An Operation Of Prtmac Computer Science Essay

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Proactive RTMAC is a cross layer framework, with an on-demand QoS extension of DSR routing protocol at the network layer and RTMAC protocol at the MAC layer. PRTMAC is a tightly coupled solution, which requires the bandwidth reservation and bandwidth availability estimation services from the underlying MAC protocol. It is designed to provide enhanced real-time traffic support and service differentiation to highly mobile ad hoc wireless networks such as that formed by military combat vehicles. The performance of real-time sessions in ad hoc wireless networks is affected by mobility of nodes in many different ways.

The two major ways in which mobility affects real-time sessions are breakaways and reservation clashs. If a node participating in a QoS session moves out of the transmission range of either or both of its upstream and downstream nodes, we say the QoS session is broken due to breakaway. Assume that node A is transmitting to node B over a given slot (say slot #1). Similarly, at some other region in the network, node C is transmitting to node D over the same slot (slot #1). Now, if node C moves into the transmission range of node B (assume no breakaway due to mobility for the session between nodes C and D), packets transmitted by nodes A and C result in a collision at node B. This problem is referred as clash.

6.4.1. Operation of PRTMAC

The PRTMAC framework is shown in Fig. 29. RTMAC [13] is used as the MAC protocol. The out-of-band signaling channel gathers additional information about the ongoing real-time sessions, such that proactive measures can be taken to protect these sessions from breakaways and clashes. A narrow band control channel that operates over a transmission range with twice that of the data transmission range is used as the out-of-band signaling channel. Every node sends out control beacons (short fixed sized packets) at regular intervals over the control channel. The information carried by the beacons, and the beacon itself, are used by the nodes to gather information about real-time sessions. Firstly, the signal strength of the received beacon is used to gain an idea about the relative distance of the node which sent the beacon. Further, the information carried by the beacon is used in predicting breakaways and clashes. The beacons carry information about each of the sessions that the originating node is carrying, and the slots in the super-frame that have been reserved for them. Each node originates periodic beacons on the control channel. The beacon has information about all on-going real-time sessions at the node. The information includes the start and end times of the reservation slot of each session, the sender and the receiver of the session, and the service class (service classes are used to provide differentiated services among the real-time sessions existing in the system, for example, the command and control sessions in a military communication system may require higher priority than the other sessions) to which the session belongs. The range of the control channel has to be sufficiently larger than that of the data channel so that all possible events that can cause a session to be interrupted can be discovered well in advance.

Fig. 29. Modules in PRTMAC Framework.

Crossover-time prediction: Crossover-time is defined as the time at which a node crosses another nodes data transmission range r. This event is defined as crossover. As apparent from Fig. 30(a) and (b), there are two different crossover-times, namely crossover-time-in and crossover-time-out.

The crossover-time-in is the expected time at which node B in Fig. 30(a), reaches the crossover- point such that a bidirectional link forms between nodes A and B. Fig. 30(b) shows the crossover-time-out, which happens at the instant node B moves away from node A such that the link between nodes A and B breaks. Each node (say node A), upon reception of every new beacon from another node (say node B), predicts the crossover time based on the signal strength history obtained from past beacons i.e., if node B is inside the range of the data channel of node A, node A predicts the crossover-time-out, and if node B is outside the range of the data channel of node A, node A predicts the crossover-time-in. The prediction of crossover-time-out of node B with respect to node A is performed by keeping track of the signal strengths of the beacons previously sent by node B to node A. A node stores a fixed number of <time, signal strength> tuples of the beacons received from any other node. Using this, it generates a polynomial on the variation of signal strength with time. The roots of the polynomial refer to the time at which the signal strength can cross a receiving threshold. When node A predicts that node B is going to cross the data channel range within the next beacon interval, it takes proactive actions described in the next section. If node B is already within the data channel range of node A, then the prediction will be for a crossover-out event, and all sessions going on between nodes A and B will be interrupted. If node B is outside the range of node A, then it is a crossover-in event, and any packets belonging to existing real-time sessions at node A and node B will collide if their reservation times overlap. Note that if the predicted time of entry is beyond the next beacon interval, no action needs to be taken as of now, since the event would be predicted again, on receipt of the next beacon.

Fig. 30. Illustration of Crossover-in and Crossover-out.

Fig. 31. (a) No clash and (b) before clash.

Handling breakaways: The event of breakaways can be handled in two different ways, first is the local reconfiguration and second is the end-to-end reconfiguration.

In local reconfiguration, the upstream node (say node U) that has detected breakaway takes the responsibility and issues fresh route probe packets to obtain a path with reservation from that node to the destination. But, in the case of end-to-end reconfiguration, node U informs the source node about the breakaway, so that the source finds a new path to the destination. In PRTMAC a combination of the above two types is attempted which is described as below: Node U checks if its routing table has another path towards the destination node (say node F). If there exists such a node, then node U makes reservations on the link Uâ€"F for the on-going session. If the session is interrupted and reconfigured locally a number of times, then end-to-end reconfiguration is attempted.

Fig. 32 Clash handling.

Handling clashes: Fig. 31(a) illustrates how two nodes can reside safely within range of each other if the reserved slots do not overlap with each other. If the reservation slots clash for the two nodes, as indicated in Fig. 31(b), then PRTMAC handles it in such way that the flow between say node N and node C is assigned to a new slot (#5) as shown in Fig. 32.

