Wireless, Mobile Networks6-1 Ch. 6: Wireless and Mobile Networks Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers

Wireless, Mobile Networks 6-1

Ch. 6: Wireless and Mobile Networks

Background: # wireless (mobile) phone subscribers now

exceeds # wired phone subscribers (5-to-1)! # wireless Internet-connected devices equals

# wireline Internet-connected devices laptops, Internet-enabled phones promise anytime

untethered Internet access two important (but different) challenges

wireless: communication over wireless link mobility: handling the mobile user who changes

point of attachment to network


Chapter 6 outline

6.1 Introduction

Wireless6.2 Wireless links,

characteristics CDMA

6.3 IEEE 802.11 wireless LANs (“Wi-Fi”)

6.4 Cellular Internet Access

Mobility6.5 Principles:

addressing and routing to mobile users

6.6 Mobile IP6.7 Handling mobility in

cellular networks6.8 Mobility and higher-

layer protocols

6.9 Summary


Elements of a wireless network

network infrastructure

cell tower in cellular network (e.g. 3G or 4G)

access pointin 802.11 wireless LAN


wireless hosts laptop, smartphone run applications may be stationary

(non-mobile) or mobile wireless does not

always mean mobility




base station typically connected

to wired network responsible for

sending packets between wired network and wireless host(s) in its “area” e.g., cell towers,

802.11 access points




wireless link typically used to

connect mobile(s) to base station

multiple access protocol coordinates link access

various data rates, transmission distance




Characteristics of selected wireless links

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m – 4 Km

Long-rangeoutdoor

5Km – 20 Km

.056

.384

1

4

5-11

54

2G: IS-95, CDMA, GSM

2.5G: UMTS/WCDMA, CDMA2000

802.15

802.11b

802.11a,g

3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO

4G: LTWE WIMAX

802.11a,g point-to-point

200 802.11n

Dat

a ra

te (

Mbp

s)


hosts “operate in infrastructure mode”

all services via base station (connects mobiles into wired network)

handoff: mobile changes base station providing connection into wired 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 network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects to base station (WiFi,

cellular) which connects to

larger Internet

no base station, noconnection to larger

Internet (e.g. Bluetooth)

host may have torelay through several

wireless nodes to connect to larger Internet: mesh net

no base station, noconnection to larger Internet. May have torelay to reach other

wireless nodeMANET (mobile ad hoc netw)


Chapter 6 outline

6.1 Introduction







6.6 Mobile IP6.7 Handling mobility in

cellular networks6.8 Mobility and higher-

layer protocols

6.9 Summary


Wireless Link Characteristics (1)

important differences from wired link ….

decreased signal strength: radio signal strength decreases as it propagates through matter (path loss)

interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (e.g. microwave) interfere as well

multipath propagation: radio signals reaching the receiving antenna by two or more paths because radio signal reflects off objects and the ground

…. make communication across (even a point to point) wireless link much more “difficult” => powerful CRC error detection & reliable data transfer protocols


Wireless Link Characteristics (2) SNR: signal-to-noise ratio

measures signal strength relative to noise => larger SNR gives easier to extract signal from background noise

three different modulation schemes: amplitude modulation of a signal:

change the amplitude of periodic waveform to encode information (QAM = Quadrature amplitude modulation)

SNR versus Bit-Error-Rate (BER) for given modulation scheme:

increase transmission power -> increase SNR->decrease BER

but: little practical gain beyond certain threshold, stronger signal costs more energy (bad for battery-powered users)

10 20 30 40

QAM256 (8)

QAM16 (4)

BPSK (1)

SNR(decibels)

BER

10-1

10-2

10-3

10-5

10-6

10-7

10-4


Wireless Link Characteristics (2) SNR versus Bit-Error-Rate (BER)

for given SNR: choose modulation scheme that meets BER requirement and gives highest throughput

SNR may change with mobility: dynamically adapt modulation technique and rate to keep BER low! (e.g. with SNR of 10 dB we have to choose BPSK to avoid high BER)

when sending at high bit rate, differences in amplitude are low => errors while we sample the signal

10 20 30 40

QAM256 (8)

QAM16 (4)

BPSK (1)

SNR(decibels)

BER

10-1

10-2

10-3

10-5

10-6

10-7

10-4


Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access):

AB

C

Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each

other => A, C unaware of their interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Fading problem: B, A hear each other B, C hear each other A, C can not hear each

other as signal strength decreases => interference at B


Code Division Multiple Access (CDMA)

uses: 3G mobile networks (UMTS, etc.) channel partitioning protocol unique “code” assigned to each user all users share same frequency, but each

user uses his own code to encode data=> allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

encoded signal = (original data) x (code) decoding: sum of product of encoded signal

and code divide time slot for transmitting a bit into

many “mini slots” view “0” are “-1”


CDMA encode/decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M8x1/8-8x1/8


CDMA: two-sender interference

using same code as sender 1, receiver recovers sender 1’s original data from summed channel data!

