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Chapter 7
Wireless and
Mobile Networks
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All material copyright 1996-2020
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A
Top-Down Approach
8th edition
Jim Kurose, Keith Ross Pearson, 2020
Wireless and Mobile Networks: context
§ more wireless (mobile) phone subscribers than fixed (wired) phone subscribers (10-to-1 in 2019)!
§ more mobile-broadband-connected devices than fixed-broadband- connected devices devices (5-1 in 2019)!
• 4G/5G cellular networks now embracing Internet protocol stack, including SDN
§ two important (but different) challenges
• wireless: communication over wireless link
• mobility: handling the mobile user who changes point of attachment to network
Wireless and Mobile Networks: 7-2
Chapter 7 outline
§ Introduction Wireless
§Wireless Links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Wireless and Mobile Networks: 7- 3
Elements of a wireless network
wired network infrastructure
Wireless and Mobile Networks: 7- 4
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Elements of a wireless network
wired network infrastructure
wireless hosts
§ laptop, smartphone, IoT
§ run applications
§ may be stationary (non-mobile) or mobile
• wireless does not always mean mobility!
Wireless and Mobile Networks: 7- 5
Elements of a wireless network
wired network infrastructure
base station
§ typically connected to wired network
§ relay – responsible for sending packets
between wired network and wireless host(s) in its “area”
• e.g., cell towers, 802.11 access points
Wireless and Mobile Networks: 7- 6
56
Elements of a wireless network
wired network infrastructure
wireless link
§ typically used to connect mobile(s) to base station, also used as backbone link
§ multiple access protocol coordinates link access
§ various transmission rates and distances, frequency bands
Wireless and Mobile Networks: 7- 7
Characteristics of selected wireless links
14 Gbps 10 Gbps
3.5 Gbps 600 Mbps
54 Mbps 11 Mbps 2 Mbps
802.11ax
802.11ac 802.11n
802.11g
802.11b Bluetooth
5G 802.11 af,ah
4G LTE
Indoor Outdoor 10-30m 50-200m
Midrange outdoor 200m-4Km
Long range outdoor 4Km-15Km
Wireless and Mobile Networks: 7- 8
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Elements of a wireless network
wired network infrastructure
infrastructure mode
§ base station connects mobiles into wired network
§ handoff: mobile changes base station providing connection into wired network
Wireless and Mobile Networks: 7- 9
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
Wireless and Mobile Networks: 7- 10
Wireless network taxonomy
single hop
multiple hops
infrastructure (e.g., APs)
no infrastructure
host connects to base station (WiFi, 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
Wireless and Mobile Networks: 7- 11
Chapter 7 outline
§ Introduction Wireless
§Wireless links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Link Layer: 6-12
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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: wireless network frequencies (e.g., 2.4 GHz) shared by many devices (e.g., WiFi, cellular, motors): interference
§ multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times
…. make communication across (even a point to point) wireless link much more “difficult”
Wireless and Mobile Networks: 7- 13
Wireless link characteristics (2)
§ SNR: signal-to-noise ratio
• larger SNR – easier to extract signal
from noise (a “good thing”)
§ SNR versus BER tradeoffs
• given physical layer: increase power ->
10-1 10-2 10-3 10-4 10-5
increase SNR->decrease BER 10-6
• given SNR: choose physical layer that meets BER requirement, giving highest throughput
• SNR may change with mobility:
dynamically adapt physical layer (modulation technique, rate)
10-7
10 20 30 40
SNR(dB)
QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps)
Wireless and Mobile Networks: 7- 14
13 14
Wireless link characteristics (3)
Multiple wireless senders, receivers create additional problems (beyond
multiple access):
A
ABC
C
B
A’s signal strength
C’s signal strength
Hidden terminal problem
§ B, A hear each other
§ B, C hear each other
§ A, C can not hear each other means A, C unaware of their interference at B
Signal attenuation:
space
§ B, A hear each other
§ B, C hear each other
§ A, C can not hear each other
interfering at B
Wireless and Mobile Networks: 7- 15
Code Division Multiple Access (CDMA)
§ 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
• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
§ encoding: inner product: (original data) X (chipping sequence)
§ decoding: summed inner-product: (encoded data) X (chipping sequence)
Wireless and Mobile Networks: 7- 16
15 16
4
BER
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CDMA encode/decode
Zi,m= di.cm
channel output Zi,m
data
bits d1 = -1
code 1 1 1 1 -1
d0 = 1
1 1 1 1 1 1 1 1
sender
receiver
-1
-1 -1 -1
slot 1
channel output
M
-1
-1 -1 -1
slot 1
slot 0
1 111111 1
-1 -1 -1
-1
-1 -1 -1
1 1 1
1
slot 0 channel output
d1 = -1
slot 1 channel output
M
Di = S Zi,m.cm m=1
received input
-1 -1 -1
-1 1
-1 1 1 1
-1
-1 -1 -1
-1 -1 -1
d0 = 1
slot 0 channel output
… but this isn’t really useful yet!
