Medium Access Control Protocols

CMPE 257: Wireless Networking SET MAC-2: Medium Access Control Protocols Spring 2006 UCSC CMPE257 1

NAMA Improvements Inefficient activation in certain scenarios. For example, only one node, a, can be activated according NAMA, although several other opportunities exist. 10 8 4 a b e 6 5 c d 1 7 3 f g h We want to activate g and d as well. Spring 2006 UCSC CMPE257 2

Node + Link (Hybrid) Activation Additional assumption Radio transceiver is capable of code division channelization (DSSS direct sequence spread spectrum) Code set is C. Code assignment for each node is per time slot: i.code = i.prio mod C Spring 2006 UCSC CMPE257 3

Hybrid Activation Multiple Access (HAMA) Node state classification per time slot according to their priorities. Receiver (Rx): intermediate prio among one-hop neighbors. Drain (DRx): lowest prio amongst one-hop. BTx: highest prio among two-hop. UTx: highest prio among one-hop. DTx: highest prio among the one-hop of a drain. Spring 2006 UCSC CMPE257 4

Transmission schedules: HAMA (cont.) BTx > all one-hop neighbors. UTx > selected one-hops, which are in Rx state, and the UTx has the highest prio among the one-hop neighbors of the receiver. DTx > Drains (DRx), and the DTx has the highest prio among the one-hops of the DRx. Spring 2006 UCSC CMPE257 5

HAMA Operations Suppose no conflict in code assignment. Nodal states are denoted beside each node: Node D converted from Rx to DTx. Benefit: one-activation in NAMA to four possible activations in HAMA. 10-BTx a 1-DRx f 8-Rx b 7-UTx g 6-Rx c 5-DTx d 4-DRx e 3-DRx h Spring 2006 UCSC CMPE257 6

Other Channel Access Protocols Other protocols using omni-directional antennas: LAMA: Link Activation Multiple Access PAMA: Pair-wise Activation Multiple Access Protocols that work when uni-directional links exist. Node A can receive node B s transmission but B cannot receive A s. Protocols using direct antenna systems. Spring 2006 UCSC CMPE257 7

Comparison of Channel Access Probability Spring 2006 UCSC CMPE257 8

Protocol Throughput Comparison Spring 2006 UCSC CMPE257 9

Comments Scheduled-access protocols are evaluated in static environments and what about their performance in mobile networks? Neighbor protocol will also have impact on the performance of these protocols Need comprehensive comparison of contentionbased and scheduled access protocols. Spring 2006 UCSC CMPE257 10

References [R01] S. Ramanathan, A unified framework and algorithm for channel assignment in wireless networks, ACM Wireless Networks, Vol. 5, No. 2, March 1999. [BG01] Lichun Bao and JJ, A New Approach to Channel Access Scheduling for Ad Hoc Networks, Proc. of The Seventh ACM Annual International Conference on Mobile Computing and networking (MOBICOM), July 16-21, 2001, Rome, Italy. [BG02] Lichun Bao and JJ, Hybrid Channel Access Scheduling in Ad Hoc Networks, IEEE Tenth International Conference on Network Protocols (ICNP), Paris, France, November 12-15, 2002. Spring 2006 UCSC CMPE257 11

References [IEEE99] IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std 802.11-1999. [TK84] H. Takagi and L. Kleinrock, Optimal Transmission Range for Randomly Distributed Packet Radio Terminals, IEEE Trans. on Comm., vol. 32, no. 3, pp. 246-57, 1984. [WV99] L. Wu and P. Varshney, Performance Analysis of CSMA and BTMA Protocols in Multihop Networks (I). Single Channel Case, Information Sciences, Elsevier Sciences Inc., vol. 120, pp. 159-77, 1999. [WG02] Yu Wang and JJ, Performance of Collision Avoidance Protocols in Single-Channel Ad Hoc Networks, IEEE Intl. Conf. on Network Protocols (ICNP 02), Paris, France, Nov. 2002. Spring 2006 UCSC CMPE257 12

