WO2004075481A1

WO2004075481A1 - Communication protocol for ad-hoc networks

Info

Publication number: WO2004075481A1
Application number: PCT/IT2004/000059
Authority: WO
Inventors: Borgonovo Flaminio; Antonio Capone; Matteo Cesane; Luigi Fratta
Original assignee: Politecnico Di Milano
Priority date: 2003-02-18
Filing date: 2004-02-16
Publication date: 2004-09-02
Also published as: ITMI20030286A1

Abstract

The present invention relates to a communication protocol for use in ad-hoc networks. In one of its embodiments, the communication protocol for ad-hoc networks includes a plurality of terminals suitable to transmit data packets with each other, having a frame subdivided into a plurality of time slots (SN). Each data packet includes the frame information (FI) relevant to a predetermined number of previous time slots (SN) starting from the one in which the packet transmission occurs; said frame information (FI) includes for each time slot (SN) and identifier (ID) of a terminal if it had performed a transmission in said slot (SN); a time slot (SN) is labeled as RESERVED for a first terminal if a second terminal finds said slot BUSY, otherwise it is labeled as AVAILABLE; a transmission between a third and a fourth terminal is successful if the slot (SN) in which said third terminal transmitted is found BUSY for said third terminal in all frame information (FI).

Description

Communication protocol for ad-hoc networks

Field of the Invention The present invention relates to a communication protocol for use in ad-hoc networks.

The design of ad hoc networks has recently attracted a lot of attention, mainly because many characteristics of such networks, especially in a highly mobile environment make the design of a prompt, efficient, flexible, and reliable MAC very difficult.

An ad hoc network is composed of mobile terminals having no access point (AP) that communicate one to the other through broadcast or circular radio transmissions, i.e., transmissions that reach all the terminals within the permitted transmission power range. However, due to radio range limitations, physical broadcasting does not cover all terminals and a multi-hop scenario, where packets are relayed by intermediate terminals to reach their destination, must be considered. Background Art

Applications of mobile ad-hoc networks can range from military field communications, where networks must be deployed immediately without the support of base stations and infrastructures, to inter-vehicle communications, designed for both traffic safety enhancement and entertainment purposes. The inter-vehicle communication application poses the more stringent requirements, due to highly variable topology and the need to provide a continuous exchange of broadcast information to support traffic control applications.

Because of the highly variable environment, all protocols and coordinating functions in mobile ad-hoc networks, must be completely distributed. This constrains the implementation of several layers. At physical layer, no central station providing a central clock, exists. Nor is there a central repeater enabling to reach all the terminals to perform functions such as central transmission synchronization or collision detection. Moreover, due to the distributed nature of the environment, transmission collisions occur among some receivers only, that is, we are in front of the hidden terminal problem. The constraints at MAC access control level are even higher. In fact, access to the channel must be completely distributed, but operate on a physical channel not showing broadcasting characteristics, and all the terminals can potentially collide with the transmitting terminal.

The use of the Carrier Sense Collision Avoidance in the Distributed Coordination Function of IEEE 802.11 standard does not solve the problem. Furthermore, no central algorithm (for instance the one called PCF of IEEE 802.11 standard) can be used. Another unsolved problem of ad-hoc networks is how to provide reliable transmissions. Many of the broadcast protocols do not work well in ad-hoc networks due to the dynamic nature of the network topology. Therefore, the preferred means proposed for broadcasting is the flooding approach and its variants. In flooding, each terminal receiving a broadcast packet retransmits it just once until all the terminals are reached. However this procedure is highly inefficient in networks with a high connectivity degree. In fact, in a network with n terminals, the flooding procedure requires n transmissions of the same information, while in fully connected networks one single transmission is sufficient to reach all the terminals. The problem is further aggravated for random access protocols, where neighbor nodes re-transmit a broadcast packet almost at the same time causing massive collisions.

The drawbacks mentioned above are particularly important in vehicular control applications continuously broadcasting background information, like cruise parameters. Also, this information is mainly directed to neighboring vehicles, and a flooding procedure would saturate the whole network with useless information for many terminals.