code 1 1 1
slot 1
1
-1
-1 -1 -1
slot 0
Wireless and Mobile Networks: 7- 17
CDMA: two-sender interference
Sender 1 Sender 2
channel sums together transmissions by sender 1 and 2
using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!
… now that’s useful!
Wireless and Mobile Networks: 7- 18
Chapter 7 outline
§ Introduction Wireless
§Wireless links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Link Layer: 6-19
IEEE 802.11 Wireless LAN
IEEE 802.11 standard
Year
Max data rate
Range
Frequency
802.11b
1999
11 Mbps
30 m
2.4 Ghz
802.11g
2003
54 Mbps
30m
2.4 Ghz
802.11n (WiFi 4)
2009
600
70m
2.4, 5 Ghz
802.11ac (WiFi 5)
2013
3.47Gpbs
70m
5 Ghz
802.11ax (WiFi 6)
2020 (exp.)
14 Gbps
70m
2.4, 5 Ghz
802.11af
2014
35 – 560 Mbps
1 Km
unused TV bands (54-790 MHz)
802.11ah
2017
347Mbps
1 Km
900 Mhz
§ all use CSMA/CA for multiple access, and have base-station and ad-hoc network versions
Wireless and Mobile Networks: 7- 20
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5
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802.11 LAN architecture
BSS 1
Internet
switch or router
BSS 2
§ wireless host communicates with base station
• base station = access point (AP) § Basic Service Set (BSS) (aka “cell”)
in infrastructure mode contains: • wireless hosts
• access point (AP): base station • ad hoc mode: hosts only
Wireless and Mobile Networks: 7- 21
802.11: Channels, association
§ spectrum divided into channels at different frequencies • AP admin chooses frequency for AP
• interference possible: channel can be same as that chosen by
neighboring AP!
§ arriving 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
• then may perform authentication [Chapter 8]
• then typically run DHCP to get IP address in
AP’s subnet
BSS
Wireless and Mobile Networks: 7- 22
802.11: passive/active scanning
BBS 1
AP1 1
BBS 2 BBS 1 BBS 2
1 AP2 AP1 2 1 2 AP2
23 34
H1
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
H1
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
selected AP to H1
Wireless and Mobile Networks: 7- 23
IEEE 802.11: multiple access
§ avoid collisions: 2+ nodes transmitting at same time § 802.11: CSMA – sense before transmitting
• don’tcollidewithdetectedongoingtransmissionbyanothernode
§ 802.11: no collision detection!
