Chapter 6 Wireless and Mobile Networks

Computer Networking: A Top Down Approach

4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007. 6: Wireless and Mobile Networks 6-1

Chapter 6: Wireless and Mobile Networks Background:

   # wireless (mobile) phone subscribers now exceeds # wired phone subscribers!

computer nets: laptops, palmtops, PDAs, Internet-enabled phone 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 6: Wireless and Mobile Networks 6-2

Chapter 6 outline

6.1 Introduction Wireless  6.2 Wireless links, characteristics   6.3 IEEE 802.11 wireless LANs (“wi-fi”) 6.4

Access   Cellular Internet architecture standards (e.g., GSM) Mobility     6.5

addressing and routing to mobile users 6.6

6.7

Principles: Mobile IP Handling mobility in cellular networks 6.8

Mobility and higher layer 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

Elements of a wireless network

network infrastructure wireless link  typically used to connect mobile(s) to base station    also used as backbone link multiple access protocol coordinates link access various data rates, transmission distance 6: Wireless and Mobile Networks 6-6

Characteristics of selected wireless link standards

200 54 5-11 4 1

802.11n

802.11a,g 802.11b

802.15

802.11a,g point-to-point 802.16 (WiMAX) UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO

.384

.056

Indoor 10-30m

UMTS/WCDMA, CDMA2000 IS-95, CDMA, GSM

Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km data 3G cellular enhanced 3G 2G 6: Wireless and Mobile Networks 6-7

Elements of a wireless network

network infrastructure infrastructure mode  base station connects mobiles into wired network  handoff: mobile changes base station providing connection into wired network 6: Wireless and Mobile Networks 6-8

Elements of a wireless 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 6: Wireless and Mobile Networks 6-9

Wireless network taxonomy

infrastructure (e.g., APs ) no infrastructure single hop host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) Mobile Adhoc Networks multiple hops host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET Vehicular Adhoc Networks 6: Wireless and Mobile Networks 6-10

Wireless Communication Systems & Networking

What complicates wireless networking vs. wired networking?

6: Wireless and Mobile Networks 6-11

1- Channel characteristics for satellite we get extended propagation delays high bit error rate ‘BER’ (higher than optical fiber and coax.) asymmetry in bandwidth and delay unidirectional links effects of wave propagation, attenuation,… etc.

2- Mobility: continuous and introduces topology dynamics 3- Power constraints in lots of the wireless devices 6: Wireless and Mobile Networks 6-12

Wireless Link Characteristics (1)

Differences from wired link ….

   decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: objects ground, arriving ad destination at slightly different times radio signal reflects off …. make communication across (even a point to point) wireless link much more “difficult” 6: Wireless and Mobile Networks 6-13

Wireless Link Characteristics (2)

10 -1   SNR: signal-to-noise ratio  larger SNR – easier to extract signal from noise (a “good thing”)

SNR versus BER tradeoffs



given physical layer:

increase power -> increase SNR->decrease BER 

given SNR:

thruput choose physical layer that meets BER requirement, giving highest • SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 20 SNR(dB) 30 QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) Quadrature Amplitude Modulation (QAM) Binary Phase Shift Keying (BPSK) 6: Wireless and Mobile Networks 40 6-14

Wireless network characteristics

Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C A’s signal strength B A Hidden terminal problem   B, A hear each other B, C hear each other  A, C can not hear each other means A, C unaware of their interference at B C C’s signal strength space Signal attenuation:    B, A hear each other B, C hear each other A, C can not hear each other interfering at B 6: Wireless and Mobile Networks 6-15

Chapter 6 outline

6.1

Introduction Wireless  6.2 Wireless links, characteristics   CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”)  6.4

cellular Internet access   architecture standards (e.g., GSM) Mobility     6.5

addressing and routing to mobile users 6.6

6.7

Principles: Mobile IP Handling mobility in cellular networks 6.8

Mobility and higher layer protocols 6.9

Summary 6: Wireless and Mobile Networks 6-16

IEEE 802.11 Wireless LAN

 802.11b

   2.4-5 GHz unlicensed spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer (CDMA: code division multiple access) • all hosts use same chipping code    802.11a