Sender 1

Sender 2

channel sums together transmissions by sender 1 and 2

(-2-2-2-2)/8 (2+2+2+2)/8


CDMA: Code Division MADrawbacks:-Synchronization of all users is required-Multipath propagation: delayed copies of signal may be received which are not orthogonal any longer!-Near-far problem: nearby users will completely swamp far away users (signal of users far away cannot be received correctly) => accurate power control needed -having J more codes, the bandwidth must J times largerMain advantage:- when using FDMA or TDMA for cellular systems, frequencies used in a cell cannot be reused in adjacent cells (to avoid interference); in CDMA one can reuse the frequencies

more details:A.Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison Wesley, 1995.


Chapter 6 outline

6.1 Introduction







6.6 Mobile IP6.7 Summary


IEEE 802.11 Wireless LAN different standards (802.11a/b/g/n) differ in modulation techniques used (=>

different maximal transmission rate) and in frequency range (2.4 or 5 GHz)

all use CSMA/CA (carrier sense multiple access with collision avoidance)

all have base-station and ad-hoc network versions


802.11 LAN architecture wireless host

communicates with base station base station = access

point (AP) Basic Service Set (BSS)

contains: wireless hosts access point (AP):

base station ad hoc mode: hosts

only

BSS 1

BSS 2

Internet

hub, switchor router


802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11

channels at different (partly overlapping) frequencies admin chooses frequency for AP interference possible: channels can be same or

overlapping host: must associate with an AP

scans channels, listening for beacon frames (beacon = “Blinklicht”) containing AP’s name (service set identifier = 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

BSS 2BSS 1

1

23

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 2AP 1

H1

BSS 2BSS 1

122

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


IEEE 802.11: multiple access random access protocol as for Ethernet (carrier sense

MAC) sense before transmitting

refrain from transmitting when channel is sensed busy no collision detection during transmission!

difficult to receive (sense collisions) when transmitting due to weak received signals that gets overwhelmed by sending signal

can’t sense all collisions in any case: hidden terminal, fading => once transmission is running, it is never aborted (as opposed

to Ethernet when collision is detected)

optional feature: try to avoid collisions when several nodes are transmitting at the same time goal: avoid collisions: CSMA/C(ollision)A(voidance)


IEEE 802.11 MAC Protocol: CSMA/CA SIFS= Short Inter-frame Spacing

Time required for a receiving station to sense the end of a frame and start transmitting an ACK (compute CRC etc.)

DIFS= Distributed Inter-frame Spacing sender waits before transmission If the medium is continuously idle

for this duration, only then a node is supposed to transmit a frame.

DIFS = SIFS + (2 * Slot time) where slot time is 20μs in 802.11b

sender receiver

DIFS

data

SIFS

ACK


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for time period 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 (binary exponential), repeat step 2

802.11 receiver

- if frame received OK

return ACK after time period SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


IEEE 802.11 MAC Protocol: CSMA

sender receiver

DIFS

data

SIFS

ACK

• Why not send directly when channel is sensed idle? Assume three senders, one is transmitting and finishes => other senders both start sending => collision

• With random backoff time, one of the senders might send directly while the other waits

• When do collisions occur: same backoff time chosen or senders are hidden from each other (can’t sense whether channel idle)


decrement by one slot time


IEEE 802.11 MAC Protocol: CSMA

source: IEEE Std 802.11TM-2012 (Revision of IEEE Std 802.11-2007)


Optional avoidance mechanism

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

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer


framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: four address fields!

Address 2: MAC addressof wireless host or AP transmitting this frame(sender: wireless host or AP)

Address 1: MAC addressof wireless host or AP to receive this frame (destination within subnet)

Address 3: MAC addressof router interface to which AP is attached (=> send to Internet; see example on next slide)

Address 4: used only in ad hoc mode

bytes


InternetrouterH1 R1

H1 MAC addr AP MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

H1 MAC addr R1 MAC addr

dest. address source address

802.3 Ethernet frame

802.11 frame: addressing

from the router’s perspective, the AP is invisible (ethernet subnet!)now, H1 knows MAC

adress of source router


InternetrouterH1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr H1 MAC addr

dest. address source address

802.3 Ethernet frame

802.11 frame: addressing

address 3 allows theAP to determinecorrect destinationMAC address

framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

expected duration of plannedtransmission (including ACKs)when RTS, CTS or data is sent

frame seq #(since frames may beretransmitted if not ACKed)

frame type(RTS, CTS, ACK, data)

802.11 frame: more

frame sent by access point

detailed specification: http://standards.ieee.org/about/get/802/802.11.html


802.11: mobility within same subnet

How does switch know which AP is associated with H1?

