05 Chapter5 Link Layer and LANs.pptx

第1页：第6章：无线和移动

第6章：无线和移动

• # wireless (mobile) phone subscribers nowexceeds # wired phone subscribers!
• computer nets: laptops, palmtops, PDAs, Internet-enabled phone promise anytimeun tethered Internet access
• two important (but different) challenges
  • wireless:communication over wireless link
  • mobility:handling the mobile user who changes pointof attachment to network

第2页：第6章路线图

第6章路线图

• 6.2 Wireless links,characteristics
  • CDMA
• 6.3IEEE 802.11 wireless LANs (“wi-fi”)
• 6.4 Cellular Internet Access
  • architecture
  • standards (e.g., GSM)Mobility
• 6.5 Principles:addressing and routingto mobile users
• 6.6 Mobile IP
• 6.7 Handling mobility incellular networks
• 6.8 Mobility and higher-layer protocols 6.9 Summary

第3页：组成要素的无线网络网络

组成要素的无线网络网络

• laptop, PDA, IP phone
• run applications
• may be stationary(non-mobile) or mobile
  • wireless doesnotalways mean mobility

第4页：组成要素的无线网络网络

组成要素的无线网络网络

• typically connected towired network
• relay-responsiblefor sending packets between wired network and wirelesshost(s) in its “area”
  • e.g., cell towers,802.11 access points

第5页：组成要素的无线网络网络

组成要素的无线网络网络

• typically used toconnect mobile(s) tobase station
• also used as backbonelink
• multiple access protocol coordinate slink access
• various data rates,transmission distance

第6页：选定的无线链路特性

选定的无线链路特性

• 4 Km Long-range outdoor 5 Km
• 20 Km.056.384145-1154IS-95, CDMA, GSM2GUMTS/WCDMA, CDMA20003G802.15802.11 b 802.11 a,g UMTS/WCDMA-HSPDA, CDMA2000-1 x EVDO3G cellular enhanced 802.16 (Wi MAX)802.11 a,g point-to-point 200802.11 n Data rate (Mbps)data

第7页：组成要素的无线网络网络

组成要素的无线网络网络

• base station connects mobiles into wired network
• handoff: mobile changes base stationproviding connectioninto wired network

第8页：Elements wireless networkad hoc

Elements wireless networkad hoc

• no base stations
• nodes can onlytransmit to othernodes within linkcoverage
• nodes organize themselves into anetwork: route among themselves

第9页：Wireless network taxonomy single

Wireless network taxonomy single

Wireless network taxonomy single hopmultiple hops infrastructure(e.g., APs)noinfrastructurehost connects tobase station (Wi Fi, Wi MAX, cellular)which connects tolarger Internetno base station, noconnection to larger Internet (Bluetooth,ad hoc nets)host may have torelay through several wireless nodes toconnect to larger Internet:mesh netno base station, noconnection to larger Internet. May have torelay to reach othera given wireless node MANET, VANET

第10页：Wireless Link Characteristics

Wireless Link Characteristics

• 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 offobjects ground, arriving ad destination atslightly different times…. make communication across (even a point to point)wireless link much more “difficult”

第11页：无线链路特性 (2)

无线链路特性 (2)

• SNR: signal-to-noise ratio
  • larger SNR
  • easier toextract signal from noise (a“good thing”)
• SNR versus BER tradeoffs
  • given physical layer:increase power-> increase SNR->decrease BER
  • given SNR:choose physical layer that meets BERrequirement, giving highestthruput
• SNR may change withmobility: dynamically adapt physical layer (modulation technique, rate)10203040QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)SNR(d B)BER10-110-210-310-510-610-710-4

第12页：无线网络特性多

无线网络特性多

• B, A hear each other
• B, C hear each other
• A, C can not hear each othermeans A, C unaware of their interference at BABCA’s signal strength space C’s signal strength Signal attenuation:
• B, A hear each other
• B, C hear each other
• A, C can not hear each otherinterfering at B

第13页：Code Division Multiple Access (CDMA)

Code Division Multiple Access (CDMA)

• used in several wireless broadcast channels(cellular, satellite, etc) standards
• unique “code” assigned to each user; i.e., code setpartitioning
• all users share same frequency, but each user hasown “chipping” sequence (i.e., code) to encode data
• encoded signal= (original data) X (chipping sequence)
• decoding:inner-product of encoded signal andchipping sequence
• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codesare “orthogonal”)

第14页：CDMA Encode/Decodeslot

CDMA Encode/Decodeslot

CDMA Encode/Decodeslot 1 slot 0 d 1=-111111-1-1-1-Zi,m= di.cmd 0= 111111-1-1-1-11111-1-1-1-11111-1-1-1-slot 0 channel outputslot 1 channel output channel output Zi,msendercodedatabitsslot 1 slot 0 d 1=-1 d 0= 111111-1-1-1-11111-1-1-1-11111-1-1-1-11111-1-1-1-slot 0 channel outputslot 1 channel output receivercode receivedinput Di= Zi,m.cmm=1MM

第15页：CDMA双发送方干扰

CDMA双发送方干扰

CDMA: two-sender interference

第16页：第6章路线图

第6章路线图

• 6.2 Wireless links,characteristics
  • CDMA
• 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
• 6.4 cellular Internet access
  • architecture
  • standards (e.g., GSM)Mobility
• 6.5 Principles:addressing and routingto mobile users
• 6.6 Mobile IP
• 6.7 Handling mobility incellular networks
• 6.8 Mobility and higher-layer protocols 6.9 Summary

第17页：IEEE 802.11 Wireless LAN

IEEE 802.11 Wireless LAN

• 802.11 b
  • 2.4-5 GHz unlicensed spectrum
  • up to 11 Mbps
  • direct sequence spread spectrum (DSSS) in physical layer
• all hosts use same chippingcode
• 802.11 a
  • 5-6 GHz range
  • up to 54 Mbps
• 802.11 g
  • 2.4-5 GHz range
  • up to 54 Mbps
• 802.11 n:multiple antennae
  • 2.4-5 GHz range
  • up to 200 Mbps
• all use CSMA/CA for multiple access
• all have base-station and ad-hoc network versions

第18页：11 LAN architecture

11 LAN architecture

• wireless host communicateswith base station
  • base station = access point (AP)
• Basic Service Set (BSS)(aka “cell”) in infrastructuremode contains:
  • wireless hosts
  • access point (AP): basestation
  • ad hoc mode: hosts only BSS 1BSS 2 Internethub, switchor router APAP

第19页：11：信道关联

11：信道关联

• 802.11 b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
  • AP admin chooses frequency for AP
  • interference possible: channel can be same asthat chosen by neigh boring AP!
• host: mustassociatewith an AP
  • scans channels, listening forbeacon frames containing AP’s name (SSID) and MAC address
  • selects AP to associate with
  • may perform authentication [Chapter 8]
  • will typically run DHCP to get IP address in AP’ssubnet

第20页：11: passive/active scanning AP 2AP

11: passive/active scanning AP 2AP

.11: passive/active scanning AP 2AP 1H1BBS 2BBS 112234 Active Scanning:(1)Probe Request frame broadcastfrom H1(2)Probes response frame sent from APs(3)Association Request frame sent:H1 to selected AP(4)Association Response frame sent:H1 to selected APAP 2AP 1H1BBS 2BBS 11231 Passive Scanning:(1)beacon frames sent from APs(2)association Request frame sent:H1 to selected AP(3)association Response frame sent:H1 to selected AP

第21页：IEEE 802.11: multiple access

IEEE 802.11: multiple access

• avoid collisions: 2+nodestransmitting at same time
• 802.11: CSMA-sense before transmitting
  • don’t collide with ongoing transmission by other node
• 802.11:nocollision detection!
  • difficult to receive (sense collisions) when transmitting dueto weak received signals (fading)
  • can’t sense all collisions in any case: hidden terminal, fading
  • goal:avoid collisions:CSMA/C(ollision)A(voidance)ABCABCA’s signal strength space C’s signal strength