  5-6 GHz range up to 54 Mbps 802.11g

  2.4-5 GHz range up to 54 Mbps 802.11n: multiple antennae  2.4-5 GHz range  up to 200 Mbps   all use CSMA/CA for multiple access all have base-station and ad-hoc network versions 6: Wireless and Mobile Networks 6-17

802.11 LAN architecture

BSS 1 AP Internet hub, switch or router AP   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 2 6: Wireless and Mobile Networks 6-18

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 service set ID (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP’s subnet 6: Wireless and Mobile Networks 6-19

802.11: passive/active scanning

BBS 1 BBS 2 BBS 1 BBS 2 AP 1

1

AP 2

2 1 3

H1

Passive Scanning:

(1) beacon frames sent from APs (2) association Request frame sent: H1 to selected AP (3) association Response frame sent: selected AP to H1 AP 1

2 1 3 2 4

AP 2 H1

Active Scanning

: (1) Probe Request frame broadcast from H1 (2) Probes response frame sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent: selected AP to H1 6: Wireless and Mobile Networks 6-20

IEEE 802.11: multiple access

   avoid collisions: 2 + nodes transmitting at same time 802.11: CSMA - sense before transmitting  don’t collide with ongoing transmission by other node 802.11: 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 goal:

avoid collisions:

CSMA/C(ollision)A(voidance) A C B A A’s signal strength B C C’s signal strength space 6: Wireless and Mobile Networks 6-21

IEEE 802.11 MAC Protocol: CSMA/CA

sender 802.11 sender 1 if sense channel idle for 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, repeat 2 802.11 receiver if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS data ACK receiver SIFS Distributed Inter-frame Spacing (DIFS) Short Inter-frame Spacing (SIFS) 6: Wireless and Mobile Networks 6-22

Hidden Terminal Problem in WLANs

6: Wireless and Mobile Networks 6-23

Avoiding collisions: RTS/CTS

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 RTS heard by all nodes   sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets!

6: Wireless and Mobile Networks 6-24

Collision Avoidance: RTS-CTS exchange

A AP B reservation collision time DATA (A) defer 6: Wireless and Mobile Networks 6-25

Check Animations on-line (applet & ns) 6: Wireless and Mobile Networks 6-26

802.11 frame: addressing

2 frame control 2 duration 6 address 1 6 address 2 6 address 3 2 6 seq control address 4 0 - 2312 payload 4 CRC Address 1: MAC address of wireless host or AP to receive this frame Address 2: MAC address of wireless host or AP transmitting this frame Address 4: Address 3: MAC address of router interface to which AP is attached used only in ad hoc mode 6: Wireless and Mobile Networks 6-27

802.11 frame: addressing

H1 Internet R1 router AP R1 MAC addr AP MAC addr dest. address source address 802.

3

frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.

11

frame 6: Wireless and Mobile Networks 6-28

802.11 frame: more

duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) 2 frame control 2 6 duration address 1 6 address 2 6 address 3 2 6 seq control address 4 0 - 2312 payload 4 CRC 2 Protocol version 2 Type 4 Subtype 1 To AP 1 From AP 1 More frag 1 Retry 1 Power mgt 1 More data 1 WEP 1 Rsvd frame type (RTS, CTS, ACK, data) 6: Wireless and Mobile Networks 6-29

802.11: mobility within same subnet

  H1 remains in same IP subnet: IP address can remain same switch: 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 BBS 1 AP 1 router hub or switch H1 AP 2 BBS 2 6: Wireless and Mobile Networks 6-30

802.11: advanced capabilities

Rate Adaptation

 base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point Rate adaptation can change rate from 100Mbps to 1Mbps !!

Does this affect higher protocol layers?