In ad-hoc networks, parallel transmissions can also be present, as it happens in disjoined networks. However, in networks making use of the carrier (CSMA) sensing or the RQS/CLS mechanism of IEEE 802.11 , some parallel transmissions can be made impossible due to the simultaneous exposure of two or more terminals even if they do not interfere among them.

The ALOHA protocol is a well known multiple access protocol (MAC) to the common means of a network. A multiple access communication is a communication among different terminals through a shared communication means. Scope of the medium access control level (MAC) is to allocate the use of the shared communication means among the terminals. Scope of the protocol is to coordinate and make the communication channel use effective.

The ALOHA protocol foresees that terminals can access the channel anytime they have information to transmit. Due to the transmission collision possibility among different terminals, each terminal shall monitor its transmission and wait for a receipt confirmation from the receiving terminal.

Making a small restriction of the transmission freedom of the ALOHA protocol, the information transmission speed can be doubled. Assuming a constant length packet, the transmission time is divided into time slots equivalent to the transmission time of one packet. This protocol is called Slotted ALOHA. A method to improve the effectiveness of the Slotted ALOHA protocol is the well- known Reservation ALOHA. In Slotted ALOHA protocol each slot is available for use by any terminal, irrespective of the previous use. In Reservation ALOHA protocol the time slot is considered the (temporary) property of the terminal that successfully used it.

The Reservation ALOHA protocol requires a central repeater, so that all the terminals are able to receive all the transmitted signals and obtain the same time slot status information, e.g. busy, free or collided.

In the application we are considering, no central repeater is present. In these conditions there is no guarantee that a terminal will hear all the transmissions due to the hidden terminal problem, and therefore destructive interference when trying to access a slot. Furthermore, the transmitting terminals do not know the outcome of their transmissions.

Object of the Invention

Considering the state of technique described, scope of the present invention is to offer a means access control (MAC) in ad-hoc networks, in particular for inter-vehicle communications, not showing the drawbacks of the background art. Summary of the Invention

According to the present invention, said scope is reached through a communication protocol, for ad-hoc network including a plurality of terminals suitable to transmit information packets among them, having a frame subdivided into a plurality of time slots, characterized in that each data packet includes the frame information relevant to a pre- determined number of previous time slots starting from the one where the packet transmission occurs; said frame information includes, for each time slot, a terminal identifier index, if this performed a transmission in said time slot; a time slot is labeled as RESERVED for a first terminal if a second terminal finds said time slot BUSY, otherwise it is labeled AVAILABLE; a transmission between a third and a fourth terminal is successful if the time slot in which said third terminal performed its transmission is found BUSY for said third terminal in all frame information.

Preferably, a terminal will enable or disable an indicator in the frame information if the packet received is a broadcast packet or is destined to the terminal itself; let S be the identifier of a terminal and D the identifier of the destination terminal, then a slot labeled as RESERVED can be BUSY if: said indicator in all frame information received is disabled and the frame information received from D signals that the slot is AVAILABLE; a transmission is successful if the time slot is coded as BUSY in the frame information of the destination terminal, otherwise the transmission has failed.

More preferably, denoted Q the set of the neighbors of i, and Si c the subset of neighbors that have not received the packet in slot k, a terminal i decides to retransmit the packet if |Si| > 0 and for all j, the following condition is not satisfied Si c Cj AND (IQJI > |Ci| OR (|Cj| = |Ci| AND IDj > ID!))

Profitably, this protocol does not make use of a central repeater.

Moreover, the frame information includes for each slot a flag if the slot is BUSY, an identifier of the station transmitting in said time slot, a priority indicator of the terminal transmitting in said slot, and an indicator if the packet received is a broadcast packet or is destined to the terminal itself.

Profitably, each terminal can occupy more than one time slot.

Thanks to the present invention we can implement a medium access control (MAC) in ad-hoc networks based on a R-ALOHA reliable protocol (RR-ALOHA), which is a completely shared access technique, capable of dynamically establishing a reliable single- hop broadcast channel on a frame structure for each terminal active on the network. The channel is used to provide the following services:

- quick and reliable information connectivity at level two on all the network terminals;

- non-contended access to a reliable single-hop broadcast service; - quick means to reserve additional bandwidth and service quality as requested by the application, in a completely distributed way;

- an efficient point-to point communication exploiting the parallel transmissions;

- an efficient multi-hop broadcast service. Brief Description of the Drawings The present invention, together with further characteristics and advantages thereof may be better understood with reference to the following detailed description of one of its practical embodiments, taken in conjunction with the accompanying drawings, in which:

Figure 1 shows the diagram of a network in which terminals can communicate one to another with maximum two transmissions and shows also the frame information Fl of the terminals;

Figure 2 shows the diagram of a network in which terminals can communicate one to another with more transmissions;

Figure 3 shows the diagram of a network in which terminals can communicate one to another with more transmissions and parallel transmissions are allowed. Detailed Description of a Preferred Embodiment of the Invention

Figure 1 shows nos. 7 terminals marked 1-7, subdivided into three clusters or sets A, B and C. Each set represents the area in which the terminals therein contained, can communicate one to another through one sole transmission (one-hop). The terminals of different sets not belonging to common subsets cannot communicate with each other. The terminals belonging to the subset common to all the three sets A, B and C can communicate with all the adjacent sets and are defined as 1 -terminals. The union of sets with a common subset is defined as the set in which the terminals can reach each other with maximum two transmissions.

A packet, preferably having the length of a time slot, includes^' information to be transmitted (payload) and frame information Fl. Each active terminal transmits every N slots the frame information Fl based on the. information received during the N previous slots. The frame information includes the status, as heard by the terminal, of each of the N previous slots (with the number of frame slot SN, k-N to k), defined sliding virtual frame (SVF). The slot status information shall be BUSY if the slot contained a transmission of a terminal, in this case also the identifier (ID) of the transmitting terminal shall be included. Otherwise the slot status information shall be AVAILABLE.

Figure 1 shows also the frame information FI-1 - FI-7 of the terminals. The frame information has been aligned, so that each column refers to the same time slot. For instance, the frame slot FI-1 of terminal 1 , shows in a 11 slots (N = 11) frame, only the transmissions of terminals 2, 4 and 5 in addition to its transmission, i.e., it represents the slots whose status is BUSY with the relevant identifier ID of the terminal that performed the transmission.

Therefore, we can say that the use of a time slot depends on the label attached to each time slot. More in particular, for each time slot k, the frame information Fl received in a virtual sliding frame (SVF) is used to determine the classification of each slot k-N in accordance with the following rule:

RULE 1 : a slot is labeied as RESERVED if coded as BUSY in at least one time slot received, otherwise it is labeled as AVAILABLE.

In this way, a slot used as channel by a terminal in a set is seen as RESERVED by all the terminals of the set. In fact, it is seen as BUSY by terminals belonging to the subset common to all the three A, B and C sets (1 -terminals) that can communicate the BUSY information to all the terminals of the adjacent sets.

The classification of the k-N slot is extended to the k slot, which, if AVAILABLE, can be used to relay a packet from any terminal that willing to set up its channel. The hidden terminal problem is thus solved and the R-ALOHA protocol can be applied. A terminal j transmitting in an AVAILABLE time slot determines, after a given time, the outcome of its transmission according to the following rule:

RULE 2: the transmission is successful if the slot is coded as BUSY for terminal j in all the frame information (Fl), otherwise the transmission has failed.

According to this rule, if only one terminal is attempting a transmission, all the terminals in the same cluster will recognize the transmission and therefore, all frame information Fl received by the terminal will mark the slot as BUSY and therefore the transmission is successful. All the other terminals in the network will recognize the slot as BUSY in the frame information of 1 -terminals, and declare the time slot as RESERVED. The RESERVED slot shall then be dedicated to that terminal until it is released.

If two or more terminals are attempting a transmission in the same time slot, the terminal that is not able to decode the transmission due to a collision, will mark the slot as AVAILABLE. It can happen that some terminals correctly decode the transmission and code the slot as BUSY. In this case, the slot is assigned to terminal j if all the frame information is in agreement, while the contending terminal will find out its failure. Otherwise, the contending terminals failed their transmission. At network starting, all the slots are AVAILABLE, and terminals start transmitting in accordance with the described protocol until all of them have accessed their channel. The channel is automatically released when the terminal goes out or out of the transmission range of all the other active terminals in the frame.

Let's now consider the more general case in which the network includes terminals communicating with each other through multiple transmissions (multi-hop) such as for instance the one shown in figure 2, where letters A, B and C represent disjoint clusters.

All the terminals in A, B (and AB) relay in their common frame slot denoted FR 1 , giving origin to the A+B set in which each terminal knows the frame status as seen by all the terminals in the set in which the terminals can communicate with each other through two-hop transmissions (two-hops).

The terminals of the BC set see both B and C transmission but not the A ones. The terminals in C do not receive the frame information of the AB set and therefore are free to use the slots already used by the terminals in A, producing the frame called FR 2. Similar is the situation for the FR 3 frame. For convenience, we have assumed that the frame information Fl is available at the end of the transmission slot. If this is not the case, a new access must be delayed up to processing of the frame information Fl.

If the field relevant to information to be transmitted (payload), in the transmission packet, is long enough, it can be used to transmit also applications or other service information, e.g. routing information. On the contrary, if the band granted to a terminal

(one channel) is not enough, two methods can be preferably used to increase it.

An additional channel can be reserved to the first already acquired using the procedure previously described. If more terminals try to reserve the same slot, the slot signaled in the frame information Fl will be assigned on the basis of the terminal priority. Otherwise, in accordance with rule 2 a collision shall be detected and the procedure newly attempted. If no free lot is available, the terminals having higher priority can acquire preemption right on low priority terminals causing a collision and signaling the new request.

Additional time slots can be used to define point-to-point channels reusing slots in disjoint clusters of the terminals of sets that can communicate with each other through a single hop transmission (one-hop), i.e., use parallel transmissions. To allow the maximum reuse, a flag, the PTP flag included in the frame information Fl of each slot is used:

RULE 3: a terminal will enable or disable the PTP flag in the frame information Fl if the received packet is a broadcast packet or is destined to the terminal itself.

To determine if the packet is destined to the terminal itself, the destination address is read in the packet header.

Free slots are reserved as previously said. However, also the RESERVED slots can be reserved as follows:

RULE 4: Let S be the identifier ID of a terminal and D the identifier ID of the destination terminal, then a RESERVED slot can be reserved if: 1) the PTP service flag in all the frame information Fl received is disabled and

2) the frame information Fl received by D signal that the slot is AVAILABLE.

The conditions mentioned above, are such that no collision can ever occur at both the receivers. This can be noticed making reference to figure 3, which shows the diagram of a network in which the terminals can reach any other terminal with more transmissions, and parallel transmissions are allowed.

Let's assume that transmission in A is the first transmission.

In case a, all frame information Fl received by the terminals in AB mark the slot as BUSY with the PTP service flag OFF.

In the case the frame information Fl received by the destination terminal in AB labels the slot as BUSY with the PTP service flag ON. Furthermore, the condition 2) is Rule 4 is always satisfied. Therefore, in case a, the transmission in B can be made, while in case b it can not.

In cases c and d, the frame information Fl received by the terminals in AB mark the slot as BUSY with the PTP service flag OFF. In case c the destination D sees the transmitter in AB, while in case d it doesn't. Therefore, the transmission in B can be made only in case d, which is a case of two exposed but not interfering terminals.

However, if several access attempts occur concurrently, collisions can still occur. The attempt outcome is determined according to the following rule:

RULE 5: the transmission is successful if the slot is coded as BUSY in the frame information of the destination terminal, otherwise transmission has failed.

In networks in which the terminals communicate one to another through multiple transmissions (multi-hop), they must retransmit the transmissions so that all the terminals of the network are reached. With flooding, all terminals relay broadcast packets at least once; the procedure is reliable, but has the drawback of an inefficient use of the available band. Assume that the terminal i receives a broadcast packet in the slot k. Let Q be the set of the neighbors of i (one-hop set) and Sj c Q the subset of the neighbors that have not received the packet in slot k. These subsets can be easily identified by i through the frame information Fl received in the sliding virtual frame (SVF) following the slot k. Also C_j, j e Q sets, are identified by the same information. RULE 6: The terminal i decides to retransmit the packet if |S|| > 0 and for all the j the following condition is not satisfied

Si c AND (|Cj| > |Cι| OR (|Cj| = |Cι| AND ID, > ID|))

It can be demonstrated that Rule 6 leads to a sole set of retransmission terminals making reference to figure 1 , where one sole terminal will result selected as re-transmitter. The condition Sj c C_j is always satisfied if j is a 1 -terminal (in ABC). In this case, if i is not a 1 -terminal, the condition |C_j| > |Cj| is always satisfied and the terminals outside the common ABC set cannot retransmit. If it is a 1 -terminal, the whole condition is satisfied by all of the terminals except for the terminal having the lowest identifier ID, which retransmits the broadcast packet if the packet itself has not been generated by the 1- terminal.

In the more general scenario of figure 2, assuming that the broadcast packet is transmitted in B, the Sj e C_j condition is always satisfied if j is a 1 -terminal in AB and i is a terminal in A+B. Proceeding as above, we can see that the transmitting terminal is a 1- terminal in set AB. Likewise, the Sj c C_j condition is always satisfied if j is a 1 -terminal in BC and i is a terminal in B+C, and the transmitting terminal is a 1 -terminal in the BC set. The selected terminals will retransmit the broadcast packet in clusters A and C, respectively. By repeating the just described procedure, the broadcast packet will reach all the terminals in the network with a reduced number of retransmissions versus the flooding approach. It must be noticed that with this procedure, making particular reference to rules 1 , 2 and 6, used with the knowledge of topology, obtained for instance through the routing mechanism of layer 3, an ad-hoc multicast service can be offered.

The proposed protocol uses a slotted channel, and can therefore use frame physical layers such as the UMTS Terrestrial Radio Access TDD. In this case the slotting information can be provided by the GPS system. However, this protocol can be used also with asynchronous physical layers such as the IEEE 802.11.

In the protocol described above, the number of slots N must be large enough to accommodate the maximum number of terminals M in any two-hop cluster. In addition, if any terminal is to set up additional channels, N must be much larger than M. 5 Note that M is fixed, but it does not limit the number of terminals that the network can support, since, within the network, slots and frames are reused, much as happens for cellular systems. Also in this case, M can be reduced to reduce the terminal transmission power.

For example we can assume M = 100 and N = 200 (and the SVF length is equal to 0 N=200). The frame information Fl of the terminals can be configured with four fields as follows:

1 ) the busy status (1 bit),

2) the identifier ID (8 bit),'

3) a priority field (2 bit), 5 4) the PTP service flag (1 bit).

The ID identifier is selected at random and changed if already in use. In the case considered, the number of bits used by the frame information Fl of the terminals is 2400 bits. Further fields can be reserved to transmit information relevant to layer 2 such as MAC addresses, frame check sequences, and physical guard times. In o this way the total overhead can be as high as 2500 bits.

With a 5000 bit long packet and with the 2500 bit frame information Fl, the transmitted information is 2500 bits.

Therefore the maximum efficiency is 75%. The overall frame duration, assuming a 10 Mb/s channel speed, will be 100 ms, yielding a bandwidth of 25 kb/s available for each 5 channel and a 5 Mb/s bandwidth available for reservation.

