Inter-Vehicle Communications
Cooperative driving utilizes onboard sensors in vehicles that communicate with road-mounted sensors, signs, and other vehicles to enhance driver safety and facilitate efficient travel from one location to another Various subapplications can be integrated into cooperative driving systems, as detailed in Table 1.4.
The integration of cooperative driving applications in vehicles relies on various factors, including sensors on other vehicles, roadways, and signs, as well as communication capabilities for data exchange Advanced radar systems are essential for monitoring vehicles lacking sensors and identifying terrain features and obstacles A future goal is to establish an ad hoc network among vehicles equipped with sensors, enabling real-time status observation and necessary corrective actions Onboard computers are crucial for processing signals from these sensors, managing vehicle communications, and assisting drivers in vehicle control While inter-vehicle communications may seem futuristic, numerous vehicle manufacturers and third-party researchers are actively developing the necessary technologies to bring this vision to life With this foundation, we can explore specific subapplications of cooperative driving.
Table Cooperative Driving Subapplications: Accident Warning
Frontal collision prevention Hazard warning
Intersection alert Overtaking and lane change assistance Rear-end collision prevention
Road departure preventionSpeed alert
While traveling on interstate highways or major roads, you will often notice various highway signs providing important information, such as AMBER Alerts for missing children.
Accident warning signs can indicate travel delays and suggest alternate routes for drivers, but they are often passive and may be overlooked during lane changes or other maneuvers In contrast, active accident warning systems provide information that is difficult to ignore, such as radio broadcasts that display warnings on vehicle dashboards Additionally, onboard radar can detect traffic congestion and automatically reduce vehicle speed Some systems utilize sensors to identify crashes through airbag deployment, relaying this information to other vehicles and services like OnStar via ad hoc communication networks Further exploration of inter-vehicle communications will be provided later in this book.
Radar and electronic light pulses enable the measurement of round-trip delays as they bounce off objects, allowing for the assessment of distance between vehicles or obstacles A decrease in this delay indicates that the gap is closing, which can trigger automatic speed adjustments in the vehicle Additionally, drivers are alerted to potential frontal collision risks through visual or audio signals.
Hazard warnings can be generated by measuring the round-trip delay of pulse transmissions from all sides of a vehicle, similar to frontal collision prevention systems However, due to cost and potential interference challenges, utilizing sensors that operate on different frequencies, like wireless local area networks (LANs), may be more effective for communicating the locations of other vehicles and the center of the roadway to moving vehicles The following chapters will explore the fundamental concepts of wireless communications technologies in vehicles.
A significant portion of vehicle accidents takes place at intersections, which can be two-way or four-way stops, as well as traffic light-controlled areas Many drivers neglect to come to a complete stop at stop signs, while others disregard yellow and red traffic signals, contributing to the high rate of collisions.
To enhance safety at intersections and reduce traffic accidents, implementing warning systems for drivers as they approach is crucial This can be achieved through active methods, such as low-powered transmitters that send signals to vehicle receivers, prompting audio or visual alerts for drivers Alternatively, passive solutions like reflector coatings can activate alerts when pulsed by onboard systems Additionally, built-in navigation systems can provide audio notifications, warning drivers of upcoming intersections.
To enhance safety at intersections, implementing an ad hoc network can be beneficial, as it allows vehicles to communicate traffic conditions to one another However, this approach requires vehicles to be equipped with a pulse system and the capability to act as nodes within the network A more feasible solution involves installing sensors at intersections that transmit a specific frequency signal when multiple vehicles are approaching, alerting other drivers tuned to that frequency about the busy traffic conditions ahead.
1.2.1.5 Overtaking and Lane Change Assistance
Overtaking or passing a vehicle poses risks due to blind spots in rearview mirrors To enhance safety during these maneuvers, an onboard system utilizing a side-mounted camera or directional pulses can effectively detect nearby vehicles, aiding drivers in safely passing or changing lanes.
Rear-end collision protection is essential, especially as the adoption of front-end collision systems in vehicles may take decades To enhance safety, alternative methods are needed to alert drivers of potential rear-end collisions One effective solution is the implementation of rear-facing cameras that improve visibility, alongside radar technology that uses pulse reflections to detect approaching vehicles This radar system can warn drivers of imminent collisions by measuring the time it takes for signals to bounce back Additionally, vehicles equipped with sensors can automatically reduce speed when closing in on another vehicle in the same lane, further enhancing safety on the roads.
There are two primary methods to alert vehicle operators when their vehicle is drifting off the roadway The first method utilizes sensors positioned at the center or edge of the road, which trigger audio or visual alarms and potentially adjust steering when a vehicle strays from these sensors The second method employs a microprocessor to monitor steering behavior; it can detect when a driver is tired or has fallen asleep, as the steering becomes minimal or erratic, prompting an alert to the driver.
To alert vehicle operators of speeding conditions, various methods can be implemented One approach involves establishing a predefined speed threshold based on roadway conditions, triggering alerts if the vehicle exceeds this limit Another method includes the automatic deceleration of the vehicle when sensors detect that it has successfully overtaken another car and returned to its original lane.
An emerging area in inter-vehicle communications is consumer assistance, which enhances the driving experience beyond traditional navigation systems While high-end vehicles typically feature built-in navigation for route planning and local points of interest, new functions are being introduced to broaden consumer support These advancements include real-time traffic information, mobile business assistance, and multimedia services, all aimed at improving connectivity and convenience for drivers.
Traditionally, drivers relied on highway signs and radio broadcasts for traffic information However, recent advancements have introduced real-time traffic updates directly to smartphones and in-dash navigation systems A notable vehicle manufacturer has partnered with a third party to offer a year of complimentary traffic information to buyers of specific models If this trend gains popularity, it's likely that other manufacturers will follow suit, providing similar services to their customers.
Radio Frequency Spectrum Allocation
Most countries have a government agency that regulates the use of the frequency spectrum Under the provisions of International Telecommunications Union
(ITU) treaties with most countries, those countries are obligated to comply with the radio frequency spectrum allocations specified by the ITU for international use
Ensuring effective communication in aviation and telecommunications requires careful management of the frequency spectrum, allowing aircraft to connect with control towers and enabling satellite ground stations to receive signals While ITU treaties permit countries to allocate their frequency spectrum domestically, these allocations must not conflict with those of neighboring countries As a result, travelers may experience varying uses of the frequency spectrum, with differences becoming more noticeable when crossing international borders or traveling between continents.