10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 20 30 SNR(dB) 40 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 6: Wireless and Mobile Networks 6-31

802.11: advanced capabilities

Power Management

  node-to-AP: “I am going to sleep until next beacon frame”  AP knows not to transmit frames to this node  node wakes up before next beacon frame beacon frame: contains list of mobiles with AP to-mobile frames waiting to be sent  node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame (typically after 100msec) 6: Wireless and Mobile Networks 6-32

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 802.15: evolved from Bluetooth specification  2.4-2.5 GHz radio band  up to 721 kbps

S S P M P P S

radius of coverage

P M

Master device

S

Slave device

P

Parked device (inactive) 6: Wireless and Mobile Networks 6-33

802.16: WiMAX

  like 802.11 & cellular: base station model   transmissions to/from base station by hosts with omnidirectional antenna base station-to-base station backhaul with point-to-point antenna unlike 802.11:   range ~ 6 miles (“city rather than coffee shop”) ~14 Mbps point-to-point point-to-multipoint 6: Wireless and Mobile Networks 6-34

802.16: WiMAX: downlink, uplink scheduling

 transmission frame  down-link subframe: base station to node  uplink subframe: node to base station DL MAP UL MAP DL burst 1 DL burst 2 … … DL burst n Initial maint.

… request conn.

SS #1 SS #2 … SS #k uplink subframe downlink subframe base station tells nodes who will get to receive (DL map) and who will get to send (UL map), and when  WiMAX standard provide mechanism for scheduling, but not scheduling algorithm 6: Wireless and Mobile Networks 6-35

Chapter 6 outline

6.1 Introduction Wireless  6.2 Wireless links, characteristics   CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”)  6.4 Cellular Internet Access   architecture standards (e.g., GSM) Mobility     6.5

addressing and routing to mobile users 6.6

6.7

Principles: Mobile IP Handling mobility in cellular networks 6.8

Mobility and higher layer protocols 6.9

Summary 6: Wireless and Mobile Networks 6-36

Components of cellular network architecture

   MSC connects cells to wide area net manages call setup (more later!) handles mobility (more later!) cell  region  analogous to 802.11 AP  to network through BS  covers geographical

base station mobile users air-interface:

mobile and BS (BS) attach physical and link layer protocol between Mobile Switching Center Mobile Switching Center Public telephone network, and Internet wired network 6: Wireless and Mobile Networks 6-37

Wireless Comm. Systems

In general a wireless communication network consists of: 1- Users (mobile station) 2- Base Station (BS): connects users to MSC 3- Mobile Switching Center (MSC): connects the base stations with each other, and to the PSTN (public switched telephone network) 6: Wireless and Mobile Networks 6-38

6: Wireless and Mobile Networks 6-39

6: Wireless and Mobile Networks 6-40

-

Cellular Comm./Networking Terminology

Hand-off: the process of transferring the mobile from one base station to another Roamer: a mobile operating in a coverage area other than the one in which it subscribed (moving to another MSC) 6: Wireless and Mobile Networks 6-41

Cellular Telephone Systems

A cellular system services a large number of users over extended geographical coverage with limited frequency spectrum.

High capacity is attained by limiting the coverage of the base station to a cell, so that the same frequency can be re-used in other cells A problem may occur when moving from one cell to another while keeping the call un interrupted. [the hand-off problem] 6: Wireless and Mobile Networks 6-42

6: Wireless and Mobile Networks 6-43

Design concepts: The Cellular Concept and Frequency Re-use

The cellular concept was introduced to solve the problem of frequency limitation (or spectral congestion) and user capacity Replace a single high power base station with several lower power base stations, each covering a smaller geographical area, a ‘cell’.

Each of the base stations is allocated a number of channels (portion of the overall system channels) 6: Wireless and Mobile Networks 6-44

Neighboring base stations (would in general) use different frequency channels to reduce interference.

(more later on interference, channel assignment and frequency planning) 6: Wireless and Mobile Networks 6-45

Frequency Re-use

A cell uses a set of frequencies A ‘cluster’ holds several cells Frequency re-use factor: 1/#cells per cluster 6: Wireless and Mobile Networks 6-46

G F B A E G F C D B A E G F C D B A E G F Cluster Cell C G D F B C A D E B A E C D Cellular frequency re-use concept: cells with the same letter use the same set of frequencies.

A cluster of cells (highlighted in bold) is replicated over the coverage area. The cluster size,

N

, is equal to 7. Since each cell contains one-seventh of the overall channels, the cell frequency re-use factor is 1/7.

This requires channel/frequency planning and allocation!

