Communication Networks: Wireless and Mobile Communication Networks. Prof. Amir Herzberg BIU, Dept. of CS

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1 Communication Networks: Wireless and Mobile Communication Networks Prof. Amir Herzberg BIU, Dept. of CS From ch.6 of Kurose and Ross, 3 rd edition; and [KMK], ch. 8. All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July : Wireless and Mobile Networks 6-1

2 Background: Wireless and Mobile Networks # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! Computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytime untethered Internet access Internet telephony: a reality, an earthquake Two important (but different) challenges Wireless link: no CD (e.g. hidden-terminal), reliability, security Mobility of computers and users; provisioning Plus: limited computing power and energy 6: Wireless and Mobile Networks 6-2

3 Wireless and Mobile Communication Networks: Outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics 6.3 IEEE wireless LANs ( wi-fi ) Sensor and personalarea networks Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-3

4 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

5 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, access points 6: Wireless and Mobile Networks 6-5

6 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

7 54 Mbps Characteristics of selected wireless link standards 5-11 Mbps 1 Mbps {a,g} b.11 p-to-p link 384 Kbps 56 Kbps UMTS/WCDMA, CDMA2000 IS-95 CDMA, GSM 3G 2G Indoor Outdoor Mid range outdoor Long range outdoor 10 30m m 200m 4Km 5Km 20Km 6: Wireless and Mobile Networks 6-7

8 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

9 Wireless network characteristics Lower Signal/Noise ratio (cf. wired networks) Limited, shared spectrum: orthogonal signals (FDMA/CDMA/TDMA) or `collisions as noise` C A B C A B A s signal strength C s signal strength Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other space Signal fading: 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 6-9

10 Wireless Link Characteristics Differences from wired link. Energy and computing-power limitations Decreased signal strength Obstacles and hidden-terminal problem Collision detection hard or impossible More noise Interference from other sources Multipath propagation different delays interferences between paths or (multipath) fading Lower signal/noise Higher bit error rate. more difficult 6: Wireless and Mobile Networks 6-10

11 Hi/Low BER States Model Wireless links often have two BER states High, Low E.g., due to (multipath) fading Model by two-state Markov (1 g) model: g Good Bad b (1 b) Simplify: all packets Ok in `Good`, fail in `Bad` 6: Wireless and Mobile Networks 6-11

12 Wireless Link Characteristics Differences from wired link. Decreased signal strength: signal attenuates as it propagates through matter (path loss) Obstacles: e.g. mountain; hidden-terminal problem Collision detection hard or impossible More noise: Interference from other sources: frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); also noise e.g. from motors Multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times Energy and computing-power limitations. make communication across (even a point to point) wireless link much more difficult 6: Wireless and Mobile Networks 6-12

13 Wireless and Mobile Communication Networks: Outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics 6.3 IEEE wireless LANs ( wi-fi ) Ad-hoc, sensor and personal-area networks Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-13

14 IEEE Wireless LANs Wireless LANs Up to 11Mbps 5-6 GHz GHz (unlicensed) b lo-cost, good propagation; but slow, interferences Up to 54 Mbps a g All use CSMA/CA for multiple access All have base-station and ad-hoc network versions 6: Wireless and Mobile Networks 6-14

15 IEEE Wireless LAN b GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions 6: Wireless and Mobile Networks 6-15

16 LAN architecture BSS 1 AP DS Internet LAN switch AP BSS 2 router wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts (mobiles) access point (AP): base station ad hoc mode: hosts only =Independent BSS (IBSS) Extended Service Set (ESS) One or more BSS Connect by LANswitch or DS DS=Distribution System 6: Wireless and Mobile Networks 6-16

17 802.11: Channels, association b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies Non-overlapping only if at least 4 channels apart At most 3 non-overlapping channels (1, 6 and 11). AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scan channels, listening for beacon frames containing SSID (ESS/IBSS name) and MAC address of AP select AP; initiate association protocol may perform authentication will typically run DHCP to get IP address in AP s subnet 6: Wireless and Mobile Networks 6-17

18 IEEE : multiple access Like Ethernet, uses CSMA: random access carrier sense: don t collide with ongoing transmission Unlike Ethernet: Ack, no Collision Detection no collision detection transmit all frames to completion ACK: to detect loss without collision detection Why 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 And loss may be due to (higher) error rate of wireless Goal: avoid collisions: CSMA/CA (Collision Avoidance) 6: Wireless and Mobile Networks 6-18

19 IEEE MAC Protocol: CSMA/CA [simplified] sender 1 if sense channel idle then - transmit entire frame (no Colli. Detect) 2 if sense channel busy then - start random backoff timer - timer counts down while channel idle - transmit when timer expires receiver if frame received OK then return ACK else ignore (no NACK!) SIFS: Short Inter-Frame Space max time to begin Ack [e.g., 16µsec in a] sender data ACK receiver SIFS (e.g. 16µs) 6: Wireless and Mobile Networks 6-19

20 IEEE MAC Protocol: CSMA/CA sender (when trying to send) 1 if sense channel idle for DIFS then - transmit entire frame (no CD) [DIFS>SIFS+2T prop for priority to ACK] 2 if sense channel busy then - count down the backoff timer - but only while channel idle - transmit when timer expires - If ACK, reduce backoff range by 1 - if no ACK, double backoff range, select time randomly from range, repeat receiver if frame received OK - return ACK (within SIFS) DIFS DIFS sender data ACK receiver SIFS (e.g. 16µs) 6: Wireless and Mobile Networks 6-20

21 MAC Operation Data Frames and their ACK DIFS Src Data SIFS Dest Other Ack DIFS Contention Window Next MPDU Defer Access Backoff after Defer Acknowledgment should arrive within DIFS Senders wait for DIFS no-carrier time, then exponential backoff delay [slot=t prop ] 6: Wireless and Mobile Networks 6-21

22 Two Additional Mechanisms in PCF (Point Coordination Function) Polling to coordinate senders, e.g. to ensure QoS SIFS < PIFS < DIFS (priorities!) RTS/CTS mechanism Can t detect collision while sending Collision for long packet is wasteful RTS (Request to Send): request to reserve channel to send long packet w/o collisions CTS (Clear to Send): approve RTS Optional mechanism 6: Wireless and Mobile Networks 6-22

23 RTS/CTS [optional in MAC] Sender sends small request-to-send (RTS) RTSs may collide with each other (but are short) Include indication of length of packet transmission AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions for time specified in CTS Q: if you hear RTS only (no CTS), should you wait? Avoid data frame collisions completely using small reservation packets! 6: Wireless and Mobile Networks 6-23

24 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) RTS(A) reservation collision RTS(B) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) 6: Wireless and Mobile Networks 6-24

25 Q: Defer on RTS, CTS or both? Idea 1: RTS contains length, defer till end A B C D RTS RTS CTS CTS data ACK ACK 6: Wireless and Mobile Networks 6-25

26 Q: Defer on RTS, CTS or both? Idea 1: RTS contains length, defer till end Problem: maybe not granted? Idea 2: defer only on CTS A B C D RTS RTS RTS CTS CTS data ACK ACK 6: Wireless and Mobile Networks 6-26

27 Q: Defer on RTS, CTS or both? Idea 1: RTS contains length, defer till end Problem: maybe not granted? Idea 2: defer only on CTS What if unheard? A B C D RTS RTS CTS CTS data ACK ACK 6: Wireless and Mobile Networks 6-27

28 Q: Defer on RTS, CTS or both? Idea 1: RTS contains length, defer till end Problem: maybe not granted? Idea 2: defer only on CTS What if unheard? Solution: Defer by CTS By length in CTS Defer by RTS But only 2 DIFS! Ok if A hears either recipient or sender A B C D RTS RTS data CTS ACK CTS ACK 6: Wireless and Mobile Networks 6-28

29 DCF MAC Example 1... Data ACK time 2 time 3 RTS CTS Data ACK time 4 RTS CTS Data ACK time node defers; backoff counter frozen backoff period 1 time RTS 2 time RTS 3 time RTS CTS Data ACK 4 time 6: Wireless and Mobile Networks 6-29

30 addressing & `switching` AP H1 R1 router Internet AP R1 MAC addr AP MAC addr AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 dest. address frame source address frame AP identified in frame (Unlike regular switch!!) 6: Wireless and Mobile Networks 6-30

31 frame: addressing frame control duration address 1 address 2 address 3 seq control address 4 payload 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 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode Why 3 addresses (with AP)? Is AP a switch or a router?? 6: Wireless and Mobile Networks 6-31

32 frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for reliable ARQ) frame control duration address 1 address 2 address 3 seq control address 4 payload CRC Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd frame type (RTS, CTS, ACK, data) 6: Wireless and Mobile Networks 6-32

33 802.11: mobility within same subnet H1 remains in same IP subnet: IP address can remain same Works fine for hub Switch: which AP is associated with H1? self-learning switch will see frame from H1 and remember port to reach H1 Solution: when H1 joins, AP2 sends switch a packet from H1 BBS 1 AP 1 router hub or switch AP 2 H1 BBS 2 6: Wireless and Mobile Networks 6-33

34 MAC Management, Beacons and Traffic Indication Map (TIM) has several MAC management frames (Re/De)Association req/response, Authentication Beacon (sent periodically by AP) Timestamp, Beacon Interval, Capabilities, SSID, Rates, Parameters, Traffic Indication Map (TIM) Allows host to select AP (or host can send probe) TIM: list of (associated but sleeping) hosts with packets queued at the access point. Even sleeping hosts (sometimes) listen to Beacon To check incoming messages in TIM, get broadcasts Sleeping to save energy when idle 6: Wireless and Mobile Networks 6-34

