Computer Networks. Wireless and Mobile Networks. László Böszörményi Computer Networks Mobile - 1

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1 Computer Networks Wireless and Mobile Networks László Böszörményi Computer Networks Mobile - 1

2 Background Number of wireless (mobile) phone subscribers now exceeds number of wired phone subscribers! Computer nets: laptops, netbooks, Internetenabled phone promise anytime unlimited Internet access Three important (but different) challenges Communication over wireless link Handling mobile user who changes point of attachment to Mobility / location managment László Böszörményi Computer Networks Mobile - 2

3 The Mobile Telephone System First Generation Mobile Phones, Analog Voice Single channel, push-to-talk IMTS (Improved Mobile Telephone System, 1960s) AMPS (Advanced MPS, Bell Labs, 1982) Cellular structure, frequency reuse, high capacity Uniform in the US, different in European Countries Second Generation MPS, Digital Voice Uniform in Europe (GSM), few systems in US (D-AMPS) Third Generation Digital Voice and Data Fourth (LTE) and Fifth Generation Fast Digital Data, Mobile Broadband Channels László Böszörményi Computer Networks Mobile - 3

4 Cellular Principle In practice: roughly circular a) Frequencies are reused, but not in adjacent cells (A, B denote the used frequency areas) b) To add more users, smaller cells (microcells) can be used Temporary microcells (portable towers) e.g. for sport events Hard / soft handoff between cells takes ca. 300 ms (incl. channel switch) László Böszörményi Computer Networks Mobile - 4

5 Cellular Network Architecture Mobile Switching Center László Böszörményi Computer Networks Mobile - 5

6 Elements of a wireless (1) infrastructure wireless hosts laptop, netbook, IP phone run applications may be stationary (nonmobile) or mobile wireless does not always mean mobility László Böszörményi Computer Networks Mobile - 7

7 Elements of a wireless (2) infrastructure base station typically connected to wired relay - responsible for sending packets between wired and wireless host(s) in its area e.g., cell towers access points László Böszörményi Computer Networks Mobile - 8

8 Elements of a wireless (3) 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 László Böszörményi Computer Networks Mobile - 9

9 Maximal Bitrate of Wireless Link Standards 150*4 Mbps n High Speed [Download] Packet Access: 3G+ Long Term Evolution: 4G 54 Mbps 5-11 Mbps 1 Mbps {a,g} b > 100 Mbps 3,6 84 Mbps 384 Kbps Wireless Ethernet Standards (Mainly Indoor m) Outdoor (50-200m) Mid-range (200m-4km) Long-range (5-200km) GPRS(General Packet Radio Service): 2.5 G EDGE (Enhanced Datarates for GSM Evolution): 2.5 G Universal Mobile Telecommunications System: 3G László Böszörményi Computer Networks Mobile - 10

10 Elements of a wireless (4) infrastructure infrastructure mode base station connects mobiles into wired Handoff (hand over): mobile changes base station providing connection into wired László Böszörményi Computer Networks Mobile - 11

11 Elements of a wireless (5) Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a : route among themselves László Böszörményi Computer Networks Mobile - 12

12 Wireless Link Characteristics Wireless links has additional burdens Decreased signal strength: radio signal attenuates as it propagates through matter (path loss) Interference from other sources: standardized wireless frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well Multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times Many wireless broadcast channel standards (cellular, satellite, etc.) use Code Division Multiple Access CDMA allows multiple users to coexist and transmit simultaneously (if codes are orthogonal ) László Böszörményi Computer Networks Mobile - 13

13 Wireless Network Characteristics Multiple wireless senders and receivers create additional problems for multiple access: CSMA/CD does not work well 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 A and C unaware of their interference at B: CSMA/CD space Signal fading B, A hear each other B, C hear each other A and C unaware of their interference at B: CSMA/CD László Böszörményi Computer Networks Mobile - 14

14 Strategies to assign frequencies Beauty contest Companies argue, why are they the best Government officials decide, may lead to corruption Lottery Non-interested companies may make odd business Auction Companies may go bankruptcy (UMTS) No allocation at all in low power range E.g. cordless mouse, microwave oven, bluetooth, Wirles LAN (802.11b) May lead to interference ISM (Industrial, Scientific, Medical bands) László Böszörményi Computer Networks Mobile - 15

15 IEEE Wireless LAN (WiFi) b GHz unlicensed radio spectrum up to 11 Mbps Widely deployed, using base stations Many WiFi access points (APs) form the WiFi jungle WiFi APs are cheap and getting ubiquitous a 5-6 GHz range 54 Mbps, smaller distance g GHz range 54 Mbps, longer distance All use CSMA/CA for multiple access CA: Collision Avoidance All have base-station and ad-hoc versions László Böszörményi Computer Networks Mobile - 16

16 LAN architecture BSS 1 AP Internet hub, switch or router AP Wireless host communicates with Base Station = Access Point (AP) Basic Service Set (BSS) (aka cell ) contains Wireless hosts In infrastructure mode also base station BSS 2 All have a MAC address László Böszörményi Computer Networks Mobile - 17

17 Avoiding collisions Sender first transmits small request-to-send (RTS) packets to BS using CSMA RTS indicates time for data + ACK 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 Gives permission to sender to transmit long data frame Instructs other stations to defer transmission Avoids collisions of long data frames even for hidden terminals László Böszörményi Computer Networks Mobile - 21

18 Collision Avoidance: RTS-CTS exchange A AP B reservation collision DATA (A) defer time László Böszörményi Computer Networks Mobile - 22

19 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 We need 3 addresses due to the 3 players: host,bs,router László Böszörményi Computer Networks Mobile - 23

20 frame: addressing 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 source address frame Ethernet frame László Böszörményi Computer Networks Mobile - 24

21 802.11: mobility within same subnet H1 remains in same IP subnet IP address and even TCP connections can remain If IC hub: AP2 takes over H1 fully transparently If IC Switch Which AP is associated with H1? Self-learning must be enforced: switch will see frame from H1 and remember which port to use to reach H f: inter-ap protocol If stations move between the subnets Mobile IP needed (see later) BBS 1 AP 1 router IC: hub or switch AP 2 H1 BBS 2 László Böszörményi Computer Networks Mobile - 25

22 What is mobility? no mobility high mobility wireless user, using same access point mobile user, connecting/ disconnecting from using DHCP. mobile user, passing through multiple access points while maintaining ongoing connections (cell phone, or TCP) Spectrum of mobility from the perspective Dynamic IP address assignment: DHCP solves this Permanent address: mobile client keeps same IP addresses The latter maybe received via DHCP (don t confuse with static IP addr.) László Böszörményi Computer Networks Mobile - 26

23 Mobility: Vocabulary (1) home : 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 (PA or MA): address in home, can always be used to reach mobile e.g., wide area correspondent László Böszörményi Computer Networks Mobile - 27

24 Mobility: Vocabulary (2) Permanent address: remains constant (e.g., ) visited : in which mobile currently resides (e.g., /24) Care-of-address (COA): address in visited. (e.g., ) wide area correspondent: wants to communicate with mobile correspondent foreign agent: entity in visited that performs mobility functions on behalf of mobile. László Böszörményi Computer Networks Mobile - 28

25 Mobility: registration home visited 2 wide area foreign agent contacts home agent home: this mobile is resident in my 1 mobile contacts foreign agent on entering visited End result: Foreign agent knows about mobile Home agent knows location of mobile László Böszörményi Computer Networks Mobile - 30

26 Mobility via Indirect (Triangle) Routing home correspondent addresses packets using home address of mobile home agent intercepts packets, encapsulates and forwards to foreign agent 1 wide area 2 foreign agent receives and decapsulates packets, forwards to mobile 4 3 visited mobile replies directly to correspondent László Böszörményi Computer Networks Mobile - 31

27 Mobility via Direct Routing home correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited correspondent requests, receives foreign address of mobile 2 1 wide area 3 4 mobile replies directly to correspondent László Böszörményi Computer Networks Mobile - 33

28 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) Agent discovery Protocols to advertise and solicit services of home and foreign (H/F) agents Registration with home agent Protocols to register/deregister COAs Indirect routing of datagrams Rules for tunneling, forwarding, error handling László Böszörményi Computer Networks Mobile - 36

29 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: László Böszörményi Computer Networks Mobile - 37

30 Mobile IP: agent discovery Agent advertisement Foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9) periodically Agent solicitation Mobile starts (type=10) 0 type = 9 8 code = checksum H,F bits: home and/or foreign agent router address standard ICMP fields R bit: registration required M,G encapsulation bits: IP-in-IP or smthg. else type = 16 registration lifetime length sequence # RBHFMGV bits 0 or more care-ofaddresses reserved mobility agent advertisement extension László Böszörményi Computer Networks Mobile - 38

31 Mobile IP: registration example home agent HA: foreign agent COA: ICMP agent adv. COA: visited : /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: 714. László Böszörményi Computer Networks Mobile - 39

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