1. Now (switched network)
LAN Wiring
Wiring Scheme
Generation
Speed
Distance
Interference
10Base5
1st Gen (1970s)
10Mbps
500m
More resistant
10Base2
2nd Gen
200m
Somewhat resistant
10BaseT
3rd Gen
100m
Susceptible
100BaseT/
1000BaseT
Now (switched network)
100Mbps/
1000Mbps
Highly susceptible
As the Ethernet speed increases, the maximum distance for a LAN segment decreases because of the increased susceptibility to interference.

2. LAN Extensions

3. Distance Limitation and LAN Design
Hardware is designed to emit a fixed amount of energy
Long distance may cause weak signals and errors due to interference
Can we use other transmission media that is insensitive to electrical noise? 3 3

4. Fiber Modems Communicate over an optical fiber
Can span long distance
Provide standard network interface (e.g., Ethernet)
Expensive and fragile, seldom used

5. Other Technologies That Extend LANs
Three key extension technologies
Repeaters (early 90s)
Bridges
Switches

6. Repeaters
A repeater is an analog device used to propagate LAN signals over long distances
A layer 1 device
A repeater does not understand packets or signal coding
Instead, it merely amplifies the signal received 6 6

7. Connecting Two LAN SegmentsB C D E F R
When there is a collision on segment 1, will NIC-F sense the collision?
Can we use 99 repeaters to connect 100 LAN segments? No
A super long hallway  too busy / too many collisions
Repeater also amplifies noise and distortion  too many errors

8. Pros and Cons of Repeater
Low cost
Simple to implement
Cons
Limited number of repeaters that can be used in a network.
General rule-of-thumb: 5 segments / 4 repeaters 8 8

9. Bridges
A bridge is a mechanism that connects two LAN segments.
A layer 2 device
Receives all packets sent on the segment.
Forwards an intact frame from one segment to the other.
Two LAN segments connected by a bridge appear to behave like a single LAN.
LAN Segment 2
LAN Segment 1
P-B A D E F
NIC-L
NIC-R 9 9

10. Bridges Steps for P-A to send “I like you” to P-F:
P-A will compose the packet
P-A forwards the packet to NIC-A
NIC-A calculates the CRC, adds SOH, EOT
NIC-A modulates the packet  send out using CSMA/CD
After a while, NIC-L will receive the m-carrier
NIC-L demodulates the m-carrier and put it into the cache
NIC-L does length and CRC checkings
If the packet passes all checkings, NIC-L will strip off SOH, EOT, CRC, and forward “|F|A|I like you|” to P-B
P-B checks the destination address of this packet, if this address is for a computer on segment 1, the packet will be discarded; otherwise, the bridge forwards the packet
NIC-R calculates the CRC, adds SOH, EOT
NIC-R modulates the packet  send out using CSMA/CD
After a while, NIC-F will receive the m-carrier…
I like you F A
CRC
I like you
SOH
EOT F A

11. Bridges
Q1: If the packet was sent from computer A to computer D, will the bridge P-B see the packet?
Q2: If there is a collision on segment 1, will P-B know of this collision?
Q3: When we use bridges, do we need to follow the 5-4 rule?
1. Yes, because NIC-L does not do address filtering
2. No, because NIC-L will do CRC checking and discard the packet
3. No. Signal distortion will not be propagated with bridges

12. Why Bridging Works Well?
A bridged network can exhibit higher overall performance than a single LAN
Each segment forms a separate collision domain
A bridge permits simultaneous transmission on each segment 12 12

13. Review: Switches Steps for P-A to send “I like you” to P-C:
P-A will compose the packet
P-A forwards the packet to NIC-A
NIC-A calculates the CRC, adds SOH, EOT
NIC-A modulates the packet  send out
After a while, NIC-1 will receive the m-carrier
NIC-1 demodulates the m-carrier and put the packet into a queue
P-S checks this packet and find destination is “C”
NIC-3 modulates the packet  send out
After a while, NIC-C will receive the m-carrier
NIC-C demodulates the packet and put it into its cache
NIC-C does two checkings; If failed, the packet will be discarded
Length checking: the payload should be between 46 and 1500 bytes
CRC checking
If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and forward “|C|A|I like you|” to P-C
I like you C A
CRC
I like you
SOH
EOT C A

14. Switching and Layer 2 Switches
Logically, an Ethernet switch is similar to a set of bridges.
Dealing with packets, not signals.
A switch as simulating a bridged network that has one computer per LAN segment. 14 14

15. Switches vs. Bridges
Switches can be thought as more efficient versions of bridges
Completely eliminate collisions
Perform forwarding via hardware
Bridges use software
Do NOT add/remove SOH, EOT, CRC
No length or CRC checking