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

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Presentation on theme: "Wireless, Mobile Networks6-1 Ch. 6: Wireless and Mobile Networks Background:  # wireless (mobile) phone subscribers now exceeds # wired phone subscribers."— Presentation transcript:

1 Wireless, Mobile Networks6-1 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

2 Wireless, Mobile Networks6-2 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 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

3 Wireless, Mobile Networks6-3 Elements of a wireless network network infrastructure cell tower in cellular network (e.g. 3G or 4G) access point in wireless LAN

4 Wireless, Mobile Networks6-4 wireless hosts  laptop, smartphone  run applications  may be stationary (non- mobile) or mobile  wireless does not always mean mobility Elements of a wireless network network infrastructure

5 Wireless, Mobile Networks6-5 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, access points Elements of a wireless network network infrastructure

6 Wireless, Mobile Networks6-6 wireless link  typically used to connect mobile(s) to base station  multiple access protocol coordinates link access  various data rates, transmission distance Elements of a wireless network network infrastructure

7 Wireless, Mobile Networks6-7 Characteristics of selected wireless links Indoor 10-30m Outdoor m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km G: IS-95, CDMA, GSM 2.5G: UMTS/WCDMA, CDMA b a,g 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 4G: LTWE WIMAX a,g point-to-point n Data rate (Mbps)

8 Wireless, Mobile Networks6-8 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 Elements of a wireless network network infrastructure

9 Wireless, Mobile Networks6-9 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 Elements of a wireless network

10 Wireless, Mobile Networks6-10 Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, cellular) which connects to larger Internet no base station, no connection to larger Internet (e.g. Bluetooth) 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 wireless node MANET (mobile ad hoc netw)

11 Wireless, Mobile Networks6-11 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 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

12 Wireless, Mobile Networks6-12 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

13 Wireless, Mobile Networks6-13 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) QAM256 (8) QAM16 (4) BPSK (1) SNR(decibels) BER

14 Wireless, Mobile Networks6-14 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 QAM256 (8) QAM16 (4) BPSK (1) SNR(decibels) BER

15 Wireless, Mobile Networks6-15 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B 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 signal strength space C’s signal strength 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

16 Wireless, Mobile Networks6-16 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”

17 Wireless, Mobile Networks6-17 CDMA encode/decode slot 1 slot 0 d 1 = Z i,m = d i. c m d 0 = slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M 8x1/8 -8x1/8

18 Wireless, Mobile Networks6-18 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 ( )/8 ( )/8

19 Wireless, Mobile Networks 6-19 CDMA: Code Division MA Drawbacks: -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 larger Main 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.

20 Wireless, Mobile Networks6-20 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 Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Summary

21 Wireless, Mobile Networks6-21 IEEE 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

22 Wireless, Mobile Networks 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, switch or router

23 Wireless, Mobile Networks : Channels, association  b: 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

24 Wireless, Mobile Networks : passive/active scanning AP 2 AP 1 H1 BSS 2 BSS passive scanning: (1)beacon frames sent from APs (2)association Request frame sent: H1 to selected AP (3)association Response frame sent from selected AP to H1 AP 2 AP 1 H1 BSS 2 BSS 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

25 Wireless, Mobile Networks6-25 IEEE : 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)

26 Wireless, Mobile Networks6-26 IEEE 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 20 μ s in b sender receiver DIFS data SIFS ACK

27 Wireless, Mobile Networks6-27 IEEE MAC Protocol: CSMA/CA 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 receiver - if frame received OK return ACK after time period SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

28 Wireless, Mobile Networks6-28 IEEE 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)

29 Wireless, Mobile Networks 6-29 decrement by one slot time

30 Wireless, Mobile Networks 6-30 IEEE MAC Protocol: CSMA source: IEEE Std TM-2012 (Revision of IEEE Std )

31 Wireless, Mobile Networks6-31 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!

32 Wireless, Mobile Networks6-32 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

33 Wireless, Mobile Networks6-33 frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control frame: four address fields! Address 2: MAC address of wireless host or AP transmitting this frame (sender: wireless host or AP) Address 1: MAC address of wireless host or AP to receive this frame (destination within subnet) Address 3: MAC address of router interface to which AP is attached (=> send to Internet; see example on next slide) Address 4: used only in ad hoc mode bytes

34 Wireless, Mobile Networks6-34 Internet router H1 R1 H1 MAC addr AP MAC addr R1 MAC addr address 1 address 2 address frame H1 MAC addr R1 MAC addr dest. address source address Ethernet frame frame: addressing from the router’s perspective, the AP is invisible (ethernet subnet!) now, H1 knows MAC adress of source router

35 Wireless, Mobile Networks6-35 Internet router H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame R1 MAC addr H1 MAC addr dest. address source address Ethernet frame frame: addressing address 3 allows the AP to determine correct destination MAC address

36 frame control duration address 1 address 2 address 4 address 3 payloadCRC seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version expected duration of planned transmission (including ACKs) when RTS, CTS or data is sent frame seq # (since frames may be retransmitted if not ACKed) frame type (RTS, CTS, ACK, data) frame: more frame sent by access point detailed specification:

37 Wireless, Mobile Networks : 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 2 BSS 1 H1 detects weakening signal from AP1  scans and finds beacon frames of AP2  dissociates with AP1, associates with AP2  remains 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)

38 Wireless, Mobile Networks : advanced capabilities Rate 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 SNR(dB) BER 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

39 Wireless, Mobile Networks6-39 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 : advanced capabilities

40 Wireless, Mobile Networks6-40 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 Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Summary

41 Wireless, Mobile Networks6-41 Mobile Switching Center Public telephone network Mobile Switching Center Components of cellular network architecture  connects cells to wired tel. net.  manages call setup (more later!)  handles mobility (more later!) MSC  covers geographical region  base station (BS) analogous to AP  mobile users attach to network through BS  air-interface: physical and link layer protocol between mobile and BS cell wired network

42 Wireless, Mobile Networks6-42 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) frequency bands time slots

43 Wireless, Mobile Networks6-43 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 Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Summary

44 Wireless, Mobile Networks6-44 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).

45 wide area network Wireless, Mobile Networks6-45 Mobility: vocabulary home network: permanent “home” of mobile (e.g., /24) permanent address: address in home network, can always be used to reach mobile e.g., home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote

46 Wireless, Mobile Networks6-46 Mobility: more vocabulary wide area network care-of-address: address in visited network. (e.g., 79, ) visited network: network in which mobile currently resides (e.g., /24) permanent address: remains constant ( e.g., ) foreign agent: entity in visited network that performs mobility functions on behalf of mobile. correspondent: wants to communicate with mobile

47 Wireless, Mobile Networks6-47 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?

48 Wireless, Mobile Networks6-48 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

49 Wireless, Mobile Networks6-49  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

50 wide area network Wireless, Mobile Networks6-50 Mobility: registration end result:  foreign agent knows about mobile  home agent knows location of mobile home network visited network 1 mobile contacts foreign agent on entering visited network 2 foreign agent contacts home agent: “this mobile is resident in my network”

51 Wireless, Mobile Networks6-51 Mobility via indirect routing wide area network home network visited network correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent source address is permanent address => usage: see Mobile IP (later)

52 Wireless, Mobile Networks6-52 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!

53 Wireless, Mobile Networks6-53 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

54 Wireless, Mobile Networks6-54 Mobility via direct routing home network visited network correspondent requests, receives foreign address of mobile correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent used for routing telephone calls to mobile users in several mobile telephone network standards, including GSM

55 Wireless, Mobile Networks6-55 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?

56 Wireless, Mobile Networks6-56 wide area network 1 foreign net visited at session start anchor foreign agent 2 4 new foreign agent 3 correspondent agent correspondent new foreign network 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

57 Wireless, Mobile Networks6-57 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 Mobility 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Summary

58 Wireless, Mobile Networks6-58 Mobile IP: indirect routing Permanent address: Care-of address: dest: packet sent by correspondent dest: dest: packet sent by home agent to foreign agent: a packet within a packet dest: foreign-agent-to-mobile packet RFC 5944

59 Wireless, Mobile Networks6-59 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

60 Wireless, Mobile Networks6-60 Mobile IP: registration example visited network: /24 home agent HA: foreign agent COA: mobile agent MA: registration req. COA: HA: MA: Lifetime: 9999 identification:714 …. registration reply HA: MA: Lifetime: 4999 Identification: 714 …. registration reply HA: MA: Lifetime: 4999 Identification: 714 …. time ICMP agent adv. COA: …. registration req. COA: HA: MA: Lifetime: 9999 identification: 714 …. now, FA knows that it should look for datagrams with COA and forward them decapsulated more details: RFC 5944

61 Current state of Mobile IP not 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 sufficient  GSM (telephone) networks have their own mobility solutions (indirect routing; phone number is permanent, roaming number is COA)  deployed in WiMAX networks (IEEE ) Wireless, Mobile Networks 6-61

62 Wireless, Mobile Networks6-62 Chapter 6 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


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