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Wireless MAC and Mobility Management Protocols

Wireless MAC and Mobility Management Protocols. Prof. Malathi Veeraraghavan Elec. & Comp. Engg. Dept/CATT Polytechnic University mv@poly.edu Date: Nov. 17, 2002. CATT: Center for Advanced Technology in Telecommunications. “Wireless” and “Mobile”.

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Wireless MAC and Mobility Management Protocols

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  1. Wireless MAC and Mobility Management Protocols Prof. Malathi Veeraraghavan Elec. & Comp. Engg. Dept/CATT Polytechnic University mv@poly.edu Date: Nov. 17, 2002 CATT: Center for Advanced Technology in Telecommunications

  2. “Wireless” and “Mobile” • How does the “wireless” dimension change the networking problem? • How does the “mobile” dimension change the networking problem? • What is the “portable” dimension?

  3. The “wireless” dimension • Naturally broadcast medium • One transmitter sends data; multiple receivers can receive the signal and obtain the data • Need a MAC protocol to share the “naturally broadcast” wireless medium • Hence the first topic: Wireless MAC protocols

  4. The “mobile” dimension • A communicating user moves • Network needs to redirect data bits/packets to the user • Handoff management problem • A “server” – node that expects incoming calls/session-opening packets– is currently not at “home” • Network needs to find user who may have “roamed away” to deliver the incoming calls/packets • Location management problem • Hence the second topic: Mobility management • Handoff management + Location management

  5. The “portable” dimension • Does the mobile location management problem arise if the endpoint that moves only runs “client” ends of applications • i.e., it always initiates communication sessions • no one ever “calls” it first

  6. get address get address Dest: 24.3.5.10; Src: yahoo My laptop My laptop My laptop address: 128.238.24.5 address: 24.3.5.10 Dest: yahoo; Src: 24.3.5.10 Dest: 128.238.24.5; Src: yahoo Dest: yahoo; Src: 128.238.24.5 How are portable nodes handled • When a portable node connects to some switch of a network, it is assigned an endpoint address on that switch • Allows the network to route to portable using summarized switch routing information • e.g., DHCP (Dynamic Host Configuration Protocol) 24.3 @Home R yahoo web server Poly network R 128.238

  7. Outline • Wireless MAC protocols • Principles of fixed-, random-, and demand-assignment MAC schemes • Cellular/PCS MAC schemes, AMPS, IS-136, GSM TDMA, CDMA • IEEE 802.11 MAC schemes • Mobility management protocols • Principles of handoff management and location management • Cellular mobility management: IS41 and GSM MAP • 802.11 IAPP and Mobile IP • DHCP: manages portable nodes

  8. Internet Reverse channels Downstream Access points Upstream Forward channels Need for wireless MAC protocols • Wireless is naturally a shared medium PSTN Base stations/cell sites

  9. Classification of wireless MAC protocols Wireless MAC protocols Fixed-assignment schemes Random-access schemes Demand assignment schemes Circuit-switched (e.g., FDMA, TDMA) Connectionless packet-switched (e.g., 802.11) Connection-oriented packet-switched (e.g., CDMA)

  10. FDMA (Frequency Division Multiple Access) • Similar to broadcast radio and TV, assign a different carrier frequency per call • Modulation technique determines the required carrier spacing • Each communicating wireless user gets his/her own carrier frequency on which to send data • Need to set aside some frequencies that are operated in random-access mode to enable a wireless user to request and receive a carrier for data transmission

  11. TDMA(Time Division Multiple Access) • Each user transmits data on a time slot on multiple frequencies • A time slot is a channel • A user sends data at an accelerated rate (by using many frequencies) when its time slot begins • Data is stored at receiver and played back at original slow rate

  12. Hybrid FDMA/TDMA TDMA Carrier Frequency Frequency Time Time Frequency vs. time FDMA Frequency Time Basic principle ofcommunication: Two regions in the time-frequency plane with equal areas can carry the same amount of information

  13. Cellular System Overview

  14. Duplex techniques • Separates signals transmitted by base stations from signals transmitted by terminals • Frequency Division Duplex (FDD): use separate sets of frequencies for forward and reverse channels (upstream and downstream) • Time Division Duplex (TDD): same frequencies used in the two directions, but different time slots

  15. A A B B 825 870 845 890 A A B A B 1 1 667 667 824 825 845 849 1 667 799 991 1023 Advanced Mobile Phone System (AMPS) cellular system • FDMA/FDD • Spectrum allocation by FCC: A and B allocations to different providers Reverse Forward Original A A B A B Extended 869 870 890 894 1 667 799 991 1023

  16. Control aspect • How is a phone user allocated frequencies for acall? • The phone competes with other phones using a random-access protocol on a control channel • A set of channels are set aside as “control channels” • channels 313-354 (21 channels in each band) • reverse control channels (RECC) • random access (many-to-one) • forward control channels (FOCC) • broadcast channels (one-to-many)

  17. IS136: NA-TDMA • NA-TDMA is a hybrid FDMA/TDMA scheme • Therefore each frequency will have time slots that are shared by multiple calls • Typical: three calls share one frequency • NA-TDMA is three times as efficient • Same frequency allocation as for AMPS • Carriers are 30khz apart

  18. 1 2 3 The TDMA aspect: frames and time slots • Every frame is 40ms long and consists of 6 time slots • 1.9ms offset: allows a terminal to perform full-duplex communications without transmitting and receiving simultaneously • done to avoid a duplexing filter that separates strong transmit signal from weak receive signal base station to mobile 6 1 2 3 4 5 6 4 45 Mhz or 80 Mhz 1.9ms mobile to base station 6 1 2 3 4 5 6 1 2 3 4 5 40ms

  19. Data rate of a carrier (frequency) • What is the data rate of a carrier (frequency) • Each time slot carries 324 bits • Data rate per carrier (frequency) • Four types of channels • A full-rate channel occupies two time slots per frame • data rate: 16.2kb/s • can have three times as many calls as in AMPS • per frame: 1, 2, 3, 1, 2, 3, 1, 2, 3,.... • Half-rate channel (8.1kbps), doublefull-rate channel (32.4kbps) triple full-rate channel (48.6kbps)

  20. GSM: a hybrid TDMA/FDMA scheme • Frequency band: Uplink: 890-915 MHz, Downlink: 935-960 MHz • Frequency range: 50 MHz (25 MHz Up, 25 MHz Down) • Carrier spacing: 200 KHz (but time shared bet. 8 subscribers) • Duplex distance: 45 MHz (FDD)

  21. GSM: The TDMA aspect • Number of carriers: 25 MHz/200KHz = 124 • Users/carrier: 8 • The reverse channel is retarded by 3 time slots relative to the forward • Time slot = 4.62/ 8 ms (or approx. 0.577 ms) • One physical channel is one time slot per TDMA frame.

  22. Traffic channels • A traffic channel (TCH) is used to carry speech and data traffic. • TCHs are defined using a 26-frame multiframe (a group of 26 TDMA frames) • The length of a 26-frame traffic multiframe is 120 ms • Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) (12 and 25 on alternate multiframes) and 1 is currently unused. • Full rate channel: 24 slots per multiframe; Half-rate channel: 12 slots per multiframe

  23. GSM transmission rate • A full-rate traffic channel has a bit rate of • Within each time slot, some bits reserved for control; only 114 bits of 156.25 bits are for voice data • Contrast this to IS136 where transmission rate is 48.6kb/s and to the full-rate channel bit rate of 16.2kbps

  24. Speech coding and channel rates

  25. Dimension of space • Control power of signals transmitted to control the distance of propagation • Sharing over time/frequency/space dimensions textbook in reference

  26. 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 The cellular concept 5 • Hexagonally tiled cells • Cannot reuse frequencies in six surrounding cells • Minimum of seven frequency sets is required if N=7 4 6 1 3 7 2 • Repeat tiling of seven-cell array • Distance between like cells must be far enough to avoid interference • Smaller cells lead to better frequency reuse • More calls per unit area • Transmitted power must be smaller to avoid interference • Requires careful power management • Requires larger number of base stations A. Ibrahim

  27. D R Reuse: use a hexagonal cell frequency plan • D: Distance between • a base station and • the nearest base station that • uses the same channels • R: Radius of a cell • Reuse distance = D/R • Channel plan: method of • assigning channels to cells • to guarantee a minimum • reuse distance between cells that • use the same channel 18dB which is the minimum reuse distance for which

  28. 6 2 7 4 3 5 6 2 7 Reuse factor • Divide available channels into N groups • N: reuse factor; select N such that cells assigned the same frequencies will have a D:R ratio greater than (D:R)req • For hexagons, reuse factor N is given by 1 1 • Practical values of N • range from 3 to 21 • most commonly used: 7 (D/R = 4.6)

  29. Spectrum efficiency • C: Number of conversations per Mhz that would be possible in a single cell with no interference from neighboring cells • N: Channel reuse factor – indicates the capacity reduction due to interference from signals transmitted in other cells • Spectrum efficiency E conversations/cell/Mhz

  30. Spectrum efficiency of AMPS • Has a total of 832/2 = 416 channels • Set aside 21 for control • Therefore 395 traffic channels • Per cell, we can have 56 and 3/7 channels (N=7) • Four cells are given 56 channels and three cells are given 57 channels • The 395 channels fit over 25Mhz • Therefore, the spectrum efficiency of an AMPS system is conversations/cell/Mhz

  31. Spectrum efficiency

  32. Processing gain Information signal (R b/s) Digital modulator Radio modulator Transmit signal Digital carrier (W chips/s) Radio-frequency carrier IS-95 CDMA: Code Division Multiple Access • Channel: binary code assigned to terminal • Same bandwidth spectrum as AMPS • The bandwidth of a CDMA signal is 1.23Mhz = 1.23Mhz/30khz = 41 AMPS channels W: 1.228Mch/s R: 9.6kb/s G = 128ch/b (2127 codes possible) TDMA/FDMA use single-stage modulation; CDMA uses two stages

  33. Add G products Correlator Demodulation • CDMA soft capacity: • System with K transmitters • b0 is transmitted by transmitter 0 • I0 increases as K increases; error if • If G is 128 chips/bit, for an error probability of 0.001, K = 14 Information signal Low-pass signal Received signal Radio demodulator +G: 1 -G: -1 Radio-frequency carrier Digital carrier aggregate interference in receiver 0 cross-correlation

  34. Capacity & Interference • Digital carrier for forward channel is a row of a 6464 Walsh Hadamard matrix; all carriers are mutually orthogonal; each base station has access to 64 physical channels (one from each row of the matrix). 20 carriers/cell (25Mhz/1.23Mhz). 20*64 =1300; but spectrum efficiency calculation shows 45*25 = 1127 per cell. • In reverse direction, a channel corresponds to a binary code assigned to a terminal. Before the digital carrier modulation, an orthogonal Walsh modulation takes place (for error coding reasons). • Reuse factor: 1 – same frequencies used in adjacent cells • At base station receiver, correlation is property of reverse-link digital carriers of transmitters within cell + from neighboring sites • At handset receiver, interference from other cells low because radio modulators at different base stations use PN sequences with different time delay offsets relative to universal time; no interference from same cell because BS uses orthogonal carriers in downstream direction.

  35. CDMA system capacity • Dependent on use of • directional base station antennas  • variable bit-rate speech (silence detection)  • interference from adjacent cells  • imperfect power control (impact of motion)  • outage margin: fraction of time signal quality can be below the system target  (imposes limits) • All these factors impact spectrum efficiency • Why is CDMA a demand-assignment scheme?

  36. Outline revisited • Wireless MAC protocols • Principles of fixed-, random-, and demand-assignment MAC schemes • Cellular/PCS MAC schemes, AMPS, IS-136, GSM TDMA, CDMA • IEEE 802.11 MAC schemes • Mobility management protocols • Principles of handoff management and location management • Cellular mobility management: IS41 and GSM MAP • 802.11 IAPP, DHCP and Mobile IP

  37. Random access MAC protocols • Comparable to connectionless packet-switching • No reservations are made; instead a wireless endpoint simply starts sending data packets • What can happen? • Collision • Need to avoid collisions or detect collisions and retransmit • What’s the cost of being too careful to avoid collisions? • Utilization will be sacrificed

  38. Various random-accessMAC schemes • ALOHA: just send & wait for ACK • Slotted ALOHA: send in slots • CSMA: sense carrier, but wait for ACK • CSMA/CD: detect collisions instead of waiting for ACK • CSMA/CA

  39. Wireless 802.11 LAN • Uses CSMA/CA • Why CA and CD? • Difficult to detect collisions in a radio environment • Hidden station problem: • Two mutually far away stations A and C want to send to B. • At A and C, channel appears idle • But collision occurs at B