6: Wireless and Mobile Networks 6-47

-

Multiple Access (MA) Techniques for Wireless Communications

MA schemes allow multiple mobile users to share a limited frequency spectrum.

Main MA schemes: FDMA, TDMA, SSMA (FHMA, CDMA [DSMA]), SDMA 6: Wireless and Mobile Networks 6-48

FDMA 6: Wireless and Mobile Networks 6-49

Frequency Division Multiple Access (FDMA)

Assigns individual channels to individual users on demand Only 1 user utilizes the channel at a time. Idle times are wasted. Capacity is not shared.

Communication is continuous Does not need synchronization Costly filters at the base station Need guard bands to alleviate interference 6: Wireless and Mobile Networks 6-50

TDMA 6: Wireless and Mobile Networks 6-51

Time Division Multiple Access (TDMA)

In a time slot only 1 user transmits (or receives) Several users share a single frequency channel Transmission is non-continuous Power consumption is lower than FDMA (e.g., the transmitter can be turned off when idle) During idle time, a mobile performs MAHO Synchronization is needed 6: Wireless and Mobile Networks 6-52

-

Spread Spectrum Multiple Access (SSMA)

Traditional communication techniques Strive to conserve bandwidth By contrast, Spread spectrum techniques use bandwidth several orders of magnitude larger than the min. required bandwidth !!

6: Wireless and Mobile Networks 6-53

Spread Spectrum Multiple Access (SSMA)

Spread spectrum techniques use bandwidth larger than the min. required bandwidth Modulation: Uses pseudo-noise (PN) sequence to convert the signal into wideband The PN is random, but can be re-produced by receiver Demodulation: Correct correlation using a PN re-produces the signal Using wrong PN sequence produces noise, hence this scheme is ‘secure’ 6: Wireless and Mobile Networks 6-54

Spread Spectrum (SS) uses two techniques: (1) FHMA: frequency hopped MA (1) DSMA: direct sequence MA (also called CDMA: code division multiple access) Frequency Hopped MA (FHMA) Frequencies of individual users are varied in a pseudo-random fashion within the wideband range The signal is broken into bursts and each burst is sent on a different frequency 6: Wireless and Mobile Networks 6-55

CDMA 6: Wireless and Mobile Networks 6-56

Code Division Multiple Access (CDMA)

      (cellular, satellite, etc) standards unique “code” assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

encoded signal

= (original data) X (chipping sequence)

decoding:

chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 6: Wireless and Mobile Networks 6-57

Speading the signal power over a wide spread of the frequency spectrum reduces fading effects only part of the spectrum, hence only part of the signal, is affected by fading No frequency planning required since users use the same frequency Soft hand-off can be provided since all the cells use the same frequency. MSC monitors signals.

In soft hand-off the channel (or frequency) remains the same and the base station changes 6: Wireless and Mobile Networks 6-58

Space Division MA (SDMA)

 Controls the radiated energy for each user in space using spot beam (directional) antennas 6: Wireless and Mobile Networks 6-59

-

Hybrid Multiple Access Systems

Time division frequency hopping (TDFH): (used in some versions of GSM) User can hop to new frequency at the start of a new TDMA frame Hence reducing interference and fading effects User hops over pre-defined frequencies 6: Wireless and Mobile Networks 6-60

FDMA/CDMA: The available bandwidth is split into subspectra. In each subspectrum CDMA is used Allows to assign subspectra on-demand 6: Wireless and Mobile Networks 6-61

FDMA/CDMA 6: Wireless and Mobile Networks 6-62

Cellular networks: the first hop

Techniques for sharing mobile-to-BS radio spectrum  combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots frequency bands time slots 6: Wireless and Mobile Networks 6-63

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 GSM Don’t drown in a bowl of alphabet soup: use this for reference only  6: Wireless and Mobile Networks 6-64

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 data rates up to 384K CDMA-2000   (phase 1) data rates up to 144K evolved from IS-95 6: Wireless and Mobile Networks 6-65

Cellular standards: brief survey

3G systems:  voice/data Universal Mobile Telecommunications Service (UMTS)   data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 Mbps CDMA-2000: CDMA in TDMA slots  data service: 1xEvlution Data Optimized (1xEVDO) up to 14 Mbps 6: Wireless and Mobile Networks 6-66

