CMPE 257 Wireless and Mobile Networking SET 3m Medium Access Control Protocols Winter 2004 UCSC CMPE252B 1 MAC Protocol Topics MAC protocols using multiple channels with one transceiver only MMAC Multi channel MAC SSCH Slotted Seeded Channel Hopping Spring 2005 CMPE257 UCSC 2 Motivation for Multi Channel Multiple orthogonal channels available in IEEE 802 11 3 channels in 802 11b 12 channels in 802 11a Utilizing multiple channels can improve throughput Allow simultaneous transmissions 1 1 defer Single channel Spring 2005 2 Multiple Channels CMPE257 UCSC 3 Problem Statement Using k channels does not translate into throughput improvement by a factor of k Constraint Each node has only a single transceiver Nodes listening on different channels cannot talk to each 1 2 other Capable of listening to one channel at a time Goal Design a MAC protocol that utilizes multiple channels to improve overall performance Modify 802 11 DCF to work in multi channel environment Spring 2005 CMPE257 UCSC 4 802 11 Power Saving Mechanism Time is divided into beacon intervals All nodes wake up at the beginning of a beacon interval for a fixed duration of time ATIM window Exchange ATIM Ad hoc Traffic Indication Message during ATIM window Nodes that receive ATIM message stay up during for the whole beacon interval Nodes that do not receive ATIM message may go into doze mode after ATIM window Spring 2005 CMPE257 UCSC 5 802 11 Power Saving Mechanism Beacon Time A B C ATIM Window Beacon Interval Spring 2005 CMPE257 UCSC 6 802 11 Power Saving Mechanism Beacon A Time ATIM B C ATIM Window Beacon Interval Spring 2005 CMPE257 UCSC 7 802 11 Power Saving Mechanism Beacon A B Time ATIM ATIM ACK C ATIM Window Beacon Interval Spring 2005 CMPE257 UCSC 8 Multi Channel Hidden Terminals Consider the following na ve protocol Static channel assignment based on node ID Communication takes place on receiver s channel Spring 2005 Sender switches its channel to receiver s channel before transmitting CMPE257 UCSC 9 Multi Channel Hidden Terminals Channel 1 Channel 2 A RTS B C A sends RTS Spring 2005 CMPE257 UCSC 10 Multi Channel Hidden Terminals Channel 1 Channel 2 A CTS B C B sends CTS C does not hear CTS because C is listening on channel 2 Spring 2005 CMPE257 UCSC 11 Multi Channel Hidden Terminals Channel 1 Channel 2 A DATA B RTS C C switches to channel 1 and transmits RTS Collision occurs at B Spring 2005 CMPE257 UCSC 12 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 Spring 2005 CMPE257 UCSC 13 Wu s Protocol Wu00ISPAN Assumes 2 transceivers per host One transceiver always listens on control channel Negotiate channels using RTS CTS RES RTS CTS RES packets sent on control channel Sender includes preferred channels in RTS Receiver decides a channel and includes in CTS Sender transmits RES Reservation Sender sends DATA on the selected data channel Spring 2005 CMPE257 UCSC 14 Wu s Protocol cont Advantage No synchronization required Disadvantage Each host must have 2 transceivers Per packet channel switching can be expensive Control channel bandwidth is an issue Spring 2005 Too small control channel becomes a bottleneck Too large waste of bandwidth Optimal control channel bandwidth depends on traffic load but difficult to dynamically adapt CMPE257 UCSC 15 Proposed Protocol MMAC Assumptions 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 2005 CMPE257 UCSC 16 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 2005 CMPE257 UCSC 17 Preferred Channel List PCL Each node maintains PCL Records usage of channels inside the transmission range High preference HIGH Medium preference MID Already selected for the current beacon interval No other vicinity node has selected this channel Low preference LOW Spring 2005 This channel has been chosen by vicinity nodes Count number of nodes that selected this channel to break ties CMPE257 UCSC 18 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 2005 CMPE257 UCSC 19 Channel Negotiation Common Channel Selected Channel A Beacon B C D Time ATIM Window Spring 2005 Beacon Interval CMPE257 UCSC 20 Channel Negotiation Common Channel A B Selected Channel ATIMATIM RES 1 Beacon ATIMACK 1 C D Time ATIM Window Spring 2005 Beacon Interval CMPE257 UCSC 21 Channel Negotiation Common Channel A B C D Selected Channel ATIMATIM RES 1 Beacon ATIMACK 1 ATIMACK 2 ATIM ATIMRES 2 Time ATIM Window Spring 2005 Beacon Interval CMPE257 UCSC 22 Channel Negotiation Common Channel A B C D ATIMATIM RES 1 Selected Channel RTS DATA Channel 1 Beacon Channel 1 ATIMACK 1 ATIMACK 2 CTS ACK CTS ACK Channel 2 Channel 2 ATIM ATIMRES 2 RTS DATA Time ATIM Window Spring 2005 Beacon Interval CMPE257 UCSC 23 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 2005 CMPE257 UCSC 24 Aggregate Throughput Kbps Wireless LAN Throughput 2500 2500 MMAC 2000 DCA 1500 1000 2000 MMAC 1500 DCA 1000 802 11 500 1 10 100 1000 Packet arrival rate per flow packets sec 802 11 500 1 10 100 1000 Packet arrival rate per flow packets se 30 nodes 64 nodes MMAC shows higher throughput than DCA and 802 11 Spring