Download Mobile and wireless communication

89-850 Communication
Networks:
Wireless and Mobile
Communication Networks
Prof. Amir Herzberg
BIU, Dept. of CS
From ch.6 of Kurose and Ross,
3rd edition; and [KMK], ch. 8.
Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.
All material copyright 1996-2004
J.F Kurose and K.W. Ross, All Rights Reserved
6: Wireless and Mobile Networks
6-1
Background: Wireless and Mobile Networks
 # wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!
 Computer nets: laptops, palmtops, PDAs,
Internet-enabled phone promise anytime
untethered Internet access
 Internet telephony: a reality, an earthquake
 Two important (but different) challenges



Wireless link: no CD (e.g. hidden-terminal), reliability,
security
Mobility of computers and users; provisioning
Plus: limited computing power and energy
6: Wireless and Mobile Networks
6-2
Wireless and Mobile
Communication Networks: Outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 Sensor and personalarea networks
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-3
Elements of a wireless network
network
infrastructure
wireless hosts
 laptop, PDA, IP phone
 run applications
 may be stationary
(non-mobile) or mobile

wireless does not
always mean mobility
6: Wireless and Mobile Networks
6-4
Elements of a wireless network
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
6: Wireless and Mobile Networks
6-5
Wireless network characteristics
Lower Signal/Noise ratio (cf. wired networks)
Limited, shared spectrum: orthogonal signals
(FDMA/CDMA/TDMA) or `collisions as noise`
C
A
B
A
B
Hidden terminal problem
C
C’s signal
strength
A’s signal
strength
space
 B, A hear each other
Signal fading:
 A, C can not hear each other
 B, C hear each other
 B, C hear each other
 B, A hear each other
 A, C can not hear each other
interfering at B
6: Wireless and Mobile Networks
6-9
Wireless Link Characteristics
Differences from wired link ….
 Energy
and computing-power limitations
 Decreased signal strength
• Obstacles and hidden-terminal problem
• Collision detection hard or impossible
 More
noise
• Interference from other sources
• Multipath propagation  different delays
interferences between paths or (multipath) fading

Lower signal/noise  Higher bit error rate
…. more “difficult”
6: Wireless and Mobile Networks
6-10
Hi/Low BER States Model
 Wireless links often have two BER states
 High,
Low
 E.g., due to (multipath) fading
 Model by two-state Markov model:
(1g)
g
Good
Bad
b
(1b)
Simplify: all packets Ok in `Good`, fail in `Bad`
6: Wireless and Mobile Networks
6-11
Wireless and Mobile
Communication Networks: Outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
 Ad-hoc, sensor and
personal-area
networks
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-13
IEEE 802.11 Wireless LANs
802.11
Wireless
LANs
5-6 GHz
2.4-5 GHz
(unlicensed)
802.11b
lo-cost, good
propagation;
but slow,
interferences
Up to
11Mbps
Up to 54
Mbps
802.11a
802.11g
 All use CSMA/CA for multiple access
 All have base-station and ad-hoc network versions
6: Wireless and Mobile Networks
6-14
802.11 LAN: Infrastructure and
Ad-Hoc modes
 Infrastructure mode:
wireless host communicates
with base station
 base station = access point
(AP)
Internet
router