第22页：IEEE 802.11 MAC Protocol: CSMA/CA802

IEEE 802.11 MAC Protocol: CSMA/CA802

IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender 1 if sense channel idlefor DIFSthentransmit entire frame (no CD)2 ifsense channel busy thenstart random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoffinterval, repeat 2802.11 receiver-if frame received OKreturn ACK after SIFS(ACK needed dueto hidden terminal problem)sender receiver DIFSdata SIFSACK

第23页：碰撞避免RTS-CTS交换

碰撞避免RTS-CTS交换

• sender first transmits smallre quest-to-send (RTS) packetsto BS using CSMA
  • RTSs may still collide with each other (but they’re short)
• BS broadcasts clear-to-send CTS in response to RTS
• RTS heard by all nodes
  • sender transmits data frame
  • other stations defer transmissions avoid data frame collisions completelyusing small reservation packets!

第24页：Collision Avoidance: RTS-CTS exchange

Collision Avoidance: RTS-CTS exchange

Collision Avoidance: RTS-CTS exchange APABtime RTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA (A)ACK(A)ACK(A)reservation collisiondefer

第25页：frame control duration address 1 address

frame control duration address 1 address

frame control duration address 1 address 2 address 4 address 3 payload CRC22666260-23124 seqcontrol 802.11 frame: addressing Address 2:MAC addressof wireless host or APtransmitting this frame Address 1:MAC addressof wireless host or APto receive this frame Address 3:MAC addressof router interface towhich AP is attached Address 4:used onlyin ad hoc mode

第26页：Internet router APH1R1AP MAC addr H1

Internet router APH1R1AP MAC addr H1

Internet router APH1R1AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3802.11 frame R1 MAC addr AP MAC addrdest. address source address 802.3 frame 802.11 frame: addressing

第27页：frame control duration address 1 address

frame control duration address 1 address

frame control duration address 1 address 2 address 4 address 3 payload CRC22666260-23124 seqcontrol Type From APSubtype To APMorefrag WEPMoredata Powermgt Retry Rsvd Protocol version 22411111111802.11 frame: moreduration of reserved transmission time (RTS/CTS)frame seq #(for reliable ARQ)frame type(RTS, CTS, ACK, data)

第28页：hub orswitch AP 2AP 1H1BBS 2BBS

hub orswitch AP 2AP 1H1BBS 2BBS

• H1 remains in same IPsubnet: IP addresscan remain same
• switch: which AP isassociated with H1?
  • self-learning (Ch. 5):switch will see framefrom H1 and“remember” which switch port can beused to reach H1

第29页：11：高级特性速率自适应

11：高级特性速率自适应

• base station, mobile dynamically change transmission rate(physical layer modulation technique)as mobile moves, SNRvaries QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)10203040SNR(d B)BER10-110-210-310-510-610-710-4 operating point 1. SNR decreases, BERincrease as node movesaway from base station 2. When BER becomes toohigh, switch to lower transmission rate but withlower BER

第30页：11：高级特性功率管理

11：高级特性功率管理

• node-to-AP: “I am going to sleep until nextbeacon frame”
  • AP knows not to transmit frames to thisnode
  • node wakes up before next beacon frame
• beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent
  • node will stay awake if AP-to-mobile framesto be sent; otherwise sleep again until nextbeacon frame

第31页：Mradius ofcoverage SSS

Mradius ofcoverage SSS

• less than 10 m diameter
• replacement for cables(mouse, keyboard,headphones)
• ad hoc: no infrastructure
• master/slaves:
  • slaves request permission tosend (to master)
  • master grants requests
• 802.15: evolved from Bluetooth specification
  • 2.4-2.5 GHz radio band
  • up to 721 kbps

第32页：WiMAX与802.16

WiMAX与802.16

• like 802.11 & cellular:base station model
  • transmissions to/frombase station by hostswith omnidirectional antenna
  • base station-to-basestation backhaul withpoint-to-point antenna
• unlike 802.11:
  • range ~ 6 miles (“cityrather than coffeeshop”)
  • ~14 Mbpspoint-to-multi point point-to-point