Further frame information Fl reduction can be obtained inserting the ID and the priority field once every k frames, including the slots that are accessed for the first time. This information is needed by the MAC in the access phase only, but must be repeated to let new active terminals learn the association of busy slots with terminals. For example, if 0 we include the ID and the priority field once every 10 frames, the frame information Fl can be reduced to 400 bits 90% of time, yielding a maximum efficiency of 93%.

If a lower channel speed is used, as for instance 3,84 Mb/s, the packet length must be further reduced to maintain the 100 ms frame duration and the related time responsiveness. 5 An important protocol performance figure is the time needed for a new terminal to acquire the BCH. According to the present procedure, a new terminal will attempt a transmission with probability p in the next available slot. The probability that one among k contending terminals gains access, i.e., its transmission does non collide, is given by:

S = kp (1-p)^k'1

That is maximized for kp = 1 , where it yields S = e^"1 = 0,376 for large values of k. Should the M-k terminals have already acquired the channel, the probability of the remaining k terminals is equal to p = 1/k. However, the outcome of an access attempt is known only after one entire frame, and, while awaiting the outcome, no new slots can be accessed. This makes the average number of attempts per lots less than 1, a condition that complicates the performance analysis. Therefore, some preliminary figures have been obtained by simulation, considering the case in which the number of the terminals and the number of the slots in a frame are 50/100, 100/100 and 100/200, respectively. In 50/100 and 100/200 cases, all the terminals achieve their slot in 6 frames, while in the 100/100 case, the period is almost doubled, due to the higher number of collisions.

Claims

1. Communication protocol, for ad-hoc networks, including a plurality suitable to transmit information packets among them, having a frame subdivided into a plurality of time slots (SN), characterized in that each data packet includes the frame information (Fl) relevant to a pre-determined number of previous time slots (SN) starting from the one where the packet transmission occurs; said frame information includes (Fl), for each time slot (SN) an index (ID) identifying a terminal if this performed a transmission in said time slot (SN); a time slot (SN) is labeled as RESERVED for a first terminal if a second terminal finds said time slot BUSY, otherwise it is labeled AVAILABLE; a transmission between a third and a fourth terminal is successful if the time slot (SN) in which said third terminal performed its transmission is found BUSY for said third terminal in all the frame information (Fl).

2. Communication protocol according to claim 1 characterized in that a terminal will enable or disable a service flag (PTP) in the frame information (Fl) if the packet received is a broadcast packet or if it is destined to the terminal itself; let S the identifier (ID) of a terminal and D the identifier (ID) of the destination terminal, then a time slot (SN) labeled as RESERVED can be busy if: said service flag (PTP) in all the frame information (Fl) received is disabled and the frame information (Fl) received by D signal that the slot is AVAILABLE; a transmission is successful if the time slot (SN) is labeled as BUSY in the frame information (Fl) of the destination terminal, otherwise the transmission has failed.

3. Communication protocol according to claim 1 characterized in that denoted Cj the set of the neighbors of i, and SJ C Cι the subset of the neighbors that have not received the packet in the k slot, a terminal i decides to retransmit the packet if |Si| > 0 and for all the j the following condition is not satisfied Si c Cj AND (|Cj| > |Ci| OR (|Cj| = |Ci| AND IDj > IDi))

4. Communication protocol according to claim 1 characterized in that said protocol does not use a central repeater.

5. Communication protocol according to claim 1 characterized in that said frame information (Fl) includes for each slot (SN) a flag status if the slot is BUSY.

6. Communication protocol according to claim 1 characterized in that said frame information (Fl) includes for each time slot (SN) an identifier (ID) of the station transmitting in said time slot (SN).

7. Communication protocol according to claim 1 characterized in that said frame information (Fl) includes for each slot (SN) a priority indicator of the terminal transmitting in said time slot (SN).

8. Communication protocol according to claim 1 characterized in that said frame information (Fl) includes for each slot (SN) a service flag (PTP) if the received packet is a broadcast packet or is destined to the terminal itself.

9. Communication protocol according to claim 1 characterized in that each terminal can occupy more than one time slot (SN).