• difficulttosensecollisions:hightransmittingsignal,weakreceivedsignal
due to fading
• can’tsenseallcollisionsinanycase:hiddenterminal,fading
• goal:avoidcollisions:CSMA/CollisionAvoidance
C A
ABC
B
A’s signal strength
C’s signal strength
space
Wireless and Mobile Networks: 7- 24
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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 DIFS
receiver
SIFS
data
ACK
Wireless and Mobile Networks: 7- 25
Avoiding collisions (more)
idea: sender “reserves” channel use for data frames using small reservation packets
§ sender first transmits small request-to-send (RTS) packet 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
Wireless and Mobile Networks: 7- 26
25 26
Collision Avoidance: RTS-CTS exchange
time
A AP B
DATA (A)
defer
reservation collision
Wireless and Mobile Networks: 7- 27
802.11 frame: addressing
2 2 6 6 6 2 6 0-2312 4
frame control
duration
address 1
address 2
address 3
seq control
address 4
payload
CRC
Address 1: MAC address of wireless host or AP to receive this frame
Address 2: MAC address of wireless host or AP transmitting this frame
Address 4: used only in ad hoc mode
Address 3: MAC address of router interface to which AP is attached
Wireless and Mobile Networks: 7- 28
27 28
7
RTS(B)
RTS(A) RTS(A)
CTS(A)
ACK(A)
CTS(A)
ACK(A)
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802.11 frame: addressing
Internet
H1
address 1
802.11WiFi frame
R1
address 3
MAC dest addr
802.3 Ethernet frame
MAC source addr
Wireless and Mobile Networks: 7- 29
R1 MAC addr
H2 MAC addr
AP MAC addr
H1 MAC addr
R1 MAC addr
address 2
802.11 frame: addressing
duration of reserved frame sequence # (for reliable data transmission time (RTS/CTS) transfer)
2 2 6 6 6 2 6 0-2312 4
22411111111
frame d control
uration
address 1
address 2
address 3
seq control
address 4
payload
CRC
protocol version
type
subtype
to AP
from AP
more frag
retry
power mgt
more data
WEP
rsvd
frame type (RTS, CTS, ACK, data)
Wireless and Mobile Networks: 7- 30
29 30
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 (Ch. 6): switch will see frame from H1 and “remember” which switch port can be used to reach H1
H1
BBS 2
Wireless and Mobile Networks: 7- 31
BBS 1
802.11: advanced capabilities
Rate adaptation
§ base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
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
10-1 10-2 10-3 10-4 10-5 10-6 10-7
10 20 30 40 SNR(dB)
QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point
Wireless and Mobile Networks: 7- 32
31 32
8
BER
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802.11: advanced capabilities
power management
§ 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
§ 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
Wireless and Mobile Networks: 7- 33
Personal area networks: Bluetooth
§less than 10 m diameter §replacement for cables (mouse,
keyboard, headphones)
§ad hoc: no infrastructure
§ 2.4-2.5 GHz ISM radio band, up to 3 Mbps
§master controller / clients devices:
• master polls clients, grants requests for client transmissions
P P
C
M
radius of coverage
CP
P
M master device
C client device
P parked device (inactive)
Wireless and Mobile Networks: 7- 34
C
Personal area networks: Bluetooth
§TDM, 625 μsec sec. slot
§ FDM: sender uses 79 frequency
channels in known, pseudo-random order slot-to-slot (spread spectrum)
• other devices/equipment not in piconet only interfere in some slots
§ parked mode: clients can “go to sleep” (park) and later wakeup (to preserve battery)
§ bootstrapping: nodes self-assemble (plug and play) into piconet
C
P
P
radius of coverage
C
M C
P
M
P C P
master device
client device
parked device (inactive)
Wireless and Mobile Networks: 7- 35
Chapter 7 outline
§ Introduction Wireless
§Wireless links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Link Layer: 6-36
35 36
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4G/5G cellular networks
§ the solution for wide-area mobile Internet § widespread deployment/use:
• more mobile-broadband-connected devices than fixed- broadband-connected devices devices (5-1 in 2019)!