LAN switch
AP
BSS 1
DS

AP
Basic Service Set (BSS) (aka
“cell”) contains:
• wireless hosts (mobiles)
• access point (AP): base
station
Extended Service Set (ESS)
• One or more BSS
• Connect by LANswitch or DS
• DS=Distribution System
 Ad hoc mode: hosts only
• =Independent BSS (IBSS)
BSS 2
6: Wireless and Mobile Networks
6-16
802.11 Media Access Coordination (MAC)
 Point Coordination Function (PCF)
 Only in Infrastructure mode
 Access point coordinates transmissions
 Allows: bounded delay , QoS
 Distributed Coordination Function (DCF)
 Ad-hoc or Infrastructure mode
 All are peers
 Like Ethernet, uses CSMA: random access, carrier sense
 Unlike Ethernet: Ack, no Collision Detection
 Optional: use RTS/CTS (Request/Clear To Send)
 No bound on delay (starvation possible)
 Our focus
6: Wireless and Mobile Networks
6-18
IEEE 802.11 DCF
 Like Ethernet, uses CSMA:
 random access
 carrier sense: don’t collide with ongoing transmission
 Unlike Ethernet: Ack, no Collision Detection
 no collision detection – transmit all frames to completion
 ACK: to detect loss without collision detection
 Why 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
 And… loss may be due to (higher) error rate of wireless
 Goal: avoid collisions: CSMA/CA (Collision Avoidance)
6: Wireless and Mobile Networks
6-19
802.11 DCF MAC Protocol: CSMA/CA
[simplified]
802.11 sender
1 if sense channel idle then
sender
- transmit entire frame (no Colli. Detect)
2 if sense channel busy then
- start random backoff timer
- timer counts down while channel idle
- transmit when timer expires
receiver
data
802.11 receiver
if frame received OK
then return ACK else ignore (no NACK!)
SIFS: Short Inter-Frame Space – max time
to begin Ack [e.g., 16μsec in 802.11a]
ACK
6: Wireless and Mobile Networks
SIFS
(e.g.
16s)
6-20
802.11 DCF MAC Protocol: CSMA/CA
sender
802.11 sender (when trying to send)
1 if sense channel idle for DIFS then
DIFS
transmit entire frame (no CD)
[DIFS>SIFS+2Tprop for priority to ACK]
2 if sense channel busy then
- count down the backoff timer
- – but only while channel idle
- transmit when timer expires
receiver
-
- If ACK, reduce backoff range by 1
data
DIFS
- if no ACK, double backoff range, select
time randomly from range, repeat 2
ACK
SIFS
(e.g.
16s)
802.11 receiver
if frame received OK
- return ACK (within SIFS)
6: Wireless and Mobile Networks
6-21
802.11 DCF MAC Operation
Data Frames and their ACK
DIFS
Src
Data
SIFS
Ack
Dest
DIFS
Contention Window
Next MPDU
Other
Defer Access
Backoff after Defer
 Src found channel idle for DIFS  send data
 Other sender found channel busy
 Wait for DIFS idle time, then exponential backoff
delay [slot=Tprop]
6: Wireless and Mobile Networks
6-22
Two Additional Mechanisms in 802.11
 PCF (Point Coordination Function)
Polling to coordinate senders, e.g. to ensure QoS
 SIFS < PIFS < DIFS (priorities!)

 DCF with RTS/CTS mechanism
 Can’t detect collision while sending…
 Collision for long packet is wasteful
 RTS (Request to Send): request to reserve
channel to send long packet w/o collisions
 CTS (Clear to Send): approve RTS
 Optional mechanism
6: Wireless and Mobile Networks
6-23
RTS/CTS [optional in 802.11 MAC]
 Sender sends small request-to-send (RTS)
 RTSs
may collide with each other (but are short)
 Include indication of length of packet transmission
 AP broadcasts clear-to-send CTS in response to RTS
 CTS heard by all nodes
 sender transmits data frame
 other stations defer transmissions for time
specified in CTS
 Q: if you hear RTS only (no CTS), should you wait?
Avoid data frame collisions completely
using small reservation packets!
6: Wireless and Mobile Networks
6-24
Collision Avoidance: RTS-CTS exchange
A
B
AP
reservation collision
DATA (A)
defer
time
6: Wireless and Mobile Networks
6-25
Q: Defer on RTS, CTS or both?
 Idea 1: RTS contains length, defer till end
A
B
RTS
C
D
RTS
CTS
CTS
data
ACK
ACK
6: Wireless and Mobile Networks
6-26
Q: Defer on RTS, CTS or both?
 Idea 1: RTS contains length, defer till end

Problem: maybe not granted?
 Idea 2: defer only on CTS
A
RTS
B
C
RTS
CTS
D
RTS
CTS
data
ACK
ACK
6: Wireless and Mobile Networks
6-27
Q: Defer on RTS, CTS or both?
 Idea 1: RTS contains length, defer till end

