Transcript 3-Physical+logical toplogy

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