MAC Layer-Wireless Networks-Lecture Slides, Slides of Wireless Networking

Download MAC Layer-Wireless Networks-Lecture Slides and more Slides Wireless Networking in PDF only on Docsity! 2 Outlines  MAC Layer ► Scalability ► Single Channel ► Multi-Channel ► Some Ideas ► Research Issues  Network Layer ► Routing ► Wish List ► Route Optimization Criteria ► Routing fairness ► Routing – Cross-layer design  QoS Support at each layer  WMN Standards docsity.com 3 Last Lecture  Introduction to WMN  Characteristics  WMN vs MANET  Architecture  Applications  Critical factors influencing performance ► Radio techniques, scalability, QoS, security, Ease of Use, Mesh connectivity docsity.com 6 Basic Techniques  Scheduled ► Fix scheduled TDMA ► Polling ► Impractical due to lack of: • Central coordination point • Reasonable time synchronization  Random Access ► CSMA – simple and popular ► RTS/CTS – protects the receiver docsity.com 7  Channels can be implemented by: ► FDMA ► CDMA (code assignment is an issue) ► SDMA (with directional antennas) ► Combinations of the above s2 s3 s1 f t c f t c f t c f t c f t c f t c f t c docsity.com 8 Single channel MAC  Improving existing MAC protocol ► By changing parameters of CSMA/CA based MACs like contention window size and backoff procedures for multi-hop. ► Contention based approaches are not scalable and throughput degrades with increase in contention.  Cross-layer design with advanced physical layer. ► MAC based on directional antenna and power control  Innovative solutions ► Need new ideas to overcome low end-end throughput for multi-hop ad hoc environment. ► TDMA or CDMA based MAC needs to be explored. ► Compatibility, cost and complexity are the important factors in designing new protocols docsity.com 11 Multi-channel MAC (MMAC)  Multi-channel single transceiver ► One channel active at a time ► Different nodes may operate on different times. ► Hence, coordination is required  Multi-channel multi-transceiver ► A radio includes multiple parallel RF chips and baseband processing modules to support several simultaneous channels. ► On top of multiple channel in physical layer, only single MAC to coordinate operations.  Multi-radios MAC ► A node has multiple radios each with its own MAC and physical. ► A virtual MAC protocol such as a multi-radio unified MAC protocol is required to coordinate communication among all. docsity.com 12 MMAC Functions  Maintaining data structure of all channels in each node. ► classified into three types depending on its status of allocation.  Negotiating channels during ad hoc traffic indication message (ATIM) window. ► Negotiations are done through a pre-defined channel known to all nodes.  Selecting a channel. ► The criterion is to use a channel with the lowest count of source–destination pairs that have selected the channel. docsity.com 13 Multi-radio Unification Protocol  Discovering neighbours. ► After the discovering procedures, neighbors are classified into MUP enabled and legacy nodes.  Selecting a NIC ► based on one-hop round trip time (RTT) measurements. MUP selects the NIC with the shortest RTT between a node and its neighbors.  Utilizing the selected NIC for a long period. ► This period is determined by a random process and in the order of 10–20 s.  Switching channels. ► After the random time period, all NICs are measured again through one-hop probe messages. If an NIC has a certain amount of quality improvement than the existing NIC, then it is selected for sending packets. docsity.com 16 Routing  Finds and maintains routes for data flows  The entire performance of the WMN depends on the routing protocol  May be the main product of a mesh company  May be missing docsity.com 17 Routing – Wish List  Scalability ► Overhead is an issue in mobile WMNs.  Fast route discovery and rediscovery ► Essential for reliability.  Mobile user support ► Seamless and efficient handover  Flexibility ► Work with/without gateways, different topologies  QoS Support ► Consider routes satisfying specified criteria  Multicast ► Important for some applications (e.g., emergency response) docsity.com 18 Existing Routing Protocols  Internet routing protocols (e.g., OSPF, BGP, RIPv2) ► Well known and trusted ► Designed on the assumption of seldom link changes ► Without significant modifications are unsuitable for WMNs in particular or for ad hoc networks in general.  Ad-hoc routing protocols (e.g., DSR, AODV, OLSR, CBR, TORA) ► Newcomers by comparison with the Internet protocols ► Designed for high rates of link changes; hence perform well on WMNs ► May be further optimized to account for WMNs’ particularities Ad Hoc Networks Wireless Mesh Networks docsity.com 21 Routing – Cross-Layer Design (cont)  Routing – Transport ► Choosing routes with low error rates may improve TCP’s throughput. ► Especially important when multiple routes are used ► Freezing TCP when a route fails.  Routing – Application ► Especially with respect of satisfying QoS constraints docsity.com 22 Network Layer - Fairness  Fairness ► Equal share of resources to all participants. ► Special case of priority based QoS.  Horizontal – nodes 1, 2 ► The MAC layer’s fairness ensures horizontal fairness.  Vertical – nodes 3, 4 ► MAC layer is no longer sufficient GW 1 2 GW 3 4 docsity.com 23 Fairness Problem Ideal Real  Unfair  Inefficient GW G G S1S2 12 docsity.com 26 WMNs Standards  IEEE 802.16a WMAN Mesh ► “mesh mode” in addition to the point-to-multipoint(PMP) mode defined in IEEE 802.16. ► Operating in the licensed and unlicensed lower frequencies of 2– 11 GHz, allowing non-line-of-sight (NLO) communications, spanning up to a 50 km range. ► Supporting multihop communications. docsity.com 27 WMNs Standards  802.11s WLAN Mesh ► MAC layer needs to be extended to a wireless DS to support broadcast/multicast ► Multi-hop capability added to 802.11g/a/b ► Auto configure on power up ► Multi-channel multi-radio operation ► Topology discovery ► MAC Path selection protocol ► Modified forwarding for QOS and mesh control docsity.com VWMNs Standards > 802.11s MCF Sublayer Mesh interworking with other 802 networks Mesh topology learning, routing and forwarding Mesh . 2 ement Medium access SNe ea coordination Mesh security Discovery and association 802.11 service integration Mesh configuration and management PHYs 28 docsity.com