Problem: maybe not granted?
 Idea 2: defer only on CTS
A
 What if unheard?
RTS
B
C
D
RTS
CTS
CTS
data
ACK
ACK
6: Wireless and Mobile Networks
6-28
Q: Defer on RTS, CTS or both?
 Idea 1: RTS contains length, defer till end
 Problem: maybe not granted?
 Idea 2: defer only on CTS
 What if unheard? A
 Solution:
 Defer by CTS
• By length in CTS
Defer by RTS…
But only 2 DIFS !
 Ok if A hears either
recipient or sender
RTS
B
C
D
RTS
CTS
CTS
data

ACK
ACK
6: Wireless and Mobile Networks
6-29
802.11 DCF MAC Example
1 ....
ACK
Data
time
2
time
3
RTS
ACK
Data
CTS
time
4
RTS
CTS
Data
ACK
time
node defers; backoff counter frozen
1
backoff period
RTS
time
2
RTS
time
3
RTS
CTS
Data
ACK
time
4
time
6: Wireless and Mobile Networks
6-30
802.11 addressing & `switching`
AP
R1 router
H1
Internet
AP
R1 MAC addr AP MAC addr
dest. address
source address
802.3 frame
AP identified in 802.11
frame
(Unlike regular switch!!)
AP MAC addr H1 MAC addr R1 MAC addr
address 1
address 2
address 3
802.11 frame
6: Wireless and Mobile Networks
6-31
802.11: mobility within same subnet
 H1 remains in same IP
subnet: IP address
can remain same
 Works fine for hub
 Switch: which AP is
associated with H1?
 self-learning
switch
will see frame from H1
and “remember” port
to reach H1
 Solution: when H1
joins, AP2 sends switch
a packet from H1
router
hub or
switch
BBS 1
AP 1
AP 2
H1
BBS 2
6: Wireless and Mobile Networks
6-34
MAC Management, Beacons and
Traffic Indication Map (TIM)
 802.11 has several MAC management frames
 (Re/De)Association req/response, Authentication…
 Beacon (sent periodically by AP)
 Timestamp, Beacon Interval, Capabilities, SSID,
Rates, Parameters, Traffic Indication Map (TIM)
 Allows host to select AP (or host can send probe)
 TIM: list of (associated but sleeping) hosts with
packets queued at the access point.
 Even sleeping hosts (sometimes) listen to Beacon
• To check incoming messages in TIM, get broadcasts
• Sleeping to save energy when idle
6: Wireless and Mobile Networks
6-35
Ad Hoc Networking
Ad hoc networks
 no base stations
 transmit to other nodes
within link coverage
 nodes organize
themselves into a
network: route among
themselves
 Supported in 802.11 but
still many open issues,
research
 WANET: Wireless AdHoc NETwork
 MANET: Mobile Ad-Hoc
Net (they move, too!)
6: Wireless and Mobile Networks
6-36
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

 Evolved from Bluetooth
P
S
P
radius of
coverage
M
S
P
S
P
M Master device
S Slave device
P Parked device (inactive)
6: Wireless and Mobile Networks
6-37
Sensor Networks
 A special interesting type of Ad-Hoc network…
 Idea: distribute low-cost `sensors` to perform
measurements, even do actions
 Applications:



Weather forecasts, natural disaster warnings
Detection of physical damages (leakage, fire,…)
Military applications: intelligence, smart mines
 Properties…
 Wireless
 Random location
 Low cost, energy
6: Wireless and Mobile Networks
6-38
Which Transmission Range?
 When using AP/Bases, nodes must reach it
 Large
transmission range
 But in sensor networks, WANET?
 Smaller transmission range
 Saves energy, allows spectrum reuse (cellular?)
 But: requires routing, forwarding by nodes
6: Wireless and Mobile Networks
6-39
Connectivity, Topology, Routing…
 Assume nodes distributed uniformly in area [?]
 One dimensional (line), two (surface), three (space)
 Let n be number of nodes
 Let r(n) be transmission range of node
 Questions:
 Probability that all/most nodes are connected
 Probability that (almost) entire area is `covered` by nodes
[connected to `base`/`edge`]
•  Number (and cost) of nodes