The Communications Act of 1934 in the United States established a division of authority over the radio frequency spectrum between the National Telecommunications and Information Administration (NTIA) under the U.S Commerce Department and the independent Federal Communications Commission (FCC) While the NTIA oversees spectrum usage for federal government purposes, the FCC manages it for non-federal entities The radio frequency spectrum spans from 9 KHz to 300 GHz and is organized into over 450 distinct frequency bands.
One common method for categorizing the frequency spectrum is by using wavelength as a power of 10 metric For instance, the ultra low frequency (ULF) band corresponds to wavelengths ranging from 10^8 to 10^7 meters, while the extremely low frequency (ELF) band covers wavelengths from 10^7 to 10^5 meters Additionally, the very low frequency (VLF) band encompasses wavelengths from 10^0 to 10^13 meters.
A second popular method used to categorize the frequency spectrum comes from one of the well-known national regulators of the frequency spectrum, the
The FCC categorizes the frequency spectrum from 0 to 400 GHz, highlighting the overlap of various frequency bands A summary of the FCC's frequency band nomenclature is presented in Table 2.8, which outlines the frequency ranges associated with distinct bands and their pairs A comparison between Tables 2.7 and 2.8 reveals a lack of direct correspondence, leading technical literature to often reference specific frequency ranges instead of band nomenclature to ensure accurate communication between parties.
Table Well-Known Frequency Bands
The electromagnetic spectrum is divided into various frequency ranges, each with distinct characteristics Ultra low frequency (ULF) spans from 10^-8 to 10^-7 Hz, while extremely low frequency (ELF) ranges from 10^-7 to 10^-5 Hz Very low frequency (VLF) encompasses 10^-5 to 10^-4 Hz, and low frequency (LF) covers 10^-4 to 10^-3 Hz Medium frequency (MF) is classified between 10^-3 and 10^-2 Hz, followed by high frequency (HF) from 10^-2 to 10^-1 Hz Very high frequency (VHF) ranges from 10^-1 to 1 Hz, and ultra high frequency (UHF) spans 1 to 10^1 Hz Super high frequency (SHF) is defined from 10^-1 to 10^-2 Hz, while extremely high frequency (EHF) ranges from 10^-2 to 10^-3 Hz Lastly, electro-optical frequency (EOF) is classified between 10^-3 and 10^-8 Hz, and high-energy frequency (HEF) spans from 10^-8 to 10^-13 Hz.
Very low frequency/low frequency (VLF/LF) 0–130 KHz
Low frequency/medium frequency (LF/MF) 130–505 kHz
Medium frequency/high frequency (MF/HF) 2107–3230 kHz
High frequency/very high frequency (HF/VHF) 33–162.0125 MHz
Very high frequency/ultra high frequency (VHF/UHF) 162.0125–322 GHz
Ultra high frequency (UHF) 322–2655 MHz
Ultra high frequency/super high frequency (UHF/SHF) 2655–3700 MHz
Super high frequency (SHF) 3200 MHz–27.5 GHz
Super high frequency/extremely high frequency (SHF/EHF) 27.5–32 GHz
Extremely high frequency (EHF) 32–400 GHz
Table 2.9 showcases 20 common and emerging wireless applications along with their corresponding frequency bands used in the United States, highlighting the extensive range of wireless applications and the diverse frequency allocations It's important to note that frequency bands may vary internationally.
In the United States, the FCC has divided the frequency spectrum into around 450 blocks, each designated for specific applications This chapter will focus on collision avoidance radar (CAR), particularly millimeter wave radar systems and sensors that operate within the 36 to 94 GHz frequency range, which is also utilized for inter-vehicle communications.
Table Common and Evolving Wireless Applications
Personnel communications 929–932 MHz Satellite telephone uplink 1610–1626.5 MHz Personnel communications 1850–1990 MHz 802.11/11b/g wireless LAN 2.4–2.4835 GHz Satellite telephone downlink 2483.5–2500 GHz Large dish satellite TV 4–6 GHz
802.11a wireless LAN 5.15–5.35 GHz; 5.725–5.825 GHz Small dish satellite TV 11.7–12.7 GHz
Radar Operations
Radar, which stands for radio detection and ranging, is a technology that sends microwave energy beams to detect moving targets When these beams hit a target, some energy is reflected back to the radar unit, allowing it to analyze the reflected signal The frequency shift of the returned signal, proportional to the target's speed, enables accurate measurement of how fast a vehicle is approaching or receding from the radar source.
Radar technology, initially developed for military use during World War II, has evolved into various applications, including radar guns and radar-controlled cameras, leading to increased speeding tickets for motorists This article will first explore police traffic radars, which are widely used to identify speeders Subsequently, we will discuss millimeter radar, which is increasingly being integrated into vehicles to enhance safety features and prevent collisions.
Since the introduction of the first radar unit for state police in 1947, radar technology has significantly advanced, allowing devices to function from both stationary and moving patrol vehicles Modern moving radars are capable of measuring oncoming traffic, with some models also tracking receding traffic and same-lane vehicles These devices can monitor one or two targets by focusing on the strongest reflection for identifying the closest or largest vehicles, or by using echo time to detect the fastest targets.
Police radar operates in a distinct frequency band Table 2.10 lists the fre- quency, tolerance, and frequency range for an obsolete (S-band) and four existing
Ka 33.400–36.000 radar bands (X, Ku, K, Ka), although for one band (Ku) no equipment has been sold in the United States.
The S-band police radar, first introduced in 1947 for Connecticut state police, has become obsolete due to its operation in the same frequency range as microwave ovens The 2.4000- to 2.4835-GHz frequency band, allocated for industrial, scientific, and medical (ISM) unlicensed operations, has rendered S-band radars outdated.
X-band police radar dates to the mid-1960s This radar operates at 10.525 GHz ±
Operating at 25 MHz, X-band traffic radars offer enhanced performance across various weather conditions due to reduced signal attenuation compared to K- or Ka-bands In Europe, certain countries utilize X-band traffic radars tuned to frequencies of 9.41 or 9.90 GHz.
The Federal Communications Commission (FCC) has designated the 13.45 GHz frequency in the Ku-band for traffic radar in the United States; however, these radars have not been sold or utilized domestically In contrast, several European countries have adopted the use of Ku-band 13.45 GHz for traffic radar applications.
K-band radar dates to 1976 and operates on a single frequency of 24.125 or 21.150
GHz with a tolerance of ±100 MHz K-band radar has a wider beam than Ka-band radar but a more narrow beam than an X-band radar.
The Ka-band represents the most recent allocation of frequency for traffic radar use
Initially, the FCC allocated the 34.2- to 35.2-GHz frequency spectrum for traffic radar use In 1992 the FCC expanded the Ka-band for traffic radar use to 33.4 to
Ka-band radar features a narrower beam compared to X- and K-band radars, functioning within a tolerance of ±100 or ±50 MHz Its unique characteristic lies in its wideband version, which operates on a single frequency for a brief moment before rapidly switching to another frequency.
34.2- to 35.2-GHz frequency spectrum using either 13 channels (2600/200) or 26 channels (2600/100).
Here frequency hopping can be viewed as a countermeasure aimed at defeating vehicles operating over the speed limit that have radar detectors.
Radar systems can be categorized based on their operating frequency and functionality Some microwave radars continuously transmit signals, such as those on the DEW line that monitor for potential threats from Russian aircraft over the polar ice cap In contrast, other radar types, like handheld radar guns used by state police, transmit only upon operator command to enforce traffic speed limits Additionally, there are radar systems that operate periodically, emitting pulses every few seconds to measure the speed of approaching vehicles.
The previous types of radar discussed were all based upon microwave technology
Laser radar, a type of technology that utilizes laser-generated light pulses in the upper infrared band, features significantly narrower beams than microwave-based radar While it is capable of measuring speed and range, its effectiveness is hindered by the need for precise aiming and its inability to function properly from within a patrol vehicle behind glass Furthermore, environmental factors such as fog, rain, dust, smoke, and humidity greatly diminish its detection capabilities As a result of these limitations, automobile manufacturers have largely dismissed the use of laser radar technology.
Collision avoidance radar (CAR) serves as a form of inter-vehicle communication, utilizing radar technology to detect nearby vehicles and objects This system aids drivers in avoiding potential collisions by scanning the surroundings With its use of millimeter wave radar, CAR benefits from very high frequencies that enable effective line-of-sight transmission capabilities.
CAR technology functions within the 36- to 94-GHz frequency range, strategically steering clear of police radar frequencies to minimize interference Radar sensors are installed on the rear of vehicles, allowing a central vehicle to gauge the distance to the vehicle ahead This central vehicle is also equipped with a sensor to detect the position of the vehicle behind it, which may lack its own sensor The passive nature of this sensor makes it a cost-effective solution compared to active systems.
IEEE Wireless LANs
Equipping all existing vehicles with collision avoidance radar (CAR) sensors presents a substantial challenge, leading the author to suggest that successful implementation may necessitate sensors on both the front and rear of vehicles While CAR technology is effective for military convoys, its practicality for commercial use in public vehicles remains questionable.
This section highlights key IEEE wireless LAN standards, essential for understanding the growing interest among vehicle manufacturers and standards organizations in utilizing wireless LAN technology This technology aims to facilitate ad hoc networking, enabling vehicles to seamlessly join and leave communication networks By connecting to these networks, vehicles can exchange critical information related to safety, traffic, and other relevant data, enhancing overall road safety and efficiency.
In the United States, the American National Standards Institute (ANSI) assigned the responsibility for developing Local Area Network (LAN) standards to the Institute of Electrical and Electronics Engineers (IEEE) This led to the creation of standards for wired Ethernet and token ring LANs in the 1980s Two decades later, the IEEE introduced its first wireless LAN standard, known as the 802.11 standard.
The IEEE's initial wireless LAN standard, 802.11, established three physical layers for wireless communication: infrared, frequency-hopping spread spectrum (FHSS), and direct-sequence spread spectrum.
In the late 1990s, vendors introduced products utilizing Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) for wireless LANs; however, there is no record of any products being developed that adhered to the IEEE 802.11 infrared communications standard.
Under frequency-hopping spread spectrum (FHSS), a station transmits for a brief period, known as dwell time, before hopping to a new frequency to maintain communication Each LAN station is familiar with the frequency-hopping algorithm, allowing them to adjust their transmitters or receivers accordingly Notably, FHSS originated from actress Hedy Lamarr, who proposed this technique to the U.S War Department during World War II as a means of enhancing transmission security.
Direct-sequence spread spectrum (DSSS) is a transmission technique initially developed by the military to counteract enemy jamming This method involves applying a spreading code to each bit of data, effectively spreading the transmission over a wider bandwidth At the receiver's end, the original signal is reconstructed by correlating the received signal with the spreading code, ensuring reliable communication even in challenging environments.
The "majority rule" principle is utilized in data transmission, where, for instance, with a five-bit spreading code, the received bits 10110 indicate a majority of three bits set to 1, leading the receiver to determine the correct bit as 1 According to the IEEE 802.11 standard, an 11-bit spreading code is used to enhance data integrity.
The original 802.11 standard operates in the unlicensed ISM band from 2.4 to
The 802.11 wireless LAN standard operates primarily at 2.4835 GHz, while the 802.11a standard utilizes higher frequency bands of 5.15-5.35 GHz and 5.725-5.825 GHz Due to the rapid attenuation of high frequencies, the 802.11 standard experiences a shorter transmission range compared to other wireless LAN standards that function in lower frequency bands.
The early adoption of 802.11 wireless LANs faced limitations due to low data rates of only 1 and 2 Mbps across its three physical layers To address the demand for faster data transmission, the IEEE introduced two key extensions to the original 802.11 standard, namely 802.11a and 802.11b, enhancing wireless performance significantly.
The IEEE 802.11a standard introduced advanced modulation techniques that achieve data transmission speeds of up to 54 Mbps This increased bandwidth is made possible through orthogonal frequency division multiplexing (OFDM), which divides the frequency spectrum into multiple subchannels for individual modulation.
The IEEE 802.11a standard operates in the 5-GHz frequency band, making it incompatible with the original 802.11 standard, which functions at 2.4 GHz Due to the rapid attenuation of high frequencies, 802.11a wireless LAN stations have a shorter range compared to those using the 2.4-GHz band Consequently, organizations need to install more access points to achieve equivalent coverage in areas served by 2.4-GHz access points.
The second extension to the basic IEEE 802.11 standard is 802.11b Under the
The IEEE 802.11b standard utilizes Direct Sequence Spread Spectrum (DSSS) technology, offering data transfer rates of 11 Mbps and 5.5 Mbps through two new modulation methods Additionally, it ensures compatibility with 802.11 DSSS devices operating at lower speeds of 2 Mbps and 1 Mbps This compatibility is achieved by operating within the 2.4-GHz frequency band.
A comparison of the IEEE 802.11a and 802.11b standards reveals both advantages and disadvantages While the 802.11a standard offers a higher data transfer rate, it operates on the 5-GHz frequency band, leading to a shorter transmission distance Conversely, the 802.11b standard, which uses the 2.4-GHz frequency band, provides a longer transmission range but at a lower data transfer rate.
The 802.11b standard offers a longer transmission distance but at a lower data rate compared to 802.11a To enhance both data rate and transmission range in wireless LANs, the IEEE developed the 802.11g standard by integrating the modulation method from 802.11a with the frequency band of 802.11b.
To provide backward compatibility with the large base of 802.11b equipment, the
802.11g standard also supports DSSS operations at 11, 5.5, 2, and 1 Mbps Thus, the relatively new IEEE802.11g standard can be considered to represent a dual standard because it provides 802.11b compatibility.
MANET
Mesh networking represents a technology that allows different types of data, to include digitized voice, to be routed between nodes that join and disengage from a network structure on a dynamic basis.
A mesh network, as depicted in Figure 3.3, consists of five nodes, with one node joining and another leaving the network This dynamic setup enables continuous connections and disconnections, allowing the mesh network to automatically reconfigure itself It identifies primary and alternate routes, facilitating data transmission by routing around blocked paths through node hopping until the destination is reached In this context, "ad hoc" is synonymous with "on the fly," as nodes can randomly join or leave the network.
In Figure 3.3, it is crucial for each node to monitor the other nodes in the network to facilitate data transmission when necessary Additionally, it is important to communicate the presence or absence of a node to the rest of the network whenever a node joins or leaves.
A basic mesh network can effectively cover areas from small campus buildings to large cities like Philadelphia, the first major North American city to offer WiFi services through this technology In a defined geographic area, the mesh network utilizes fixed access points, while the wireless nodes are mobile, enhancing connectivity and coverage.
A mesh network is an effective system for routing data between nodes, typically characterized by fixed locations However, another variant known as a Mobile Ad Hoc Network (MANET) features stations that are frequently in motion and only stop briefly MANETs are particularly suitable for meeting the connectivity needs of vehicles operating in dynamic environments.
The key difference between a MANET, or what some persons refer to as a
Vehicular Ad Hoc Networks (VANETs) differ from conventional wireless mesh networks in two key aspects Firstly, while a Mobile Ad Hoc Network (MANET) operates without fixed points, a traditional wireless mesh network relies on access points for connectivity Secondly, communication capabilities vary, as MANET nodes can communicate directly with each other, whereas nodes in a mesh network must connect through an access point With this understanding of the distinctions between these network types, we can now explore the evolving IEEE standard for wireless mesh networks before delving deeper into MANETs.
IEEE 802.11s is an evolving standard designed to enhance mesh networking, addressing challenges similar to those faced by Mobile Ad Hoc Networks (MANET) Understanding this standard allows us to identify key functions essential for supporting a self-configuring multi-hop topology.
The IEEE 802.11s standard, anticipated to be released in 2008, aims to establish a framework and protocol for the automatic configuration of pathways between access points in a wireless environment using multi-hop nodes This initiative began with a call for proposals in June.
2005 resulted in 15 submissions that were pared down to 4 by September 2005 In
January 2006 two competing proposals referred to as SEE Mesh, led by Intel and
Firetide, and Wi-Mesh, proposed by Nortel’s Wi-Mesh Alliance, were merged and used as the starting point for the development of the actual standard.
Configuration Control and Management Routing
Figure 3.4 highlights the key components that form the foundation of the 802.11s architecture It includes established standards for data transmission, specifically the physical layer associated with IEEE802.11 methods (802.11a/b/g/n) Additionally, it addresses configuration, control, and management (CCM), which outline the processes for configuring, controlling, and managing nodes, including the routing and measurement updates necessary for maintaining accurate routing tables.
The IEEE 802.11 standard features three distinct Distributed Coordination Functions (DCFs) that facilitate medium access These DCFs assess the availability of the radio frequency before transmission to minimize contention If the channel is occupied, the transmitting station must wait a random back-off period, and if the channel remains busy, the contention window doubles with each attempt until it reaches its maximum limit.
The hybrid coordination function (HCF) is employed in wireless systems that support quality of service (QoS), operating in two distinct modes: Enhanced Distributed Channel Access (EDCA) and another mode.
HCF-Controlled Channel Access (HCCA) operates alongside the Enhanced Distributed Channel Access (EDCA), which is a contention-based channel access method While EDCA allows for prioritized traffic management, HCCA focuses on supporting parameterized traffic through a polling mechanism.
In the architecture diagram illustrated in Figure 3.4, measurements encompass the pathways between nodes and their entry and exit points within an ad hoc network, which are essential for routing packets from the source to the destination Furthermore, the CCM management process utilizes these measurements for effective topology discovery Additional functions of CCM include channel allocation, path selection, and packet forwarding.
The emergence of wireless mesh networking has introduced new terminology to the field A mesh point is defined as a node capable of supporting mesh services When a mesh point additionally offers access point services, it is designated as a mesh access point Conversely, a mesh point that connects to a wired network is known as a mesh portal.
In a mesh network, each mesh point must discover and connect with its peers, while also determining the most efficient path for forwarding frames to their intended destinations The evolving 802.11s standard introduces a path selection protocol known as Hybrid Wireless Mesh, which facilitates this process.
(HWM) is being considered To enhance interoperability, vendors can also use their own protocols for path selection.
