1. Repeaters and Bridges Relay Stations

2. Repeaters Add delays comparable to BIT times Clean and reamplify the digital signal Do not examine the contents of the message Function at the physical level of OSI physical Other Layers physical Other Layers physical REPEATER

3. LAN Performance LANs typically have a queueing nature Vary by WHERE the elbow of the curve lies Ability to increase performance (decrease delay) is directly related to successful partitioning of traffic load delay

4. Traffic Partitioning Separate nodes into groups according to traffic One partitioning of nodes Messages to nodes in another group Messages from nodes in another group Messages to nodes in the same group A LAN or group of computers

5. Example of Partitioning Group AGroup B Group AGroup B 80% A 20% B 70%B 30%A Good Partitioning..Localized traffic Poor Partitioning..Nonlocal traffic 30% A 70% B 25%B 75%A

6. Performance Comparison A Generalization load delay 100% local AB 80% local 60% local

7. Bridge Pros and Cons Major advantage is better performance when traffic can be successfully partitioned A less important advantage is security.. preventing sniffers etc. Major disadvantage: Bridges add DELAY Delay must be offset by one of the other two advantages.

8. Example where payback is NEGATIVE load AB Wo/Bridge W/Bridge Traffic in both cases 80% local Bridge delay actually makes the total delay worse delay

9. How could the bridge make it WORSE? Consider ethernet A node in group A must compete for a slot before transmitting Once the message is received at the bridge, the message is –error-checked –assessed for forwarding –must COMPETE AGAIN for a slot on B

10. Bridges and OSI Bridges function at the link layer Typically do not connect LANs of different types, i.e. ethernet and token ring, but CAN. Delay greater than the length of the message. As with some LANs (ethernet) do not guarantee delivery but only accept/forward messages received without errors

11. physical Other Layers physical Other Layers physical BRIDGE link MAC link How Does a Bridge Work? Anything DESTINED for nodes on this side are NOT forwarded A Consider Messages from A Anything DESTINED for nodes on this side ARE forwarded ALL messages from BOTH SIDES are received at the bridge!

12. How Does a Bridge Know Where the Nodes are Located? Different techniques exist Static –administrator types them in and maintains them –too much trouble Dynamic –Bridges learn themselves –administrators don’t have to maintain them! –transparent operation.. Plug and Go!

13. How can a Bridge LEARN? BRIDGE Initially does not know anything! Source Destination …... When it receives the first message, which is helpful - source - destination ? ANSWER: SOURCE Not sure where destination is but KNOWS source is on that side Assume worst case and send on other side until it knows better

14. How can a Bridge LEARN? BRIDGE Source Destination …... Bridge Table A E... D G... A D... A B A... B D E... D C E... C STILL does NOT know about E or G!

15. Other “Routing” techniques Redundant links can lead to LOOPS LOOPS can lead to a flood messages while learning Use Minimal Spanning Tree algorithms from data structures and discrete math Also use Source Routing and make sender determine path and put it in the message.