35 Ad Hoc Networking Ad hoc networks no base stations transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves Supported in but still many open issues, research WANET: Wireless Ad- Hoc NETwork MANET: Mobile Ad-Hoc Net (they move, too!) 6: Wireless and Mobile Networks 6-35

36 802.15: 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 Evolved from Bluetooth S M S P S P M P S radius of coverage 6: Wireless and Mobile Networks 6-36 P Master device Slave device P Parked device (inactive)

37 Sensor Networks A special interesting type of Ad-Hoc network Idea: distribute low-cost `sensors` to perform measurements, even do actions Applications: Weather forecasts, natural disaster warnings Detection of physical damages (leakage, fire, ) Military applications: intelligence, smart mines Properties Wireless Random location Low cost, energy 6: Wireless and Mobile Networks 6-37

38 Which Transmission Range? When using AP/Bases, nodes must reach it Large transmission range But in sensor networks, WANET? Smaller transmission range Saves energy, allows spectrum reuse (cellular?) But: requires routing, forwarding by nodes 6: Wireless and Mobile Networks 6-38

39 Connectivity, Topology, Routing Assume nodes distributed uniformly in area One dimensional (line), two (surface), three (space) Let n be number of nodes Let r(n) be transmission range of node Questions: Probability that all/most nodes are connected Probability that entire area is `covered` by nodes connected to `base`/`edge` Routing, scheduling, broadcast protocols for nodes Using minimal resources (energy, storage) Minimize collisions 6: Wireless and Mobile Networks 6-39

40 Sensor Network Tasks/Protocols Routing, forwarding, broadcast Neighbor/topology discovery, organization E.g. setup spanning tree for efficient broadcast Optimization tasks Optimize communication Load balancing (also to save energy) Location measurement Clock synchronization Distributed computation E.g. to detect image Handling mobility (MANET) 6: Wireless and Mobile Networks 6-40

41 Chapter 6 outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics CDMA 6.3 IEEE 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 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-41

42 Components of cellular network architecture cell covers geographical region base station (BS) analogous to AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS MSC connects cells to wide area net manages call setup (more later!) handles mobility (more later!) Mobile Switching Center Mobile Switching Center Public telephone network, and Internet wired network 6: Wireless and Mobile Networks 6-42

43 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 6: Wireless and Mobile Networks 6-43

44 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 CDMA-2000 GPRS IS-136 TDMA/FDMA EDGE GSM IS-95 UMTS Don t drown in a bowl of alphabet soup: use this for reference only 6: Wireless and Mobile Networks 6-44

45 Cellular standards: brief survey 2.5 G systems: voice and data channels for those who can t wait for 3G service: 2G extensions general packet radio service (GPRS) evolved from GSM data sent on multiple channels (if available) enhanced data rates for global evolution (EDGE) also evolved from GSM, using enhanced modulation Date rates up to 384K CDMA-2000 (phase 1) data rates up to 144K evolved from IS-95 6: Wireless and Mobile Networks 6-45

46 Cellular standards: brief survey 3G systems: voice/data Universal Mobile Telecommunications Service (UMTS) GSM next step, but using CDMA CDMA more (and more interesting) cellular topics due to mobility (stay tuned for details) 6: Wireless and Mobile Networks 6-46

47 Wireless and Mobile Communication Networks: Outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics 6.3 IEEE wireless LANs ( wi-fi ) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-47

48 What is mobility? Covered in [KR], not in [KMK] 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 6-48

49 Mobility: Vocabulary home network: permanent home of mobile (e.g., /24) home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote Permanent address: address in home network, can always be used to reach mobile e.g., wide area network correspondent 6: Wireless and Mobile Networks 6-49

50 Mobility: more vocabulary Permanent address: remains constant (e.g., ) visited network: network in which mobile currently resides (e.g., /24) Care-of-address: address in visited network. (e.g., ) wide area network correspondent: wants to communicate with mobile Foreign agent: entity in visited network that performs mobility functions on behalf of mobile. 6: Wireless and Mobile Networks 6-50

51 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 6-51

52 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 6-52

53 Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence not via usual routing table exchange. scalable to millions of mobiles 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 care-ofaddress of mobile, sends directly to mobile 6: Wireless and Mobile Networks 6-53

54 Mobility: registration home network visited network wide area network foreign agent contacts home agent home: this mobile is resident in my network 2 1 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 6-54

55 Mobility via Indirect Routing home network correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area network 2 foreign agent receives packets, forwards to mobile 4 3 visited network mobile replies directly to correspondent 6: Wireless and Mobile Networks 6-55

