CHAPTER 4 CROSS LAYER INTERACTION

Transcription

1 38 CHAPTER 4 CROSS LAYER INTERACTION The cross layer interaction techniques used in the lower layers of the protocol stack, solve the hidden and exposed terminal problems of wireless and ad hoc networks. All these techniques used in the lower layers improve not only the lower layer functionalities, but also the TCP congestion control mechanisms in wireless networks. The cross layer interaction i.e. the combination of TCP-AL with IEEE PHY and MAC is known to be TCP-WPAL. 4.1 IEEE PHY The IEEE PHY provides two services: the PHY data service and the PHY management service, interfacing the physical layer management entity (PLME). The PHY data service enables the transmission and reception of the PHY protocol data units (PPDU) across the physical radio channel. The functions of the PHY are the activation and deactivation of the radio transceiver energy detection (ED), link quality indication (LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets across the physical medium. The receiver energy detection (ED) measurement is intended for use by a network layer, as part of the channel selection algorithm. It is an estimate of the received signal power within the bandwidth of an IEEE channel. No attempt is made to identify or decode signals on the channel. The ED time should be equal to 8 symbol periods. Upon reception

2 39 of a packet, the PHY sends the PSDU length, the PSDU itself, and the link quality (LQ) in the PD-DATA indication primitive. The LQI measurement is a characterization of the strength and/or quality of a received packet. The measurement may be implemented using the receiver ED, a signal-to-noise estimation or a combination of these methods. The use of the LQI result is up to the network or application layers. A clear channel assessment (CCA) is performed according to at least one of the following three methods: Energy above threshold: the CCA shall report a busy medium upon detecting any energy above the ED threshold. Carrier sense only: the CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of the IEEE This signal may be above or below the ED threshold. Carrier sense with energy above threshold: the CCA shall report a busy medium only upon the detection of a signal with the modulation and spreading characteristics of the IEEE , with energy above the ED threshold. Each PPDU packet consists of the following basic components: the SHR, which allows a receiving device to synchronize and lock into the bit stream the PHR, which contains frame length information a variable length payload, which carries the MAC sublayer frame.

3 IEEE MAC The IEEE MAC sublayer provides two services: the MAC data service and the MAC management service interfacing to the MAC sublayer management entity (MLME) service access point (SAP) (MLMESAP). The MAC data service enables the transmission and reception of the MAC protocol data units (MPDU) across the PHY data service. The features of the MAC sublayer are beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association and disassociation. Figure 4.1 Superframe Structure The IEEE allows the optional use of a superframe structure, which is shown in Figure 4.1. The format of the superframe is defined by the coordinator. The superframe is bounded by the network beacons and is divided into 16 equally sized slots. The beacon frame is sent to the first slot of each superframe. If a coordinator does not want to use the superframe structure, it may turn off the beacon transmissions. The beacons are used to synchronize the attached devices, to identify the PAN and to describe the structure of the superframes.

4 41 The superframe can have an active and an inactive portion. During the inactive portion, the coordinator shall not interact with its PAN and may enter a low-power mode. The active portion consists of a contention access period (CAP) and a contention free period (CFP). Any device wishing to communicate during the CAP shall compete with other devices using a slotted CSMA/CA mechanism. On the other hand, the CFP contains guaranteed time slots (GTSs). The GTSs always appear at the end of the active superframe, starting at a slot boundary immediately following the CAP. The PAN coordinator may allocate up to seven of these GTSs, and a GTS can occupy more than one slot period. The CFP, if present, shall start on a slot boundary immediately following the CAP, and extend to the end of the active portion of the superframe. The length of the CFP is determined by the total length of all the combined GTSs. No transmissions within the CFP shall use a CSMA-CA mechanism. A device transmitting in the CFP shall ensure that its transmissions complete one IFS period before the end of its GTS. IFS time is the amount of time necessary to process the received packet by the PHY. Transmitted frames shall be followed by an IFS period. The length of the IFS depends on the size of the frame that has just been transmitted. Frames of up to amaxsifsframesize in length shall be followed by a short interframe space (SIFS), whereas frames of greater length shall be followed by a Long interframe space (LIFS). The duration of different portions of the superframe are described by the values of macbeaconorder and macsuperframeorder. macbeaconorder describes the interval at which the coordinator shall transmit its beacon frames. The beacon interval, BI, is related to the macbeaconorder, BO, as follows:

5 42 abasesuperframeduration, BI=2 BO, 0 BO 14 (4.1) The superframe is ignored if BO = 15. The value of macsuperframeorder describes the length of the active portion of the superframe. The superframe duration, SD, is related to macsuperframeorder, SO, as follows: ABaseSuperFrameDuration, SD=2 SO, 0 SO 14 (4.2) If SO = 15, the superframe should not remain active after the beacon. The PANs that do not wish to use the superframe in a nonbeaconenabled shall set both macbeaconorder and macsuperframeorder to 15. In this kind of network, a coordinator shall not transmit any beacons, all transmissions except the acknowledgement frame shall use the unslotted CSMA-CA to access the channel. GTSs shall not be permitted. 4.3 CROSS LAYER INTERACTION Cross layer interaction means that the TCP-AP which is the concentrating the link layer and, LRED which is the concentrating data link layer techniques, are combined with the IEEE PHY and IEEE MAC. Cross layer interaction exploits the dependencies and interactions between layers to increase the performance in certain scenarios of wireless networks. Cross layer interaction does the sharing of knowledge about the layer state and conditions, which are a promising paradigm for performance optimization in wireless systems. It also provides knowledge about the channel conditions of PHY and MAC to routing, transport and application layers, which allow to design more sophisticated allocation and optimization algorithms.

6 43 The proposed work combines the concepts of the TCP-AP, LRED with the IEEE WPAN. Here, the Cross layer interaction is achieved as the TCP-AP which is the concentrating link layer, and LRED which is the concentrating data link layer techniques are combined with the IEEE PHY and IEEE MAC. In the Data link layer, the Link Random Early Discard (LRED) technique seeks to react earlier to link overload, and it solves the hidden terminal problem. The LRED algorithm is developed based on the observation that the TCP can potentially benefit from the built-in dropping mechanism of the MAC. The main idea is to further tune up the wireless link s drop probability, based on the perceived link drops. While the wired RED provides a linearly increasing drop curve as the queue exceeds a minimum value min_th, the LRED does so as the link drop probability exceeds a minimum threshold. In the link layer, the adaptive pacing technique seeks to improve spatial reuse and it solves the exposed terminal problem. In the current protocol, a node is constrained from contending for the channel, by a random backoff period, plus a single packet transmission time, that is announced by its immediate downstream node. However, the exposed receiver problem persists, due to lack of coordination between nodes that are two hops away from each other. Adaptive pacing solves this problem, without requiring nontrivial modifications to the , or a second wireless channel. The basic idea is to let a node further back-off, an additional packet transmission time when necessary, in addition to its current deferral period (i.e., the random backoff, plus one packet transmission time). This extra backoff interval helps in reducing contention drops caused by exposed receivers, and extends the range of the link-layer coordination from one hop to two hops, along the packet forwarding path.

7 44 In the Physical layer and MAC layer, the WPAN physical layer (IEEE PHY) and MAC enhancement (IEEE MAC) had been employed to interact with the network layer and application layer. The main features of IEEE are network flexibility, low cost, very low power consumption, and low data rate in an adhoc self-organizing network, among inexpensive fixed, portable and moving devices. It is developed for applications with relaxed throughput requirements which cannot handle the power consumption of heavy protocol stacks. The routing algorithm can be thought of as a hierarchical routing strategy with table-driven optimizations applied where possible. The routing layer is said to start with the well-studied public domain algorithm Ad hoc On Demand Distance Vector (AODV) and Motorola s Cluster-Tree algorithm. Here, the data link layer, Network layer, MAC layer, Application layer and Physical layer are modified, to improve the performance of the transport layer protocol. Because of the concentration in various interlinked layers improves the performance of the TCP in wireless networks, is improved. 4.4 ANALYSIS OF CROSS LAYER INTERACTION Normally, the TCP timer management maintains a variable RTT (Round Trip Time) by R = R TT new TTold + (1- ) M (4.3) where M is the time taken for receiving the acknowledgement and typical value of = 7/8.