Chapter 6 outline

6.1 Introduction Wireless  6.2 Wireless links, characteristics   CDMA 6.3 IEEE 802.11 wireless LANs (“wi-fi”)  6.4

Cellular Internet Access   architecture standards (e.g., GSM) Mobility  6.5 Principles: addressing and routing to mobile users    6.6

6.7

Mobile IP Handling mobility in cellular networks 6.8

Mobility and higher layer protocols 6.9

Summary 6: Wireless and Mobile Networks 6-67

What is mobility?

 spectrum of mobility, from the

network

perspective: no mobility high mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from network using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections ( like cell phone) 6: Wireless and Mobile Networks 6-68

Mobility: Vocabulary

home network:

permanent “home” of mobile (e.g., 128.119.40/24)

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

wide area network

Permanent address:

address in home network, can always be used to reach mobile e.g., 128.119.40.186

correspondent 6: Wireless and Mobile Networks 6-69

Mobility: more vocabulary

Permanent address:

remains constant ( e.g., 128.119.40.186)

visited network:

in which mobile currently resides network (e.g., 79.129.13/24)

Care-of-address:

in visited network.

(e.g., 79,129.13.2) address

correspondent: mobile wants to communicate with

wide area network

foreign agent: that performs entity in visited network mobility functions on behalf of mobile.

6: Wireless and Mobile Networks 6-70

How do you contact a mobile friend:

Consider friend frequently changing addresses, how do you find her?

   search all phone books?

call her parents?

expect her to let you know where he/she is?

I wonder where Alice moved to?

6: Wireless and Mobile Networks 6-71

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 6: Wireless and Mobile Networks 6-72

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 6: Wireless and Mobile Networks 6-73

Mobility: registration

home network visited network 1 2 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 6-74

Mobility via Indirect Routing

home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile home network 3 wide area 2 1 4 correspondent addresses packets using home address of mobile visited network mobile replies directly to correspondent 6: Wireless and Mobile Networks 6-75

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 triangle routing: mobile correspondent-home-network  inefficient when correspondent, mobile are in same network 6: Wireless and Mobile Networks 6-76

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!

6: Wireless and Mobile Networks 6-77

Mobility via Direct Routing

correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile home network 4 correspondent requests, receives foreign address of mobile 2 1 wide area network 3 4 visited network mobile replies directly to correspondent 6: Wireless and Mobile Networks 6-78

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 6-79

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 correspondent correspondent agent foreign net visited at session start anchor foreign agent 2 5 4 3 new foreign agent new foreign network 6: Wireless and Mobile Networks 6-80

Chapter 6 outline

6.1

Introduction Wireless  6.2 Wireless links, characteristics    6.3

wireless LANs (“wi-fi”) 6.4

CDMA IEEE 802.11 Access   Cellular Internet architecture standards (e.g., GSM) Mobility   6.5

Principles: addressing and routing to mobile users 6.6 Mobile IP   6.7 Handling mobility in cellular networks 6.8

Mobility and higher layer protocols 6.9

Summary 6: Wireless and Mobile Networks 6-81

Mobile IP

   RFC 3344 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 6-82

Mobile IP: indirect routing

packet sent by home agent to foreign agent: a packet within a packet dest: 79.129.13.2

dest: 128.119.40.186

foreign-agent-to-mobile packet dest: 128.119.40.186

Permanent address: 128.119.40.186

dest: 128.119.40.186

packet sent by correspondent Care-of address: 79.129.13.2

6: Wireless and Mobile Networks 6-83

Mobile IP: agent discovery

 agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) 0 8 16 24 H,F bits: home and/or foreign agent type = 9 code = 0 = 9 router address c hecksum = 9 standard ICMP fields R bit: registration required type = 16 length registration lifetime sequence # RBHFMGV bits reserved 0 or more care-of addresses mobility agent advertisement extension 6: Wireless and Mobile Networks 6-84

Mobile IP: registration example

home agent HA: 128.119.40.7

time foreign agent 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 …. registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format …. 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 …. visited network: 79.129.13/24 Mobile agent MA: 128.119.40.186 registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 …. 6: Wireless and Mobile Networks 6-85

Mobile IP

   RFC 2002.

