1. CS4470 Computer Networking Protocols
12/31/2018
CS Computer Networking Protocols
5. Data Link Layer 3
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Overview of data link layer LANs Framing Error detection
12/31/2018
Outline
Overview of data link layer
Framing
Error detection
LANs
Topologies
Ethernet
Extended LANs
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3. LAN Distance Limitations
LANs use shared medium - Ethernet, token ring
Why limit the length of LANs
Fare Access
CSMA/CD –with minimum frame size, if propagation delay too long we won’t detect collision while transmitting
Hardware is engineered to emit a fixed amount of electrical power
An electrical signal gradually becomes weaker as it travels along a copper wire.
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4. LAN Distance Limitations
Standards set a maximum distance for LANs
10BaseT 100m per segment
10Base m per segment
10Base m per segment
Can extend distances
Repeaters/Hubs
Bridges/Switches
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5. Repeaters
A bidirectional, analog amplifier that retransmits analog signals
Simply copy signals between segments
Do not understand frame formats or addresses
Act on bits
One repeater can effectively double the length of an LAN segment, e.g. 100m to 200m
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6. Hubs Hubs act as repeaters Connect multiple LAN segments hub
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7. Limits on Repeaters/Hubs
Can't extend Ethernet with repeaters infinitely
At most 4 repeaters
CSMA/CD requires low delay
if medium is too long, CSMA/CD won't work
Run into the problem of not being able to detect collisions while transmitting the frame
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8. Limits on Repeaters/Hubs
12/31/2018
Limits on Repeaters/Hubs
Repeaters/Hubs don’t understand frames
Don’t distinguish between signals that correspond to a valid frame and other electrical signals
If a collision or electrical interference occurs on one segment, repeaters cause the same problem to occur on all other segments
Even two computers are in the different LAN segments, they cannot transmit at the same time
Aside from hubs, bridges are used today in favor of repeaters
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9. Bridges A device that connects two LANs Act on frames
List to traffic on each segment
When it receives a frame from LAN1, it checks the destination address
In LAN1? Discard!
In LAN2? Go to 3)
The frame arrived intact?
Yes! Forward it to LAN2
No! discard
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10. Bridge Operation
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11. Advantages of Bridges They help isolate problems
Don’t forward interference signals in one segment to another
Don’t forward collisions from one segment to another
Only forward complete and correct frames
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12. Frame filtering A bridge doesn’t forward a frame unless necessary
How to determine whether to forward a frame?
A bridge receives a frame from LAN1
Checks the destination address
If the destination address is in LAN1, discard the frame
If not, forward the frame on the other segment
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13. Frame filtering (cont.)
How does a bridge know which computers are attached to which segment?
Adaptive learning
Bridges can learn automatically
No configurations required
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14. Bridge Learning Algorithm
Receive some data packet on some input port
Look at the packet’s source MAC address. If it’s not a broadcast address then:
Lookup address in internal lookup table
If not in the table, store it in the table using the address and the port it is on
If it is in the table, change the entry if it’s on a different port
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15. Bridge Operation Example
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16. Exercise
Four LANs are connected by three bridges. Each bridge maintains two tables which describe what stations are in each LAN.
Initially, all the tables are empty.
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17. Exercise
Show how the tables of the four bridges change after each of the following events happen in sequence.
A sends a frame to B
A sends a frame to F
F sends a frame to A
E sends a frame to I
G sends a frame to H
H sends a BROADCAST frame.
D sends a frame to G
G sends a frame to D
H sends a frame to C
A moves to LAN3 and then sends a frame to B
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18. 5. Link CS4470 Actions Bridge 1 Bridge 2 Bridge 3 LAN 1 LAN 2 LAN2
a.A sends a frame to  B  A A
b. A sends a frame to F
c. F sends a frame to A  F F
A,F
d. E sends a frame to I
 F,E A 
F, E
A,F,E
e. G sends a frame to H
  F,E,G
 A,G
 F, E
 A,F,E G
f. H sends a BROADCAST frame.
  F,E,G,H
A,G,H
G,H
g. D sends a frame to G
 F,E,G,H,D
A,G, H,D
A,F,E,D
 G,H
h. G sends a frame to D
 A,G, H,D
 A,F,E,D
i. H sends a frame to C
F,E,G,H,D 
F, E 
j. A moves to LAN3 and then sends a frame to B
 F,E,G,H,D,A
G, H,D 
F, E,A
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19. Multiple LANs
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20. Multiple Bridges
Anything wrong with this picture?
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21. A cycle of bridges
Bridged LANs contain a loop and all bridges forward broadcast frames
frames flow around the cycle forever
Computers on all segments receive an infinite number of copies
How to prevent this?
A bridged network must NOT allow both of the following conditions to occur simultaneously
All bridges forward all broadcast frames
The bridged network contains a cycle of bridged segments.
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22. A cycle of bridges A network is managed by more than one administrator
In such a setting, it is possible that no single person knows the entire configuration of the network
A bridge that closes a loop might be added without anyone knowing
Loops are built into the network on purpose
Redundancy is provided to make the network more immune o failure
Bridges must be able to handle loops
Run a distributed spanning tree algorithm
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23. Switches Switches are similar to bridges
They both operate at the link layer
They both know frame addresses
The two terms are often used interchangeably
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24. Combining switches and hubs
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25. Switch example Suppose C sends a frame to D
address
interface 1 2 3 A B E G 1 2 3
hub
hub
hub A I D F B G C H E
Switch receives frame from from C
notes in the switch table that C is on interface 1
because D is not in table, switch forwards frame into interfaces 2 and 3
frame received by D
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26. Switch example Suppose D replies back with frame to C.
address
interface 1 2 3 A B E G C 1 2 3
hub
hub
hub A I D F B G C H E
Switch receives frame from from D
notes in switch table that D is on interface 2
because C is in table, switch forwards frame only to interface 1
frame received by C
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27. Switches and Bridges? Most switches act just like a bridge
Segments traffic
Typically implements spanning tree algorithm
Has more ports than a bridge
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28. Hub vs. Bridge vs. Switch A B C D E F hub A B C A B C D E F switch
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