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Ch. 6: Wireless and Mobile Networks

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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 Wireless, Mobile Networks

2 Chapter 6 outline Mobility Wireless 6.1 Introduction
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 Wireless, Mobile Networks

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

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

5 Elements of a wireless network
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 network infrastructure Wireless, Mobile Networks

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

7 Characteristics of selected wireless links
200 802.11n 54 802.11a,g 802.11a,g point-to-point 5-11 802.11b 4G: LTWE WIMAX 4 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO Data rate (Mbps) 1 802.15 .384 2.5G: UMTS/WCDMA, CDMA2000 .056 2G: IS-95, CDMA, GSM Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km Wireless, Mobile Networks

8 Elements of a wireless network
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 network infrastructure Wireless, Mobile Networks

9 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 Wireless, Mobile Networks

10 Wireless network taxonomy
single hop multiple hops host connects to base station (WiFi, cellular) which connects to larger Internet host may have to relay through several wireless nodes to connect to larger Internet: mesh net infrastructure (e.g., APs) no base station, no connection to larger Internet. May have to relay to reach other wireless node MANET (mobile ad hoc netw) no infrastructure no base station, no connection to larger Internet (e.g. Bluetooth) Wireless, Mobile Networks

11 Chapter 6 outline Mobility Wireless 6.1 Introduction
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 Wireless, Mobile Networks

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 Wireless, Mobile Networks

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) 10-1 10-2 10-3 BER 10-4 10-5 10-6 10-7 10 20 30 40 SNR(decibels) Binary phase-shift keying (BPSK) In QAM-modulation the number of different wave forms used (in brackets) determines the bandwidth efficiency (=data rate / bandwidth), i.e. with QAM16 we have a 4 times higher throughput (4 times more bits per pulse). Concrete throughput depends on chosen frequency (e.g. 2.4 GHz => number of waveforms sent per sec) and number of streams used. QAM256 (8) QAM16 (4) BPSK (1) Wireless, Mobile Networks

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 10-1 10-2 10-3 BER 10-4 10-5 10-6 10-7 10 20 30 40 SNR(decibels) QAM256 (8) QAM16 (4) BPSK (1) Wireless, Mobile Networks

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

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” Wireless, Mobile Networks

17 CDMA encode/decode channel output Zi,m sender receiver Zi,m= di.cm
1 - Zi,m= di.cm data bits d0 = 1 1 - 1 - 1 - sender slot 1 channel output slot 0 channel output code slot 1 slot 0 Di = S Zi,m.cm m=1 M -8x1/8 8x1/8 received input 1 - 1 - d0 = 1 d1 = -1 slot 1 channel output slot 0 channel output code receiver slot 1 slot 0 Wireless, Mobile Networks

18 CDMA: two-sender interference
channel sums together transmissions by sender 1 and 2 Sender 1 Sender 2 ( )/8 ( )/8 using same code as sender 1, receiver recovers sender 1’s original data from summed channel data! Wireless, Mobile Networks

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 power control uses up bits we could otherwise send information with advantages in multicell system (like cellular networks) because if user sends nothing, bandwidth is not wasted because one has less interference more details: Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison Wesley, 1995. Wireless, Mobile Networks

20 Chapter 6 outline Mobility Wireless 6.1 Introduction
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 Wireless, Mobile Networks

21 IEEE 802.11 Wireless LAN different standards (802.11a/b/g/n)
differ in modulation techniques used (=> different maximal transmission rate) and in frequency range (2.4 or 5 GHz) all use CSMA/CA (carrier sense multiple access with collision avoidance) all have base-station and ad-hoc network versions Wireless, Mobile Networks

22 802.11 LAN architecture wireless host communicates with base station
Internet 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 hub, switch or router BSS 1 BSS 2 Wireless, Mobile Networks

23 802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different (partly overlapping) frequencies admin chooses frequency for AP interference possible: channels can be same or overlapping host: must associate with an AP scans channels, listening for beacon frames (beacon = “Blinklicht”) containing AP’s name (service set identifier = SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP’s subnet Wireless, Mobile Networks

24 802.11: passive/active scanning
AP 2 AP 1 H1 BSS 2 BSS 1 1 2 3 4 active scanning: Probe Request frame broadcast from H1 Probe Response frames sent from APs Association Request frame sent: H1 to selected AP Association Response frame sent from selected AP to H1 BSS 1 BSS 2 1 1 AP 1 AP 2 2 3 H1 passive scanning: beacon frames sent from APs association Request frame sent: H1 to selected AP association Response frame sent from selected AP to H1 For battery-driven devices: passive scan is preferred to save energy (APs anyway broadcast beacon frames several times per second) Wireless, Mobile Networks

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) Wireless, Mobile Networks

26 IEEE 802.11 MAC Protocol: CSMA/CA
SIFS= Short Inter-frame Spacing Time required for a receiving station to sense the end of a frame and start transmitting an ACK (compute CRC etc.) DIFS= Distributed Inter-frame Spacing sender waits before transmission If the medium is continuously idle for this duration, only then a node is supposed to transmit a frame. DIFS = SIFS + (2 * Slot time) where slot time is 20μs in b sender receiver DIFS data SIFS ACK finally frame is discarded if ACK never arrives Wireless, Mobile Networks

27 IEEE 802.11 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 2 receiver - if frame received OK return ACK after time period SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK binary exponential backoff (as for CSMA/CD) counter remains frozen while channel is sensed busy when timer expires, channel must be idle Wireless, Mobile Networks

28 IEEE 802.11 MAC Protocol: CSMA
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) sender receiver DIFS data SIFS ACK Wireless, Mobile Networks

29 decrement by one slot time Wireless, Mobile Networks

30 IEEE 802.11 MAC Protocol: CSMA
source: IEEE Std TM-2012 (Revision of IEEE Std ) Wireless, Mobile Networks

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! Wireless, Mobile Networks

32 Collision Avoidance: RTS-CTS exchange
B AP RTS(A) RTS(B) reservation collision RTS(A) CTS(A) defer DATA (A) ACK(A) The amount of time the node should wait before trying to get access to the medium is included in both the RTS and the CTS frame. time Wireless, Mobile Networks

33 802.11 frame: four address fields!
bytes frame control duration address 1 2 4 3 payload CRC 6 seq Address 4: used only in ad hoc mode 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) payload: IP datagram or ARP packet; typically less than 1500 Bytes (even though 2312 is allowed) Address 2: MAC address of wireless host or AP transmitting this frame (sender: wireless host or AP) Wireless, Mobile Networks

34 802.11 frame: addressing Internet router H1 R1 H1 MAC addr R1 MAC addr
H1 MAC addr AP MAC addr R1 MAC addr address 1 address 2 address 3 frame router H1 R1 H1 MAC addr R1 MAC addr dest. address source address 802.3 Ethernet frame from the router’s perspective, the AP is invisible (ethernet subnet!) now, H1 knows MAC adress of source router Wireless, Mobile Networks

35 802.11 frame: addressing Internet router H1 R1 R1 MAC addr H1 MAC addr
AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 frame H1 R1 R1 MAC addr H1 MAC addr dest. address source address 802.3 Ethernet frame address 3 allows the AP to determine correct destination MAC address Wireless, Mobile Networks

36 802.11 frame: more frame seq # (since frames may be
retransmitted if not ACKed) expected duration of planned transmission (including ACKs) when RTS, CTS or data is sent frame control duration address 1 2 4 3 payload CRC 6 seq Type From AP Subtype To More frag WEP data Power mgt Retry Rsvd Protocol version 2 4 1 frame type (RTS, CTS, ACK, data) frame sent by access point detailed specification:

37 802.11: mobility within same subnet
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 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 better solutions are currently being developed (so that switch is informed when H1 disassociates/associates) Wireless, Mobile Networks

38 802.11: 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 10-1 10-2 QAM256 10-3 QAM16 BPSK BER 10-4 operating point 10-5 10-6 10-7 10 20 30 40 SNR(dB) 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 Wireless, Mobile Networks

39 802.11: advanced capabilities
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 Wireless, Mobile Networks

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 Wireless, Mobile Networks

41 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 Mobile Switching Center Public telephone network base stations must not be in middle (can be in corner and use directional antennas); a single BS can even serve 3 cells wired network Wireless, Mobile Networks

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 FxT Wireless, Mobile Networks

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 Wireless, Mobile Networks

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, connecting/ disconnecting from network using DHCP (e.g. moving to different room). mobile user, passing through multiple access point while maintaining ongoing connections (e.g. cellular access in an ICE) Wireless, Mobile Networks

45 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 wide area network permanent address: address in home network, can always be used to reach mobile e.g., Wireless, Mobile Networks

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

47 How do you contact a mobile friend:
Consider friend frequently changing addresses, how do you find her? I wonder where Alice moved to? search all phone books? call her parents? expect her to let you know where he/she is? Wireless, Mobile Networks

48 Mobility: approaches search all phone books 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 call her parents advertise highly specific route to permanent address e.g. Saarbrücken phone number prefix in Lausanne? expect her to let you know Wireless, Mobile Networks

49 Mobility: approaches let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systems let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile not scalable to millions of mobiles Wireless, Mobile Networks

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

51 Mobility via indirect routing
foreign agent receives packets, forwards to mobile home agent intercepts packets, forwards to foreign agent visited network home network 3 4 1 2 wide area network correspondent addresses packets using home address of mobile mobile replies directly to correspondent source address is permanent address => usage: see Mobile IP (later) Wireless, Mobile Networks

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! Wireless, Mobile Networks

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 Wireless, Mobile Networks

54 Mobility via direct routing
foreign agent receives packets, forwards to mobile correspondent forwards to foreign agent visited network 1 2 3 4 home network mobile replies directly to correspondent correspondent requests, receives foreign address of mobile used for routing telephone calls to mobile users in several mobile telephone network standards, including GSM Wireless, Mobile Networks

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? 1 2 3 4 Wireless, Mobile Networks

56 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 foreign net visited at session start anchor foreign agent wide area network 2 1 4 3 5 new foreign network correspondent agent new foreign agent correspondent Wireless, Mobile Networks

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 Wireless, Mobile Networks

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

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 Wireless, Mobile Networks

60 Mobile IP: registration example
visited network: /24 home agent HA: foreign agent COA: mobile agent MA: 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 registration req. COA: HA: MA: Lifetime: 9999 identification: 714 …. more details: RFC 5944 lifetime: in sec (time registr. is valid) registration reply HA: MA: Lifetime: 4999 Identification: 714 …. registration reply HA: MA: Lifetime: 4999 Identification: 714 …. time Wireless, Mobile Networks

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 ) WiMAX : broadband wireless access (2.4 – 66GHz) replace cable and DSL services fourth-generation wireless => competes with LTE Wireless, Mobile Networks

62 Chapter 6 summary Wireless Mobility 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 Wireless, Mobile Networks


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