The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

A LINK-DIRECTIONALITY-BASED DUAL CHANNEL MAC PROTOCOL WITH A POWER EXCHANGE ALGORITHM FOR IEEE 802.11 AD-HOC NETWORKS

Ping Chung Ng Department of Engineering Science,

University of Oxford ping.ng@eng.ox.ac.uk

David J. Edwards Department of Engineering Science,

University of Oxford david.edwards@eng.ox.ac.uk

Soung Chang Liew Dept. of Information Engineering,

The Chinese University of Hong Kongsoung@ie.cuhk.edu.hk

Abstract When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be . However, most of the multi-channel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% of the

C⋅2

C⋅2 targeted capacity. In our paper [1], we proposed a link-directionality-based dual channel MAC protocol (DCP) to boost the network capacities up to 78% of our targeted capacities, 78%* = . However, DCP still failed to reach the capacity target due to the overheads incurred by the protocol. In this paper, we implement a power exchange algorithm on top of the DCP in an attempt to double the capacities of networks using the single-channel 802.11 protocol. This algorithm incurs relatively small overheads and can further release the protocol constraints imposed by DCP. Simulations show that the proposed scheme (DCPwPEA) can achieve more than 132% of our targeted capacities, 132%* = . We believe this protocol can be extended to allow multi-link simultaneous transmissions which can multiply the network capacities of ad-hoc networks by using only two channels.

C⋅2 C⋅56.1C⋅2

C⋅2 C⋅64.2

.

I. INTRODUCTION

When a wireless network uses more channel resources for transmissions, it should achieve a higher network capacity. If an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using n channels should be . However, most of the multi-channel 802.11 protocols proposed in the literature simply compared their performance achievements with the original single-channel 802.11 protocol, without considering the channel resources they had used. In fact, most of them (e.g. [2],[3]) only appear to be able to reach less than 60% of the targeted capacities ( ). This can be attributed to three reasons: i) an additional control channel is used to allocate transmission channels; ii) the overhead induced by the information added to the packet headers; and iii) the transmissions of RTS/DATA and the receptions of CTS/ACK by a node are assigned to the same channel which limit the potential of simultaneous transmissions (details will be explained in Sections II). In the paper [1], we proposed a dual channel protocol which can achieve more than 78% of the targeted capacity, . Although pair-wise

simultaneous transmissions are allowed by the protocol, the achievable capacities are limited by the overheads incurred. Thus, it failed to double the network capacities to

Cn ⋅

Cn ⋅

C⋅2CC ⋅=⋅ 56.12*78.0

C⋅2 . To compensate the penalties due to protocol overheads, in this paper, we further release the bundles of the DCP protocol by adopting a power exchange algorithm.

II. PREVIOUS WORK

A. The Concept In this section, we will outline the concept and the protocol proposed in our paper [1]. To avoid simultaneous transmissions that may lead to collisions, the 802.11 protocol uses short request-to-send (RTS) and clear-to-send (CTS) messages to notify other nodes within the virtual carrier-sensing range (VCSRange) to update their Network Allocation Vector (NAV). The NAV includes the duration time of the ongoing transmission. Thus, no other nodes within the VCSRange can begin transmissions before the NAV expires. Figure 1a shows an example. Under the 802.11 protocol with RTS/CTS access mode, none of the links B, C or D can transmit at the same time with link A. As a result, only one link inside the VCSRange region can transmit at one time. This is because A has to receive the DATA packet from AT while

has to wait for the ACK from AR . Any other simultaneous transmissions within the VCSRange region of and in the same channel will lead to collision of the transmission between and .

R

AT

AR AT

AR ATTo release the above bundles, we can split the transmissions between two nodes of a link into two channels based on their directionalities. Let us consider the case where there are two channels, s and t. Nodes transmit RTS and DATA in one channel (e.g., channel s) as they are in the same direction (from to ) while CTS and ACK are transmitted in another channel (e.g. channel t). The channels are assigned dynamically based on the directionality, network topology and who else are transmitting the neighbourhood. RTS and DATA can be transmitted in either channel s or t, and thus CTS and ACK will be sent in the other channel (t or s). The main idea is to allow the simultaneous transmission of another link i within the VCSRange region of AR and AT provided that the transmissions of link i do not interfere with the receptions of the ACK on or the DATA on . There

AT AR

AT AR

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

are two possible cases: Case 1: the transmissions of link i within the VCSRange use a different channel, and thus these do not affect the reception of R ( T ) in another channel. A A

Case 2: the transmissions of link i use the same channel as the reception of ( ) but those transmissions are far enough from ( ).

AR AT

AR ATFor Case 2, let be the distance between and ,

RATB be the distance between BT and A , and assume the capture threshold (CPThreshold) is set to be 10dB. From [4], in a two-ray propagation model, assuming noise is negligible if the signal-to-interference ratio at is larger than the CPThreshold, can capture the signal from when is transmitting. That is,

RATAd − AT ARd − R

AR

AR AT

BTdCPThresholddSNR RATARATB >= −−

4)/(

RATARATB dd −− > *78.1 (1) In the worst case that and are separated by the maximum transmission range (250m), AR can capture the signal from if is located at more than 1.78*250m=445m away from . In our simulation, the VCSRange is set to be 550m, if BT can not receive the CTS from , must be far enough so that its signal can not interfere with the reception of signal from at . Our proposed MAC protocol in [1] utilized this property to assign transmission channels for links. Figure 1b shows the same scenario as Fig. 1a with the channel assignments based on the Cases 1 and 2 for simultaneous transmissions of links A, B and C.

AT AR

AT BT

AR

AR BT

AT AR

a) b) Figure 1. A network topology using a) original 802.11 and b) our

proposed scheme

B. Dual Channel Protocol (DCP) The proposed dual channel protocol (DCP) in [1] assigns the transmission channels of each link based on the availabilities of the receptions of RTS and CTS from other links. The protocol is modified from the original 802.11 MAC protocol and it attempts to seek opportunities for simultaneous transmissions. Assume all nodes use the same power for transmissions and each node has two half-duplex transceivers that are monitoring both channels at the same time. Consider two links, link i and link j. When a node (e.g., ) of link i receives the but not the

of another link j, it will assign its to the same channel as that of . Thus, link can transmit simultaneously with because receiver is located

far enough away from the transmitter (Case 2) and is receiving CTS or ACK in another channel (Case 1). Thus, there is no collision between link i and link j. Similarly, when a node (e.g., ) of link i receives the but not the of another link j, it will assign its to the same channel as that of . If a node can receive both the and of another link j, it will fall back to the original 802.11 protocol and will resume transmissions only after the completion of the sensed signal. In this case, links i and j have to take turn to transmit. Simulations in [1] showed that DCP can significantly boost capacities up to 78% of the targeted capacities,

iT jRTS

jCTS iRTS

jRTS RTi >−

RTj >− jR

iT jT

iT jCTS

jRTS iRTS

jCTS

jCTS jRTS

C⋅2CC ⋅=⋅ 56.12*78.0 .

III. DUAL CHANNEL PROTOCOL WITH A POWER EXCHANGE ALGORITHM (DCPWPEA)

A. The Concept The previous section has outlined the DCP protocol

which assigns transmission channels based on availabilities of receptions of RTS and CTS packets. In other words, pair-wise simultaneous transmission links must have one of their two nodes outside the VCSRanges of the nodes of the other link (see links B and C in Fig. 1b). This, however, induces unnecessary protocol constraints which limit the chances for simultaneous transmissions. Consider two links again, link i and link j. Deriving from the equation (1), we can define the interference range of link j as

(2) RjTjRjTjj dddCPThresholInRange −− =∗= *78.1)( /1 α

where the path loss exponent 4=α and dBdCPThreshol 10= .

Simultaneous transmissions are actually permitted if a node of link i using the channel s (or t) for transmissions is apart from a node of link j using the same channel s (or t) for reception with a distance larger than . For example, in Fig. 2, links A and B can transmit concurrently as B is located at more than AInRange away from A . From equation (2), InRange varies with the distance between two nodes of a link. The closer the distance between and ( ), the smaller is the . Simultaneous transmission links can then be packed closer to each other. This can drastically improve the network capacities.

jInRange

T R

jT jR RjTjd − jInRange

Consider Fig. 2 again. Assuming AT uses channel 1 to transmit RTS and DATA to AR while A uses another independent channel 2 to send CTS and ACK back to AT . Since is outside the interference range of link A ( A ), it can also transmit RTS and DATA via channel 1. This does not affect the receptions of signals of

A from AT because the signal from at A is more than 10dB stronger than the signal from BT . In addition, BR uses another independent channel (channel 2) for transmissions which do not interfere the signal receptions

R

BTInRange

R AT R

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

of A . Therefore, links A and B can transmit at the same time.

R

Comparing link B in Figures 1b and 2, simultaneous transmission links can now be packed closer to each other. This can significantly improve the network capacities. However, the protocol can no longer assign transmission channels based on availabilities of receptions of RTS and CTS packets as both nodes BT and B can now receive the CTS packets from node A . To solve this problem, we implement a power exchange algorithm on top of DCP to identify possible simultaneous transmission opportunities within VCSRanges.

RR

B. DCP with a Power Exchange Algorithm (DCPwPEA) In this sub-section, we will outline a power exchange

algorithm for releasing the protocol constraints of the DCP protocol. To realize the concept mentioned in the previous sub-section, nodes seek simultaneous transmission opportunities based on the information included in the packets received from other links. As shown in Fig. 2, we divide the regions for simultaneous transmissions into two parts:

I. a node of link j using channel s (or t) for transmission is more than VCSRange away from a node of link i using channel s (or t) for reception. (Section II)

II. a node of link j using channel s (or t) for transmission is more than InRange but less than VCSRange away from a node of link i using channel s (or t) for reception. (Section III)

For region I, the DCPwPEA follows the DCP and keeps using the availabilities of receptions of RTS and CTS packets to decide channel assignments. For region II, we introduce a power exchange algorithm to assign channels based on the information included in the packets received from other links. The DCPwPEA first utilizes the information from the Power Exchange Algorithm to look for simultaneous transmission opportunities (for region II). In the case that one of two nodes of a link is located at region I (outside the VCSRange of another link) such that RTS/CTS packets can not be received, the DCPwPEA will then fall back to use the availabilities of receptions of RTS and CTS packets to select channels.

Consider a pair of links, link i and link j, within region II. When ( ) of link i receives a RTS (CTS) packet from ( ), it records the ( ) of the packet received and add an additional SIR header in its MAC header of the replied CTS (DATA) packet. When other nodes (e.g. ) receive the CTS (DATA) packet, they will extract the

RiTiSIR >−(

TiRi) information from the

MAC header. In addition, they will record the ( ) of the CTS (DATA) packets received.

Owing to the reciprocal nature of the wireless radio link, we can state that ( ).

The algorithm then compares ( ) with ( ). If

iR iT

iT iR RiTiSIR >− TiRiSIR >−

jTSIR >−

TjRiSIR >− TjTiSIR >−

TjRiRiTj SIRSIR >−>− = TjTiTiTj SIRSIR >−>− =

RiTjSIR >− TiTjSIR >−

RiTiSIR >− TiRiSIR >− dCPThresholSIRSIR RiTiRiTj >− >−>−

( ), simultaneous transmissions are allowed and the protocol will then assign the same transmission channels to . Otherwise, it will wait for the completions of the sensing signals and then resume the transmission process.

dCPThresholSIRSIR TiRiTiTj >− −>−

jT

Figure 2. Closer packing of simultaneous transmissions are allowed after adopting a power exchange algorithm in DCP.

C. Overheads of DCPwPEA Comparing the DCPwPEA with the DCP, additional

header overheads are incurred since DCPwPEA includes information about the SIR of the received packets in the headers of the replied packets (CTS or DATA). Figure 3 shows the structures of the DATA and CTS packets. The size of SIR in the packet, however, is much smaller than the size of the sum of the PHY header, MAC header and DATA/CTS. In contrast, the power exchange algorithm allows a closer packing of simultaneous transmissions which significantly overrides the penalty incurred by header overheads.

