Announcements: Assignment 4 due now Lab 4 due next Tuesday Assignment 5 posted, due next Thursday

Transcription

1 ECE/CS 372 introduction to computer networks Lecture 15 Announcements: Assignment 4 due now Lab 4 due next Tuesday Assignment 5 posted, due next Thursday Credit for lecture slides to Professor Bechir Hamdaoui Adapted from Jim Kurose & Keith Ross (original copyright) Chapter 6, slide 20

2 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Chapter 6, slide 21

3 IEEE : multiple access There are two access operating modes Infrastructure based mode Ad hoc based mode Chapter 6, slide 22

4 IEEE : multiple access There are two multiple access functions: Point Coordination Function (PCF) TDMA-like access Point Coordinator (PC) polls users in a roundrobin fashion No contention Synchronous Infrastructure mode Distributed Coordination function (DCF) CSMA-like access Random access: listenbefore-talk Contention-like medium Asynchronous Both infrastructure and adhoc modes Chapter 6, slide 23

5 IEEE multiple access Contention and contention free periods: CFP CP CFP CP Contention-Free Period (CFP) Synchronous traffic Point Coord. Fct (PCF) is the access method Contention Period (CP) Asynchronous traffic Distr. Coord. Fct (DCF) is the access method Access point alternates between CFP and CP modes Chapter 6, slide 24

6 IEEE DCF MAC one at a time: 2 + nodes send at same time => collision CSMA - sense before transmitting don t collide with ongoing transmission by other node no collision detection difficult to sense collision when transmitting due to weak sigl goal: avoid collisions: CSMA/C(ollision)A(voidance) use Acknowledgment mechanism to recover from collision C A B C A B A s signal strength C s signal strength space Two scenarios where collision cannot be detected Chapter 6, slide 25

7 IEEE DCF MAC Simple MAC if channel idle for SIFS=10 µsec then sender transmits frame receiver waits for SIFS=10 µsec and sends ACK SIFS sender receiver (SIFS stands for Short InterFrame Space, and allows HW to switch from rx to tx) data Challenges/issues (1) more than one communication higher collision rate (100%) (2) when collision occurs, it at least should not occur when ACK is being sent allow ACK to be sent successfully ACK SIFS Chapter 6, slide 26

8 IEEE DCF MAC Simple MAC w/ minor improvement if channel idle for DIFS=50 µsec then sender transmits frame receiver waits for SIFS=10 µsec only and sends ACK SIFS < DIFS ACK gains access before (DIFS stands for DCF InterFrame Space) sender DIFS data receiver Challenges/issues (1) sure, it now prevents collision between DATA & ACK (2) but collision can still occur between DATAs How to reduce collision even further? ACK SIFS Chapter 6, slide 27

9 IEEE DCF MAC MAC w/ backoff After medium sensed busy don t send right away wait for a random time, then transmit idea: different senders will hopefully pick different times to transmit so as to avoid colliding with each other again Chapter 6, slide 28

10 IEEE DCF MAC MAC w/ backoff After medium sensed busy DIFS DIFS W = contention window Medium busy Random counter Data ready Defer access Backoff counter b Transmit! contention window W: define new parameter W = 8, 16, 32, etc backoff counter b: pick a random number b from [0,W] E.g., if W = 64, perhaps b = 23 Decrement counter b by 1 every idle slot If someone else goes first, reset b but don t change W If counter reaches 0, transmit Chapter 6, slide 29

11 IEEE DCF MAC MAC w/ backoff After collision DIFS DIFS W = contention window Medium busy Random counter Collision? Defer access Backoff counter b Transmit! contention window W: define new parameter W = 8, 16, 32, etc backoff counter b: pick a random number b from [0,W] E.g., if W = 64, perhaps b = 23 Decrement counter b by 1 every idle slot If counter reaches 0, transmit If another collision, reset b & increase W Chapter 6, slide 30

12 IEEE DCF MAC MAC w/ backoff After collision DIFS DIFS W = contention window Medium busy Random counter Collision? Defer access Backoff counter b Transmit! Challenge How to choose W? lower W higher collision waste of bandwidth higher W lower collision but wasted, unused slots Solution: W should adapt/adjust to loads/# of users W increases as # of users increases and vice-versa Exponential Backoff Chapter 6, slide 31

13 IEEE DCF MAC MAC w/ exponentional backoff exponential backoff: (1) increase W when collision (2) decrease W when success This is how it works: set W = W 0 (this is an initial value) pick random backoff counter b from [0,W] transmit when counter reaches 0 if collision occurs, set W 2W (double window) and repeat Chapter 6, slide 32

