20 - Mobility
6: Wireless and Mobile Networks 6-1
802.15: personal area network
less than 10 m diameter
replacement for cables
(mouse, keyboard, S
P
headphones) P
radius of
ad hoc: no infrastructure
M
coverage
master/slaves: S S P
P
slaves request permission to
send (to master)
master grants requests
802.15: evolved from M Master device
Bluetooth specification S Slave device
2.4-2.5 GHz radio band P Parked device (inactive)
up to 721 kbps
6: Wireless and Mobile Networks 6-2
Components of cellular network architecture
MSC
connects cells to wide area net
manages call setup (more later!)
handles mobility (more later!)
cell
covers geographical
region
base station (BS) Mobile
analogous to 802.11 AP Switching
mobile users attach
Center
Public telephone
to network through BS network, and
air-interface: Internet
physical and link layer Mobile
protocol between Switching
mobile and BS Center
wired network
5: DataLink Layer 5-3
Cellular networks: the first hop
Two techniques for sharing
mobile-to-BS radio
spectrum
combined FDMA/TDMA:
divide spectrum in time slots
frequency channels, divide
each channel into time
slots frequency
CDMA: code division
bands
multiple access
5: DataLink Layer 5-4
Cellular standards: brief survey
2G systems: voice channels
IS-136 TDMA: combined FDMA/TDMA (north
america)
GSM (global system for mobile communications):
combined FDMA/TDMA
most widely deployed
IS-95 CDMA: code division multiple access
Don’t drown in a bowl
GSM of alphabet soup: use this
for reference only
5: DataLink Layer 5-5
Cellular standards: brief survey
2.5 G systems: voice and data channels
for those who can’t wait for 3G service: 2G extensions
general packet radio service (GPRS)
evolved from 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 144K
evolved from IS-95
5: DataLink Layer 5-6
Cellular standards: brief survey
3G systems: voice/data
Universal Mobile Telecommunications Service (UMTS)
GSM next step, but using CDMA
CDMA-2000
….. more (and more interesting) cellular topics due to
mobility (stay tuned for details)
5: DataLink Layer 5-7
What is mobility?
spectrum of mobility, from the network perspective:
no mobility high mobility
mobile wireless user, mobile user, mobile user, passing
using same access connecting/ through multiple
point disconnecting access points while
from network maintaining ongoing
using DHCP. connections (like cell
phone)
5: DataLink Layer 5-8
Mobility: Vocabulary
home network: permanent home agent: entity that will
“home” of mobile perform mobility functions on
(e.g., 128.119.40/24)
behalf of mobile, when mobile
is remote
wide area
network
Permanent address:
address in home
network, can always be
used to reach mobile
e.g., 128.119.40.186 correspondent
5: DataLink Layer 5-9
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:
agent discovery
registration with home agent
indirect routing of datagrams
5: DataLink Layer 5-21
Mobile IP: agent discovery
agent advertisement: foreign/home agents advertise
service by broadcasting ICMP messages (typefield = 9)
0 8 16 24
type = 9 code = 0 checksum
=9 =9
H,F bits: home standard
and/or foreign agent router address ICMP fields
R bit: registration
required type = 16 length sequence #
RBHFMGV
registration lifetime reserved
bits mobility agent
advertisement
0 or more care-of- extension
addresses
5: DataLink Layer 5-22
Mobile IP: registration example
visited network: 79.129.13/24
home agent foreign agent
HA: 128.119.40.7 COA: 79.129.13.2 ICMP agent adv.
Mobile agent
COA: 79.129.13.2 MA: 128.119.40.186
….
registration req.
registration req. COA: 79.129.13.2
COA: 79.129.13.2 HA: 128.119.40.7
HA: 128.119.40.7 MA: 128.119.40.186
MA: 128.119.40.186 Lifetime: 9999
Lifetime: 9999 identification:714
identification: 714 ….
encapsulation format
….
registration reply
time HA: 128.119.40.7 registration reply
MA: 128.119.40.186
Lifetime: 4999 HA: 128.119.40.7
Identification: 714 MA: 128.119.40.186
encapsulation format Lifetime: 4999
…. Identification: 714
….
