CMPE 257: Wireless and Mobile Networking SET 3m:

Winter 2004 UCSC CMPE252B

1

CMPE 257: Wireless and Mobile NetworkingSET 3m:

Medium Access Control Protocols

Spring 2005 CMPE257 UCSC 2

MAC Protocol Topics MAC protocols using multiple

channels with one transceiver only MMAC (Multi-channel MAC) SSCH (Slotted Seeded Channel Hopping)


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

Motivation for Multi-Channel

1

defer

1

2

Single channel Multiple Channels


Problem Statement Using k channels does not translate into

throughput improvement by a factor of k Nodes listening on different channels cannot talk to

each other

Constraint: Each node has only a single transceiver 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

1 2


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



A

B

C

Time

Beacon

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM Window

Beacon Interval



A

B

C

Time

Beacon

ATIM

ATIM-ACK

ATIM Window

Beacon Interval


Multi-Channel Hidden Terminals Consider the following naïve protocol

Static channel assignment (based on node ID)

Communication takes place on receiver’s channel

Sender switches its channel to receiver’s channel before transmitting


Multi-Channel Hidden Terminals

A B CRTS

A sends RTS

Channel 1

Channel 2



A B CCTS

B sends CTS

Channel 1

Channel 2

C does not hear CTS because C is listening on channel 2



A B CDATA

C switches to channel 1 and transmits RTS

Channel 1

Channel 2

Collision occurs at B

RTS


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


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


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

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


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


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


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


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


Channel Negotiation

A

B

C

DTime

ATIM Window

Beacon Interval

Common Channel Selected Channel

Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

Time

ATIM Window

Beacon Interval


Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval


Beacon


Channel Negotiation

A

B

C

D

ATIM

ATIM-ACK(1)

ATIM-RES(1)

ATIM-ACK(2)

ATIM ATIM-RES(2)

Time

ATIM Window

Beacon Interval


Beacon

RTS

CTS

RTS

CTS

DATA

ACK

ACK

DATA

Channel 1

Channel 1

Channel 2

Channel 2


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


Wireless LAN - Throughput

30 nodes 64 nodes

MMAC

DCA

802.11

MMAC shows higher throughput than DCA and 802.11

802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)

1 10 100 1000 1 10 100 1000

2500

2000

1500

1000

500

Agg

rega

te T

hrou

ghpu

t (K

bps)

2500

2000

1500

1000

500


Multi-hop Network – Throughput

3 channels 4 channels

MMAC

DCA

802.11802.11

DCA

MMAC

Packet arrival rate per flow (packets/sec)1 10 100 1000

Packet arrival rate per flow (packets/sec)1 10 100 1000

Agg

rega

te T

hrou

ghpu

t (K

bps)

1500

1000

500

0

2000

1500

1000

500

0


Throughput of DCA and MMAC(Wireless LAN)

DCA MMAC

2 channels

802.11

MMAC shows higher throughput compared to DCA

6 channels

802.11

2 channels

6 channels

Agg

rega

te T

hrou

ghpu

t (K

bps) 4000

3000

2000

1000

0

4000

3000

2000

1000

0

Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec)


Analysis of Results DCA

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 MMAC

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


Conclusion and Future Work Conclusion:

MMAC requires a single transceiver per host to work in multi-channel ad hoc networks

MMAC achieves throughput performance comparable to a protocol that requires multiple transceivers per host

Future work Dynamic adaptation of ATIM window size

based on traffic load for MMAC Efficient multi-hop clock synchronization


SERIAL ETHERNET

SERIAL ETHERNET

SERIAL ETHERNET

Motivation: Improving CapacityTraffic on orthogonal channels do not interferee.g. Channels 1, 6 and 11 for IEEE 802.11b

Channel 11

Channel 1

Channel 6

Example: An IEEE 802.11b network with 3 Access Points

Can we get the benefits of multiple channels in ad hoc networks?

Channel 6


Channel Hopping: Prior Work

Using multiple radios: DCA (ISPAN’00): a control and a data channel MUP (Broadnets’04): multiple data channels

Consumes more power, expensive Using non-commodity radios:

HRMA (Infocom’99): high speed FHSS networks Nasipuri et al, Jain et al: listen on many channels

Expensive, not easily available

Using a single commodity radio: Multi-channel MAC (MMAC) (Mobihoc’04)


Channel Hopping: MMACMMAC Basic idea:

Periodically rendezvous on a fixed channel to decide the next channel

Issues Packets to multiple destinations high delays Control channel congestion Does not handle broadcasts

Channel 1

Channel 6

Channel 11

Data DataControl DataControl


SSCH

A new channel hopping protocol that Increases network capacity using multiple

channels Overcomes limitations of dedicated control

channel– No control channel congestion– Handles multiple destinations without high delays– Handles broadcasts for MANET routing


SSCH: Slots and Seeds Divide time into slots: switch channels at beginning of a slot

3 channelsE.g. for 802.11bCh 1 maps to 0Ch 6 maps to 1

Ch 11 maps to 2

1 0 2 1 0 2 1 0

0 1 2 0 1 2 0 1

New Channel = (Old Channel + seed) mod (Number of Channels)seed is from 1 to (Number of Channels - 1)

Seed = 2

Seed = 1

(1 + 2) mod 3 = 0

(0 + 1) mod 3 = 1

A

B

• Enables bandwidth utilization across all channels• Does not need control channel rendezvous


SSCH: Syncing Seeds• Each node broadcasts (channel, seed) once every slot • If B has to send packets to A, it adjusts its (channel, seed)

