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Lecture 7

CSE 123 Lecture Notes - Lecture 7: Local Area Network, Collision Domain, Collision Detection


Department
Computer Science and Engineering
Course Code
CSE 123
Professor
Alex Snoeren
Lecture
7

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One link is NOT enough!
- aka Local Area Network (LAN)
- link is “multiplexed” across time
- frames have a link-layer header with addresses of source and dest of hosts on the LAN
- one shared link (bus) limits scale in terms of
1. Distance - (i.e Ethernet spec is up to 2500 m)
2. Performance - the more computers hook up to the link, the more their “share” is going to
go down!
- Better alternative is to have multiple busses
- we don’t have to share “air spaces” in time
- need ability to connect multiple busses together!
Hubs/Repeaters
- Physical layer device
- one “port” for each LAN
- Repeat received bits on one port out ALL other ports
Hub: when it hears a bit come in on one wire, it copies that bit to all the other wires
- We may have to “amplify” the signal (repeat it)
- Needs electrical current
Still One Big Bus
- single collision domain
- no improvement in MAX throughput
- can’t share this link “at the same time” (frames will hit!)
- average throughput < as # nodes increases
- still limited in distance and # of hosts
- collision detection requirements (gets harder as the medium gets larger)
- synchronization requirements
- requires performance”homogeneity”
- all have to speak the same Ethernet!
- can’t connect 10 Mbps and 100 Mbps networks
Note: Hub doesn’t even know the protocol!
Bridges - receives stuff and copies it over, frame-by-frame
- it SPEAKS the Link Layer Protocol!
- store and forward device
- can read the ENTIRE frame and “hold it” (to prevent collisions)
- “buffers” entire packet and THEN rebroadcasts it on other ports
- Creates separate collision domains
- in situations where frames don’t need to be copied over, it can check the addresses in
the header to KNOW not to forward to wires where they’re not needed
- can accommodate different speed interfaces! (10, 100, etc.)
-
Selective Forwarding
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