Computer Networks Wireless and Mobile Networks László Böszörményi Computer Networks Mobile - 1
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 network Mobility / location managment László Böszörményi Computer Networks Mobile - 2
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 Generation Fast Digital Data László Böszörményi Computer Networks Mobile - 3
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
Cellular Network Architecture Mobile Switching Center László Böszörményi Computer Networks Mobile - 5
Elements of a wireless network (1) network 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 - 6
Elements of a wireless network (2) network infrastructure base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers 802.11 access points László Böszörményi Computer Networks Mobile - 7
Elements of a wireless network (3) 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 László Böszörményi Computer Networks Mobile - 8
rate 150*4 Mbps 802.11n 54 Mbps 5-11 Mbps 1 Mbps 384 Kbps 56 Kbps 802.11{a,g} 802.11b 802.15 Wireless link standards High Speed Packet Access HSPA ( 84) 3G+ HSDPA (3,6-7,2) HS Download PA Universal Mobile Telecommunications System Wideband Code Division Multiple Access 3G UMTS / W-CDMA, CDMA2000 2G IS-95 CDMA, GSM Indoor Outdoor Mid range outdoor Long range outdoor range 10 30m 50 200m 200m 4Km 5Km 20Km GPRS(General Packet Radio Service), 2.5 G EDGE (Enhanced Datarates for GSM Evolution), 2.5 G László Böszörményi Computer Networks Mobile - 9
Elements of a wireless network (4) network infrastructure infrastructure mode base station connects mobiles into wired network Handoff (hand over): mobile changes base station providing connection into wired network László Böszörményi Computer Networks Mobile - 10
Elements of a wireless network (4) 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 László Böszörményi Computer Networks Mobile - 11
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 network 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 - 12
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 - 13
Collision Avoidance: RTS-CTS exchange A AP B reservation collision DATA (A) defer time László Böszörményi Computer Networks Mobile - 15
What is mobility? no mobility high mobility wireless user, using same access point mobile user, connecting/ disconnecting from network using DHCP. mobile user, passing through multiple access points while maintaining ongoing connections (cell phone, or TCP) Spectrum of mobility from the network perspective Dynamic IP address assignment: DHCP solves this Permanent address: mobile client keeps same IP addresses László Böszörményi Computer Networks Mobile - 16
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 - 17
Mobile IP: indirect routing foreign-agent-to-mobile packet packet sent by home agent to foreign agent: a packet within a packet dest: 128.119.40.186 dest: 79.129.13.2 dest: 128.119.40.186 Permanent address: 128.119.40.186 dest: 128.119.40.186 packet sent by correspondent Care-of address: 79.129.13.2 László Böszörményi Computer Networks Mobile - 18
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 = 0 16 24 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 - 19
Mobile IP: registration example home agent HA: 128.119.40.7 foreign agent COA: 79.129.13.2 ICMP agent adv. COA: 79.129.13.2. visited network: 79.129.13/24 Mobile agent MA: 128.119.40.186 registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format. registration req. COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714. registration reply time HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format. registration reply HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714. László Böszörményi Computer Networks Mobile - 20