Cross-layer design for packet fast forward in Mobile Ad Hoc Network

TAN Wei, QIN Dan-yang, SHA Xue-jun, XU Yubin Department of Communication Engineering Harbin Institute of Technology Heilongjiang P. R. China

{tan_wei,qindanyang,shaxujun, ybxu }@hit.edu.cn

Abstract In this paper, we propose a novel cross-layer scheme for

packet fast forward protocol in mobile ad hoc networks called the PFFMAC (Packet Fast Forward MAC) protocol. PFFMAC is a cross-layer solution in which the MAC layer chooses different packet forward process according to routing table by estimating middle nodes. In PFFMAC, packet forward process in MAC layer is dynamically adjusted from routing information obtained in routing tables. RTS are sent with ACK at middle nodes in available routing which never waits for a DIFS and back-off slots, leading to higher throughput, smaller delay and also lower overhead. This cross-layer based protocol can be easily used in any proactive and reactive routing without any additional control messaging. Formulation analysis shows that the cross-layer architecture with PFFMAC outperforms traditional 802.11 protocol performance. Also, Extensive simulations are used to compare the two kind protocols. The results indicate that PFFMAC achieves significant improvements in network throughput, overhead, and packet delay and PFFMAC performs much better with larger nodes and higher node mobile rate.

Keywords: Cross-layer design, PFFMAC, MANET

1. Introduction

Mobile ad hoc network (MANET) is a collection of mobile nodes that communicate with one another over wireless links without requiring support of a fixed infrastructure. Instead, nodes agree to relay one another’s packets toward their destinations, acting as routers and automatically organizing into a cooperative network. MANETs have been proposed for use in military, disaster relief and emergency operations.

Nodes in MANET typically communicate over the same wireless channel, which prevents closely positioned nodes from transmitting simultaneously. The most common distributed MAC protocol used in MANETs today is the IEEE 802.11 Distributed Coordination Function (DCF) [1]. Because of low throughput, many efforts have been made on enhancing the throughput of MANETs through concurrent scheduling at the MAC layer. The existing works can be divided into two main classes. In the first class, transmission power control is used per-packet to increase the spatial channel reuse. Transmission power control based schemas can further be divided into two sub-categories: single-channel based or multi-channel based. However, as pointed out in [2], single-channel based transmission power control can degrade the network throughput. Even optimized by periodically increasing the transmit power during the DATA transmission

to inform the nodes in the carrier sensing zone. The real throughput enhancement through transmission power control is obtained in multi-channel based transmission power control schemas [3, 4].

Although the simulations results in [3,4] indicate impressive improvements in throughput over the 802.11 scheme, there are some major design problems with these schemes, such as how to deal with the huge latency introduced by transmission power control [5], the unrealistic assumption of same channel gain for both the control channel and data channel, the hardware complexity for the wireless communication node to be equipped with two transceivers, and incompatible with existing standards and hardware.

In the second class, the approach to improve throughput is to insert additional control gaps between RTS/CTS and DATA packets for successfully scheduled transmission, such as the MACA-P [6]. POWMAC [7] is a single-channel and single transceiver protocol, which combines the approach of additional control gap and transmission power control. Besides the problems introduced by transmission power control, POWMAC protocol adds the control packet for all nodes which is not always necessary

This paper presents a novel cross-layer technique that improves throughput and delay simultaneously with single channel and transceiver. We implement a cross-layer control architecture, in which the routing layer notifies middle nodes in the routing information, and the applications adjust their MAC layer according to the feedback value with PFFMAC protocol. Cross-layer control is extremely simple, but powerful. Formulation analysis shows that the cross-layer control architecture with PFFMAC outperforms traditional 802.11 protocol performance. Moreover, as the number of nodes increases, PFFMAC does much better than traditional MAC protocol which is without cross-layer design.

The rest of this paper is organized as follows. Section 2 describes Cross-Layer design scheme for both packet fast forward MAC and routing layer. Section 3 gives problem formulation. Section 4 presents experimental results. Finally, Section 5 concludes the paper and discusses opportunities for future research.

2. Cross-Layer Design and PFFMAC Protocol

In IEEE802.11b DCF, when a node wishes to transmit a packet, it first defers its transmission for a randomly chosen interval, and then senses the transmission medium. If the medium is idle, the node transmits a Request-To-Send (RTS) packet. On receiving the RTS, the destination node replies with a Clear-To-Send (CTS) packet. On receiving the CTS packet, the source node starts the data transmission. Once the DATA packet was successfully received, the destination node

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sends an acknowledged (ACK) to the source node. The RTS-CTS exchange is needed to let the neighbor nodes of both the source and the destination know that the data transmission is about to begin, seen in figure 2. The traditional scheme is given in figure 1, in which takes three nodes for short to present two hop in mobile Ad Hoc network. Node A and C are source node and destination node respectively, while node B acts middle node. It’s also the same when there are several middle nodes for multi-hop. IEEE802.11 employs monolayer design in which every layer works singly, so network layer, MAC layer and PHY layer never change information.

Figure1. Traditional scheme

Figure2. IEEE802.11 MAC mechanism

Unfortunately, after every middle node sends ACK they must wait a time of DIFS and several back-off slots to send RTS to find destination node, which waste channel bandwidth and raise delay. In Packet Fast Forward MAC Protocol (PFFMAC), a cross-layer design is used to optimize network performance seen in figure 3.

Network layer can exchange forward routing information with MAC layer, for which MAC can adjust packet forward process dynamically from routing information obtained from routing tables. Fortunately, route table is usually maintained by the underlying routing layer, and therefore can be shared with interested applications. Applications that are interested in adapting a callback function at the underlying routing layer. The callback function is invoked whenever the route at middle nodes. In figure 4, because of forward information exchange, PFFMAC piggybacks RTS on ACK, leading to more time to send packet and less possibly to exchange control messages. Thus, PFFMAC get higher throughput, smaller delay and also lower overhead. This cross-layer based protocol can be easily used in any proactive and reactive routing without any additional control messaging. This is totally different form other works mentioned in section 1

which is only for on kind of routing protocol or need to exchange messages such as RREQ.

We have analyzed and evaluated the cross-layer design with PFFMAC both in formulation and in ns-2 simulation. Next section presents problem formulation.

Figure3. Cross-layer scheme

Figure4. PFFMAC mechanism

3. Problem Formulation

We consider a conventional IEEE 802.11 ad hoc network in Figure 5, where the nodes’ transmission range is r and the wireless channel data rate is R. Let us assume that the packet generation rate inside the cell is described with a mean value of g packets/s, while the payload size of the transmitted packets with a mean value of P. If we ignore the hidden terminals and supposing that all nodes are in line of sight with each other, the utilization of the cell, is

0

USB I

=+

(1)

As defined in [8], U is the expected duration for which packet payload is transmitted, B is the expected duration of a busy period and I is the expected duration of an idle period. Let p be the probability that no packet arrives in an IEEE 802.11 time slot,τ be the duration of an IEEE 802.11 time slot. Thus,

( ) 1

1

11

n

n

I n p pp

ττ∞

−

=

= − =−∑ (2)

Forward Info. With Routing

N

MAC

PHY

Forward

Info. With R ti

Forward Info. With R ti

CCrroossss--llaayyeerr ddeessiiggnn

Node C

Forward With

Routing

N

MAC

PHY

CCrroossss--llaayyeerr ddeessiiggnn

Node C

Forward With

Routing

Forward With

Routing

Node A Node

MAC

N W

PHY

Node A Node CNode B

Remove/Add MAC header + route lookup/ARP + queuing wait

Timer Expiry

AA

BB

CC

DIFS

RRTTSS

CCTTSS ACK

DATA

RRTTSS

CCTTSS

DIFS

DATA

AACCKK

This latency should be reduced

AA

BB

CC CCTTSS ACK

DATA RRTTSS

CCTTSS

DIFS

DATA

AACCKK//RRTTSS

Timer Expiry

PPFFFFMMAACC:: ppiiggggyybbaacckk RRTTSS oonn AACCKK

FFoorrwwaarrdd IInnffoo.. eexxcchhaannggee

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Figure5. IEEE802.11 Network

Busy slots are given as,

( )1

1

11n

Bn

t np pp

−∞

=

= − =∑ (4)

Given sT as the time needed for a successful transmission and cT as the time a collision takes, the expected duration of a busy period is

( ) ( )1 c s s c

s B s c B s

T p T TB Tt p Tt p

p+ −

= + − = (5)

The transmission time of the packet payload is

pPTR

= (6)

The average time for which payload information is transmitted in the channel is given by

1

1p

p B s

T pU T t p

p= =

− (7)

Let us now consider a system in which each packet is transmitted by means of basic access mechanism, RTS/CTS access mechanism and PFFMAC access mechanism, which we only need to discuss sT and cT with different access mechanisms. Let H = PHY head + MAC head, δ is transmit delay. We define BAS

sT and BAScT for basic one

BASsT H P SIFS ACK DIFSδ δ= + + + + + + (8)

BAScT H P DIFS δ= + + + (9)

RTSsT and RTS

cT for RTS/CTS one. RTS

sT RTS SIFS C TS SIFSH P SIFS AC K D IFS

δ δδ δ

= + + + + ++ + + + + + +

(10)

RTScT RTS DIFS δ= + + (11)

RTSsRT for PFFMAC, which send RTS after ACK at middle

nodes.