In the absence of any measures taken to resolve a clash, both the sessions that experience a clash will be reconfigured from the source to the destination, resulting in degradation of performance. PRTMAC prevents such an occurrence to the extent possible, by pro-actively shifting one of the sessions to a new slot, so that the two sessions do not clash. This benefit of clash resolution is more important when a higher priority session clashes with a lower priority session. In such a case, the node having the low priority session has to reconfigure it to a new slot. As illustrated in Fig. 32, the node whose responsibility it is to reconfigure the session is denoted by node N, the other node, whose session clashes with node Ns session, is denoted by node O, and the counterpart of node N in its session by node C. Node N goes through its reservation tables and its neighbor reservation table corresponding to node C and tries to come up with a free reservation slot in both nodes N and C large enough to accommodate the session to be shifted. If it succeeds in finding such a free slot, the existing reservations for the session have to be dropped and new reservations have to be made for the session in the free slot. This is achieved by the originator of the session freeing the earlier reservation and issuing a request for the reservation of the slots belonging to the free slot.

If both the sessions that clash have high priority and node N cannot come up with a free slot enough to accommodate the session, it informs node O about its failure in shifting the session. Now node O executes the above process with its counterpart, and tries to shift the session.

If one of the sessions that clash is a high priority session and the other a low priority one, and the node that has a low priority session (here it is node N) is unable to find a new slot to shift the session, the low priority session undergoes end-to-end reconfiguration. This is to ensure that the low priority session would not hinder the high priority sessions.

7. Comparison

S.

N.

Framework Name

Type of Service support

Type of signaling

used

Routing Protocol used

MAC Protocol Used

End-to-end delay

Throughput

%

1.

INSIGNIA

Adaptive services, packet audio, video, and real-time data applications

In-band signaling

TORA

IEEE 802.11e MAC protocol

Long delays.

40-50

2.

INORA

Real-time audio, video and data

In-band signaling

TORA

IEEE 802.11e MAC protocol

Lesser average delay

50-60

3.

SWAN

Real time UDP traffic, and best effort UDP and TCP traffic

ECN-based

regulation

AODV

IEEE 802.11e MAC protocol

Average delay

60-70

4.

PRTMAC

Enhanced real-time traffic support and service differentiation to highly mobile ad hoc wireless networks. such as military combat vehicles

out-of-band

DSR

RTMAC

Minimum delay

70-80

8. Conclusion

In this paper available solutions are described in the literature for QoS provisioning in AWNs were discussed. First the issues and challenges involved in providing QoS in AWNs were identified. Then the existing QoS approaches were classified according to several criteria such as interaction between routing protocol and resource reservation signaling, interaction between network and MAC layer, and routing information update mechanism. A layer-wise classification of the existing QoS solutions was also provided. Then Component of QoS Frameworks is discussed in layer wise manner. Where QoS signaling is classified in three categories. After that some existing QoS framework are discussed in a detail (e.g. example INSIGNIA, INORA, SWAN, and PRTMAC). At last comparison of QoS framework is given in tabular format.

INSIGNIA framework provides an integrated approach to QoS provisioning by combining in band signaling, call admission control, and packet scheduling together. The soft state reservation scheme used in this framework ensures that resources are quickly released at the time of path reconfiguration. But, this framework supports only adaptive applications, for example, multimedia applications. This framework is transparent to any MAC protocol. Also as this framework assumes that routing protocol provides new routes in the case of topology changes. If enough resources are not available because of the changing network topology, the enhanced QoS application may be downgraded to base QoS or even to best-effort service. As this framework uses in-band signaling, resources are not reserved before the actual data transmission begins. Hence INSIGNIA is not suitable for real-time applications that have stringent QoS requirements.

INORA is better than INSIGNIA in that it can search multiple paths with lesser QoS guarantees. It uses the INSIGNIA in-band signaling mechanism. Since no resources are reserved before the actual data transmission begins and since data packets have to be transmitted as best-effort packets in case of admission control failure at the intermediate nodes, this model may not be suitable for applications that require hard service guarantees.

SWAN gives a framework for supporting real-time applications by assuming a best-effort MAC protocol and not making any resource reservation. It uses feedback based control mechanisms to regulate real-time traffic at the time of congestion in the network. As best-effort traffic serves as a buffer zone for real-time traffic, this model does not work well in scenarios where most of the traffic is real-time in nature. Even though this model is scalable (because the intermediate nodes do not maintain any per flow or aggregate state information), it cannot provide hard QoS guarantees due to lack of resource reservation at the intermediate nodes. An admitted real-time flow may encounter periodic violations in its bandwidth requirements

9. Problem Definition

In the literature we have study that in QoS provisioning techniques used in existing QoS framework. Security was not provided for any QoS provisioning techniques like resource reservation, flow restoration, and adaptation. While these techniques tend to be vulnerable to a number of threats and attacks likes, Denial of service (DoS), Impersonation, Disclosure, Attacks on information in transit, Attacks against routing, and node hijacking. QoS frameworks are typically subjected to first level of attack; the adversary focuses on disrupting the basic mechanisms of the QoS provisioning, such as resource reservation, flow restoration, and adaptation, which are essential for proper QoS operation.

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