A Speech-Optimised Multiple Access Scheme for a Mobile Ad Hoc Network

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V. N. Muthiah and W. C. Wong Department of Electrical Engineering

National University of 5ingapore 10 Kent Ridge Crescent, Singapore 119260

Abstract-We propose a multiple access scheme based on Time Division Duplexing (TDD) for peer-to-peer speech communications in a short-range mobile ad hoc network of wireless nodes. This reservation-based packet-switched scheme aims to improve the utilisation of radio channel by taking into account the on-off patterns of speech. The TDD frame suggested for the channels is divided into a control sub-frame and an information sub-frame. The Busy Indication control packets used to indicate the slot reservation is aimed at disseminating the channel status to new nodes joining the network and to ensure a collision-free data channel.

I. INTRODUCTION A Mobile Ad Hoc network [ 1,2] is a type of wireless network, in

which nodes can communicate with one another without relying on any preexisting infrastructure. Many of the multiple access schemes [3-71 proposed for such networks have been for data oriented transmissions, where a terminal has sole access to the radio channel for the duration of its transmissions. For speech communications, however, this is not required due to the inherent bursty nature of speech traffic [8]. In two-way conversations, it is often the case that only one party is in active state, i.e. in a talkspurt. The key factors to be considered in design of a multiple access scheme for a mobile ad hoc networks are - 1) fully distributed operation 2) synchronisation of nodes and 3) addressing of the hidden terminal and exposed terminal problems. In addition, a reservation-based mechanism would help to reduce collisions and ensure guaranteed access to the channel, which is critical in real-time applications. If the channel reservation status were made available to all nodes in the network, it would be highly beneficial in such ad hoc scenarios.

11. PROTOCOL DESCRIPTION

A.

The protocol is based on a TDD frame structure consisting of minislots for control purposes and payload slots for transmission of speech packets generated in a talkspurt as shown in Fig.1.

Frame Format and Call Setup Procedure

Control Sub-channel (12Tx + 1 2 R x

Minislots)

Information Sub-channel

(12 speech slots)

It 6mS-l- 12ms -1 Fig. 1 Protocol frame structure

A global clock provided by a Global Positioning System (GPS) is assumed for synchronisation of the nodes in the network. The initial call setup and reservation of a slot in the information subchannel for the source node’s frst talkspurt is obtained through an RTS (Request To Send) / CTS (Clear To Send) handshake. This dialogue .

0-7803-6534-8/00/$10.00 0 2000 IEEE

14- 3ms ----*I4-- 3 m s -4 Fig. 2 Control sub-channel

B.

On the successful completion of the above RTS / (XS exchange, the source node transmits a Tx-Busy Indication packet on the forward minislot (corresponding to the reserved information slot) in every frame until the end of transmission, i.e. end of the talkspurt. The other party in the conversation sends &-Busy Indication on the reverse minislot in a similar manner. At the end of the talkspurt the busy indications are stopped and the information slot is now available to all users for contention. Thus these Busy Indication packets provide the channel reservation status to all nodes and address the hidden terminal and exposed terminal problems. A node intending to setup a call with another node in the network scans one complete control sub-channel and selects randomly one of the ‘free’ forward minislots and contends for it by sending an RTS packet. These minislots are thus subjected to collision, while the reverse minislots and information sub-channel are collision-free.

C. Talkspurt-level Reservations

At the onset of subsequent talkspurts (after the first talkspurt from the node that setup the call), the affected node contends by a Talkspurt RTS (TS-RTS) packet in the forward minislot to the other party. The destination node responds with a Talkspurt CTS (TS-CTS), thus reserving the speech slot. The Busy Indication packets are sent until the end of the talkspurt as mentioned above. For the entire duration of the call, the nodes are set to be in a ‘Busy’ status, during which they cannot respond to other incoming call setup requests or setup other new calls.

D. Contention, Backoff and Call Completion

During call setup, when no ‘free’ forward minislot is available or when the expected CTS is absent, the source node goes into a backoff mode (its status is still ‘Busy’). After a uniform random backoff period that is selected between predefined lower and upper limits, the node tries to send an RTS. A maximum number of attempts is set, after which, the call is cleared from the system, i.e. the call is dropped. When no ‘free’ forward minislot is available for a TS-RTS

Usage of Channel Busy Indication Packets

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contention, the talking node scans the control channel on the next frame and repeats the procedure in the second frame from the current one. If these two attempts fail, the speech packet at the head of the transmit speech buffer is dropped. This gives a maximum speech packet delay limit DmU, of 36ms. As reservations are done only on talkspurt basis, no extra control exchange for call completion is required.

E. Definition of Terms Used

Ignored calls are the calls that arrive at a node when that node is already in a ‘Busy’ state. These calls are cleared and the rest of the calls that attempt setup are designated as Processed calls. Some of the processed calls are dropped after the unsuccessful call setup attempts. Call dropping probability is defined as the ratio of calls dropped to the calls processed in the system. The call setup delay is the time taken for a successful RTS-CTS exchange. The offered load to the system is taken as the load (from the call durations) in Erlangs from the processed calls. The carried load is based on the entire duration (this includes the silence gaps) of the successful calls.

HI. SIMULATION MODEL,

The network considered here is one where all nodes are assumed to be within radio range of each other. The Brady 6- state conversational speech model [9] with the modified values adopted in [lo] is used. The call arrivals at each node are independent Poisson processes with an exponential mean call interarrival time set to a range of values from 200 to 1000 s. The mean call duration is set to 60 s. The destination IDS of the nodes for call setup are selected randomly from the number of nodes in the system. The Block Oriented Network Simulator (BONeS) DESIGNER is used to model and carry out event driven Monte Carlo simulations of the network. Most analyses of other schemes assume the calls (system loading) to be placed to nodes outside the region in consideration. Our simulations take into account the activities of both parties in a call. The performance for networks with 20,40 and 100 nodes are evaluated. A warm-up time of 10% of the mean Poisson call inter-arrival duration at each node of the network is imposed before collecting statistics. The maximum number of attempts for call setup is set to 5 s with a maximum backoff period of 2.5 s.

Iv. RESULTS AND DISCUSSION

We observe that at lower loads the outcome of call setup attempt mainly depends on the availability of the destination node rather than channel availability. The call ignoring probability that gives an idea of how ‘busy’ the nodes are, is seen to increase with load as expected. The dropped calls are observed to be mainly due to the destination node being ‘busy’, except when the system is heavily loaded which leads to a high probability that all slots are occupied. At higher call arrival rates, more nodes and eventually all nodes would be in a ‘busy’ state trying to setup calls to one another. The scheme provides very low collision probabilities for light loading whereas high collision probabilities are observed in the 100- node network due to the repetitive contention by the nodes

for the 12 available slots. The packet dropping probability performance is seen to be good except in the 100-node network where a maximum value of 0.12 is observed at higher loads due to severe overloading. Thus statistical multiplexing gain is achieved at the expense of speech packet dropping when congestion occurs. The normalised throughput achieved is seen to reach a maximum value of 1.18 (since the carried load is based on the total call duration which includes the silence gaps). This gives an estimate of the calls that can be supported in the system, an indication of the multiplexing gain achieved by the scheme.

v. CONCLUSIONS

The efficient usage of the control channel for call setup and subsequent transmission of Busy Indication packets yields low probabilities of control packet collision, while also ensuring a collision-free data channel. The channel status information made available by the Busy Indication packets is highly useful in the ad hoc network considered. Talkspurt-based reservation leads to multiplexing gain thereby giving an edge against circuit switched schemes. Except in an overloaded system, the speech packet dropping Probability is maintained at a satisfactory level achieving good recovered speech quality.

REFERENCES

[l] Z. J. Haas, et al., Guest Editorial, Wireless Ad Hoc Networks, IEEE JSAC, Vo1.17, No.8. August 1999.

[2] Z. J. Haas and S. Tabrizi, “On Some Challenges and Design Choices in Ad-Hoc Communications”, Proc. Military Communications Conference, 1998.

[3] P802.11, IEEE Draft Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification, D2.0 (July 1995).

[4] P. Karn, “MACA - A new channel access method for packet radio”, Proc. ARRUCRFU Amateur Radio 9th Computer Networking Conf, 1990.

[5] V. Bhargavan, A. Demers, S. Shenker and L.Zhang, “MACAW: A Media Access Protocol for Wireless LAN’s’’, Proc. ACM SIGCOMM’ 94, August 1994, London, UK, pp. 212-25.

[6] C. L. Fullmer and J. J. Garcia-Luna-Aceves, “Floor Acquisition Multiple Access (FAMA) for Packet-Radio Networks”, Proc. of ACM SIGCOMM’95, 1995.

[7] J. Deng and Z. J. Haas, “Dual Busy Tone Multiple Access (DBTMA): A New Medium Access Control for Packet Radio Networks”, Proc. IEEE ICUPC ’94, ACM,

[8] D. J. Goodman, R.A. Valenzuela, K.T. Gayliard and B. Ramamurthi, “ Packet Reservation Multiple Access for Local Wireless Communications”, IEEE Trans. Commun., vol. 37.August 1989, pp.885-890.

[9] P. T. Brady, “A model for generating on-off speech patterns in two-way conversations”, Bell Syst. Tech. J., Vol. 48, No. 7, September 1969, pp. 2245-2472.

[ 101 W. C. Wong, “Packet Reservation Multiple Access in a Metropolitan Microcellular Radio Environment”, IEEE J. Select. Areas Commun., vol. 11, No.6. August 1993,

1994, pp.212-225.

pp. 918-925.

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