MSIT 413: Wireless Technologies Week 9

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1 MSIT 413: Wireless Technologies Week 9 Michael L. Honig Department of EECS Northwestern University March 2017

2 Schedule for Today Finish discussion of random access Core networking; mobility management Resource management, scheduling 2

3 Random Access Access Point (AP) Station A Station C Station B Terminals send/receive messages (packets) to/from the AP at random times (i.e., when they appear). 3

4 Carrier Sense Multiple Access (CSMA) Packet arrives Sense channel Busy? no Transmit packet yes Delay transmission (non-persistent) Listen before talk (LBT) protocol Collisions occur if transmitters cannot sense the other transmission (e.g., due to large propagation delay) Lower probability of collision/higher throughput than ALOHA Long propagation times è more collisions ALOHA preferred for wide area applications

5 CSMA with Collision Detection (CSMA/CD) Nodes detect a collision in progress, and stop transmitting before the entire packet is transmitted. Assumes nodes can hear each other when they are transmitting. Appropriate for wired channels. Problems with wireless channels: Nodes cannot transmit and receive at the same frequency at the same time. Not all nodes may be in range of each other.

6 Hidden Terminal Problem Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D.

7 Hidden Terminal Problem Coverage area for station C. Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D. C may not sense A s transmission, causing a collision at B.

8 Example: Cognitive Radio Primary users of spectrum share with secondary users Secondary users cannot disrupt primary service Proposed for TV white space unused TV channels available for secondary use TV transmitter TV receiver

9 Exposed Terminal Problem Station A Station B Station D Station C B is transmitting to A. C wants to transmit to D

10 Exposed Terminal Problem Coverage area for station C. Station A Station B Station D Station C B is transmitting to A. C wants to transmit to D. C senses B s transmission, and does not transmit even though it would not cause interference at A.

11 Basic Problem Carrier sensing determines whether or not there are interfering sources near the transmitter, whereas we want to know if there are active receivers nearby.

12 Solutions Busy-tone multiple access (BTMA) Separate control channel used to indicate that the channel is idle or busy. An active station transmits a busy tone on the control channel. Each receiver that senses a busy tone turns on its own busy tone. Used in ad hoc networks. Digital or Data Sense Multiple Access (DSMA) Used in FDD cellular mobile data networks Forward channel periodically broadcasts a busy/idle bit for the reverse link. Mobile transmits if bit is in idle state; base station sets bit to busy. Not carrier sensing: sensing is performed after demodulation. Multiple Access with Collision Avoidance (MACA)

13 Hidden Terminal Problem Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D.

14 Revealing the Hidden Terminal Station A RTS Station B Coverage area for station C. Station D Station C A first sends a Request to Send (RTS) packet to B.

15 Revealing the Hidden Terminal CTS Coverage area for station C. Station A Station B Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C!

16 Revealing the Hidden Terminal Station A data Station B Coverage area for station C. Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C! C is silent for duration of A s transmission (specified in CTS) 16

17 Revealing the Hidden Terminal Coverage area for station C. Station A Station B Station D Station C What if C hears RTS, but not CTS?

18 Exposed Terminal Station A RTS Station B Coverage area for station C. Station D Station C C will not hear the CTS from A.

19 RTS Collision Station A RTS Station B Station E RTS Station C RTS messages from E and B collide à exponential backoff

20 Corrupted CTS Station A CTS Station B Station E Data, RTS, or CTS Station C CTS message from A is corrupted due to interference from E à exponential backoff by B

21 MACA Protocol (RTS/CTS) Transmitter Receiver Request to Send (RTS), packet length Clear to Send (CTS), packet length Data Ack Terminals receiving either an RTS or CTS must not transmit for the duration of the packet. (What if the terminal hears RTS but not CTS?) Collision occurs if multiple nodes transmit an RTS, or the CTS is not heard due to other interference. Collision è binary exponential back-off

22 Wireless Local Area Networks (WLANs) Low mobility, high data rates within confined region (building or campus) Competitive with other wireless data systems (4G, fixed wireless access) Unlicensed bands Industrial, Scientific, Medical (ISM): 2.4 GHz National Information Infrastructure (UNII): 5 GHz Must accept interference, therefore uses spread spectrum signaling, or random access with collision avoidance. 22

23 Comparison of Wireless Systems 23

24 Overview of Standard IEEE GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE b Extension 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 24