Routing, scheduling, broadcast protocols for nodes
• Using minimal resources (energy, storage)
• Minimize collisions
6: Wireless and Mobile Networks
6-40
Sensor Network Tasks/Protocols
 Routing, forwarding, broadcast
 Neighbor/topology discovery, organization
 E.g. setup spanning tree for efficient broadcast
 Optimization tasks
• Optimize communication
• Load balancing (also to save energy)
 Location measurement
 Clock synchronization
 Use to save energy (listen only once per interval)
 Distributed computation
 E.g. to detect image
 Handling mobility (MANET)
6: Wireless and Mobile Networks
6-41
Wireless and Mobile
Communication Networks: Outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-48
What is mobility?
Covered in [KR], not in [KMK]
 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)
6: Wireless and Mobile Networks
6-49
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
wide area
network
correspondent
6: Wireless and Mobile Networks
6-50
Mobility: more vocabulary
Permanent address: remains
constant (e.g., 128.119.40.186)
visited network: network
in which mobile currently
resides (e.g., 79.129.13/24)
Care-of-address: address
in visited network.
(e.g., 79.129.13.2)
wide area
network
correspondent: wants
to communicate with
mobile
Foreign agent: entity
in visited network
that performs
mobility functions on
behalf of mobile.
6: Wireless and Mobile Networks
6-51
How do you contact a mobile friend:
Consider friend frequently changing
addresses, how do you find her?
I wonder where
Alice moved to?
 search all phone
books?
 call her parents?
 expect her to let you
know where he/she is?
6: Wireless and Mobile Networks
6-52
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
 `breaks` routing (aggregation), allows MITM
 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
6: Wireless and Mobile Networks
6-53
Mobility: approaches
 Let routing handle it: routers advertise permanent
Breaks
not
address
via usual
Allowsof mobile-nodes-in-residence
route
scalable
routing
table
exchange.
MITM
aggregation
to millions of
attacks
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 care-ofaddress of mobile, sends directly to mobile

6: Wireless and Mobile Networks
6-54
Mobility: registration
visited network
home network
1
2
wide area
network
foreign agent contacts home
agent home: “this mobile is
resident in my network”
mobile contacts
foreign agent on
entering visited
network
End result:
 Foreign agent knows about mobile
 Home agent knows location of mobile
6: Wireless and Mobile Networks
6-55
Mobility via Indirect Routing
foreign agent
receives packets,
forwards to mobile
home agent intercepts
packets, forwards to
foreign agent
home
network
visited
network
3
wide area
network
correspondent
addresses packets
using home address
of mobile
1
2
4
mobile replies
directly to
correspondent
6: Wireless and Mobile Networks
6-56
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 often done by mobile itself
 triangle routing: correspondent-home-networkmobile
 inefficient when
correspondent, mobile
are in same network

6: Wireless and Mobile Networks
6-57
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: ongoing connections can be
maintained!
6: Wireless and Mobile Networks
6-58
Mobility via Direct Routing
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
home
network
4
wide area
network
2
correspondent
requests, receives
foreign address of
mobile
visited
network
1
3
4
mobile replies
directly to
correspondent
6: Wireless and Mobile Networks
6-59
Mobility via Direct Routing: comments
 overcome triangle routing problem
 non-transparent to correspondent:
correspondent must get care-of-address
from home agent

