02 Topology

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Physical and Logical Topologies

Lecture 2

Mariusz Nowostawski

INFO333

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Content

Network Architecture

Physical Topology

Bus, Ring, Star, Mesh, Tree, Hybrid

Logical Topology

Circuit Switching

Message Switching

Packet Switching

Ethernet

CSMA/CD

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Network Architecture

Network physical topology Physical layout

Media

Network logical topology Network access methods: how data is transmitted

between nodes: Framing + signal pulses viatransmission medium

At OSI Data Link Layer

Most common Network Architectures Ethernet, Token Ring

FDDI, ATM

We will focus on Ethernet

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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.

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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

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Bus Topology

No connecting device

Two end-points: terminators = 50 Ohm resistors

Terminators absorb signal No signal reflection (noise)

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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

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Bus Topology

Taken from: http://www.edrawsoft.com/images/network/Bus-Network-Topology.png

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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

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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

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Star Topology

taken from: http://www.teach-ict.com/as_a2/topics/networks/pages/chap5_files/star.gif

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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

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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

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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 breaksentire ring breaksnetwork

down Lack of scalability: more nodeshigher response time

because of token passing

More difficult to configure than a Star: node adjunctionRing shutdown and reconfiguration

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Ring Topology

Taken from: http://www.brainbell.com/tutorials/Networking/images/01fig04.gif

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Mesh Topology

Each node to every other node

Often used in Backbone/WAN to interconnect

LANs

Taken from: http://studynotes.net/images/mesh.gif

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Mesh Topology

Advantages

Fault tolerance: communication not stopped if

one link breaks

Good for Backbone

Disadvantages

Expensive

Difficult for installation, management,

troubleshooting

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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

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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

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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

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Circuit Switching

Connection between two nodes is createdbefore 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 videoapplications

Used by ISDN and ATM

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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

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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

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Ethernet

Ethernet originally developed by Xerox in 1970s Widely used today for network architectures

IEEE 802 standards Data Link and Physical layers

LAN, WAN and Wireless networks

Several specifications of 802 standards 802.3: CSMA/CD (see next slides)

803.5: Token Ring 802.11: Wireless technologies

not exhaustive: Evolving specifications

We will focus on 802.3 since it is used by mostEthernet networks

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802.3: CSMA/CD (1)

Carrier Sense Multiple Access with Collision

Detection

Defines Ethernet network access method

Developed in 1960s at University of Hawaii

Adopted by Xerox in 1970 and further developed by

Xerox, DEC and Intel

Standardized at beginning of 1980s by IEEE

Specifications precise transmission rates, cabletypes, maximum and minimum cable lengths

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802.3: CSMA/CD (2)

Carrier Sense

NICs listen to network to sense if a signal (carrier) isdetectedanother node is transmitting

If no signal, channel is freethe node can send data

Multiple Access Several nodes can access the media and sense signal

simultaneously

Collision Detection

Nodes NICs can detect collision: when two nodestransmit data in the network at the same time

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802.3: CSMA/CD (3)

Collision Detection If a node NIC detects a collision:

It stops transmission

It send 32-bit jam message Informs other nodes that a collision happened & that it

will not transmit for random time periodpreviousframes are ignored

If channel free, it will retransmit frames

Collision Domain

Ethernet segment where collisions can happen

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802.3: CSMA/CD (4) Sources of collisions

Too much traffic

If too many collisions this can indicate a technicalproblem

A poorly designed network, a malfunctioning NIC

Unnecessary traffic

Collision information itself is repeated by repeaters

Propagation delay

If too long, nodes may transmit even if channel is notfree

Example: 100BaseT network max specified distancebetween two nodes to avoid delay is 100 meters

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802.3: CSMA/CD (5)

CSMA/CD

Logical bus topology for access method

Data go to all nodes from sending node

Physical topology may be Bus or Star

5 specifications

10BaseT

10Base2

10Base5

100BaseT

Gigabit Ethernet

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802.3: CSMA/CD (6)

10BaseT Twisted-pair cables

Transmission rate: 10 Mbps baseband

Physical topology: Star

Segment length: 2 to 100 meters

10Base2 (Thin Ethernet) Coaxial cables

Transmission rate: 10 Mbps baseband Physical topology: Bus

Max segment length: 185 meters

Max: 30 hosts per segment

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802.3: CSMA/CD (7)

10Base5 (Thick Ethernet)

Coaxial cables

Transmission rate: 10 Mbps baseband

Physical topology: Bus

Max segment length: 500 meters bigger cable diameter less attenuation

Max: 100 hosts per segment

100BaseT (Fast Ethernet) Twisted-pair cables

Transmission rate: 100 Mbps baseband

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802.3: CSMA/CD (8)

Gigabit Ethernet

Transmission rate: 1 Gbps baseband

IEEE 802.3ab Gigabit Ethernet over UDP

(1000BaseT)

IEEE 802.3z Gigabit Ethernet over fiber

(1000BaseX)

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References

CCNAGuide to Cisco NetworkingThird

editionKelly Claude and Kelly Cannon