Multiple Access Control & Mobility Portions from Chapters 5 & 6

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1 Multiple Access Control & Mobility Portions from Chapters 5 & 6 A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April Thanks and enjoy! JFK/KWR All material copyright J.F Kurose and K.W. Ross, All Rights Reserved 5: DataLink Layer 5-1 Multiple Access Links and Protocols Two types of links : point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) 5: DataLink Layer 5-2 1

2 Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination 5: DataLink Layer 5-3 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. simple 5: DataLink Layer 5-4 2

3 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions recover from collisions Taking turns nodes take turns, but nodes with more to send can take longer turns 5: DataLink Layer 5-5 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame : DataLink Layer 5-6 3

4 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle FDM cable frequency bands time 5: DataLink Layer 5-7 Random Access Protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes collision, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA 5: DataLink Layer 5-8 4

5 Slotted ALOHA Assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot if no collision: node can send new frame in next slot if collision: node retransmits frame in each subsequent slot with prob. p until success 5: DataLink Layer 5-9 Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization 5: DataLink Layer

6 Slotted Aloha efficiency Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send) suppose: N nodes with many frames to send, each transmits in slot with probability p prob that given node has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 max efficiency: find p* that maximizes Np(1-p) N-1 for many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives: Max efficiency = 1/e =.37 At best: channel used for useful transmissions 37% of time!! 5: DataLink Layer 5-11 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t 0 collides with other frames sent in [t 0-1,t 0 +1] 5: DataLink Layer

7 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [t 0-1,t 0 ]. P(no other node transmits in [t 0,t 0 +1] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) choosing optimum p and then letting n -> infty... = 1/(2e) =.18 even worse than slotted Aloha! 5: DataLink Layer 5-13 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don t interrupt others! 5: DataLink Layer

8 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability 5: DataLink Layer 5-15 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: received signal strength overwhelmed by local transmission strength human analogy: the polite conversationalist 5: DataLink Layer

9 CSMA/CD collision detection 5: DataLink Layer 5-17 Taking Turns MAC protocols channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead taking turns protocols look for best of both worlds! 5: DataLink Layer

10 Taking Turns MAC protocols Polling: master node invites slave nodes to transmit in turn typically used with dumb slave devices concerns: polling overhead latency single point of failure (master) slaves data data poll master 5: DataLink Layer 5-19 Taking Turns MAC protocols Token passing: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) (nothing to send) T T data 5: DataLink Layer

11 Summary of MAC protocols channel partitioning, by time, frequency or code Time Division, Frequency Division random access (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD collision detection: easy in some technologies (wire), hard in others (wireless) taking turns polling from central site, token passing Bluetooth, FDDI, IBM Token Ring 5: DataLink Layer 5-21 IEEE : multiple access avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C A B A s signal strength C s signal strength space 6: Wireless and Mobile Networks

12 IEEE MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver DIFS sender data ACK receiver SIFS - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) 6: Wireless and Mobile Networks 6-23 Avoiding collisions (more) idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! 6: Wireless and Mobile Networks

13 Collision Avoidance: RTS-CTS exchange A RTS(A) RTS(A) AP reservation collision B RTS(B) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) 6: Wireless and Mobile Networks 6-25 What is mobility? spectrum of mobility, from the perspective: no mobility high mobility mobile wireless user, using same access point mobile user, connecting/ disconnecting from using DHCP. mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone) 6: Wireless and Mobile Networks

14 Mobility: Vocabulary home : 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, can always be used to reach mobile e.g., wide area correspondent 6: Wireless and Mobile Networks 6-27 Mobility: more vocabulary Permanent address: remains constant (e.g., ) visited : in which mobile currently resides (e.g., /24) Care-of-address: address in visited. (e.g., 79, ) wide area correspondent: wants to communicate with mobile foreign agent: entity in visited that performs mobility functions on behalf of mobile. 6: Wireless and Mobile Networks

15 How do you contact a mobile friend: Consider friend frequently changing addresses, how do you find her? search all phone books? call her parents? expect her to let you know where he/she is? I wonder where Alice moved to? 6: Wireless and Mobile Networks 6-29 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: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile 6: Wireless and Mobile Networks

16 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 routing tables indicate mobiles where each mobile located no changes to end-systems let end-systems handle it: indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote direct routing: correspondent gets foreign address of mobile, sends directly to mobile 6: Wireless and Mobile Networks 6-31 Mobility: registration home visited 2 wide area foreign agent contacts home agent home: this mobile is resident in my 1 mobile contacts foreign agent on entering visited End result: Foreign agent knows about mobile Home agent knows location of mobile 6: Wireless and Mobile Networks

17 Mobility via Indirect Routing home correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent 1 wide area 2 foreign agent receives packets, forwards to mobile 4 3 visited mobile replies directly to correspondent 6: Wireless and Mobile Networks 6-33 Indirect Routing: comments Mobile uses two addresses: permanent address: used by correspondent (hence mobile location is transparent to 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- -mobile inefficient when correspondent, mobile are in same 6: Wireless and Mobile Networks

18 Indirect Routing: moving between s suppose mobile user moves to another 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 (but with new care-of-address) mobility, changing foreign s transparent: on going connections can be maintained! 6: Wireless and Mobile Networks 6-35 Mobility via Direct Routing correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile visited home 4 correspondent requests, receives foreign address of mobile 2 1 wide area 3 4 mobile replies directly to correspondent 6: Wireless and Mobile Networks

19 Mobility via Direct Routing: comments overcome triangle routing problem non-transparent to correspondent: correspondent must get care-of-address from home agent what if mobile changes visited? 6: Wireless and Mobile Networks 6-37 Accommodating mobility with direct routing anchor foreign agent: FA in first visited data always routed first to anchor FA when mobile moves: new FA arranges to have data forwarded from old FA (chaining) correspondent wide area 1 correspondent agent anchor foreign agent new foreign agent foreign net visited at session start 2 new foreign 6: Wireless and Mobile Networks