  40. Mechanisms for CA • Use of Request-To-Send (RTS) and Confirm-to-Send (CTS) mechanism • When a station wants to send a packet, it first sends an RTS. The receiving station responds with a CTS. Stations that can hear the RTS or the CTS then mark that the medium will be busy for the duration of the request (indicated by Duration ID in the RTS and CTS) • Stations will adjust their Network Allocation Vector (NAV): time that must elapse before a station can sample channel for idle status • this is called virtual carrier sensing • RTS/CTS are smaller than long packets that can collide • Use of InterFrame Spaces (IFS)

  41. stretching CP CFP Frame CFP Super-frame Foreshortened CFP 802.11 MAC • IEEE 802.11 combines a demand-assignment MAC protocol with random access • PCF (Point Coordination Mode) – Polling • CFP (Contention-Free Period) in which access point polls hosts • DCF (Distributed Coordination Mode) • CP (Contention Period) in which CSMA/CA is used

  42. DCFDistributed Coordination Function • This mode of 802.11 is a random access MAC • When a node needs to send data, it senses the medium. If idle, wait for a period of DIFS and if the medium is still idle after DIFS, send immediately. • If when the medium is sensed it is busy; then • wait for medium to be idle for a DIFS (DCF IFS) period • then decrement backoff timer until • medium becomes busy again; freeze timer, OR • timer reaches 0; transmit frame • if two stations have their timers reach 0, collision will occur; for every retransmission attempt, increase the contention window (CW), idle period after a DIFS, exponentially; 2i –1 starting with CWmin e.g., 7, 15, 31.

  43. DIFS DIFS SIFS CW Random backoff time DCF mode transmission without RTS/CTS Data source Ack destination NAV other Defer access Exercise: Show timing diagram for DCF mode with RTS/CTS

  44. DCF MAC • Send immediately (after DIFS) if medium is idle • If medium was busy when sensed, wait a CW after it becomes idle (because many stations may be waiting when medium is busy; if they all send the instant the medium becomes idle, chances of collision are high)

  45. PHY layer • Three physical layer specifications are part of 802.11 • Spread spectrum • Frequency hopping (FH) • Direct Sequence (DS) • Infrared (IR)

  46. FH • What is FH? • Modulate the data signal such that it occupies different frequency bands as transmission progresses • e.g., send a song over many FM radio channels with some dwell time per channel • Why not FDMA? • Multipath fading affects narrow frequency bands so that some channels offer very poor transmission • In FH, time spent in each channel is small

  47. 802.11 FH PHY • 79 non-overlapping 1Mhz channels used • 1Mbps signals transmitted over the 2.4Ghz band • 2400 – 2483.5Mhz • 83.5 Mhz of bandwidth (US: starts 2.402Ghz to 2.480 – so 79) • A channel hop occurs every 224 s • 78 hopping patterns • Divided into 3 sets of 26 patterns each • The sets are designed to avoid prolonged collision periods between hopping sequences in a set • Hopping patterns collide 3 times on average, and 5 times in the worst case over a hopping cycle; each hop is a jump of a minimum of 6 channels • Each 802.11 LAN must use a particular hopping pattern • The hopping patterns allow for 26 networks to be collocated and still operate simultaneously

  48. +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 DS • Modulate data signal by a signal that occupies a much larger bandwidth • Chip rate: time to transmit a +1 or –1 • To transmit a data bit, need 11 chip times • 11 chip Barker sequence To transmit +1, send To transmit -1, send +1 +1 +1 +1 +1 -1 -1 -1 -1 11 symbol times 11 symbol times

  49. 802.11 DS • Takes a 1Mbps data signal and converts it into a 11 Mbps signal • 11 channels in the 2.4Ghz band (5Mhz spacing) • Channels separated by center frequencies at least 30Mhz apart can operate without interference • If total bandwidth is only 83.5 Mhz, only 3 802.11 LANs using DS can have overlapping cells • FCC only allocates between 2412 and 2462

  50. Address 1 Address 2 Address 3 Address 4 Seq. control Pwr Mgmt More Frag More Data Order Retry WEP Type 802.11 MAC frame format bytes 2 2 6 6 6 2 6 0-2312 4 Frame control Duration/ ID FCS Frame body MAC header Protocol version To DS From DS Sub-type bits 2 2 4 1 1 1 1 1 1 1 1

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