5: DataLink Layer 5-23
Mobile IP: indirect routing
foreign-agent-to-mobile packet
packet sent by home agent to foreign dest: 128.119.40.186
agent: a packet within a packet
dest: 79.129.13.2 dest: 128.119.40.186
Permanent address:
128.119.40.186
Care-of address:
79.129.13.2
dest: 128.119.40.186
packet sent by
correspondent
5: DataLink Layer 5-24
Components of cellular network architecture
recall: correspondent
wired public
telephone
network
MSC MSC
MSC
MSC
MSC
different cellular networks,
operated by different providers
5: DataLink Layer 5-25
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
visited network: network in which mobile currently
resides
visitor location register (VLR): database with
entry for each user currently in network
5: DataLink Layer 5-26
GSM: indirect routing to mobile
home
HLR
network correspondent
2
home
Mobile
home MSC consults HLR, Switching
gets roaming number of Center
mobile in visited network
1 call routed
to home network
3 Public
VLR switched
Mobile
telephone
Switching
network
Center
4
home MSC sets up 2nd leg of call
to MSC in visited network
mobile
user MSC in visited network completes
visited call through base station to mobile
network
5: DataLink Layer 5-27
GSM: handoff with common MSC
Handoff goal: route call via
new base station (without
interruption)
VLR Mobile reasons for handoff:
Switching stronger signal to/from new
Center
BSS (continuing connectivity,
less battery drain)
old new
routing
load balance: free up channel
routing
old BSS
in current BSS
new BSS
GSM doesn’t mandate why to
perform handoff (policy), only
how (mechanism)
handoff initiated by old BSS
5: DataLink Layer 5-28
GSM: handoff with common MSC
1. old BSS informs MSC of impending
handoff, provides list of 1+ new BSSs
2. MSC sets up path (allocates resources)
to new BSS
VLR Mobile 3. new BSS allocates radio channel for
Switching
Center 2 use by mobile
4 4. new BSS signals MSC, old BSS: ready
1
7
8 5. old BSS tells mobile: perform handoff to
3
old BSS 5 6
new BSS
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 6-29
GSM: handoff between MSCs
anchor MSC: first MSC
visited during call
home network
correspondent call remains routed
Home
MSC through anchor MSC
new MSCs add on to end
anchor MSC
PSTN of MSC chain as mobile
moves to new MSC
MSC
MSC
MSC
IS-41 allows optional
path minimization step
to shorten multi-MSC
chain
(a) before handoff
5: DataLink Layer 5-30
GSM: handoff between MSCs
anchor MSC: first MSC
visited during call
home network
correspondent call remains routed
Home
MSC through anchor MSC
new MSCs add on to end
anchor MSC
PSTN of MSC chain as mobile
moves to new MSC
MSC
MSC
MSC
IS-41 allows optional
path minimization step
to shorten multi-MSC
chain
(b) after handoff
5: DataLink Layer 5-31
GSM Mobile IP
home network
correspondent
Home
MSC
anchor MSC
PSTN
MSC
MSC MSC
5: DataLink Layer 5-32
Mobility: GSM versus Mobile IP
GSM element Comment on GSM element Mobile IP element
Home system Network to which the mobile user’s permanent Home network
phone number belongs
Gateway Mobile Home MSC: point of contact to obtain routable Home agent
Switching Center, or address of mobile user. HLR: database in
“home MSC”. Home home system containing permanent phone
Location Register number, profile information, current location of
(HLR) mobile user, subscription information
Visited System Network other than home system where Visited network
mobile user is currently residing
Visited Mobile Visited MSC: responsible for setting up calls Foreign agent
services Switching to/from mobile nodes in cells associated with
Center. MSC. VLR: temporary database entry in
Visitor Location visited system, containing subscription
Record (VLR) information for each visiting mobile user
Mobile Station Routable address for telephone call segment Care-of-
Roaming Number between home MSC and visited MSC, visible address
(MSRN), or “roaming to neither the mobile nor the correspondent.
number”
5: DataLink Layer 5-33
Wireless, mobility: impact on higher layer protocols
logically, impact should be minimal …
best effort service model remains unchanged
TCP and UDP can (and do) run over wireless, mobile
… but performance-wise:
packet loss/delay due to bit-errors (discarded
packets, delays for link-layer retransmissions), and
handoff
• TCP interprets loss as congestion, will decrease congestion
window un-necessarily
• delay impairments for real-time traffic
limited bandwidth of wireless links
5: DataLink Layer 5-34
Chapter 6 Summary
Wireless Mobility
wireless links: principles: addressing,
capacity, distance routing to mobile users
channel impairments home, visited networks
CDMA direct, indirect routing
IEEE 802.11 (“wi-fi”) care-of-addresses
CSMA/CA reflects case studies
wireless channel mobile IP
characteristics mobility in GSM
cellular access impact on higher-layer
architecture protocols
standards (e.g., GSM,
CDMA-2000, UMTS)
5: DataLink Layer 5-35