•self-learning (Ch. 5): switch will

see frame from H1 and “remember”

which switch port can be used to

reach H1

H1 BSS 2BSS 1

H1 detects weakening signal from AP1scans and finds beacon frames of AP2 dissociates with AP1, associates with AP2remains in same IP subnet: keep IP address and all ongoing TCP connections

• better solutions are currently being developed (so that switch is informed when H1 disassociates/associates)


802.11: advanced capabilitiesRate adaptation base station, mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

2 unack’d frames in a row => next lower modul. scheme

10 frames ack’d in a row => next higher modul. scheme

QAM256 QAM16

BPSK

10 20 30 40SNR(dB)

BE

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


power management node-to-AP: “I am going to sleep until next

beacon frame” (power manag. bit in header) AP knows not to transmit frames to this node

(buffers frames!) node wakes up before next beacon frame (every

100 msec) beacon frame: contains list of mobiles with

buffered AP-to-mobile frames node will stay awake if AP-to-mobile frames to be

sent (=> has to send request to AP) otherwise sleep again until next beacon frame

802.11: advanced capabilities


Chapter 6 outline

6.1 Introduction




6.4 Cellular Internet access





Mobile Switching

Center

Public telephonenetwork

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


Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum

(1) combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots (used in 2G systems)

(Having F different sub-bands and T time slots, how many simultaneous calls can be supported?)

(2) CDMA: code division multiple access (used in 3G UMTS)

frequencybands

time slots


Chapter 6 outline

6.1 Introduction









Various degrees of 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 (e.g. cellular access in an ICE)

mobile user, connecting/ disconnecting from network using DHCP (e.g. moving to different room).

wide area network


Mobility: vocabularyhome network: permanent “home” of mobile(e.g., 128.119.40/24)

permanent address: address in home network, can always be used to reach mobilee.g., 128.119.40.186

home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote


Mobility: more vocabulary

wide area network

care-of-address: address in visited network.(e.g., 79,129.13.2)

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

search all phone books

call her parents

expect her to let you know


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

Mobility: approaches

wide area network


Mobility: registration

end result: foreign agent knows about mobile home agent knows location of mobile

home networkvisited network

1

mobile contacts foreign agent on entering visited network

2

foreign agent contacts home agent: “this mobile is resident in my network”


Mobility via indirect routing

wide area network

homenetwork

visitednetwork

3

2

41

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

correspondentsource address is

permanent address

=> usage: see Mobile IP (later)


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:

on going connections can be maintained!


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 may be done by mobile itself

inefficient when correspondent and mobile are in same network

1 23

4


Mobility via direct routing

homenetwork

visitednetwork

correspondent requests, receives foreign address of mobile

correspondent forwards to foreign agent

foreign agent receives packets, forwards to mobile

mobile replies directly to correspondent

used for routing telephone calls to mobile users in several mobile telephone network standards, including GSM


Mobility via direct routing: comments

more efficient if correspondent is in same or ”close” network

non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited network? how to inform correspondent?

1 23

4


wide area network

1

foreign net visited at session start

anchorforeignagent

2

4

new foreignagent

3

correspondentagent

correspondent

new foreignnetwork

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) requires additional coordination among the foreign

agents

5


Chapter 6 outline

6.1 Introduction









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 packetRFC 5944


Mobile IP: agent discovery

agent advertisement: foreign/home agents advertise service for mobile nodes by broadcasting ICMP messages (type field = 9)=> mobile node gets care-of-address


Mobile IP: registration example

visited network: 79.129.13/24home agent

HA: 128.119.40.7foreign agentCOA: 79.129.13.2

mobile agentMA: 128.119.40.186

registration req. COA: 79.129.13.2

HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification:714….

registration reply HA: 128.119.40.7MA: 128.119.40.186Lifetime: 4999Identification: 714 ….

registration reply HA: 128.119.40.7

MA: 128.119.40.186Lifetime: 4999Identification: 714….

time

ICMP agent adv.COA:

79.129.13.2….

registration req. COA: 79.129.13.2HA: 128.119.40.7MA: 128.119.40.186Lifetime: 9999identification: 714 ….

now, FA knowsthat it should look for datagrams with COA and forward them decapsulated

more details:RFC 5944

Current state of Mobile IPnot widely deployed yet because:need for seamless handover is given for few applications only (e.g. VoIP, video conferencing)handover is nice to have, re-connection is mainly sufficientGSM (telephone) networks have their own mobility solutions (indirect routing; phone number is permanent, roaming number is COA)deployed in WiMAX networks (IEEE 802.16)



Chapter 6 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

Documents

Wireless, Mobile Networks6-1 Ch. 6: Wireless and Mobile Networks Background: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers