1. Local Area Networks: Topologies

2. Packet Identification & MAC Addresses
Each packet specifies an intended recipient with an identifier.
Demultiplexing uses the identifier known as an address.
IEEE (Institute for Electrical and Electronic Engineers) allocates a unique address for each piece of network interface hardware, i.e., a network interface card (NIC)
Media Access Control (MAC) address
A device may have multiple MAC addresses depending on how many network interface cards it has.

3. 48 bits in hexadecimal number

4. MAC Addresses Composition Vendor ID + NIC ID IEEE assigns a vendor ID
VV:VV:VV:NN:NN:NN
IEEE assigns a vendor ID
Organizationally Unique ID (OUI)
The vendor assigns a unique NIC value to each device

5. Exercise: Find MAC address of your computer
Start Command Prompt (Start Runcmd) and type ipconfig /all
Do a google search to find the vendor name of the NIC of your computer

6. Compose a Packet: Step 1 I like you
A packet consists of two conceptual parts:
Header that contains metadata, such as an address
Payload that contains the data being sent
header
payload
54:DD:91
11:CA:3B
E8:25:32
46:C2:8F
I like you
6 bytes
dest. address
6 bytes
source address
bytes

7. Compose a Packet: Step 2
Framing: using extra bits to delineate the start and end of a frame
SOH: Start of Header (ASCII code: )
EOT: End of Transmission (ASCII code: )
To enable computer to distinguish two packets, the SOH and EOT must be unique and different from the payload.
Frame: Layer-2 packet
SOH
header
payload
EOT
1 byte
1 byte

8. Compose a Packet: Step 3 Add error checking code Cable
Ethernet use Cyclic Redundancy Check (CRC), a form of channel coding for high-speed data network
SOH
header
payload
EOT
CRC
Cable
modulation
More on CRC: Chapter 8.13 – 8.14

9. How to Transmit a Packet?
Depends on the network type and topology
Three main categories of packet switching network:

10. LAN Example

11. LAN Topologies
Each LAN is classified into a category according to its topology or general shape

12. Star Topology in Detail
P-B
NIC-B
P-S
P-A
NIC-2
P-C
NIC-A
NIC-1
NIC-3
NIC-C
NIC-4
P-D
NIC-D

13. Star Topology in Detail
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. Bus Topology P-B P-D P-A P-C
Bus topology usually consists of a single cable to which computers are attached.
The ends of a bus network must be terminated.
Preventing electrical signals from reflecting back along the bus.
P-B
P-D
NIC-B
NIC-D
P-A
NIC-A
P-C
NIC-C

15. Bus Topology in Detail 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-C will receive the m-carrier
NIC-C demodulates the packet and put it into its cache
NIC-C does address checking; if the packet is not for P-C, discard it
NIC-C does length and CRC checkings; If failed, the packet will be discarded
If the packet passes all checkings, NIC-C will strip off SOH, EOT, CRC, and send “|C|A|I like you|” to P-C
I like you C A
CRC
I like you
SOH
EOT C A

16. Bus Topology
When there is a signal on the bus, everybody can sense it.
What if multiple computers are sending packets simultaneously?
Collisions may happen!

17. Bus Topology: The Collision Problem
Basic solution: CSMA (Carrier Sense Multiple Access)
Before any packet is sent out, the sender will first senses its connecting point to see if the bus is idle.
If nothing is sensed, it will send out a packet, otherwise it will wait until nothing is sensed.
Q1: Do you believe that CSMA can avoid all collisions? No

18. Bus Topology: The Collision Problem
Advanced solution: CSMA/CD (CD = Collisions Detection)
During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus.
If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit
Q2: If collision occurs, who will detect the collision?
Every computer will sense the collision

19. Bus Topology: The Collision Problem
Advanced solution: CSMA/CD (CD = Collisions Detection)
During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus.
If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit
Q3: Why do we wait a random amount of time?
So that we are sure that there isn’t as great a chance that the packets collide again

20. Bus Topology: The Collision Problem
Advanced solution: CSMA/CD (CD = Collisions Detection)
During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus.
If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit
Q4: When a NIC senses a collided m-carrier, will the NIC card demodulate it?
Yes. You have to demodulate the packet in order to know if it is corrupted

21. Bus Topology: The Collision Problem
Advanced solution: CSMA/CD (CD = Collisions Detection)
During the whole transmission period of any packet (after it is send out) the sender keeps on sensing the bus.
If any other m-carrier is sensed, stop transmitting instantly; wait for a random amount of time; then retransmit
Q5: When a NIC senses a collided m-carrier, will the NIC forward the packet to the computer?
No. The packet will be discarded.

22. Why Different Topologies?
Ring:
Easy for computers to coordinate access and to detect whether the network is operating correctly
An entire ring network is disabled if one of the cables is cut
Star:
Protecting the network from damage to a single cable
No collisions at all
Requiring more wires
Bus:
Fewer wires
A network is disabled if someone accidentally cuts the main cable
Difficult to add new devices if no tap exists.
As such, bus topology is losing popularity
but Plenty still around.