A Survey of Energy efficient Network Protocols for Wireless Networks Presentation by – Sanjay Acharya Course – CS 898T Instructor – Dr. Chin-Chih Chang

Topics Covered… Need for saving power in wireless networks. Sources of power consumption in hardware. Power consumption in wireless network operation. The network architecture considered… Power conservation techniques in various layers. Conclusion.

The need for saving power in wireless networks. Wireless devices have become a necessity and play an important role in wireless communication. Wireless technology has in recent years grown in leaps and bounds. Their source of power supply?? An integrated source of power supply, a battery. Technology w.r.t battery has not improved much since many years as compared to other areas. The battery requires frequent charging from an external source. Thus power conservation is a necessity for continuous operation of the wireless device.

Sources of power consumption. A lot of research has already been done in designing power efficient hardware. Some more power can be saved by designing energy efficient network protocols. Source%Consumption Display36 CPU/Memory21 Wireless Interface18 Hard Drive18

Sources of power consumption in wireless network operation. Power consumption in wireless network operation can be broadly classified as, –Communication related. This involves the transceiver. Transmitter sends control & data packets while the receiver receives them. The radio may be in 3 states viz. transmit, receive and standby. Typical consumption can be 1.5W, 0.75W and 0.01W respectively. The goal should be to optimize transceiver usage. –Computation related. This is related to protocol processing and involves the CPU and memory e.g. Data compression. Data compression reduces packet length thus saving energy but may increase the computation power. Techniques to reduce communication cost might increase computation costs. Hence a balance between the two needs to be established.

The network architecture considered… Infrastructure wireless network Ad hoc wireless network

Physical Layer. Most of the recent research has been related to this layer. Not much advancement done w.r.t battery. Some of the techniques are, –Variable clock speed cpu. energy/task α voltage 2 If voltage can be increased or decreased proportional to speed then, energy/task α speed 2 –Use flash memory and disk spin down. By using flash memory to cache files in memory instead of hard drive, the hard drive can be stopped from spinning. Power can be saved up to 30%. One of the complexities is asynchronously updating the hard drive blocks with t he flash memory.

Data Link Layer (Mac Sublayer) The MAC layer is responsible for providing shared access to the wireless channels. Some techniques for power conservation are, IEEE –Node wishing to sleep informs the base station about it by setting power management frame bits. –The base station then buffers all the data packets for that node. –The node also periodically listens to beacon signals from the base station. –The beacon also includes “Traffic Indication Map” (TIM). TIM includes the nodes for which the base station has buffered data. –If TIM indicates positive, the node responds to the base station which then forwards the packets to the node.

Data Link Layer EC MAC protocol (Energy Conserving MAC Protocol). –A reservation and scheduling based protocol. –Transmission frame is divided into a number of phases. –FSM (frame synchronization message) is transmitted by base station for synchronization and transmission order for request/update phase. –In the request/update phase, each mobile transmits (transmission order defined in FSM) new connection requests. –In the “New User Phase” new mobiles register with the base station. This is a contention phase and uses SLOTTED ALOHA. –In the “Schedule Message” phase, base station broadcasts slot permissions for the data transmission phase. –Energy consumption is considerably reduced by the use of a centralized scheduler. Collisions are prevented which reduces retransmissions and also transceivers may be switched off as the data phase is scheduled between the different mobiles.

Data Link Layer PAMAS (Power aware multi-access protocol). –This modifies the MACA protocol by providing separate data and signaling channels. –When a mobile needs to transmit data it sends a RTS. If it does not receive a CTS, it goes into a back off state. –If a CTS is received, the mobile sends data while the receiver emits a busy tone. –The other neighboring mobile nodes on receiving the busy tone, refrain from any transmissions and go off to sleep. –The neighboring nodes emit periodic probes to which the transmitter responds with packet containing time ‘t’ when the transmission will end. The nodes then can go off to sleep for time ‘t’.

Data Link Layer (LLC layer) LLC Layer. –LLC layer provides a reliable connection over the MAC layer in addition to flow and error control. –Normally in LLC, error is handled by retransmissions but these lead to energy consumption. Adaptive error control with ARQ On error, probes are sent periodically till a correct ACK received. Data transmission resumes after an ACK is received. Method superior to normal ARQ and saves energy by reducing retransmissions.

Network Layer The main task is routing of packets. Shortest route is A-E-D causing excessive energy drainage of E. If E fails, then network partitions. Protocol should not consider shortest routes but route packets on an energy perspective. Unicast Traffic. Protocol design includes a number of metrics like, Minimize energy consumed per packet. Short hops cost less while long hops cost more. Maximize time to partition of network. Route packets in such a way that load is shared equally among the nodes participating in the partition. Same priority of operation of mobiles so that all mobiles remain powered for as long as possible. Packet cost: Do not route packets through low power mobile routes. Goal is to consider these metrics in determining ideal routes for packets.

Network Layer Multicast traffic. Traffic destined to multiple receivers. Flooding algorithm cannot be used because of unnecessary retransmissions leading to energy loss. Good design is to use power-aware broadcast tree. –Tree constructed starting from source. –Give priority to nodes that consume less energy and have high outgoing degree. –Simulations indicate power savings of up to 20% than flood based routing.

Transport Layer Transmission control protocol most commonly used transport protocol for wired networks. This cannot be used for wireless networks because of retransmissions. Confusion of handover and link errors to channel congestion leading to retransmissions and thus energy loss. In order to minimize retransmissions the following techniques are used, Split Connection protocol. Split TCP over wireless link Use modified version of TCP for wireless part. e.g. Berkeley snoop module

Transport Layer Link Layer Approach Local transmissions tuned for wireless networks. Hide errors from TCP. Link layer can use FEC to minimize retransmissions. End to End protocol approach Use modified versions of TCP. Selective acknowledgements and explicit loss notification scheme used (ELN). Using ELN identifies the reason for packet loss.

Transport Layer TCP probing. On error stop data transmissions. Send probes to receiver. If sender receives for 2 RTT’s, then resume transmission If there are random errors, then resume transmission If there are persistent errors then invoke congestion control mechanisms.

OS/Middleware & Applications Function of OS is to manage physical resources. Under low power circumstances, the operating system can use a reduced cpu voltage. When the system is idle, shutdown the mobile system. Use page replacement algorithms for power aware DRAM chips. Such a memory chip can exist in a number of power states like “active”, “standby”, “nap” and “power down”.

Applications –Load partitioning: Power intensive part of application executed at base station –Proxies: Such agents adapt to changes in power condition. Stop video transmission and allow only audio. –Video Transmission: Use low bit rate video for simple encoding and computation, thus saving energy. Discard few packets while maintaining tolerable video quality.

Conclusions A battery has a limited source of supply and not enough progress has been made. There is a lot of scope in optimizing network protocols for reducing power consumption. This area has immense potential for research till an effective solution is developed for lasting energy source.

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