Real-time Conversational Applications
Advanced Network Technologies
Wireless 2
School of Computer Science
Dr. | Lecturer
1
IEEE 802.11 Wireless LA Fi
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
3
802.11 LAN architecture
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
BSS 1
BSS 2
Internet
hub, switch
or router
4
BSS Base station system
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
5
Channel overlap, except 1, 6, 11.
802.11: passive/active scanning
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
3
1
passive scanning:
beacon frames sent from APs
association Request frame sent: H1 to selected AP
association Response frame sent from selected AP to H1
AP 2
AP 1
H1
BBS 2
BBS 1
1
2
2
3
4
active scanning:
Probe Request frame broadcast from H1
Probe Response frames sent from APs
Association Request frame sent: H1 to selected AP
Association Response frame sent from selected AP to H1
6
Beacon: to provide illumniation guidance.
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
7
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
8
CSMA/CD (collision detection)
spatial layout of nodes
9
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
10
Carrier sense multiple access (CSMA)
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
11
DIFS: Distributed inter-frame space: period of time waited before transmitting its frame
SIFS: Shorter inter-frame spacing
As opposed to CSMA/CD, 802.11 sends the entire frame and tries to avoid collision rather than aborting.
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!
12
Clear-to-send CTS, request-to-send RTS
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?
13
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)
BER
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
14
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
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
20
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
21
Frame division/ time division/ code division FDMA/TDMA/CDMA
BSC
BTS
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
voice network unchanged in core
data network operates in parallel
3G (voice+data) network architecture
General Packet Radio Service
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
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)
VoIP issue: IP is best effort whereas voice requires timely constraints.
OFDM: orthogonal frequency division multiplexing [Rhode 2008; Ericsson 2011]
24
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
LTE Radio Access Networks: uses a combination of orthogonal frequency-division multiplexing (OFDM) and time division multiplexing.
VoIP issue: IP is best effort whereas voice requires timely constraints.
OFDM: orthogonal frequency division multiplexing [Rhode 2008; Ericsson 2011]
25
4G: Long-Term Evolution (LTE)
Two important innovations over 3G
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
VoIP issue: IP is best effort whereas voice requires timely constraints.
OFDM: orthogonal frequency division multiplexing [Rhode 2008; Ericsson 2011]
26
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.
28
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
30
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
31
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?
32
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
33
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
34
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”
35
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
36
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
37
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!
38
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
39
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
40
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
41
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
43
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
44
Mobile IP: agent discovery
agent advertisement: foreign/home agents advertise service by broadcasting ICMP (Internet Control Message Protocol) messages (typefield = 9)
R bit: registration required
H,F bits: home and/or foreign agent
45
ICMP router discovery message
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
….
46
Mobile node sends Mobile IP registration message to foreigna getn
The FA receives the message and records the mobile node’s permanent IP address and FA sends to Home Agent the IP registration (UDP) with COA (care-of-address), HA, Mobile Agent encapsulation format.
The Home Agent receives registration request and check for authenticity and correctness: tunneling to mobile node; sends response with HA, MA, registration lifetome and identification.
FA receives the reply and forwards to mobile node
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:
48
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
49
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
50
GSM: Groupe Spécial Mobile -> Global System for Mobile communications [Mouly, Pautet. The GSM system for Mobile Communications, Cell and Sys. Palaiseau, 1992].
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
51
Mobile
Switching
Center
VLR
old BSS
1
3
2
4
5
6
7
8
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
52
Handoff algorithm: a brief overview
Signal Strength of Two Base Stations: when to handoff?
53
Handoff algorithm: a brief overview
Naive way: Compare the RSSs (Received Signal Strength) of two B at
Pnew> Pold
54
Ping-pong effect
Handoff back and forth.
55
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
56
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
58
RBHFMGV
bits
reserved
type = 16
type = 9
code = 0
= 9
c
hecksum
= 9
router address
standard
ICMP fields
mobility agent
advertisement
extension
length
sequence #
registration lifetime
0 or more care
-
of
-
addresses
0
8
16
24
type = 9
code = 0 = 9
checksum = 9
router address
standard
ICMP fields
mobility agent advertisement
extension
type = 16
length
sequence #
registration lifetime
RBHFMGV bits
reserved
0 or more care-of-addresses
0
8
16
24
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