• 4G availability: 97% of time in Korea (90% in US)
§ transmission rates up to 100’s Mbps
§ technical standards: 3rd Generation Partnership Project (3GPP)
• wwww.3gpp.org
• 4G: Long-Term Evolution (LTE)standard
Wireless and Mobile Networks: 7- 37
4G/5G cellular networks
similarities to wired Internet
§ edge/core distinction, but both
below to same carrier
§ global cellular network: a
network of networks
§ widespread use of protocols we’ve studied: HTTP, DNS, TCP, UDP, IP, NAT, separation of data/control planes, SDN, Ethernet, tunneling
§ interconnected to wired Internet
differences from wired Internet § different wireless link layer
§ mobility as a 1st class service § user “identity” (via SIM card) § business model: users
subscribe to a cellular provider
• strongnotionof“homenetwork”
versus roaming on visited nets
• globalaccess,withauthentication
infrastructure, and inter-carrier settlements
Wireless and Mobile Networks: 7- 38
37 38
Elements of 4G LTE architecture
Mobile device:
§ smartphone, tablet, laptop, IoT, … with 4G LTE radio
§ 64-bit International Mobile Subscriber Identity (IMSI), stored on SIM (Subscriber Identity Module) card
§ LTE jargon: User Equipment (UE)
Mobile device (UE)
Base station
(eNode-B)
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
to Internet
PDN gateway (P-GW)
radio access network
all-IP Enhanced Packet Core (EPC)
Wireless and Mobile Networks: 7- 39
…
Elements of 4G LTE architecture
Base station:
§ at “edge” of carrier’s network
§ manages wireless radio
resources, mobile devices in its coverage area (“cell”)
§ coordinates device authentication with other elements
§ similar to WiFi AP but:
• activeroleinusermobility
• coordinateswithnearlybase
stations to optimize radio use § LTE jargon: eNode-B
Mobile device (UE)
Base station
(eNode-B)
…
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
to Internet
PDN gateway (P-GW)
Wireless and Mobile Networks: 7- 40
39 40
10
Elements of 4G LTE architecture
Home Subscriber Service
§ stores info about mobile devices for which the HSS’s network is their “home network”
§ works with MME in device authentication
Mobile device (UE)
Base station
(eNode-B)
…
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
to Internet
PDN gateway (P-GW)
Wireless and Mobile Networks: 7- 41
Elements of 4G LTE architecture
Serving Gateway (S-GW), PDN Gateway (P-GW)
§ lie on data path from mobile to/from Internet
§ P-GW
• gateway to mobile cellular
network
• Looks like nay other
internet gateway router • provides NAT services
§ other routers:
• extensive use of tunneling
Mobile device (UE)
Base station
(eNode-B)
…
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
to Internet
PDN gateway (P-GW)
Wireless and Mobile Networks: 7- 42
41 42
Elements of 4G LTE architecture
Mobility Management Entity
§ device authentication (device-to-network, network- to-device) coordinated with mobile home network HSS
Mobile device (UE)
Base station
(eNode-B)
…
Mobility Management Entity (MME)
Serving Gateway (S-GW)
Home Subscriber Service (HSS)
to Internet
PDN gateway (P-GW)
Wireless and Mobile Networks: 7- 43
§ mobile device management:
• device handover between cells • tracking/paging device location
§ path (tunneling) setup from mobile device to P-GW
LTE: data plane control plane separation
HSS
control plane
§ new protocols for mobility management , security, authentication (later)
data plane
§ new protocols at link, physical layers
§ extensive use of tunneling to facilitate mobility
Wireless and Mobile Networks: 7- 44
base station
base station
MME
S-GW
S-GW
IP tunnels
P-GW
P-GW
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11
LTE data plane protocol stack: first hop
LTE link layer protocols:
§ Packet Data Convergence: header compression, encryption
§ Radio Link Control (RLC) Protocol: fragmentation/reassembly, reliable data transfer
§ Medium Access: requesting, use of radio transmission slots
data plane
Wireless and Mobile Networks: 7- 45
Application
Transport
IP
Packet Data Convergence
Radio Link
Medium Access
Physical
IP
Packet Data Convergence
Radio Link
Medium Access
Physical
base station
S-GW P-GW
LTE data plane protocol stack: first hop
Application
Transport
IP
Packet Data Convergence
Radio Link
Medium Access
Physical
IP
Packet Data Convergence
Radio Link
Medium Access
Physical
base station
LTE radio access network:
§ downstream channel: FDM, TDM within frequency channel (OFDM – orthogonal frequency division multiplexing)
• “orthogonal”: minimal interference between channels
• upstream: FDM, TDM similar to OFDM
§ each active mobile device allocated two or more 0.5 ms time slots over 12 frequencies
• schedulingalgorithmnotstandardized–up to operator
• 100’sMbpsperdevicepossible
Wireless and Mobile Networks: 7- 46
45 46
LTE data plane protocol stack: packet core
GTP-U
tunneling:
§
§ §
mobile datagram encapsulated using GPRS Tunneling Protocol (GTP), sent inside UDP datagram to S-GW
S-GW re-tunnels datagrams to P-GW
supporting mobility: only tunneling endpoints change when mobile user moves
Wireless and Mobile Networks: 7- 47
GTP-U
GTP-U
UDP
UDP
IP
IP
link
link
Physical
Physical
UDP
IP
Packet Data Convergence
IP
link
Radio Link
Medium Access
Physical
Physical
\
base station
S-GW P-GW
LTE data plane: associating with a BS
1
2
3
data plane
S-GW P-GW
base station
1 BS broadcasts primary synch signal every 5 ms on all frequencies
§ BSs from multiple carriers may be broadcasting synch signals
2 mobile finds a primary synch signal, then locates 2nd synch signal on this freq.