第33页：16: Wi MAX: downl

16: Wi MAX: downl

• transmission frame
  • down-link subframe: base station to node
  • uplink subframe: node to base stationpream. DL-MAPUL-MAPDLburst 1SS #1DLburst 2DLburst n Initialmaint.requestconn.downlink subframe SS #2SS #kuplink subframe…………base station tells nodes who will get to receive (DL map)and who will get to send (UL map), and when
• Wi MAX standard provide mechanism forscheduling, but not scheduling algorithm

第34页：第6章路线图

第6章路线图

• 6.2 Wireless links,characteristics
  • CDMA
• 6.3IEEE 802.11 wireless LANs (“wi-fi”)
• 6.4 Cellular Internet Access
  • architecture
  • standards (e.g., GSM)Mobility
• 6.5 Principles:addressing and routingto mobile users
• 6.6 Mobile IP
• 6.7 Handling mobility incellular networks
• 6.8 Mobility and higher-layer protocols 6.9 Summary

第35页：Mobile Switching Center Public telephone

Mobile Switching Center Public telephone

• connects cells to wide area net
• manages call setup (more later!)
• handles mobility (more later!)MSC
• covers geographical region
• base station(BS)analogous to 802.11 AP
• mobile usersattachto network through BS
• air-interface:physical and link layer protocol between mobile and BScellwired network

第36页：蜂窝网络第一跳 Two

蜂窝网络第一跳 Two

• combined FDMA/TDMA:divide spectrum infrequency channels, divideeach channel into timeslots
• CDMA:code division multiple accessfrequencybandstime slots

第37页：蜂窝标准 brief 概览2G 系统

蜂窝标准 brief 概览2G 系统

• 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 IS-136GSMIS-95GPRSEDGECDMA-2000UMTSTDMA/FDMADon’t drown in a bowlof alphabet soup: use thisfor reference only

第38页：Cellular standards: brief survey 2.5 G

Cellular standards: brief survey 2.5 G

• 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
  • data rates up to 384K
• CDMA-2000(phase 1)
  • data rates up to 144K
  • evolved from IS-95

第39页：蜂窝标准 brief 概览3G 系统

蜂窝标准 brief 概览3G 系统

• Universal Mobile Telecommunications Service (UMTS)
  • data service: High Speed Uplink/Downlink packet Access (HSDPA/HSUPA): 3 Mbps
• CDMA-2000: CDMA in TDMA slots
  • data service: 1 x Evlution Data Optimized (1 x EVDO)up to 14 Mbps….. more (and more interesting) cellular topics due to mobility (staytuned for details)

第40页：第6章路线图

第6章路线图

• 6.2 Wireless links,characteristics
  • CDMA
• 6.3IEEE 802.11 wireless LANs (“wi-fi”)
• 6.4 Cellular Internet Access
  • architecture
  • standards (e.g., GSM)Mobility
• 6.5 Principles:addressing and routingto mobile users
• 6.6 Mobile IP
• 6.7 Handling mobility incellular networks
• 6.8 Mobility and higher-layer protocols 6.9 Summary

第41页：什么移动性

什么移动性

• spectrum of mobility, from thenetwork perspective:no mobilityhigh mobility mobile wireless user,using same access point mobile user, passing through multiple access point while maintaining ongoing connections (like cellphone)mobile user,connecting/disconnectingfrom networkusing DHCP.

第42页：移动性术语

移动性术语

Mobility: Vocabularyhome network:permanent“home” of mobile(e.g., 128.119.40/24)Permanent address:address in homenetwork,can alwaysbeused to reach mobilee.g., 128.119.40.186 home agent:entity that willperform mobility functions onbehalf of mobile, when mobileis remotewide area network correspondent

第43页：Mobility: more vocabulary

Mobility: more vocabulary

Mobility: more vocabulary Care-of-address:addressin visited network.(e.g., 79,129.13.2)wide area network visited network:networkin which mobile currently resides(e.g., 79.129.13/24)Permanent address:remains constant (e.g., 128.119.40.186)foreign agent:entityin visited networkthat performs mobility functions onbehalf of mobile.correspondent:wantsto communicate withmobile

第44页：如何联系移动节点

如何联系移动节点

• search all phonebooks?
• call her parents?
• expect her to let youknow where he/she is?I wonder where Alice moved to?Consider friend frequently changing addresses, how do you find her?