8 45 Whenever the acknowledgement comes in, the difference between the expected and observed values R TT -M is computed and the deviation, D is calculated as Difference, D = D + (1- ) R TT -M (4.4) and the Timeout = R TT + 4 * D (4.5) whereas in LRED that works in link layer, by monitoring a single parameter, the average number of retries in the packet transmissions at the link-layer, accomplishes the above said three goals. Retry values are computed by the function GetMacRetries and the average retry value is computed as: avg_retry new = 7/8 avg_retry old + 1/8 retry (4.6) With the use of avg_retry, the mark probability value is changed by using mark_prob = min { avg _ retry min_ th, max_p} (4.7) max_ th min_ th In a multihop wireless network, it is the link-layer contention induced packet loss, that offers the first sign of network overload. The drop probability will decide the average TCP window size at which TCP stabilizes eventually. Network overload actually has different implications in the multihop wireless context. Because of the nature of wireless networks, the consideration of the drop probability and counting the retries will improve the performance of TCP. Not only considering the RTT which is done in traditional TCP, avg_retry and mark_probability are considered to improve the performance of

9 46 TCP in wireless networks since in wireless networks packets can be received by the destination by retransmitting the packets whenever the nodes are having hidden terminal and exposed terminal problem. Also pacing is provided when avg_retry < min_th. Pacing means providing extra time than the normal wait time to receive ACK as shown below. This change in time interval reduces the packet loss when the nodes are in mobile nature. When pacing ON, extra Backoff = TX Time(DATA) + overhead (4.8) backoff random Backoff + extra Backoff (4.9) In IEEE , a frame transmitted with the acknowledgement request field set to 1 shall be acknowledged by the recipient. If the intended recipient correctly receives the frame, it shall generate and send an acknowledgement frame containing the same Sequence Number from the data or MAC command frame that is being acknowledged. The transmission of the ACK shall commence between RTT and RTT + aunitbackoffperiod symbols after the reception of the last symbol of the data or MAC command frame. Because of the increase in the time interval for symbols improves the performance of TCP by PDR gets increased and delay gets decreased. Because these new parameters are included in calculation, the time complexity will be increased. This can be minimized by applying the concept of the IEEE , especially in the MAC and Physical layer. The main features of this standard are network flexibility, low cost, very low power consumption, and low data rate in an adhoc self-organizing network, among inexpensive fixed, portable and moving devices. Large size packets are divided into some number of smaller size packets that are transmitted to the destination, so that the Packet Delivery Ratio (PDR) is increased. It is developed for applications with relaxed throughput requirements, which

10 47 cannot handle the power consumption of heavy protocol stacks. This is the motivation for the Cross Layer Interaction. In this work, cross layer interaction improves the performance of the TCP by the modification on the traditional TCP by the various parameters min_th, avg_retry, mark_probability, extra backoff and backoff period per symbol. Those parameters are considered in various layers, i.e., data link layer, link layer, network layer, MAC layer and physical layer, and prove that the cross layer interaction improves the performance of the TCP in wireless networks. 4.5 CONCLUSION The cross layer interaction in the TCP improves its performance considerably in wireless networks. The concentration is given in various lower layers, with the use of a minimum number of parameters. The complexity involved in the inclusion of these parameters does not affect the TCP performance. All the lower layers support the transport layer to work efficiently. Even though the cross layer interaction minimizes the TCP congestion control, it has some limitations, i.e., reducing the throughput of the transmission. The next chapter deals the performance of TCP-AL and TCP-WPAL analytically.