MAC Protocols Using Directional Antennas Basic protocols Directional Virtual Carrier Sensing (DVCS) Directional MAC (D-MAC) in UDAAN Spring 2006 UCSC CMPE257 13

MAC Protocols Using Directional Antennas The MAC protocols so far assume that a node s transmissions reach all of its neighbors. With powerful antenna systems, it is possible to limit transmissions and receptions to desired directions only. This can increase spatial reuse and reduce interferences to neighbors nodes. Caveat: Not all neighbor nodes defer access. Directional receiving is not always desired. Spring 2006 UCSC CMPE257 14

Omni-Directional and Directional Transmissions Node A Node B Node C Node A Node B θ Node C Omni-directional transmission Directional transmission Spring 2006 UCSC CMPE257 15

Antenna systems Directional Antenna Models Switched beam fixed orientation Adaptive beam forming any direction Simulation models: Complete signal attenuation outside the directional transmission beamwidth (θ) ``Cone plus ball model Directional transmissions have higher gains Possible to use power control to reduce the gain Various medium access control schemes have been proposed and/or investigated (see Refs). Spring 2006 UCSC CMPE257 16

Basic Scheme One OTOR (omni-transmit, omni-receive) The usual omni RTS/CTS based collision avoidance All packets are transmitted and received omni-directionally. IEEE 802.11 MAC protocol uses such scheme. DIFS SRC RTS DATA DEST SIFS SIFS SIFS CTS ACK DIFS OTHER NAV(RTS) NAV(CTS) Defer Access Spring 2006 UCSC CMPE257 17

Basic Scheme Two DTOR (directional-transmit, omni-receive) Packets are transmitted directionally. Packets are received omni-directionally. Increased spatial reuse (+) and collisions (-). Talks btw. A & B, C & D can go on concurrently; More collisions may occur; + Spatial reuse is increased; + Nodes spend less time waiting. Spring 2006 UCSC CMPE257 18

Basic Scheme Three DTDR (directional-transmit, directional-receive) All packets are transmitted and received directionally. Aggressive spatial reuse Talks btw. A & B, C & D can go on concurrently; More collisions may occur; Channel status info. is incomplete; + Aggressive spatial reuse; + Nodes spend less time waiting. Spring 2006 UCSC CMPE257 19

Basic Scheme Four MTDR (mixed transmit, directional receive) CTS packets are transmitted omni-directionally while other packets are transmitted directionally. Tradeoff between spatial reuse and collision avoidance D sends RTS to C directionally; C replies with omni-cts; + A and G defer their access and won t cause collisions; However, A cannot talk with B at the same time. Spring 2006 UCSC CMPE257 20

Predictions from the Analysis [WG03] The DTDR scheme performs the best among the schemes analyzed. Increased spatial reuse and reduced interference through directional transmissions. Directional receiving cancels much interferences from neighbors and hidden terminals. Throughput of the DTDR scheme with narrow beamwidth has a slightly increase when N increases. Spatial reuse effect is more conspicuous. Scalability problem is mitigated. Spring 2006 UCSC CMPE257 21

Simulation Results [WG03] Higher-gain directional transmissions have negative effects on throughput and delay. More nodes are affected. Influence of side lobes can be almost canceled out if: The level of side lobes is reasonably low through the advancement of antenna systems. Carrier sensing threshold is raised such that nodes are less sensitive to channel activities. Spring 2006 UCSC CMPE257 22

Advanced Schemes Directional Virtual Carrier Sensing ([TMRB02]) Angle-of-Arrival (AoA) information available Nodes record direction information and defer only to non-free directions (directional NAV) UDAAN ([RRSWP05]) Switched beam antenna Experimental system was built to test the effectiveness of directional antenna systems Spring 2006 UCSC CMPE257 23

Details on Directional NAV Physical carrier sensing still omni-directional Virtual carrier sensing be directional directional NAV When RTS/CTS received from a particular direction, record the direction of arrival and duration of proposed transfer Channel assumed to be busy in the direction from which RTS/CTS received Spring 2006 UCSC CMPE257 24