25 Overview of Standard IEEE discontinued 2.4 GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE b Extension discontinued 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 25

26 802.11b vs g b g Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels Max Speed 11 Mbps Direct Sequence Spread Spectrum (DSSS) CSMA/CA same same channels 6 to 54 Mbps via OFDM 5.5 and 11 Mbps via DSSS CSMA/CA 26

27 802.11g vs a g a Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels 6 to 54 Mbps via OFDM fallback to DSSS CSMA/CA Indoor/outdoor range: 38m/140m 5 Ghz range 5 channels Max Speed 54 Mbps Orthogonal Frequency Division Multiplexing (OFDM) CSMA/CA Shorter range 27

28 802.11g vs n g n Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels (20 MHz) 6 to 54 Mbps via OFDM fallback to DSSS CSMA/CA Indoor/outdoor range: 38m/140m 2.4 or 5 Ghz MIMO enhancement to 11g/11a Rates (20 MHz/40MHz): 72.2 / 150 Mbps per MIMO stream Up to 4 MIMO streams Indoor/outdoor range: 70m / 250m 28

29 802.11n vs ac n ac 2.4 or 5 Ghz MIMO enhancement to 11g/11a Rates (20 MHz/40MHz): 72.2 / 150 Mbps per MIMO stream Up to 4 MIMO streams Indoor/outdoor range: 70m / 250m 64 QAM 5 Ghz Enhancement to 11a with multi-user MIMO Rates (20 to 160 MHz): 96.3 to Mbps per MIMO stream Up to 8 MIMO streams Indoor/outdoor range: 35m / 115m 256 QAM 29

30 Peer-to-Peer (Ad Hoc) Configuration Single Cell Station A Station C Station B Mobile devices are referred to as Stations. Each Station can communicate directly with another Station. System referred to as Independent Basic Service Set (IBSS) 30

31 Infrastructure Configuration Access Point Single Cell Station A Station C Station B Access Point is analogous to a cellular Base Station System referred to as Infrastructure Basic Service Set (BSS) 31

32 Extended BSS (EBSS) Configuration Distribution System (DS) BSS Connect APs via a wired network System referred to as Extended BSS APs have a BSSID System has a SSID Basic Service Set 32

33 802.11b: Physical Layer CCK (Complementary Code Keying): Each 8-bit symbol is mapped to 8 four-phase (QPSK) symbols. 33 The signatures (codes) are orthogonal.

34 Protocol Architecture Logical link control Contention-free service Contention service MAC Layer Point Coordination Function (PCF) Distributed Coordination Function (DCF) Air interface (802.11/11a/11b/11g/11n) PCF: Point Coordination Control Function (infrastructure) 34

35 Point Coordination Function (PCF) Access Point Single Cell Station A Station C Station B Access Point Becomes a Scheduler Contention Free Mode (TDMA) Optional MAC feature not widely available in products. 35

36 Protocol Architecture Logical link control Contention-free service Contention service MAC Layer Point Coordination Function (PCF) Distributed Coordination Function (DCF) Air interface (802.11/11a/11b/11g) PCF: Point Coordination Control Function (infrastructure) DCF: CSMA/CA with additional virtual carrier sensing (ad hoc) DCF is more prevalent; PCF and DCF can co-exist 36

37 Basic Access Method DIFS Contention window PIFS DIFS Busy medium SIFS Backoff window Next frame Slot time Time Defer access Select slot using binary exponential backoff 37

38 Inter Frame Spacing (IFS) Station has to detect a minimum idle time before transmit Time depends on type of Frame, type of MAC SIFS: Short IFS (used by ACKs, CTS) PIFS: PCF IFS used by PCF Frames DIFS: DCF IFS used by DCF Frames SIFS < PIFS < DIFS Relative values of IFS used to prioritize Medium Access 38

39 CSMA/CA (Collision Avoidance) 39

40 Distributed Control Function (DCF) Network Allocation Vector: Prevents transmissions by other terminals. Virtual carrier sensing: RTS/CTS packets state the duration of the packet. All stations within range must remain silent during that period. 40

41 Frame Structure Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size 41

42 Frame Structure Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) 42

43 Frame Structure Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) Frame Control: indicates if frame is data, RTS, CTS, or other type of control. 43

44 Frame Structure Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) Frame Control: indicates if frame is data, RTS, CTS, or other type of control. Duration: indicates time in microseconds that the channel will be allocated for transmission (virtual carrier sensing) 44

