Other important element in wireless network is the power consumption, indeed energy efficiency of communication networks is becoming an increasingly important topic, especially with the expected explosion of traffic to be carried by those networks. In order to validate the proposed approach, we also compare the WMP performance with the benchmark provided by the native Broadcom’s firmware in terms of power consumption. For this features we work together with NITlab group, the NITlab proposes NITOS, a Energy consumption Monitoring Framework (EMF) able to support online monitoring of energy expenditure, along with the experiment execution [21]. We run some tests to measure the power consumption of the different MAC versions discussed in our previous section. The experimental scenario consists of 2 NITOS Icarus nodes, attached with the BCM4311 m-PCIe chipset. We generate saturated traffic for 15 secs and measure the power consumption at the NIC level for the transmitter node. Power consumption is monitored for 25 secs, just to make sure that all frame transmissions are properly captured.
More specifically, we compared the power consumption of 5 different MAC versions: • Original Firmware
• DCF-master WMP state machine
• TDMA WMP state machine with time frame 10ms • TDMA WMP state machine with time frame 0.1ms
In the figure 7.13 we plot the average power consumption of each scheme. We confirm that DCF state machine implementation (DCF-Master - 0.9417 W) consumes even less amount of Power, compared to the standard firmware implementation (Original Firmware - 1.0127 W).
(a) (b)
Figure 7.11: DLS parameters 7.11(a), DCLS parameters 7.11(b)
Figure 7.12: DL setup compairson
Chapter 8
High level flexibility
8.1
Introduction
The 802.11 standard specifies a common medium access control (MAC) Layer, which provides a variety of functions that support the operation of 802.11-based wireless LANs. In general, the MAC Layer manages and maintains communications between 802.11 stations (radio network cards and access points) by coordinating access to a shared radio channel and utilizing protocols that enhance communications over a wireless medium. The basis function performed from the MAC layer is the
Medium access, defined the technique to shared the medium. We have shown the basis function
in the chapter 1, and in other previous chapters we present the motivation to required flexibility in the MAC layer medium access function, then we showed the own solution based on WMP platform [61]. Other functions performed by MAC layer are:
• Scanning: In this function, un-associated STAs (e.g., STAs just turned their power on) scan
the medium and listen to the beacons of the available APs (passive scanning). The 802.11 standard defines both passive and active scanning; Passive scanning is mandatory where each NIC scans individual channels to find the best access point signal. Periodically, access points broadcast a beacon, and the radio NIC receives these beacons while scanning and takes note of the corresponding signal strengths. The beacons contain information about the access point, including service set identifier (SSID), supported data rates, etc. The radio NIC can use this information along with the signal strength to compare access points and decide upon which one to use. Optional active scanning is similar, except the radio NIC initiates the process by broadcasting a probe frame, and all access points within range respond with a probe response. Active scanning enables a radio NIC to receive immediate response from access points, without waiting for a beacon transmission. The issue, however, is that active scanning imposes additional overhead on the network because of the transmission of probe and corresponding response frames.
• Authentication: Authentication is the process of proving identity, and the 802.11 standard
specifies two forms: Open system authentication and shared key authentication. We report a detail of this process in the section 8.3.
• Association: Once authenticated, the radio NIC must associate with the access point be-
fore sending data frames. Association is necessary to synchronize the radio NIC and access point with important information, such as supported data rates. The radio NIC initiates the association by sending an association request frame containing elements such as SSID and supported data rates. The access point responds by sending an association response frame containing an association ID along with other information regarding the access point. Once the radio NIC and access point complete the association process, they can send data frames to each other.
• RTS/CTS: The optional request-to send and clear-to-send (RTS/CTS) function allows the ac-
cess point to control use of the medium for stations activating RTS/CTS. The use of RTS/CTS alleviates hidden node problems, that is, where two or more radio NICs can’t hear each other and they are associated with the same access point. If the radio NIC activates RTS/CTS, it will first send a RTS frame to access point before sending a data frame. The access point will then respond with a CTS frame, indicating that the radio NIC can send the data frame. With the CTS frame, the access point will provide a value in the duration field of the frame header
that holds off other stations from transmitting until after the radio NIC initiating the RTS can send its data frame. This avoids collisions between hidden nodes.
• Power Save Mode: The optional power save mode that a user can turn on or off enables
the radio NIC to conserve battery power when there is no need to send data. With power save mode on, the radio NIC indicates its desire to enter "sleep" state to the access point via a status bit located in the header of each frame. The access point takes note of each radio NIC wishing to enter power save mode, and buffers packets corresponding to the sleeping station. In order to still receive data frames, the sleeping NIC must wake up periodically (at the right time) to receive regular beacon transmissions coming from the access point. These beacons identify whether sleeping stations have frames buffered at the access point and waiting for delivery to their respective destinations. The radio NICs having awaiting frames will request them from the access point. After receiving the frames, the radio NIC can go back to sleep. The MAC layer together with the Logical Link Control (LLC) composed the Data-Link layer, this layer initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully.
The layer above the Data-Link layer is the network layer, it is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for me- dia access control, flow control and error checking. The network layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions. Functions of the network layer include the connection model, the host addressing and the message forwarding. Host addressing is important function, because every host in the network must have a unique ad- dress that determines where it is. This address is normally assigned from a hierarchical system. On the internet, addresses are known as Internet Protocol (IP) addresses, because managed from the IP protocol. The IP is responsible for addressing hosts and for routing datagrams (packets) from a source host to a destination host across one or more IP networks. A precise protocol is been developed to assign automatically the IP address, it name is Dynamic Host Configuration Protocol (DHCP) and allows a computer to join an IP-based network without having a pre-configured IP address. DHCP is a protocol that assigns unique IP addresses to devices, then releases and renews these addresses as devices leave and re-join the network.
Also, in the other functions of the Data-Link layer and in the network layer we can add flexibility to improve the feature for the network nodes. This could lead to a quick and clever adaptation of the nodes, when the environment changing dynamically.
In the traditional network, protocol stacks are logically organized in layers. These layers are strictly separated, and the cooperation between them is restricted by concise interfaces, which the application work only a single layer. In principle, all these layers have been designed to fulfill their functionality without interaction across the layers. History shows that this works well in wired and static environments. However, today’s and upcoming networks consist of wireless links and highly mobile nodes. In order to adapt to the rapidly and frequently changing network conditions under those circumstances, a more sophisticated interaction between protocols than in a traditional layered architecture is desirable.
For example, the vehicular network are a system in which the nodes work in environment that changing mostly rapidly, in this network access can be obtain roadside installed Internet Gate- ways(Access Point) or from other vehicles. However, several difficulties must be addressed in such a scenario, the speed of the vehicles reduce the time to exchange the informations. For this rea- son is needed that all operation for connection set-up are really fast. In normal WLANs system
the connection set-up phases are handled by different application and different protocol level, the set-up connection process is not optimized, because several actors work not synergically for the process. In generally, all these programs are developed for a specific part of the process, every level not interact with other layers.
If we consider the network vehicular example above, current solutions are not able to dynam- ically change of the environment i.e., adaptation of connection set-up phase at different environ- ment during runtime. Moreover, an optimizations is possible through a cross-layer frameworks that manage synergically the connection set-up at different levels.
We propose new dynamic association and reassociation procedure that manage the association for both layers, data-link layer and network layer, the own solution is based on a monitoring status of the presence of AP in the environment. We presentwpa_fast , a unique program that handled
wpa_fast is a system to optimize the connection set up time in vehicular network, reduce the
connection set-up time, and improve the time that the node use for exchange the information, in this chapter, we characterize the WiFi connection set-up process and we are focusing on technique to improve the connection set-up. After, we show the results of a measurement evaluation of
wpa_fast in different environments.
For this framework we work carried out with the Network Research Lab (NRL) at UCLA Com- puter Science Department. The NRL supports research projects in a broad range of topics in net- work communications including, car-to-car networks, VANETs and vehicular network.