56 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: correspondent-home-networkmobile inefficient when correspondent, mobile are in same network 6: Wireless and Mobile Networks 6-56

57 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: ongoing connections can be maintained! 6: Wireless and Mobile Networks 6-57

58 Mobility via Direct Routing home network correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited network correspondent requests, receives foreign address of mobile 2 1 wide area network 3 4 mobile replies directly to correspondent 6: Wireless and Mobile Networks 6-58

59 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 6-59

60 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) correspondent wide area network 1 correspondent agent anchor foreign agent new foreign agent foreign net visited at session start 2 new foreign network 6: Wireless and Mobile Networks 6-60

61 Response (mobile corresponding) Triangle routing: mobile corresponding Using mobile host s IP address Foreign network may block for `IP spoofing` (egress filtering) Indirect via foreign: mobile FA corresp. Requires FA (Foreign agent) to `spoof` Indirect via home: mobile home corresp. Overhead but works Direct: mobile corresponding Use temporary IP address (and mobile IP) Exercise: which are supported in mobile-ip? GSM? 6: Wireless and Mobile Networks 6-61

62 Wireless and Mobile Communication Networks: Outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics 6.3 IEEE wireless LANs ( wi-fi ) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-62

63 Mobile IP RFC 3220 has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) three components to standard: indirect routing of datagrams agent discovery registration with home agent 6: Wireless and Mobile Networks 6-63

64 Mobile IP: indirect routing foreign-agent-to-mobile packet packet sent by home agent to foreign agent: a packet within a packet dest: dest: dest: Permanent address: dest: packet sent by correspondent Care-of address: : Wireless and Mobile Networks 6-64

65 Mobile IP: agent discovery agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) H,F bits: home and/or foreign agent type = 9 code = 0 = 9 router address checksum = 9 standard ICMP fields R bit: registration required type = 16 registration lifetime length sequence # RBHFMGV bits reserved 0 or more available Care-Of-Addresses (COA) mobility agent advertisement extension 6: Wireless and Mobile Networks 6-65

66 Mobile IP: registration example home agent HA: foreign agent COA: ICMP agent adv. COA: visited network: /24 Mobile agent MA: registration req. COA: HA: MA: Lifetime: 9999 identification: 714 encapsulation format. registration req. COA: HA: MA: Lifetime: 9999 identification:714. registration reply time HA: MA: Lifetime: 4999 Identification: 714 encapsulation format. registration reply HA: MA: Lifetime: 4999 Identification: : Wireless and Mobile Networks 6-66

67 Wireless and Mobile Communication Networks: Outline 6.1 Introduction Wireless 6.2 Wireless links, characteristics 6.3 IEEE wireless LANs ( wi-fi ) Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Cellular networks 6.8 Mobility and higherlayer protocols 6.9 Summary 6: Wireless and Mobile Networks 6-67

68 Components of cellular network architecture cell covers geographical region base station (BS) analogous to AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS MSC connects cells to wide area net manages call setup (more later!) handles mobility (more later!) Mobile Switching Center Mobile Switching Center Public telephone network, and Internet wired network 6: Wireless and Mobile Networks 6-68

69 Multiple operators (providers) recall: wired public telephone network correspondent MSC MSC MSC MSC MSC different cellular networks, operated by different providers 6: Wireless and Mobile Networks 6-69

70 Handling mobility in cellular networks home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon) home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (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 6: Wireless and Mobile Networks 6-70

71 GSM: indirect routing to mobile home MSC consults HLR, gets roaming number of mobile in visited network mobile user HLR 4 2 home network home Mobile Switching Center VLR Mobile Switching Center visited network 3 correspondent 1 call routed to home network Public switched telephone 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 6-71

72 GSM: handoff with common MSC Handoff goal: route call via new base station (without interruption) old BSS VLR old routing Mobile Switching Center new routing new BSS 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 6-72

73 GSM: handoff with common MSC old BSS 1 VLR 8 Mobile Switching Center 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 6-73

74 GSM: handoff between MSCs home network Home MSC anchor MSC MSC correspondent PSTN MSC 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 Or: optional path minimization step to shorten multi-msc chain 6: Wireless and Mobile Networks 6-74

75 GSM: handoff between MSCs home network Home MSC anchor MSC MSC correspondent PSTN MSC MSC (b) after 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 6-75

76 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 the mobile user s permanent phone number belongs 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 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. Mobile IP element Home network Home agent Visited network Foreign agent Care-ofaddress 6: Wireless and Mobile Networks 6-76

77 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 noise, collisions, 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 6-77

78 Summary Wireless wireless links: capacity, distance channel impairments CDMA IEEE ( 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 mobility in GSM impact on higher-layer protocols 6: Wireless and Mobile Networks 6-78

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