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