Hidden Terminal Problem And Exposed Terminal Problem

Published Date: 02 Nov 2017

A mobile ad hoc network is a wireless network without centralized control where every node acts as a router, forwarding packets to the destination when necessary [40]. MANets have several advantages over conventional wired networks. First of all, MANets are very convenient. The operator doesn’t have worries such as running wires in tight places or obtaining low-voltage permits. Secondly, the deployment range of MANets is impressive compared to wired networks whose length of wires is limited. However, some valuable characteristics of wired networks (e.g., reliability, cost, speed) are traded off in achieving this.

3.1 Properties of MANETs

MANets differ from their traditional wired counterpart in several aspects and they are itemized as follows.

3.1.1 Dynamic Topology:

Nodes in MANets may move arbitrarily which leads to a changing topology. This is quite different from traditional wired networks. Typically in high mobility applications like vehicular communication, topology changes rapidly. On one hand, dynamic topology may increases the cost of maintaining routes due to link breakages. On the other hand, node mobility may reduce the effects of network partitioning as shown in Figure 3.1

(a) Network partition

(b) Route establishment

Figure 3.1 Node mobility aided route establishment [40]

Due to limited transmission range of radio and arbitrary movement of nodes, the wireless link becomes unpredictable and unstable, leading to difficulties in maintaining the route. Therefore, the MANet routing protocol should be capable of dealing route break besides route discovery.

3.1.2 Unpredictable Link Quality:

Wireless media is time and location dependent. Signals experience fading, interference and multipath cancellation during transmission [41],[42]. In addition, wireless links have lower capacity than their wired counterpart, increasing the possibility of network congestion. Since MANets are regarded as an extension of the existing wired network in many applications, the bandwidth problem should be considered. Unpredictable link quality, together with limited bandwidth makes providing bandwidth and delay guarantees a really challenging task.

3.1.3 Limited Energy Resource:

Many mobile devices in MANets depend on batteries or other finite energy sources for their energy supply. Sometimes, frequent recharging or battery replacement may be undesirable or even impossible [43]. Therefore, many energy efficient protocols have been proposed in different layers, such as [44] and [45] in physical layer, [46] and [47] in link layer, [48] and [49] in network layer and [50] in transmission layer.

3.1.4 Hidden Terminal Problem And Exposed Terminal Problem:

The hidden terminal problem and exposed terminal problem are experienced frequently in MANets. As shown in Figure 3.2(a), the hidden terminal problem happens when two nodes A and C stay out of each other’s transmission range and send packets simultaneously to a same destination B. Those packets collide and thereby are dropped by node B. Figure 3.2(b) describes the exposed terminal problem. As seen, when node B is transmitting packets to A, node C has to defer its transmission for node D even if such transmission will not disturb the reception process in node A. RTS/CTS acknowledgement and handshake in 802.11 partly solve the hidden terminal problem at the cost of throughput reduction.

(a) Hidden terminal problem

(b) Exposed node problem

Figure 3.2 Hidden and exposed terminals

3.2 Various Types of Routing Models

Routing models can also be divided in following ways:

3.2.1 Proactive Routing:

In this mechanism of network which continuously monitors network topology and maintains current routing tables even in presence of instantaneous demand. DV and LS schemes comes under this category. The routing information is always available for a sender. However, the network is has continuous and huge overhead of routing management traffic, unfortunately much of this traffic is not useful and serious wastage of precious bandwidth..

3.2.2 Reactive Routing:

In this network mechanism gather s routing information only on demand. Moreover routing management traffic is very less as the routes are discovered only when needed. Currently, these schemes are very popular as they substantially reduce overheads of route management traffic.

3.2.3 Hybrid Routing:

This network routing scheme is a combination of proactive as well as reactive protocol philosophy and present best of both. A proactive table driven strategy is employed for establishment and maintenance of routes between nodes of the same area, where communication among nodes of different zones is achieved by using reactive on-demand strategy. It is also found that the approach can be highly applicable in larger networks and applications on such networks that are characterized by a comparatively high degree of locality of communication. In other words for te networks where communication between nodes with close vicinity to each other is extensively high as compared to member nodes which are sparsely distributed over the network zone.

3.3 Routing protocols for MANETs

Routing is an important issue in networks. In wired networks, dynamic routing approaches are prevalent among which distance vector routing as well as link state routing are among most popular models [51]. Distance vector routing is based on the Bellman-Ford algorithm in which each node maintains a routing t information is advertised periodically.

The source adopts the shortest route when it has packets for a destination. In link state routing, every node propagates its current status of links to all reachable nodes. Whenever a link status in one node changes, a corresponding advertisement will be broadcasted based on the routing table which is refreshed. In wired networks, both distance vector and link state routing behave well due to comparatively stable link quality and topology. However, properties such as link quality and topology in MANets become unpredictable, degrading the performance of some distance vector and link state routes. As a consequence, protocols have been proposed and well-studied, five of which are described below.

3.3.1 Destination Sequenced Distance Vector protocol (DSDV):

DSDV [52] is a typical proactive routing protocol in which each node has to maintain a routing table for all available destinations. Routing updates are broadcast periodically. DSDV relies on a sequence number to indicate the freshness of the corresponding item to guarantee loop-freedom. When a route breakage between two nodes, say A and B, is detected by node A, it increases the corresponding sequence number and sets the distance to node B as infinite and this information will be further broadcasted. In DSDV, the routing information broadcasts introduce a large number of control packets which increases the overhead. At the same time, it takes some time before a route can be used, the so called the convergence time. In wired networks where the topology is comparatively stable, this convergence time is minor and it can be neglected. However, in a network where topology changes rapidly, the convergence time is sufficiently long that there will likely be a lot of dropped packets.

3.3.2 Dynamic Source Routing (DSR):

DSR is a reactive protocol which establishes routes on demand [53]. It initializes a route request process when a route to the destination is not known in the route cache. Up on receiving a route request packet (RREQ) packet, intermediate nodes either generate a route reply packet (RREP) while it caches the corresponding route or it adds its own address to the RREQ and forwards the RREQ until it reaches the destination or the packet live time expires. Where bidirectional links exist, the reverse path will be used when the destination or intermediate node doesn’t have a route to the source in the cache. In the case of a route breakage, an error packet is generated by the node which detects it and the corresponding item in the route cache is erased. Compared to DSDV, DSR doesn’t use periodic broadcasts and thereby reduces routing overhead, saves energy and partly eases network congestion. However, each data packet carries routing information in DSR, increasing the overhead.

3.3.3 Ad hoc On-demand Distance Vector (AODV):

AODV [54] is a reactive protocol, based on the distance vector algorithm. The source in AODV originates a RREQ packet when a route to the destination is not available in the cache. The RREP packet is forwarded until it arrives at the destination or an intermediate node which has a fresh enough route. When a stale route is detected, the corresponding routing item is removed and a link failure message is sent out, triggering the route discovery process. HELLO messages are generated periodically to indicate the presence of a node to its neighbors.

Compared to conventional distance vector protocols, the number of advertisement packets in AODV is largely reduced. Two main disadvantages of AODV are HELLO induced routing overhead increase and an assumption of bidirectional links.

3.3.4 Temporally-Ordered Routing Algorithm (TORA):

TORA [55] is a reactive MANET protocol, aimed at minimizing routing overhead by controlling the receiving scope of routing messages when the topology changes. In TORA, each node is assigned a height. All messages flow downstream like water, from a node with a higher height to another one with a lower height. When a node happens to have packets for a destination but it has no downstream links, it broadcasts a Query (QRY) packet which will then be forwarded until it reaches a node that either knows a valid route or is the destination. Such a node will broadcast an update (UPD) packet containing its own height. Other nodes receiving this UPD packet will set their own heights with higher values compared with that in the UPD packet and broadcast this new height. In this manner, the route is established.

In TORA, only one route will be discovered even if multiple routes are available because each node only has one height value that is initially based on the distance from the destination [56].

3.3.5 Optimized Link State Routing (OLSR):

OLSR [57] is protocol with proactive routing that utilizes Hello messages and Topology Control (TC) messages to explore & exchange information related to link state. On the basis of this information individual nodes are informed about the next hop node for destinations. Being a proactive routing algorithm, the route establishment time for OLSR is short since routes are known before use. Two disadvantages of OLSR are a potentially long convergence time, periodic information broadcast induced extra energy consumption and additional routing overhead.

Table 3.1 summarizes the five well-studied protocols described above. As seen, all of them are loop free, avoiding the waste of limited resources in MANets. DSDV and OLSR are two proactive protocols and more energy and bandwidth are consumed for routing information advertisements. DSDV and OLSR are more suitable for slowly changing networks in which it takes less

Table 3.1 Comparison of protocols for MANets

Property

Protocol

DSDV

DSR

AODV

TORA

OSLR

Loop-free

Yes

Yes

Yes

Yes

Yes

Reactive/Proactive

Proactive

Reactive

Reactive

Reactive

Proactive

Unidirectional link support

No

Yes

No

No

No

Power conservation

No

No

No

No

No

Adaptive

No

No

No

No

No

QoS support

No

No

No

No

No

time to converge. DSR is the only protocol that supports unidirectional links. Although energy is of great importance for many mobile devices, it is not considered in all protocols. None of the protocols above are adaptive, indicating that they do not contain any smart routing schemes. Meanwhile, it is observed that QoS issues are not considered in any of those protocols. With the development of MANets, several adaptive protocols have been proposed [58][59][60].

3.4 QoS in MANETs

As stated in last section, many routing protocols such as DSDV, DSR and AODV have paid little attention to QoS support in the early development of MANETs. However, QoS provision is becoming more important nowadays due to the rising popularity of real-time applications.

3.4.1 Rising necessity for QoS provision:

In the past decades mobile traffic, which by definition refers to data generated by handsets, laptops and mobile broadband gateways, has been growing rapidly annually.

According to a survey by Cisco, mobile data in 2010 was triple the volume of the entire global Internet traffic in 2000. The growth rate in the previous year was 159%, which is 10% higher than anticipated in 2009. This rapid growth in mobile data is forecast to continue for the next five years with an average annual growth of 92% [61]. There are several reasons why mobile traffic has grown so quickly. Firstly, mobile video, which requires high bit rates, is considered to lead to the increase of mobile traffic. It is reported that mobile video reached as high as 49.8% of total mobile traffic in 2010 and will account for two thirds of mobile traffic by 2015 [61]. Moreover, Internet gaming, which consumes, on average, 63 PB per month in 2009, also results in a growth in mobile traffic and it is expected to achieve an annual growth of 37% in the coming five years [62]. Last but not the least, Voice over IP (VoIP) which includes phone-based VoIP services direct from or transported by a third party to a service provider, and software-based internet VoIP such as Skype, leads to the expansion of mobile traffic. Many of those applications described above are real-time applications which demand certain guarantees for performance metrics for acceptable operation. Those metrics specify the Quality of Service.

3.4.2 QoS routing in MANETs:

The rapid growth of video in mobile traffic has resulted in a shift of research interests from best effort service to the provision of higher and better quality of service in MANets. QoS routing algorithm design is challenging because it has to deal with unfavourable conditions such as time-dependent wireless links, dynamic topology and energy constraints. Considerable efforts have been devoted to this which leads to the emergence of a number of QoS routing techniques.

Generally speaking, two schemes, new protocol design and QoS-aware extension, are adopted to implement QoS routing. New protocol design refers to developing an algorithm with a new methodology while QoS-aware extension means combining QoS guarantee schemes with some well-studied protocols (e.g., DSDV, DSR and AODV).

The performance comparison of these protocols considering all the characteristics that should be possessed by routing protocols is the fundamental step towards the invention of new routing protocol. In this research, I have done the detailed comprehensive review of routing protocols. Much of literature indicate that AODV protocol performs better among all studied routing protocols for network scenario with greater mobility, high node density, large area, high amount of network traffic and network that operate over relatively long time. Hence, AODV protocol was chosen for further study.