IEEE 802.11P (Physical Layer)

Physical layer is the bottommost layer from WAVE architecture, and it is very similar to OSI model seventh layer. It provides a link between the MAC layer and the medium used for sending and transmitting data block. It is responsible for data formatting according to network requirements.

This layer consists of two layers [5,6]:

  • 1. Physical Layer Convergence Protocol (PLCP): This protocol is responsible for communication with MAC layer and transforms data packet coming from MAC layer to OFDM frame.
  • 2. Physical Medium Access (PMD): It is a link between the physical transmission medium and fiber links.

Physical layer in DSRC is a modification in the existing IEEE standards to satisfy the vehicular network requirement. Challenges faced by the vehicular network are collision avoidance between vehicles, message formatting speed, and low latency requirement. IEEE 802.IIP, i.e., DSRC for VANET, states the latency of 50-100 ms. It improves the performance of physical layer and gives robust, scalable, and low latency in communication. Low latency avoids accidents and saves life in the vehicular environment. It also achieves a minimum bit error rate. Basic technical factors in communication like encoding, modulation, and unutilized subcarrier affect the performance of physical layer. Physical layer in VANET is critical and responsible for providing better performance of network.

IEEE 802.11P (MAC Layer)

MAC layer is present between data link layer and physical layer in OSI model. It provides easy control to the nodes for communication. MAC layer in IEEE 802.1 lp plays an important role in faster and efficient communication.

In VANET, vehicles and RSU transmit different messages. Messages are mainly classified as safety and nonsafety messages. Single channel is used to transmit all these messages, which make it difficult to guarantee quality of service. It may cause congestions in network due to less important messages and avoid important/emergency messages from transmitting. Safety messages in VANET play an important role in avoiding accidents, traffic jam situation, and other on-road situations. Performance of vehicular network will be affected due to single-channel use for different messages. As multiple users can access channel at the same time, there are chances of attack on messages. Solution to resolve the problem of safety and effective transmission of messages is multichannel MAC. Multichannel MAC facilitates nodes with different channels to communicate with each other. It makes access more easy and flexible. Vehicular network with multichannel MAC achieve better performance in the form of throughput and less delay than a single-channel network [7-10].

IEEE 1609.3 (Networking Services)

IEEE 1609.3 standard protocol is used to provide network services. It consists of two services: data plane and management plane services. Service includes two devices, namely, provider device that sends WSA messages to indicate its availability for use services of SCHs. Provider device is WSA service initiator; and user device that monitors the received WSAs. As per the availability of SCH channel, user device receives WSA and joins the service [11,12].

Data Plane Services

Data plane network services support two protocol stacks: WSMP and IPv6 protocol stacks.

I. WAVE Short Message Protocol (WSMP): WAVE Short Message Protocol (WSMP) is used to achieve time-efficient and priority- based message transmission. It is a WAVE network-layer unique protocol. When WAVE short message data units are received from upper layers, WSMP generates WSMP header and is added to receive unit, and makes packet transmission request to LLC.

LLC sets the ether type field value to encapsulate the packet and pass the data to the lower layers. When the LLC receives MAC data unit, it checks the ether type field value, then delivers it to either IPv6 or WSMP stack [11,12].

2. Internet Protocol Version 6 IPv6: WAVE standards support User Datagram Protocol (UDP) or Transmission Control Protocol or Internet Protocol. LLC sublayers are utilized to send and receive IP traffic.