Directional NAV (DNAV) Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) C CTS D X Y Spring 2006 UCSC CMPE257 25

Directional NAV (DNAV) Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) D C DNAV X Y Spring 2006 UCSC CMPE257 26

Directional NAV (DNAV) New transmission initiated only if direction of transmission does not overlap with DNAV, i.e., if ( > 0 ) B A D DNAV RTS C Spring 2006 UCSC CMPE257 27

D-MAC Forced Idle is to avoid starvation FI-Busy ``aggressive Tight integration with power control Spring 2006 UCSC CMPE257 28

Directional Neighbor Discovery Three kinds of links (neighbors) N-BF, without beam forming T-BF, using only transmit-only beamforming TR-BF, using transmit and receive beamforming Two methods for discovery Informed discovery Blind discovery Spring 2006 UCSC CMPE257 29

Directional Packet Transmission A B D-O transmission B s omni receive range A B D-D transmission B s directional receive beam Spring 2006 UCSC CMPE257 30

Related topics Neighbor protocol and topology management Energy efficiency Routing Spring 2006 UCSC CMPE257 31

References [KSV00] Ko et al., Medium Access Control Protocols Using Directional Antennas in Ad Hoc Networks, in IEEE INFOCOM 2000. [NYH00] Nasipuri et al., A MAC Protocol for Mobile Ad Hoc Networks Using Directional Antennas, in IEEE WCNC 2000. [R01] R. Ramanathan, On the Performance of Ad Hoc Networks with Beamforming Antennas, ACM MobiHoc '01, Oct. 2001. [TMRB02] Takai et al., Directional Virtual Carrier Sensing for Directional Antennas in Mobile Ad Hoc Networks, ACM MobiHoc 02, June 2002. [CYRV02] Choudhury et al., Medium Access Control in Ad Hoc Networks Using Directional Antennas, ACM MobiCom '02, Sept. 2002. [WG03] Yu Wang and JJ, Collision Avoidance in Single-Channel Ad Hoc Networks Using Directional Antennas, in IEEE ICDCS '03. [RRSWP05] Ramanathan et al., Ad Hoc Networking With Directional Antennas: A Complete System Solution, IEEE JSAC 2005. Spring 2006 UCSC CMPE257 32

Acknowledgments Parts of the presentation are adapted from the following sources: Prasant Mohapatra, UC Davis, http://www.cs.ucdavis.edu/~prasant/ecs257/notes/ad Spring 2006 UCSC CMPE257 33

MAC Protocols for Networks with Multiple Channels Motivation and issues Summary of approaches Examples Open research problems Spring 2006 UCSC CMPE257 34

Motivation for Using Multiple Channels Multiple orthogonal channels available in IEEE 802.11 3 channels in 802.11b 12 channels in 802.11a Multiple channels available in general Utilizing multiple channels can improve throughput and reduce delays Allow simultaneous transmissions in the same 2-hop neighborhood. 1 1 Single channel defer 2 Multiple Channels Spring 2006 UCSC CMPE257 35

Issues with Multiple Channels Senders and receivers need to meet in one of many channels Hidden and exposed terminals are now problems involving more than one channel Similar problems than with space division Each node may have one or multiple radios Half duplex nodes, even with multiple radios per node: At best, a node can receive or transmit over one or multiple radios, but not both Synchronization is hard to avoid w/o a dedicated control channel. Spring 2006 UCSC CMPE257 36

Approaches* 1. Dedicated Control Channel Dedicated control radio & channel for all control messages DCA[Wu2000], DCA-PC[Tseng2001], DPC[Hung2002]. 2. Split Phase Fixed periods divided into (i) channel negotiation phase on default channel &(ii) data transfer phase on negotiated channels MMAC[J.So2003], MAP [Chen et al.] 3. Common Hopping All non-busy nodes follow a common, well-known channel hopping sequence -- the control channel changes. HRMA[Tang & JJ 98], CHMA, CHAT, RICH [Tzamaloukas & JJ] 4. Parallel Rendezvous Each node publishes its own channel hopping schedule SSCH [Bahl04], McMAC [So et al] * Mo, So and Walrand, Comparison of Multi-Channel MAC Protocols, MSWIM 05. Spring 2006 UCSC CMPE257 37