Goals: Attempts to provide support for host mobility while maintaining ‘transparency’:   the correspondent node need not know the location of the mobile node the connection already established should be maintained during movement even if the mobile node changes its network point of attachment 6: Wireless and Mobile Networks 6-86

Mobile IP

 Each mobile node has a home network, home address and home agent Correspondent Node Home Network Home Agent (HA) Mobile Node 6: Wireless and Mobile Networks 6-87

• When mobile node (MN) moves to a foreign network it obtains a care-of-address (COA) from the foreign agent (FA) that registers it with the home agent (HA) • COA is used by HA to forward packets destined to MN Correspondent Node Foreign Network Register Register (HA) Foreign Agent (FA) Advertisement (FA,COA) Solicitation Mobile Node Home Agent Home Network 6: Wireless and Mobile Networks 6-88

Packets sent by MN go directly to CN Correspondent Node (CN) Mobile Node (MN) Packets to MN are picked up by the HA and tunneled to MN Home Agent (HA) • Triangle Routing in Mobile-IP 6: Wireless and Mobile Networks 6-89

Triangular routing can be very inefficient, especially when C << B+A, where A (as shown) is the shortest path from CN to MN C Correspondent Node (CN) Mobile Node (MN) A B Home Agent (HA) • Triangle Routing in Mobile-IP 6: Wireless and Mobile Networks 6-90

Drawbacks of Mobile IP

 Other than (the main problem) of triangular routing  Mobile IP incurs lots of communication with the home agent with every movement   so, may not be fit for ‘micro’ mobility [e.g., move between rooms or buildings within the same network domain] handoff delays are significant since registration/packets need to go through the home agent first 6: Wireless and Mobile Networks 6-91

Suggested solutions

 To avoid triangular routing   use ‘route optimization’ use micro-mobility architectures • Cellular IP (CIP) • Hawaii • Multicast-based Mobility (M&M) 6: Wireless and Mobile Networks 6-92

(4) CN changes the destination address of the packets to go to MN’s new address Correspondent Node (CN) (3) When MN gets packets from CN it sends a

Binding Update

to CN with its new address Mobile Node (MN) (2) Initial packets to MN are sent through HA to MN (1) MN registers with HA as in basic Mobile IP.

Home Agent (HA) • Route Optimization (simple illustration) 6: Wireless and Mobile Networks 6-93

 With route optimization    Triangular routing is avoided Still have problems with micro mobility and smooth hand-off Need additional mechanisms to deal with these issues, which makes the protocol complex.

6: Wireless and Mobile Networks 6-94

Micro-Mobility

 Hierarchical approach to mobility:   During frequent, intra-domain, movement only local efficient handoff is performed without notifying the home agent (HA) or the correspondent node (CN) For inter-domain mobility use Mobile IP. Notify HA or CN only during inter-domain movement 6: Wireless and Mobile Networks 6-95

FA or CN Distribution tree dynamics while roaming Domain Root Wireless link Mobile Node 6: Wireless and Mobile Networks 6-96

M&M: Join/Prune dynamics to modify distribution Domain Root Wireless link Mobile Node 6: Wireless and Mobile Networks 6-97

Components of cellular network architecture

recall: MSC wired public telephone network correspondent MSC MSC MSC MSC different cellular networks, operated by different providers 6: Wireless and Mobile Networks 6-98

Handling mobility in cellular networks

 

home network:

network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) 

home location register (HLR):

(could be in another network) database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location

visited network:

resides   network in which mobile currently

visitor location register (VLR):

database with entry for each user currently in network could be home network 6: Wireless and Mobile Networks 6-99

GSM: indirect routing to mobile

home MSC consults HLR, gets roaming number of mobile in visited network HLR 2 home network home Mobile Switching Center mobile user correspondent 4 VLR Mobile Switching Center visited network 3 1 Public switched telephone network call routed to home network home MSC sets up 2 nd leg of call to MSC in visited network MSC in visited network completes call through base station to mobile 6: Wireless and Mobile Networks 6-100

