Network Topology

A computer network topology is the physical communication scheme used by connected devices. These pages illustrate the common computer network topologies including bus, ring and star topology diagrams. More complex networks can be built as hybrids of two or more of these basic topologies.

 

•      LANs and WANs - Geographical coverage

•      LANs

– A single geographical location, such as office building, school, etc

– Typically High speed and cheaper.

•      WANs

– Spans more than one geographical location often connecting separated LANs

– Slower

– Costly hardware, routers, dedicated leased lines and complicated implementation procedures.

•      Topology - Physical and logical network layout

– Physical – actual layout of the computer cables and other network devices

– Logical – the way in which the network appears to the devices that use it.

•      Common topologies:

– Bus, ring, star, mesh and wireless

        

         Bus topology

•      Uses a trunk or backbone to which all of the computers on the network connect.

•      Systems connect to this backbone using T connectors or taps.

•      Coaxial cablings ( 10Base-2, 10Base5) were popular options years ago.

 

Advantages	Disadvantages
Cheap and easy to implement	Network disruption when computers are
	added or removed
Require less cable	A break in the cable will prevent all
	systems from accessing the network.
Does not use any specialized network	Difficult to troubleshoot.
equipment.

 

Ring Topology

•      Logical ring

– Meaning that data travels in circular fashion from one computer to another on the network.

– Typically FDDI, SONET or Token Ring technology are used to implement a ring network

– Ring networks are most commonly wired in a star configuration

Token Ring has multi-station access unit (MSAU),equivalent to hub or switch. MSAU performs the token circulation internally.

  

Advantages	Disadvantages
Cable faults are easily located, making	Expansion to the network can cause
oubleshooting easier	network disruption
Ring networks are moderately easy to	A single break in the cable can disrupt the
install	entire network.

Star Topology

•     All computers/devices connect to a central device called hub or switch.

•      Each device requires a single cable

•      point-to-point connection between the device and hub.

•      Most widely implemented

•      Hub is the single point of failure 

 

Advantages	Disadvantages
Easily expanded without disruption	Requires more cable
to the network
Cable failure affects only a single	A central connecting device allows
user	for a single point of failure
Easy to troubleshoot and isolate	More difficult to implement
problems

 

Mesh Topology

•      Each computer connects to every other.

•      High level of redundancy.

•      Rarely used.

– Wiring is very complicated

– Cabling cost is high

– Troubleshooting a failed cable is tricky

– A variation hybrid mesh – create point to point connection between specific network devices, often seen in WAN implementation. 

 

Advantages	Disadvantages
Provides redundant paths between devices	Requires more cable than the other LAN topologies
The network can be expanded without disruption to current uses	Complicated implementation

Wireless networking

•      Do not require physical cabling

•      Particularly useful for remote access for laptop users

•      Eliminate cable faults and cable breaks.

•      Signal interference and security issue. 

Advantages	Disadvantages
Allows for wireless remote access	Potential security issues associated with
	wireless transmissions
Network can be expanded without	Limited speed in comparison to other
disruption to current users	network topologies

  

IEEE and Networking standards

•      Institute of Electrical and Electronic Engineers (IEEE) developed a series of networking standards

– Networking technologies developed by manufacturers are Compatible

– Cabling, networking devices and protocols are all interchangeable under the banner of a specific IEEE

	Specification	Name
	802.1	Internetworking
	802.2	The LLC(Logincal Link Control) sublayer
	802.3	CSMA/CD ( Carrier Sense Multiple Access with Collision
		Detection) for Ethernet networks
	802.4	A token passing bus
	802.5	Token Ring networks
	802.6	Metropolitan Area Network (MAN)
	802.7	Broadband Technical Advisory Group
	802.8	Fiber-Optic Technical Advisory Group
	802.9	Integrated Voice and Data Networks
	802.10	Standards for Interoperable LAN/MAN Security (SILS) (Network
		Security)
	802.11	Wireless networks
	802.12	100Mbps technologies, including 100BASEVG-AnyLAN

 

802.3    IEEE standard

•       Defines characteristics for Ethernet networks.

•       New additions,

– 802.3u for Fast Ethernet

– 802.3z for Gigabit Ethernet, referred to as 802.3x.

– … …

– 802.3ac 10gbits/s, expe. 2009

– … …

•       Speed: Original 10Mbps, Fast Ethernet 100Mbps, Gigabit Ethernet 1000Mbps

•       Topology: bus or star.

•       Media: Coaxial and twisted pair cabling, also fiber optic cable.

•       Access method: CSMA/CD

  

802.5   IEEE standard

•      Specifies the characteristics for Token Ring Networks.

•      Introduced by IBM in the mid 80s, network topology of choice until the rise of the popularity of Ethernet.

•      Speed: 4 to 16Mbps

•      Topology: logical ring and most often a physical star. Logical ring is often created in the Multistation Access Unit (MSAU)

•      Media: twisted pair cabling.

• Access method: token passing.

802.11b IEEE Standard  - Wireless

•       Specifies the characteristics of wireless LAN Ethernet networks.

– Special devices called wireless access points to allow communicate.

– Also connect to wired networks to create wireless portions of entire networks.

– Access method: Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA ), a variation of CSMA/CD.

– Topology: physical wireless, logical bus

•       Protocol

– 802.11b 1999 2.4GHz, 11Mbits/s

– 802.11g 2003 2.4GHz, 54 Mbits/s

– 802.11n 2008 2.4G,5GHz, 248Mbits/s

– ... …

FDDI

•      Fiber Distributed Data Interface (FDDI) standard was developed by American National Standards Institute (ANSI)

•      Dual ring technology for fault tolerance

•      Speed: 100Mbps or higher

•      Topology: dual ring topology

•      Media: fiber optic cable, > 2 kilometers. Also possible use copper wire as Copper Distributed Data Interface (CDDI).

Access method: token-passing access method