14 IEEE DCF MAC MAC w/ exponentional backoff Example Initial window W 0 = 8 1 st collision: set W 1 =W 0 =8; pick b from [0,1,,7= W 1-1] 2 nd collision: set W 2 =2W 0 =16; pick b from [0,1,,15= W 2-1] m th collision: set W m =2 (m-1) W 0 ; pick b from [0,1,, W m -1] Maximum window size = W max = 2 (m-1) W 0 After m th collision, Window is set to W max For example: (m+1) th collision, W m+1 = W max at i th collision: W i = min {2 (i-1) W 0, W max } Chapter 6, slide 33

15 IEEE DCF MAC MAC w/ exponentional backoff How do you detect a collision without collision detection? You don t! You intuit them! Any failed transmission (i.e. you didn t get an ACK) is interpreted as a collision. Chapter 6, slide 34

16 More challenges: Hidden terminal problem 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, and hence, may interfere at B space Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each Problem: If A is transmitting to B, C won t be able to sense that medium is busy Chapter 6, slide 35

17 RTS/CTS handshaking mechanism idea: allow sender to reserve channel rather than random access of data frames Solves the hidden terminal problem Avoids collisions of long data frames Sender: transmits small request-to-send (RTS) packets Receiving neighbors all hear this RTS defer receipt RTSs may still collide with each other (but they re short) Receiver: transmits small clear-to-send (CTS) packets in response to RTS Sending neighbors all hear CTS defer transmission Chapter 6, slide 36

18 Collision Avoidance: RTS-CTS exchange A B C reservation collision DATA (A) defer time Chapter 6, slide 37

19 Final words Carrier sense mechanisms: there are two types Physical carrier sensing actual sensing of medium to determine whether it is busy or not Virtual carrier sensing provided by MAC via RTS/CTS frames. Predicts future traffic based on information/duration indicated in RTS/CTS frames Chapter 6, slide 38

20 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Chapter 6, slide 39

21 What is mobility? spectrum of mobility, from the network perspective: no mobility high mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from network using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) Chapter 6, slide 40

22 Mobility: Vocabulary home network: 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: address in home network, can always be used to reach mobile e.g., wide area network correspondent Chapter 6, slide 41

23 Mobility: more vocabulary Permanent address: remains constant (e.g., ) visited network: network in which mobile currently resides (e.g., /24) Care-of-address: address in visited network. (e.g., 79, ) wide area network correspondent: wants to communicate with mobile foreign agent: entity in visited network that performs mobility functions on behalf of mobile. Chapter 6, slide 42

24 How do you contact a mobile friend: Consider a friend who frequently changes addresses, how do you find her? I wonder where Alice moved to? expect her to update the new phone book: So, search all phone books for new address? expect her to let you know where she is? So, use her new address expect her to let her parents know So, call her parents? Chapter 6, slide 43

25 Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange. routing tables indicate where each mobile located no changes to end-systems Let end-systems handle it: Let mobile inform his home agent E.g. let Alice inform her parents Alice s friend must contact her parent before Chapter 6, slide 44

26 Mobility: approaches Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence not via usual routing table exchange. scalable to millions of mobiles routing tables indicate where each mobile located no changes to end-systems Let end-systems handle it: Let mobile inform his home agent E.g. let Alice inform her parents Alice s friend must contact her parent before Chapter 6, slide 45

27 Mobility: registration home network visited network wide area network foreign agent contacts home agent home: this mobile is resident in my network 2 1 mobile contacts foreign agent on entering visited network End result: Foreign agent knows about mobile Home agent knows location of mobile Chapter 6, slide 46

28 Mobility via Indirect Routing indirect routing: communication from correspondent to mobile goes via home agent, then forwarded to remote home network correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area network 2 foreign agent receives packets, forwards to mobile 4 3 visited network mobile replies directly to correspondent Chapter 6, slide 47

29 Mobility via Direct Routing direct routing: correspondent gets foreign address of mobile, then sends directly to mobile home network correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile 4 visited network correspondent requests, receives foreign address of mobile 2 1 wide area network 3 5 mobile replies directly to correspondent Chapter 6, slide 48

30 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Chapter 6, slide 49

31 Mobile IP RFC 3344 has many features we ve seen: home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet) Uses indirect routing of datagrams Chapter 6, slide 50

32 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: Chapter 6, slide 51

33 Wireless, mobility: last words Challenges/issues packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff congestion: TCP interprets loss as congestion, will decrease congestion window unnecessarily bandwidth: limited bandwidth of wireless links Chapter 6, slide 52