§ mobile then finds info broadcast by BS: channel bandwidth, configurations; BS’s cellular carrier info
§ mobile may get info from multiple base stations, multiple cellular networks
3 mobile selects which BS to associate with (e.g., preference for home carrier)
4 more steps still needed to authenticate, establish state, set up data plane
Wireless and Mobile Networks: 7- 48
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12
Link
Link
49 50
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LTE mobiles: sleep modes
ZZZZ…
data plane
as in WiFi, Bluetooth: LTE mobile may put radio to “sleep” to conserve battery:
§ light sleep: after 100’s msec of inactivity
§ wake up periodically (100’s msec) to check for downstream transmissions § deep sleep: after 5-10 secs of inactivity
§ mobile may change cells while deep sleeping – need to re-establish association Wireless and Mobile Networks: 7- 49
Global cellular network: a network of IP networks
Home Subscriber Server
home mobile
carrier network P-GW
public Internet and inter-carrier IPX
P-GW
visited mobile carrier network
home network HSS:
§ identify & services info, while in home network and roaming
all IP:
§ carriers interconnect with each other, and public internet at exchange points
§ legacy 2G, 3G: not all IP, handled otherwise
Wireless and Mobile Networks: 7- 50
in home network
SIM card: global identify info in home network
roaming in visited network
On to 5G!
§ goal: 10x increase in peak bitrate, 10x decrease in latency, 100x increase in traffic capacity over 4G
§ 5G NR (new radio):
§ two frequency bands: FR1 (450 MHz–6 GHz) and FR2 (24 GHz–52 GHz): millimeter wave
frequencies
§ not backwards-compatible with 4G
§ MIMO: multiple directional antennae
§ millimeter wave frequencies: much higher data rates, but over shorter distances
§ pico-cells: cells diameters: 10-100 m
§ massive, dense deployment of new base stations required
Wireless and Mobile Networks: 7- 51
Chapter 7 outline
§ Introduction Wireless
§Wireless links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Link Layer: 6-52
51 52
13
…
…
…
53 54
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What is mobility?
§ spectrum of mobility, from the network perspective:
no mobility
device moves between networks, but powers down while moving
device moves within same AP in one provider network
device moves among APs in one provider network
high mobility
device moves among multiple provider networks, while maintaining ongoing connections
Wireless and Mobile Networks: 7- 53
We’re interested in these!
Mobility approaches
§ let network (routers) handle it:
• routers advertise well-known name, address (e.g., permanent 32-
bit IP address), or number (e.g., cell #) of visiting mobile node via
usual routing table exchange
• Internet routing could do this already with no changes! Routing
tables indicate where each mobile located via longest prefix match!
Wireless and Mobile Networks: 7- 54
Mobility approaches
§ let network (routers) handle it:
• routers advertise well-known noat me, address (e.g., permanent 32-
bit IP address), or number (e.g., cell #) of visiting mobile node via
mobiles
• Internet routing could do this already with no changes! Routing tables indicate where each mobile located via longest prefix match!
§ let end-systems handle it: functionality at the “edge”
• indirect routing: communication from correspondent to mobile
goes through home network, then forwarded to remote mobile
• direct routing: correspondent gets foreign address of mobile, send directly to mobile
Wireless and Mobile Networks: 7- 55
usual routing table exchange
scalable
to billions of
Contacting a mobile friend:
Consider friend frequently changing locations, how do you find him/her?
§search all phone books? §expect her to let you know
where he/she is?
§call his/her parents? § Facebook!
The importance of having a “home”:
§ a definitive source of information about you
§ a place where people can find out where you are
I wonder where Alice moved to?