Figure 3. SIR information added to headers of DATA and CTS packets

Figure 4. Multiplying the network capacity by our proposed channel assignment scheme (DCPwPEA)

D. Allowing Tri-link Simultaneous Tranmsissions In previous sub-sections, we have considered pair-wise

simultaneous transmissions. In this section, we will discuss the hardware requirements for permitting tri-link

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

simultaneous transmissions to further boost the network capacity.

Figure 4 shows a scenario with the channel assignments based on the directionalities of the links. Links 1 to 4 can transmit simultaneously and this can multiply the network capacity by using only two channels. To assign transmission channels, nodes have to extract the SIR information from the CTS and DATA packets received from other links. This requires the use of the 802.11g chip set to extract packets from overlapping signals. Consider Links A, C and D in Fig. 4. Suppose link A first begins the transmission. Both links C and D can receive the CTS from node A and thus they (C & D) decide that they can transmit simultaneously with link A. Link C then starts the transmission while link A is still transmitting. From link D’s point of view, the signal from links A and C are overlapped and thus link D fails to extract the CTS from link C. Therefore, link D cannot decide if it can transmit simultaneously with link C. To solve this problem, a possible solution is to use the widely deployed IEEE 802.11g DSSS-OFDM chip set. Nodes use direct sequence spread spectrum (DSSS) for transmitting RTS/CTS and MAC headers, and Orthogonal Frequency Division Multiplexing (OFDM) for sending DATA and ACK packets. Figure 5 shows parts of the DSSS-OFDM chip set [5]. Consider Fig. 4 again. When link A is sending DATA OFDM signal while link C is transmitting CTS DSSS signal, the filter (as shown in Fig. 5) of node DT can extract the CTS signal of link C from the DATA signal of link A. If , then links C and D can transmit currently. Otherwise, link D will halt its transmission process and resume after link C completes its transmissions.

R

dCPThresholSIRSIR TCRCTCRD >− >−>−

Figure 5. Parts of the IEEE 802.11g DSSS-OFDM chip set

IV. ANALYSIS OF CAPACITY IMPROVEMENT

In this section, we analyze the capacity enhancement of our proposed protocol without taking the protocol overheads into account. The protocol overheads are incurred when nodes halt their transmission progresses for sensing nearby signals and by inserting SIR information in packet headers. Since the capacity improvements achieved by DCPwPEA highly depend on the network topologies, here we examine the upper and lower bounds of numbers of simultaneous transmissions that can be packed in the VCSRanges of a link using the original 802.11 protocol.

Figure 6 shows a link of two nodes ( A and A ) separated by a distance, d. In the original 802.11 protocol, when a link is transmitting packets, no other links in its VCSRange can transmit at the same time. Let be the

area covered by the VCSRanges of link A and let N be the number of additional simultaneous transmission links that can be packed within the VCSRanges of link A as shown in Fig. 6. The capacity of DCPwPEA then becomes

T R

vcA

1+N times of the capacity of the original 802.11 protocol. In DCPwPEA, simultaneous transmissions within the VCSRanges of link A are permitted if at least one node of another link is located outside the interference ranges (InRanges) of link A.

1) The Lower Bound Consider links A, B and C in Fig. 8a. When BR and

CT of links B and C are located just slightly outside the InRanges of link A while and are within the InRanges of link A, at most two other simultaneous transmission links (links B and C) are allowed in the area . This is because the signal from and will induce a SIR just slightly larger than the CPThreshold (10dB) at AR and AT respectively. Thus, . Let C be the capacity achieved by the original single-channel 802.11 protocol. Theoretically, the capacity can reach

BT CR

vcA BR CT

2≥N

CCN ⋅≥⋅+ 3( )1 . However, the protocol overheads degrade the capacity to (more details will be shown in the simulation results of Section V).