The evolving 802.11s standard is designed as a single-radio, shared mesh extension specifically for indoor access points, distinguishing it from the extensive outdoor wireless infrastructure utilized by cities for public Internet access Additionally, it contrasts with ad hoc mobile networks primarily employed by vehicles Therefore, this chapter will conclude with a focus on Mobile Ad Hoc Networks (MANET).
The effort in developing a series of specifications for MANET dates to 2001, when the MANET Working Group (WG), part of the Internet Engineering Task Force
(IETF), published a draft version of the Ad Hoc On-Demand Distance Vector
(AODV) Routing protocol In July 2003 the latest draft of AODV was published as
RFC 3561 To date, the MANET WG has developed six Internet draft documents and five RFCs.
The LIN Specification
This article explores the transmission of data and the rationale for creating a networking technology that, despite its relatively low data transfer rates, remains relevant in an age dominated by high-speed technologies.
The LIN specification is structured like a troika, comprising three essential components: the LIN Protocol Specification, which outlines the physical and data-link layers; the LIN Configuration Language, detailing the format of the LIN configuration file for network setup; and the LIN Application, which defines the functional aspects of the network.
An Application Programming Interface (API) defines the interaction between the network and application programs Additionally, developers and technicians have access to a set of tools that enhance the LIN Configuration Language, including a network configuration generator and a bus analyzer/emulator, commonly known as the LIN spector.
The LIN (Local Interconnect Network) specification encompasses both hardware and software aspects, providing essential tools for the development and testing of applications A block diagram in Figure 4.1 highlights the major components of the LIN specification.
LIN minimizes costs by utilizing a single conductor wire to create a 12-V bus, leveraging the vehicle's body as a common ground Communication is facilitated through the serial communications interface (SCI) using a universal asynchronous protocol.
Tools Signal Database Manager Configuration Language
Intra - Vehicle LIN Network Note, the Bus Analyzer/Emulation is the LIN spector
LIN Physical Layer LIN Physical Layer
Bus Transceiver The LIN Protocol
Operating System Application Software The LIN API
The LIN specification includes key components such as the receiver-transmitter (UART) data format, which features clock synchronization for nodes lacking a stable time base It supports a maximum data rate of 20 kbps and allows for a transmission distance of up to 40 meters.
LIN, or Local Interconnect Network, is a broadcast serial network architecture that features one master node and up to 16 slave nodes Data transmitted on a shared bus allows all connected nodes to receive the information, characterizing it as a broadcast network.
In systems lacking a collision detection mechanism, communication is initiated solely by the master node, which sends out messages that are responded to by one or more slave nodes based on a predefined message identifier Typically, the master node is a moderately powerful microcontroller, while the slave nodes can either be dedicated application-specific integrated circuits (ASICs) or less powerful microcontrollers.
Modern automobiles utilize multiple Local Interconnect Networks (LINs), each supporting up to 16 nodes These LINs connect through their master nodes to a more advanced Controller Area Network (CAN), allowing LIN data to be transmitted to a central hub This integration facilitates the display of warning information on console screens, supports diagnostic testing, and enables various centralized functions within the vehicle.
The 20-kbps maximum data rate can be exceeded; however, doing so can result in electromagnetic interference Thus, the data rate is kept at or below 20 kbps as a mechanism to minimize electromagnetic interference (EMI) Through self-syn- chronization, neither crystals nor ceramic resonators are required in slave nodes that are controlled by a master node in a master–slave relationship Thus, the abil- ity to avoid the use of crystals or ceramics resonators results in a significant cost reduction.
4.2.1.4 Examining the Master–Slave Relationship
The LIN bus architecture features a single master node that can manage up to 16 slave nodes The master node relies on predefined scheduling tables to initiate data transmission and reception on the LIN bus These tables provide essential timing information for the commencement of message transmissions.
Figure 4.2 depicts a master-slave relationship, showcasing a master node that oversees four slave nodes Each master node is equipped with both a master task and a slave task, whereas the slave nodes are designed to handle only slave tasks.
The master task determines when and what frames should be transferred onto the bus In comparison, the slave task provides the data transported by each frame.
The master task governs the entire bus and protocol, controlling the timing and transfer of messages It initiates communication by sending a SYNC BREAK to signal the start of a message frame, followed by a SYNC byte to establish timing, and a message identifier that includes sender and receiver information, purpose, and data length Additionally, the master task handles error management, monitors data and check bytes, and responds to wake-up BREAK signals from inactive slave nodes requesting action.
The slave task is generally simpler than the master task, as it either responds to the master task or disregards it if the requested action pertains to other slaves.
In the International Standards Organization (ISO) Open Systems Interconnection
In the OSI model, the data-link layer sits above the physical layer, playing a crucial role in the transmission of frames over the bus in the LIN protocol This layer is essential for understanding how LIN frames are structured and transported efficiently.
Each LIN frame consists of a header and a response The header is transmitted by the LIN master (master task), while the response is returned by one LIN slave (slave
Message Frames
The CAN data frame is the primary message type on the CAN bus, as defined by the ISO 11519 specification It features an 11-bit identifier field and a one-bit remote transmission request (RTR) field, which together establish message priority among multiple nodes vying for bus access Standard CAN operates at data rates of up to 125 kbps.
An extended CAN data frame utilizes a 29-bit identifier, achieved by adding an 18-bit identifier field to the standard CAN frame This frame type includes three modifications and supports data rates of up to 1 Mbps.
CAN data frame the arbitration field, which is employed to determine the priority of messages when two or more nodes contend for access to the bus, consists of a
29-bit identifier field formed by separate 11-bit and 18-bit identifier fields and the
RTR bit Now that we have a basic appreciation for the two types of data frames, let us examine their composition in detail.
Figure 5.5 illustrates the fields in the standard CAN data frame Both the low-speed
CAN, defined by the ISO 11519 specification, and CAN 2.0A, defined by the ISO
The 11898 specification supports an 11-bit identifier field, distinguishing it from the original standard CAN, which operates at 125 kbps, while CAN 2.0A functions at a higher speed of 1 Mbps.
In analyzing the standard and extended CAN data frame formats, it's important to note the absence of an address field This is due to the broadcast nature of CAN messages, which eliminates the need for addressing Instead, CAN messages are content addressed, meaning that the message's content dictates whether a node will respond to it.
It's important to understand that the presence of an ACK bit does not confirm that the intended nodes have received the message Instead, this bit can be set by any controller that successfully received the message.
ACK bit at the end of the message Thus, the ACK bit only informs us that one or more nodes on the bus correctly received the message. n n
The extended CAN data frame employs a 29-bit identifier, enhancing the standard CAN's 11-bit identifier This extension includes an additional 18-bit identifier field, which is separated from the original identifier by two fields: the substitute remote request (SRR) field and the identifier extension.
Figure 5.6 depicts the structure of the extended CAN message frame, which features an 18-bit identifier field that expands the identifier to 29 bits In contrast to the standard CAN data frame shown in Figure 5.4, the extended CAN data frame introduces three additional fields.
11-bit identifier 18-bit identifier r0 r1 0 8 data bytes
Figure The extended CAN message frame.
The Standard CAN data frame format includes several key components: the Start of Frame (SOF) bit, which signals the beginning of a frame and helps synchronize nodes on an idle bus; the identifier, an -bit value that determines message priority, with lower values indicating higher priority; and the Remote Transmission Request (RTR) bit, which is activated when information is needed from another node Although all nodes on the bus receive the RTR request, the identifier specifies which node will respond.
The IDE (Identifier Extension Bit) is utilized to establish a standard CAN identifier without an extension The R0 bit is reserved for future use, while the DLC (Data Length Code) indicates the number of bytes being transmitted The data can range from 0 to 8 bits (1 byte), and the CRC (Cyclic Redundancy Check) is a crucial component that contains the checksum for error detection.
The error detection in data transmission involves a CRC field, which includes a bit for the CRC and a recessive delimiter bit to signify the end of the field Each node receiving a correct message replaces a designated bit with a dominant bit to confirm error-free receipt; if an error is detected, the message is discarded and retransmitted The acknowledgment (ACK) field spans two bits, with the first bit serving as the acknowledgment and the second as a delimiter Additionally, a seven-bit end-of-frame (EOF) field indicates the conclusion of a CAN message, while a seven-bit inter-frame separator (IFS) denotes the time needed for a controller to transfer a correctly received frame into its message buffer.
SRR — Substitute remote request bit, which replaces the RTR bit in the stan- dard message location as a placeholder in the extended frame
IDE — Identifier extension (IDE) bit, which indicates that an 18-bit extension identifier follows
In both standard and extended CAN frames, the arbitration field serves as a pseudo-field that determines message priority when multiple nodes compete for bus access.
The Controller Area Network (CAN) protocol features an 11-bit identifier along with a dominant Remote Transmission Request (RTR) bit for data frames In the extended CAN format, the arbitration field is expanded to include a 29-bit identifier, two recessive bits known as the Substitute Remote Request (SRR) and Identifier Extension (IDE), in addition to the RTR bit.
Bit stuffing in both standard and extended CAN frames involves inserting a bit of opposite polarity following a sequence of five consecutive bits of the same polarity This process applies to the entire frame, starting from the start-of-frame bit field and extending through the 15-bit cyclic redundancy code field.
A third type of message that can be transmitted on a CAN bus is the remote frame The remote frame is similar to the standard and extended CAN data frames
The remote frame differs from other data frames in two significant ways: it lacks a data field and is distinctly identified by the RTR bit being set to recessive.
Remote frames facilitate a request-response mechanism in bus traffic For instance, when Node A sends a remote frame with an arbitration field value of 246, any node that recognizes the need for a response will reply with a data frame, maintaining the same arbitration field value of 246.
Error Handling
The error frame is a crucial component of the CAN protocol, transmitted by any node that detects an error in a message This special frame deviates from standard CAN message rules and prompts all other nodes in the network to send their own error frames When a node identifies an error, it automatically retransmits the original message To prevent continuous error frame transmission that could congest the bus, CAN controllers utilize error counters, which will be discussed in the following section.
The error frame is composed of two key fields, with the first field containing error flags These flags are generated through the combination of error signals from various nodes on the bus.
Error flags in networking are categorized into two types: active and passive An active error flag is sent by a node that identifies an error while in the "error active" state, indicating a detected issue on the network Conversely, a passive error flag is transmitted by a node that recognizes an active error frame while in the "error passive" state, reflecting an acknowledgment of the existing error.
The overload frame is the fifth type of frame that can be transmitted on the CAN bus It is sent by a node that is too busy to handle more data, serving to introduce an additional delay between messages.
In concluding our exploration of the Controller Area Network (CAN), we will focus on a crucial aspect: its error handling mechanisms Before delving into this topic, it's essential to briefly review how traditional communication technologies detect and correct errors, as this will offer a valuable comparison to CAN's error management strategies.
In today's communication systems, error handling is achieved through two primary methods: parity, which checks for errors in independently transmitted bytes, and checksums, which verify data integrity by grouping bytes into blocks for transmission.
Parity checking involves adding an extra bit, known as a parity bit, to each byte before transmission This method can be classified into two types: even and odd parity In even parity checking, the parity bit is assigned a binary 0 if the byte contains an even number of set bits; conversely, it is assigned a binary 1 if the byte has an odd number of set bits, ensuring that the total count of set bits remains even Conversely, in odd parity checking, the parity bit is set to binary 1 when the byte has an even number of set bits and to binary 0 when it has an odd number, maintaining an odd total of set bits.
Parity checking can only detect single-bit errors, making it difficult to correct byte errors without visual inspection or retransmitting the entire document To address these limitations, most error detection and correction methods have developed by blocking bytes and incorporating a checksum calculated using a predefined algorithm.
The Xmodem protocol is a widely used communications method that utilizes a fixed number of bytes to create a block for error checking This block checking technique ensures data integrity during transmission by verifying that the sent and received data match correctly.
A block is composed of 128 bytes, and if the final block contains less than 128 bytes of data, it is completed with pad characters (ASCII 127) to ensure it reaches the full 128-character size.
Block checking involves applying an algorithm to each data block to generate a checksum, which is then appended and transmitted alongside the block Upon receipt, the receiver computes a local checksum using the same algorithm and compares it to the transmitted checksum If they match, the data block is deemed error-free, the checksum is removed, and the block is processed The receiver also sends an acknowledgment to the sender, allowing for the next data block to be sent Conversely, if the checksums do not match, it indicates potential bit errors, prompting the receiver to send a negative acknowledgment to the sender, which triggers a retransmission of the data block along with its checksum to correct the errors.