what if mobile changes visited network?
6: Wireless and Mobile Networks
6-60
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)
foreign net visited
at session start
wide area
network
anchor
foreign
agent
1
2
4
5
correspondent
agent
correspondent
3
new foreign
agent
new
foreign
network
6: Wireless and Mobile Networks
6-61
Response (mobilecorresponding)
 Triangle routing: mobile  corresponding
 Using mobile host’s `home’ IP address
 Foreign network may block for `IP spoofing`
(egress filtering)
 Indirect via foreign: mobileFA corresp.
 Requires FA (Foreign agent) to `spoof`
 Indirect via home: mobile homecorresp.
 Overhead… but works
 Direct: mobile corresponding
 Use temporary IP address (and mobile IP)
6: Wireless and Mobile Networks
6-62
Wireless and Mobile
Communication Networks: Outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-63
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
6: Wireless and Mobile Networks
6-64
Mobile IP: indirect routing
foreign-agent-to-mobile packet
packet sent by home agent to foreign
agent: a packet within a packet
dest: 79.129.13.2
dest: 128.119.40.186
dest: 128.119.40.186
Permanent address:
128.119.40.186
dest: 128.119.40.186
Care-of address:
79.129.13.2
packet sent by
correspondent
6: Wireless and Mobile Networks
6-65
Mobile IP: agent discovery
 agent advertisement: foreign/home agents advertise
service by broadcasting ICMP messages (typefield = 9)
0
type = 9
H,F bits: home
and/or foreign agent
R bit: registration
required
16
8
24
checksum
=9
code = 0
=9
standard
ICMP fields
router address
type = 16
length
registration lifetime
sequence #
RBHFMGV
reserved
bits
0 or more available
Care-Of-Addresses (COA)
mobility agent
advertisement
extension
6: Wireless and Mobile Networks
6-66
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
….
6: Wireless and Mobile Networks
6-67
Wireless and Mobile
Communication Networks: Outline
6.1 Introduction
Wireless
 6.2 Wireless links,
characteristics
 6.3 IEEE 802.11
wireless LANs (“wi-fi”)
Mobility
 6.5 Principles:
addressing and routing
to mobile users
 6.6 Mobile IP
 6.7 Cellular networks
 6.8 Mobility and higherlayer protocols
6.9 Summary
6: Wireless and Mobile Networks
6-68
Components of cellular network architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 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
6: Wireless and Mobile Networks
6-69
Multiple operators (providers)
recall:
correspondent
wired public
telephone
network
MSC
MSC
MSC
MSC
MSC
different cellular networks,
operated by different providers
6: Wireless and Mobile Networks
6-70
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
6: Wireless and Mobile Networks
6-71
GSM: indirect routing to mobile
home
network
HLR
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
correspondent
home
Mobile
Switching
Center
1
3
VLR
Mobile
Switching
Center
4
Public
switched
telephone
network
call routed
to home network
home MSC sets up 2nd leg of call
to MSC in visited network
mobile
user
visited
network
MSC in visited network completes
call through base station to mobile
6: Wireless and Mobile Networks
6-72
GSM: handoff with common MSC
 Handoff goal: route call via
new base station (without
interruption)
 reasons for handoff:
VLR Mobile
Switching
Center
old
routing
old BSS

new
routing

new BSS

stronger signal to/from new
BSS (continuing connectivity,
less battery drain)
load balance: free up channel
in current BSS
GSM doesn’t mandate why to
perform handoff (policy), only
how (mechanism)
 handoff initiated by old BSS
6: Wireless and Mobile Networks
6-73
GSM: handoff with common MSC
VLR Mobile
Switching
Center 2
4
1
8
old BSS
5
7
3
6
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
6: Wireless and Mobile Networks
6-74
GSM: handoff between MSCs
 anchor MSC: first MSC
visited during cal
home network
correspondent
Home
MSC

call remains routed
through anchor MSC
 new MSCs add on to end
anchor MSC
PSTN
MSC
MSC
MSC
of MSC chain as mobile
moves to new MSC
 Or: optional path
minimization step to
shorten multi-MSC chain
6: Wireless and Mobile Networks
6-75
GSM: handoff between MSCs
 anchor MSC: first MSC
visited during cal
home network
correspondent
Home
MSC

call remains routed
through anchor MSC
 new MSCs add on to end
anchor MSC
PSTN
MSC
MSC
MSC
(b) after handoff
of MSC chain as mobile
moves to new MSC
 IS-41 allows optional
path minimization step
to shorten multi-MSC
chain
6: Wireless and Mobile Networks
6-76
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 noise, collisions, handoff
 TCP interprets loss as congestion, will decrease
congestion window un-necessarily
 delay impairments for real-time traffic
 limited bandwidth of wireless links

6: Wireless and Mobile Networks
6-78
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
 mobility in GSM
 impact on higher-layer
protocols
6: Wireless and Mobile Networks
6-79