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Figure 20. Vehicular communication networks with multiple paths of V2I and V2C

Vehicular communication networks provide for a wide range of applications designed to solve several problems related to:

Public Safety. Road safety can be improved by messages exchanged by vehicles, e.g., in the case of accidents/collisions, bad weather conditions (ice/water on the road) unexpected events (e.g., low bridges, oil on the road), or to assist the drivers in lane change/overtaking operations.

Traffic Information. V2X and C-V2X can be utilised to provide traffic monitoring/ shaping (including traffic light management), i.e., aimed at adjusting the scheduling of traffic lights to

help the drivers move in the green phase, thus also contributing to the reduction of environmental pollution.

Infotainment. Providing travellers with on-board information and entertainment services such as Internet access or music download.

So, vehicle communication can be divided into Public Road Safety Communications Resilience and ad-hoc vehicular communications Resilience.

Vehicular networks provide communications for a wide range of applications, as shown in Figure 20, public road communication includes V2N, V2I and C-V2X.

7.2.1. Understand Public Communications Resiliency

Communications resilience is that networks can recover from damage, accident or attack, hereby minimising the possibility of the service outage. There are three key elements of communications resilience90_:

1. Route Diversity. Route diversity is defined as routing communications between two

vehicles over more than one physical path (RF communication channels). As shown in Figure.6.4 vehicles can upload and download data to the cloud via V2N and V2I. Meanwhile, vehicles can act as a relay to communicate with infrastructure by V2V. 2. Redundancy. Redundancy means that additional or duplicate communications assets

share the load or provide back-up to the primary asset. In the purpose of resilience, network redundancy means dedicated resource blocks (RB) for the recovery or emergency communication use only.

3. Protective/Restorative Measures. Protective measures decline the probability that a

threat will affect the network, while therapeutic measures enable rapid restoration if commercial services are lost or congested.

7.2.2. Network Failure Management

Network failure management includes: Fault detection, Fault localization, and Fault notification91_.

1. Fault Detection. Parameters and counters can be used to detect the communication

network failure at different network layers.

• Physical layer: signal loss, modulation loss and synchronous clock loss.

• Signal strength: the signal deterioration at the receiver side during the specified period; it can be detected from Signal-to-Interference Plus Noise Ratio (SINR), channel BER, the dispersion level, the crosstalk, or the attenuation level. • Service Quality: Package loss ratio, channel throughput or package delays, etc. 2. Fault Localisation. During fault localisation, where the failure occurred is determined,

i.e., a faulty item is recognised.

90_{https://www.dhs.gov/safecom/blog/2018/02/07/public-safety-communications-resiliency-ten-keys-obtaining-}

resilient-local

91 _{D. Papadimitriou and E. Mannie, Eds. (2006) ‘Analysis of Generalized Multi-Protocol Label Switching}

3. Fault Notification. Fault notification is used to inform the control centre that there

was a failure in the network. This triggers the appropriate procedures to resolve the fault quickly and try to prevent it happening in the future, if possible.

7.2.3. Cost of Resilience

The cost of a recovery is very important for the operator and should be taken into

consideration as an important factor to determine the different resilience methods based on several parameters.

Generally, the most direct way is to base it on the network redundancy for resilience, such as the extra network resource usage for supporting the specific recovery method. Normally, it is the dedicated resource blocks (RB). There are some other elements to measure the cost of communication resilience, e.g., additional software, the increased Operational Expenditures (OPEX) related to the new staff or higher expenses on device operation92_.

7.2.4. Ad-hoc Vehicular Communications Resilience

Vehicular communications can be provided either without or with the support of a roadside infrastructure, also referred to as vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) wireless networking. Based on data travel via vehicles the V2V communication can be classified as either: (1) single hop (sender to receiver directly), or (2) multi-hop V2V (between sender to receiver there are vehicles acting as relays.)

The advanced applications of intelligent transportation systems require both reliable and

low-latency communication. An example is road safety warning (e.g. related to collision warning or traffic coordination issues). If the information delay is high, it may increase risk to human life or injury.

7.2.5. Reliability Requirements of V2V Communication

The V2V communication can be considered as self-organising, self-optimizing, and with a short transmission range.

• Dynamic network topology: with frequent topology changes resulting in common path unavailability, or even causing network disconnections/partitioning.

• A relatively sufficient resource of energy and data storage. Compared with other mobile communication devices (mobile phone, pads), vehicles have higher energy and data storage.

• Geographic-based message distribution provides fast dissemination of time-critical information to other vehicles.

• Strict data delay requirement, because of the safety applications.

The categories of safety applications are identified by the Vehicle Safety Communications Consortium (VSCC)93_{. Safety applications require low delay communications because the}

92 _{H. Lønsethagen, A. Solem, and B. Olsen (2005) ‘Feasibility of Bandwidth on Demand. Case Study}

Approach, Models and Issues’ EU FP6 IP IST-NOBEL Project internal presentation, Sept. 19–21.

93 _{Delgrossi, Luca, and Tao Zhang (2012) ‘Vehicle safety communications: protocols, security, and privacy’,}

validity of information (e.g., post-crash warnings) expire very fast, and any such delayed information shortly may shortly become useless for surrounding vehicles. Therefore,100ms is the maximum latency of safety message delivery, while 10 Hz is the minimum frequency of message exchange.

Safety-related notifications can be either event-driven or periodic. Event-driven messages are disseminated after identification of an event94_{. Safety applications data is normally a}

one-hop broadcasting communication. it should be to send out safety-related messages over 150m by one-hop broadcasting. In the case of multi-hop distribution of

safety messages. The total coverage distance of safety applications is in the range between 300 m and 20 km95_{. The requirement of non-safety applications is shown in Table 6.3.}

Table 11.Classification of non-safety applications

Categories Applications Frequency (Hz) Latency (ms)

Traffic efficiency Enhanced route guidance and navigation

10 <100

Green light optimal speed advisory

10 <100

V2V merging assistance 10 <100

Infotainment Internet access in vehicle 1 <500

Point of interest notification 1 <500

Remote diagnostics 1 <500

7.2.6. Resilience of End-to-End V2V Communications

The three elements of end to end V2V communication resilience are: (1) communications path stability, (2) multipath routing.

Network Path Stability. The main factor for measuring the V2V network path stability is the path outage probability. An outage occurs if data from the source vehicle cannot reach the destination vehicle. Specifically, the transmission vehicle fails to find the destination vehicle within the vehicle’s maximum communication range. The maximum communication range is defined as the range that both the receiver signal level and signal quality SINR is higher than a required Quality-of-Service (QoS) threshold.

The approach of enhancing the network capacity is to decrease the network outage probability. For example, in a single-hop network, usually one can increase the

94 _{Vijayakumar, Pandi, Victor Chang, L. Jegatha Deborah, Balamurugan Balusamy, and P. G. Shynu (2018)}

‘Computationally efficient privacy preserving anonymous mutual and batch authentication schemes for vehicular ad hoc networks’ Future generation computer systems 78, 943-955.

95 _{ETSITR102638 (June 2019) Intelligent Transport System (ITS); Vehicular Communications; Basic Set of}

transmission power to increase the communication rang. However, this can increase the interference to other co-frequency users.

Multipath Routing. It improves the reliability of end-to-end transmission, and multipath routing can transmit information via a multiple hop relay network. Additionally, multipath routing can also improve network throughput, load balancing, and packet delivery ratio. However, multipath can cause a data delay because of the longer transmission path and relay processing. The multipath routing is suitable for the delay-tolerant service. Such as V2V store-carry-forward (SCF) network96_.

In recent years store-carry-forward (SCF) relaying has received attention for its potential to deliver extra mobile capacity for delay-tolerant data delivery. The principle idea is to

transmit data close to the intended destination by physically carrying the data packets across most of the original transmission distance. It has been shown that it can lead to higher energy efficiency for transmission. What has been lacking, however, is the design of route selection algorithms that are optimised and efficient for application in large scale urban simulations, using real vehicular traffic, to examine performance trade-offs.