Using a checksum can minimize the chances of undetected errors, but simple algorithms may still allow such errors to occur To enhance error detection, modern communication systems implement a polynomial-based error checking method In this approach, the data block is treated as a long polynomial, which is then divided by a predetermined polynomial The quotient is ignored, and the remainder, known as the cyclic redundancy check (CRC), is stored in a dedicated CRC field.
Now that we have an appreciation for the manner by which conventional com- munications systems perform error handling, let us turn our attention to CAN error handling.
The CAN protocol incorporates error handling capabilities similar to traditional communication systems, utilizing five methods for error detection—two at the bit level and three at the message level When a controller detects an error in a message on the CAN bus, it transmits an error flag to notify other controllers to discard the faulty message, effectively reducing bus traffic The original transmitter then retransmits the erroneous message, making the error flag analogous to a negative acknowledgment, with errors being rectified through retransmission.
One intriguing aspect of CAN error handling is a node's ability to disconnect from the CAN bus under specific conditions Each node tracks two error counters: the transmit error counter, which increases with transmit errors, and the receive error counter, which increases with receive errors Typically, the transmit error counter will rise more quickly than the receive error counter, as it is likely that the transmitter is responsible for the detected errors When the transmit error counter reaches a set threshold, the node transitions into an error passive state, where it refrains from sending error flags Subsequently, the node enters a "bus off" state, ceasing all participation in bus traffic.
The CAN protocol employs five methods for error detection, which can be classified into bit-level and message-level detection This section focuses on how the CAN protocol identifies errors, with a summary of these detection methods provided in Table 5.2.
Intra-Vehicle Communications
Wireless Communications
The integration of the Media Oriented Systems Transport (MOST) with an audio gateway to the Controller Area Network (CAN) enables seamless communication between the CAN-based in-vehicle network and infotainment devices like radios, CD changers, and DVD navigation displays Understanding these intra-vehicle wired communications sets the stage for exploring advancements in wireless communications within vehicles.
In the past decade, the adoption of wireless technology in vehicles has surged dramatically, with millions of cars now utilizing various wireless applications These technologies facilitate hands-free phone operation, remote door unlocking, and real-time navigation and traffic updates While some wireless communications, like satellite services, extend beyond the vehicle, they are considered part of intra-vehicle communications due to their direct integration This section will focus on Bluetooth technology and satellite services, including satellite radio and satellite-based vehicle solutions.
Bluetooth represents an industry standard for the creation and operation of wireless
Personal Area Networks (PANs) are low-speed, short-range wireless networks that utilize frequency hopping, distinguishing them from wireless LANs This section explores the key parameters of PAN standards, highlighting the similarities and differences between Bluetooth technology and wireless LANs, along with various intra-vehicle applications enabled by Bluetooth.
The Bluetooth specification was initially created by Ericsson in Lund, Sweden, in 1994 In 1998, the development was handed over to the Bluetooth Special Interest Group (SIG), which has since grown to include over 6,000 member companies.
SIG and the IEEE has standardized the technology as the IEEE 802.15.1 standard.
Early Bluetooth versions, like 1.0 and 1.0B, faced interoperability issues that hindered widespread adoption However, Bluetooth 1.1 and 1.2 addressed many of these problems, with version 1.2 introducing enhancements that led to its integration into various devices, including cell phones, computers, and vehicle radio systems Notably, version 1.2 featured adaptive frequency-hopping spread-spectrum (FHSS) technology, which significantly enhanced Bluetooth's resistance to radio frequency interference by avoiding congested frequencies This improvement was crucial as wireless LANs operate on similar frequencies but with higher power, potentially disrupting Bluetooth connections.
Released in 2004, Bluetooth 2.0 significantly enhanced transmission capabilities, extending the range to 100 meters and achieving data rates up to three times faster than Bluetooth 1.2 This version maintained backward compatibility with 1.2 while also improving bit error rate performance and reducing transmission errors.
Bluetooth represents a low-power-consumption communications protocol that enables devices to exchange data with one another Currently there are three defined
Bluetooth classes that designate the maximum permitted power in mW and dBm, and range in meters Table 6.4 summarizes the three classes Note that Class 1 requires support of Bluetooth 2.0.
In examining the entries in Table 6.4 you will note that the most powerful
Bluetooth class is Class 1, as it provides a maximum power level that can extend the range of transmission to approximately 100 m For most in-vehicle operations,
Class 2 Bluetooth should be more than sufficient, as it enables a transmission range of up to approximately 10 m.
A Bluetooth master device can connect with up to seven slave devices, creating a network known as a piconet This piconet is an ad hoc network formed automatically when Bluetooth-enabled devices come within range of each other The limitation on the number of active nodes in a piconet is due to the use of a three-bit Media Access Control system.
(MAC) address to identify active devices, resulting in a maximum of eight devices
Maximum Permitted Power (dBm) Range
In a network configuration, one master device can manage up to seven active slave devices, with the capability to activate an additional 255 inactive or parked slave devices However, the total number of active devices in the network must not exceed eight at any given time.
Bluetooth technology facilitates both unicast and broadcast transmission methods In unicast transmission, the master device sends data to slave devices in a round-robin fashion, allowing for direct communication between a slave and the master node While broadcast transmission is technically supported, its practical application is limited, as Bluetooth devices typically have minimal need to receive data intended for other devices.
As previously discussed, Bluetooth uses the same frequency band as wireless LANs
Bluetooth technology functions within the 2.4-GHz band, known as the industrial, scientific, and medical (ISM) unlicensed frequency band It enables communication between devices through a method called frequency hopping, utilizing 79 distinct frequencies ranging from 2.402 to 2.480 GHz, with each frequency separated by 1 MHz in the United States.
In other countries the frequency band range may be reduced, resulting in a lesser number of frequency hops available for use.
The Bluetooth protocol is capable of rapidly changing channels up to 1600 times per second, ensuring efficient data transmission While Bluetooth versions 1.1 and 1.2 support a maximum data transfer rate of 723.1 kbps, version 2.0 significantly enhances this capability, achieving speeds of up to 2.1 Mbps This high data rate is particularly advantageous for applications such as connecting a cell phone to a vehicle's audio system, as voice data can be digitized at rates as low as 8 kbps, well within Bluetooth's robust transfer capabilities.