45 Frame Structure Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) Frame Control: indicates if frame is data, RTS, CTS, or other type of control. Duration: indicates time in microseconds that the channel will be allocated for transmission (virtual carrier sensing) Sequence Control: identifies place in packet sequence for fragmentation and reassembly CRC: Cyclic Redundancy Check (for error detection) 45

46 Association/Disassociation Active scanning: laptop scans different frequencies Passive scanning: laptop waits for beacon on a particular frequency 46

47 Association/Disassociation Probe Request 47

48 Association/Disassociation Probe Request Probe Response 48

49 Association/Disassociation Probe Request Probe Response Authenticate 49

50 Association/Disassociation Probe Request Probe Response Authenticate Associate Request 50

51 Association/Disassociation Probe Request Probe Response Authenticate Associate Request Associate Response 51

52 Association/Disassociation Probe Request Probe Response Authenticate Associate Request Associate Response Disassociate (No ack) 52

53 Power Management Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). 53

54 Power Management Beacon announces packet arrival. Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). Wakes up at designated times to receive a burst. 54

55 Power Management Transmits packet(s). Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). Wakes up at designated times to receive a burst. 55

56 Core Networking Functions Call setup Mobility management Handoff 56

57 Wireline Call Setup Switch wire pair Helen s phone off hook PSTN Information Flow network address Bob s phone 57

58 Wireline Call Setup Switch wire pair Helen s phone off hook dial tone PSTN Information Flow network address Bob s phone 58

59 Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes PSTN Information Flow network address alert signal Bob s phone 59

60 Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication PSTN Information Flow network address alert signal Bob s phone 60

61 Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication remove ring indication PSTN Information Flow network address alert signal off hook Bob s phone 61

62 Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication remove ring indication PSTN Information Flow conversation network address alert signal off hook Bob s phone 62

63 Cellular Call Setup 1. Call Request 63

64 Cellular Call Setup 1. Call Request 2. Send numbers to switch 3. Page Receiver 4. Request Channel 64

65 Cellular Call Setup (cont.) 5. Switch assigns channels 6. Cellular conversation is set up 65

66 Information Flow for Cellular Call 66

67 Mobility Management Location management How to track mobile users? Handoff management How to change the wireless access point? Uses wireline resources: Uses wireless resources: 67

68 Mobility Management Location management How to track mobile users? Handoff management How to change the wireless access point? Uses wireline resources: database and signaling link capacity Uses wireless resources: paging, registration 68

69 Location Areas (LAs) LA-1 LA-2 LOCATION UPDATE NO LOCATION UPDATE Mobiles announce changes in LAs Requires database updates 69

70 Paging LA-1 LA-2 LOCATION UPDATE NO LOCATION UPDATE Identifies cell of mobile user. Mobile is paged within reported LA. 70

71 Location Area Tradeoff Small location areas: Mobile users update frequently Fewer wireless resources spent on paging Large location areas: Mobile users update infrequently More wireless resources spent on paging Can also implement dynamic update rules (e.g., depending on time since last update, distance traveled, mobility and call patterns) 71

72 Location Databases HLR Service provider footprint Local service area VLR Home Location Register: Stores and manages all mobile subscriptions for a specific operator Contains directory number, profile information, current location, and validation period Visitor Location Register: Stores and manages subscription information for visiting subscribers. Directs calls to and from visiting subscribers. Both are accessed through the Mobile Switching Center (MSC). 72

73 Interconnection Between a Cellular Network and the PSTN STP SCP SCP: Service Control Point STP: Signal Transfer Point SSP: Service Switching Point STP SCP ( HLR ) SSP Trunk PSTN MSC Cellular switch 73

74 Interconnection Between a Cellular Network and the PSTN SCP STP STP SCP ( HLR ) SSP: Service Switching Point MSC: Mobile Switching Center The SSP can be a central office in the PSTN, or another MSC. SSP Trunk PSTN MSC Cellular switch 74

75 Interconnection Between a Cellular Network and the PSTN SCP STP STP SCP SCP: Service Control Point ( HLR ) Contains HLR/VLR databases Answers queries from SSP Connected to SSP through the STP SSP Trunk PSTN MSC Cellular switch 75

76 Interconnection Between a Cellular Network and the PSTN SCP STP STP SCP ( HLR ) STP: Signal Transfer Point Relays messages from network switch to databases. Come in matched pairs to reduce chances of failure. SSP Trunk PSTN MSC Cellular switch 76