Management Plane Services

  • WAVE Management Entity (WME): WME provides network management services to WAVE. It serves all service request of higher layers and channel assignments; monitors WAVE Service Announcements (WSA); configures IPv6 using data received from other WAVE devices; and maintains Management Information Base (MIB).
  • Management Information Base (MIB): MIB provides a channel- related information like all transmitter details in tabular form with their available timing info. It also has system configuration and system status information. Number of channels supported, advertiser identity, registration port, and WMS maximum length are part of system configuration information. Group of status tables, namely, Provider Service Request Table (PSRT), User Service Request Table (USRT), CCH Service Request Table (CSRT). and WSM Service Request Table (WSRT), is part of system status information. PSRT is table located in MIB and maintains provider information such as Provider Service Identifier (PSID) of the registered application, IPv6 address, port number, and so on. MIB consists of USRT for maintaining information such as user PSIDs, advertiser identifier, link quality, channel number, and so on; CSRT is responsible to manage CCH interval, request priority, and request status; and WSRT consists of PSIDs of the registered applications [11,12].
  • WAVE Services Request: WAVE service requests are of six types. It mainly includes services of adding, updating, and deleting. WME accepts the provider service requests which are generated by higher- layer entity and assigns the SCH access. It also triggers the MLME to start generating WSAs.
  • User Service Request: User service request is made by higher layer, when WME broadcasting for WSAs. If required SCH is available, it assigns SCH access. When a higher layer generates a WSM service request and shows an interest to receive WSM of a particular PSID, the WME accepts the request and ongoing of its monitoring; it delivers any received WSMs with matching PSID to the requested higher entity.
  • CCH Service Request: Whenever a higher layer requires ongoing CCH access during a particular interval for a WSM activity or WSA reception, CCH service request is generated, which will be considered by WME.
  • Management Data Service Request: WME accepts all the request of management data entity for vendor-specific action (VSA) frames. It assigns SCH to serve request and trigger the MLME to generate VSA frame.
  • Timing Advertisement Service Request: WME accepts the request from management data entity for timer advertisement service request and transmits ТА frame. WME assigns SCH or CCH and triggers the MLME to generate the ТА frame [11,12].
  • WSM Frame Format: Data structure used to send WAVE short messages is called as WSM frame. This frame consists of different fields, namely, version field WSMP which shows the version of the WAVE protocol, Provider Service Identifier field (PSID), Channel Number field which defines the channel that is used for communication, Data Rate field which specifies the data rate used in transmission, WAVE element ID field which represents WSMP header, WAVE Length field which determines length of the data field, and WSM Data field which contains the payload data, e.g., “Hello World.”
  • WSA Frame Format: Availability of service is announced by provider by sending WSA frame. This frame is also a data structure like other frames.

IEEE 1609.4 (Multichannel Operations)

Basic IEEE 802.11 is extended to this standard. It gives multichannel services to the vehicular environment. This standard is making possible to manage coordination of channel selection and maximum utilization of available spectrum. It avoids congestion and also makes available separate channel of high- est-priority messages.

Multichannel operations under IEEE 1609.4 standard in VANETs define a number of channels. Every channel has distinct applications and distinct characteristics, as shown in Figure 2.10. As shown in diagram, every channel not only uses distinct frequencies but also has different transmission powers.


A set of channels for multichannel operations in WAVE

FIGURE 2.10 A set of channels for multichannel operations in WAVE.

Channel used for safety-critical control messages has the highest potential transmission power, while channel used for noncritical control messages has less-priority applications and short-range safety.

Applications are provided/use channels with smaller allowed transmit powers. IEEE 1609.4 time multiplex division method is used to determine SCH and CCH. Every alternate time slot is given to CCH. SCH utilizes the remaining time slot as per the system requirement. The IEEE 1609.4 multichannel operation is implemented with priority scheme in IEEE 802.1 lp MAC layer. It is very similar to the IEEE 802.1 le. Channel has four access categories, denoted by AC0-AC3. AC3 has the highest priority among these categories. For the plain enhanced distributed coordination function (EDCA), frames with different data contents get placed in different queues. Internal contention procedure is applied to serve these contents, as shown in Figure 2.11.

Different channel and different access categories’ queues have different timer settings related to the internal contention procedure.

Internal contention process for 802.11pMAC

FIGURE 2.11 Internal contention process for 802.11pMAC.


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