CMPE 257: Wireless and Mobile Networking

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1 CMPE 257: Wireless and Mobile Networking Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 3 CMPE 257 Winter'11 1

2 Announcements Accessing secure part of the class Web page: User id: cmpe257. Passwd: OWY5OG CMPE 257 Winter'11 2

3 Today Medium access control (MAC). CMPE 257 Winter'11 3

4 Medium Access Control MAC protocols. Aka, multiple access protocols. CMPE 257 Winter'11 4

5 When do we need MAC? 2 types of links: Point-to-point. Shared. shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) CMPE 257 Winter'11 5

6 Shared Links If more than 1 node transmits at the same time: Collision at receiver! MAC protocol: Arbitrate access to medium. Determine who can transmit when. CMPE 257 Winter'11 6

7 Expanded Data Link Layer Sublayers of data link layer: Logical Link Control (LLC): flow and error control. Multiple Access Control (MAC): multiple access resolution. CMPE 257 Winter'11 7

8 Types of MAC Who makes decision? Centralized versus distributed MACs. Channel access policy. CMPE 257 Winter'11 8

9 Types of MAC Channel Access Policy Random access (or contention-based) No scheduled time for transmissions. No order for transmissions. Controlled access Stations coordinate access to channel. Station only transmits when it has right to send. Channelization Bandwidth of channel is statically partitioned. CMPE 257 Winter'11 9

10 Another way to look at it Control: Distributed. Centralized. How they coordinate access to medium: Round-robin. Scheduled-access. Contention-based. A-priori partitioning channel. CMPE 257 Winter'11 10

11 Contention-Based Protocols Aka, Random-Access MAC: ALOHA Slotted Aloha CSMA CSMA/CD (Ethernet IEEE 802.3) CSMA/CA (WiFi IEEE ) CMPE 257 Winter'11 11

12 CSMA Carrier sense: Stations listen first whether another transmission is in progress. If medium in use, wait. If not, transmit. Can collisions still occur? CMPE 257 Winter'11 12

13 CSMA/CD CSMA + collision detection. Performance improvement over CSMA. How? Listen while transmitting. If collision, transmit brief jamming signal and abort transmission. How does this improve performance? Wait random time and try again. CMPE 257 Winter'11 13

14 Collision Avoidance (CA) CSMA/CD assumes stations can detect collision. Not valid in all contexts (e.g., wireless): Attenuation too great to detect collision at all stations. Hard for transmitter to distinguish its own transmission from incoming weak signals and noise. Radios are usually half-duplex ($$$). CSMA/CA tries to avoid collisions. CMPE 257 Winter'11 14

15 CSMA/CA Tries to avoid collisions by avoiding the hidden terminal problem. CMPE 257 Winter'11 15

16 Hidden-Terminal Problem CMPE 257 Winter'11 16

17 Hidden- and Exposed Terminals Hidden Terminals Exposed Terminals CMPE 257 Winter'11 17

18 CSMA/CA: RTS-CTS Solution Channel reservation With collision avoidance, stations exchange small control packets to determine which sender can transmit to a receiver. CMPE 257 Winter'11 18

19 CSMA-CA CSMA/CA means both physical- and virtual carrier sensing. Physical: CS. Virtual: CA. CMPE 257 Winter'11 19

20 How does CSMA-CA work? CMPE 257 Winter'11 20

21 IEEE Provides 2 types of medium access: DCF: distributed coordination function. PCF: point coordination function. DCF is contention-based. PCF is polling-based. CMPE 257 Winter'11 21

22 IEEE MAC Protocol: CSMA/ sender CA 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) CMPE 257 Winter'11 22

23 IEEE Wireless LAN b GHz unlicensed spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps n: multiple antennae GHz range up to 200 Mbps all use CSMA/CA for multiple access all have base-station and ad-hoc network modes. CMPE 257 Winter'11 23

