Topologies, Backbones, Switching, and Ethernet ITNW 1325, Chapter V, Part I

Physical Topologies

Overview:  Reflect geometry of physical connections only – without devices, connectivity methods, or addressing  Don’t reflect device types, connectivity methods, or addressing schemes in use  Three fundamental types are bus, ring, and star – can be mixed to create hybrid topologies  Important to understand in order to troubleshoot related problems or change communications infrastructure  Differ from logical topologies – reflect how digital data propagates between nodes

Physical Topologies Overview (continued):  Physical and logical topologies used within the same network may be very different  The term topology commonly refers to a physical topology when used alone – with logical used explicitly  Too restrictive – rarely seen in their pure form in medium-sized and large networks

Physical Topologies Bus:  Implies nodes connected by a single cable without employing connectivity devices  Provides only one communications channel – baseband transmission is supported only  Enables only one node to transmit at a time – nodes compete for the right to transmit  Requires each node to passively listen for and accept data directed to it – passive topology  Nodes other than sending and receiving ones sense the transmission but ignore the information sent

Physical Topologies Bus (continued):  A broadcast transmission would be processed by all connected nodes – parts of a single broadcast domain  Requires resistors – terminators – at the cable ends to prevent endless travel of the signal (signal bounce)  Without terminators, old signals would keep bouncing off the wire ends – prevent propagation of new signals  Must be grounded at one end – helps to remove static electricity that could adversely affect the signal  Example – nodes connected with a coaxial cable and sharing the available bandwidth (50-Ohm terminators)

Physical Topologies Bus (continued):  Not scalable – performance degrades as more nodes are added and compete for the right to transmit  Hard to troubleshoot – errors are easily detected but their exact source or location are difficult to locate  Not fault tolerant – any single break or defect affects the entire network disrupting transmissions  Lack security – every connected node can read any data transmission destined to it or to someone else  The least expensive topology to set up – rarely used today due to multiple carried drawbacks

Physical Topologies Bus (continued):

Physical Topologies Bus (continued): BNC T-Connector BNC Terminator

Physical Topologies Ring:  Implies that each node is connected to the two nearest ones – with the entire topology forming a circle  Each node accepts and responds to frames addressed to it – while forwarding other packets to the next node  Implies that each node to participates in delivery acting as a repeater – active topology  Employs twisted pair of fiber optic cable as medium

Physical Topologies Ring (continued):  Not scalable – performance degrades as more nodes are added and introduce additional transmission delays  Not fault tolerant – a single malfunctioning node would break the ring and disable the entire network  Used by obsolete Token Ring networks

Physical Topologies Ring (continued):

Physical Topologies Star:  Implies nodes connected through a central connectivity device – forwards frames to the recipient’s segment  Requires more cabling – twisted pair of fiber optic – and more configuration than bus or star topologies  Requires proper configuration and constant availability of the central device  Enables connecting two devices only to each physical segment – a cabling problem affects two nodes at most  Enables many nodes to transmit at a time – depending on the ability of the central device to handle the load

Physical Topologies Star (continued):  The most scalable topology – can be easily easily moved, isolated, or interconnected with other networks  The most fault tolerant – a malfunctioning node would not affect any other node or a communication device  The easiest to troubleshoot – having one node per segment makes an error easier to locate  Carries single point of failure – a problem with the central connectivity device affects all connected nodes  More expensive to set up and maintain – requires more cabling and administration than other topologies

Physical Topologies Star (continued):  Limits the number of nodes per segment – may result in reduced or eliminated competition for the medium  Most widely used topology on modern networks

Physical Topologies Star (continued):

Logical Topologies

Overview:  Reflect how information propagates between nodes – may differ from a physical topology used  Important to understand when building networks, troubleshooting them, or optimizing their performance  Represented by two fundamental types – bus and ring

Logical Topologies Bus (“Local Broadcast”):  Data travels from one network device to all other ones on the segment – each connected node can access data  Commonly supported by networks that use a bus, a star, or a star-wired bus physical topology Ring:  Data follows a circular path between sender and receiver – even in case physical connections form a star  Supported by networks that use a ring or a star-wired ring physical topology

Logical Topologies Bus (continued):

Logical Topologies Ring (continued):

Hybrid Physical Topologies

Overview:  Complex combinations of fundamental physical topologies – more suitable for modern networks  Minimize weaknesses and increase scalability of networks – better fit large and growing networks  Two primary kinds – star-wired ring and star-wired bus

Hybrid Physical Topologies Star-Wired Bus:  Implies groups of nodes that are star-connected to connectivity devices that are connected via a bus  Enables covering longer distances and interconnecting or isolating different network segments  Inherits fault-tolerance, scalability, and manageability from a star topology  Requires more cabling and more connectivity devices than a star or a bus – more expensive than basic ones  A basis for modern midsize and large Ethernet networks

Hybrid Physical Topologies Star-Wired Bus (continued):

Hybrid Physical Topologies Star-Wired Ring:  Implies groups of nodes that are star-connected to connectivity devices – and the ring logical topology  Data flows in a circular pattern over the star-like wiring  Inherits fault-tolerance, scalability, and manageability from a star topology  A basis for obsolete Token Ring networks

Hybrid Physical Topologies Star-Wired Ring (continued):

Backbone Networks

Overview:  Cabling that interconnects various parts of enterprise – local and remote offices, departments, and computers  Commonly carry substantially more traffic than cables connecting to workstations – possess increased capacity  Designed for continuous high throughput to avoid congestion – complex and require careful planning  Four fundamental types – serial, distributed, collapsed, and parallel

Backbone Networks Serial:  Implies two or more internetworking devices connected to each other in a daisy-chain fashion (linked series)  Used for extending networks and adding device ports to connect more user workstations  Requires to observe the maximum number of connected devices and segments – depends on the network type  Not scalable – delays in information delivery increase as more devices are added to the backbone  Not fault tolerant – any single break or defect affects the entire backbone disrupting transmissions

Backbone Networks Serial (continued):  The simplest logically, the least expensive, and the easiest to implement backbone type

Backbone Networks Distributed:  Consists of a number of connectivity devices connected to multiple central devices in a hierarchy  More devices can be added to existing layers – allows for simple expansion at lower costs of adding networks  Can employ advanced devices for connecting LAN segments – raise effectiveness of data transmissions  Maps onto the structure of a building – with some devices serving floors and/or departments and other ones connecting these segments together  Enables segregation and easy management of networks

Backbone Networks Distributed (continued):  May include a daisy-chain linked bus – inherits its limitations requiring to place it thoughtfully  Device at the upper layers represent potential single points of failure – can damage the entire network  Brings relatively simple, quick, and inexpensive implementation – popular on today’s LANs and MANs

Backbone Networks Distributed (continued):

Backbone Networks Collapsed:  Implies having the single central connection point for multiple networks – connects multiple LANs together  Makes the central device the highest level of the backbone – must be able to handle heavy traffic loads  Scalable – makes addition of new segments easy, with potential necessity to upgrade the central device only  The central network device represents single point of failure for the entire network – must be available  Fault tolerant – a failed segment does not affect others

Backbone Networks Collapsed (continued):  Centralizes maintenance and troubleshooting and enables interconnecting networks of different types

Backbone Networks Collapsed (continued):

Backbone Networks Parallel:  Resembles other backbone types – implies duplicate connections between connectivity devices  Doubles the amount of cable needed and physical ports used on network devices – can be quite expensive  Provides network load balancing, redundancy, and increased performance  Most robust backbone type – commonly implemented within critical segments of the network