration reply HA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714 ….
registration reply HA: 128.119.40.7
MA: 128.119.40.186Lifetime: 4999Identification: 714….
time
ICMP agent adv.COA:
79.129.13.2….
registration req. COA: 79.129.13.2HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification: 714 ….
now, FA knowsthat it should look for datagrams with COA and forward them decapsulated
more details:RFC 5944
Current state of Mobile IPnot widely deployed yet because:need for seamless handover is given for few applications only (e.g. VoIP, video conferencing)handover is nice to have, re-connection is mainly sufficientGSM (telephone) networks have their own mobility solutions (indirect routing; phone number is permanent, roaming number is COA)deployed in WiMAX networks (IEEE 802.16)
Wireless, Mobile Networks 6-61
Wireless, Mobile Networks 6-62
Chapter 6 summary
Wireless wireless links:
capacity, distance channel impairments CDMA
IEEE 802.11 (“Wi-Fi”) CSMA/CA reflects
wireless channel characteristics
cellular access
Mobility principles: addressing,
routing to mobile users home, visited networks direct, indirect routing care-of-addresses
case studies mobile IP