LAN technology

Note n These slides contain a number of (basic) animations… –They do not therefore translate well to handouts...

Post-conditions n Be able to describe a number of solutions to the Media Access Control “problem” –Probabilistic ALOHA(decentralised) Slotted ALOHA(centralised) Ethernet(decentralised) –Deterministic Token Ring n Understand basic LAN topologies –physical –logical n Be able to differentiate fundamental LAN devices –repeaters, bridges, switches, hubs

Topologies

Physical topologies - mesh The cost of maintaining and adding nodes is expensive!

Physical topologies - bus

Physical topologies - star

Physical topologies - ring

A self-repairing ring A redundant ring (with opposing directionality) is used in case of problems with either a part of the ring itself or with one of the nodes.In this example we assume a break in the cable...

Virtual topologies (virtual ring) A particular physical topology may exhibit behaviour that suggests an entirely different topology. For example, token ring networks often use a hub that generates ring behaviour in a star topology.

Virtual topologies (virtual bus) As another example, a star topology may be used to generate bus behaviour as would be expected on an Ethernet network..

The Media Access Control “problem”

Solution space... Very broadly, solutions may be classed as: n Centralised or decentralised –is there some mediating node that controls access, or is access control distributed? n Probabilistic or deterministic –is there an orderly algorithm or is access more stochastic in nature?

ALOHA Decentralised Probabilistic

ALOHA Time

Slotted-ALOHA Centralised Probabilistic

Slotted-ALOHA Time Green machine wants to transmit but has to wait for the next slot

Ethernet Decentralised Probabilistic

Ethernet ABCD FE ‘A’ and ‘C’ both want to transmit at similar times. A collision results. A random back-off occurs and they retry; on this occasion ‘C’ is successful. A C A C C

Token Ring Decentralised (although monitor stations may be used)… Deterministic

Token Ring A “token” is passed around the ring... When stations want to transmit, they wait for the token to pass... A “free” token may be grabbed, and a message attached... The token is “released” by the token holder on the next pass... And so on...

Devices

Repeater / hub (ethernet) The effect is that the two LANs now behave as larger one.

The “collision domain” collision domain n The concept of a collision domain emerges from the connection of Ethernet networks n Consider the effect that connectivity devices have on the LANs they connect: –physical-layer devices extend the collision domain they simply amplify bits to give greater range –higher layer devices generate multiple collision domains they pass on frames/packets only as appropriate

Bridge (ethernet) The use of a bridge results in two collision domains. In this case the bridge knows the DA is in the top LAN and drops the frame.

Bridge (token ring) This slide shows how a bridge (as a level 2 device) “talks” the protocol of the LANs that it is attached to.

Other devices - Ethernet switches switches n In the generic sense, hubs, bridges and routers are all “switches”... ethernet switch n An ethernet switch is typically full duplex –It can eliminate many of the characteristics of “traditional” Ethernet carrier sensing collision detection –This is achieved by use of duel links and a star topology, resulting in no MAC issues a basic link-to-link protocol results

Recall the MAC problem where ‘A’ and ‘C’ want near-simultaneous access... ABCD FE A C A C C

Other devices - Ethernet switches The use of an ethernet switch transforms the problem from a Media Access Control one to a (simpler) link-to-link one. D A BC SA = ‘A’; DA = ‘B’ SA = ‘C’; DA = ‘D’