24 CSMA Variants 1-persistent (IEEE 802.3): If medium idle, transmit. If medium busy, keep listening; when medium idle, transmit with probability 1. p-persistent: Same as above but with probability p. Non-presistent: If medium idle, transmit. If medium busy, wait a random period before retrying. CMPE 257 Winter'10 24

25 MAC: A Bird s Eye View CMPE 257 Winter'10 25

26 Solutions to Hidden/Exposed Nodes in CSMA Use control packets: RTS/CTS (Request-To-Send/Clear-To-Send) Used by MACA (Multiple Access Control Avoidance) and MACAW (MACA for Wireless LANs). Use both control packets and carrier sense: CSMA/CA, IEEE CMPE 257 Winter'10 26

27 Dynamic Reservation Approaches: Sender- vs. Receiver-initiated Sender-initiated: A node wanting to send data takes the initiative of setting up the reservation. Most existing schemes. Receiver-initiated: A receiving node polls a potential transmitting node for data. A node can send data after being polled. E.g., MACA-By Invitation. CMPE 257 Winter'10 27

28 Single vs. Multiple Channel Protocols Single channel protocols: control and data use the same channel. Multiple channel protocols: separate channels for control & data transmission; data transmission on separate channels. CMPE 257 Winter'10 28

29 Other criteria for classification Power-aware. E.g., PAMAS. Directional or omnidirectional antennas. QoS-aware End-to-end (E2E) delay Packet loss rate (or the probability) Available bandwidth Challenges: lack of centralized control, limited bandwidth, node mobility, power/computational constraints, error-prone nature of wireless media. CMPE 257 Winter'10 29

30 MACAW [Bharghavan, 1994]. Proposed as improvement to MACA [Karn, 1990]. Note that first IEEE standard (IEEE legacy ) released in CMPE 257 Winter'10 30

31 MACA Introduced CA. RTS/CTS handshake (2-way). CMPE 257 Winter'10 31

32 MACA If node A wants to transmit to B, it first sends an RTS packet to B, indicating the length of the data transmission to follow. B returns a CTS packet to A with the expected length of the transmission. A starts transmission when it successfully receives CTS. RTS and CTS packets are much shorter than data packets. A neighboring node overhearing an RTS defers its own transmission until the corresponding CTS would have been finished. A node hearing the CTS defers for the expected length of the data transmission. CMPE 257 Winter'10 32

33 MACA (Cont d) Nodes close to sender: If no CTS heard, OK to transmit. Avoid exposed terminal problem: nodes that hear only RTS can transmit simultaneously with RTS sender. Nodes close to receiver: Upon hearing CTS, defer till after data. Avoid hidden terminal. Binary exponential backoff (BEB). Possible unfair channel allocation (starvation). CMPE 257 Winter'10 33

34 MACAW Inspired basic changes to MACA: Additional signaling. Modified backoff algorithm. CMPE 257 Winter'10 34

35 MACAW Backoff Tries to avoid BEB s unfairness. Proposed fix: sharing congestion information among nodes. Backoff counter information propagated in packet header. After successful transmission, neighbors have the same backoff counter. CMPE 257 Winter'10 35

36 Data Transmission in MACAW Added ACK. Reliability at layer 2. If ACK not received: Retransmit frame. Increment backoff timer. CMPE 257 Winter'10 36

37 Data Transmission in MACAW Added small Data Sending (DS) control frame. Addresses exposed terminal problem. Example: S1->R1 and S2->R2 CTS from R2 may collide with transmission S1->R1. S2 backs-off. Fix: make sure S2 knows RTS-CTS exchange between S1 and R1 was successful. S1 sends small control frame, DS with data exchange duration. When S2 receives DS, defers its transmission. R1 S1 S2 R2 CMPE 257 Winter'10 37

38 Data Transmission in MACAW Added Request for Request-to-Send (RRTS). R2 contends on behalf of S2 if it received RTS from S2 if it could not have responded because deferring due to S1->R1 exchange. When S2 receives RRTS from R2, proceeds with RTS, etc. RRTS RRTS S2 R2 R1 S1 CMPE 257 Winter'10 38

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