77 Common Channel Signaling (CCS) SCP STP STP SCP ( HLR ) CCS control channels Used for call-setup, database queries, addressing, traffic control. SSP Use different channels from data traffic (out-of-band signaling). Trunk PSTN MSC Cellular switch 77

78 Mobile Station (MS) Registration (IS-41) NEW VLR HLR OLD VLR Chicago Los Angeles, CA New York City MS was registered in NYC, then travels to Los Angeles. HLR shows location as NYC. VLR in NYC contains MS. 78

79 Mobile Station (MS) Registration (IS-41) NEW VLR 1 HLR Chicago OLD VLR Los Angeles, CA New York City 1. MS turns on, registers with local (new) VLR 79

80 Mobile Station (MS) Registration (IS-41) NEW VLR 2 HLR OLD VLR 1 Chicago New York City Los Angeles, CA 1. MS turns on, registers with local (new) VLR 2. New VLR informs user s HLR of new location. HLR sends an ack, which includes the MS s profile, to the new VLR. 80

81 Mobile Station (MS) Registration (IS-41) NEW VLR 2 HLR OLD VLR 1 3 Chicago Los Angeles, CA New York City 1. MS turns on, registers with local (new) VLR 2. New VLR informs user s HLR of new location. HLR sends an ack, which includes the MS s profile, to the new VLR. 3. The new VLR informs the MS of the successful registration. 81

82 Mobile Station (MS) Registration (IS-41) NEW VLR 2 HLR 4 OLD VLR 1 3 Chicago Los Angeles, CA 1. MS turns on, registers with local (new) VLR New York City 2. New VLR informs user s HLR of new location. HLR sends an ack, which includes the MS s profile, to the new VLR. 3. The new VLR informs the MS of the successful registration. 4. After step 2, the HLR also sends a deregistration message to cancel 82 the obsolete location record in the old VLR.

83 Call Delivery (IS-41) PSTN MSC Mobile places a call to another mobile in a different location area. What are the steps for call delivery? 83

84 Call Delivery (IS-41) 1 SSP 1 HLR MSC 1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. 84

85 Call Delivery (IS-41) 1 SSP 1 1 HLR VLR MSC 1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address. 85

86 Call Delivery (IS-41) 1 SSP 1 2 HLR 1 2 VLR MSC 1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address. 2. The VLR returns the address to the SSP through the HLR. 86

87 Call Delivery (IS-41) PSTN 1 2 HLR 1 2 VLR 3 MSC 1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address. 2. The VLR returns the address to the SSP through the HLR. 3. A trunk (voice circuit) is set up from the originating switch to the MS through the visited MSC. 87

88 Call Delivery (IS-41) PSTN 1 2 HLR 1 2 VLR 3 MSC 1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address. 2. The VLR returns the address to the SSP through the HLR. 3. A trunk (voice circuit) is set up from the originating switch to the MS through the visited MSC. 4. Page is sent in location area to determine base station of MS. 88

89 Pointer Fowarding Move Operation SCP ( HLR ) MSC SCP ( VLR ) MSC SCP ( VLR ) MSC SCP ( VLR ) Find Operation SCP ( HLR ) MSC SCP ( VLR ) MSC SCP ( VLR ) MSC SCP ( VLR ) 1. Move operation (registration): Pointer is created from the old VLR to the new VLR. No HLR registration is required. 2. Find operation (call delivery): The pointer chain is traced to locate MS. Pointer from HLR moved to destination VLR. 89

90 Mobile IP Internet Enables computers to maintain internet connectivity (TCP connections) while moving among internet attachments. Applies to both mobile use and to nomadic use (e.g., with wired connections). Forwards packets to new IP address via tunneling. Some analogies with cellular mobility management: HLR! Home agent VLR! Foreign agent 90

91 Packet Delivery to Node A Mobile node A Home network for A Home agent Internet or other topology of routers and links Foreign network Foreign agent Server X Home agent routes traffic to devices associated with network A. Foreign agent routes traffic to visiting devices. 91

92 Packet Delivery to Node A Mobile node A Home network for A Home agent 1 Internet or other topology of routers and links Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 92

93 Packet Delivery to Node A Mobile node A Home network for A Home agent Internet or other topology of 2 routers and links 1 Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 2. IP datagram is intercepted by home agent and tunneled to A s Care-Of-Address (COA). 93

94 Packet Delivery to Node A Mobile node A Home network for A Home agent Internet or other topology of 2 routers and links 1 3 Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 2. IP datagram is intercepted by home agent, and tunneled to A s Care-Of-Address (COA). 3. Foreign agent delivers IP datagram to A across the foreign network. 94

95 Packet Delivery From Node A Mobile node A Home network for A Home agent Internet or other topology of 2 routers and links Foreign network Foreign agent Server X 4. A sends IP traffic to X through foreign agent. 5. IP datagram is delivered to X over internet. 95

96 Mobile IP: Basic Operations 1. Discovery: Identifies prospective Home Agents and Foreign Agents for mobile node (MN). 2. Registration: Authenticated procedure to inform the home agent of its COA. 3. Tunneling: Forwards IP datagrams from a home address to a COA. 96

97 Determines whether or not attachment point has changed (e.g., due to handoff). MN continually listens for advertisements from foreign and home agents. Eligible routers issue periodic broadcast messages. Advertisements include: IP address of router If registration is required The maximum lifetime of registration request If router is busy Nature of agent (home and/or foreign ) Maximum allowable time of registration request COAs supported Discovery: Details MN compares the network portion of IP address with its own home address. Mismatch implies that the MN is on a foreign network. MN can solicit an agent advertisement (e.g., if timer has expired). If no foreign agents are available, then MN may act as its own foreign agent by using a co-located COA. 97

98 Registration: Details Message is sent to Home Agent to set up COA. MN sends registration request to Foreign Agent. Foreign agent relays request to MN s Home Agent. The HA accepts or denies the request and sends a registration reply to the FA. The FA relays this reply to the MN. The HA creates a mobility binding between the MN s home address and the current COA. Registration request message includes: Request to retain old bindings (e.g., for handoff) Request to receive broadcast datagrams in home network If the mobile node is using a co-located COA Lifetime of binding IP addresses of the HA and FA (i.e., the COA), and the home address of the MN 98 Authentication extension for security.

99 Tunneling Home Agent steals identity of MN Example: R3 is the HA for a host H attached to a foreign network R3 informs IP layer in LAN Z that datagrams destined for H s address should be sent to R3 R2 and D (connected to LAN Z) insert address of R3 at the MAC-level for all packets transmitted to H. LAN X A B INTERNET R1 LAN Y R3 C R2 D LAN Z 99

100 Packet Delivery to Node A Mobile node A Home network for A Home agent 1 Internet or other topology of routers and links Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. Source / destination addresses? 100

101 Source / Destination Addresses (Step 1) IP version number, and other header fields Tunnel source IP address (HA) COA (for FA) IP version number and other header fields Original source IP address Home IP address of MN TCP and rest of packet 101

102 Packet Delivery to Node A Home network for A Home agent Internet or other topology of 2 routers and links 1 Mobile node A Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 2. IP datagram is intercepted by home agent and tunneled to A s Care-Of-Address (COA). 102

103 Tunneling: IP-within-IP Encapsulation IP version number, and other header fields Tunnel source IP address (HA) COA (for FA) IP version number and other header fields Original source IP address Home IP address of MN TCP and rest of packet 103

104 Packet Delivery to Node A Mobile node A Home network for A Home agent Internet or other topology of 2 routers and links 1 3 Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 2. IP datagram is intercepted by home agent, and tunneled to A s Care-Of-Address (COA). 3. Foreign agent delivers datagram to A across foreign network: 104

105 Packet Delivery to Node A Mobile node A Home network for A Home agent Internet or other topology of 2 routers and links 1 3 Foreign network Foreign agent Server X 1. Server X transmits IP datagram to mobile node A s home address. 2. IP datagram is intercepted by home agent, and tunneled to A s Care-Of-Address (COA). 3. Foreign agent delivers datagram to A across foreign network: Strips off outer IP header, encapsulates the original IP datagram in a network-level packet. 105

106 Delivery to A (Step 3) IP version number, and other header fields Foreign agent IP address Foreign network IP address of MN IP version number and other header fields Original source IP address Home IP address of MN TCP and rest of packet 106

107 Issues Triangle routing may be inefficient. Handoff during registration. Old data is dropped by old FA, retransmitted, and re-tunneled. Other possibilities: Smooth handoff: old FA tunnels to new FA Old FA may tunnel back to HA Packets from MN may have to be tunneled through the HA. Foreign network may have a firewall Called Reverse tunneling 107

108 Handoff Decision Depends on RSS, time to execute handoff, hysteresis, and dwell (duration of RSS) Proprietary methods Handoff may also be initiated for balancing traffic. 1G (AMPS): Network Controlled Handoff (NCHO) Handoff is based on measurements at BS, supervised by MSC. 2G, GPRS: Mobile Assisted Handoff (MAHO) Handoff relies on measurements at mobile Enables faster handoff Mobile data, WLANs (802.11): Mobile Controlled Handoff (MCHO) Handoff controlled by mobile 108

109 Generic Handoff Procedure Home database (4) (3) Anchor point (5) Old visiting database Old (1) New (6) New visiting database (2) 1. Decision is made to handoff. 2. MS registers with the new visiting database. 3. New visited database requests subscriber profile from home database. 4. Home database responds with authentication of mobile. New up/downlink channels are assigned (circuit-switched). The two databases are updated with new location. 5. Home database sends a message to the old visited database to flush or redirect packets sent to or associated with MS. 109

110 Handoff in : BSS Transition Distribution System (DS) BSS BSS Transition Basic Service Area 1 Basic Service Area 2 Mobile station (MS) moves to a different BSS within the same Extended Service Set (ESS) MS sends dissociate message to AP 1. Need not be received. MS sends re-associate message to AP 2. AP 1 notified about change of location via 110 wired network.

111 Handoff in GSM Mobile Station (MS) Base Station Subsystem (BSS) Network and Switching Subsystem (NSS) MS BTS BSC VLR HLR AuC MS MS BTS BSC MSC OMC EIR U m BTS Radio interface A bis A Interface to other networks PSTN etc. Internal handoff: between BTSs controlled by the same BSS. External handoff: between BSSs controlled by the same MSC. Mobile monitors the RSS for channels in adjacent cell, reports to MSC. BTS also monitors RSS from mobile. 111

112 Handoff Information Flow Measurement report Handoff command Handoff required Handoff command Handoff complete Clear command Clear complete Handoff request Handoff request ACK Handoff Handoff complete MS BSS1 MSC BSS2 112

113 Intersystem Handoff BS1 BS2 BS1 BS2 Base Stations Base Stations PSTN Anchor MSC MSC A Trunk MSC B PSTN MSC A Trunk MSC B Before the handoff After the handoff 1. MSC A requests MSC B to set up voice channel with BS2. 2. MS synchronizes to BS2. 3. MSC A connects the call path (trunk) to MSC B. 113

114 Forward/Backward Handoffs MSC A X MSC B MSC A X X MSC B Handoff forward Handoff backward MSC A MSC B X MSC C MSC A X MSC B X X MSC C Handoff through 3 rd switch Path minimization 114

115 Handoff Issues Intersystem handoffs cellular (e.g., GPRS) "! WLAN "! (WMAN) Appropriate metrics? Performance in mobile data networks Different metrics than for voice (e.g., outage, average number of handoffs) Throughput, maintaining QoS Latency generally not an issue Can retransmit 115

116 Voice versus Data Voice Quality of Service metrics: Data QoS metrics: Performance Objective: Performance Objective: 116

117 Voice versus Data Voice QoS metrics: Blocking probability Outage probability Delay (strict) Performance Objective: Total capacity (Erlangs/Hz/km 2 ) Data QoS metrics: Throughput Outage probability Delay (statistical) Performance Objective: Total throughput (bits/sec/hz/km 2 ) Voice plus Data? 117

118 Radio Resource Allocation Problem: Allocate radio resources to satisfy Quality of Service requests 118

119 Radio Resource Allocation Problem: Allocate radio resources to satisfy Quality of Service requests Radio resources: power, bandwidth, time slots, codes (signatures), access points 119

120 Radio Resource Allocation Problem: Allocate radio resources to satisfy Quality of Service requests Radio resources: power, bandwidth, time slots, codes (signatures), access points Quality of Service request: target rate (no retransmissions e.g., for voice); throughput, delay (data) 120

121 Voice Resource Allocation (TDMA/FDMA) 1 channel/ time slot Resource Pool: channels/time slots Channel assignment problem: Maximize total voice capacity subject to QoS constraints. Static vs. dynamic assignment General conclusion: Dynamic better than static if the load is not too large 121