3 channels1 0 2 1 0 2 1 0

0 1 2 1 0 2 1 0

Seed

Seed

Follow A: Change next (channel, seed) to (2, 2)

A

B

2 2 2 2 2 2 2 2

1 2 2 2 2 2 2 2

2

2

1

Stale (channel, seed) info simply results in delayed syncing

B wants to start a flow with A

2


Nodes might not overlap!If seeds are same and channels are different in a slot:

3 channels

Seed = 2

Seed = 2

Nodes are off by a slot Nodes will not overlap

1 0 2 1 0 2 1 0

1 0 2 1 0 2 12

A

B


SSCH: Parity Slots

3 channels

Seed = 1

Seed = 1

Every (Number of Channels+1) slot is a Parity Slot

In the parity slot, the channel number is the seed

Parity Slot Parity Slot

Guarantee:

If nodes change their seeds only after the parity slot,

then they will overlap

0 1 2

0 12 2

2

0 1 1 1

111 0A

B


SSCH: Partial SynchronizationSyncing to multiple nodes, e.g., A sends packets to B & C• Each node has multiple seeds• Each seed can be synced to a different node

Parity Slot Still Works• Parity slot: (Number of Channels)*(Number of Seeds) + 1• In parity slot, channel is the first seed• First seed can be changed only at parity slot

If the number of channels is 3, and a node has 2 seeds: 1 and 2

2 2 1 0 110 2 2 11 00

(1 + 1) mod 3 = 2

(2 + 2) mod 3 = 1

Parity Slot= seed 1


Illustration of the SSCH Protocol

2 2 1 0 110 2 2 11 00Node A

2 0 1 2 120 2 2 11 00Node B

Seeds

B wants to start a flow with A

Complete Sync

(sync 1st seed)

Seeds (1, 2)

Channels: (1, 2)

Partial Sync

(only 2nd seed)

Seeds: (2, 2)

Channels: (2, 1)

1 2 1 2 1 2 1 2 1 2 1 2

Seeds 2 1 2 2 2 2 1 2 1 2 1 2

Suppose each node has 2 seeds, and hops through 3 channels.


SSCH: Handling BroadcastsA single broadcast attempt will not work with SSCH

since packets are not received by neighbors on other channels

2 1 0 10

0 1 2 20

B’s broadcast

Node A

Node B

Seeds

Seeds

1 2 1 2

2 2 2 2

SSCH Approach

Rebroadcast the packet over ‘X’ consecutive slots

a greater number of nodes receive the broadcast

B’s broadcast in SSCH


Simulation EnvironmentQualNet simulator: IEEE 802.11a at 54 Mbps, 13 channels Slot Time of 10 ms and 4 seeds per

node a parity slot comes after 4*13+1 = 53 slots, 53 slots is: 53*10 ms = 530 ms

Channel Switch Time: 80 µs Chipset specs [Maxim04], EE literature [J. Solid State Circuits 03]

CBR flows of 512 byte packets per 50 µs


SSCH: Stationary ThroughputPer-Flow throughput for disjoint flows

0

2

4

6

8

10

12

14

0 5 10 15

# Flows

Th

rou

gh

pu

t (i

n M

bp

s)

IEEE 802.11a

SSCH

SSCH significantly outperforms single channel IEEE 802.11a


SSCH Handles Broadcasts

0

0.1

0.2

0.3

0.4

2 3 4 5 6 7 8 9

# Broadcasts

Ro

ute

Dis

cove

ry T

ime

(in s

ec)

0

1

2

3

4

5

6

7

2 3 4 5 6 7 8 9

# Broadcasts

Ave

rag

e R

ou

te L

eng

th

(# h

op

s)

10 Flows in a 100 node network using DSR

For DSR, 6 broadcasts works well (also true for AODV)

Average discovery time for IEEE 802.11a

Average route length for IEEE 802.11a


SSCH in Multihop Mobile Networks

0

0.5

1

1.5

2

2.5

0.2 0.4 0.6 0.8 1

Speed (in m/s)

Flo

w T

hrou

ghpu

t (in

Mb

ps)

Random waypoint mobility: Speeds min: 0.01 m/s max: rand(0.2, 1) m/s

0

1

2

3

4

5

0.2 0.4 0.6 0.8 1

Speed (in m/s)

Ave

rag

e R

ou

te L

eng

th

(# h

op

s)

Average flow throughput for IEEE 802.11a

Average route length for IEEE 802.11a

SSCH achieves much better throughputalthough it forces DSR to discover slightly longer routes


Conclusions

SSCH is a new channel hopping protocol that:

Improves capacity using a single radio Does not require a dedicated control

channel Works in multi-hop mobile networks

Handles broadcasts Supports multiple destinations (partial sync)


Future Work

Analyze TCP performance over SSCH Study interoperability with non-SSCH

nodes Study interaction with 802.11 auto-rate Implement and deploy SSCH (MultiNet)


References [SV04] So and Vaidya, Multi-

Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using a Single Transceiver, in Proc. of ACM MobiHoc 2004.

[BCD04] Bahl et al., SSCH: Slotted Seeded Channel Hopping for Capacity Improvement in IEEE 802.11 Ad-Hoc Wireless Networks, in Proc. of ACM MobiCom 2004.


Acknowledgments

Parts of the presentation are adapted from the following sources: So’s ACM MobiHoc 2004 presentation Ranveer Chandra, Cornell University,

http://www.cs.cornell.edu/people/ranveer/multinet/ssch_mobicom.ppt



Documents

CMPE 257: Wireless and Mobile Networking SET 3m: