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Transcript
Physical Topology • Physical layout of the network nodes – Broad description of the network: no detail about device types, connection methods, addressing, ... – 3 most common topologies: – Bus, Star, Ring – Network administrator needs to understand physical topology • Troubleshooting, upgrading network infrastructure, effect on chosen logical topology, etc. Bus Topology • One cable (the bus) connecting all network nodes – Usually coaxial cables – One communication channel shared between nodes/workstations • Shared cable capacity • Data is sent via the bus by broadcast and each node responsible to accept the data frame when it detects its MAC address as destination address in the frame • Other nodes ignore data Bus Topology – No connecting device – Two end-points: terminators = 50 Ohm resistors • Terminators absorb signal ⇨ No signal reflection (noise) Bus Topology • Advantages – Easy and inexpensive to set-up • Disadvantages – Lack of scalability • more nodes ⇨ performance degrades on unique channel – Difficulty to troubleshoot • Error may occur anywhere along the bus – No fault-tolerance • Network down if cable breaks ⇨ Usually for network limited to 10 nodes ⇨ Often combined with other topologies Bus Topology Taken from: http://www.edrawsoft.com/images/network/Bus-Network-Topology.png Star Topology • All nodes are connected to central device called concentrator (or hub) or Multi-station Access Unit (MAU) – One cable connects two devices – No terminator • Usually twisted-pair cables or fiber cables Star Topology • Advantages – Better resilience per segment: problem isolation – More expensive than Bus: hubs cost more than Bus connectors – Easier to troubleshoot than Bus – Scalable • Disadvantages – More cabling than Bus and Ring – More configuration – Failure at concentrator will affect all the network ⇨ Frequent topology: lots of support Star Topology taken from: http://www.teach-ict.com/as_a2/topics/networks/pages/chap5_files/star.gif Ring Topology • Similar to the Bus but all the devices connected to a common cable forming a closed loop: no begin/end • Usually twisted-pair cables or fiber optic cables Ring Topology • Packets are transmitted in one direction of ring – Each node accepts/responds to its packets and forward remaining packets to next node in ring – Usually a token (3-byte packet) is used • Sending node with token transmits: data + token through ring • Destination node picks-up data frame and returns ACK via ring to sending node • Sending node releases token to next node in ring Ring Topology • Advantages – Fault tolerance: no collision because media access method, fault isolation – Economical (N nodes, N links) – Also provides redundant paths • Disadvantages – More cables than a bus – Failure: One node breaks ⇨ entire ring breaks ⇨ network down – Lack of scalability: more nodes ⇨ higher response time because of token passing – More difficult to configure than a Star: node adjunction ⇨ Ring shutdown and reconfiguration Ring Topology Taken from: http://www.brainbell.com/tutorials/Networking/images/01fig04.gif Mesh Topology • Each node to every other node • Often used in Backbone/WAN to interconnect LANs Taken from: http://studynotes.net/images/mesh.gif Mesh Topology • Advantages – Fault tolerance: communication not stopped if one link breaks – Good for Backbone • Disadvantages – Expensive – Difficult for installation, management, troubleshooting Tree Topology • In a tree each node connected to a concentrator: similar to a star • Concentrators connected together to form a hierarchy Taken from: http://www.teach-ict.com/as_a2/topics/networks/pages/chap5_files/tree.gif Hybrid Topologies • Simple topology are too restrictive – Scalability, performance, etc. • Usually physical topology combines Bus, Star and Ring • Two examples – Star-Wired Bus • Groups of nodes are star-connected hubs • Hubs are connected together via a Bus – Star-Wired Ring • Physically nodes are connected via a Star • Data is transmitted between node using token passing method Logical Topology • Network access methods – How data is transmitted between nodes • Three methods used for all network architectures for connection creation – Circuit switching – Message switching – Packet switching Circuit Switching • Connection between two nodes is created before nodes transmit: circuit • Bandwidth is dedicated to the circuit until end of connection – Not economical: waste of bandwidth • Data follows the same circuit – Dedicated path ideal for audio and video applications • Used by ISDN and ATM Message Switching • Uses the store and forward principle – Connections is established between two nodes – Information is sent from node 1 to node 2 – Connection is broken between node 1 and node 2 – Node 2 stored and forward the information it received to node 3 • Nodes need to have enough resources: memory and processing to store and forward data Packet Switching • Data is broken as packets • Packets are transported using any path of the network to the destination – Usually the fastest path is used based on routing method • No bandwidth waste due to open connection – Use of destination address and sequence number to get and rebuild packets at destination node • Takes time: may be not suitable for live data (audio and video) • Intermediary nodes do not process data • Internet is a packet-switched network