Approach 1: Dedicated Control Channel Dedicated control radio and control channel DCA[Wu2000], DCA-PC[Tseng2001], DPC[Hung2002]. 3. [Control Chan]: S ---- RTS (Suggested Data Chan.) --> R 4. [Control Chan]: S <-- CTS (Agreed Data Chan.) ---- R 5. [Control Chan]: (optional) S --- Reservation (broadcast) ---> All 6. [Data Chan]: Sender --- Data Packet ---> Receiver Spring 2006 UCSC CMPE257 38

Dedicated Control Channel Channel Ch3 Rendezvous & contention occur on the control channel. Data Ack Ch2 Node 1+2 Data... Ack Ch1 Rts (2,3) Cts (2) Rsv (2) Rts (3) Cts (3) Rsv (3) Time Legend: Node 1 Node 2 Note 3 Node 4 Spring 2006 UCSC CMPE257 39

Nasipuri s Protocol Assumes N transceivers per host Capable of listening to all channels simultaneously Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC] Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC] Disadvantage: High hardware cost (today!)* * In the future (~5 to 10 years), having 4 radios per node will be affordable Spring 2006 UCSC CMPE257 40

Approach 2: Split-Phase Time is divided into (equal) periods. Each period consists of 2 phases: control (channel negotiation), data transfer. Examples ae: MMAC[J.So2003] (UIUC), MAP [Chen2003] Channel negotiation happens on a default channel. Nodes negotiate the channels to use. RTS/CTS/Data on the separate channels Spring 2006 UCSC CMPE257 41

Split-Phase Channel Channel negotiation on a common channel Ch2 Rts Cts Data Ack Ch1 Ch0 Hello (1,2,3) Ack (1) Rsv (1) Hello (2,3)... Rts Cts Data Ack Time Control Phase Data Transfer Phase Legend: Node 1 Node 2 Note 3 Node 4 Spring 2006 UCSC CMPE257 42

Assumptions MMAC (So and Vaidya) Each node is equipped with a single transceiver The transceiver is capable of switching channels Channel switching delay is approximately 250us Per-packet switching not recommended Occasional channel switching not to expensive Multi-hop synchronization is achieved by other means Spring 2006 UCSC CMPE257 43

MMAC Idea similar to IEEE 802.11 PSM Divide time into beacon intervals At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window) Nodes negotiate channels using ATIM messages Nodes switch to selected channels after ATIM window for the rest of the beacon interval Spring 2006 UCSC CMPE257 44

Preferred Channel List (PCL) Each node maintains PCL Records usage of channels inside the transmission range High preference (HIGH) Already selected for the current beacon interval Medium preference (MID) No other vicinity node has selected this channel Low preference (LOW) This channel has been chosen by vicinity nodes Count number of nodes that selected this channel to break ties Spring 2006 UCSC CMPE257 45

Channel Negotiation In ATIM window, sender transmits ATIM to the receiver Sender includes its PCL in the ATIM packet Receiver selects a channel based on sender s PCL and its own PCL Order of preference: HIGH > MID > LOW Tie breaker: Receiver s PCL has higher priority For LOW channels: channels with smaller count have higher priority Receiver sends ATIM-ACK to sender including the selected channel Sender sends ATIM-RES to notify its neighbors of the selected channel Spring 2006 UCSC CMPE257 46

Channel Negotiation Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval Spring 2006 UCSC CMPE257 47

Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) Beacon B ATIM- ACK(1) C D Time ATIM Window Beacon Interval Spring 2006 UCSC CMPE257 48

Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) Beacon B ATIM- ACK(1) C ATIM- ACK(2) D ATIM ATIM- RES(2) Time ATIM Window Beacon Interval Spring 2006 UCSC CMPE257 49

Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) RTS DATA Channel 1 Beacon B ATIM- ACK(1) CTS ACK Channel 1 C ATIM- ACK(2) CTS ACK Channel 2 D ATIM ATIM- RES(2) RTS DATA Channel 2 Time ATIM Window Beacon Interval Spring 2006 UCSC CMPE257 50

Simulation Model ns-2 simulator Transmission rate: 2Mbps Transmission range: 250m Traffic type: Constant Bit Rate (CBR) Beacon interval: 100ms Packet size: 512 bytes ATIM window size: 20ms Default number of channels: 3 channels Compared protocols 802.11: IEEE 802.11 single channel protocol DCA: Wu s protocol MMAC: Proposed protocol Spring 2006 UCSC CMPE257 51

Wireless LAN - Throughput Aggregate Throughput (Kbps) 2500 2500 2000 1500 1000 MMAC DCA 2000 1500 1000 500 802.11 500 30 nodes 64 nodes MMAC DCA 802.11 1 10 100 1000 1 10 100 1000 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) MMAC shows higher throughput than DCA and 802.11 Spring 2006 UCSC CMPE257 52

Multi-hop Network Throughput Aggregate Throughput (Kbps) 1500 MMAC 1000 DCA 500 802.11 0 1 10 100 1000 Packet arrival rate per flow (packets/sec) 2000 1500 1000 500 MMAC DCA 802.11 0 1 10 100 1000 Packet arrival rate per flow (packets/sec) 3 channels 4 channels Spring 2006 UCSC CMPE257 53

Throughput of DCA and MMAC (Wireless LAN) Aggregate Throughput (Kbps) 4000 3000 2000 1000 0 6 channels 2 channels 802.11 Packet arrival rate per flow (packets/sec) 4000 3000 2000 1000 0 6 channels 2 channels 802.11 Packet arrival rate per flow (packets/sec) DCA MMAC MMAC shows higher throughput compared to DCA Spring 2006 UCSC CMPE257 54

Analysis of Results DCA MMAC Bandwidth of control channel significantly affects performance Narrow control channel: High collision and congestion of control packets Wide control channel: Waste of bandwidth It is difficult to adapt control channel bandwidth dynamically ATIM window size significantly affects performance ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval reduced overhead Compared to packet-by-packet control packet exchange in DCA ATIM window size can be adapted to traffic load Spring 2006 UCSC CMPE257 55

Further Work Needed Dynamic adaptation of ATIM window size based on traffic load for MMAC Efficient multi-hop clock synchronization Better uses of data segment Multipoint communication support Spring 2006 UCSC CMPE257 56

Approach 3: Common Hopping All idle nodes follow the same channel hopping sequence E.g., HRMA[Tang98], CHMA[Tzamaloukas2000], CHAT [Tzamaloukas2000] 3. [Common Channel]: S ---- RTS ---> R 4. [Next Common Channel]: everyone else [Same Channel]: S <-- CTS ---- R 5. [Same Channel]: S ---- Data ---> R All return to Current Common Channel after sending/receiving Spring 2006 UCSC CMPE257 57

3. Common Hopping Channel Idle nodes hop together in common channel Ch3 Ch2 Ch1 Ch0 RTS (c to d) RTS (b to a) Cts, Data, Ack 1 2 3 4 5 6 7 8 9 10 11 Enough for one RTS Legend: Node a Node b Note c Node d Spring 2006 UCSC CMPE257 58 Time

Main Limitations The time any dialogue can last must be shorter than the time it takes for the common hopping sequence to revisit the channel being used. Approach is useful only if sufficiently large numbers of channels are available. Time sync is needed. Spring 2006 UCSC CMPE257 59

Approach 4: Parallel Rendezvous Nodes choose their own hopping sequences. Nodes publish the seeds of their hopping sequences so nodes can track each other. Key: Parallel rendezvous on multiple channels Examples: SSCH [Bahl et al.]: senders transmits only when receivers are on the same channel. McMAC [So & Walrand]: senders can deviate from their published schedule temporarily to transmit. Spring 2006 UCSC CMPE257 60

