Control Access to Shared Medium - Lecture Slides | ENEE 426, Study notes of Organizational Communication

Download Control Access to Shared Medium - Lecture Slides | ENEE 426 and more Study notes Organizational Communication in PDF only on Docsity! Centralized MAC and Switching ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Multiple Access • Infrastructure: – Control access to uplink • Distributed: – Control access to shared medium – Distributed networks are half-duplex – No central controller -> users = network Slide # 2 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Token Ring Performance • Deterministic – Same amount of time to transmit every packet – Finite maximum amount of time between transmissions • Compared to CSMA/CD – Better overall performance – No chance of collisions – But more overhead to maintain tokens • Switched Ethernet put Token Ring out of business Slide # 5 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Quality of Service • Each node has a priority • To transmit – If received token/data frame has higher priority already, wait for next token – Otherwise change priority to match yours – If empty token arrives with your priority, transmit – When token returns, reset priority to original value Slide # 6 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 FDDI and RPR • FDDI was fiber version of token ring – Fiber Distributed Data Interface • Initial interest, but died out • RPR is ring-like topology for SONET/SDH – Resilient Packet Ring (IEEE 802.17) • Used in some MANs Slide # 7 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Comparison • FDMA – Typically requires fixed-width channels – Good for circuit switched networks – Too few users, wasted capacity – Too many users, some users receive no service • TDMA – Size of time blocks typically fixed – Vary who gets assigned to which blocks when – Users get 1/n of the available bandwidth – If underutilized, others can increase speed • CDMA – Power levels fixed – More users means more interference, so more coding necessary, reducing overall rate Slide # 10 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Switched Networks • Virtual Circuits – Connection-oriented communications on packet-switched network – Network preconfigured with identifiers for source-destination pairs – Virtual Circuit IDs (VCIs) – Packets transmitted with virtual circuit ID in the header – Devices map this VCI to physical interfaces as the packet transits to properly route it • Good for QoS – Can guarantee enough capacity to support all the VCs at particular rate • Bad for dynamic networks – Overhead associated with setup/teardown of VCs Slide # 11 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Switched Networks • Source Routing – Each packet includes a list of each intermediate switch, so switch knows how to transmit packet through the network – Avoids setup overhead, but no QoS guarantees – Dumb switches, but smart hosts Slide # 12 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Spanning Tree Protocol • Used to generate loop-free switching topologies – Prevent broadcast storms – Prevent bridging table failure • Basic goal – Compute a TREE that SPANS the entire network without introducing any loops – Not all trees are optimal • May take advantage of low-rate links • Introduce bottlenecks – Protocols try to compute minimum weight spanning tree Slide # 15 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 Spanning Tree Protocol • Each bridge has unique ID and priority number • Basic Idea – Elect a root bridge (lowest priority) – Each bridge computes least-cost path from itself to the root bridge (provisions for breaking ties) – Cost is function of link speeds • Protocol – STP has unique MAC address (broadcast to other switches) – Every two seconds STP messages are exchanged • Weight to believed root bridge • Notification of topology change Slide # 16 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 STP Example Slide # 17 ENEE 426 | Communication Networks | Spring 2009 Lecture 5 STP Example Slide # 20 DP = designated port Lowest cost path to root ENEE 426 | Communication Networks | Spring 2009 Lecture 5 STP Example Slide # 21 BP = blocked port ENEE 426 | Communication Networks | Spring 2009 Lecture 5 STP Example Slide # 22 Link failure recompute