Wireless and Mobile Networks: 7- 56
55 56
14
Home network, visited network: 4G/5G
Home Subscriber Server
home mobile
carrier network P-GW
public Internet and inter-carrier IPX
P-GW
visited mobile carrier network
home network:
§ (paid) service plan with cellular provider, e.g., Verizon, Orange
§ home network HSS stores identify & services info
visited network:
§ any network other than your home network
§ service agreement with
other networks: to provide access to visiting mobile
Wireless and Mobile Networks: 7- 57
in home network
SIM card: global identify info including home
roaming in network visited network
Home network, visited network: ISP/WiFi
ISP/WiFi: no notion of global “home”
attach
authentication access
server
attach
public Internet
authentication
access server
§ credentials from ISP (e.g., username, password) stored on device or with user
§ ISPs may have national, international presence
§ different networks: different credentials
• someexceptions(e.g., eduroam)
• architecturesexist(mobile IP) for 4G-like mobility, but not used
Wireless and Mobile Networks: 7- 58
57 58
Home network, visited network: generic
Home Network
e.g.,: 128.119/16
Visited Network
e.g.,: 79.129/16
Permanent IP: 128.119.40.186
Home Subscriber Server
public or private Internet
Correspondent
NAT IP:
10.0.0.99 Mobility
IMSI 78:4f:43:98:d9:27
IMSI manager 78:4f:43:98:d9:27
Mobility
Visited network gateway
manager Home network
Home gateway gateway
Wireless and Mobile Networks: 7- 59
Registration: home needs to know where you are!
Home Network
e.g.,: 128.119/16
end result:
Visited Network
Permanent IP: 128.119.40.186
Home Subscriber Server
public or private Internet
1
e.g.,: 79.129/16
Mobility manager
mobile associates with visited mobility manager
visited mobility manager registers mobile’s location with home HSS
Wireless and Mobile Networks: 7- 60
IMSI 78:4f:43:98:d9:27
2
NAT IP: 10.0.0.99
IMSI 78:4f:43:98:d9:27
Mobility
Visited network gateway
manager Home network
Home gateway gateway
§ visited mobility manager knows about mobile § home HSS knows location of mobile
59 60
3/4/21
15
…
…
…
…
…
…
Mobility with indirect routing
Home Network
e.g.,: 128.119/16
Visited Network
e.g.,: 79.129/16
Permanent IP: 128.119.40.186
Home Subscriber Server
2
4a
public or private
NAT IP:
10.0.0.99 3
Mobility manager
IMSI 78:4f:43:98:d9:27
IMSI 78:4f:43:98:d9:27
visited gateway router forwards to mobile
Mobility
Visited network gateway
manager Home network
home gateway receives datagram, forwards (tunnels) to remote gateway
correspondent uses home address as datagram destination address
Home gateway gateway
1
Internet
4b
Correspondent
visited gateway router forwards reply to correspondent via home network (4a) or directly (4b)
Wireless and Mobile Networks: 7- 61
Mobility with indirect routing: comments
§triangle routing:
• inefficient when correspondent
and mobile are in same network
§mobile moves among visited networks: transparent to correspondent!
• registers in new visited network
• new visited network registers with home HSS
• datagrams continue to be forwarded from home network to mobile
in new network
• on-going (e.g., TCP) connections between correspondent and mobile can be maintained!
Wireless and Mobile Networks: 7- 62
61 62
3/4/21
Mobility with direct routing
Home Network
e.g.,: 128.119/16
Visited Network
e.g.,: 79.129/16
Permanent IP: 128.119.40.186
Home Subscriber Server
2
public or private
NAT IP:
10.0.0.99 4
Mobility manager
IMSI 78:4f:43:98:d9:27
IMSI 78:4f:43:98:d9:27
visited gateway router forwards to mobile
Mobility manager
Home gateway
Visited network gateway
3
correspondent contacts home HSS, gets mobile’s visited network
1
Internet
Correspondent
Correspondent addresses datagram to visited network address
Wireless and Mobile Networks: 7- 63
Mobility with direct routing: comments
§ overcomes triangle routing inefficiencies
§ non-transparent to correspondent: correspondent must get care-of-
address from home agent
§ what if mobile changes visited network?
• can be handled, but with additional complexity
Wireless and Mobile Networks: 7- 64
63 64
16
Mobility in 4G networks: major mobility tasks
Home Subscriber Server
Home network
3
Internet
Mobility manager
MME
S-GW P-GW
2
base station
1 4
1
2
base station association: § covered earlier
§ mobile provides IMSI –
identifying itself, home network
control-plane configuration:
§ MME, home HSS establish control-plane state – mobile is in visited network
P-GW
4
Visited network
Streaming
server 3
mobile handover:
data-plane configuration:
§ MME configures forwarding tunnels for mobile § visited, home network establish tunnels from
home P-GW to mobile
§ mobile device changes its point of attachment to visited network
Wireless and Mobile Networks: 7- 66
65 66
Configuring LTE control-plane elements
Home Subscriber Server
Home
network P-GW
Mobility manager
MME 2
base station
P-GW S-GW Visited network
§Mobile communicates with local MME via BS control-plane channel §MME uses mobile’s IMSI info to contact mobile’s home HSS
• retrieve authentication, encryption, network service information
• home HHS knows mobile now resident in visited network
§BS, mobile select parameters for BS-mobile data-plane radio channel
Wireless and Mobile Networks: 7- 67
Configuring data-plane tunnels for mobile
§S-GW to BS tunnel: when Home
Mobility manager
MME
base station
S-GW
P-GW Visited network
mobile changes base stations, simply change endpoint IP address of tunnel
§S-GW to home P-GW tunnel: implementation of indirect routing
Subscriber Server
Home
network P-GW
Internet
Streaming server
§tunneling via GTP (GPRS tunneling protocol): mobile’s datagram to streaming server encapsulated using GTP inside UDP, inside datagram
Wireless and Mobile Networks: 7- 68
67 68
3/4/21
Chapter 7 outline
§ Introduction Wireless
§Wireless links and network characteristics
§WiFi: 802.11 wireless LANs §Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles
§ Mobility management: practice • 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Link Layer: 6-65
17
69 70
3/4/21
Handover between BSs in same cellular network
data path before hando S-GW
1
current (source) BS selects target BS, sends Handover Request message to target BS
target BS pre-allocates radio time slots, responds with HR ACK with info for mobile
ver
source BS
14
P-GW
data path after handover
2
target BS
3
2
MME
3 source BS informs mobile of new BS
§ mobile can now send via new BS – handover
looks complete to mobile
4
source BS stops sending datagrams to mobile, instead forwards to new BS (who forwards to mobile over radio channel)
Wireless and Mobile Networks: 7- 69
Handover between BSs in same cellular network
S-GW
5
source BS
1
2
4
3 7
5 target BS informs MME that it is new BS for mobile
§ MME instructs S-GW to change tunnel endpoint to be (new) target BS
P-GW
6 7
6
MME
5
target BS
target BS ACKs back to source BS: handover complete, source BS can release resources
mobile’s datagrams now flow through new tunnel from target BS to S-GW
Wireless and Mobile Networks: 7- 70
Mobile IP
§mobile IP architecture standardized ~20 years ago [RFC 5944]
• long before ubiquitous smartphones, 4G support for Internet protocols
• did not see wide deployment/use
• perhaps WiFi for Internet, and 2G/3G phones for voice were “good enough” at
the time
§mobile IP architecture:
• indirect routing to node (via home network) using tunnels
• mobile IP home agent: combined roles of 4G HSS and home P-GW
• mobile IP foreign agent: combined roles of 4G MME and S-GW
• protocols for agent discovery in visited network, registration of visited location in home network via ICMP extensions
Wireless and Mobile Networks: 7- 71
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 handover loss
• TCP interprets loss as congestion, will decrease congestion window un-
necessarily
• delay impairments for real-time traffic
• bandwidth a scare resource for wireless links
Wireless and Mobile Networks: 7- 72
71 72
18
3/4/21
Chapter 7 summary
Wireless
§Wireless Links and network characteristics §WiFi: 802.11 wireless LANs
§Cellular networks: 4G and 5G
Mobility
§ Mobility management: principles § Mobility management: practice
• 4G/5G networks
• Mobile IP
§ Mobility: impact on higher-layer protocols
Wireless and Mobile Networks: 7- 73
73
19