C⋅64.2

2) The Upper Bound In Fig. 6, all other links (links 1 to N) are located just

slightly inside the VCSRanges of link A so as to minimize the interferences on node AT and A . Consider AR with N/2 number of links interfering its signal receptions. In a two-ray propagation model, AR can receive packets successfully from if the signal-to-noise ratio at is

R

AT AR

dCPThresholdv

NSIR >= 4)(

2/1

As a result, the maximum number of simultaneous transmissions (N) within becomes

vcA

4)(2dv

dCPThresholN <

Since N is an integer,

⎥⎦⎥

⎢⎣⎢≤ 4)(2

dv

dCPThresholN (4)

Figure 7 plots N against the distance (d) between two nodes of a link when VCSRange (v) is set to 550m. The closer the distance between two nodes of a link, the larger number of additional simultaneous transmission links (N) is allowed since interferences from other links become less influential. Thus, the upper bound varies with the separations between links (d) and highly depends on the network topologies. For example, when d is set to 250m, at most four other simultaneous transmission links (N=4) can be packed inside . Comparing to the original 802.11 protocol, our protocol (DCPwPEA) can potentially multiply network capacities.

vcA

The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06)

Figure 6. Packing simultaneous-transmission links in the VCSRanges of a link.

Figure 7. The number of additional simulatneous-transmission links (N) versus the distance between two nodes of a link (d)

V. SIMULATION RESULTS

We have implemented our proposed MAC protocol (DCPwPEA) in the NS-2 [6] simulator. For fair comparisons with the settings of the original 802.11 protocol in NS-2, we assume the same values for the VCSRange and the TxRange by setting the data rate (for sending DATA/ACK) at 12Mbps (OFDM, QPSK) and the basic rate (for transmitting RTS/CTS and MAC headers) at 2Mbps (DSSS, DQPSK). All data sources are saturated UDP traffic stream with fixed packet size of 1460bytes. Figure 8a shows three links in a string topology. As shown in Fig. 9a, using the original single-channel 802.11 protocol results in 4.85Mbps total network throughput, thus the targeted capacity for using dual channels is by definition 4.85Mbps*2=9.7Mbps. With our proposed scheme, a total network throughput of 12.85Mbps is achieved, which is 132% of the targeted capacity. Figures 8b and 9b shows another example of an irregular topology. Our proposed scheme obtains total network throughput at 21.71Mbps, which is 219% of the targeted capacity (4.95Mbps*2=9.9Mbps). In addition to the capacity enhancement, our proposed protocol achieves a fair bandwidth allocation in both cases.

VI. CONCLUSION

This paper has adopted a power exchange algorithm in the link-directionality-based dual channel protocol (DCP) proposed in our paper [1] in an attempt to double the network capacities. The algorithm releases unnecessary protocol constraints imposed by DCP. The capacity

improvements by allowing closer packing of simultaneous transmission links significantly override the overheads incurred by the algorithm. We have shown that our proposed scheme can boost the network capacities of single channel IEEE 802.11 ad-hoc networks by more than 264%. We have also demonstrated that the potential of multiplying the network capacities by two channels.

a)

b)

Figure 8. a) Three links in a string topology and b) five links in an irregular network topology

a) b)

Figure 9. Per-link throughput of the networks of Fig. 8a and 8b with the original 802.11 protocol and our proposed MAC protocol (DCPwPEA)

REFERENCES [1] P.C. Ng, D. Edwards, S. C. Liew, “A Link-directionality-based

Dual Channel MAC Protocol for IEEE 802.11 Ad-hoc Networks”, IEEE INFOCOM’06 Student Workshop, April 2006.

[2] N. Choi et al., “Multi-channel MAC protocol for mobile ad hoc networks”, IEEE VTC 2003-Fall 2003.

[3] A.Baiocchi et al., “Why a Multichannel Protocol can boost IEEE 802.11 Performance”, ACM MSWiM’04.

[4] T. Rappaport, “Wireless Communications: Principles and Practice”, Prentice Hall, New Jersey, 2002

[5] “802.11g”, http://www.vocal.com [6] “The Network Simulator – ns2”, http://www.isi.edu/nsnam/ns.