Week 9 wireless
Advanced Network Technologies
Wireless 2
School of Computer Science
Dr. Wei Bao | Lecturer
IEEE 802.11 Wireless LANs
WiFi
IEEE 802.11 WiFi
Wireless and Mobile Networks: 7- 3
IEEE 802.11
standard
Year Max data
rate
Range Frequency
802.11b 1999 11 Mbps 30m 2.4 Ghz
802.11a 1999 54 Mbps 30m 5 Ghz
802.11g 2003 54 Mbps 30m 2.4 Ghz
802.11n (WiFi 4) 2009 600 Mbps 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
802.11 LAN architecture
v wireless host
communicates with base
station
§ base station = access point
(AP)
v Basic Service Set (BSS) (aka
“cell”) in infrastructure
mode contains:
§ wireless hosts
§ access point (AP): base
station
§ ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switch
or router
802.11: Channels, association
› 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at
different frequencies
– AP admin chooses frequency for AP
– interference possible: channel can be same as that chosen by neighboring AP!
› 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
– may perform authentication
– will typically run DHCP to get IP address in AP’s subnet
802.11: passive/active scanning
AP 2AP 1
H1
BBS 2BBS 1
1
2 3
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 2
AP 1
H1
BBS 2BBS 1
1
22
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
collisions
› collisions can occur:
propagation delay means
two nodes may not hear
each other’s transmission
› collision: frame transmission
time wasted
spatial layout of nodes
Wired Networks: CSMA/CD (collision detection)
CSMA/CD:
– collisions detected within short time
– colliding transmissions aborted, reducing channel wastage
› collision detection:
– wired LANs: measure signal strengths, compare transmitted, received
signals
– Can transmit and sense at the same time
– wireless LANs: received signal strength overwhelmed by local
transmission strength
– CSMA-CD cannot be used in wireless LAN
CSMA/CD (collision detection)
spatial layout of nodes
IEEE 802.11: multiple access
› 802.11: no collision detection!
– difficult to receive (sense collisions) when transmitting due to weak
received signals (fading)
– can not sense all collisions in any case: hidden terminal, fading
– goal: avoid collisions: CSMA/C(ollision)A(voidance)
space
A
B
C A B C
A’s signal
strength
C’s signal
strength
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender
1 if sense channel idle for DIFS （Distributed
inter-frame space） 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
802.11 receiver
– if frame received OK
return ACK after SIFS (Shorter inter-frame
spacing)
Sender： if no ACK, increase random
backoff interval, repeat 2
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)
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!
Collision Avoidance: RTS-CTS exchange
AP
A B
time
RTS(A)
RTS(B)
RTS(A)
CTS(A) CTS(A)
DATA (A)
ACK(A) ACK(A)
reservation collision
defer
Please think: How does A (B) know that RTS collide?
Rate adaptation
› base station, mobile
dynamically change
transmission rate (physical
layer modulation technique)
as mobile moves, SNR
varies
802.11: advanced capabilities
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
10 20 30 40
SNR(dB)
B
E
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
Exposed Terminal
Source: Wikipedia
Exposed Terminal
Ideal: S1->R1 and S2->R2 simultaneously
However: S2 can sense the carrier of S1 so that it keeps silence
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Can RTS-CTS fail? Yes
Source: http://www.cs.jhu.edu/~cs647/mac_lecture_3.pdf
Cellular Internet Access
Architecture and standards
Mobile
Switching
Center
Public telephone
network
Mobile
Switching
Center
Components of cellular network architecture
v connects cells to wired tel. net.
v manages call setup (more later!)
v handles mobility (more later!)
MSC
v covers geographical
region
v base station (BS)
analogous to 802.11 AP
v mobile users attach to
network through BS
v air-interface: physical
and link layer protocol
between mobile and BS
cell
wired network
Cellular networks: the first hop
Two techniques for sharing mobile-
to-BS radio spectrum
› combined FDMA/TDMA: divide
spectrum in frequency channels,
divide each channel into time
slots
› CDMA: code division multiple
access
frequency
bands
time slots
BSCBTS
Base transceiver station (BTS)
Base station controller (BSC)
Mobile Switching Center (MSC)
Mobile subscribers
Base station system (BSS)
Legend
MSC
Public
telephone
network
Gateway
MSC
G
2G (voice) network architecture
radio
network
controller
MSC
SGSN
Public
telephone
network
Gateway
MSC
G
Serving GPRS Support Node (SGSN)
Gateway GPRS Support Node (GGSN)
Public
Internet
GGSN
G
Key insight: new cellular data
network operates in parallel
(except at edge) with existing
cellular voice network
v voice network unchanged in core
v data network operates in parallel
3G (voice+data) network architecture
General Packet Radio Service
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
1. Evolved packet core (EPC): simplified all-IP core network that
unifies the cellular circuit-switched voice network and the
packet switched cellular data network.