GSM: handoff with common MSC

VLR Mobile Switching Center old BSS old routing new routing new BSS    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 GSM doesn’t mandate why to perform handoff (policy), only how (mechanism) handoff initiated by old BSS 6: Wireless and Mobile Networks 6-101

GSM: handoff with common MSC

1 VLR Mobile Switching Center 4 2 7 8 old BSS 5 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 6-102

GSM: handoff between MSCs

home network Home MSC anchor MSC MSC MSC correspondent PSTN MSC (a) before handoff   

anchor MSC:

first MSC visited during cal  call remains routed through anchor MSC new MSCs add on to end 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 6-103

GSM: handoff between MSCs

home network Home MSC anchor MSC MSC MSC correspondent PSTN (b) after handoff MSC   

anchor MSC:

first MSC visited during cal  call remains routed through anchor MSC new MSCs add on to end 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 6-104

Mobility: GSM versus Mobile IP

GSM element Home system Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR) Visited System Visited Mobile services Switching Center.

Visitor Location Record (VLR) Mobile Station Roaming Number (MSRN), or “roaming number” Comment on GSM element

Network to which mobile user’s permanent phone number belongs

Mobile IP element

Home MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information

Home network Home agent

Network other than home system where mobile user is currently residing Visited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent.

Visited network Foreign agent Care-of address

6: Wireless and Mobile Networks 6-105

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 6: Wireless and Mobile Networks 6-106

Chapter 6 Summary

Wireless  wireless links:    capacity, distance channel impairments CDMA   IEEE 802.11 (“wi-fi”)  CSMA/CA reflects wireless channel characteristics cellular access   architecture standards (e.g., GSM, CDMA-2000, UMTS) 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 6-107

Code Division Multiple Access (CDMA)

      (cellular, satellite, etc) standards unique “code” assigned to each user; i.e., code set partitioning all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

encoded signal

= (original data) X (chipping sequence)

decoding:

chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 6: Wireless and Mobile Networks 6-108

CDMA Encode/Decode

channel output Z i,m Z i,m = d i .

c m sender data bits d 1 = -1 code 1 1 1 1 1 slot 1 d 0 = 1 1 1 1 1 1 slot 0 1 1 slot 1 1 1 1 1 1 1 channel output 1 1 slot 0 channel output received input 1 1 1 1 1 1 1 1 1 1 receiver code 1 1 1 1 1 slot 1 1 1 1 1 1 slot 0 D i = M S m=1 Z i,m .

c m M d 1 = -1 slot 1 channel output d 0 = 1 slot 0 channel output 6: Wireless and Mobile Networks 6-109

CDMA: two-sender interference

6: Wireless and Mobile Networks 6-110

Direct Sequence Spread Spectrum

   Original signal is m(t) The spreading signal is p(t) [the PN sequence] The spread spectrum signal is S

ss (t)

A single pulse or symbol of the PN waveform is called a

chip

6: Wireless and Mobile Networks 6-111

S

ss (

t

) ~

m

(

t

)

p

(

t

)cos(2 

f

c t+  )

B

: is the bandwidth of

m

(

t

)cos(2 

f

c t+  )

W

ss : is the bandwidth of

S

ss (

t

)

W

ss >>

B

Data

m(t)

Phase modulation

S

ss (

t

) Transmitted Signal

p(t)

PN Code Generator Oscillator

f

c Chip Clock Block diagram of a DS-SS system with binary phase modulation Transmitter 6: Wireless and Mobile Networks 6-112

(A) Channel Symbol (B) encoder (C) Chip Symbol duration for

m(t)

:

Ts

Chip duration for

p(t)

:

Tc f

(B,C) Processing Gain

PG=W ss /B=Ts/Tc

, a measure of interference rejection capability 6: Wireless and Mobile Networks 6-113

Bit stream (A) Encoded stream (B)

Tc

Pseudo-noise sequence (C)

Ts m

(

t

)

p

(

t

) 6: Wireless and Mobile Networks 6-114

 Example:   f(B,C)=B  C, where • 1  • 1  • 0  1= 0 0 = 1 0 = 0 if we have received f(B,C) and we are able to re-generate the PN (C), then we can get B.

6: Wireless and Mobile Networks 6-115