McMAC Every node has a its own random hopping sequence called the home channel. Hopping freq. is a parameter. To Tx, sender S leaves its home channel to meet its receiver R with some probability p_tx. If R s channel is busy or R is away from home, S tries again later. Otherwise, S and R exchanges Data/Ack. Spring 2006 UCSC CMPE257 61

McMAC (no hopping) t=1 2 3 4 5 6... Ch 1?? Ch 2 Ch 3 Ch 4 Sender needs to know the home channel of the receiver, but time sync. is not needed. Spring 2006 UCSC CMPE257 62

McMAC (with hopping) Original schedule t=1 2 3 4 5 6 7 8 9 Ch1 Ch2 Spring 2006 UCSC CMPE257 63

McMAC (with hopping) Original schedule t=1 2 3 4 5 6 7 8 9 Ch1 Ch2 1. Data arrives 2. RTS/ CTS/ Data 3. Hopping stopped during data transfer 4. Hopping resumes Spring 2006 UCSC CMPE257 64

Qualitative Comparison (True? Let s Discuss!) Control Channel Split- Phase Common Hopping Parallel Rendez-vous # Radios 2 1 1 1 Contention Bottleneck Y Y Y N Time Sync. N Loose Very Tight Param. Track neighbors N N N Y Spring 2006 UCSC CMPE257 65

N: # nodes M: # channels Simulation Parameters L: T_sw: T_slot: S: avg. packet length channel switch time slot time = RTS/CTS link speed/channel (Mbps) Spring 2006 UCSC CMPE257 66

Simulation Scenarios N: # nodes M: # chans L: Avg Pkt Len T_sw: Switch Time T_slot: Slot Time Scenario #1 Similar to 802.11b 20 3 1024B /10240B (5/50 slots) 100us 810us Scenario #2 Similar to 802.11a 40 12 1024B /10240B (6.8/68 slots) 100us 200us S: Link Speed 2Mbps 6Mbps Spring 2006 UCSC CMPE257 67

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Dedicated Control Channel (short pkts) Channel Time (slot) Spring 2006 UCSC CMPE257 69

Dedicated Control Channel (long pkts) Channel Time (slot) Spring 2006 UCSC CMPE257 70

Split Phase (30 control / 120 data slots) Channel Spring 2006 UCSC CMPE257 71 Time (slot)

Split Phase (12 control / 48 data slots) Channel Spring 2006 UCSC CMPE257 72 Time (slot)

Common Hopping (Short Pkts) Channel Spring 2006 UCSC CMPE257 73 Time (slot)

Common Hopping (Long Pkts) Channel Spring 2006 UCSC CMPE257 74 Time (slot)

McMAC hopping (Short Pkts) Channel Spring 2006 UCSC CMPE257 75 Time (slot)

McMAC hopping (Long Pkts) Channel Spring 2006 UCSC CMPE257 76 Time (slot)

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Dedicated Control Channel (short pkts) Channel Time (slot) Spring 2006 UCSC CMPE257 78

Common Hopping (short pkts) Channel Time (slot) Spring 2006 UCSC CMPE257 79

McMAC hopping (short pkts) Channel Time (slot) Spring 2006 UCSC CMPE257 80

Conclusions by Mo, So, and Walrand Dedicated control channel: works surprisingly well esp. for long packets! Split-phase: depends heavily on the control/data phase durations Common-hopping: fragmentation problem Parallel rendezvous: good potential, but synchronization is an issue. Spring 2006 UCSC CMPE257 81

Research Opportunities Few schemes based on contention have not addresses collision issues. Efficient time sync is a requirement for multi-channel MACs, unless a dedicated control channel is used. Can we map prior approaches for distributed code assignment for CDMA networks to multi-channel MACs by making a code to equal a hoping sequence? Is topology-dependent scheduling inherently better than multiple rendezvous? Impact of updating 2-hop neighborhood What happens when radios become really cheap and a node can receive multiple concurrent transmissions? (Same if we have MUD with MIMO) What scheduling advantages do we gain? What MACs can we propose? Spring 2006 UCSC CMPE257 82