Public
telephone
network
Public
Internet
Evolved Packet Core
(all-IP)
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
2. LTE Radio Access Networks: uses a combination of
orthogonal frequency-division multiplexing (OFDM) and time
division multiplexing.
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
2. LTE Radio Access Networks: uses a combination of
orthogonal frequency-division multiplexing (OFDM) and time
division multiplexing.
0 0.5 1 1.5 2 2.5
f1
f2
f3
f4
f5
f6
time slots (ms)
frequency carriers
Mobility principles:
Addressing and routing to mobile
users
What is 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 (like cell
phone)
mobile user,
disconnecting from
network when
moving.
Should Address always remain the same?
› Mobile phone: the phone number remains the same
at all time when you travel
› How about IP Address?
wide area
network
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
Mobility: more vocabulary
wide area
network
care-of-address: address
in visited network.
(e.g., 79,129.13.2)
Foreign (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
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?
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
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
not
scalable
to millions of
mobiles
wide area
network
Mobility: registration
end result:
› foreign agent knows about mobile
› home agent knows location of mobile
home network
visited network
1
mobile contacts
foreign agent on
entering visited
network
2
foreign agent contacts home
agent home: “this mobile is
resident in my network”
Mobility via indirect routing
wide area
network
home
network
visited
network
3
2
4
1
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
correspondent
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
› triangle routing: correspondent-home-network-mobile
– inefficient when correspondent, mobile are in same network
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)
› changing foreign networks transparent: on going connections can
be maintained!
1 2
3
4
Mobility via direct routing
home
network
visited
network
correspondent
requests, receives
foreign address of
mobile
correspondent forwards
to foreign agent
foreign agent
receives packets,
forwards to mobile
mobile replies
directly to
correspondent
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?
1 2
3
4
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)
wide area
network
1
foreign net visited
at session start
anchor
foreign
agent 2
4
new foreign
agent
3
correspondent
agent
correspondent
new
foreign
network
5
Mobile IP
Mobile IP
› RFC 3344
› has many features we have seen:
– home agents, foreign agents, foreign-agent registration, care-of-addresses
› three components to standard:
– indirect routing of datagrams
– agent discovery
– registration with home agent
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 packet
Mobile IP: agent discovery
› agent advertisement: foreign/home agents advertise service by
broadcasting ICMP (Internet Control Message Protocol) messages
(typefield = 9)
RBHFMGV
bits reserved
type = 16
type = 9 code = 0
= 9
checksum
= 9
router address
standard
ICMP fields
mobility agent
advertisement
extension
length sequence #
registration lifetime
0 or more care-of-
addresses
0 8 16 24
R bit: registration
required
H,F bits: home and/or
foreign agent
Mobile IP: registration example
visited network: 79.129.13/24
home agent
HA: 128.119.40.7
foreign agent
COA: 79.129.13.2
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
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
encapsulation format
….
registration reply
HA: 128.119.40.7
MA: 128.119.40.186
Lifetime: 4999
Identification: 714
….
time
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
….
Mobility in Cellular Networks
Components of cellular network architecture
correspondent
MSC
MSC
MSC MSC
MSC
wired public
telephone
network
different cellular networks,
operated by different providers
recall:
Handling mobility in cellular networks
› home network: network of cellular provider you subscribe to (e.g., Vodafone)
– 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
Public
switched
telephone
network
mobile
user
home
Mobile
Switching
Center
HLR home
network
visited
network
correspondent
Mobile
Switching
Center
VLR
GSM: indirect routing to mobile
1 call routed
to home network
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
3
home MSC sets up 2nd leg of call
to MSC in visited network
4
MSC in visited network completes
call through base station to mobile
› handoff goal: route call via new
base station (without
interruption)
› reasons for handoff:
– stronger signal to/from new BSS
(continuing connectivity, less battery
drain)
– load balance: free up channel in current
BSS
› handoff initiated by old BSS
Mobile
Switching
Center
VLR
old BSS
new BSS
old
routing
new
routing
GSM: handoff with common MSC
Mobile
Switching
Center
VLR
old BSS
1
3
2
4
5 6
78
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
GSM: handoff with common MSC
Handoff algorithm: a brief overview
Signal Strength of Two Base Stations: when to handoff?
Handoff algorithm: a brief overview
› Naive way: Compare the RSSs (Received Signal Strength) of two BSs
Handoff at
› Pnew> Pold
Ping-pong effect
Handoff back and forth.
Smarter ways
› RSS: initiate handoff to BS new if
› Pnew> Pold
› RSS with threshold(PT): choose BS new if
› Pnew> Pold and Pold< PT
› RSS with hysteresis(PH): choose BS new if
› Pnew> Pold+PH
› RSS with threshold(PT) and hysteresis(PH): choose BS new if
› Pnew> Pold+PH and Pold< PT › Even better: Add a Dwell Timer to the above algorithms: start timer when above condition is met; initiate handoff if condition persists when timer expires 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