第45页：移动性方法

移动性方法

• Let routing handle it:routers advertise permanent address of mobile-nodes-in-residence via usualr outing 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 homeagent, then forwarded to remote
  • direct routing:correspondent gets foreign address of mobile, sends directly to mobile

第46页：移动性方法

移动性方法

• Let routing handle it:routers advertise permanent address of mobile-nodes-in-residence via usualr outing 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 homeagent, then forwarded to remote
  • direct routing:correspondent gets foreign address of mobile, sends directly to mobilenot scalableto millions ofmobiles

第47页：Mobility: registration End result

Mobility: registration End result

• Foreign agent knows about mobile
• Home agent knows location of mobilewide areanetworkhome network visited network 1 mobile contacts foreign agent onentering visited network 2 foreign agent contacts homeagent home: “this mobile isresident in my network”

第48页：Mobility via Indirect Rout

Mobility via Indirect Rout

Mobility via Indirect Routingwide area networkhome network visited network 3241 correspondent addresses packetsusing home addressof mobilehome agent intercepts packets, forwards toforeign agent foreign agent receives packets,forwards to mobile mobile repliesdirectly tocorrespondent

第49页：Indirect Routing: comments

Indirect Routing: comments

• Mobile uses two addresses:
  • permanent address:used by correspondent (hence mobile location istransparentto 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-network-mobile
  • inefficient when correspondent, mobileare in same network

第50页：间接路由 moving 之间网络

间接路由 moving 之间网络

• 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 (butwith new care-of-address)
• mobility, changing foreign networks transparent:on going connections can bemaintained!

第51页：Mobility via Direct Rout

Mobility via Direct Rout

Mobility via Direct Routingwide area networkhome network visited network 4241 correspondentre quests, receives foreign address ofmobile correspondent forwardsto foreign agent foreign agent receives packets,forwards to mobile mobile repliesdirectly tocorrespondent 3

第52页：Mobility via Direct Routing: comments

Mobility via Direct Routing: comments

• overcome triangle routing problem
• non-transparent to correspondent:correspondent must get care-of-addressfrom home agent
  • what if mobile changes visited network?

第53页：蜂窝网络移动性切换（示意图）

蜂窝网络移动性切换（示意图）

• anchor foreign agent: FA in first visited network
• data always routed first to anchor FA
• when mobile moves: new FA arranges to have dataforwarded from old FA (chaining)

第54页：第6章路线图

第6章路线图

• 6.2 Wireless links,characteristics
  • CDMA
• 6.3IEEE 802.11 wireless LANs (“wi-fi”)
• 6.4 Cellular Internet Access
  • architecture
  • standards (e.g., GSM)Mobility
• 6.5 Principles:addressing and routingto mobile users
• 6.6 Mobile IP
• 6.7 Handling mobility incellular networks
• 6.8 Mobility and higher-layer protocols 6.9 Summary

第55页：移动 IP

移动 IP

• RFC 3344
• 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

第56页：移动 IP 间接路由永久地址

移动 IP 间接路由永久地址

Mobile IP: indirect routing Permanent address:128.119.40.186 Care-of address:79.129.13.2 dest: 128.119.40.186 packet sent bycorrespondentdest: 79.129.13.2 dest: 128.119.40.186 packet sent by home agent to foreign agent: apacket within a packetdest: 128.119.40.186 foreign-agent-to-mobile packet

第57页：移动 IP 代理发现

移动 IP 代理发现

• agent advertisement:foreign/home agents advertise service by broadcasting ICMP messages(typefield = 9)R bit: registration required H, F bits: homeand/or foreign agent

第58页：移动 IP 注册示例

移动 IP 注册示例

Mobile IP: registration example

第59页：蜂窝网络体系结构组成

蜂窝网络体系结构组成

Components of cellular network architecture correspondent MSCMSCMSCMSCMSCwired public telephone network different cellular networks,operated by different providersrecall:

第60页：蜂窝网络中的移动性处理

蜂窝网络中的移动性处理

• home network:network of cellular provider yousubscribe to (e.g., Sprint PCS, Verizon)
  • home location register (HLR):database in homenetwork 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 withentry for each user currently in network
  • could be home network

第61页：Public switched telephone network mobile

Public switched telephone network mobile

Public switched telephone network mobileuserhome Mobile Switching Center HLRhome network visited network correspondent Mobile Switching Center VLRGSM: indirect routing to mobile 1 call routedto home network 2 home MSC consults HLR,gets roaming number ofmobile in visited network 3 home MSC sets up 2 ndleg of callto MSC in visited network 4MSC in visited network completescall through base station to mobile

第62页：Mobile Switching Center VLRold BSSnew

Mobile Switching Center VLRold BSSnew

• Handoff goal: route call vianew base station (without interruption)
• reasons for handoff:
  • stronger signal to/from new BSS (continuing connectivity,less battery drain)
  • load balance: free up channelin current BSS
  • GSM doesn’t mandate why toperform handoff (policy), onlyhow (mechanism)
• handoff initiated by old BSS

第63页：Mobile Switching Center VLRold

Mobile Switching Center VLRold

Mobile Switching Center VLRold BSS13245678GSM: handoff with common MSCnew BSS1. old BSS informs MSC of impending handoff, provides list of 1+new BSSs 2. MSC sets up path (allocates resources)to new BSS3. new BSS allocates radio channel foruse by mobile 4. new BSS signals MSC, old BSS: ready 5. old BSS tells mobile: perform handoff tonew BSS6. mobile, new BSS signal to activate newchannel 7. mobile signals via new BSS to MSC:handoff complete. MSC reroutes call 8 MSC-old-BSS resources released

第64页：家庭网络家庭 MSC

家庭网络家庭 MSC

• anchor MSC:first MSCvisited during cal
  • call remains routed through anchor MSC
• new MSCs add on to endof MSC chain as mobilemoves to new MSC
• IS-41 allows optionalpath minimization stepto shorten multi-MSCchain

第65页：家庭网络家庭 MSC

家庭网络家庭 MSC

• anchor MSC:first MSCvisited during cal
  • call remains routed through anchor MSC
• new MSCs add on to endof MSC chain as mobilemoves to new MSC
• IS-41 allows optionalpath minimization stepto shorten multi-MSCchain

第66页：Mobility: GSM versus Mobile IPMobile IP

Mobility: GSM versus Mobile IPMobile IP

Mobility: GSM versus Mobile IPMobile IP element Comment on GSM element GSM element Homenetwork Network to which mobile user’s permanent phone number belongs Home system Home agent Home MSC: point of contact to obtain routable address of mobile user. HLR: database inhome system containing permanent phone number, profile information, current location ofmobile user, subscription information Gateway Mobile Switching Center, or“home MSC”. Home Location Register(HLR)Visited network Network other than home system where mobile user is currently residing Visited System Foreign agent Visited MSC: responsible for setting up callsto/from mobile nodes in cells associated with MSC. VLR: temporary database entry invisited system, containing subscription information for each visiting mobile user Visited Mobile services Switching Center. Visitor Location Record (VLR)Care-of-address Routable address for telephone call segment between home MSC and visited MSC, visibleto neither the mobile nor the correspondent. Mobile Station Roaming Number(MSRN), or “roaming number”

第67页：无线移动性影响 higher 层

无线移动性影响 higher 层

• logically, impact shouldbe 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 bit-errors (discardedpackets, delays for link-layer retransmissions), andhandoff
  • TCP interprets loss as congestion, will decrease congestion window un-necessarily
  • delay impairments for real-time traffic
  • limited bandwidth of wireless links

第68页：第6章小结

第6章小结

• wireless links:
  • capacity, distance
  • channel impairments
  • CDMA
• IEEE 802.11 (“wi-fi”)
  • CSMA/CA reflects wireless channel characteristics
• cellular access
  • architecture
  • standards (e.g., GSM, CDMA-2000, UMTS)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