RTSsRT RTS SIFS CTS SIFS

H P SIFS ACKδ

δ δ= + + + +

+ + + + + + (12)

In order to consider hidden terminal problem, we use additional probability 0e , which be proved [9] that for the IEEE 802.11 case this probability is:

( )1

0sTIe p

B I

β βτ

− = +

(13)

According to the above, the utilization of a cell is now: 0 0S S e= ⋅ (14)

β is the fraction of nodes that are hidden to node A, which can calculate as follows

( ) ( )22 22 2 3 4r r rβ π π π = − =

(15)

So there are ( )1 1 β− identical cells of nodes that utilize the wireless channel. So the utilization of the whole channel is

( )1tS S β= − (16)

Figure6. Theory and simulation analysis for PFFMAC

From above we can see that the utilization of the whole channel is only decided by sT and cT while the smaller sT and cT , the higher utilization. PFFMAC works with the smallest sT and cT , so it can obtain the highest utilization. Figure 6 presents the results by theory and simulation. 4. Performance Evaluations

In this study we use the extended version of UCB/LBNL network simulator NS-2[10]. We simulate 50 and 100 nodes respectively in a virtual environment of 1000 * 1000 m for 600 second. The channel data rate set to 2 Mbps and 15 nodes send CBR data packet with packet length of 512 Byte. The speed varies from 1 to 20m/s. We take throughput, delay and overhead as metric for network performance.

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Figure7. Throughput for different protocols

Figure8. Delay for different protocols

Figure9. Overhead for different protocols

Figure 7 shows that throughput of network decreases with

an increase mobile rate of nodes for different protocols when number of nodes are 50 and 100. 50n-802.11 and 50n-PFFMAC in figures stand for performance of 802.11 and PFFMAC respectively with 50 nodes. Because of the same payload in network, the more nodes are and the lower throughput is, of which each protocol outperforms with 50 nodes than 100 nodes. Note that PFFMAC employs cross-layer design to piggyback RTS on ACK at middle nodes, it can save more channel bandwidth and delay to send packets, leading to improve throughput even delay and overhead seen

in figure 8 and figure 9. When network size become larger and higher mobile rate, the throughput even become better with PFFMAC. Such as at mobile rate of 20, PFFMAC improves throughput from 50000 to 50500 with 50 nodes, while from 48200 to 49500 with 100 nodes. We can draw the same conclusion from figure 8 and figure 9.

5. Conclusion

This paper presented a cross-layer based packet fast forwarding MAC protocol PFFMAC for mobile ad hoc network. This protocol can work with both proactive routing protocol and reactive one without any changes. The protocol does not require any additional control messaging and solely rely on routing table and MAC layer contention information. Problem Formulation showed that the cross-layer scheme with PFFMAC outperforms traditional 802.11 protocol performance. Our simulation results reveal that adopting PFFMAC protocol significantly improves throughput, delay and overhead performance of the network by avoiding waiting time at middle nodes. PFFMAC performs much better when number of nodes becomes large and also when mobile rate of node is higher.

6. References [1]IEEE”IEEE std 802.11 - wireless LAN medium access control (MAC) and physical layer (PHY) specifications” 1997 [2]E.-S. Jung and N. H. Vaidya “A Power Control MAC Protocol for Ad Hoc Networks” ACM Wireless Networks (WINET), Volume 11, Issue 1-2, pp. 55-66, 2005. [3] J. Deng and Z. Haas, “Dual busy tone multiple access (DBTMA) a multiple access control scheme for ad hoc networks”, IEEE Transactions on Communications, Volume 50, Issue 6, June 2002. [4]A. Muqattash and M. Krunz “Power controlled dual channel (PCDC) medium access protocol for wireless ad hoc networks” In Proceedings of the IEEE INFOCOM Conference, pp 470–480, 2003 [5]V Kawadia and P. R. Kumar “Principles and protocols for power control in ad hoc networks” IEEE JSAC, Special Issue on Ad Hoc Networks, Vol 1, pp.76-88, 2005. [6]Acharya, Misra and S Bansal “Design and Analysis of a Cooperative Medium Access Scheme for Wireless Mesh Networks” In Proceedings of the First International Conference on Broadband Networks, 2004 [7]Alaa Muqattash and Marwan Krunz “POWMAC: A Single-Channel Power-Control Protocol for Throughput Enhancement in Wireless Ad Hoc Networks” IEEE JSAC,Vol. 23, Issue 5, pp. 1067-1084, 2005 [8]Vassis, D.; Kormentzas, G.; Consumer Communications and Networking Conference, 2006. CCNC 2006 3rd IEEE Volume 2, 8-10 Jan. 2006 Page(s):1273 - 1276 [9]F. Tobagi, L. Kleinrock, “Packet Switching in Radio Channels: Part II the Hidden Terminal Problem in Carrier Sense Multiple Access and the Busy-Tone Solution”, IEEE Transactions on Communications, vol. com-23, No 12, Dec 1975. [10]K. Fall and K. Varadhan, Ns Notes and Documentation Technical Report, University of California Berkeley, LBL, USC/ISI, and Xeron PARC, 2003

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