Network Topologies Lecture 2
Dr John C. Murray Principal Lecturer
Recap: Circuit Switching
• Establishes a point to point connection between two devices
– Channel is dedicated
– Full bandwidth given
– Remains connected for duration of communication
• Minimal delays
– Channel is dedicated
• Highly inefficient
Packet Switching
• Much more efficient than Circuit Switching • Data segmented into small ‘chunks’ of
data
– Each ‘chunk’ is called a packet
– Has a destination (and source) address
• Has lots of other information we’ll discuss later
– Allows for better use of the network
– More people can communicate at any given time
Packet Switching
• Used to maximise the bandwidth of the network
• Proposed for military use in the 1960’s, implemented in general ‘home’ networks in 1968.
• Allow for much more resilient networks – How?
Network Expansion
• Networks rapidly grew exponentially… – Remember last weeks IoT graph…
• “I think there is a world market for maybe five computers“
– Thomas John Watson, Sr. (February 17, 1874 – June 19, 1956) was the chairman and CEO of International Business Machines (IBM)
Network Topologies
• Three main topologies we will look at first:
– Ring Network – Bus Network – Star Network
Ring Network
Ring Network
• Each ‘node’ connects to EXACTLY two other nodes
• Data travels in a ring
– Each node handles ALL traffic
– Every packet sent can be received and handled by every node
BNC Connectors
Advantages
• Easy to configure
• Easy to fault find
• No central management required • Easy to add and remove nodes
Disadvantages
• A failure of a node, breaks the network
• Communication delay is directly proportional to number of nodes
• Bandwidth is shared
Bus Network
• Connected nodes are daisy chained
• Just like a ring network all nodes receive all network traffic
Advantages
• Easy to add nodes
• Easy-ish to fault find
Disadvantages
• All network is received by all nodes
• If the network is severed no node can receive data
• All nodes have the same transmission priority
– Is this a problem?
– Who gets access to the network?
– Uses Media Access Control
• Carrier Sense Multiple Access (CSMA)
Star Network
Star Network
• One of the most common computer networks
– Uses the Ethernet standard
• Each node connects to a central ‘hub’ – Acts as a ‘gateway’ for the traffic
• More resilient than previous networks
• Central ‘hub’ broadcasts messages to all nodes in the network
Advantages
• A failure of a node does not affect the remainder of network
• Easy to detect faults
• Expanding and changing is easy
• Improved performance
– Less nodes and cables when two nodes communicate
• Isolation of devices
Disadvantages
• The ‘hub’ can be a bottle neck
• If the hub fails, the network is unusable • Expensive to purchase
• Large amounts of cable
Token Ring vs Ethernet
• What happens when we increate the number of nodes on a network?
– More Data
– More traffic
– More Errors?
• Why?
• What is a collision?
– When two ‘packets’ collide on the network
• What is a result of this?
– We lose the packets
– We have to resend the packets
• Congesting the network
Token Ring Solution
• Token Ring – Solution
– A ‘token’ is passed around the network, allowing nodes the right to transmit
Ethernet Solution
• Ethernet–Solution
– Uses CSMA/CD
– Carrier-Sense Multiple Access w/ Collision Detection
• CSMA/CD is a modification of carrier sense multiple access (CSMA).
– Improves performance by terminating transmission as soon as a collision is detected
– shortening the time required before a retry can be attempted.
CSMA/CD
• Carrier-Sense
– Determine if another transmission is in progress
• If there is a carrier wait for it to end • “Listen before talking”
10 Is frame ready for transmission?
Goto 20, else goto 10
20 Is media idle?
Goto 30, else goto 20
30 Collision?
JSR CollisionDetect, else goto 40
40 Transmission complete
CSMA/CD Flowchart
Questions
Mesh Network
Mesh Network
• All nodes are connected to all other nodes
– Provides a point-to-point connection
• Between every single node
• This is known as a fully connected network
– Provides excellent security
• Due to the point-to-point nature
• Used in security conscious scenarios
– If a node goes down
• Employs rerouting of the network
• Reduces security, but still better than other topologies
Mesh Network
• Withouteverynodefullyconnected – Known as a partial network
– Due to cost of fully connected
– Each node increases complexity exponentially • c = (n(n-1))/2
• For a network with 8 nodes
• Each node will have 7 connections
– What about the machines in this building? – ≈1000
– Partial networks are used in areas where security isn’t needed
• But resilience is, such as?
MANet
• Mobile Ad Hoc Networks
• A self configuring mobile network
– Self-configuring?
• Each node is a router
• The network structure changes as the nodes move
– No infrastructure needed • Cables
• Power
• Static routers
• Applications
– Network Failure – Military
– Deserted Areas
• One laptop per child (XO)
– Car Technology
• Inter-car-communication
– Entertainment • Games
• Chat applications 😉 » WeChat
MANet
Mobile Ad Hoc Network
• Definition
– A collection of wireless mobile hosts forming a temporary network without the aid of any centralised administration or standard support services
• OftentheMANETtopologyisdynamic—nodes enter and leave the network continuously
• No centralised control or fixed infrastructure to support :-
– network configuration or – reconfiguration
Ad-hoc Networks
• Why MANET?
– Mobile nodes have limited communication range • Reducesbatterydrain
– Enables spatial reuse of limited bandwidth increased network capacity
– To connect all nodes in the network, each node is a
• Packet source
• Packet sink • Router
– Nodes must route packets for other nodes to keep the network fully connected
– In MANETs, a big problem is how to determine where a destination node is located relative to a sending node
Ad-hoc Networks
• Route-finding is a current area of much research
– Want to determine an “optimal” way to find “optimal” routes
• Dynamic links
– Broken links must be updated when a node moves out of communication range with another node
– New links must be formed when a node moves into communication range with another node
• Based on this new information, routes must be modified – Frequency of route changes a function of node mobility
Videos
Video
3D Complexity
It’s not all good!
• Routing performance
– Routes change over time due to node mobility
– Need to avoid long delays when sending packets
• But would like to avoid lots of route maintenance overhead
– Want as many participating nodes as possible for greater aggregate throughput, shorter paths, and smaller chance of partition
It’s not all good!
• Quality of service – Link variability
– route variability
– Collisions
– Congestion
• Security – interesting new vulnerabilities and complexities
– Routing denial of service
• Nodes may agree to route packets • Nodes may then fail to do so
– Broken, malicious, selfish nodes
Questions