Local Area Networks

LAN

Why LANs?

• Provide a means of DIRECT connection to other machines

• Manage access

• Provide reasonable performance

• Hopefully allow for interconnection with other LANs

LAN criteria

• Resolve access

• Fair access

• Quick access

• Fast transmission

• Security

• Robustness

Do LANs meet these criteria?

• There are a huge number of network designs which have been proposed

• Few are actually in operation

• Most LAN designs excel in a few of the previous areas but not all

• OSI allows for integration of newer technologies as they evolve.

• For now-> price & performance drive us to ethernet.

Ethernet

What you are likely to see

Ethernet

• Broadcast medium– Everyone sees all transmissions*

• Orderly free-for-all to determine access

• Cheap

• High performance (few Mbps to Gbps)

• Many standards– Speed– Medium

Basic Principles

• Access strategies vary significantly.

• Centralized– Some main controlling unit deciding– Less Robust

• Decentralized– Group decisions– Controlled behavior

• Moving centralized control

Other classifications

• Reserved vs non-reserved

• Static versus dynamic

• Broadband (FDM) vs baseband (TDM-ethernet)

• LAN, MAN, WAN

• Optical vs electrical

What strategy does ethernet use?

• Less time required to decide access means faster access and less delay.

• Simple algorithm like using a “party-line” in a telephone system

• Pick up the phone and see if it’s in use

• If not send – checking to make sure no one else did the same

Assume each at extreme ends

A B

A begins transmission (a<<1)

A B

A message almost reaches B and B starts

A B

B sees the collision

A B

A sees the collision

After “2a”, either collision or not

• Within “2a” frame transmission times, the initiator will – See a collision and stop transmitting– See NO collision and proceed with the

assurance that NO ONE ELSE will transmit as others will look before beginning.

• But what happens after a collision?– No central organizer– Wait a random amount of time and try again– If another collision -> WAIT LONGER!

And what does that tell us?

How many collisions can occur?

• Eventually the physical layer responds that it can’t deliver and the (OSI) layer above decides what to do next

• There is NO UPPER BOUND on how long it will take to get access

• Can’t service some types of traffic well– Video and voice

How does it work so well?

• If “a” is small enough, a number of contention cycles (“2a”) can occur for each transmission.

• E.g. if 2a= .002 and it takes 5 collision cycles, the overhead is 5x.002=.01 of the time and 1/(1+.01) is still very good.

• Capacity typically far exceeds demand which means most of the time the network is free and no collisions occur.

• Degrades in a heavily loaded environment!

What does it look like?

• Originally a bus architecture

• Now a logical bus but a physical star

computer computer computer computer

HUB

computer computer computer computer

SWITCH

computer computer computer computer

SWITCH

What impact does that have on network security?

What impact does that have on performance?

Queueing Revisited Offered Load Versus Throughput

Offered Load

Throughput

100% 200%

100%

ideal

more typical

ethernet

Token Rings

An alternative

token

In order to use the net, you must seize the token

token

message

token

message

token

message

message

token

What is the improvement?

• A little delay in getting access

• An upper bound on access as long as all behave well (and they do .. Same program)

• Used in environments where upper bound is required like in automated manufacturing.

• Previous queueing graph is more like the ideal, does not degrade to zero.