Bluetooth uses Gaussian frequency shift keying (GFSK) modulation Under GFSK a binary 1 is represented by a positive frequency deviation, while a binary 0 is rep- resented by a negative frequency derivation.
Bluetooth utilizes a pseudo-random frequency-hopping sequence for communication, similar to methods used in wireless LANs Each piconet's hopping sequence is based on the Bluetooth device address of the master, creating a unique frequency pattern for communication between the master and slave devices This design allows multiple piconets to operate in close proximity without causing interference.
The hopping sequence is determined by the clock of the master Bluetooth node, which divides the channel into time slots Each time slot lasts 625 ms and is associated with a specific radio frequency hop Consequently, a series of consecutive hops will utilize various RF hop frequencies.
Bluetooth supports five logical channels that can be used to transfer different types of information Table 6.5 indicates the logical channels supported by Bluetooth and their use.
Bluetooth devices can be assigned four types of addresses, including the three-bit MAC address, which is only valid when a slave device is active on a channel.
Bluetooth addresses include a device address, parked member address, and access request address.
The Bluetooth device address is a unique 48-bit identifier, while the parked member address is an eight-bit address that can identify up to 255 parked slaves, remaining valid only while the slave is parked Additionally, the access request address allows a parked slave to identify the appropriate slave-to-master half slot within the access window for transmitting access request messages, and like the parked member address, it is only valid during the parked state.
Inter-Vehicle Communications
The Intelligent Roadway
AODV and TBRPF are compared in terms of routing overhead in both low- and high-mobility scenarios, as shown in Table 7.8 In low mobility, a low traffic rate is expected, whereas high mobility leads to a higher traffic rate The routing overhead is significantly influenced by the mobility mode of operation AODV, which sends numerous small routing control packets, is effective for both low and high mobility Similarly, TBRPF's proactive approach ensures a consistent routing overhead, making it suitable for varying mobility conditions.
The latency of AODV for low-mobility and low-traffic operations is slightly better than that of TBRPF, because AODV uses small routing control packets
In high-mobility and high-traffic operations, the latency of both protocols tends to converge Additionally, as highlighted in Table 7.8, AODV is characterized by its use of smaller routing control packets, which impacts routing overhead.
TBRPF is particularly effective in low-traffic environments, but as traffic levels rise, its proactive protocol demonstrates increased efficiency due to its consistent routing overhead.
In this concluding section, we will explore how vehicles can be equipped with intelligence to anticipate road and highway conditions Additionally, we will discuss the role of ad hoc networks in enhancing information dissemination and their relationship with this technology.
Through the use of wireless access points or even simple broadcast stations located at strategic locations, such as exit ramps or at the beginning of a curved incline or
Routing Protocol Mobility/Traffic Level Feature AODV TRBPF
Low/low Packet delivery ratio High High
Routing overhead Low Medium High/high Packet delivery ratio High High
Routing overhead can vary based on terrain, with high medium steep hills posing unique challenges for information dissemination to approaching vehicles Table 7.9 outlines various roadway locations where information can be effectively broadcasted directly to vehicles or through a Vehicle Mobile Ad Hoc Network (VANET).
Lane markers serve a dual purpose by guiding vehicles along designated paths and enhancing safety by preventing drift that could lead to collisions However, factors such as driver fatigue, intoxication, or distraction can cause individuals to veer into adjacent lanes, increasing the risk of accidents.
Lane markers play a crucial role in accident prevention, but they function as passive devices By integrating wireless transmitters with these markers, they can actively communicate with vehicle operators This technology would provide audio or visual warnings when lane drift is detected, significantly improving road safety.
Highway "Do Not Enter" signs are crucial for preventing head-on collisions, especially for inexperienced drivers, but they can be hard to see at night or in bad weather To enhance safety, implementing GPS technology could allow vehicles to receive warning messages only when they are at risk of entering restricted lanes This approach could reduce unnecessary alerts, ensuring that drivers are alerted only when truly needed, thus improving overall roadway safety.
Table Locations on Roadways that Could Communicate with Vehicles
Lane markersLane directionRoad junctionTraffic lightRoad exitTemporary obstacles
A road junction, where multiple roads intersect, is statistically more susceptible to accidents, even with warning and stop signs in place Therefore, implementing a warning broadcast system at these intersections could enhance safety In the future, this broadcast could be integrated into vehicle electronics, allowing vehicles to automatically slow down as they approach the junction.
Traffic lights have evolved from passive devices that merely cycled through green, yellow, and red to sophisticated systems equipped with video cameras These modern traffic lights, commonly found in urban areas, strategically capture images of drivers and their license plates when vehicles run red lights.
After running a red light, a motorist may receive a traffic fine in the mail, which includes evidence of the violation and a form to complete for payment.
Integrating video cameras with traffic lights allows for the broadcasting of traffic signal states, enabling vehicles to automatically adjust their speed With the right software, cars can gradually stop as the light shifts from yellow to red or accelerate slightly to pass through green lights This technology not only enhances safety but also helps regulate traffic flow, ultimately improving vehicle fuel efficiency.
When exiting a highway, drivers face diverse scenarios, including stop signs, traffic lights, and construction obstructions Additionally, road signs provide crucial information about nearby gas stations, restaurants, and rest stops Broadcasted traffic updates can alert approaching vehicles to ongoing construction and guide them to available gas and service stations, as well as dining options.
Utilizing vehicle-to-vehicle communication can enhance information sharing by alerting approaching vehicles, giving drivers more time to decide on taking an exit ramp.
As vehicles navigate various road conditions, they often face resurfacing lanes, pothole repairs, and other obstacles that necessitate merging into different lanes and reducing speed Installing broadcast transmitters at these locations can effectively alert oncoming traffic to slow down and change lanes as needed.
Thus, temporary obstacles represent another traffic condition where transmission of broadcast information could supplement conventional traffic signs.
There are two basic methods by which information can be transmitted to approach- ing vehicles: infrastructure to vehicle and vehicle to vehicle When a Vehicle Mobile
Ad Hoc Network is formed, it also becomes possible for the initial infrastructure-to- vehicle transmission to be relayed through the formed VANET.