122 CDMA Power Control (Voice) Near-far problem P 1 P 3 P 4 P 2 Adjust (P 1,,P K ) to satisfy Signal-to-Interference Plus Noise Ratio (SINR) constraints: Signal Power Noise + Interference target value (e.g.,3 to 5 db for 3G) 122

123 Issues Preferences, priorities Mixed services 123

124 Data Resource Allocation uses random access (CSMA/CA) Rate control (depends on distance to AP) No power control (so far) Cellular systems (3G, WiMax) use scheduling. Base station allocates time/frequency slots across users. Mainly applies to downlink, since downlink traffic is typically much greater than uplink traffic. 124

125 How to Maximize Throughput? d 1 d 3 d 4 d 2 125

126 How to Maximize Throughput? d 1 d 3 d 4 d 2 Transmit to the user with the best channel with all available power! 126

127 Maximizing Throughput with Mobile Users d 1 d 2 Received power user 1 user 2 transmit to user 2 transmit to user 1 transmit to user 2 transmit to user 1 time Base station schedules packets based on received signal strength. 127 This exploits multi-user diversity.

128 Maximum Rate Scheduler Transmit to the user with the best channel (maximum achievable rate). Maximizes total throughput Penalizes users with bad channels (e.g., far from base) Time Division Multiplexing Exploits multi-user diversity Different users have different channels. What about fairness? 128

129 Opportunistic Scheduling (Downlink Data) Frequency incoming packets Base station time Base station scheduler assigns time/frequency slots to mobiles. mobiles Exploits variations in channels to increase throughput. 129

130 TDM Scheduler Scheduler must assign users to time slots. Rates: R 1 (user 1) > R 2 (user 2) > R 3 (user 3) Objectives: Time Time slots (Time-Division Multiplexing (TDM)) (typically around 5 msec, which is < channel coherence time) Efficiency (e.g., total throughput) Fairness (e.g., variations in throughput across users) How to assign users? 130

131 Scheduler Assignments (user): Max-Rate Scheduler Rates: R 1 (user 1) > R 2 (user 2) > R 3 (user 3) Time slots (Time-Division Multiplexing) Time Assigning user 1 all slots maximizes rate. 131

132 Equal-Rate (Fair) Scheduler Example: R 1 = 3R 2 = 4R 3 Scheduler Assignments (user): Time slots (Time-Division Multiplexing) Time 132

133 Proportional Fair Scheduler (1xEVDO) Rates: R 1 (user 1) > R 2 (user 2) > R 3 (user 3) Scheduler Assignments (user): Time T (around 5 msecs) Time window (8T) Time Windowed throughputs: User 1: 5R 1 /8; User 2: 2R 2 /8 User 3: R 3 /8; User 4: 0 Assign the user with the largest scheduling metric: rate / assigned fraction of time window 133

134 Proportional Fair Scheduler (1xEVDO) Rates: R 1 (user 1) > R 2 (user 2) > R 3 (user 3) Scheduler Assignments (user): Time T Time window (8T) Time Windowed throughputs: User 1: 5R 1 /8; User 2: 2R 2 /8 User 3: R 3 /8 Scheduling metric: User 1: 8/5 User 2: 4 User 3: 8 If the rates stay the same, the scheduler tends to divide the time slots equally among the users. 134

135 Throughput (Max-Rate Scheduler) Throughput number of users K 135

136 Throughput (Max-Rate Scheduler) Throughput proportional to log (K) with Rayleigh fading number of users K This increase is due to random fading. Instead of being detrimental, random fading across users is being exploited to provided higher rates! 136

137 Rate-Based Schedulers Proportional Fair (1XEVDO) The user scheduled at time slot i has the largest Rate (or S/I)/(windowed throughput) If the windowed throughput decreases, then the user s priority increases. Largest Weighted Delay First The user scheduled at time slot i has the largest priority factor x Rate x (Delay for packet so far) As the packet delay increases, so does the user s priority. 137

138 OFDM/TDMA: subchannels Scheduling in OFDMA t m TDMA TDMA\OFDMA Each color represents a different user, which is assigned particular time slots. Different sub-carriers can be assigned to different users. t N time slot Each user can be assigned a time/frequency slice. Exploits multiuser diversity across time and frequency. Requires a time/frequency scheduler. What information does the scheduler need? 138

139 WiMax OFDMA Frame Structure (TDD example) (downlink) (uplink) 139

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