Time division multiple access is a multiple access method for shared the channel by dividing the signal into different time slots. TDMA is successful works in cellular mobile communication for several years ago. Recently has been combined with OFDM to introduce OFDMA. TDMA also ensure fairness between nodes in the network. In vehicular scenario, we proposed TDMA protocol to work with CSMA/CA to mitigate and cope with some of the challenges in vehicular communications. In this chapter we will discuss the design of the protocol, connection messages, protocol flow and cross intercommunication between the new TDMA sublayer with CSMA/CA and PHY layers. In section 3.2 a general explanation of proposed TDMA protocol, the design of Wi-Fi-based 802.11p is discussed in Section 3.3. In Section 3.4, implementations of TDMA protocol in the simulation environment is presented. Simulation problems and implementation improvements is discussed in Section 3.5. The chapter summarization is given in Section 3.6.
4.2. EXPLANATIONS OF TDMA PROTOCOL
The TDMA protocol is representing as a provider client protocol, which means the protocol is centralized. The other possibility is to define a distributed or an ad hoc protocol as it is done in (Fan Yu, 2007) and (Katrin, 2009). We mean by centralized that the provider will be the only one handles the information that has given in both channels. This does not mean that all communication is going to be only unidirectional (from the provider to the client), but sometimes is going to be bidirectional communication. The provider in our case here is the RSU (Road Side Unit) and the client/station is OBUs (Onboard Units). Form now we may always use the term RSU to provider or centralized node and the OBU to client or mobile station.
Here we need to implement frame of 10 ms, those frame consist of two main time slots, one of them for the control and the second one for the service or data channel, and both are using different duration. Why we chose value of 10 ms because this is currently used in many TDMA implementations i.e. WIMAX.
4.3. DESIGN OF WI-FI BASED TDMA PROTOCOL
IEEE802.11 has two modes DCF and PCF. Distributed Coordination Function (DCF) relies on CSMA/CA distributed algorithm and an optional virtual carrier sense using RTS and CTS control frames (IEEE Std 802.11, 1999). If the channel is busy during the DIFS (DCF Interframe Space) interval, the station defers its transmission. Point Coordination Function (PCF) is used for “infrastructure” mode, which provides contention-free frame transfer for processing time-critical information transfers (W. Wang, 2003). PCF is optional in the standard and only few vendors implemented it in their adapters. Two different periods defined at PCF mode: contention-free period (CFP) and contention Period (CP). CFP uses contention free-poll frames to give stations the permission to transmit. However, PCF has many drawbacks and limitations in long distance applications (i.e. up to tens of kilometers) this due to sensitivity of the acknowledgement (ACK) messages to propagation delay which is designed for contention-free local area networks purposes. Also, once a station reserves the access to the medium, it may occupy the medium for long time without any station can interrupt its transmissions even in the high priority traffics case; i.e. if the remote station has lower data rate due to the distance, then it will take long time to release the channel (Pravin, 2003).
Consequently, it has been shown that (S. Sharma, 2002) (Sridhar, 2006) TDMA based MAC is suitable for long distance propagation delay. Most of the implemented solution for long distance Wi-Fi-based network was used WiMAX like TDMA frame for conducting the PMP scenario. However, using WiMAX/TDMA above Wi-Fi is increasing the system complexity and overhead since the WiMAX/TDMA has been built for the licensed-based and Wi-Fi is built with unlicensed environment. In this research a design of TDMA over the 802.11 is presented. The function of the proposed TDMA is to disable the contention behavior of 802.11 (CSMA/CA) for contention-less MAC. In this research a new cross layer design is introduced between CSMA/CA and new logical TDMA layer, which the Wi-Fi MAC frame is encapsulated in a logical TDMA header before forwarded to IP layer. The proposed protocol stack is shown in Figure4.1. The CSMA/CA peer-to-peer protocol is disabled and replaced with TDMA peer-to-peer protocol as shown with the dot-lines.
Figure.4.1. Protocol flow of the TDMA-based PMP
The logical TDMA header is added between IP header and MAC header. The function of the new header is to disable the random access feature of the CSMA/CA in 802.11 and replace it by logical TDMA function, which is maintains the synchronization of the local timers in the stations and delivers protocol related parameters. The frame is shown in Figure 4.2. The proposed TDMA header contains BCCH (broadcast control channel), FCCH (frame control channel) and RA (random access).
BCCH: contains general information i.e. timestamp through time_stamp_update(), SSID, BS-node capabilities and random access time interval ra_interval(). All this parameters (except the RA time interval) is prepared and copied from the beacon frame (using beacon_content()) from the Wi-Fi MAC device driver. The BCCH information helps the APs in the sleep, wakeup, transmitting and receiving times.
Figure.4.2. Additional TDMA header is added to Wi-Fi frame
FCCH: carries the information about the structure and format of the ongoing frame i.e. scheduler () and time_slot_builder(); containing the exact position of all slots and Tx/Rx times and guard time between them and scheduling.
RACH: contains a number of random access channels (RCH). This field is uses when no schedule has been assigned to the APs in the UL fields. Non-associated APs use RA for the first contact with an AP using slot_time_request(). The flow diagram of logical control and data channels is shown in Figure 4.3.
Figure 4.3: the flow of the virtual channels for the TDMA frame,
First, the RACH frame is receiving if there any connection request from APs to BS. Then, BCCH, FCCH and AGCH broadcast their information, then transmit and receive user’s payload. Timer is controlling all the transmitted and received signals. Although, the new TDMA header is introduced at the cost of the performance due to the overhead, however, in the long distance applications with point-to-multiple-point infrastructure scenarios usually the numbers of stations are not too high compared with end-user part. In our scenario we consider 4 remote access points and one central access point (BS-node). By implementing TDMA_module() each APs would assigned with time slot within the TDMA frame. TDMA also saves power because each STA only needs to wake-up during these time slots in each frame. If new node (AP) wants to join the network it listens to the BCCH frame to get the initial parameters from the BS-node. Then it uses the RA period to send time_slot_request() request to the BS-node to request for time slot. The BS-node uses the FCCH field to update the new scheduling table in scheduler(). The TDMA_module() assigns time slots for APs by taking copy of the NAV (network allocation vector) information (NAV_update()) from the Wi-Fi MAC layer and modifying it according to the schedule scheme. NAV is considered as virtual carrier sensing which is limits the need for contention-based physical carrier sensing. This is done by setting new back_off_counter() and NAV_new() in the TDMA_module() which indicates the amount of time that medium will be reserved for each time slots. The BS-node set the NAV value to the frame length time plus any other necessary messages to complete the current operation to make sure that no station (AP) will access the channel during the frame transmission. Other stations count down from the NAV to 0. When the NAV has nonzero value, the scheduler () send back to the Wi-Fi MAC that indication that the medium is busy; before the NAV reaches 0, the back_off() and NAV_new() update the Wi-Fi MAC with the new NAV. The destination address (DA) and source address (SA) in the MAC frame header and in the SSID is modified according to the new NAV and RR scheduling information. Figure4.4 shows illustrate the flow of the process in cross-layer concept, which is consisting of three layers: TDMA source code, wireless driver and hardware abstraction layer (HAL). The cross layer is performed between wireless driver and the source code. HAL is different for each hardware platform. The procedure of this approach is also below: