Logical & Physical Topologies

• A logical technology is how the hosts are connected logically example token Ring can be laid out in a physical star but it is still a logical ring

• A physical topology is how the hosts are laid out physically in a room or building.

Topologies and protocols

• Most topologies and protocols are linked

• Token passing protocols must have some form of returning the token back to the start such as a ring

• Broadcast protocols such as ethernet must not have a ring as this protocol relies on listening to the media as it broadcasts and therefore in a ring would hear itself. So that topology used tends to be a star or point

Topologies • Bus

• Ring

• Star

• Point-to-point

• Full-mesh / Partial-mesh

Ethernet Topology

• LAN Topology• Half-Duplex must listen to the wire to make sure no

one else is sending• Media type BNC (British Navel Connector)• Speed 10MBps

Ethernet

• This was taken from www.wown.com

Ethernet Topology• Ethernet Digital, Intel, Xerox (DIX) and 802.3

– Defined the original Ethernet standards– CSMA/CD

• Carrier Sense Multiple Access with Collision Detection

– All versions use a Bus topology– Half-duplex operation (full-duplex capable with the right equipment)

• Standards– 10Base2 – Thin Ethernet – 185m– 10Base5 – Thick Ethernet –500m– 10BaseT – Twisted-pair Ethernet – 100m

• The original Ethernet speed was 4-bit data path at 2.5Mhz• Current processors are above 2 GHz

CSMA/CD

• Carrier sense - stations sense whether there is traffic on the wire. Wait until medium idle then begin to transmit frame.

• Multiple access - more than one station can simultaneously be connected and contend for the right to send. Each uses same access algorithm.

• Collision detection - individual stations detect when there has been a collision (transmissions from two stations interfere with each other) and react appropriately.

LISTEN TONETWORK

---WAIT UNTIL

QUIET

START SENDINGMESSAGE

STOP SENDINGMESSAGE

----SEND "JAM" SIGNAL

WAIT RANDOMINTERVAL

(Rand nbr of slottimes)

HEARCOLLISION?

Yes

No DONE

Wait plus InterframeGap

CSMA/CD• Listen to medium during transmission

• Detect whether another station’s signal interferes

• Back off from interference and try again

Collision Domain

• This was taken from www.wown.com

Ethernet Topology• Ethernet technologies

• 802.3 Standard Ethernet (10Mb)

• 802.3u Fast Ethernet (100Mb)– Defines a bus topology– Half- or full duplex operation

• 100Base-TX Cat5/FE UTP – 100 meters

• 100Base-FX Multi-mode fibre – 400 meters

– Maximum raw transmission rate of 100Mb• Aggregate rates double when full-duplex

• 4-bit data path at 25Mhz

Ethernet Topology• 802.3z Gigabit Ethernet (1000Mb)

– Defines a bus topology– Half- or full duplex operation

• 1000Base-TX Cat5e/Cat6 UTP – 100 meters

• 1000Base-SX Multi-mode fibre – 260 meters

• 1000Base-LX Single mode fibre – 10 kilometres

– Maximum raw transmission rate of 1Gb• Aggregate rates double when full-duplex

• 8-bit data path at 125Mhz

• 802.3ae 10Gig Ethernet (10000Mb)

Using a switch to separate Collision Domains therefore increasing traffic

Ring Topology

• The Token controller detects that there are no tokens on the network and will generate a Token that is passed in one direction around the loop of computers

Ring Topology• When a computer has

data to send it waits for the Token then attaches its data this goes around the network to the destination required where it is acknowledged and sent back to the originator who then releases the token back on to the network

Ring Topology• IBM’s token ring was first seen in the late

1970’s• Later IEEE 802.5 became the standard• Available in 4 Mbps and 16Mbps speed• Uses a token to access the network• Token is passed around a logical ring of

network devices• System must acquire the token to transmit data• Data always travels around the ring in the same

direction, therefore , no collisions

FDDI (Fibre Distributed Data Interface)

• Similar to Token Ring but this carries a redundant ring so that if a station goes down the information is still able to continue

B Node Single Attachment System (SAS)

A Node Dual Attachment System (DAS)

FDDI/CDDI (Copper)

Star

• LAN / WAN Topology

• Sometimes referred to as Hub & Spoke

• Active device at the centre of the star

Point –to –point

• WAN Topology

• Used by switched networks such as X.25 and Frame Relay or Leased line connectivity such as T1

Full-Mesh• WAN Topology• Used by circuit-switched

networks such as X.25 and Frame Relay

• All areas have connectivity between them

• Must have an interface for each connection and therefore is more expensive but has inbuilt fault tolerance

Partial-Mesh

• WAN Topology• Not all areas have

connectivity between them

• Used by circuit-switched networks such as X.25 and Frame Relay

Any Questions?